In Re: Tmi Litigation Lori Dolan Joseph Gaughan Ronald Ward Estate of Pearl Hickernell Kenneth Putt Estate of Ethelda Hilt Paula Obercash Jolene Peterson Estate of Gary Villella Estate of Leo Beam, No. 96-7623 in Re: Tmi Litigation All Except Lori Dolan, Joseph Gaughan, Ronald Ward, Estate of Pearl Hickernell, Kenneth Putt, Estate of Ethelda Hilt, Paula Obercash, Jolene Peterson, Estate of Gary Villella and Estate of Leo Beam, No. 96-7624 in Re: Tmi Litigation

193 F.3d 613 (3rd Cir. 1999)

IN RE: TMI LITIGATION

LORI DOLAN; JOSEPH GAUGHAN; RONALD WARD; ESTATE OF PEARL HICKERNELL; KENNETH PUTT; ESTATE OF ETHELDA HILT; PAULA OBERCASH; JOLENE PETERSON; ESTATE OF GARY VILLELLA; ESTATE OF LEO BEAM, APPELLANTS NO. 96-7623

IN RE: TMI LITIGATION

ALL PLAINTIFFS EXCEPT LORI DOLAN, JOSEPH GAUGHAN, RONALD WARD, ESTATE OF PEARL HICKERNELL, KENNETH PUTT, ESTATE OF ETHELDA HILT, PAULA OBERCASH, JOLENE PETERSON, ESTATE OF GARY VILLELLA AND ESTATE OF LEO BEAM, APPELLANTS NO. 96-7624

IN RE: TMI LITIGATION

ALL PLAINTIFFS; ARNOLD LEVIN; LAURENCE BERMAN; LEE SWARTZ APPELLANTS NO. 96-7625

Nos. 96-7623/7624/7625

U.S. Court of Appeals, Third Circuit

ARGUED: June 27, 1997

Opinion filed November 02, 1999

1

ON APPEAL FROM THE UNITED STATES DISTRICT COURT FOR THE MIDDLE DISTRICT OF PENNSYLVANIA (Civil No. 88-cv-01452) (District Judge: Honorable Sylvia H. Rambo)[Copyrighted Material Omitted][Copyrighted Material Omitted][Copyrighted Material Omitted][Copyrighted Material Omitted][Copyrighted Material Omitted][Copyrighted Material Omitted]

2

Arnold Levin, Esq. Laurence S. Berman, Esq. (Argued) Craig D. Ginsburg, Esq. Levin, Fishbein, Sedran & Berman 510 Walnut Street, Suite 500 Philadelphia, PA 19106

3

Lee C. Swartz, Esq. Hepford, Swartz & Morgan 111 North Front Street P.O. Box 889 Harrisburg, PA 17108 Attorneys for Appellants in No. 96-7623/7624/7625

4

Lou Tarasi, Esq. Tarasi & Johnson, P.C. 510 Third Avenue Pittsburgh, PA 15129 Of Counsel for Certain Appellants Identified in the Entry of Appearance in Appeal No. 96-7624/7625

5

Stephen A. Saltzburg, Esq. (Argued) Howrey Professor of Trial Advocacy, Litigation and Professional Responsibility George Washington Law School 720 20th Street, N.W. Washington, D.C. 20052 Of Counsel for Appellants in No. 96-7623/7624/7625

6

Daniel J. Capra, Esq. Reed Professor of Law Fordham University School of Law Lincoln Center 140 West 62nd Street New York, NY 10023 Of Counsel for Certain Appellants Identified in the Entry of Appearance in Appeal No. 96-7625

7

A.H. Wilcox, Esq. (Argued) Ellen K. Scott, Esq. (Argued) Eric J. Rothschild, Esq. (Argued) Pepper, Hamilton & Scheetz, Llp 3000 Two Logan Square 18th and Arch Streets Philadelphia, PA 19103 Lewis S. Kunkel, Jr., Esq. Thomas B. Schmidt, III, Esq. Pepper, Hamilton & Scheetz Llp 200 One Keystone Plaza North Front & Market Streets P.O. Box 1181 Harrisburg, PA 17108 Attorneys for Appellees in Nos. 96-7623/7624/7625

8

Reuben A. Guttman, Esq. Provost & Umphrey 1350 New York Avenue, N.W. Suite 1040 Washington, D.C. 20005 Ned Miltenberg, Esq. Associate General Counsel Association of Trial Lawyers of America ("atla") 1050 31st Street, N.W. Washington, D.C. 20007 Amicus Curiae, Association of Trial Lawyers of America ("atla"), in Support of Appellants

9

Before: Greenberg and McKEE, Circuit Judges, and Greenaway, District Judge *

10

TABLE OF CONTENTS

I. INTRODUCTION 622

II. PROCEDURAL HISTORY 623

III. SCIENTIFIC BACKGROUND 629

A. Overview of Relevant Principles of Nuclear Physics 629

1. Atomic and Nuclear Structure 629

2. Radioactivity 632

3. Ionizing Radiation 634

4. Radiation Quantities and Units 636

5. Health Effects of Ionizing Radiation 638

i. Deterministic Effects 640

ii. Stochastic Effects 642

6. Radiation in the Environment 644

i. Natural Radiation 644

ii. Man-made Radiation 647

IV. NUCLEAR ENGINEERING 648

A. Nuclear Reaction 648

B. The Operation of Nuclear Power Plant 651

C. Barriers to Release of Radioactive Materials into the Environment 655

V. THE ACCIDENT AND ITS AFTERMATH 655

A. The Accident at TMI-2 655

B. Radioactive Materials Released to the Environment 657

C. Pathways of Exposure to Radioactive Materials 658

VI. LEGAL DISCUSSION 659

A. The Trial Plaintiffs' Appeal 659

1. Background 659

2. Standards Governing the Admissibility of Scientific Evidence 662

3. Trial Plaintiffs' Dose Exposure Expert Witnesses 666

i. Ignaz Vergeiner 666

a. Qualifications 666

b. Vergeiner's Opinion. 667

c. Discussion and Conclusions 667

ii. Charles Armentrout and Victor Neuwirth 672

a. Qualifications 672

b. Armentrout's Observations and Experiences 672

c. Discussion and Conclusion 673

d. Neuwirth's Soil Sample Analyses and Armentrout's Dose

Estimates. 674

e. Discussion and Conclusion 675

iii. James Gunckel 677

a. Qualifications 677

b. Gunckel's Opinion 678

c. Discussion and Conclusions 680

iv. Vladimir Shevchenko 683

a. Qualifications 683

b. Shevchenko's Tree Study 684

c. Discussion and Conclusions 686

d. The Cytogenetic Analysis 688

e. Discussion and Conclusions 690

v. Gennady Kozubov 693

11

a. Qualifications 693

b. Kozubov's Opinion 693

c. Discussion and Conclusions 694

vi. Olga Tarasenko 695

a. Qualifications 695

b. Tarasenko's Opinion 695

c. Discussion and Conclusions 697

vii. Bruce Molholt 698

a. Qualifications 698

b. Molholt's Opinions 699

c. Discussion and Conclusions 701

viii. Sigmund Zakrzewski 704

a. Qualifications 704

b. Zakrzewski's Opinion 704

c. Discussion and Conclusions 705

ix. Theodor Sterling 706

a. Qualifications 706

b. Sterling's Opinion 706

c. Discussion and Conclusions 707

x. Steven Wing 708

a. Qualifications 708

b. Wing's Mortality Study 709

c. Discussion and Conclusions 710

d. Wing's Cancer Incidence Study 711

e. Discussion and Conclusions 712

xi. Douglas Crawford-Brown 713

a. Qualifications 713

b. Crawford-Brown's Opinion 714

c. Discussion and Conclusions 714

4. Effect of the Exclusion of Wing's Lung Cancer Testimony 716

5. Exclusion of Experts' Submissions as Untimely 717

6. Conclusion 722

B. The Non-Trial Plaintiffs' Appeal 723

C. The Monetary Sanctions Appeal 728

D. Reassignment Upon Remand 728

VII. CONCLUSION 729

OPINION OF THE COURT

I. INTRODUCTION

12

These three appeals arise out of the nuclear reactor accident which occurred on March 28, 1979, at Three Mile Island in Dauphin County, Pennsylvania. 1 Two of the appeals concern the personal injury claims of more than 2,000 Three Mile Island area residents who allege that they have developed neoplasms 2 as a result of the radiation released into the environment as a result of the reactor accident. The first appeal is that of a group of ten trial plaintiffs who were selected by the parties after the District Court adopted the plaintiffs' case management order, which called for a "mini-trial" of the claims of a group of "typical" plaintiffs (the "Trial Plaintiffs"). The critical issue there is the trial plaintiffs' ability to demonstrate that they were exposed to doses of radiation sufficient to cause their neoplasms. Proof of that causation depended on the admissibility of the testimony of several experts that the Trial Plaintiffs retained. These experts attempted to testify about the amount of radiation released into the environment by the nuclear reactor accident, and thereby correlate the plaintiffs' neoplasms to that accident.

13

Defendants challenged the admissibility of the experts' testimony and the District Court was therefore required to hold extensive in limine hearings pursuant to its "gatekeeping" role under Daubert v. Merrell Dow Pharmaceuticals, Inc., 509 U.S. 579 (1993). Following those hearings, the court excluded the overwhelming majority of the Trial Plaintiffs' proposed expert testimony as to dose exposure. Following the exclusion of the dose exposure testimony, the defendants moved for summary judgment alleging that Trial Plaintiffs could not establish causation absent the excluded expert testimony regarding dose.

14

The District Court agreed and held that, as a result of its rulings under Daubert, Trial Plaintiffs were unable to connect their neoplasms to the TMI accident. Accordingly, the court granted summary judgment in favor of defendants and against the Trial Plaintiffs. In re TMI Litigation Consolidated Proceedings, 927 F. Supp. 834 (M.D. Pa. 1996). The District Court then reasoned that its Daubert rulings would be binding on all of the other plaintiffs, i.e., the Non-Trial Plaintiffs, if there were evidentiary issues common to all plaintiffs, Id. at 837 . Therefore, the court therefore extended its Trial Plaintiff summary judgment decision to the Non-Trial Plaintiffs, and granted summary judgment to the defendants on all of the claims of the approximately 2,000 remaining TMI personal injury plaintiffs. The propriety of that extension is the subject of the second appeal.

15

The third and last appeal concerns the propriety of the District Court's imposition of monetary sanctions against certain of the plaintiffs' counsel for violations of pre-trial discovery requirements and orders. The sanctioned counsel have requested that the TMI personal injury litigation be reassigned to another trial Judge upon remand, if we reverse the District Court in either or both of the first two appeals.

16

For the reasons that follow, we will affirm the grant of summary judgment to the defendants on the claims of the Trial Plaintiffs (No. 96-7623). We will, however, reverse the grant of summary judgment to the defendants on the claims of the Non-Trial Plaintiffs (No. 96-7624), but we will affirm the imposition of monetary sanctions and deny the request for reassignment (No. 96-7625).

II. PROCEDURAL HISTORY 3

17

On March 28, 1979, radioactive materials were released into the environment as the result of an accident which occurred at Unit 2 of the Three Mile Island nuclear power generating station in Dauphin County ("TMI-2"). Three Mile Island is a small island in the Susquehanna River, approximately fifteen miles downstream from Harrisburg, Pennsylvania. Following the accident, thousands of personal injury and other non-personal injury claims 4 were filed against the owners and operators of the nuclear facility. 5

18

As noted, more than 2,000 plaintiffs filed claims for personal injuries 6 purportedly caused by exposure to the radioactive materials released during the accident. Some of these personal injury claims were originally filed in the early 1980's in state and federal district courts in Pennsylvania, New Jersey and Mississippi. The defendants removed the state court actions to federal district courts in Pennsylvania and New Jersey, under the authority of the Price-Anderson Act, Pub.L. No. 85-256, 71 Stat. 576 (1957). 7

19

After removal, the District Court for the Middle District of Pennsylvania ordered, inter alia, that all pending TMI personal injury cases in the Middle District be "consolidated for pretrial proceedings only." App. 13097. The District Court also ordered that the caption of every subsequent personal injury pleading should be identified as a personal injury claim. Id.

20

After we held that the Price-Anderson Act did not create a cause of action as a federal tort and was not intended to confer jurisdiction on federal district courts, see Stibitz v. General Public Utilities Corp., 746 F.2d 993, 997 (3d Cir. 1984) and Kiick v. Metropolitan Edison Co., 784 F.2d 490, 493 (3d Cir. 1986), the state court actions were remanded, and the federal court actions were transferred to the appropriate state courts. The cases originally removed to the Middle District of Pennsylvania, and those originally filed in the Middle District, were either remanded or transferred to the Court of Common Pleas of Dauphin County. Thereafter, in 1985 and 1986, the bulk of the personal injury claims which are the subject of this appeal were filed in the state courts. 8

21

On October 15, 1985, the Dauphin County Common Pleas Court entered a case management order. In that order, the Court of Common Pleas ordered that all cases be consolidated for pretrial purposes, and also required that all pleadings be captioned to identify which plaintiffs' group they applied to. That is, all personal injury cases received from the federal court were consolidated under the caption "Cases Consolidated I" and the cases filed in state court after our decision in Stibitz, were consolidated under the caption "Cases Consolidated II."

22

In 1988, Congress enacted the Price-Anderson Amendments Act of 1988, Pub.L. No. 100-408, 102 Stat. 1066. Those amendments to the Price-Anderson Act created a federal cause of action for "public liability actions" 9 and provided that all such suits arise under the Price-Anderson Act, 42 U.S.C. § 2014 (h). The Act also provided for consolidation of such actions, including those already filed, in one federal district court. 42 U.S.C. § 2210 (n). 10 Following enactment of that Act, the defendants removed all the pending state actions to the United States District Court for the Middle District of Pennsylvania.

23

Thereafter, the District Court for the Middle District of Pennsylvania conducted a case management conference. The personal injury cases known as "Cases Consolidated I" and "Cases Consolidated II" which had been removed from the Court of Common Pleas of Dauphin County were then pending in the Middle District along with the companion actions to the "Cases Consolidated II" which had been filed by forty-two plaintiffs in Mississippi federal and state court to take advantage of the more lenient Mississippi statute of limitations. 11 As a result of Discussions during the conference, the District Court entered an order which required counsel to

24

meet to streamline the record with an eye toward reducing the number of duplicative plaintiffs and suits, assigning fewer case numbers for the various actions, and deciding which cases needed new complaints to be filed and which actions do not need answers filed.

25

Supp. App. at 78. In response to the order, counsel for plaintiffs and defendants submitted a Stipulation which provided, inter alia, that the pending TMI personal injury cases referred to as "Cases Consolidated I" and "Cases Consolidated II," together with the companion Mississippi cases, would be consolidated under a single civil action number "for administrative purposes" (emphasis added). App. Vol. I, at 440. The Stipulation required that pleadings dealing with issues common to all plaintiffs, or a legal issue potentially applicable to all plaintiffs, bear the caption "In re TMI Consolidated Proceedings" as well as the additional legend: "This document Relates to: All Plaintiffs." Id. The Stipulation further required that pleadings dealing with issues relating to one or more identified plaintiffs be captioned "In Re TMI Consolidated Proceedings" and identify lead counsel, the number of plaintiffs represented by lead counsel and the number of plaintiffs to whom the pleadings refer. Id. The Stipulation also expressly provided that

26

3. Nothing in... this Stipulation... shall be deemed to constitute or affect any waiver of claim, defense or issue, including but not limited to the statute of limitations, choice of law and bifurcation or consolidation for trial of claims, defenses, issues, parties or proceedings.

27

Id. The Stipulation was subsequently approved by the District Court.

28

Thereafter, in July of 1992, the defendants filed a motion for summary judgment directed to the forty-two plaintiffs who had sued in Mississippi state and federal courts. Defendants alleged that those claims were untimely under Section 11(b) of the Price-Anderson Amendments Act of 1988, codified at 42 U.S.C. § 2014 (hh) (the choice of law provisions), which provides that "the substantive rules of decision in [any public liability action] shall be derived from the law of the State in which the nuclear incident involved occurs," and under Section 20(b) of that Act, (the effective date provision), which provides that "the amendments made by Section 11" of the Act "shall apply to nuclear incidents occurring before, on, or after the date of the enactment of this Act." 42 U.S.C. § 2014 note. The District Court ruled that the Mississippi actions were time-barred, dismissed the respective claims, and granted summary judgment in favor of the defendants because it reasoned that § 20(b), read in conjunction with § 11, compelled the retroactive application of Pennsylvania's two-year statute of limitation to the plaintiffs' claims. In re TMI Cases Consolidated II, No. 88-14532, slip op. at 2-6 (M.D. Pa. Aug. 16, 1993).

29

On appeal, the Mississippi plaintiffs argued, inter alia, that retroactive application of the choice of law provision violated constitutional guarantees of due process. We disagreed, and held that the retroactive application of the choice of law provision was a rational exercise of Congress' legislative power. Accordingly, we affirmed the District Court's grant of summary judgment, and its dismissal of the claims of the forty-two plaintiffs. In re TMI, 89 F.3d 1106 (3d Cir. 1996), cert. denied, 519 U.S. 1077 , 117 S. Ct. 739 (1997).

