Environmental Radiation Protection Standards for Yucca Mountain, Nevada
Federal RegisterAug 27, 1999
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SUMMARY: We, the Environmental Protection Agency (EPA), are proposing
public health and safety standards for radioactive material stored or
disposed of in the potential repository at Yucca Mountain, Nevada.
Section 801 of the Energy Policy Act of 1992 (EnPA) directed the
Administrator of EPA to develop these standards. The EnPA also required
EPA to contract with the National Academy of Sciences (NAS) to conduct
a study to provide findings and recommendations on reasonable standards
for protection of the public health and safety. On August 1, 1995, NAS
released its report (the NAS Report) entitled, ``Technical Bases for
Yucca Mountain Standards.'' We have taken the NAS Report into
consideration as directed by the EnPA.
After we finalize these standards, the Nuclear Regulatory
Commission (NRC or ``the Commission'') will incorporate them into its
licensing regulations. The Department of Energy (DOE or ``the
Department'') will be responsible for demonstrating compliance with
these standards. The Commission will use its licensing regulations to
determine whether the Department has demonstrated compliance with our
standards prior to receiving the necessary licenses to store or dispose
of radioactive material in Yucca Mountain.
DATES: Comments. We must receive your comments at the address given
below on or before November 26, 1999 to assure their consideration.
Hearings. We will hold public hearings upon today's action in
Amargosa Valley, Nevada, Las Vegas, Nevada, and Washington, DC. The
dates will be announced in the Federal Register as soon as they are
determined.
ADDRESSES: Comments. Send two copies of your comments to the Central
Docket Section (6102), ATTN: Docket A-95-12, U.S. Environmental
Protection Agency, 401 M Street, SW, Washington, D.C. 20460-0001.
Documents relevant to the rulemaking. Materials relevant to this
rulemaking are contained in: (1) Docket No. A-95-12, located in Room M-
1500 (first floor in Waterside Mall near the Washington Information
Center), U.S. Environmental Protection Agency, 401 M Street, SW,
Washington, DC 20460-0001; (2) an information file in the Government
Publications Section, Dickinson Library, University of Nevada-Las
Vegas, 4504 Maryland Parkway, Las Vegas, Nevada 89154; and (3) an
information file in the Public Library in Amargosa Valley, Nevada
89020.
Background documents for this action. We have prepared additional
documents that provide more detailed technical background in support of
these proposed standards. You may obtain copies of the draft background
information document (BID), the draft economic impact evaluation, and
the Executive Summary of the NAS Report by requesting them in writing
from the Office of Radiation and Indoor Air (6602J), U.S. Environmental
Protection Agency, Washington, DC 20460-0001. We have also placed these
documents into the docket and information files. You may also find them
on our Internet site for Yucca Mountain (see the Additional Docket and
Electronic Information section later in this notice).
FOR FURTHER INFORMATION CONTACT: Ray Clark, Office of Radiation and
Indoor Air, U.S. Environmental Protection Agency, Washington, D.C.
20460-0001; telephone 202-564-9300.
SUPPLEMENTARY INFORMATION:
Who Will Be Regulated by These Standards?
The Department is the only entity directly regulated by these
standards. To utilize the Yucca Mountain repository, DOE must obtain
licensing approval from NRC. Thus, DOE will be subject to our standards
which NRC will implement through its licensing proceedings. The NRC is
only affected because, under the Energy Policy Act of 1992 (EnPA, Pub.
L. 102-486), it must modify its licensing requirements, as necessary,
to be consistent with our final standards.
Additional Docket and Electronic Information
When may I examine docket information? You may inspect the
Washington, D.C. docket (phone 202-260-7548) on weekdays (8 a.m.-5:30
p.m.). As provided in 40 CFR part 2, the docket personnel may charge a
reasonable fee for photocopying docket materials.
The information file located in the University of Nevada-Las Vegas,
Government Publications Section (702-895-3409) may be inspected when
classes are in session, Monday through Thursday (9 a.m.-8 p.m.), Friday
(9 a.m.-6 p.m.), Saturday (9 a.m.-9 p.m.), and Sunday (11 a.m.-8 p.m.).
However, since the hours vary based upon the academic calendar, you
should call ahead to be certain of the time.
The information file in the Public Library in Amargosa Valley,
Nevada (phone 775-372-5340) may be inspected Monday through Thursday
(11 a.m.-7 p.m.) and Friday (9 a.m.-5 p.m.). The library is closed from
12:30 p.m.-1 p.m. each day. It is also closed Saturday and Sunday.
Can information be accessed by telephone or the Internet? Yes, we
have established a toll-free information line that is accessible 24
hours per day. By dialing 800-331-9477, you can listen to a brief
update describing our rulemaking activities for Yucca Mountain, leave a
message requesting that your name and address be added to the Yucca
Mountain mailing list, or request that an EPA staff person return your
call. You can also find information on the World Wide Web at http://
www.epa.gov/radiation/yucca.
Acronyms
There are many acronyms used in this notice. They are listed below
for your reference and convenience.
ALARA--as low as reasonably achievable
BID--background information document
CAA--Clean Air Act
CEDE--committed effective dose equivalent
CG--critical group
DOE--U.S. Department of Energy
EIS--environmental impact statement
EnPA--Energy Policy Act of 1992
EPA--U.S. Environmental Protection Agency
GCD--greater confinement disposal
HLW--high-level radioactive waste
IAEA--International Atomic Energy Agency
ICRP--International Commission on Radiological Protection
LLW--low-level radioactive waste
MCL--maximum contaminant level
MCLG--maximum contaminant level goal
NAS--National Academy of Sciences
NCRP--National Council on Radiation Protection and Measurements
NEPA--National Environmental Policy Act
NESHAPs--National Emission Standards for Hazardous Air Pollutants
NID--negligible incremental dose
NIR--negligible incremental risk
NRC--U.S. Nuclear Regulatory Commission
[[Page 46977]]
NRDC--Natural Resources Defense Council
NTS--Nevada Test Site
NTTAA--National Technology Transfer and Advancement Act
NWPA--Nuclear Waste Policy Act of 1982
NWPAA--Nuclear Waste Policy Amendments Act of 1987
OMB--Office of Management and Budget
RCRA--Resource Conservation and Recovery Act
RME--reasonable maximum exposure
RMEI--reasonably maximally exposed individual
SDWA--Safe Drinking Water Act
SNF--spent nuclear fuel
TDS--total dissolved solids
UIC--underground injection control
UMRA--Unfunded Mandates Reform Act of 1995
USDW--underground source of drinking water
WIPP LWA--Waste Isolation Pilot Plant Land Withdrawal Act of 1992
Outline of Proposed Action
I. What Led up to Today's Action?
II. Background Information
II.A. What Are the Sources of Radioactive Waste?
II.B. What Types of Health Effects Can Radiation Cause?
II.C. What Are the Major Features of the Geology of Yucca
Mountain and the Disposal System?
II.D. Background on and Summary of the NAS Report
II.D.1. What Were the NAS Findings and Recommendations?
II.D.2. How Has the Public Participated in Our Review of the NAS
Report?
II.D.3. What Were the Public Comments on the NAS Report?
III. What Are We Proposing Today?
III.A. What Is the Proposed Standard for Storage of the Waste?
(Proposed Subpart A)
III.B. What Is the Standard for Protection of Individuals?
(Proposed Secs. 197.20 and 197.25)
III.B.1. Should the Limit Be on Dose or Risk?
III.B.2. What Should the Level of Protection Be?
III.B.3. What Factors Can Lead to Radiation Exposure?
III.B.4. Who Will Be Representative of the Exposed Population?
III.B.5. How Will the General Population Be Protected?
III.B.6. What Should Be Assumed About the Future Biosphere?
III.B.7. How Far Into the Future Is It Reasonable To Project
Disposal System Performance?
III.C. What Are the Requirements for Performance Assessments and
Determinations of Compliance? (Proposed Secs. 197.20, 197.25, and
197.35)
III.C.1. What Limits Are There on Factors Included in the
Performance Assessments?
III.C.2. Is Expert Opinion Allowed?
III.C.3. What Level of Expectation Is Required for NRC To
Determine Compliance?
III.D. Are There Qualitative Requirements To Help Assure
Protection?
III.E. What Is the Standard for Human Intrusion? (Proposed
Sec. 197.25)
III.F. How Will Ground Water Be Protected? (Proposed
Sec. 197.35)
III.F.1. Is the Storage or Disposal of Radioactive Material in
the Yucca Mountain Repository Underground Injection?
III.F.2. Does the Class-IV Well Ban Apply?
III.F.3. Which Ground Water Should Be Protected?
III.F.4. How Far Into the Future Should Compliance Be Projected?
III.F.5. How Will the Point of Compliance Be Identified?
III.F.6. Where Will the Point of Compliance Be Located?
IV. Specific Questions for Public Comment
V. Regulatory Analyses
V.A. Executive Order 12866
V.B. Executive Order 12875
V.C. Executive Order 12898
V.D. Executive Order 13045
V.E. Executive Order 13084
V.F. National Technology Transfer and Advancement Act
V.G. Paperwork Reduction Act
V.H. Regulatory Flexibility Act/Small Business Regulatory
Enforcement Fairness Act of 1996
V.I. Unfunded Mandates Reform Act
I. What Led up to Today's Action?
Spent nuclear fuel (SNF) and high-level radioactive waste (HLW)
have been produced since the 1940s, mainly as a result of commercial
power production and defense activities. Since then, the proper
disposal of these wastes has been the responsibility of the Federal
government. The Nuclear Waste Policy Act of 1982 (NWPA, Pub. L. 97-425)
formalized the current Federal program for the disposal of SNF and HLW
by:
(1) Making DOE responsible for siting, building, and operating an
underground geologic repository for the disposal of SNF and HLW;
(2) Directing us to set generally applicable environmental
radiation protection standards based upon authority established under
other laws; and
(3) Requiring NRC to implement our standards by incorporating them
into its licensing requirements for SNF and HLW repositories.