30

The defendants then moved for summary judgment against all the TMI plaintiffs, claiming that they had not breached the duty of care owed to the plaintiffs. The District Court denied the motion. The court held that state law on that issue was preempted, and that federal law determines the standard of care. In re TMI Litigation Cases Consolidated II, 904 F.Supp. 379, 395 (M.D. Pa. Feb. 18, 1994). The court also held that federal regulations 12 set the standard of care, and that each plaintiff must prove his or her individual exposure to radiation in order to establish causation, but not to establish a breach of the duty of care. Id. at 30-31. Upon defendants' motion, the District Court certified the duty of care and causation questions for interlocutory appeal. 13 On that appeal, we held that plaintiffs must establish that (1) the defendants released radiation into the environment in excess of the levels permitted by the federal regulations in effect in 1979; (2) the plaintiffs were exposed to this radiation, although not necessarily at the levels prohibited by those regulations; (3) they have injuries; and (4) radiation was the cause of those injuries. In re TMI, 67 F.3d 1103, 1119 (3d Cir. 1995), cert. denied, 516 U.S. 1154 (1996).

31

After remand, the District Court conducted lengthy in limine hearings in November of 1995 and in February and March of 1996, pursuant to Daubert v. Merrell Dow Pharmaceuticals, Inc., 509 U.S. 579 (1993). Those hearings all relate to plaintiffs' radiation dose and medical causation expert witnesses. In January and April of 1996, the District Court issued several opinions granting the majority of the defendants' motions in limine. See In re TMI Cases Consolidated II, 166 F.R.D. 8 (M.D. Pa. 1996) (granting in part defendants' motions to exclude plaintiffs' medical causation experts); Id., 922 F. Supp. 1038 (M.D. Pa. 1996)(same); Id., 922 F. Supp. 997 (M.D. Pa. 1996) (granting in part defendants' motions to exclude plaintiffs' radiation dose and medical causation experts); Id., 911 F. Supp. 775 (M.D. Pa. 1996) (granting in part defendants' motions to exclude plaintiffs' radiation dose experts); Id., 910 F. Supp. 200 (M.D. Pa. 1996)(same). Although the District Court was convinced that the majority of the plaintiffs' expert witnesses were well-qualified, the court nonetheless "found many of their opinions to be based on methodologies that were scientifically unreliable and upon data that a reasonable expert in the field would not rely upon." In re TMI Litigation Consolidated Proceedings, 927 F. Supp. 834, 839 (M.D. Pa. 1996). Accordingly, it ruled that the much of the expert testimony was inadmissible under Daubert, and its progeny. In April of 1996, the defendants filed a motion for summary judgment. They based the motion upon their contention that the District Court's Daubert rulings left the plaintiff's with no admissible evidence as to the radiation dose levels resulting from the TMI accident.

32

A subsidiary issue arose during the summary judgment proceedings as to whom the summary judgment rulings would apply. That dispute had its beginnings in June of 1993, when the District Court adopted the plaintiffs' proposed case management plan which called for an "initial mini-trial of the claims of twelve `typical' plaintiffs," half chosen by plaintiffs and half chosen by defendants. App. at 168. Under the plaintiffs' plan (which was adopted by the District Court), discovery would proceed immediately as to all issues, including punitive damages and, upon completion of discovery, "the twelve illustrative Plaintiffs would then proceed to trial on all their claims." Id. Ultimately, ten test plaintiffs, 14 who have been diagnosed with the listed illnesses, were chosen. 15

33

When the defendants filed their motion for summary judgment, they captioned it as pertaining to "All Plaintiffs" and argued that the District Court's summary judgment motion should be binding on all plaintiffs, not just the ten trial or test case plaintiffs. The District Court agreed, stating:

34

The court finds that resolution of the issue before it turns on the grounds upon which the court ultimately grants or denies summary judgment. Defendants are correct that to the extent the ruling turns on broad evidentiary issues common to all Plaintiffs, the ruling will be binding on all Plaintiffs. Likewise, Plaintiffs are correct that insofar as a ruling is based on a more narrow, Plaintiff-specific inquiry, the ruling will apply only to certain Plaintiffs. The court's reading of documents related to the June 15, 1993 order, in conjunction with subsequent case management orders and evidentiary rulings, indicates that discovery and evidentiary matters were to proceed on an "All Plaintiffs" basis. A contrary intention or result would obviate all benefits of having consolidated the many separate actions. Each Plaintiff's case depends upon expert testimony to prove both exposure and medical causation. Expert discovery is complete, and all expert reports have been filed. Thus, to the extent that the expert testimony of record fails to meet the test Plaintiffs' evidentiary burden at this state of the litigation, it will fail to meet the same burden as to every Plaintiff. It would be an exercise in futility and a waste of valuable resources to allow the many separate actions consolidated under this caption to proceed if it were clear that the cases could not withstand a motion for summary judgment. Under such circumstances, the court's summary judgment ruling will be applicable to all Plaintiffs.

35

927 F. Supp. at 838 .

36

The District Court ruled on the merits of the summary judgment motion that the Trial Plaintiffs had failed to present either direct or indirect evidence of the doses of cancer inducing levels of radiation that they were exposed to. Id. at 870 . Accordingly, the court extended its grant of summary judgment to all of the plaintiffs' cases.

37

Because the court finds the quantum of evidence on the issue of dose to be insufficient, and because no Plaintiff will be able to state a prima facie case without adequate dose evidence, the instant ruling is binding on all Plaintiffs.

38

Id. at 838 . Accordingly, the court granted summary judgment against all of the plaintiffs, both trial and nontrial.

39

These appeals followed.

40

Appeal Number 96-7623 is the appeal of the ten Trial Plaintiffs. They argue that the District Court improperly excluded their proffered expert witnesses' testimony on dose exposure, thereby erroneously subjecting them to summary judgment. They do not argue that summary judgment was improper given the District Court's Daubert rulings. Thus, if we determine that the District Court's exclusion of their dose exposure testimony was proper, we must affirm the summary judgment for the defendants against the trial plaintiffs. Consequently, the primary issue for our determination in case number 96-7623 is the propriety of the District Court's exclusion of testimony of the dose exposure experts. If, however, we decide that the court improperly excluded some or all of that evidence, we must then decide whether the evidence that was admissible is sufficient to create a genuine issue of material fact.

41

Appeal Number 96-7624 is the appeal of all of the TMI personal injury plaintiffs except the ten Trial Plaintiffs. Appellants there argue that the District Court improperly extended its Trial Plaintiffs' summary judgment decision to them. Appeal Number 96-7625 is the appeal of sanctioned counsel for the majority of the plaintiffs. Counsel argue that the District Court's imposition of monetary sanctions against them for discovery violations was improper. Each appeal is considered separately.

42

It is both impractical and unwise to begin our analysis of the Daubert challenge to the scientific testimony without first providing a brief Discussion of the fundamental principles of nuclear physics, nuclear engineering, the TMI-2 accident, ionizing radiation, and the health effects of ionizing radiation on the human body. These scientific principles are at the center of the damage that plaintiffs claim they suffered as a result of the TMI accident and the District Court's Daubert rulings. Total immersion in the complexities of these disciplines is neither required, nor possible. Accordingly, we offer the following overview of the controlling principles with an awareness that doing so stretches the boundaries of our institutional competence, and with a recognition of our need to borrow heavily from others in academic disciplines far from the familiar confines of the law.

III. SCIENTIFIC BACKGROUND

43

A. Overview of Relevant Principles of Nuclear Physics.

44

1. Atomic and Nuclear Structure.

45

Plaintiffs alleged that the accident at TMI resulted in a release of radiation into the atmosphere that caused changes to the atomic structure of their chromosomes and resulted in the formation of neoplasms. Their allegations thus implicate the structure of the atom -- the basic building block of matter -- and the physics of orbiting electrons. 16

46

The atom consists of a small but massive central nucleus surrounded by one or more orbital electrons. JOHN R. LAMARSH, INTRODUCTION TO NUCLEAR ENGINEERING 8 (2d ed. 1983). Orbiting electrons are negatively charged, much smaller in mass than the neutron, and their distances from the nucleus are much larger than the radius of the nucleus. DAVID BODANSKY, NUCLEAR ENERGY: PRINCIPLES, PRACTICES AND PROSPECTS 346 (1996). The average distance from the nucleus to the place where the outermost electron is found provides an approximate measure of atomic size. This distance is approximately the same for all atoms, except a few of the lightest atoms, and is about 2 x 10 -8 centimeters. 17 LAMARSH, at 11.

47

The nucleus has two constituent parts of approximately equal mass -- the neutron and the proton. 18 BODANSKY, at 346. Each is much more massive than the electron. LAMARSH, at 6-7. Together, they are called nucleons. BODANSKY, at 346. The neutron and proton differ in that the neutron is neutral while the proton has a positive charge equal in magnitude to the negative charge of the electron. Id. An atom is neutral or "un-ionized" when the number of positively charged protons equals the number of negatively charged electrons. D. J. BENNET, ELEMENTS OF NUCLEAR POWER 1 (2d ed. 1981). 19 "Nuclides" are very important to our Discussion. They are differing "species" of atoms whose nuclei contain particular numbers of protons and neutrons. LAMARSH, at 8. A nuclide is given the shorthand notation A Z X, where X is the symbol for the chemical element, Z is the atomic number and A is the atomic mass number. KNIEF, at 29. In general practice, however, the subscript Z is omitted because once the element, X, is given, so is the atomic number, Z. BODANSKY, at 346. Nuclides whose nuclei contain the same number of protons, i.e., the same Z, but different numbers of neutrons, i.e., different N and therefore a different mass number, A, are called isotopes of the element. BENNET, at 2. All elements have a number of isotopes, Id., and they are virtually identical in their chemical properties to the elements they are isotopes of. BODANSKY, at 346. However, the masses and other characteristics of their nuclei are different. BENNET, at 2. An isotope of an element is given the same shorthand notation as the nuclide. For example, naturally occurring oxygen, whose chemical symbol is "O", consists of three isotopes, 16 O, 17 O, and 18 O. Id. Each has 8 protons and electrons, i.e., the same atomic number, Z, but they have 8, 9 and 10 neutrons respectively, i.e., different N (N = A - Z). The nuclei of a given element can have the same mass number, A, but have a different atomic number, Z, in which case it is called an isobar. BODANSKY, at 346.

48

Though counterintuitive in the extreme, it is nevertheless a fact of atomic structure that the mass of an atom is less than the sum of the masses of its constituent parts. BENNET, at 4; BODANSKY, at 350; KNIEF, at 29; LAMARSH, at 28. The difference between the mass of the assembled atom and the sum of the mass of the component atomic parts is known as the "mass defect". KNIEF, at 29. However, mass is not really lost in the assembly of an atom from its component parts. Rather, the mass defect is converted into energy when the nucleus is formed. Id. The conversion is explained by the "principle of the equivalence of mass and energy in which Einstein stated that mass and energy are different forms of the same fundamental quantity." 20 BENNET, at 4. Therefore, in any reaction where there is a reduction in mass, the decrease is accompanied by a release of energy. Id. The energy associated with the mass defect is called "binding energy" and it represents the total energy that would be required to disassemble a nucleus into its constituent neutrons and protons. BODANSKY, at 350. Binding energy increases in a nucleus as the number of particles in the nucleus increase. In other words, binding energy increases with a corresponding increase in atomic mass number. LAMARSH, at 28. However, the rate of increase is not uniform. KNIEF, at 30.

49

The amount of binding energy in a nucleon is important when determining possible sources of nuclear energy. LAMARSH, at 28. A nuclei is stable or tightly bound when the binding energy per nucleon is high. Accordingly, a relatively large amount of energy must be supplied to break the stable nuclei apart. Id. When a tightly bound nucleus is broken apart and two nuclei of intermediate mass are formed, a relatively large amount of energy is released. BENNET, at 7. In contrast, nuclei with low binding energy per nucleon are easily broken apart, and less energy is released. LAMARSH, at 29.

50

The now familiar term, "nuclear fission" refers to the process of causing a tightly bound nucleus to split into two nuclei of intermediate mass. Id. The process proceeds in the direction of increased binding energy per nucleon. BENNET, at 7. That is, the nuclei of intermediate mass created by the fission process have greater binding energy than the original nucleus. LAMARSH, at 30. When the nuclei of intermediate mass have greater binding energy than the original nucleus, energy is released during the formation of the final nuclei. BODANSKY, at 351. This energy that is released as a result of the fission process is the source of energy in a nuclear reactor. LAMARSH, at 30. It is what we commonly refer to as "nuclear energy".

51

Atoms can exist only in certain states or configurations, with each state having its own specific energy. BODANSKY, at 351. The different energy states correspond to different electron orbits of different radii, LAMARSH, at 15, each with an energy level equal to the sum of the kinetic and potential energies of the electron in its orbit. BODANSKY, at 351. The lowest state of energy is called the "ground state" and it is the state in which the atom is normally found. LAMARSH, at 15. However, an electron can, as a result of a nuclear reaction, jump from its normal orbit to an orbit that is farther from the nucleus. An increase in energy corresponds to this "jump", and when an atom has more energy than its ground state it is said to be in an "excited state". BENNET, at 8. An atom can have a number of excited states which correspond to the number of jumps the electron has made. Id. The highest energy state occurs when the electron is completely removed from the atom. LAMARSH, at 15. The complete removal of an electron from an atom is called "ionization" and the resulting atom is said to be "ionized". Id.

52

The nucleons in the nuclei also move in orbits; however, the orbits of nucleons are not as well defined, and are not as well understood, as the orbits of electrons. L AMARSH, at 16. Like atoms, nuclei normally exist in the ground state. BENNET, at 8; BODANSKY, at 352. However, nuclei can reach excited states just as atoms can. BENNET, at 8; BODANSKY, at 352. The process is more complicated in nuclei than in atoms because excitation of nuclei can result in several nucleons being raised to excited levels simultaneously. BENNET, at 8. Although it is not yet possible to account theoretically for the exact energy levels of nuclei, as it is possible to do so for atoms. BODANSKY, at 352. It is generally true that the energies of the excited states and the energies between states are much greater for nuclei than for atoms. LAMARSH, at 16. The greater energy results from the greater forces acting between nucleons. These forces are much stronger that the forces acting between electrons and the nucleus. Id.

53

With a few exceptions, excited states in either atoms or nuclei exist for only a very short time, about 10-14 seconds. BENNET, at 9. Excess energy is quickly emitted and the system, either atomic or nuclear, decays to states of lower energy until it ultimately returns to its ground state. LAMARSH, at 15. The process of going from one state to another is called a "transition". Id. The energy lost in a transition is usually carried off by electromagnetic radiation, 21 BENNET, at 9; BODANSKY, at 352, with the lost energy equal to the difference in the energies of the two states. 22 LAMARSH, at 15.

54

2. Radioactivity.

55

As suggested by our Discussion thus far, nuclei are either stable or unstable. For all practical purposes, stable nuclei remain unchanged forever. Unstable nuclei decay spontaneously into lighter nuclei pursuant to a time scale that is unique for every element (the "half-life"). 23 The half-life for a given element is defined as the time required for one-half of a given sample of the element to "decay." If the half-life is greater than some undefined fraction of a second, the process of decay is called "radioactivity." Half-lives vary from less than a second to many billions of years. BODANSKY, at 353. Radioactivity is then, the process by which unstable nuclei seek stability. KNIEF, at 31. Frequently, the original unstable nucleus, called the "parent nucleus", decays to another radioactive nucleus, called the "daughter nucleus." LAMARSH, at 19. There may be more than one radioactive daughter nuclei produced until stability is reached. BENNET, at 11. This process of the creation and subsequent decay of several daughter nuclei is referred to as a "decay chain". LAMARSH, at 19. 24

56

The exact time at which any single nucleus will decay cannot be determined. KNIEF, at 34. However, the average behavior of a very large sample of radioactive material can be described statistically. BENNET, at 15. For a given nuclide, there is an average time, called the "decay constant", which characterizes its rate of decay. Id. The decay constant is defined as the probability per unit of time that a decay will occur. KNIEF, at 34. The amount of radioactivity present during a decay is referred to as "activity". F RED A. METTLER, JR., M.D., AND ARTHUR C. UPTON, M.D., MEDICAL EFFECTS OF IONIZING RADIATION 7 (2d ed. 1995) (hereinafter "MEDICAL EFFECTS"). The activity of a given sample is the average number of disintegrations per unit of time. For a large sample, the activity is the product of the decay constant and the number of atoms present. Id. The traditional unit for measuring radioactivity is the curie (Ci), which is defined as 3.7 x 10 10 disintegrations per second. 25

57

A radioactive nuclide is called a "radionuclide." KNIEF, at 32. During the process of radioactive decay, the nucleus spontaneously emits an alpha (a) particle or a beta () particle. BODANSKY, at 354. The emission of these particles is often accompanied by the emission of one or more gamma (g) rays. Id. An alpha (a) particle is a highly stable nucleus of the isotope helium 4 ( 4 He), consisting of two protons and two neutrons. LAMARSH, at 20. 26 Alpha (a) particles have a double positive charge and are emitted in a discrete energy spectrum. Id. They have a low level of energy and, therefore, are only capable of penetrating matter a small distance. 27

58

Decay by alpha particle emission is rather rare for nuclides lighter than lead (Pb) which has an atomic number (Z) of 82. BODANSKY, at 355. However, many of the naturally occurring radioactive elements with atomic numbers between 84 (polonium) and 92 (uranium), i.e., the heavier elements, decay by alpha particle emission. BENNET, at 13. When these elements decay, the daughter product is closer to the stability region than the parent. Id. In addition, the daughter nucleus of these heavier elements is frequently formed at an excited state of energy so that the excited nucleus immediately decays further to its ground state by the emission of gamma (g) radiation. Id. Thus, the decay of a heavy radioactive isotope by alpha particle emission also produces gamma (g) radiation. Id.

59

A beta () particle is an electron of nuclear, not orbital, origin, KNIEF, at 33, but it is identical to the electrons that orbit the nucleus. BODANSKY, at 355. Because it is an electron, it has much less mass than an alpha particle. Id. A neutron that is bound into the nucleus is not stable. LAMARSH, at 7. During decay, a neutron in the nucleus is transformed into a proton and an electron and it is this electron which is emitted as a beta () particle. Id.; BENNET, at 13.

60

Because beta () particle decay has the effect of transforming one of the neutrons into a proton, the resulting daughter nucleus has the same mass number (A) as the parent, but its atomic number (Z) is greater by one. Id. Moreover, the daughter nucleus may be formed in an excited state, and decay to its ground state by the emission of gamma (g) radiation. Id.

61

In most cases, beta particles are negatively charged and are more properly designated as - particles. Positive electrons, called "positrons" or + particles, are emitted from artificial radionuclides that are produced when positive particles, such as protons or alpha (a) particles, combine with a nucleus to form an unstable proton-rich nucleus. BODANSKY, at 355. These beta particles are very rare in naturally existing material. Id.

62

Beta () particles do not all have the same energy. BENNET, at 13. The spectrum of the energy of these particles, ranges from zero to a fixed maximum or "endpoint energy." BODANSKY, at 357. 28 However, the average energy of beta particles is about one-third, BENNET, at 13, to one-half, BODANSKY, at 357, the endpoint energy. The remaining two-thirds to one-half of maximum possible beta () particle energy is shared with another particle called the neutrino. 29 BENNET, at 13; BODANSKY, at 357. A neutrino is one of nature's more curious phenomena. It has no charge, and virtually no mass. KNIEF, at 33. It was once thought to have no mass; however, it is now believed that the neutrino may have mass, albeit very small mass. BODANSKY, at 357; Malcolm W. Browne, Los Alamos Experiment Shows Neutrino Probably Has Mass, N.Y. Times, May 7, 1996.

63

Beta () particle decay usually occurs when a nuclide has an excess of neutrons. BENNET, at 13; BODANSKY, at 358. A beta particle has greater penetrating ability than an alpha particle, BENNET, at 21, with average penetration distances ranging from 0.1 to 1 g/cm 2 , increasing with increasing energy. BODANSKY, at 355. A neutrino, however, has great penetrating power and can pass through very large amounts of material without stopping. Id. at 358.

64

As discussed earlier, gamma (g) radiation is electromagnetic radiation emitted in the form of photons by nuclei in excited states of energy. Except as noted below, gamma (g) emission is not a primary process of radioactive decay. Instead, it follows alpha (a) particle or beta () particle emission. Gamma (g) rays do not have mass or charge, and they are therefore capable of much greater penetration of matter than alpha (a) or beta () particles. 30 BODANSKY, at 355.

65

Earlier, we noted that excited states in nuclei exist for a very short time (about 10-14 seconds). Consequently, half-lives for gamma (g) ray emission are typically very short. BODANSKY, at 359. However, some nuclei have long-lived excited states, called "isomeric states", with half-lives ranging from a fraction of a second to many years. Id. In fact, in some cases, the excited state is so long that the nuclei appear semi-stable. LAMARSH, at 21. The decay to a lower state of energy by gamma (g) ray emission in a nuclei in an isomeric state is called an "isomeric transition". Id. In such a case, gamma (g) ray emission appears to be the primary radioactive process of, rather than incident to, alpha (a) or beta () particle emission. Gamma ray emission can, however, ultimately be traced back to either initiating process. 31 BODANSKY, at 359.

66

3. Ionizing Radiation.

67

The legal dispute before us is rooted in the damage that purportedly resulted from defendants' release of ionizing radiation into the atmosphere. There are a number of ways in which an ion, or charged particle, can interact with an atom. First, because it is charged, the particle exerts an electrostatic or "Coulomb force" on the atom's electrons. The exertion of Coulomb force has various effects upon an atom. One or more of the electrons may move to an outer orbit, leaving the atom in an excited state of energy or an electron may be entirely ejected from the atom. The latter event results in the formation of an ionized atom. LAMARSH, at 88. When an atom is ionized, it is split into an ion pair. The negatively charged electron of this pair is the negative ion, and the atom minus its negatively charged electron is the positive ion. BENNET, at 20. This process of ionization produces ionizing radiation. 32 MEDICAL EFFECTS, at 1.

68

The second possible result is that the charged particle may penetrate the cloud of orbiting electrons and collide with the nucleus. After collision, the charged particle is scattered from the nucleus, and, since momentum and energy are conserved in the collision, the nucleus recoils. If the charged particle has sufficient mass and energy, the recoiling nucleus may be ejected from its own electron cloud and itself become a charged particle. LAMARSH, at 88. In addition, under certain circumstances, the charged particle, particularly if it is an alpha (a) particle, may undergo a nuclear reaction when it collides with the nucleus. The charged particle may also be accelerated by the electrostatic or Coulomb field of the electrons or the nucleus and a photon may be emitted. 33 Id.

69

Whichever of these alternative results occurs, a charged particle is created. When a charged particle passes through matter, it excites and ionizes atoms in its path. Id. However, these charged particles lose energy by virtue of the electrostatic forces created by their interaction with the atoms that comprise the matter through which the charged particles pass. KNIEF, at 70. The electrostatic forces acting upon the charged particles are proportional to the product of the charges and inversely proportional to the square of the distance between them. Thus, the force decreases rapidly with distance, but becomes negligible only at very large distances. Id. At any given interval, a charged particle experiences forces from a very large number of electrons. The resulting energy losses are well defined for each charged particle and each material medium. Id. The net macroscopic effect of charged-particle interactions is characterized by range and linear energy transfer ("LET"). Id. Range is the average distance traveled by a charged particle before it completely stops. The LET is the amount of energy deposited per unit of particle track, which gives rise to the excitation and ionization. LAMARSH, at 89. The range and the LET of a specific radiation contribute to the effect they have on a material, with the range determining the distance of penetration and the LET determining the distribution of energy deposited along the path. KNIEF, at 70.

70

The LET is of particular significance to an inquiry into the biological effects of radiation. Those effects depend upon the extent to which energy is deposited by radiation as excitation and ionization within a given biological system. LAMARSH, at 89. The LET increases with the mass and charge of a moving particle. Id. Consequently, heavy charged particles, such as alpha (a) particles, are referred to as high LET radiation. Id.

71

Charged particles, are referred to as "directly ionizing radiation" because they are directly responsible for producing ionization. LAMARSH, at 88; BENNET, at 20; BODANSKY, at 354. Uncharged particles, such as gamma (g) rays, lead to excitation and ionization only after interacting with matter and producing a charged particle. Accordingly, uncharged particles are referred to as "indirectly ionizing radiation." LAMARSH, at 88.

72

While gamma (g) rays can interact with matter in a variety of ways, there are, for purposes of our analysis, three important types of interaction between gamma (g) radiation and matter -- the "photoelectric effect", "pair production" and "Compton scattering." BENNET, at 21. Because very short-range forces govern electromagnetic mechanisms, a gamma (g) ray must essentially "hit" an electron for an interaction to occur. KNIEF, at 71. In the photoelectric effect, which is the most important process at low gamma (g) ray energies, BENNET, at 199, the gamma (g) ray interacts with the entire atom, the gamma (g) ray disappears and one of the atomic electrons is ejected from the atom. LAMARSH, at 79. As a result, the energy of the gamma (g) ray or photon is converted completely to kinetic energy of an orbital electron. KNIEF, at 71. If the gamma (g) ray ejects an inner electron, the resulting hole in the electron cloud is filled by one of the outer electrons. LAMARSH, at 16, 79. This transition is accompanied either by the emission of an X ray or by the ejection of another electron.

73

Pair production occurs only for high-energy gamma (g) rays and only in the vicinity of a heavy nucleus. Id. at 80; BENNET, at 21. The gamma (g) ray is annihilated; and an electron pair -- a positron and a negatron -- is created. LAMARSH, at 80. When this occurs the energy of the gamma (g) ray converted to mass, and kinetic energy of the electron pair. KNIEF, at 71. Once they are formed, the positron and negatron move around and ultimately lose energy as a result of collisions with atoms in the surrounding matter. LAMARSH, at 80. After the positron has slowed to very low energies, it combines with a negatron, the two disappear and two photons are produced. LAMARSH, at 80-81. The photons that are produced are called "annihilation radiation." Id. at 7.

74

Compton scattering occurs when the gamma (g) ray strikes an electron and is scattered. The electron that is struck in this process recoils and acquires some of the kinetic energy of the gamma (g) ray, Id. at 81, thus reducing the energy level of the reaction. KNIEF, at 71. Since the gamma (g) ray does not disappear as it does during the photoelectric effect, and is not annihilated as it is in pair production, the Compton-scattered gamma (g) ray is free to interact again. LAMARSH, at 82.

75

Although uncharged particles cause indirect ionizing radiation, it is nonetheless possible to refer to the LET of uncharged particles. However, because they have a relatively low rate of energy loss when compared to the rate of energy loss of charged particles, gamma rays (g) are referred to as "low LET radiation." LAMARSH, at 89. The distinction between high LET radiation and low LET radiation has important biological consequences. Id. Given the same dose of radiation, biological damage from high LET radiation is much greater than damage from low LET radiation. Id. at 402.

76

4. Radiation Quantities and Units.

77

Radiation can be measured by counting the number of ionized particles it produces as it passes through air. INTERNATIONAL ADVISORY COMMITTEE, THE INTERNATIONAL CHERNOBYL PROJECT, TECHNICAL REPORT 20 (1991) (hereinafter "CHERNOBYL "). Originally, the amount of radiation exposure for X- and gamma (g) radiations was measured in units of the roentgen (R), KNIEF, at 72, which is defined as the number of electrical charges produced in a unit mass of air. CHERNOBYL, at 20. 34 Alternatively, a roentgen can be defined in terms of the amount of energy released in the production of ions with a total charge of one electrostatic unit of either sign. BENNET, at 197. 35 Thus, the roentgen is a unit of exposure in air and not a unit of radiation dose to body tissue. Moreover, it is not applicable to higher energy X-rays or to particulate radiations. MEDICAL EFFECTS, at 8. Consequently, the roentgen is not very useful for comparing the effects of various radiations on biological systems, particularly the human body. KNIEF, at 73.

78

When radiation penetrates material, its energy is absorbed and released by the constituent atoms of the material that is penetrated. CHERNOBYL, at 20. The absorbed energy per unit mass of material is termed the "absorbed dose." Id. 36 Two units are used to measure absorbed dose of any type of radiation. The original unit of absorbed dose is the "rad" (radiation absorbed dose) and is defined as 100 ergs of energy per gram of material. LAMARSH, at 401. The new unit of absorbed dose under the Systeme International d'Unites ("SI") 37 is the gray ("Gy"), which is defined as one joule of energy absorbed per kilogram of matter. CHERNOBYL, at 20. Because a rad and a gray are defined in terms of energy, it is possible to equate rads with grays, with one gray being equivalent to 100 rads (1Gy = 100 rads), or one rad equivalent to 10 milligrays (1 rad = 10 mGy). 38 MEDICAL EFFECTS, at 8.

79

However, the absorbed dose is not the only factor to be considered in estimating radiation effects on the human body. The effects also depend on the LET of the radiation. KNIEF, at 73; LAMARSH, at 402. Even when the amounts of energy absorbed are the same, alpha (a) particles are more damaging to human tissue than gamma (g) radiation because of the higher LET of alpha (a) radiation. BENNET, at 198. The fact that different types of radiation have different biological effects for the same absorbed dose is described in terms of the relative biological effectiveness ("RBE") of the radiation. LAMARSH, at 402. The RBE depends on the dose, the dose rate, the physiological condition of the subject, and various other factors. The RBE is determined through experimentation. KNIEF, at 73; LAMARSH, at 403. Accordingly, there is no one RBE for a given type of radiation, and the unit is used almost exclusively in radiobiology. LAMARSH, at 403.

80

RBE is, however, used to approximate the quality factor ("Q") of radiation, which is usually the upper limit of RBE for a specific type of radiation. Id.; KNIEF, at 73. For example, X-rays and gamma (g) rays have a Q of 1, beta () particles have a Q of 1 to 1.7, depending on their energy, and alpha (a) particles have a Q of 20. C HERNOBYL, at 20; KNIEF, at 74. To estimate the effect of a given type of radiation on body tissue, it is necessary to determine the dose equivalent. The dose equivalent is arrived at by multiplying the absorbed dose by the quality factor of the radiation. The original unit of dose equivalence is the "rem" (roentgen equivalent man) and is the product of the absorbed dose in rad and the Q of the particular radiation. LAMARSH, at 404. Thus, if the radiation is gamma (g) radiation, then an absorbed dose of 1 rad produces a dose equivalent of 1 rem, and if the radiation is alpha (a) particle radiation, then an absorbed dose of 1 rad produces a dose equivalent of 20 rem. The new SI unit of dose equivalence is the sievert (Sv) and is the product of the absorbed dose in gray (Gy) and the Q of the radiation. BENNET, at 198. Since one gray equals 100 rads (1 Gy = 100 rads), then one sievert equals 100 rem (1 Sv = 100 rem), LAMARSH, at 404, or one rem equals 10 millisieverts (1 rem = 10 mSv). MEDICAL EFFECTS, at 8.

81

The effect of a given dose equivalent varies depending on the tissue or organ exposed to the radiation. CHERNOBYL, at 20. For example, a given dose of radiation to the hand may have a different and far less serious effect than the same dose delivered to a blood-forming organ. Similarly, the biological effect of a given dose of radiation to a blood-forming organ will be different from a like exposure to reproductive tissue. LAMARSH, at 404. However, equal dose equivalents from different sources of radiation, if delivered to the same point in the body, should have approximately the same biological effect. Id. at 403.

82

The "effective dose" (E), is a unit that is derived from the equivalent dose in an attempt to indicate the combined effect of different doses of radiation upon several different tissues or body parts. CHERNOBYL, at 20. The effective dose is the product of the equivalent dose in a tissue or organ (T) multiplied by a factor called the "tissue weighing factor" (WT), which represents the contribution of that tissue or organ to the total harm resulting from uniform radiation exposure to the whole body. Id. 39

83

Each of the preceding units, (i.e., absorbed dose, equivalent dose and effective dose) relate to the radiation exposure of an individual. There are, however, units of exposure for groups of people. They are arrived at by multiplying the average dose to the exposed group by the number of people in the group. CHERNOBYL, at 20-21. The units are the "collective equivalent dose," which relates to a specified tissue or organ, and the "collective effective dose," which relates to all the people exposed to the radiation. Id. Both units are expressed in terms of man-rems or man-sieverts. LAMARSH, at 405, and they represent the total consequences of the exposure of a population or group. CHERNOBYL, at 21.

84

5. Health Effects of Ionizing Radiation.

85

Soon after the discovery of x-rays and natural radioactivity, clinical evidence suggested that ionizing radiation is harmful to human tissue. ANNALS OF THE INTERNATIONAL COMMISSION ON RADIOLOGICAL PROTECTION, ICRP PUBLICATION 60, 1990 RECOMMENDATIONS OF THE INTERNATIONAL COMMISSION ON RADIOLOGICAL PROTECTION 94 (1990)(hereinafter "ICRP 60"). The initial evidence was mainly noted from the effect of ionizing radiation on human skin. 40 Id. at 92. Later, scientists realized that exposing germinal tissue in plants and animals to ionizing radiation produced effects not only in the plants and animals that were actually exposed, but also in subsequent generations of the exposed plants and animals. Id. Scientific studies and investigations over the last century, have now given us a wealth of information about the effects of radiation on humans. 41 These studies include extensive in vitro and in vivo animal experiments, Id., the comprehensive epidemiological studies of the survivors of the atomic bombings of Hiroshima and Nagasaki, studies of x-rayed tuberculosis patients; and studies of people exposed to ionizing radiation during treatment for ankylosing spondylitis, cervical cancer and tinea capitis. NATIONAL RESEARCH COUNCIL, COMMITTEE ON THE BIOLOGICAL EFFECTS OF IONIZING RADIATIONS, HEALTH EFFECTS OF EXPOSURE TO LOW LEVELS OF IONIZING RADIATION 2(1990) (hereinafter "BEIR V"). These studies have allowed science to "narrow the range of uncertainties in human radiobiology." CHERNOBYL, at 37.

86

As noted earlier, an atom is ionized when an electron is ejected from its orbit and expelled from the atom. As ionizing radiation passes through human tissue, it can transfer its energy along the tracks of the charged particles to the atoms and molecules of the tissue and ionize the atoms and molecules of that tissue. CHERNOBYL, at 37.