Those responsibilities are generally maintained under the EnPA.
Thus, NRC will implement the standards that we are proposing today, and
DOE will submit a license application to NRC. The Commission will then
determine whether DOE has met the standards and whether to issue an
operating license for Yucca Mountain. We anticipate that NRC will
require compliance with all of the applicable provisions of 40 CFR part
197 prior to allowing receipt of radioactive material onto the Yucca
Mountain site.
In 1985, we established generic standards for the management,
storage, and disposal of SNF, HLW, and transuranic radioactive waste.
These standards are found in 40 CFR part 191 (50 FR 38066, September
19, 1985). The term ``generic'' meant that the standards applied to any
applicable facilities in the United States, including Yucca Mountain,
Nevada. In 1987, the U.S. Court of Appeals for the First Circuit
invalidated the disposal standards and remanded them to us (NRDC v.
EPA, 824 F.2d 1258 (1st Cir. 1987)). Also in 1987, the Nuclear Waste
Policy Amendments Act (NWPAA, Pub. L. 100-203) amended the NWPA by,
among other actions, selecting Yucca Mountain, Nevada as the only
potential site to be characterized.
In October 1992, the Waste Isolation Pilot Plant Land Withdrawal
Act (WIPP LWA, Pub. L. 102-579) and the EnPA became law. The statutes
changed our obligations concerning certain radiation standards. The
WIPP LWA:
(1) Reinstated the 40 CFR part 191 disposal standards except those
that were the specific subject of the remand by the First Circuit;
(2) Required us to issue standards to replace those that were the
subject of judicial remand; and
(3) Exempted the Yucca Mountain site from the 40 CFR part 191
disposal standards. We issued the final disposal standards in 40 CFR
part 191 on December 20, 1993 (58 FR 66398) to address the judicial
remand.
The EnPA gave us the responsibility to set public health and safety
radiation standards for Yucca Mountain. Specifically, section 801(a)(1)
of the EnPA directed us to ``promulgate, by rule, public health and
safety standards for the protection of the public from releases from
radioactive materials stored or disposed of in the repository at the
Yucca Mountain site.'' The EnPA also directed us to contract with NAS
to give us findings and recommendations on reasonable standards for
protection of public health and safety. Moreover, the statute provided
that our standards shall be the only such standards applicable to the
Yucca Mountain site and are to be based upon and consistent with NAS'
findings and recommendations. On August 1, 1995, NAS released its
report, ``Technical Bases for Yucca Mountain Standards'' (the NAS
Report).
[[Page 46978]]
II. Background Information
II.A. What Are the Sources of Radioactive Waste?
Radioactive wastes are the result of using nuclear fuel and other
radioactive material. Today's action proposes standards pertaining to
SNF, HLW, and other radioactive waste (these are collectively referred
to after this as ``radioactive material'' or ``waste'') which may be
stored or disposed of in the Yucca Mountain repository. (When storage
or disposal are discussed in this notice in reference to Yucca
Mountain, it is to be understood that no decision has been made
regarding the acceptability of Yucca Mountain for storage or disposal.
To save space and excessive repetition, the description of Yucca
Mountain as a ``potential'' repository will not be used but is
intended.) These standards do not apply to facilities other than those
related to Yucca Mountain.
Once enough uranium or other fissionable material in nuclear
reactor fuel has been consumed through nuclear reactions, it is no
longer useful. The product is known as ``spent'' nuclear fuel (SNF).
Sources of SNF include:
(1) Commercial nuclear power plants;
(2) Government-sponsored research and development programs in
universities and industry;
(3) Experimental reactors, such as, liquid metal fast breeder
reactors and high-temperature gas-cooled reactors;
(4) Federal Government-controlled, nuclear-weapons production
reactors;
(5) Naval and other Department of Defense reactors; and
(6) U.S.-owned, foreign SNF.
Spent nuclear fuel can be dissolved in a chemical process called
``reprocessing,'' which is used to recover desired radionuclides.
Radionuclides which are not recovered become part of the acidic liquid
wastes that DOE plans to convert into various types of solid materials.
The highly radioactive liquid or solid wastes from reprocessing SNF are
called HLW. If SNF is not reprocessed prior to disposal, it becomes the
waste form without further modification. The only commercial
reprocessing facility to operate in the United States, the Nuclear Fuel
Services Plant in West Valley, New York, closed in 1972. Since that
time, no commercial SNF has been reprocessed in the United States. In
1992, DOE decided to phase out reprocessing of its SNF which supported
the defense nuclear weapons and propulsion programs.
Where are the wastes stored now? Today, most SNF is stored in water
pools or above-ground in dry concrete or steel canisters at more than
70 commercial nuclear-power reactor sites across the Nation. High-level
waste is stored underground in steel tanks at four Federal facilities
in Idaho, Washington, South Carolina, and New York.
What types of wastes will be placed into Yucca Mountain? We
anticipate that most of the waste in Yucca Mountain will be SNF and
solidified HLW (in the rest of this notice, HLW will refer to
solidified HLW unless otherwise noted). Under current NRC regulations
(10 CFR 60.135), liquid HLW will have to be solidified, through
processes such as vitrification (mixing the waste into glass), since
non-solid waste forms would not be allowed to be stored or disposed of
in Yucca Mountain. The Department estimates that by the year 2010,
about 64,000 metric tons of SNF and 284,000 cubic meters (containing
450 million curies of radioactivity) of HLW in predisposal form and
2,600 cubic meters (containing 189 million curies) of the disposable
form of HLW will be in storage (DOE/RW-0006, Rev. 12, December 1996).
We are aware that other radioactive materials might be stored or
disposed of in the Yucca Mountain repository. These materials include
highly radioactive low-level waste (LLW), known as greater-than-Class-C
waste, and excess plutonium or other fissile materials resulting from
the dismantlement of nuclear weapons. In the future, other types of
radioactive materials could be identified for storage or disposal.
Since the plans for the disposal of these materials have not been
finalized, their impact upon the design and performance of the disposal
system has not been analyzed by NRC or DOE. However, whatever types of
radioactive materials are finally disposed of in Yucca Mountain, the
disposal system must comply with these standards.
II.B. What Types of Health Effects Can Radiation Cause?
Ionizing radiation can cause a variety of health effects. These
effects are classified as either ``non-stochastic'' or ``stochastic.''
Non-stochastic effects are those for which the damage increases with
increasing exposure, such as destruction of cells or reddening of the
skin. They are seen in cases of exposures to large amounts of
radiation. Stochastic effects are associated with long-term exposure to
low levels of radiation. Their type or severity does not depend upon
the amount of exposure. Instead, the chance that an effect, for
example, cancer, will occur is assumed to increase with increasing
exposure.
The three categories of stochastic effects are cancer, mutations,
and teratogenic effects. Cancers caused by radiation are
indistinguishable from those occurring from other causes. Cancers
caused by radiation have been observed in humans. However, the risk of
cancer at the exposure levels normally encountered by members of the
public must be estimated using indirect evidence, that is,
extrapolation from higher doses.1
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\1\ The general term ``dose'' is used to mean the dose
equivalent, effective dose equivalent, or committed effective dose
equivalent, depending upon the surrounding text. When precision is
necessary, the exact term is used.
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Mutations, the second category of stochastic effects, are created
in the reproductive cells of exposed individuals and are transmitted to
their descendants. The severity of hereditary effects can range from
inconsequential to fatal. Although hereditary effects have been
observed in animal studies at relatively high doses, hereditary effects
in humans exposed to relatively small amounts of radiation have not
been confirmed statistically in epidemiological studies. Finally, we
assume that at low levels of exposure, the probability of incurring
either cancer or hereditary effects increases as the dose increases and
that there is no lower threshold, that is, a linear, non-threshold,
dose-response relationship (this is discussed below in more detail).
Teratogenic effects, the third category of stochastic effects, can
occur following exposure of fetuses. We believe that the fetus is more
sensitive than adults to the induction of cancer by radiation. The
fetus also is subject to various radiation-induced, physical
malformations such as small brain size (microencephaly), small head
size (microcephaly), eye malformations and slow growth prior to birth.
Recent studies have focused upon the apparently increased risk of
severe mental retardation as measured by the intelligence quotient.
These studies indicate that the sensitivity of the fetus is greatest
during 8 to 15 weeks following conception, and continues, at a lower
level, between 16 and 25 weeks.2 Although we do not know
exactly how mental retardation is related to dose, it is prudent to
assume that there is a linear, non-threshold, dose-response
relationship between these effects and the dose delivered to the fetus
during the 8- to 15-week period.
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\2\ Health Effects of Exposure to Low Levels of Ionizing
Radiation, National Academy Press, Washington, D.C., 1990.
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The NAS published its reviews of human health risks from exposure
to low levels of ionizing radiation in a
[[Page 46979]]
series of reports between 1972 and 1990. However, scientists still do
not agree upon how best to estimate the probability of cancer occurring
as a result of the doses encountered by members of the public
3 because these effects must be estimated based upon the
effects observed at higher doses (such as effects seen in the survivors
of the Hiroshima and Nagasaki atomic bombs). The linear model for
estimating effects has been endorsed by many organizations, including
NAS, the International Commission on Radiological Protection (ICRP),
the United Nations Scientific Committee on the Effects of Atomic
Radiation, and the National Radiological Protection Board of the United
Kingdom.