87

There are two mechanisms by which ionizing radiation can alter human cells. LAMARSH, at 409. First, the ionization can directly alter biological structures by the disruption or breakage of molecules. Id.; ICRP 60, at 96. Second, biological structures can be altered indirectly by chemical changes set in motion by the transfers of energy to the medium as the ions pass through the molecular structure of human tissue. ICRP 60, at 96. Most of this energy transfer takes place in the water of our cells simply because water is the major component of the human body. 42 MEDICAL EFFECTS, at 13; BEIR V, at 12. When an ionizing particle passes through a water molecule, it may ionize it and produce an ionized water molecule, H2O+, and an electron. The electron can be trapped and produce a hydrated electron, eaq. BEIR V, at 12. However, the ionized water molecule, H2O+, reacts with another water molecule to produce a free radical called the "hydroxyl radical, OH." Id. 43 This particular free-radical is very reactive because it has an unpaired electron and seeks to pair its electron in order to stabilize itself. BEIR V, at 13. At high initial concentrations, back reactions occur which produce hydrogen molecules, hydrogen peroxide and water. Id. However, the water molecule is not always ionized in this process. It can also simply become excited and break up into the hydrogen radical, H., and the hydroxyl radical, OH.. Id.

88

The result of this chemical process is the formation of the three highly reactive species: the hydrated or aqueous electron, eaq, the hydroxyl radical, OH., and the hydrogen radical, H.. Id. All three are highly reactive and can damage the molecular structure of human cells. Id. Free radicals are produced almost immediately after an energy transfer. 44 They move rapidly in the medium, can travel some distance from the site of the original event that creates them, and they can cause chemical changes in the medium. Id. However, even though free radicals are highly reactive and potentially very dangerous to the structure of cells in human tissue most recombine to form oxygen and water in about 10-5 seconds without causing any injury. MEDICAL EFFECTS, at 13.

89

Ionization radiation can damage cells whether the radiation results directly from the electrons set in motion or indirectly by the chemical production of free radicals. CHERNOBYL, at 37. A great deal of evidence suggests that DNA is the principal target in an irradiated cell, and is the most critical site for lethal damage. ICRP 60, at 96; BEIR V, at 13. DNA is believed to be the "critical cellular component injured," as low doses of radiation. MEDICAL EFFECTS, at 16. The random character of energy absorption events caused by ionizing radiation can damage vital parts of DNA in several ways including single-strand or double-strand breaks in the DNA molecule. ICRP, at 96. However, it has been postulated that the majority of DNA strand breaks are not due to the direct effects of ionizing radiation, but rather are caused by the hydroxyl radical. MEDICAL EFFECTS, at 14; see also BEIR V, at 14. Irradiation can also cause a number of recombinational changes to cells. ICRP 60, at 96.

90

Not all irradiation-caused damage to DNA is harmful. Cells have evolved complex repair systems and when a single-strand break occurs, it is quite possible that the site of the damage can be identified and the break very quickly repaired. Id.; CHERNOBYL, at 38. In such a case, the DNA structure is returned to its original form, and there is no long term cellular consequence. ICRP 60, at 96. For example, if ionizing radiation affects a single protein within a cell, the cell can simply produce a new protein and there is no functional change. CHERNOBYL at 37. Alternatively, the repair may not return to DNA to its original form, but DNA integrity may be retained. Id.

91

While it is possible for double strand breaks in DNA to be repaired, the consequences of a double strand break are very serious. ICRP 60, at 96. Chromosomal aberrations are a result of DNA that is damaged by irradiation. These aberrations can be measured quantitatively as a function of absorbed dose. Id. at 97. The outcome could be cell reproductive death, misrepair reflected in a mutation or extensive gene deletion. Id. at 96.

92

If cellular damage is not repaired, it may prevent the cell from surviving or reproducing, or it may result in a viable but modified cell. CHERNOBYL, at 38. The two outcomes have severe, and different, implications for the human body, leading to either "deterministic" or "stochastic" effects. Id. Deterministic effects are entirely predictable and their severity is an inevitable consequence of a given dose. LAMARSH, at 409. Stochastic effects are those that occur at random, i.e., they are of an aleatory or statistical nature. CHERNOBYL, at 38. Thus, stochastic effects are those whose probability of occurrence, as opposed to severity, is determined by dose. LAMARSH, at 409.

93

i. Deterministic Effects.

94

Deterministic effects result when an organism can no longer compensate for the extent of dead cells by proliferating viable cells. ICRP 60, at 99. Cell death or cell killing is the main process involved in deterministic effects. Id. Unless the dose is very high, most types of cells are not immediately killed, but continue to function until they attempt to divide. Id. The attempt to divide will fail, probably because of severe chromosome damage, and the cell will die. 45 Id. Cell death usually becomes apparent within a few hours or days after irradiation. Id. at 97.

95

Cell death is not always life threatening because most body organs and tissues are unaffected by the loss of even a substantial number of cells. CHERNOBYL, at 38. It is only when a tissue or organ absorbs a certain threshold dose high enough to kill or impair the reproduction of a significant fraction of vital cells within the tissue or organ that there is a clinically detectable impairment of function. ICRP 60, at 99. If enough cells are killed, the function of the tissue or organ is impaired. Id. at 97. In extreme cases the organism dies. Id. The severity of the effect is dependent on the dose. Id. Thus, the likelihood of a deterministic effect is zero at a dose lower than some threshold, but the likelihood increases to certainty above such a threshold dose, with the severity of the harm increasing with dose. CHERNOBYL, at 38-39.

96

Cells that divide rapidly are very sensitive to radiation and it is in these cells that the damage from radiation appears to be the greatest. KNIEF, at 75. Such cells include lymphocytes, immature bone marrow cells and intestinal epithelium. Slightly less sensitive cells include those of the lens of the eye and the linings of the stomach, esophagus, mouth and skin. Cells of intermediate sensitivity include those of the liver, kidneys, lungs, thyroid and fibrous tissue. Cells that divide slowly or not at all are the least sensitive to radiation. CHERNOBYL, at 39. These include mature red blood cells, muscle connective tissue as well as bone, cartilage and nervous tissue. Id. Thus, if a person receives a whole body absorbed dose of 1 Gy or 100 rad, generally only those cells with very high sensitivity would be killed. However, as the whole body absorbed dose is increased, additional cells and organs could die and thereby alter the person's clinical presentation. Id. Obviously, if exposure to ionizing radiation results in damage to vital organs or tissues, it may well be fatal. Id.

97

The likelihood of a deterministic effect is practically zero for absorbed doses below 1 Gy or 100 rad. Above that absorbed dose level, deterministic effects occur. Some examples of deterministic effects are erythema, bone marrow depression, radiation cataracts, sterility, and acute radiation syndrome. Id.; MEDICAL EFFECTS, at 280. 46

98

Clinically significant bone marrow depression has a threshold for acute absorbed doses of about 0.5 Gy or 50 rad and for protracted exposure over many years of about 0.4 Gy or 40 rad per year. CHERNOBYL, at 39. Absent appropriate medical care, bone marrow depression will result in half of the people in a heterogeneous population who are acutely exposed to a dose of about 3 to 5 Gy or 300 to 500 rad. 47 Id. The threshold for opacities significant enough to cause vision impairment (which occurs after some delay) appears to be in the range of 2 to 10 Gy or 200 to 1000 rad for an acute exposure to x-rays or gamma (g) rays. The threshold for chronic exposure over many years is thought to be about 0.15 Gy or 15 rad per year. Id.

99

Death is almost certainly the deterministic effect for an individual exposed to a whole body dose of about 6 Gy or 600 rad or higher over a short period. Id. A 3 Gy or 300 rad dose would be lethal for half of an irradiated population who receive little or no medical care, the so called "median lethal dose". Id. However, it has been postulated that for people in good health who receive very intensive medical treatment, the median lethal dose may be from 5 Gy or 500 rad to as high as 9 Gy or 900 rad. Id.

100

ii. Stochastic Effects.

101

Stochastic effects are those which result when an irradiated cell is modified rather than killed. C HERNOBYL, at 39. Even at very low doses it is possible that ionizing radiation may deposit sufficient energy into a cell to modify it. ICRP 60, at 98. Thus, there is a finite possibility for the occurrence of a stochastic event even at very small doses. Id. Consequently, it is assumed that there is no threshold for the initiation of a stochastic event. Id., at 98; MEDICAL EFFECTS, at 73. Put another way, it is believed that stochastic effects can occur even at the lowest doses and, therefore, the possibility of a stochastic effect has to be taken into account at all doses. ICRP 60, at 67. The probability that cancer will result from radiation increases proportionally with dose. ICRP 60, at 69. CHERNOBYL, at 40. However, it is currently believed that there is no threshold dose below which the probability of cancer induction is zero. ICRP 60, at 69; CHERNOBYL, at 40. It is presumed that any transformed cell can become cancerous and become a malignant tumor. CHERNOBYL, at 40.

102

There are two generally recognized types of stochastic events. The first can occur in somatic cells and may result in the induction of cancer in the exposed person. The second can occur in cells of the germinal tissue and may result in hereditary disorders in the descendants of the irradiated. 48 CHERNOBYL, at 39-40; ICRP 60, at 69, 106-07. However, even though hereditary stochastic effects have been demonstrated on highly irradiated mice, CHERNOBYL, at 42, hereditary stochastic effects have not yet been clearly demonstrated in humans. BEIR V, at 4. Thus, any such effects are based on extrapolation from mice to humans. 49 Genetic studies of the almost 15,000 children of the survivors of the atomic bombing in Japan have not yielded evidence of a statistically significant increase in severe hereditary effects. CHERNOBYL, at 42; BEIR V, at 4. Of course, the difficulties encountered in studying the probability of hereditary effects are formidable and include the need to monitor very large numbers of people in irradiated and controlled populations. The difficulty is increased because hereditary effects caused by radiation may be indistinguishable from hereditary disease due to other causes. CHERNOBYL, at 42.

103

It is cancer induction -- the first stochastic event -- that it as issue here. The cell modification caused by ionizing radiation is presumably the result of specific molecular DNA changes by a process known as "neoplastic transformation." It is assumed that there is no threshold for the induction of the molecular changes at the DNA site. ICRP 60, at 97, 107. The potential for unlimited cellular proliferation characteristically results from molecular changes. Id., at 107. Nevertheless, this change alone does not result in a malignant transformation because other changes occur in a malignant transformation. Id. Carcinogenesis is currently believed to be a multistep process requiring two or more intracellular events to transform a normal cell into a cancer cell. BEIR V, at 135. The changes that occur are believed to proceed sequentially. ICRP 60, at 97. The initial events in the production of a cell or cells with the potential to develop into a cancer are known as "initiation". Id. The initiated cell or cells must undergo further changes, usually after a long time and possibly after stimulation by a promoting substance or catalyst, before becoming a cell with malignant potential. Id. Thereafter, the division and multiplication of such cells gives rise to an occult tumor in the "progression" stage. Id. at 97-98. "Progression" refers to the proliferation of a subpopulation of cells within a tumor. BEIR V, at 137. This subpopulation expands and overgrows the less aggressive cells. Id. The carcinogenic process, includes the growth of a primary cancer to a detectable size (e. g., about 1 cm in diameter and containing billions of cells). In humans it can take many years for such a tumor to spread to other tissues. ICRP 60, at 98.

104

The period between exposure to radiation and possible detection of a resulting cancer is called the "latency period". Id. at 107. By way of example, the median latency period for induced leukemia may be about 8 years. The latency period for many induced solid tumors, such as tumors of the breast or lung. Id. The "minimum latency period" is the shortest time in which a specified radiation-induced tumor is believed to occur after exposure. Id. It is about two years for acute myeloid leukemia, and between 5 and 10 years for other types of cancers. Id.

105

Significantly, the severity of a cancer does not depend on the level of the dose that triggered it. ICRP 60, at 60; CHERNOBYL, at 40. The mathematical model used to describe radiation induced cancer is the "linear risk model". BEIR V, at 4; CHERNOBYL, at 40. It is expressed as y = ax, where y is the incidence of excess cancer, a is a constant, and x is the dose. MEDICAL EFFECTS, at 81. The linear risk model posits that each time energy is deposited in a cell or tissue, there is a probability of the induction of cancer. Id. Thus, the effect of each small dose is additive, and therefore spreading a given dose out over time does not reduce the ultimate risk. Id.

106

Although there is scientific consensus that ionizing radiation can cause cancer, ionizing radiation, is not currently known to leave a tell-tale marker in those cells which subsequently become malignant. NATIONAL COUNCIL ON RADIATION PROTECTION AND MEASUREMENTS, NCRP STATEMENT NO. 7, THE PROBABILITY THAT A PARTICULAR MALIGNANCY MAY HAVE BEEN CAUSED BY A SPECIFIED IRRADIATION 1 (1992) (hereinafter "NCRP 7"). Medical examinations and laboratory tests can determine the type and extent of a cancer, suggest an optimal treatment, and provide a likely prognosis, but they rarely (if ever) provide definite information as to its cause. Id. Consequently, medical evaluation, by itself, can neither prove nor disprove that a specific malignancy was caused by a specific radiation exposure. Id. Therefore, the primary basis to link specific cancers with specific radiation exposures is data that has been collected regarding the increased frequency of malignancies following exposure to ionizing radiation. Id. In other words, causation can only be established (if at all) from epidemiological studies of populations exposed to ionizing radiation. Id.; LAMARSH, at 413.

107

However, the task of establishing causation is greatly complicated by the reality that a given percentage of a defined population will contract cancer even absent any exposure to ionizing radiation. In industrialized countries where the life expectancy averages about 70 years, about 30% of the population will develop cancer and about 20% of the population will die of cancer. CHERNOBYL, at 42. It is estimated that if 100,000 persons with an age and sex distribution typical of the United States are exposed to a whole body dose of 0.1 Sv and are followed over their lifetimes, between 770-810 people would develop fatal cancers in excess of the normal incidence. BEIR V, at 6.

108

6. Radiation in the Environment.

109

The inquiry into cause is further complicated by the fact that radiation is a "constituent element" of our environment, and mankind has been exposed to it since our first appearance on this planet. CHERNOBYL, at 23. Obviously, natural environmental radiation has been, and continues to be, augmented by man-made radiation. Consequently, "the radiation environment of today differs from that of yesterday, and it will continue to be transformed in the future." Id. The total average annual dose, from both natural radiation and man-made radiation, is 3.6 mSv or 360 mrem. BEIR V, at 18.

110

i. Natural Radiation.

111

There are two major sources of natural radiation. These are cosmic radiation and terrestrial radiation. LAMARSH, at 427. Cosmic radiation is highly energetic radiation that bombards the earth from outer space. Terrestrial radiation originates in radionuclides found in the earth and in our own bodies. Id. Together, cosmic and terrestrial radiation deliver the highest radiation dose that people normally receive. 50 CHERNOBYL, at 23.

112

Cosmic radiation consists primarily of a highly-energetic mixture of protons (about 87%), alpha (a) particles (about 11 percent), and a trace of heavier nuclei (about 1%) and electrons (about 1%). LAMARSH, at 427. However, the atmosphere acts as a shield, greatly weakening cosmic rays before they reach earth. MEDICAL EFFECTS, at 32. About 26 cosmogenic radionuclides have been identified. They are produced by the action of cosmic radiation. LAMARSH, at 429. However, of the 26, only tritium ( 3 H) 51 , beryllium-7 ( 7 Be), sodium-22 ( 22 Na) and carbon-14 ( 14 C), contribute appreciably to irradiation, MEDICAL EFFECTS, at 32, and only carbon-14 ( 14 C), is responsible for significant radiation doses. LAMARSH, at 429. Fortunately, carbon-14 is a relatively short-lived radionuclide. 52 It primarily results from the atmospheric interaction of thermalized cosmic ray neutrons and nitrogen. Id. The concentration of 14C is about the same in all living species, i.e., 7.5 picocuries per gram of carbon. Id. Because approximately 18% by weight of the human body is carbon, 14C contributes an estimated annual dose of 0.007 mSv or 0.7 mrems. Id.

113

The average dose of cosmic radiation at or near sea level is 0.37 mSv per year or 37 mrems per year. Id.; CHERNOBYL, at 23. However, the dose rate increases with altitude, doubling about every 1500 meters. CHERNOBYL, at 23. Consequently, people living at high altitudes 53 may have an average annual dose level reaching 1 mSV or 100 mrems. Id.

114

Terrestrial radiation accounts for as much as 85% of the total average annual dose of natural radiation, i.e., a little over 2.0 mSv or 200 mrems annually Id. There are approximately 340 naturally-occurring nuclides on earth, and of these, about 70 are radioactive. LAMARSH, at 429. They are called primordial radionuclides because they have existed in the earth's crust since the earth was formed. Id. Those now present on earth have half-lives comparable to the age of the universe. MEDICAL EFFECTS, at 33. Accordingly, primordial radionuclides with half-lives of less than about 10 8 years can no longer be detected. Id. Primordial radionuclides with half-lives of more than 10 10 years have decayed very little up to now. Id.

115

Primordial radionuclides produce secondary radionuclides through the process of radioactive decay. There are three distinct chains of primordial radionuclides: (1) the uranium series, which originates with 238 U; (2) the thorium series which originates with 232 Th; and (3) the actinium series, which originates with 235 U. Together, the parent of each chain and its respective daughter products contribute significantly to terrestrial irradiation. Id.

116

Uranium is found in various quantities in most rocks and soils, and it is the main source of radiation exposure to people out-of-doors. CHERNOBYL, at 23. The uranium isotopes are alpha (a) particle emitters and, therefore, they do not contribute to gamma (g) ray exposure. 54 MEDICAL EFFECTS, at 33. Since uranium isotopes are generally present in low concentrations, they do not contribute significantly to the internal alpha (a) ray dose delivered to humans. Id. However, since these isotopes are found in soil and fertilizers, they migrate into our food chain, and therefore, into our tissue. Id. at 35.