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\3\ The risk of interest is not at or near zero dose, but that
due to small increments of dose above the pre-existing background
level. Background in the U.S. is typically about 3 millisievert
(mSv), that is, 300 millirem (mrem), effective dose equivalent per
year, or 0.2 Sv (20 rem) in a lifetime. Approximately two-thirds of
this dose is due to radon, and the balance comes from cosmic,
terrestrial, and internal sources of exposure.
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Over the past decade, the scientific community has performed an
extensive reevaluation of the doses and effects in the Hiroshima and
Nagasaki survivors. These studies have resulted in increased estimates
(roughly threefold between 1972 and 1990) of the extrapolated risk of
cancer arising from exposure to environmental levels of radiation, that
is, background levels of radiation. Nonetheless, the estimated number
of health effects induced by small incremental doses of radiation above
natural background levels remains small compared with the total number
of fatal cancers that occur from other causes. In addition, because
cancers are the same as those resulting from other causes, identifying
them in human epidemiological studies may never be possible. This
difficulty in identifying stochastic radiation effects does not mean
that such effects do not occur. However, there is the possibility that
effects do not occur as a result of these small doses, that is, there
might be an exposure level below which there is no additional risk
above the risk that is posed by natural background radiation.
Sufficient data to prove either possibility scientifically is lacking.
As a result, we believe that the best approach is to assume that the
risk of cancer increases linearly starting at zero dose. That is, any
increase in exposure to ionizing radiation results in a constant and
proportionate increase in the potential for developing cancer.
The NAS Report stated that radiation causes about five cancers for
every severe hereditary disorder. Also, NAS concluded that nonfatal
cancers are more common than fatal cancers. Despite this, the NAS cited
an ICRP study which judged that non-fatal cancers contribute less to
overall health impact than fatal cancers ``because of their lesser
severity in the affected individuals.'' (NAS Report pp. 37-39). Our
risk estimates for exposure of the population to low-dose-rate
radiation is based upon fatal cancers rather than all cancers.
For radiation-protection purposes, we estimate (using a linear,
non-threshold, dose-response model) an average risk for a member of the
U.S. population of 5.75 in 100 (5.75 x 10-2) fatal cancers
per sievert (Sv) 4 (5.75 x 10-4 fatal cancers
per rem) delivered at low dose rates.5 (For example, if
100,000 people randomly chosen from the U.S. population were each given
a uniform dose of 1 millisievert (mSv) (0.1 rem) to the entire body at
a low rate, approximately five to six people are assumed to die of
cancer during their remaining lifetimes because of that exposure. This
is in addition to the roughly 20,000 fatal cancers that would occur in
the same population from other causes.) The risk of fatal childhood
cancer, resulting from exposure while in the fetal stage, is about 3 in
100 (3 x 10-2) per Sv (that is, 3 x 10-4
effects per rem). The risk of severe hereditary effects in offspring is
estimated to be about 1 x 10-2 per Sv (1 x
10-4 effects per rem).6 The risk of severe mental
retardation from doses to a fetus is estimated to be greater per unit
dose than the risk of cancer in the general population.7
However, the period of increased sensitivity is much shorter. Hence, at
a constant exposure rate, fatal cancer risk in the general population
remains the dominant factor.
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\4\ The traditional unit for dose equivalent has been the rem.
The unit ``sievert'' (Sv), a unit in the International System of
Units which was adopted in 1979 by the General Conference on Weights
and Measures, is now in general use throughout the world. One
sievert is equal to 100 rem. The prefix ``milli'' (m) means one-
thousandth. The individual-protection limit being proposed today may
be expressed in either unit.
\5\ ``Low dose rates'' here refer to dose rates on the order of
or less than those from background radiation.
\6\ The risk of severe hereditary effects in the first two
generations, for exposure of the reproductive part of the population
(with both parents exposed), is estimated to be 5 x
10-3 per Sv (5 x 10-5 per rem). For all
generations, the risk is estimated to be 1.2 x 10-2 per
Sv (1.2 x 10-4 per rem). For exposure of the entire
population, which includes individuals past the age of normal child-
bearing, each estimate is reduced to 40% of the cited value.
\7\ Assuming a linear, non-threshold dose response, estimated
risk for mental retardation due to exposure during the 8th through
15th week of gestation is 4 x 10-1 per Sv (4 x
10-3 per rem); under the same assumption, the estimated
risk from the 16th to 25th week is 1 x 10-1 per Sv (1
x 10-3 per rem).
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We note that there is, of course, uncertainty in our risk
estimates. A recent uncertainty analysis published by the National
Council on Radiation Protection and Measurements (NCRP Report 126)
estimated that the actual risk of cancer from whole-body exposure to
low doses of radiation could be between 1.5 times higher and 4.8 times
lower (at the 90-percent confidence level) than our basic estimate of
5.75 x 10-2 per Sv (5.75 x 10-4 per rem).
Further, existing epidemiological data does not rule out the existence
of a threshold. If there is a threshold, exposures below that level
would pose no additional risk above the risk that is posed by natural
background radiation. The risks of genetic abnormalities and mental
retardation are less well known than those for cancer and, thus, may
include a greater degree of uncertainty. However, in spite of
uncertainties in the data and its analysis, estimates of the risks from
exposure to low levels of ionizing radiation are more clearly known
than those for virtually any other environmental carcinogen.
II.C. What Are the Major Features of the Geology of Yucca Mountain and
the Disposal System?
The geology. The Yucca Mountain site is located in southwestern
Nevada approximately 90 miles northwest of Las Vegas. The eastern part
of the site is on the Nevada Test Site, the northwestern part of the
site is on the Nellis Air Force Range, and the southwestern part of the
site is on Bureau of Land Management land. The area has a desert
climate with topography typical of the Basin and Range province. See
the BID for more information.
Yucca Mountain is made of layers of ashfalls from volcanic
eruptions which happened more than 10 million years ago. The ash
consolidated into a rock type called ``tuff'' which has varying degrees
of compaction and fracturing depending upon the degree of ``welding''
caused by temperature and pressure when the ash was deposited. Regional
geologic forces have tilted the tuff layers and formed Yucca Mountain's
crest (Yucca Mountain's shape is actually a ridge rather than a peak).
Below the tuff is carbonate rock. The carbonate rock was formed from
sediments laid down at the bottom of ancient seas which existed in the
area.
There are two general hydrologic zones within and below Yucca
Mountain. The upper zone is called the ``unsaturated zone'' because the
pore
[[Page 46980]]
spaces and fractures within the rock are not filled entirely with
water. Below the unsaturated zone, beginning at the water table, is the
``saturated zone'' in which the pores and fractures are filled
completely with water. Fractures in both zones could act as pathways
which allow for faster contaminant transport than would the pores. The
Department plans to build the repository in the unsaturated zone about
300 meters below the surface and about 300 to 500 meters above the
current water table.
There are two major aquifers in the saturated zone under Yucca
Mountain. The upper one is in tuff, while the lower one is in carbonate
rock. Regional ground water in the vicinity of Yucca Mountain is
believed to flow generally in a south-southwesterly direction. The
aquifers are more fully discussed in the BID.
The disposal system. The NAS Report described the current
conception of the potential disposal system as a system of engineered
barriers for the disposal of radioactive waste located in the geologic
setting of Yucca Mountain (NAS Report pp. 23-27). Entry into the
repository for waste emplacement would be on gradually downward sloping
ramps which enter the side of Yucca Mountain. The NWPAA limits the
capacity of the repository to 70,000 metric tons of SNF and HLW.
Current DOE plans project that about 90 percent (by mass) would be
commercial SNF and 10 percent defense HLW. Within 100 years after
starting to put waste in place, the repository would be sealed by
backfilling the tunnels, closing the opening to each of the tunnels,
and sealing the entrance ramps and shafts.
We expect the engineered barrier system to consist of at least the
waste form (that is, SNF assemblies or borosilicate glass containing
the HLW), internal stabilizers for the SNF assemblies, the waste
packages holding the waste, and backfill in the space between the waste
packages and adjacent host rock. Spent nuclear fuel assemblies are
comprised of uranium oxide, fission products, fuel cladding, and
support hardware, all of which will be radioactive. (see the What are
the Sources of Radioactive Waste? section above.)
II.D. Background on and Summary of the NAS Report
Section 801(a)(2) of the EnPA directed us to contract with NAS to
conduct a study to provide findings and recommendations on reasonable
standards for protection of public health and safety. Section 801(a)(2)
of the EnPA specifically called for NAS to address the following three
issues:
(A) whether a health-based standard based upon doses to individual
members of the public from releases to the accessible environment (as
that term is defined in the regulations contained in subpart B of part
191 of title 40, Code of Federal Regulations, as in effect on November
18, 1985) will provide a reasonable standard for protection of the
health and safety of the general public;
(B) whether it is reasonable to assume that a system for post-
closure oversight of the repository can be developed, based upon active
institutional controls, that will prevent an unreasonable risk of
breaching the repository's engineered or geologic barriers or
increasing the exposure of individual members of the public to
radiation beyond allowable limits; and
(C) whether it is possible to make scientifically supportable
predictions of the probability that the repository's engineered or
geologic barriers will be breached as a result of human intrusion over
a period of 10,000 years.
On August 1, 1995, NAS submitted to us its report entitled
``Technical Bases for Yucca Mountain Standards.'' The NAS Report is
available for review in the dockets and information file described
earlier. You can order the Report from the National Academy Press by
calling 800-624-6242 or on the World Wide Web at http://www.nap.edu/
bookstore/isbn/0309052890.html#title.
II.D.1. What Were the NAS Findings and Recommendations?
The NAS Report provided a number of conclusions and
recommendations. (The EnPA used the term ``findings,'' however, the NAS
Report used the term ``conclusions.'')
Conclusions. The conclusions in the Executive Summary of the NAS
Report (pp. 1-14) were:
(a) ``that an individual-risk standard would protect public health,
given the particular characteristics of the site, provided that policy
makers and the public are prepared to accept that very low radiation
doses pose a negligibly small risk'' [later termed ``negligible
incremental risk'']. This is the response to the issue identified in
section 801(a)(2)(A) of the EnPA;
(b) that the Yucca Mountain-related ``physical and geologic
processes are sufficiently quantifiable and the related uncertainties
sufficiently boundable that the performance can be assessed over time
frames during which the geologic system is relatively stable or varies
in a boundable manner;''
(c) ``that it is not possible to predict on the basis of scientific
analyses the societal factors required for an exposure scenario.
Specifying exposure scenarios therefore requires a policy decision that
is appropriately made in a rulemaking process conducted by EPA;''
(d) ``that it is not reasonable to assume that a system for post-
closure oversight of the repository can be developed, based on active
institutional controls, that will prevent an unreasonable risk of
breaching the repository's engineered barriers or increasing the
exposure of individual members of the public to radiation beyond
allowable limits.'' This is the response to the issue identified in
section 801(a)(2)(B) of the EnPA;
(e) ``that it is not possible to make scientifically supportable
predictions of the probability that a repository's engineered or
geologic barriers will be breached as a result of human intrusion over
a period of 10,000 years.'' This is the response to the issue
identified in section 801(a)(2)(C) of the EnPA; and
(f) ``that there is no scientific basis for incorporating the ALARA
[as low as reasonably achievable] principle into the EPA standard or
USNRC [U.S. Nuclear Regulatory Commission] regulations for the
repository.''
Recommendations. The recommendations in the Executive Summary of
the NAS Report were:
(a) ``the use of a standard that sets a limit on the risk to
individuals of adverse health effects from releases from the
repository;''
(b) ``that the critical-group approach be used'' (see the Who Will
Be Representative of the Exposed Population? section later in this
notice);
(c) ``that compliance assessment be conducted for the time when the
greatest risk occurs, within the limits imposed by long-term stability
of the geologic environment;'' and,
(d) ``that the estimated risk calculated from the assumed intrusion
scenario be no greater than the risk limit adopted for the undisturbed-
repository case because a repository that is suitable for safe long-
term disposal should be able to continue to provide acceptable waste
isolation after some type of intrusion.''
Other Conclusions and Recommendations. There were other conclusions
and recommendations in addition to those summarized in the Executive
Summary. Most were related to or supported those presented in the
Executive Summary.
II.D.2. How Has the Public Participated in Our Review of the NAS
Report?
We are committed to providing ample opportunity for public
participation in our Yucca Mountain rulemaking activities. We announced
the first opportunity for public participation on September 11, 1995 in
the Federal
[[Page 46981]]
Register (60 FR 47172) where we requested comments upon the NAS Report
and announced the times and locations of three public meetings. Along
with the general request for public comments, we asked five questions:
(1) did the Report sufficiently answer the questions posed in the
EnPA;
(2) was there sufficient rationale to support the NAS' findings and
conclusions;
(3) do provisions other than those found in NAS' findings and
conclusions need to be included in the EPA standards;
(4) are any of NAS' findings or conclusions inappropriate or
inaccurate regarding Yucca Mountain; and
(5) would the cost of imposing the findings and recommendations be
justifiable when compared with the benefits provided?
We held the public meetings to inform the public of our role, to
outline the issues associated with setting standards for Yucca
Mountain, and to seek comments upon the NAS Report. The meetings were
held on September 20, 1995, in Amargosa Valley, Nevada; on September
21, 1995, in Las Vegas, Nevada; and on September 27, 1995, in
Washington, DC. We also have established several other information
sources and given directions, in the ADDRESSES and Additional Docket
and Electronic Information sections earlier in this notice, on how to
access them.
II.D.3. What Were the Public Comments on the NAS Report?
We received comments regarding the NAS Report both orally and in
writing at the public meetings and in response to the September 11,
1995, Federal Register notice, respectively. All written comments are
in the docket and information files. The oral comments were summarized
in a separate document, copies of which are also in the docket and
information files.
Some commenters believed that the NAS inadequately supported its
conclusion that there is no scientific basis for including the ``as low
as reasonably achievable'' (ALARA) principle and subsystem requirements
in the standards and, therefore, that we should include them in the
proposed standards. The ALARA principle is a radiation-protection
concept which states that exposures to radiation should be kept as low
as can be done taking into account the costs and benefits of exposure
reduction methods. ``Subsystem requirements'' refers to regulation of
individual components of the overall disposal system. Other comments
indicated that there was inadequate rationale to support NAS' concept
of negligible incremental risk (NIR). The NIR concept is based upon an
NCRP concept known as ``negligible incremental dose'' (NID, discussed
in more detail later in this notice) which was described by NAS ``as a
level of effective dose that can, for radiation protection purposes, be
dismissed from consideration'' (NAS Report pp. 59-60). Commenters also
stated that they did not support the NAS'' rejection of a collective-
dose standard. Comments were divided upon requiring quantitative or
qualitative assessment of human intrusion.
With regard to the three questions posed in the EnPA: (1) There
were mixed responses upon whether a standard to protect individuals
could adequately protect the general public; (2) there was nearly
unanimous agreement that active institutional controls cannot prevent a
breach of the repository; and (3) there was nearly unanimous agreement
that it is impossible to predict the probability of future human
intrusion into the repository.
Commenters also expressed views related to a number of other
issues. The majority favored:
(1) A standard expressed in terms of dose;
(2) The highest level of protection possible;
(3) Measuring compliance at the time of peak risk of the maximally
exposed individual;
(4) A reference biosphere to be specified by EPA;
(5) Including other local sources of man-made radiation in
determining an acceptable level of protection;
(6) Protection equal to that specified for WIPP, that is, that in
40 CFR part 191 (WIPP is a geologic disposal system in New Mexico for
defense-related transuranic waste but, unlike Yucca Mountain, WIPP is
subject to our generic radioactive-waste standards codified at 40 CFR
part 191; see also 61 FR 5224, February 9, 1996);
(7) Using a collective-dose limit to restrict exposure to the
general population while ignoring the NIR concept;
(8) Including assurance requirements; and
(9) Including ground water protection requirements.
We have taken into consideration all comments received during
preparation of these proposed standards. If you submitted comments in
response to the September 11, 1995, Federal Register notice or at the
September 1995 public hearings, you should submit additional comments
in response to today's notice to convey any concerns or views about
this proposal.
III. What Are We Proposing Today?
We are proposing, and requesting comment upon, public health and
safety standards governing the storage and disposal of SNF, HLW, and
other radioactive material in the repository at Yucca Mountain, Nevada.
We are also announcing a public comment period and public hearings to
gather comments upon the proposal.
As noted earlier, section 801(a)(1) of the EnPA gave us rulemaking
authority to set ``public health and safety standards for the
protection of the public from releases from radioactive materials
stored or disposed of in the repository at the Yucca Mountain site.''
The statute also directed us to develop standards ``based upon and
consistent with the findings and recommendations of the National
Academy of Sciences.'' Section 801(a)(2) of the EnPA directed us to
contract with NAS to conduct a study to provide findings and
recommendations on reasonable standards for protection of the public
health and safety. Because the EnPA called for us to act ``based upon
and consistent with'' the NAS findings, a major issue in this
rulemaking is whether we are bound to follow the NAS determinations
without exception or whether we have discretionary decision-making
authority.
As a practical matter, the difficulty of this issue is reduced
because some of the findings and recommendations in the NAS Report are
expressed in a non-binding manner. In other words, NAS stated its
findings and recommendations as starting points for the rulemaking
process or recognized those that involve public policy issues that are
more properly addressed in this public rulemaking proceeding. However,
the Report also contains some findings and recommendations stated in
relatively definite terms. It is these issues that most squarely
present the question of whether we are to treat the views of NAS as
binding.
Whether the EnPA binds us to following exactly the NAS findings and
recommendations is a question that warrants close attention at this
stage of the rulemaking because it affects the scope of our rulemaking.
If we are required to follow every view expressed in the NAS Report,
any such issue would be treated as addressed conclusively by NAS. We
would not need to entertain public comment upon the affected issues
since the outcome would be predetermined.
[[Page 46982]]
We believe that the EnPA does not bind us absolutely to follow the
NAS Report. Instead, we have used the NAS Report as the starting point
for this rulemaking. Today's proposal is based upon and consistent with
the findings and recommendations of NAS. We have developed this
proposal guided by the findings and recommendations of NAS because of
the special role given NAS by Congress and the scientific expertise of
NAS. However, the entirety of our proposed standards for the Yucca
Mountain disposal system is the subject of this rulemaking. We do not
intend to treat the views expressed by NAS as necessarily dictating the
outcome of this rulemaking, thereby foreclosing public scrutiny of
important issues. For the reasons described below, we believe this
proposed interpretation of the EnPA is consistent with the statute and
prudent in that it avoids potential Constitutional issues. Further,
this proposed interpretation supports an important EPA policy
objective--ensuring an opportunity for public input upon all aspects of
the issues presented in this rulemaking.