117

Another significant source of terrestrial radiation exposure is radium-226 ( 226 Ra)-- an isotope which originates in the uranium series -- and its daughter products. Radium-226, with a half-life of 1622 years, is an alpha (a) emitter and is present in all rocks, soils and water. Id. Radium is chemically similar to calcium and it passes through the food chain into humans because plants absorb it from the soil. 55 The annual dose attributable to the intake of 226 Ra is 7 microsieverts (Sv) or 0.7 mrem. Id. at 36.

118

Approximately 95% of the world's people live in areas where the annual average dose from outdoor external radiation sources is about 0.4 mSv or 40 mrems. CHERNOBYL, at 24. However, there are areas in the world where people are exposed to very high levels of terrestrial radiation. For example, thorium-rich monozite sands in certain areas of Brazil and India also have exceptionally high levels of irradiation. LAMARSH, at 429. 56

119

Radium-226 is also an important source of terrestrial radiation exposure because it decays to radon-222 ( 222 Rn), a noble gas radionuclide with a half-life of 3.8 days that emits alpha (a) particles and contributes to gamma (g) radiation through its gamma-emitting descendants. Id. Radon is an odorless, colorless, nonreactive gas that poses no significant biological threat. Id. Alpha (a) particles emitted by radon outside the body do not penetrate skin. Id. However, the daughter elements formed as radon decays can be a significant source of natural irradiation and potential biological damage. CHERNOBYL, at 24. Once inhaled, the radon daughters may be deposited in the tracheo-bronchial tree. Id. Some of the radon daughters -- the polonium isotopes, 218 (radon A) and 214 (radon C 1) --emit alpha (a) particles. 218 Po (radon A) provides the major alpha (a) particle dose to the tracheo-bronchial tree, and it therefore poses an increased risk of lung cancer. C HERNOBYL, at 24.

120

Because it is a noble gas, radon diffuses from its point of origin. LAMARSH, at 430. Moreover, because it is the immediate daughter product of the decay of radium-226 ( 226 Ra), it can be present in radium-bearing rocks, soils and home construction materials. Id at 429-30. Radon enters buildings primarily through the underlying and surrounding soils and, secondarily from building materials, outdoor air, tap water and natural gas. CHERNOBYL, at 24. Since concentration increases in enclosed spaces, radon concentration is much higher indoors than outdoors. MEDICAL EFFECTS, at 37-38. Radon is the largest contributor to terrestrial radiation because people spend most of their time indoors. 57 Levels of radon in the air vary from place to place, season to season, day to day and hour to hour. Id.

121

Both lead-210 ( 210 Pb) and polonium-210 ( 210 Po), which, as noted above, are decay products of radon-222 ( 222 Rn), are introduced into the human body through inhalation as well as through the food chain. LAMARSH, at 429-30. 222 Rn is a noble gas and therefore, tends to diffuse into the atmosphere where it can travel large distances before decaying into 210 Pb. Id. at 430. 210 Pb is not inert and attaches to dust and moisture particles in the atmosphere soon after it is formed. Consequently, it can be inhaled directly into the body or fall onto leafy vegetables or pasture grasses from where it can enter the food chain. Id. at 430, 433. Lead-210 does not lead to significant internal radiation doses because it is a rather weak beta () particle emitter. However, its daughter product, 210Po, is a powerful, highly-energetic alpha (a) particle emitter and it provides very significant doses of radiation. Id. at 431.

122

Because both radionuclides can enter the body through ingestion, internal radiation exposure is influenced by dietary patterns. CHERNOBYL, at 24. For example, both radionuclides are present in seafood thus, in countries such as Japan, where seafood is a dietary staple, annual intakes of both radionuclides are significantly higher than in countries where seafood is not a staple. Id. Both radionuclides concentrate in lichens. Accordingly, people in the extreme northern hemisphere who eat the meat of animals that graze on lichens (caribou and reindeer) have levels about ten times higher than the norm. Id. Both 210Pb and 210Po are found on broadleaf tobacco plants, Id. at 433. Both of these radionuclides have also been detected in commercial tobacco products and in cigarette smoke. CHERNOBYL, at 24.

123

There is a dispute within the scientific community as to whether background radiation produces stochastic effects. The International Commission on Radiological Protection assumes that stochastic effects may be induced by natural radiation and man-made radiation. ICRP 60, at 93. It has been inferred that about 3% of cancer deaths each year in the United States are attributable to background radiation, with 1.5 to 2% due to natural radiation, 0.5% to medical uses and 1% or less to occupational sources. See Luis Felipe Fajardo, Ionizing Radiation and Neoplasia, in NEW CONCEPTS IN NEOPLASIA AS APPLIED TO DIAGNOSTIC PATHOLOGY 99 (Cecilia M. Fenoglio-Preiser, Ronald S. Weinstein and Nathan Kaufman, eds., 1986). However, it has also been reported that natural background has not yet been proven to be cancer inducing, and, some scientists claim that natural background radiation does not cause cancer. Id.

124

ii. Man made Radiation.

125

Here, of course, we are most directly concerned with radiation from the nuclear power plant at TMI. It is undisputed that the production of electricity by nuclear power can add to the radioactivity in our environment. Irradiation occurs in the production of electricity, and in all stages of the fuel cycle, i.e., mining, fuel fabrication, transportation, reactor operation and reprocessing CHERNOBYL, at 26-27. However, under normal circumstances, and without considering the effect of nuclear power plant accidents, the overall impact of nuclear power generation on the total population is reported to be very small. MEDICAL EFFECTS, at 45.

126

There are three major sources of man-made radiation other than nuclear power plants. These are: industrial processes other than nuclear power generation that also use radionuclides, medical irradiation, and nuclear weapons testing. CHERNOBYL, at 24.

127

Medical irradiation is generally divided into three categories: (1) diagnostic x-ray examinations; (2) the use of radiopharmaceuticals in nuclear medicine; (3) therapeutic applications of radiation. MEDICAL E FFECTS, at 47. 58

128

Nuclear weapons testing occurs either above ground ("atmospheric testing"), or underground. 59 Radionuclides released in atmospheric testing can enter the body directly or be deposited on the earth's surface from whence they may later be absorbed via the food chain, or be absorbed through by way of external radiation. MEDICAL EFFECTS, at 44.

129

Generally, estimates of human exposure to fallout are more concerned with atmospheric (and more particularly stratospheric), fallout than with local or tropospheric fallout because radionuclides in the stratosphere result in fallout worldwide. Id. In fact, stratospheric particulate fallout accounts for most of mankind's worldwide exposure to fission products. Id. Fallout consists of numerous radioactive byproducts of atomic reactions. However, only four of these have half-lives of sufficient length to be of significant concern to present and future populations: 14C, with a half-life of 5730 years; 137 Cs and 90 Sr, both with a half-life of 30 years; and 3 H, with a half-life of 12 years. CHERNOBYL, at 25. 14 C provides almost two-thirds of the dose exposure because of the relatively short half-lives of the other three radionuclides. Id. The average annual dose to individuals from atmospheric testing is 0.01 mSv or 1 mrems. Id. There have been approximately 1300 underground nuclear weapons tests. MEDICAL EFFECTS, at 44. However, a well-contained underground explosion delivers little, if any, radionuclides to the environment, except for occasional venting. Id.

130

Industrial processes, such as electricity production, mining, and the use of certain building materials and fertilizers produce above average concentrations of natural radionuclides. CHERNOBYL, at 25. Coal contains more radionuclides than other fossil fuels and burning coal produces a large amount of particulate emissions. M EDICAL EFFECTS, at 39. Other sources of industrial irradiation include certain consumer products, such as luminous timepieces, electronic and electrical devices, video display terminals, antistatic devices, and smoke detectors. MEDICAL EFFECTS, at 42. However, tobacco products probably contribute the greatest radiation dose of all consumer products. Id. at 43. It has been postulated that the radionuclides 210Pb and 210Po are responsible for the high incidence of lung cancer in smokers. BEIR V, at 19; LAMARSH, at 433.

131

It is generally conceded that atmospheric weapons testing has contributed more to man-made radiation than nuclear power plants. Id. at 45. On average, the annual dose from all facets of the nuclear fuel cycle is less than 0.1% of that from natural radiation, CHERNOBYL, at 26, or less than 10 mSv or 1 mrem a year. KNIEF, at 88. In fact, it has been postulated that atmospheric releases of radionuclides from fossil fuel plants, especially coal plants without scrubber systems, may be greater than the releases of radionuclides from nuclear power plants. MEDICAL EFFECTS, at 45.

132

Nevertheless, it is beyond dispute that nuclear power plants in general, and nuclear accidents in particular, can release harmful radioactivity into the environment. Irradiation occurs not only in the production of electricity but in all stages of the fuel cycle, i.e., mining, fuel fabrication, transportation, reactor operation and reprocessing CHERNOBYL, at 26-27. 60

133

Accordingly, before proceeding with our Discussion of the District Court's application of Daubert to the expert testimony that was offered to prove that TMI-2 released radiation that caused the Trial Plaintiffs' neoplasms we will briefly discuss the operation of a nuclear power plant in an effort to better determine if Trial Plaintiffs proffered sufficient evidence to connect their injuries to the nuclear reactions that took place inside the nuclear generator at TMI-2. For purposes of assessing the Daubert challenges to the experts in this case, we will limit our Discussion of nuclear fission to reactions initiated by neutrons. 61

IV. NUCLEAR ENGINEERING

A. Nuclear Reaction. 62

134

The bulk of electricity generated in the United States is the result of thermal energy (i.e., heat) produced in either fossil-fueled boilers or nuclear power plants. ANTHONY V. NERO, JR., A GUIDEBOOKTO NUCLEAR REACTORS 3 (1979). Nuclear power plants generate energy through nuclear fission. Id. Nuclear fission provides nearly one hundred million times as much energy as the burning of one carbon atom of fossil fuel. KNIEF, at 4. Fission therefore has obvious advantages over fossil-fuel based energy production. It has been estimated that the complete fission of just one pound of uranium would release approximately the same amount of energy as the combustion of 6,000 barrels of oil or 1,000 tons of high-quality coal. NERO, at 4. However, the major disadvantage of the fission process is now painfully obvious. It requires mankind to harness and control one of the most awesome physical powers in the universe. In addition, potentially deadly radioactive materials are produced in the process. KNIEF, at 4Neutron interactions with nuclei are possible because the absence of a charge allows a neutron to approach a nucleus without repulsion from an opposing force. BENNETT, at 23. The important reactions occur at relatively low energies and include "elastic scattering", "inelastic scattering", "neutron capture" and fission. BODANSKY at 46. Chance determines which of these reactions occur for a given neutron. Id. at 48.

135

In elastic scattering, a neutron and nucleus collide without any change in the structure of the target nucleus. Id. It is, therefore, like "the collision of two billiard balls of unequal mass." BENNET, at 23. Although the structure of the nucleus is unchanged in elastic scattering, the laws of motion cause the neutron to change direction and speed and the nucleus recoils. BODANSKY, at 47. The total kinetic energy of the system is unchanged, but some of the neutron's energy is transferred to the target nucleus. Id. Elastic scattering can occur with any target nucleus, but in nuclear reactors it is most significant when the target nucleus is relatively light, and the loss of kinetic energy is, therefore, relatively large. Id. In that scenario, elastic scattering effectively reduces the energy of the neutrons without depleting their number.

136

Inelastic scattering differs from elastic scattering because the target nucleus is left in an excited state. Id. The target nucleus decays, usually quickly, to the ground state with the emission of gamma (g) rays. The total kinetic energy of the neutron and the target nucleus after the scattering is less than that of the neutron before the scattering, with the difference equaling the energy of the gamma (g) rays. Id. An important characteristic of inelastic scattering is that neutrons lose on average much more energy per collision than they do in elastic scattering. BENNET, at 28.

137

In neutron capture, which occurs in the first stage of many reactions, the neutron combines with the target nucleus to form an excited compound nucleus. BODANSKY, at 47. The term "neutron capture" is usually restricted to cases where the excited compound nucleus decays by the emission of gamma rays. 63 Id. The number of gamma rays emitted in the de-excitation varies. Id.

138

Neutron capture can occur for almost any target nucleus and it plays two general roles in nuclear reactors. First, it consumes neutrons that might otherwise initiate fission. Second, it transforms nuclei produced in fission into other nuclei. Id. Further, as will be discussed below, the reaction is significant because it is the first step in the production of plutonium-239 in reactors. Id.

139

As noted earlier, a nuclear reactor produces energy from fission. However, fission is possible for only a very few target nuclei, the most important being isotopes of uranium and plutonium. Id. at 48. If an appreciable fission yield is produced by neutron incidents upon a target nucleus, that nuclear species is termed fissile. Id. at 61. In a typical fission reaction, the excited compound nucleus divides into two fragments plus several neutrons. 64 Id. at 48. The energy released in fission comes from the large kinetic energy of the fission fragments. Id. An average fission event produces nearly 200 million electron volts of energy (200 MeV). KNIEF, at 45. In contrast, approximately 2-3 electron volts (eV) of energy is released for each carbon atom burned with oxygen. Id. Neutrons produced by the fission event have an average energy of 2 million electron volts (2 MeV). BENNET, at 55. The fission fragments come to rest within about 10-3 centimeters of the fission site so that all of their energy is converted into heat. LAMARSH, at 76. It is important to remember that he fission products are all radioactive. BENNET, at 30. The formation of each fission product is followed by a series of beta decays that continues with successive emission of - particles until a stable isobar is reached. BODANSKY, at 48. In addition, gamma rays are emitted in the de-excitation of the fission fragments, assuming they are formed in excited states, as well as in the de-excitation of the products of the successive -decays. Id..

140

The release of significant amounts of energy from fission requires a chain reaction. Id. at 69. Fission forms the backbone of the required chain reaction. NERO, at 5. Earlier, it was noted that neutrons produced by fission in the uranium fuel have an average energy of 2 MeV. However, fission in uranium will not result in a chain reaction if the neutrons interact at energies close to those in which they were emitted. BODANSKY, at 73. Consequently, it is necessary to reduce the energy of the neutrons from this average energy region to a more favorable region below 1 electron volt (eV) by elastic collisions with the nuclei of a moderator. Id. Commonly used moderators are hydrogen in water, deuterium in heavy water or carbon in graphite. Id. at 74. After a sufficient number of elastic collisions with the moderator, the neutrons have reached a low or "thermal energy" and a chain reaction can occur. NERO, at 7.

141

A chain reaction is sustained by the emission of low energy neutrons from fissioning nuclei. BODANSKY, at 50. A nuclear reactor such as TMI-2 is "merely" the system in which a controlled chain reaction takes place. LAMARSH, at 103. In order for a self-sustaining chain reaction to occur, at least one of the neutrons produced in one fission event must cause a second fission event from which one neutron causes a third fission event, and so on. Id. at 102. Each such generation must have more fission events than the preceding one if a continuing and useful chain reaction is to occur. BODANSKY, at 69. 65 The condition for establishing a chain reaction is commonly expressed as the "achievement of criticality". Id. "Criticality" is described quantitatively in terms of the multiplication factor, denoted by the symbol k, LAMARSH, at 102, or the criticality factor, BODANSKY, at 69, which is defined as the ratio of the number of fissions (or fission neutrons) in one generation divided by the number of fissions (or fission neutrons) in the preceding generation. LAMARSH, at 102. A system is critical if k equals 1. Id. If k is greater than 1, the system is "supercritical," "and a divergent chain reaction exists in which the neutron density and fission rate increase, possibly at an explosive rate as in an atomic bomb." BENNET, at 54. If k is less than 1, the system is "subcritical" and the chain reaction decreases and eventually stops. Id

142

In a nuclear reactor, the operator can vary the value of k by varying the rate at which neutrons are produced within the reactor with the rate at which they are absorbed or disappear. 66 LAMARSH, at 103. To increase the power being produced by the reactor, the operator increases k to a value greater than 1 so that the reactor becomes supercritical. Id. When the desired level has been reached, the operator adjusts k to one so that the reactor is critical and maintains the desired level. Id. To reduce power or shut the reactor down, the operator reduces k to less than 1, making the reactor subcritical. Id.

143

Earlier, we noted that neutron capture wastes neutrons if we consider only fission. However, neutron capture plays an important role in reactors because through it nonfissionable nuclei become fissile. BODANSKY, at 61, 78. Neutron capture in thorium-232 ( 232 Th) and uranium-238 ( 238 U) leads, with intervening - decays, to the production of the fissile nuclei uranium-233 ( 233 U) and plutonium-239 ( 239 Pu). Id. at 61. Therefore, 232 Th and 238 U are termed fertile. Id. Where uranium is used as the fuel, plutonium-239 ( 239 Pu) is ultimately produced by the capture of neutrons in fertile uranium-238 ( 238 U). The 239 Pu can be used for the production of atomic weapons or in other reactors. BODANSKY, at 78. It also contributes fissile material which is consumed in the reactor before the fuel is removed, supplementing the original fissile 235 U in the fresh fuel. Id. Essentially, through neutron capture, the fertile nuclei become fissile nuclei enabling the nuclear reaction to continue in the reactor.

B. The Operation of Nuclear Power Plant. 67

144

At its most elementary level, a nuclear reactor is a deceptively simple apparatus. Simply put, a nuclear power plant produces heat energy that is converted to steam in a boiler. Affidavit of John A. Daniel at ¶ 17. The steam is used to turn a turbine, which is connected to, and turns an electrical generator that produces electrical power. Id. The apparent simplicity of this basic operation is made even more deceptive when one considers the awesome power of the forces at work within nuclear reactors such as TMI-2. However, beyond this elementary level, a nuclear power plant is an extraordinarily complex system. The heat energy is produced in a vessel called a "reactor," because it contains the nuclear reactions. Id. The reactor vessel is a steel pressure vessel with walls that are 8-1/2 inches thick, surrounded by a concrete and steel shield over 8 feet thick. Id. at 23.