Section 801(a)(2) of the EnPA required a study by NAS that provides
``findings and recommendations on reasonable standards for protection
of the public health and safety.'' While this section of the EnPA calls
for NAS to address three specific issues, Congress did not place any
restrictions upon other issues NAS could address. The report of the
Congressional conferees underscored that ``the National Academy of
Sciences would not be precluded from addressing additional questions or
issues related to the appropriate standards for radiation protection at
Yucca Mountain beyond those that are specified.'' (H.R. Rep. No. 1018,
102nd Cong., 2d Sess. 391 (1992)). Thus, given the potentially
unlimited scope of the NAS inquiry under the statute, NAS could have
provided findings and recommendations that would dictate literally all
aspects of the public health and safety standards for Yucca Mountain,
rendering our function a ministerial one.
Section 801(a)(1) of the EnPA plainly gave EPA the authority to
issue, by rulemaking, public health and safety standards for Yucca
Mountain. If at the same time that Congress gave NAS the authority to
provide findings and recommendations on any issues related to the Yucca
Mountain public health and safety standards, Congress also intended
that NAS' findings and recommendations be binding upon us, then
Congress would have effectively delegated to NAS a standard-setting
authority that overrides our delegated rulemaking authority. Carried to
its logical conclusion, under this view of the statute, NAS would have
authority to establish the public health and safety standards, and to
do so without a public rulemaking process. Then the direction for EPA
to set standards ``by rule'' would be unnecessary or relatively
meaningless. This tension in the statute can be reasonably resolved by
interpreting the NAS' findings and recommendations as non-binding, but
highly influential, expert guidance to inform our rulemaking.
Thus, we do not believe the statute forces our rulemaking to adopt
mechanically the NAS' recommendations as standards. If it did, the
statutory provisions would allow us to consider only those issues that
NAS did not address. Further, the provisions calling for us to use
standard rulemaking procedures in issuing the standards would be
unnecessary to reach results that NAS already established.
The report of the conferees also indicates that Congress did not
intend to limit our rulemaking discretion. The Conference Report
provides that Congress intended NAS to provide ``expert scientific
guidance'' on the issues involved in our rulemaking and that Congress
did not intend for NAS to establish the specific standards:
The Conferees do not intend for the National Academy of
Sciences, in making its recommendations, to establish specific
standards for protection of the public but rather to provide expert
scientific guidance on the issues involved in establishing those
standards. Under the provisions of section 801, the authority and
responsibility to establish the standards, pursuant to rulemaking,
would remain with the Administrator, as is the case under existing
law. The provisions of section 801 are not intended to limit the
Administrator's discretion in the exercise of his authority related
to public health and safety issues. (H.R. Rep. No. 1018 at p. 391)
Our proposed interpretation of the EnPA as not limiting the issues
for consideration in this rulemaking is consistent with the views we
expressed to Congress during deliberations over the legislation. The
Chairman of the Senate Subcommittee on Nuclear Regulation requested our
views of the bill reported out of conference. The Deputy Administrator
of EPA indicated that the NAS Report would provide helpful input.
Moreover, EPA's Deputy Administrator pointed to the language, cited
above, stating the intent of the conferees not to limit our rulemaking
discretion and assured Congress that any standards for radioactive
materials that we ultimately issue would be the subject of public
comment and involvement and would fully protect human health and the
environment. (138 Cong. Rec. S33,955 (daily ed. October 8, 1992)).
Our proposed interpretation also is consistent with the role that
both NAS and Congress understood NAS would fulfill. During the
Congressional deliberations over the legislation, NAS informed Congress
that while it would conduct the study, it would not assume a standard-
setting role because that is properly the responsibility of government
officials. (138 Cong. Rec. S33,953 (October 8, 1992)).
Our proposed interpretation of the NAS Report also avoids
implicating potentially significant Constitutional issues. Construing
the EnPA as delegating to NAS the responsibility to determine the
health and safety standards at Yucca Mountain may violate the
Appointments Clause of the Constitution (Art. II, sec. 2, cl. 2), which
imposes restrictions against giving Federal governmental authority to
persons not appointed in compliance with that Clause. In addition, the
Constitution places restrictions arising under the separation of powers
doctrine upon the delegation of governmental authority to persons not
part of the Federal government. We are not concluding, at this time,
that an alternative interpretation would necessarily run afoul of
Constitutional limits. However, we believe it is reasonable both to
assume that Congress intended to avoid these issues when it adopted
section 801 of the EnPA and to interpret the EnPA accordingly.
In summary, we do not believe we must, in this rulemaking, adopt
all of the positions advanced by NAS. At the same time, the statute
does give NAS a special role. As noted, the NAS' findings and
recommendations have been the starting point for this rulemaking and
our proposal is consonant with those findings and recommendations. In
fact, the NAS Report influenced us heavily during the development of
this proposed rule. We have included many of the findings and
recommendations in whole in today's proposal, and we intend to continue
to weigh the NAS Report heavily throughout the course of this
rulemaking. We will tend to give greatest weight to the judgments of
NAS about issues having a strong scientific component, the area where
NAS has its greatest expertise. In addition, we will reach final
determinations that are congruent with the NAS analysis whenever we can
do so without departing from the Congressional delegation of authority
to us to
[[Page 46983]]
promulgate, by rule, public health and safety standards for protection
of the public, which we believe requires the consideration of public
comment and our own expertise and discretion.
We request public comment upon how we should view and weigh the
NAS' findings and recommendations in this rulemaking. Public commenters
should also address this issue in the context of the specific issues
presented in this rulemaking. Commenters should indicate whether we
have given proper consideration to the NAS' findings and
recommendations, whether we should give them more or less weight, and
what the resulting outcome should be.
The following sections describe our proposed public health and
safety standards for Yucca Mountain and the considerations which
underlie the set of standards we are proposing today. The next section
addresses the storage portion of the proposed standards. All of the
other sections pertain to the disposal portion of the standards.
III.A. What Is the Proposed Standard for Storage of the Waste?
(Proposed Subpart A)
Section 801(a)(1) of the EnPA calls for EPA's public health and
safety standards to apply to radioactive materials ``stored or disposed
of in the repository at the Yucca Mountain site.'' (The repository is
the mined portion of the facility constructed underground within the
Yucca Mountain site. Hereafter, the term ``repository'' refers to the
Yucca Mountain repository.) The EnPA differentiates between waste that
is ``stored'' and waste that is ``disposed,'' although it indicates
that we must issue standards that apply to both types of activity.
Congress was not clear regarding its intended use of the word
``stored'' in this context. Also, NAS did not address the issue of
storage (see proposed Secs. 197.2 and 197.12 for our proposed
definitions of ``storage'' and ``disposal''). The Yucca Mountain
repository currently is conceived to be a disposal facility, not a
storage facility, but that could change. Therefore, we propose to
interpret this language as directing us to develop standards that apply
to waste that DOE either stores or disposes of in the Yucca Mountain
repository. The public health and safety standards we issue under
section 801 of the EnPA would, therefore, apply to waste inside of the
repository, whether it is there for storage or disposal.
The Department will also handle and might store radioactive
material aboveground (that is, outside the repository). Those
activities are covered by our previously promulgated standards for
management and storage, codified at subpart A of 40 CFR part 191. The
40 CFR part 191 standards require that DOE manage and store SNF, HLW,
and transuranic radioactive wastes at a site, such as Yucca Mountain,
in a manner that provides a reasonable expectation that the annual dose
equivalent to any member of the public in the general environment will
not exceed 25 millirem (mrem) to the whole body. This is the standard
which DOE must meet for WIPP and the greater confinement disposal (GCD)
facility. (The GCD facility is a group of 120-feet deep boreholes
located within the Nevada Test Site (NTS) which contains disposed
transuranic wastes.)
The storage standards in 40 CFR 191.03(a) are stated in terms of an
older dose-calculation method and are set at an annual whole-body-dose
limit of 25 mrem/yr. The proposed storage standards for Yucca Mountain
use a modern dose-calculation method known as ``committed effective
dose equivalent'' (CEDE).8 Even though today's proposal uses
the modern method of dose calculation, we believe that the proposed
dose level essentially maintains a similar risk level as in 40 CFR
191.03(a) at the time of its promulgation (see the discussion of the
different dose-calculation methods in the What Should the Level of
Protection Be? section later in this notice). The difference between
these dose calculation procedures presents a problem in combining the
doses for regulatory purposes. However, we have begun a rulemaking to
amend both 40 CFR Parts 190 and 191. That rulemaking would update these
limits to the CEDE methodology. We anticipate that we will finalize the
amendments to parts 190 and 191 prior to the finalization of this
rulemaking. If that does not occur, we would need to address the
calculation of doses under the two methods in another fashion. For
example, we could require that the doses occurring as a result of
activities outside the repository be converted into annual CEDE for
purposes of determining compliance with the storage standard. We
request comments upon such an approach.
---------------------------------------------------------------------------
\8\ The term ``committed effective dose'' in this rulemaking has
the same meaning as the term ``committed effective dose equivalent''
which was used prior to the publication of ICRP Publication No. 60.
It is used here since the term is less complicated and more compact.
Also, the use of ``committed effective dose'' is consistent with
subpart B of 40 CFR part 191 (58 FR 66398, 66402, December 20,
1993).
---------------------------------------------------------------------------
Section 801 of the EnPA specifically provides that the standards
that we issue shall be the only ``such standards'' that apply at Yucca
Mountain. Thus, the statute provides that the EnPA is the exclusive
authority for ``such standards'' and, in turn, replaces our generally
applicable standards for radiation protection to the extent that
section 801 requires site-specific standards. Otherwise, our generic
standards are not affected. As noted, we propose to interpret the scope
of section 801 as applying to both storage and disposal of waste in the
repository. Thus, waste inside the repository would be subject to the
standards proposed in today's notice. Our generic standards in subpart
A of 40 CFR part 191 will apply to waste outside of the repository.
Using this interpretation, we have considered the differences
between the conditions covered by the storage standards in 40 CFR
191.03(a) and the conditions which could affect storage in the Yucca
Mountain repository. The most significant difference is that the
storage in Yucca Mountain would be underground whereas most storage
covered under 40 CFR part 191 is aboveground. Otherwise, the technical
situations we anticipate under both the existing generic standards and
the proposed Yucca Mountain standards are essentially the same. Also,
one of our goals in issuing 40 CFR parts 190 and 191 was to bring the
entire uranium fuel cycle under consistent EPA standards. Therefore, we
are proposing that the part 197 standards continue the coverage of the
uranium fuel cycle because SNF, a large part of the waste planned for
emplacement in Yucca Mountain, is part of that fuel cycle. Therefore,
we are proposing to extend a similar level of protection as in the 1985
version of subpart A of 40 CFR part 191. In other words, under the part
197 storage standards, exposures of members of the public from waste
storage inside the repository would be combined with exposures
occurring as a result of storage outside the repository but within the
Yucca Mountain site. The total dose could be no greater than 150
microsieverts (Sv) (15 mrem) CEDE per year (CEDE/yr).
Our application of subpart A of 40 CFR part 191 to storage
activities outside of the repository at the Yucca Mountain site is
supported by the WIPP LWA. Section 8 of the WIPP LWA excludes Yucca
Mountain from our generic disposal standards but not from the generic
management and storage standards found in subpart A of 40 CFR part 191.
If we finalize the proposed interpretation of section 801 of the EnPA
as applying to radioactive material stored or disposed of in the
repository, we would apply subpart A of 40 CFR part 191 to the storage
activities outside of the repository at the site without further public
notice.
[[Page 46984]]
We request comment upon our proposed interpretation that section
801 of the EnPA directs us to develop new standards that apply only to
radioactive materials stored in the repository. We also request public
comment upon whether we should instead construe section 801 of the EnPA
as providing for the establishment of new storage standards, rather
than applying the existing storage standards in 40 CFR part 191 to
storage, or handling, of radioactive materials at the Yucca Mountain
site prior to their movement into the repository. If we decide, based
upon the alternative interpretation of section 801, to promulgate new
storage standards for the site, we anticipate that we would adopt
standards essentially the same as those in 40 CFR 191.03(a). Thus, we
request public comment upon whether we should develop and adopt in this
rulemaking, under section 801 of the EnPA, new standards for management
and storage activities at the site, and request comments upon the
adoption of such standards based upon those in 40 CFR 191.03(a).
III.B. What Is the Standard for Protection of Individuals? (Proposed
Secs. 197.20 and 197.25)
III.B.1. Should the Limit Be on Dose or Risk?
Although a standard for limiting exposure of people to radiation
can take many forms, NAS narrowed its final considerations to risk and
dose, that is, a risk-based or dose--based standard. The numeric level
of the proposed standard for protecting individual members of the
public from radioactive materials disposed of in the Yucca Mountain
disposal system is addressed in the What Should the Level of Protection
Be? section later in this notice. The discussion here explains why we
selected a dose-based standard rather than a risk-based standard, as
recommended by NAS.
Two forms of radiation exposure can occur depending upon the
location of the source relative to the body `` internal and external.
Internal exposures occur when a person inhales or ingests contaminated
air, food, water, or soil. External exposures occur because a person is
near a radionuclide which is emitting X-rays, gamma rays, beta
particles, or neutrons. ``Dose'' is a measure of the amount of
radiation received by individuals resulting from exposure to
radionuclides. ``Risk'' is the probability of an individual incurring
an adverse health effect from exposure to radiation. The NAS defined
``risk'' as the product of two parameters: (1) the probability of an
individual receiving a dose, and (2) the probability of incurring a
health effect because of that dose (NAS Report p. 42). This rulemaking
takes both of these factors into account. (The probability of an
individual receiving a dose is part of the performance assessment and
is discussed in the What Are the Requirements for Performance
Assessments and Determinations of Compliance? section later in this
notice.) As mentioned in the previous section, these standards state
radiation risk estimates as the probability of an individual developing
a fatal cancer, since fatal cancers are the greatest harm to
individuals from low-dose-rate radiation (NAS pp. 37-39).
Section 801(a)(1) of the EnPA directed that our standards for Yucca
Mountain ``shall prescribe the maximum annual effective dose equivalent
to individual members of the public from releases to the accessible
environment from radioactive materials stored or disposed of in the
repository....'' At the same time, the EnPA calls for us to issue our
standards ``based upon and consistent with'' the findings and
recommendations of NAS. The NAS recommended that we adopt a standard
expressed as risk rather than the dose standard that Congress
prescribed. The NAS offered two reasons for its recommendation. First,
a risk-based standard is advantageous relative to a dose-based standard
because it ``would not have to be revised in subsequent rulemakings if
advances in scientific knowledge reveal that the dose-response
relationship is different from that envisaged today'' (NAS Report p.
64). Second, a standard in the form of risk more readily enables the
public to comprehend and compare the standard with human-health risks
from other sources.
We have reviewed and evaluated the merits of a risk-based standard
as recommended by NAS. However, we are proposing a dose-based standard
for the following reasons. First, both national and international
radiation protection guidelines developed by bodies of non-governmental
radiation experts, such as ICRP and NCRP, generally have recommended
that radiation standards be established in terms of dose. Also,
national and international radiation standards, including the
individual-protection requirements in 40 CFR part 191, are established
almost solely in terms of dose or concentration, not risk. Therefore, a
risk-based standard will not allow a convenient comparison with the
numerous existing radiation guidelines and standards that are stated in
terms of dose.
Second, we have an established methodology for calculating dose
that is described in Federal Guidance Reports Nos. 11 and 12 (Federal
Guidance). The development of this methodology was a combined effort of
many Federal agencies involved in radiation protection and has become
Federal policy. The guidance provides a consistent methodology for
calculating doses for regulatory purposes. By contrast, there is
currently no Federal Guidance Report, in final form, for calculating
risk from radiation exposure.
Third, we have based the proposed dose-based standard upon the risk
of developing a fatal cancer as a result of that level of exposure
based upon a linear, non-threshold, dose-response relationship. We
would establish a risk-based standard in the same manner. Thus, a risk-
based standard, like a dose-based standard, depends upon current
knowledge and assumptions about the chance of developing fatal cancer
from a particular exposure level. Dose and risk are closely related;
one can be converted to the other simply by using the appropriate
factor. Therefore, both dose- and risk-based standards are based upon
scientific assumptions that could change and no matter how it is
expressed, the standard is based upon risk.
Finally, section 801(a)(1) of the EnPA specifically calls for a
dose-based standard. Most commenters supported this by asking for a
dose-based standard rather than a risk-based standard.
Accordingly, we are proposing a standard expressed as a limit on
dose. We are requesting comments upon the proposed form of the
standard, including whether the standard should be expressed as risk.
III.B.2. What Should the Level of Protection Be?
As noted previously, section 801(a)(1) of the EnPA calls for our
Yucca Mountain standards to ``prescribe the maximum annual effective
dose equivalent to individual members of the public from releases of
radioactive materials.'' Development of the individual-protection
standard requires us to evaluate and specify several factors. These
factors include the level of protection, who the standards should
protect, and how long the standards should provide protection.
Determining the appropriate dose level is ultimately a question of both
science and public policy. The NAS stated in its Report: ``The level of
protection established by a standard is a statement of the level of the
risk that is acceptable to society. Whether posed as ``How safe is safe
enough?'' or as ``What is an acceptable
[[Page 46985]]
level?'', the question is not solvable by science'' (NAS Report p. 49).
We seek to find answers to these questions for the Yucca Mountain
disposal system through this rulemaking.
We considered the NAS findings and recommendations in our
determination of the CEDE level that would be adequately protective of
human health. We also reviewed established EPA standards and guidance,
other Federal agencies' actions for both radiation and non-radiation-
related actions, and other countries' regulations. In addition, we
evaluated guidance on dose limits provided by National and
international, non-governmental, advisory groups of radiation experts.
The NAS recommended a range of risk levels that we could use as a
reasonable starting point in this rulemaking (NAS Report p. 5). The
range of annual risk of fatal cancer suggested by NAS was 1 chance in
100,000 (1 x 10-\5\) to 1 chance in 1,000,000 (1 x
10-\6\) (this corresponds to a range of 20 to 2 mrem CEDE/
yr). The NAS based its recommendation upon its review and evaluation of
our actions, other Federal actions, guidelines developed by National
and international groups, and regulations of other countries. For these
standards, we are proposing a limit of 150 Sv (15 mrem) CEDE/
yr. This limit corresponds approximately to an annual risk of 7 chances
in 1,000,000 (7 x 10-\6\)--within the range that NAS
recommended as a starting point for consideration.
Table 1 below lists the dose limits of other current EPA and NRC
regulations (adapted from NAS Report p. 50). Today's proposed standard
of 150 Sv (15 mrem) CEDE/yr is within the range of these
established standards. Further, it is consistent with the individual-
protection standard at 40 CFR 191.15 in our generic disposal standards
which limits the annual CEDE to 150 Sv (15 mrem)/yr.