145

The TMI-2 nuclear reactor at issue here was a standard Babcock & Wilcox pressurized water reactor ("PWR"). 68 It used uranium dioxide (U02) as a fuel. Id. at ¶¶ 17, 18; J. A. Daniel, Noble Gas Transport During the TMI-2 Accident (1993) 87. The UO2 is formed into ceramic pellets, each of which is about one-half inch in diameter. Daniel Aff. at ¶ 17. Fuel pellets are stacked into metal rods called "fuel pins", which are arranged into square "fuel assemblies." Id. The fuel assemblies are approximately twelve feet high, contained in the reactor, and are collectively referred to as the "reactor core". Id. The fuel assemblies contain several control rods with instruments, some of which monitor the reactor, and others which speed up or slow down the reaction. Id. These control rods, contain materials with large thermal neutron absorption capacities. BODANSKY, at 80. The rods are either inserted or withdrawn to maintain the appropriate level of criticality, (where k equals 1), and enable the reactor to operate at steady power for long periods of time. BENNET, at 107-08. The reactor contains water some of which serves to cool the reactor during the nuclear reaction, and some of which is heated to steam by the chain reaction. The fuel rods containing the uranium fuel are sheathed in rods that prevent the fuel pellets from coming into direct contact with the water in the reactor core and are called the fuel cladding. MITCHELL ROGOVIN, NUCLEAR REGULATORY COMMISSION INQUIRY GROUP, NUREG/CR-1250, TMI REPORT TO THE COMMISSIONERS AND TO THE PUBLIC 10 (1980)(hereinafter "NUREG/CR-1250"). Daniel Aff. at ¶ 17.

146

The nuclear reaction in the core is generated by a neutron source which emits neutrons in a manner designed to initiate a chain reaction. Most fission products are solid at fuel temperature, but some are gases. Id. at ¶ 18. Some of the fission products, especially the noble gases krypton and xenon, migrate to the edge of the fuel pellet and collect in the space between the fuel and the cladding. Since some of the fission products may escape from the cladding into the water in the reactor, the reactors are designed to contain cooling water within its own closed loop. Id.

147

In a PWR of the TMI-2 type, there are three cooling systems. Daniel Aff. at ¶ 19. A PWR coolant system 69 is a circuit or closed loop of distilled water with a small amount of boric acid. NUREG/CR-1250 at 10. The primary circuit, called the "primary coolant" or "reactor coolant", circulates water through the reactor core. Daniel Aff. at ¶ 19. During normal reactor operation, the water in the primary coolant is kept at an average temperature of 575o F and pressures high enough, (around 2200 pounds per square inch), to keep the water from boiling to steam. NUREG/CR-1250 at 10. The water in the primary coolant circulates and recirculates through this loop. Id. While it is circulating and recirculating, the primary coolant water picks up heat from the fission reaction in the reactor core and carries the heat from the core through two "hot leg" pipes to two steam generators. Daniel Aff. at ¶ 19; NUREG/CR-1250 at 10. The steam generators are tanks, approximately 35 feet tall, in which the primary coolant water passes through a large number of narrow tubes that transfer heat to water contained in another, separate circuit, called the "secondary circuit" or "feedwater loop." Daniel Aff. at ¶ 19; NUREG/CR-1250 at 10. The water in the secondary circuit, called "feedwater," is maintained at a lower pressure than the water in the primary coolant circuit, Daniel Aff. at ¶ 19, allowing the feedwater to boil to steam. Id.; NUREG/CR-1250 at 10. The steam in the secondary circuit is called the "main steam system". Daniel Aff. at ¶ 19.

148

The primary coolant water, having lost some of its heat in the secondary circuit, is then returned to the reactor core through four pipes, known as "cold legs." NUREG/CR-1250 at 11. Once the primary coolant water is returned to the reactor core, it is heated once again by the nuclear reaction, and the cycle repeats. Id. The primary coolant water is kept moving at high speed in the primary circuit by four reactor coolant pumps -- enormous devices each of which requires enough electricity for its own operation to light a small town. Id.

149

At a point between the reactor and the steam generator, there is a pipe leading to the bottom of a large vessel or tank called a "pressurizer". NUREG/CR-1250 at 11. During normal operation, coolant water does not circulate back and forth through this pipe. Id. The pressurizer is normally kept a bit more than half full of coolant water. Above the water is a cushion, or "bubble," of steam. Id.; Daniel Aff. at ¶ 20. The pressurizer is a means of keeping the pressure in the reactor coolant system relatively constant, to prevent the reactor coolant water from boiling. NUREG/CR-1250 at 11. The steam cushion at the top of the pressurizer can be made larger or smaller by slight heating or cooling of the pressurizer water just beneath it. Id. When the bubble temperature and pressure are increased, the bubble tends to push water from the pressurizer out into the primary reactor coolant loop, thereby increasing pressure in the loop. Id. When the temperature and pressure in the pressurizer are lowered, steam condenses and the bubble shrinks. Id. Consequently, water tends to come from the reactor coolant loop into the pressurizer, and the overall system pressure is lowered. Id.

150

A relief valve, referred to as a "power-operated relief valve" ("PORV"), is located in a pipe leading out of the top of the pressurizer, at the top of the space normally occupied by the steam bubble. Daniel Aff. at ¶ 21, NUREG/CR-1250 at 11. The PORV is designed to open automatically when the system begins to overpressurize. NUREG/CR-1250 at 11. Theoretically, if the pressure in the coolant system rises very abruptly, the PORV will open, some of the steam will rush out to a drain tank, thus shrinking the bubble, more water will move up into the pressurizer from the primary coolant loop, and system pressure will decrease. Id.; Daniel Aff. at ¶ 21. A rupture disk is provided on the drain tank to relieve pressure if the drain tank becomes too full. Daniel Aff. at ¶ 21. When the system pressure is back to normal, the PORV is supposed to close automatically. NUREG/CR-1250 at 11. If the PORV fails to close, there is a backup, called a "block valve". Daniel Aff. at ¶ 21. There are two other safety valves in addition to the PORV. NUREG/CR-1250 at 11; Daniel Aff. at ¶ 21.

151

The heart of the secondary circuit or feedwater loop is the steam generator. NUREG/CR-1250 at 11. After the water in the secondary circuit is heated to steam by the hot water from the primary coolant system, the steam moves through the secondary circuit or feedwater loop to a steam turbine which turns the power generator that produces the electricity. NUREG/CR-1250 at 11. The steam then passes through a condenser, where it is cooled and condensed into water once again and recycled through the steam generators. Id. at 11-12. The pipe line returning the water from the condenser to the steam generator also contains the condensate polishers or "demineralizers". Id. at 12. The demineralizers are basically water softeners, that use ion exchange resins to purify the feedwater. Id.

152

The condenser itself is cooled by water from the third circuit, i.e., the cooling towers, Id. at 12; Daniel Aff. at ¶ 19, which are the familiar landmarks of nuclear plants. The water in the condenser circuit, having been warmed in the process of condensing steam back into water, is pumped up to the cooling towers. NUREG/CR-1250 at 12. The condenser water is thus cooled by exposure to the atmosphere as it tumbles down a steep run of steps and then is pumped back into the condensers. Id. The escaping water vapor is the plume or cloud which can often be seen coming out of the cooling tower. Id. 70

153

In addition to the systems peculiar to a PWR, the TMI-2 reactor had a general safety system common to all reactors called the "emergency core cooling system" ("ECCS"), which is designed to supply cooling water to the hot reactor core if there is a loss of water due to a break in the reactor coolant system. NUREG/CR-1250 at 12. If the break is small, the leakage flowing from the break may not be significant and the system's internal pressure will stay high, making it difficult for the ECCS to pump water in. Id. Consequently, the plant has high pressure injection ("HPI") pumps to deliver water from a large borated water storage tank ("BWST"). Id. However, the HPI pumps are not able to deliver enough water if the break is large. Id. Therefore, the system has a set of low pressure injection ("LPI") pumps that can deliver water to the core rapidly when a large pipe break has caused the system's internal pressure to drop. Id. The LPI pumps also draw water from the BWST. Id.

154

The LPI pumps also have another purpose. When a reactor is shut down, the radioactive waste in the fuel continues to produce a considerable, but diminishing, amount of decay heat for days following shutdown. This heat must be removed if the core is to be kept from melting. Id. Even after the reactor is shut down, the accumulated fission products continue to decay and release energy within the reactor. LAMARSH, at 350. This fission product decay is called "decay heat". Id. A method for handling the decay heat and for cooling the reactor core after shutdown must be provided or the temperature of the fuel may rise to a point where the fuel's integrity is compromised and fission products are released. Id.

155

After shutdown, the reactor is first cooled by the continued normal operation of the steam generators until the reactor coolant system is cooled to a temperature of about 300F and an internal system pressure below 400 psi. NUREG/CR-1250, at 12. Then, valves are opened to let one of the LPI pumps circulate coolant through the reactor core and out to a special heat exchanger to bring the system temperature down to about 120-140F. Id. 71

156

Finally, there is an ECCS system called the "coreflood tanks." These are two tanks almost completely filled with water under a medium-pressure of about 600 psi. Id. They stand above the reactor and a check valve prevents the higher reactor core system pressure from driving more water into them. Id. If a large pipe break occurs, it takes a few minutes for the HPI and LPI pumps to deliver cooling water to the core. Id. Therefore, the coreflood tanks are designed to drop their thousands of gallons of water to cool the core until the HPI and LPI pumps deliver cooling water. Id.

157

C. Barriers to Release of Radioactive Materials into the Environment.

158

A nuclear reactor must obviously have barriers designed to prevent the fission products from entering the working areas of the reactor or escaping into the environment. NUREG/CR-1250 at 342. The first such barrier is the ceramic fuel matrix in which the fission products are produced. Id. The uranium dioxide (UO 2 ) used in the TMI-2 reactor is a ceramic fuel which has microscopic boundaries ("grain boundaries") between the molecules. Daniel, Noble Gas Transport at 87. These boundaries serve as microscopic roadways for certain fission products to travel out of the fuel. Id. Thus, some of the elements that are volatile or gaseous at the operating temperature of the fuel are able to migrate through the ceramic fuel. NUREG/CR-1250 at 342. However, the majority of the fission products that are produced are either trapped or chemically bound. Id.

159

The second barrier to the release of fission products is the fuel cladding. The UO 2 ceramic pellets are sealed in the fuel rods to prevent the fuel pellets from directly contacting the water in the reactor core. There is a small gap between the fuel and the fuel cladding. Noble gases such as krypton and xenon, and other volatile nuclides are contained in that gas. However, if a defect or rupture develops in the fuel cladding, volatile fission products can be released into the coolant. Id. at 343. At the time of the TMI-2 accident, the Nuclear Regulatory Commission generally allowed operation of a reactor with up to 1% of the fuel having a defect in its cladding. 72 Id.

160

The third barrier is the reactor coolant. Many of the volatile fission products, the radioiodines and radiohalogens, are soluble in the coolant in "ionic" (electrically charged) form. Id. at 343. These materials can be removed by demineralizers, such as those in the makeup and purification system of the reactor, or they may remain dissolved in the coolant. Id. The majority of these radionuclides are contained in the primary coolant system, Id., and are soluble in the coolant. Id. Their solubility decreases and they tend to precipitate or "plate out" as the pH of the primary coolant is increased. Id. The noble gas radionuclides, kryptons and xenons, have very low solubility in the coolant, particularly in the presence of other gases such as hydrogen, and they evolve into a gas or vapor phase above the coolant or wherever the coolant is depressurized. Id.

161

The fourth barrier is the reactor pressure vessel and the piping of the primary coolant system, which are made of heavy walled steel. Id. The fifth barrier is the containment building itself. It is designed to withstand over pressurization and external impacts and contain or delay fission product releases during an accident. Id.

V. THE ACCIDENT AND ITS AFTERMATH

A. The Accident at TMI-2. 73

162

What has been described as the "[n]ation's worst nuclear accident" 74 began at about 4:00 a.m. on Wednesday, March 28, 1979, The Comptroller General Report to the Congress, Three Mile Island: The Most Studied Nuclear Accident in History 1 (1980). Ironically, the "nation's worst nuclear accident" grew out of a minor malfunction, or transient that occurred in the nonnuclear part of the system. NUREG/CR-1250 at 3. For some reason, several feedwater pumps, 75 that normally drew heat from the PWR's cooling water, shut-off automatically. 76 Id. at 310. The system was designed so that when the feedwater pumps tripped, the main turbine and electrical generator also tripped. Id. Thus, by design, the turbine and generator tripped approximately one second later. Id. Three seconds after the turbine tripped, the pressure in the reactor coolant system increased to a level that caused the PORV to open in order to release the pressure. Id. When the PORV opened, the fission process in the reactor core automatically shut down. Id. Consequently, the heat generation in the reactor core dropped to decay heat levels. J. A. Daniel, Noble Gas Transport at 5.

163

However, the PORV did not close as it should have when the system pressure was reduced to acceptable levels. Instead, it remained open for approximately 2 hours. Comptroller General Report ("CGR") at 2. Unfortunately, the personnel operating Unit 2 did not realize that the PORV had not closed. They believed that it had automatically closed when the system was depressurized. Id.; NUREG/CR-1250 at 324. Because the PORV remained open, reactor coolant water flowed from the reactor coolant system into the reactor coolant drain tank, which is designed to collect reactor coolant that is released from the reactor coolant system through the PORV during power operation. Daniel, Noble Gas Transport at 6. The continued flow of reactor coolant water into the reactor coolant drain tank caused a safety valve to lift on the drain tank and a drain tank rupture disk to burst. Id. This rupture disk burst allowed the reactor coolant water to be discharged directly into the reactor building, which overfilled along with its sump pumps. Id. The reactor building sump pumps were on automatic and aligned with the auxiliary building sump tank. Id. at 31. When the reactor building sump pumps overfilled, some coolant water was transferred to the aligned auxiliary building sump tank. Id. For some reason, there was no rupture disk on the sump tank and reactor coolant water was discharged directly into the auxiliary building. Id. at 31; CGR at 2. The contaminated coolant water continued to flow from the reactor building into the auxiliary building for several days. CGR at 2. Estimates of the amount of radioactive water discharged into the reactor and auxiliary buildings range from 700,000 gallons, Nuclear Energy Institute, The TMI 2 Accident: Its Impact, Its Lessons, http://www.nei.org/pressrm/facts/TMI-2.htm>, to 5,000,000 gallons. NUREG/CR-1250 at 339.

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Approximately 2 minutes into the accident, the emergency core cooling system ("ECCS") began pumping water into the reactor core. CGR at 2. However, operations personnel, still believing that the PORV had closed, and therefore unaware that reactor coolant water was escaping from the reactor coolant system, turned off most of the water flowing to the core through the ECCS. Id. They did so believing that they were preventing the reactor system from becoming filled with water -- a condition they were required to prevent. Id.

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However, there was not enough coolant water being circulated through the reactor coolant system to cool the reactor core because reactor coolant water was being discharged into the reactor building. Consequently, the core reaction was producing more heat than the coolant system was removing, and the core began to heat up. Daniel, Noble Gas Transport at 6. The loss of reactor coolant water allowed the reactor core to become uncovered. Id. Within three hours of the beginning of the accident, as much as two-thirds of the twelve-foot high core was uncovered. Temperatures reached as high as 3500 to 4000 degrees Fahrenheit or more in parts of the core during its maximum exposure. THE REPORT OF THE PRESIDENT'S COMMISSION ON THE ACCIDENT AT THREE MILE ISLAND, THE NEED FOR CHANGE: THE LEGACY OF TMI 100 (1979) (THE KEMMENY REPORT).

166

About 2-1/2 hours into the accident, some of the fuel rods in the reactor cracked, releasing xenon and other fission product gases, which had accumulated in the fuel rod gap between the fuel and the cladding, into the coolant water. Daniel Aff. at ¶ 28. Over the next few hours, more fuel rods cracked, releasing radioactive iodine and cesium into the primary coolant water as well as additional noble gases. Id.

167

A series of events then unfolded involving various reactions, valves and controls. The end result was that, nearly 10 hours into the accident, there was a sudden spike of pressure and temperature in the reactor building. Id. at 329. Initially, the spike was dismissed as some type of instrument malfunction. Id. at 330. However, operations personnel learned on March 29th that the spike was caused by the explosion of hydrogen gas in the reactor building. Id. at 329; CGR at 2. Fears of another hydrogen explosion developed when a hydrogen gas bubble was later found in the reactor system. CGR at 2; NUREG/CR-1250 at 336, 338. Presumably, there was a concern that another hydrogen explosion would damage the reactor vessel, leading to further releases of radioactive material. NUREG/CR-1250 at 338. However, the fears about another hydrogen explosion were later learned to be unfounded. Id.; CGR at 2.

168

During the last days of March and the first week of April, operations personnel began to regain control and contain the radioactive releases caused by the accident. See NUREG/CR-1250 at 334-39. However, it was not until the afternoon of April 27, 1979 that stable conditions were finally established in TMI-2. Id. at 339.

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B. Radioactive Materials Released to the Environment.

170

The parties generally agree that the radioactive fission products released to the environment as a result of the accident escaped from the damaged fuel and were transported in the coolant through the letdown line into the auxiliary building. NUREG/CR-1250 at 343; Daniel, Noble Gas Transport at 33 ("The major pathway for fission product transport to the auxiliary building was through the letdown piping of the makeup and purification system."); Daniel Aff. at ¶ 22. Once in the auxiliary building, the radioactive fission products were released into the environment through the building's ventilation system. 77 NUREG/CR-1250 at 343; Daniel, Noble Gas Transport at 60. Because of the volatility of noble gases and radioiodines, those elements were the primary radionuclides available for release from the auxiliary building. NUREG/CR-1250 at 343. Two krypton isotopes, 87 and 85, were not released in significant quantities because of the short half-life of 87 Kr and because of the small amount of 85 Kr in the reactor core. Id. Nonetheless, despite the various filters, radioiodines were released. 78 After the first day, the quantities of 88 Kr and 135 Xe were reduced by radioactive decay. Id. All of the 133I contained in the coolant which was released to the auxiliary building eventually decayed to 133 Xe and 133m Xe. Id. These radionuclides were the predominate ones released from the plant to the environment. Id. at 344; Daniel Aff. at ¶ 35.

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C. Pathways of Exposure to Radioactive Materials.

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The various mechanisms of human exposure to radioactive materials after such materials have been released into the environment are called "pathways". NATIONAL RESEARCH COUNCIL, RADIATION DOSE RECONSTRUCTION FOR EPIDEMIOLOGIC USES 28 (1995) (hereinafter "RADIATION DOSE RECONSTRUCTION"). The pathways are "qualitatively well-known and their relative importance is understood." CHERNOBYL, at 31. Generally, when radioactive materials from nuclear power plants are released into the atmosphere, they are released into a region called the "planetary boundary layer." This is an area between the surface of the ground and an elevation of 100 meters. RADIATION DOSE RECONSTRUCTION, at 28. Once the radioactive material is released, turbulence in the atmosphere mixes the effluent particles and gases within the resulting contaminated cloud or "plume" and the plume is transported downwind. Id. The mixing of the radioactive particles and the transport of the resulting plume are called "dispersion". Id.

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Obviously, the extent and direction of a dispersion depends on many factors including wind direction, wind speed and weather, as well as the heat content of the plume, and the characteristics of a given terrain over which the plume may be carried. CHERNOBYL, at 32. As the radioactive cloud is dispersed and transported by prevailing winds, exposure to the radionuclides first occurs through external and internal irradiation. Id., at 31; RADIATION DOSE RECONSTRUCTION, at 29. The contents of the plume are depleted over time as the radionuclides settle to the ground in response to gravitational forces ("dry deposition") of through precipitation or combination with airborne moisture such as fog (wet deposition). CHERNOBYL, at 31-32. However, as suggested by some of our Discussion above, exposure can continue by external irradiation from deposits of radioactive material, inhalation of any materials suspended in the atmosphere, or transfer of the radioactive material through the terrestrial and aquatic environment to food and water, and then to internal irradiation. Id.

VI. LEGAL DISCUSSION

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With the foregoing Discussion in mind, we are ready to begin our Discussion of the District Court's evidentiary rulings. The Daubert challenge to the plaintiffs' experts implicates the reliability of the expert testimony that plaintiffs sought to admit into evidence. We now begin our analysis of that testimony, the issues of causation and the scientific principles implicated by the plaintiffs' attempt to establish the requisite nexus between the accident at TMI-2 and their injuries.

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A. The Trial Plaintiffs' Appeal.

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1. Background.

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The 10 Trial Plaintiffs claim that their diseases, or the fatal diseases of the decedents whose claim their personal representatives assert, were caused by the radioactive materials released into the environment as result of the TMI-2 accident. More specifically, they allege their conditions were caused by gamma (g) ray exposure from radioactive iodine, xenon and krypton. In re TMI Litigation Consolidated Proceedings, 927 F. Supp. 834, 840 (M.D. Pa. 1996)(the "Summary Judgment Opinion").

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We have previously held that plaintiffs seeking to recover for injuries allegedly caused by TMI-2 must show that: (1) the defendants released radiation into the environment in excess of the levels permitted by federal regulations in effect in 1979, i.e., 0.5 rems (500 mrems) or 5 mSv; (2) the plaintiffs were exposed to this radiation (although not necessarily at levels prohibited by those regulations); (3) the plaintiffs have injuries; and (4) radiation was the cause of those injuries. See In re TMI, 67 F.3d 1103, 1119 (3d Cir. 1995), cert. denied, 516 U.S. 1154 (1996). We have also held that the "exposure element requires that plaintiffs demonstrate they have been exposed to a greater extent than anyone else, i.e., that their exposure levels exceeded the normal background level." Id. (citation and internal quotations omitted).

179

Throughout this litigation, the defendants have conceded that radioactive materials were released into the environment, and that the releases at the plant boundaries exceeded 0.5 rem (500 mrem). However, they claim that no plaintiff was in an area where he or she could have been exposed to dose in excess of 0.5 rem. In re TMI, 67 F.3d at 1109 . Appellees point to a number of studies undertaken by governmental entities in the years immediately following the TMI-2 accident. The Nuclear Regulatory Commission ("NRC"), the Department of Energy ("DOE"), the Environmental Protection Agency ("EPA"), the Department of Health, Education and Welfare ("HEW"), a special President's Commission (the "Kemmeny Commission"), a special NRC investigation (the "Rogovin Report"), and the Pennsylvania Department of Health, all studied various aspects of the accident. Appellees' Br. at 3-4. The government entities studied the timeline of events recorded by in-plant computers and strip charts during the accident to determine what was happening minute by minute. Id. at 4. The inquiry examined the records from monitors located in several locations inside the plant including the ventilation stack through which the radioactive releases occurred. Offsite monitors at multiple locations where either the utility operating the plant or the Commonwealth of Pennsylvania had instruments collecting and recording data on doses in the communities surrounding Three Mile Island were also studied. 79 Id. The governmental entities also collected and assessed thousands of environmental samples of milk, water, soil, vegetation, and food. Id. They made whole body counters available at no charge to the public, and over seven hundred people were scanned to see if they had any radionuclides in their bodies from the accident. Id. Local hospitals provided free thyroid scans to anyone interested, and hundreds of people took advantage of these scans to discover any radioactive iodine that might have accumulated in their thyroids. Id. The DOE also conducted an extensive examination of the damaged reactor core in an attempt to quantify the amount of fission products remaining so as to better calculate the quantity that was released into the environment. Id.

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Defendants insist that all of these studies consistently concluded that the accident released something less than nine million curies of the noble gases, xenon and krypton, and radioactive iodine resulting in exposures of no more that 100 mrem in the immediate vicinity of the plant, and dropping quickly to tens or just a few millirems within a few miles of the plant. Id. at 5. For example, the NRC Special Inquiry Group Report concluded that the radioactive releases resulted in an average equivalent dose of 1.4 mrem to the approximately two million people living in the area. NUREG/CR-1250 at 153. Defendants contend that these studies conclusively demonstrate that the accident did not cause in releases of radionuclides in sufficient amount to pose a significant threat to the health of the people living around Three Mile Island. Appellees' Br. at 5. For example, the Ad Hoc Population Dose Assessment Group 80 concluded that the predominant exposure to people outside the plant boundaries occurred in the north-northwest sectors, the east-northeast sectors and the south-southeast sectors, with the east-north east sector registering the highest cumulative dose of 83 mrem. AD HOC POPULATION DOSE ASSESSMENT GROUP, POPULATION DOSE AND HEALTH IMPACT OF THE ACCIDENT AT THE THREE MILE ISLAND NUCLEAR STATION 44 (1979). The Ad Hoc Group also concluded the following regarding the potential health effects on the population living around Three Mile Island:

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The projected total number of fatal cancers is less than 1 (0.7). The additional number of non-fatal cancers in also less than 1 (0.7). The additional number of genetic effects for all generations is also less than 1 (0.7).... All of these values are small compared to either the existing annual incidence of similar effects or the potential effects estimated to result from the natural background radiation.... Comparing the total potential health impact of the accident with the estimated lifetime natural risk indicates that these effects, if they were to occur, would not be discernible. The uncertainties in the risk from low-level ionizing radiation would not alter this Conclusion.

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Id. at 60.

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As the years passed and health concerns about the accident persisted, the Commonwealth of Pennsylvania's Department of Health conducted several epidemiological 81 studies of the community surrounding Three Mile Island. 82 The Department of Health examined pregnancy outcomes, cancer incidence and cancer mortality in the Three Mile Island area and compared them with state and national norms. The Department's studies concluded there were no significant differences between the studied groups and the state and national norms. Appellees' Br. at 5. The defendants believe that these epidemiological studies further support their claim that the radionuclides released by the accident did not pose any significant health risks.

184

However, the Trial Plaintiffs rely, in part, on a report of the TMI Public Health Fund, 83 which opined, inter alia, that the aforementioned studies were seriously flawed insofar as they attempted to calculate dose exposures to the individuals in the communities surrounding Three Mile Island. See THREE MILE ISLAND PUBLIC HEALTH FUND, A REVIEW OF DOSE ASSESSMENTS AT THREE MILE ISLAND AND RECOMMENDATIONS FOR FUTURE RESEARCH, APPENDIX A 1 (1984). Plaintiffs gathered their own experts to conduct many of the tests recommended by the TMI Public Health Fund in its report. Appellants' Br. at 11. The scientific team that the Trial Plaintiffs assembled focused on "biological indicators of radiation dose." Trial Plaintiffs contend that those biological indicators constitute a "body of evidence... greatly overlooked or ignored in early studies...." Id. at 12. According to the Trial Plaintiffs, the biological indicators of radiation dose conclusively demonstrate that area residents were exposed to an equivalent dose of over 100 rem or 1 Sv. Summary Judgment Opinion, 927 F. Supp. at 848 . Perhaps because of their faith in the reliability of these biological indicators, Trial Plaintiffs proceeded to try their respective claims on the theory that each of the Trial Plaintiffs had been exposed to an equivalent dose of at least 10 rem or 100 mSv each. 84 See Brief of Non-Trial Plaintiffs (No. 96-7624), at 18, 38.

185

To support their contention that they were each exposed to significantly higher doses of ionizing radiation than the governmental studies calculated, and the defendants admit to, the Trial Plaintiffs developed a "blowout" theory. Under that theory, one or more unfiltered hydrogen blowouts occurred on the afternoon of the first day of the accident, whereby large quantities of radioactive noble gases and other radioactive nuclides, such as iodine and cesium, were expelled into the environment. Summary Judgment Opinion, 927 F. Supp. at 857 . They assert that, after the blowout, an extremely dense, yet narrow, plume of radioactive effluents traveled through the atmosphere evading all of the radiation monitors in place in the areas surrounding the plant and the communities. Id.

186

The "blowout" theory was first developed by Trial Plaintiffs' expert, Richard Webb, who opined in a report that a total of 106 million curies of noble gases were released during the accident, more than half of which escaped during a two and one-half hour "blowout" on the afternoon of the first day of the accident. Webb did not testify at the in limine Daubert hearing. After the first round of hearings ended, Webb left a voice-mail message for defendants' counsel recanting his proposed testimony. The District Court concluded that Webb's recantation only confirmed its intention to exclude Webb's proffered testimony as unreliable. 85 In re TMI Litigation Cases Consolidated II, 911 F. SUPP. 775, 791 (M.D. Pa. 1996).

187

However, because of the significance of the "blowout" theory to plaintiffs' case, the District Court did permit the Trial Plaintiffs to use another witness -- a nuclear engineer named David Lochbaum -- to replace Webb's testimony about a blowout. Lochbaum testified at an in limine hearing that significantly more than 10 million curies of noble gases reached the environment as a result of the accident. He opined that these gases were released from steam generator B in the early hours of the accident. However, somewhat contradictorily, he also testified that he "did not believe that there was evidence of a blowout." In re TMI Litigation Cases Consolidated II, 922 F. Supp. 997, 1052 (M.D. Pa. 1996). He testified, however, that if a blowout did occur, it was of limited length -- on the order of minutes and not over the two or three hours Webb believed. Id. Nonetheless, he testified that the blowout did release significant amounts of noble gases even though it lasted only a short time. Id.

188

The District Court concluded that Lochbaum's testimony was dependent upon other of the Trial Plaintiffs' experts being able to demonstrate that significant amounts of radionuclides were emitted. The court reasoned that if other experts were able to competently testify about significant amounts of noble gases being emitted, then Lochbaum's testimony was admissible on the issue of the source of the emissions. Thus, the court concluded that if no other expert could competently testify about significant releases of radionuclides, Lochbaum's proffered testimony about a blowout must be excluded. Id. Consequently, the trial plaintiffs proffered several witnesses whose testimony was relevant to the existence of a blowout. It is those witnesses for the most part, whose testimony was challenged under Daubert, and whose reliability is now at issue.

189

With this background established, the stage is properly set to begin our analysis of the District Court's Daubert decisions regarding the Trial Plaintiffs' dose exposure experts.

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2. Standards Governing the Admissibility of Scientific Evidence.

191

Federal Rule of Evidence 702 provides that:

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If scientific, technical, or other specialized knowledge will assist the trier of fact to understand the evidence or to determine a fact in issue, a witness qualified as an expert by knowledge skill, experience, training, or education, may testify thereto in the form of an opinion or otherwise.In Daubert v. Merrell Dow Pharmaceuticals, Inc., 509 U.S. 579 (1993), the Court set forth parameters for determining when proffered expert testimony can be admitted into evidence. 86 The Court held

193

an inference or assertion must be derived by the scientific method. Proposed testimony must be supported by appropriate validation -- i.e., "good grounds," based on what is known. In short, the requirement that an expert's testimony pertaining to "scientific knowledge" establishes a standard of evidentiary reliability.

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Id. at 590 . 87 Rule 702 also requires that the evidence or testimony "assist the trier of fact to understand the evidence or to determine a fact in issue." "This condition goes primarily to relevance." Id. at 591 . This "consideration has been aptly described... as one of `fit' ". Id. Rule 702's " `helpfulness' standard requires a valid scientific connection to the pertinent inquiry as a precondition to admissibility. Id. at 591-92 .

195

The Court in Daubert concluded that Rule 702 "clearly contemplates some degree of regulation of the subjects about which an expert may testify." Id. at 589 . Thus, the Court established a "gatekeeping role for the Judge." Id. at 597 . The Court wrote:

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Faced with a proffer of expert scientific testimony,... the trial Judge must determine at the outset, pursuant to Rule 104(a), 88 whether the expert is proposing to testify to (1) scientific knowledge that (2) will assist the trier of fact to understand or determine a fact in issue. This entails a preliminary assessment of whether the reasoning or methodology underlying the testimony is scientifically valid and of whether that reasoning or methodology properly can be applied to the facts in issue.

197

Id. at 592-93 . The Court held that these matters should be established "by a preponderance of proof," Id. at 593 n.10, and identified some "general observations," relevant to the proponent's burden, while acknowledging that the factors it identified were not all-inclusive. Id. ("[m]any factors will bear on the inquiry.").

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First, "a key question to be answered in determining whether a theory or technique is scientific knowledge that will assist the trier of fact will be whether it can be (and has been) tested." Id. "Another pertinent consideration is whether the theory or technique has been subjected to peer review and publication." Id. Publication, which is an element of peer review, "is not a sine qua non of admissibility: it does not equate with reliability." Id.

199

However, submission to the scrutiny of the scientific community is a component of "good science." Id. Accordingly, "[t]he fact of publication (or lack thereof) in a peer reviewed journal...will be a relevant, though not dispositive, consideration in assessing the scientific validity of a particular technique or methodology upon which an opinion is premised." Id. at 594 . Third, "in the case of a particular scientific technique, the court ordinarily should consider the known or potential rate of error, and the existence and maintenance of standards controlling the technique's operation." Id. Fourth, and finally, "general acceptance" can have bearing on the inquiry. Id. "Widespread acceptance can be an important factor in ruling particular evidence admissible, and a known technique which has been able to attract only minimal support with the community may properly be viewed with skepticism." Id. However, "general acceptance" is "not a necessary precondition to the admissibility of scientific evidence." Id. at 597 . Indeed, the Court specifically declined to require general acceptance when it rejected the Frye rule. See n.86 supra. Rather, general acceptance is but one factor that is considered along with all other factors relevant to the 702 inquiry.

200

The Court concluded by emphasizing that the "inquiry envisioned by Rule 702 is... a flexible one," and by reminding the trial courts that the "focus... must be solely on principles and methodology, not on the Conclusions they generate." Id. at 595. The Court also noted that the District Court should be mindful of other applicable rules in assessing a proffer of expert scientific testimony under Rule 702. Specifically, Rule 703 which provides that expert opinions based on otherwise inadmissible hearsay are to be admitted only if the facts or data relied upon are of a type reasonably relied upon by experts in the particular field in forming opinions; Rule 706 which allows the court in its discretion to procure the assistance of an expert of its own choosing; and Rule 403 which permits the exclusion of relevant evidence if its probative value is substantially outweighed by the danger of unfair prejudice, confusion of the issues, or misleading the jury. Id.

201

We applied the teachings of Daubert in deciding In re Paoli Railroad Yard PCB Litigation, 35 F.3d 717 (3d Cir. 1994), cert. denied, 115 S. Ct. 1253 (1995)(hereinafter "Paoli II"). There, we held that Rule 702 has two major requirements. First of all, the proffered "expert" must be qualified to express an expert opinion. This "qualifications" requirement is liberally interpreted and includes "a broad range of knowledge, skills, and training," Paoli II, at 741. 89 However, "the level of expertise may affect the reliability of the expert's opinion." Id.