Table 1.--Current EPA and NRC Dose Limits on Various Environmental
Concerns
------------------------------------------------------------------------
Environmental concern Limit*
------------------------------------------------------------------------
Low-Level Waste (10 CFR part 61)....... 250 Sv (25 mrem)/yr
License Termination (10 CFR part 20)... 25 mrem TEDE**/yr
Uranium Fuel Cycle (40 CFR part 190)... 25 mrem/yr
Generic Standard for Management and 25 mrem/yr
Storage of SNF and HLW (40 CFR 191.03).
Generic Individual-Dose Standard for 150 Sv (15 mrem) CEDE/
Disposal of SNF and HLW (40 CFR yr
191.15).
National Emission Standards for 10 mrem CEDE/yr
Hazardous Air Pollutants (40 CFR part
61, subparts H and I).
SNF and HLW Disposal Limit for 4 mrem/yr for man-made beta-
Underground Sources of Drinking Water and photon-emitting
(40 CFR 191.24). radionuclides
------------------------------------------------------------------------
*Unless otherwise noted, only whole-body dose limits are listed; there
may also be other requirements for any particular environmental
concern. The 25-mrem/yr, whole-body-dose limit established in 1985 is
essentially equivalent to the risk associated with today's dose rate
of 150 Sv (15 mrem) CEDE/yr (58 FR 66402, December 20, 1993).
**TEDE (total effective dose equivalent) is NRC's term for CEDE. This
regulation was not included in the NAS Report.
We note that, except for 40 CFR 191.15, 40 CFR part 61, and 10 CFR
part 20, the dose limits in Table 1 are stated in terms of an old dose
system. For example, the annual limits in 40 CFR 191.03(a) are 25 mrem
for the whole body, 75 mrem for the thyroid, or 25 mrem for any other
organ (only the whole-body limit is listed in Table 1). We established
these dose levels in 1985 (50 FR 38085, September 19, 1985) under a
different system for calculating doses than the more recent rulemakings
that use the CEDE concept. We estimate that the 25-mrem/yr, whole-body-
dose limit established in 1985 is essentially equivalent to the risk
associated with today's proposed limit of 150 Sv (15 mrem)
CEDE/yr (58 FR 66398, 66402, December 20, 1993).
In addition, the proposed 150-Sv (15 mrem)-CEDE/yr limit
in today's proposal is consistent with other current standards. For
example, our limits on radiation exposure through the air is part of
the set of limits for pollutant releases known as the National Emission
Standards for Hazardous Air Pollutants (NESHAPs, 40 CFR part 61). Since
our NESHAPs limit of 10 mrem/yr covers radionuclide releases into only
the air, the 150 Sv (15 mrem) CEDE/yr standard being proposed
for 40 CFR part 197 is consistent with the NESHAPs limit because it
applies to all potential pathways, that is, the dose limit is higher
but includes other pathways in the analysis.
In summary, based upon our review of the guidance, regulations, and
standards cited above, and the NAS Report, we are proposing a standard
of 150 Sv (15 mrem) CEDE/yr for the Yucca Mountain disposal
system. We request comment upon the reasonableness of this level of
protection.
III.B.3. What Factors Can Lead to Radiation Exposure?
Protection of the public from exposure to radioactive pollutants
requires knowledge and understanding of three factors: the source of
the radiation, the pathways leading to exposure, and the recipients of
the radiation. This section provides a discussion of the source of
radiation and pathways of exposure. The following two sections discuss
the recipients of the dose. The development of standards to protect
public health and safety from radionuclides released from waste
disposed of in the Yucca Mountain disposal system must include
consideration of the sources of radiation and pathways which could lead
to exposure of humans. The mechanisms of exposure are the basis of an
analysis called the performance assessment. The performance assessment
is the quantitative analysis of the projected behavior of the disposal
system.
Source. The waste disposed of in Yucca Mountain will contain many
different radionuclides including unconsumed uranium, fission products
(for example, cesium-137 and strontium-90), and transuranic elements
(for example, plutonium and americium).
The inventory of radionuclides over time will depend upon the type
and amount of radionuclides originally disposed of in the disposal
system, the half-lives of the radionuclides, and the amount of any
radionuclides formed from the decay of parent radionuclides (see the
BID). In the time frame of tens-to hundreds-of-thousands of years, most
[[Page 46986]]
radionuclides initially present in SNF and HLW will decay to
essentially no radioactivity. Therefore, the waste will eventually have
radiologic characteristics similar to a large uranium ore body (see the
BID).
To delay the movement of radionuclides into the biosphere, DOE
plans to use multiple barriers. These barriers would be man-made
(engineered) and natural based upon the design of, and conditions in
and around, the disposal system.
Engineered barriers must be designed to delay release of
radionuclides from the repository. For example, an engineered barrier
could be the waste form. The Department plans to convert liquid HLW
derived from reprocessing of SNF into a solid by entraining the
radionuclides into a matrix of borosilicate glass; NRC will likely
consider this an engineered barrier. The molten glass then would be
poured into and hardened in a second man-made barrier, a metal
container (see the BID). In addition, it is possible to have other man-
made barriers in the repository to serve as part of the disposal system
(see the BID).
Natural barriers at Yucca Mountain also could slow the movement of
radionuclides into the accessible environment. For instance, the
Department plans to construct the repository in a layer of tuff located
above the water table. The relative dryness of the tuff around the
repository would limit the amount of water which comes into contact
with the waste. It also would retard the future movement of
radionuclides from the waste into the underlying aquifer. Any
radioactive material that dissolved into infiltrating water,
originating as surface precipitation, still would have to be moved to
the saturated zone. Minerals, such as zeolites, contained within the
tuff beneath the repository could act as molecular filters and ion-
exchange agents for some of the released radionuclides, thereby slowing
their movement. Such minerals also could limit the amount of water that
contacts the waste and could help retard the movement of radionuclides
from the waste to the water table. This mechanism would be most
effective if flow was predominantly through the pores in the rock, also
known as the matrix (see the BID).
Pathways. Once radionuclides have left the waste packages, they
could be carried by water or air and reach the public. Upon release
from the waste packages, most radionuclides will be carried by ground
water away from the repository. However, those in a gaseous form, such
as carbon-14 (\14\C) in the form of carbon dioxide, will be carried by
air moving through the mountain.
Movement via water. Radionuclides will not be moved into the water
table instantaneously. The length of time it takes depends partly upon
how much the water moves via fractures or through the matrix of the
rock. Once radionuclides reach the saturated zone, they would move away
from the disposal system in the direction of ground water flow.
There are currently no perennial rivers or lakes adjacent to Yucca
Mountain to further transport contaminants. Therefore, based upon
current knowledge and conditions, ground water and its usage will
likely be the main pathway leading to exposure of humans. Current
knowledge suggests that the two major ways that people would use the
contaminated ground water are: (1) drinking and domestic uses; and (2)
agricultural uses (see the BID). In other words, radionuclides that
reach the public could deliver a dose if an individual: (1) Drinks
contaminated ground water or uses it directly for other household uses;
(2) drinks other liquids containing contaminated water; (3) eats food
products processed using contaminated water; (4) eats vegetables or
meat raised using contaminated water, or (5) is otherwise exposed as a
result of immersion in contaminated water or air or inhalation of wind-
driven particulates left following the evaporation of the water.
Movement via air. Some radionuclides could be carried by moving
air. The largest known source of potential movement by air in Yucca
Mountain is carbon dioxide containing \14\C. Airborne radionuclides
might move through the tuff overlying the repository and exit into the
atmosphere following release from the waste package. Once the
radioactive gas enters the atmosphere, it would disperse. This
dispersion would probably be global and, therefore, become greatly
diluted. The major pathway for exposure of people by \14\C is the
uptake of radioactive carbon dioxide by plants that humans subsequently
eat (see the BID).
III.B.4. Who Will Be Representative of the Exposed Population?
To determine whether the Yucca Mountain disposal system complies
with the standard, it will be necessary for DOE to calculate the dose
to some individual or group of individuals exposed to releases from the
repository and compare the calculated dose with the limit established
in the standard. The standard must specify, therefore, the individual
or group of individuals for whom the dose calculation is to be made.
The NAS definition of critical group. The NAS Report recommended
that we base the standards for protection of individuals upon risk
incurred by a critical group (CG). The CG would be the group of people
which, based upon cautious, but reasonable, assumptions, has the
highest risk of incurring health effects due to releases from the
disposal system. The ICRP introduced the concept of a CG in order to
account for the variation of dose which may occur in a population due
to differences in age, size, metabolism, habits, and environment. In
other words, the ICRP recommends the use of a group of people because
individuals might have personal traits which make them much more or
less vulnerable to releases of radiation than the average within a
small group of the most highly exposed individuals. The ICRP defines
the CG as a relatively homogeneous group of people whose location and
habits are such that they represent those individuals expected to
receive the highest doses as a result of the discharge of
radionuclides. The NAS adapted the CG concept to a risk framework for
the development of an individual-risk standard and recommended the
following description of the CG (NAS Report p. 53):
The critical group for risk should be representative of those
individuals in the population who, based on cautious, but
reasonable, assumptions, have the highest risk resulting from
repository releases. The group should be small enough to be
relatively homogeneous with respect to diet and other aspects of
behavior that affect risks. The critical group includes the
individuals at maximum risk and is homogeneous with respect to risk.