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Secondly, the proffered expert opinion must be reliable. Thus, "an expert's testimony is admissible so long as the process or technique [as opposed to the Conclusion] the expert used in formulating the opinion is reliable." Id. at 742 (emphasis added). We listed various factors enunciated in Daubert that assist in evaluating whether a given scientific methodology is reliable, and we also relied upon several factors we had previously identified in United States v. Downing, 753 F.2d 1224 (3d Cir. 1985). Paoli II, at 742. We held that the District Court's inquiry under Rule 702 should be guided by the criteria set forth in Daubert and Downing as well as other factors that may be relevant to a given inquiry. The factors we specifically identified include:

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"(1) whether a method consists of a testable hypothesis; (2) whether the method has been subject to peer review; (3) the known or potential rate of error; (4) the existence and maintenance of standards controlling the technique's operation; (5) whether the method is generally accepted; (6) the relationship of the technique to methods which have been established to be reliable; (7) the qualifications of the expert witness testifying based on the methodology; and (8) the non-judicial uses to which the method has been put."

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Paoli II, at 742 n. 8. We also noted that the proffered expert testimony must assist the trier of fact. In other words, admissibility depends in part on "the proffered connection between the scientific research or test result to be presented and particular disputed factual issues in the case." Id. at 743.

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Furthermore, we cautioned that the standard for determining reliability "is not that high," Id. at 745, even given the evidentiary gauntlet facing the proponent of expert testimony under Rule 702. Thus, plaintiffs do not "have to prove their case twice -- they do not have to demonstrate to the Judge by a preponderance of the evidence that the assessments of their experts are correct, they only have to demonstrate by a preponderance of evidence that their opinions are reliable." 90 Id. at 744. In other words, "the evidentiary requirement of reliability is lower than the merits standard of correctness." Id.

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"The grounds for the expert's opinion merely have to be good, they do not have to be perfect. The Judge might think that there are good grounds for an expert's Conclusion even if the Judge thinks that there are better grounds for some alternative Conclusion, and even if the Judge thinks that a scientist's methodology has some flaws such that if they had been corrected, the scientist would have reached a different result."

Id. Thus, in Paoli II we explained

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the primary limitation on the Judge's admissibility determinations is that the Judge should not exclude evidence simply because he or she thinks that there is a flaw in the expert's investigative process which renders the expert's Conclusions incorrect. The Judge should only exclude the evidence if the flaw is large enough that the expert lacks the `good grounds' for his or her Conclusions.

Id. at 746. 91

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The test of admissibility is not whether a particular scientific opinion has the best foundation, or even whether the opinion is supported by the best methodology or unassailable research. Rather, the test is whether the "particular opinion is based on valid reasoning and reliable methodology." Kannankeril v. Terminix International Inc., 128 F.3d 802, 806 (3d Cir. 1997) (emphasis added). The admissibility inquiry thus focuses on principles and methodology, not on the Conclusions generated by the principles and methodology. Id. (citing Paoli II at 744). The goal is reliability, not certainty. Once admissibility has been determined, then it is for the trier of fact to determine the credibility of the expert witness. Id. (citing Paoli II at 743-746). "The analysis of the Conclusions themselves is for the trier of fact when the expert is subjected to cross-examination." Id. Therefore, if the methodology and reasoning are sufficiently reliable to allow the fact finder to consider the expert's opinion, it is that trier of fact that must assess the expert's Conclusions. The inquiry is a factual one, not a legal one.

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Nonetheless, "Conclusions and methodology are not entirely distinct from one another." General Electric Co. v. Joiner, 522 U.S. 136 , 117 S. Ct. 512 , 519 (1997). The court "must examine the expert's Conclusions in order to determine whether they could reliably flow from the facts known to the expert and the methodology used." Heller v. Shaw Industries, Inc., 167 F.3d 146, 153 (3d Cir. 1999). "A court may conclude that there is simply too great a gap between the data and the opinion proffered." Joiner, at 519. However, such an opinion will be excluded not because it is necessarily incorrect, but because it is not sufficiently reliable and therefore too likely to lead the factfinder to an erroneous Conclusion.

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Here, the District Court assessed the disputed expert testimony under Rule 702 and held that it did not meet the conditions precedent to admissibility under Daubert. We subject the District Court's interpretation of Rule 702 to plenary review. Paoli II at 749. However, we review the District Court's decision to admit or exclude scientific evidence for an abuse of discretion. Joiner, at 519. 92 An abuse of discretion arises when the District Court's decision "rests upon a clearly erroneous finding of fact, an errant Conclusion of law or an improper application of law to fact." Hanover Potato Products, Inc. v. Shalala, 989 F.2d 123, 127 (3d Cir. 1993). An abuse of discretion can also occur "when no reasonable person would adopt the district court's view." Id. However, we do not interfere with the District Court's exercise of discretion "unless there is a definite and firm conviction that the court below committed a clear error of judgment in the Conclusion it reached upon a weighing of the relevant factors." Id.

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With the parameters of our inquiry in mind of our review in mind, the teachings of Daubert and the aforementioned scientific principles as our guideposts, we can now proceed to apply yardstick of Daubert to the expert opinions at issue here and determine if they were properly excluded under the Rules of Evidence.

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3. Trial Plaintiffs' Dose Exposure Expert Witnesses.

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i. Ignaz Vergeiner.

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a. Qualifications.

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Ignaz Vergeiner is a meteorologist with undergraduate degrees in mathematics and physics, and a Ph.D. in meteorology; all of which were earned at the University of Innsbruck in Austria. He is an Associate Professor in the Department of Meteorology and Geophysics at the University of Innsbruck and has taught graduate and undergraduate courses at that University for twenty years. App. Vol. V., at 3578. He was proffered as an expert in boundary level meteorology in alpine regions. 93 His testimony was offered to explain how the hypothesized plume containing the highly radioactive release that is part of "blow out," traveled and dispersed throughout the area surrounding Three Mile Island.

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b. Vergeiner's Opinion.

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Vergeiner's proffered testimony covered three areas. First, as an expert in boundary level meteorology, he sought to testify about the weather conditions at Three Mile Island and the surrounding areas during, and immediately following, the accident. He was to testify about his plume dispersion hypothesis based upon his studies of the weather conditions. Secondly, he sought to estimate radiation doses in certain areas surrounding Three Mile Island. Finally, he sought to testify about his distrust and skepticism of the original plant data from TMI-2 concerning plume dispersion and radiation releases in an effort to counter the defendants' evidence regarding exposure.

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Essentially, Vergeiner opined that a weather inversion, in combination with the alpine terrain that surrounds Three Mile Island, prevented the radioactive plume from rising high into the atmosphere, spreading out and dispersing in the expected "Gaussian" manner. 94 App. Vol. V., at 4005-09. Instead, he believed that the radioactive plume remained narrow, concentrated and intense and moved erratically in a north, northwest direction from Three Mile Island. He believed that it frequently came in contact with hilly terrain which caused it to reconcentrate, and that after it reconcentrated, it touched down on the ground and exposed the population to high levels of radiation. Id. at 3583-85; 3761-68; 4017-29. To illustrate his plume dispersion theory, Vergeiner produced a water model and a "plume movie". The latter is not a "movie" at all, but is rather a series of sketches he drew by which he illustrated the plume movement he hypothesized. Id. at 3769-775; 4076-78. The water model is a video of a large scale model he built in which colored water was injected into a tank filed with clear water that contained a model of alpine terrain. Id. at 3756-60.

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Vergeiner's qualifications as an expert in meteorology were not in dispute. Nonetheless, the defendants moved to exclude all of his proffered testimony under Rule 702. After a hearing, the District Court subjected his proffered testimony to an exhaustive and rigorous Daubert/Paoli II analysis and excluded the majority of it, including the plume movie and the water model. In re TMI Litigation Cases Consolidated II, 911 F. Supp. 775, 791-799 (M.D. Pa. 1996). However, the court held that, as an expert in meteorology, Vergeiner could testify about the weather conditions during and immediately following the accident if the Trial Plaintiffs could demonstrate how that testimony would assist the jury in determining a pertinent fact. Id. at 799 .

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c. Discussion and Conclusions.

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The Trial Plaintiffs argue that by excluding the overwhelming majority of Vergeiner's proposed testimony, the District Court "elevate[d] its opinion of science over that of Dr. Vergeiner's, even though the court had no training in the complex meteorological issues that he discussed...." Trial Plaintiffs' Br. at 24. We disagree. Preliminarily, we note that a proponent of rejected expert testimony could always level such a challenge against an unfavorable Daubert ruling. However, an adverse ruling under Daubert does not, in and of itself, suggest that a court substituted its opinion for that of a trained scientist. Here, we conclude that the trial court did not substitute its scientific opinion for Vergeiner's. Rather, the court correctly applied the Daubert/Paoli II guidelines and properly found the bulk of Vergeiner's testimony "unreliable and therefore inadmissible under Rule 702." 911 F. Supp. at 799 . Although we find it unnecessary to review the District Court's application of each Daubert/Paoli II criteria, we believe a few examples demonstrate that the District Court's decision to exclude the bulk of Vergeiner's proposed testimony was not an abuse of discretion.

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First, in formulating his plume dispersion hypothesis, Vergeiner discarded standard and generally accepted computer models, especially the Gaussian plume model. 95 App. Vol. V, at 4003-07. At the hearing, Vergeiner testified that the Gaussian plume model was not an adequate model to hypothesize plume dispersion given the weather conditions on the day of the accident, and considering the terrain surrounding Three Mile Island. Id. at 4007; 4051-54. In rejecting the standard computer models, Vergeiner wrote in one of his two reports:

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Clearly, available synoptic meteorological 96 observations cannot determine the flow field and dispersion characteristics down to a scale of hundreds of meters or a few kilometers, as needed to estimate local transport and diffusion from TMI releases....

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So why not straightway model the flow numerically using the power of modern computers? Let me remind the reader of the enormous complexity of such a task. Transport and dispersion models exist to the hundreds, many of them in the nuclear industry or in the scientific "grey zone" around it....

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Quite a few of these models are global in scale, and some apparently have succeeded in simulating the path and contaminating action of the Chernobyl clouds reasonably well, after years of tuning and verification on the many observations available....

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Documentation is a problem, as well as the need for special graphics and internal routines, or compatibility of various computer languages. Not all applications have been successes....

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It was my judgment, therefore, not only that it did not seem feasible to obtain access and results within a limited time span and financial frame, but that relatively simpler, well-tested, more robust and accessible models might be just as good, even preferable. This may appear to do in Justice to the more than 50 man-years' expert work condensed in this enormous structure. There is no doubt that each of these models is capable of computing flow structures very suggestive of real nature, but I couldn't convince myself that the enormously increased expense would bear a sound relation to similarly improved results.

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Ignaz Vergeiner, Treatise on the TMI-2 Accident of March 28, 1979, Particularly its Meteorological Aspects Including Transport and Dispersion of the Radionuclides Released 49-50 (July 1994)(unpublished)(hereinafter "Vergeiner I"); App. Vol. V, at 3634-35.

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Rather than using the standard computer models, Vergeiner chose to use a "numerical model" which he initially referred to by the acronym "AMBIMET," Vergeiner I, at 51; App. Vol. V, at 3636, but which he later called, in his second report, the "FITNAH model operated by AMBIMET." Ignaz Vergeiner, Treatise on the Meteorological Aspects of the TMI-2 Accident 49 (February 1995) (unpublished) (hereinafter "Vergeiner II"); App. Vol. V, at 3752. In any event, he described the model he used, whether AMBIMET or FITNAH, as "a regional one, not designed to simulate local flows on the scale of hundreds of meters" that "requires proper synoptic input." Vergeiner I, at 51-52; App. Vol. V, at 3636-37.

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However, Vergeiner never provided any testimony, documentation or any other evidence that the numerical models he did use are generally accepted within the meteorological or the broader scientific community. Although the "general acceptance" test of Frye v. United States, 54 App. D. C. 46, 293 F. 1013 (1923), was displaced by the Federal Rules of Evidence, Daubert, 509 U.S. at 589 , "general acceptance" in the scientific community can "yet have a bearing on the inquiry," and be an "important factor in ruling particular evidence admissible." Daubert, at 594. "[A] known technique which has been able to attract only minimal support within the community may properly be viewed with skepticism." Id. (quoting United States v. Downing, 753 F.2d 1224, 1239 (3d Cir. 1985)). Thus, while general acceptance is not the focus of the inquiry, it is a relevant factor which may be considered. Accordingly, a court may well cast a jaundiced eye upon a technique which is not supported by any evidence of general acceptance absent other indicia of reliable methodology. Here, it is impossible to know whether the disputed model's methodology can or has been tested or whether the model has been subjected to peer review or publication. Neither can we determine its known or potential rate of error. Consequently, we can hardly conclude that the plume dispersion model Vergeiner hypothesized meets the Daubert requirement of evidentiary reliability.

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Second, Vergeiner's "plume movie" (which, as noted earlier, is but rather a series of sketches 97 he drew to illustrate his hypothesized plume movements) is based on pure speculation. In his second report, Vergeiner presented his opinion as to behavior of the plume. He wrote:

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For Conclusion, I present my own tentative TMI plume "movie" for the first few hours... Its chief purpose is visualization of possible plume shifts and exposures, and realization of the kind of information we would need to be reasonably sure about transport and dispersion of TMI-2 effluents. [The plume movie] is the beginning of an investigation, not the end.

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App. Vol. V, at 3769 (emphasis added). The speculative nature of the plume movie was made even more apparent during Vergeiner's deposition when he described the plume movie.

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I make it clear that the [plume movie] and following are not meant to be -- I think the way I write it is that they are the beginning of a Discussion and not the end of a Discussion.... And I realize it's absolutely clear, and I state it, that this, this is an assumption, I think it's not an unreasonable one, it has some foundation, but at this stage it is just a, well, it's more than a provocation, but this --

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Q: It's the articulation of a hypothesis yet to be explored?

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A: Of a hypothesis, and it is an illustration, certainly an illustration of winds turning rapidly, which they did, that one is for sure, and the consequences of a plume, I wanted to illustrate how distorted a plume can become. I wanted to illustrate the effects. I just don't have enough of a database to prove details of this. This is absolutely clear and conceded. Absolutely clear.

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Id. at 3941.

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Rule 702 not only requires that the scientific opinion proffered by the expert be supported by "good grounds," Daubert, at 590, it also mandates that the challenged testimony "assist the trier of fact to understand the evidence or to determine a fact in issue." This requirement is one of relevance and expert evidence which does not relate to an issue in the case is not helpful. Id. at 591. The expert's testimony must "fit," and admissibility depends, in part, on a connection between the expert opinion offered and the particular disputed factual issues in the case. Paoli II, at 743. "Fit is not always obvious, and scientific validity for one purpose is not necessarily validity for other unrelated purposes." Id. Here, Vergeiner's report and testimony make clear that his plume movie was merely an assumption visualizing possible plume movements. Given its speculative character, the plume movie was properly excluded under Daubert.

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We note that in order for expert testimony to be reliable, and therefore admissible, it must be based on the methods and procedures of science rather than subjective belief or speculation. Kannankeril, 128 F.3d at 806 (citing Paoli II, 35 F.3d at 744 ). Consequently, Vergeiner's plume movie, and, (as will be discussed), his water model, are also lacking in scientific reliability and are inadmissible because of their speculative character. Nevertheless, we believe that the plume movie and the water model are more appropriately inadmissible because they lack fit.

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The water model does not "fit." The water model is a video of a large scale model tank, the bottom of which is a topographical map of alpine terrain. The tank is filled with water, a dye is injected into the water and a current is run through the water to simulate air flow. Its intended purpose is to demonstrate how a material will disperse in the atmosphere in relation to terrain and air patterns. See 911 F. Supp. at 792 n.10. However, the water model is just as speculative as the plume movie. In his deposition, Vergeiner testified that the water model was a demonstration and a "tool for visualization," but was not intended "to exactly simulate flows at the time of the TMI accident." App. Vol. V, at 3930-31. In fact, Vergeiner testified that"[t]here's no way to [simulate the complete atmospheric structure] in a simple shallow water model." Id., at 3930. Simply put, the water model does not assist the finder of fact and is, therefore, not admissible under Rule 702.

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Most importantly, we note that Vergeiner's proffered testimony about the amount of radioactive materials delivered to the areas where the plume traveled was totally unreliable. That testimony was intended to explain how the hypothesized plume containing the high concentrations of radionuclides (believed to be part of the "blowout") dispersed throughout the Three Mile Island Area exposing the population to high levels of radiation.

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In the field of radiation dose reconstruction, 98 "the amount of radionuclides released from a site over a specific period" is called the "source term". RADIATION DOSE RECONSTRUCTION, at 16. A proper dose reconstruction study should determine the amount of radionuclides released over a specified period as well as the rate of release as a function of time. Id. Consequently, a complete description of the source term "includes what was released and in what form and where and when the release occurred." Id. The National Research Council's Committee on an Assessment of [Center for Disease Control and Prevention] Radiation Studies, has noted that if dose reconstruction studies are credible, they "must rely on solid science, state-of-the-art methods, and careful peer review." Id. at 14. The Committee further noted that "[u]ltimately, a dose reconstruction study will be Judged by the scientific community on the basis of the technical quality of the

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