A group can be considered homogeneous if the distribution of
individual risk within the group lies within a total range of a
factor of ten and the ratio of the mean of individual risks in the
group to the standard is less than or equal to one-tenth. If the
ratio of the mean group risk to the standard is greater than or
equal to one, the range of risk within the group must be within a
factor of 3 for the group to be considered homogeneous. For groups
with ratios of mean group risk to the standard between one-tenth and
one, homogeneity requires a range of risk interpolated between these
limits.
The NAS also recommended that the CG risk calculated for purposes
of comparison with the risk limit established in the standard is the
average of the risks of all the members in the group. Using the average
risk avoids the problem of the outcome being unduly influenced by
unusual habits of individuals within the group.
The NAS indicated that in order to select a CG, the person or
persons likely
[[Page 46987]]
to be at highest risk from among the larger, exposed population must be
specified. To accomplish this, one must make assumptions about the
nature of human activities, lifestyles, and pathways that affect the
level of exposure. The set of circumstances that affects the dose
received, such as where people live, what they eat and drink, and other
lifestyle characteristics, is a very important part of the exposure
scenario. Many human behavior factors important to assessing repository
performance vary over periods that are short in comparison with the
compliance period proposed for these standards. The past several
centuries have seen radical changes in human technology and behavior,
many of which were not reasonably predictable. Given this potential for
rapid change, we believe that it is not possible to know what patterns
of human activity and changes in human biology might occur thousands of
years from now. For the purpose of compliance with the standard,
therefore, we are proposing that it is appropriate to use many of the
current characteristics of members of the public in the vicinity of
Yucca Mountain in the compliance assessments required by these
standards (see the What Should Be Assumed About the Future Biosphere?
section later in this notice).
The NAS Report presented two illustrative approaches for
formulating an exposure scenario for determining compliance. The NAS
also clearly stated that there might be other methods to reach the same
objective (NAS Report p. 100). One approach, described in Appendix C of
the NAS Report, A Probabilistic Critical Group, used statistical
methods and probabilities to characterize a CG. The second, The
Subsistence-Farmer Critical Group, described in Appendix D, identified
a subsistence farmer as a principal representative of the CG.
The NAS probabilistic critical group. Appendix C of the NAS Report
described a ``probabilistic critical group.'' This section describes
the contents of Appendix C of the NAS Report.
The NAS probabilistic CG approach would require use of a
theoretical population distribution which we would, or require DOE to,
develop by using a mathematical method known as ``Monte Carlo.'' The
Monte Carlo method is a mechanism to randomly select values of
parameters which have a range of possible values. The parameters would
be present-day environmental parameters, including soil quality, land
slope, growing season, depth to the aquifer, and population
distribution and lifestyles. The individuals who comprise the CG may
represent a variety of economic lifestyles and activities. The analysis
would then use the variability of those parameters in the region around
Yucca Mountain to arrive at the theoretical population for the
calculation of radiation exposure. This theoretical population would
then, according to NAS, be combined with Monte Carlo simulations of the
distribution of contaminated ground water in time and space (NAS Report
p. 148). According to NAS, each simulation would generate a plume path
which could be overlain on a map of potential farm density or water use
to determine a potential exposure area. Each of these potential plume
paths is known as a ``realization.'' Values for parameters, including
well depths, rates of water use, food sources, and consumption rates,
are determined by sampling from the parameter-value distributions. For
each plume realization of the contamination in the aquifer, the results
of the exposure simulations are combined to give a spatial distribution
of maximum exposures for the locations likely to be inhabited. This
approach would use a large number of simulations of plume realizations
to identify critical subgroups with the highest risk. It would then be
used to calculate the arithmetic average of the risk of all critical
subgroups over all plume realizations to estimate the risk for the CG.
In determining compliance, the Commission would compare this estimate
with the risk limit in the standard.
We considered proposing the probabilistic CG approach but are not
doing so for the following reasons. First, there is no relevant
experience in applying the probabilistic CG approach. Second, the
approach is very complex and difficult to implement in a manner that
assures it would meet the requirements of defining a CG. Third, we are
concerned that this approach does not appear to identify clearly who is
being protected. Finally, a significant majority of the comments that
we have received upon the NAS Report opposes the probabilistic CG
approach.
The NAS subsistence-farmer critical group. The approach in Appendix
D of the NAS Report specified one or more subsistence farmers as the
CG. It made assumptions designed to define the farmer at maximum risk
to be included in the CG. This section describes the contents of
Appendix D of the NAS Report.
The subsistence-farmer CG is a definable, highly exposed segment of
the larger, exposed population. The subsistence farmer would be assumed
to: (1) be a person with eating habits and response to doses of
radiation that would be average for present-day people and (2) obtain
all potable water and grow all of his or her own food using water
withdrawn from the aquifer contaminated with radionuclides from the
disposal system. The water used by this CG would be withdrawn at a
location downgradient from and outside the footprint of the repository
at the point of maximum potential concentration of ground water
contamination, provided that no natural geologic features preclude
drilling for water at that location. (The footprint of the repository
is the circumscription of the outermost, original emplacement locations
of the waste.)
Concentrations of radionuclides in the extracted ground water may
be smaller than in undisturbed ground water due to pumping; this
possibility could be used when evaluating exposures (NAS Report p.
155). As a result of uncertainty, there will be probabilistic
distributions of radionuclide concentrations, as they vary in time and
space in the aquifer outside the repository footprint, which are the
input variables needed to estimate the risk. The radionuclide
distributions in the aquifers, in turn, depend upon the performance of
the components of the natural and engineered barrier systems.
Projections of their performance also contain uncertainty and likely
will be subject to probabilistic assessment. Any assessment of the
potential doses from the repository, therefore, must consider the
probability of processes and events that influence eventual
concentrations of radionuclides in aquifers supplying water to the CG.
Overall, the ``expected'' risk for the average member of this CG
would be about one-half that of the most-exposed subsistence farmer
(NAS Report p. 158). This average risk to the members of the CG would
be compared with the standard selected for compliance.
We considered proposing that the protected individual(s) be the
subsistence-farmer CG. The CG concept has been utilized within the U.S.
in various ways. The NRC uses the CG concept in assessing compliance
with NRC standards for radionuclide releases from nuclear facilities.
For example, the Commission uses the CG concept in: (1) licensing
actions involving dose calculations under 10 CFR part 40, appendix A;
(2) its radiological criteria for license termination of all NRC-
licensed facilities at 10 CFR part 20, subpart E; and (3) its draft
guidance for LLW disposal under 10 CFR part 61. The State of Washington
recently
[[Page 46988]]
implemented the CG concept in actions relating to U.S. Ecology's LLW
site at Hanford, and the State of Texas endorses CG in its
decommissioning standards. Also, a great deal of international guidance
exists that discusses the use of CG. The ICRP endorses CG, and has
recommended the CG concept in numerous documents, both recent and
dating back as far as 1977. Canada, Sweden, Switzerland, and the United
Kingdom are among those individual nations that have adopted the CG
methodology for radioactive waste storage and disposal.
We prefer an approach to exposure assessment that is consistent
with other Agency programs (Guidance on Risk Characterization for Risk
Managers and Risk Assessors, Deputy Administrator F. Henry Habicht II,
February 26, 1992) and which we believe provides a level of protection
substantially equivalent to that which would be achieved by the CG
concept.
Our proposal for the protection of individuals. Most of our
programs use an approach for the development of exposure scenarios that
involves determining the high-end range of doses or exposures.
Conceptually, this range is that above the 90th percentile of the
entire (either measured or estimated) distribution of potential doses
within the exposed population. Conversely, the NESHAPs program for
radionuclides and the individual-protection requirements in the generic
SNF and HLW disposal standards at 40 CFR 191.15 require calculation of
the individual dose for a person assumed to reside at a location where
that person would receive the highest dose. However, other Agency
programs use a different approach to protect individuals by using
``reasonable, maximum exposure'' (RME) conditions. The National
Contingency Plan describes an approach to be used for the RME scenario
to protect individuals as ``a product of factors, such as concentration
and exposure frequency and duration, that are an appropriate mix of
values that reflect averages and 95th percentile distributions'' (55 FR
8666, 8710, March 8, 1990). In the past, we have defined ``reasonable
maximum'' to mean potential exposures that are likely to occur. The
method for calculating the RME is to estimate the high-end range of
possible exposures by identifying the factors which have the greatest
effect upon the size of the dose, and using maximum or nearly maximum
values for one or a few of these factors, leaving the others at their
average values (57 FR 22888, 22922, May 29, 1992). In this approach, we
select a hypothetical individual who would be representative of the
most highly exposed individuals. We call this individual the reasonably
maximally exposed individual (RMEI). To be effective, the RMEI approach
must avoid incompatible combinations of parameter values, such as, low
body weight used in combination with high intakes.
Thus, we intend for this procedure to project doses that are within
a reasonably expected range rather than projecting the most extreme
case. However, the procedure is also meant to identify an individual
dose which is well above the average dose in the exposed population.
The ultimate goal and purpose is to estimate a level of exposure that
is protective of the vast majority of individuals at a site, but is
still within a reasonable range of potential exposures.
For the preceding reasons, we are proposing the RMEI concept as our
preferred approach instead of the CG approach. The United States and
other countries have used the concept of a hypothetical individual to
represent future populations in radioactive-waste management programs.
This is consistent with widespread practice, current and historical, of
estimating dose and risk to highly exposed individuals even when the
exposure habits of future people cannot be specified or accurately
calculated, as in this case where doses must be projected for very long
periods. The approach is straightforward and relatively simple to
understand. We believe that this approach provides protection similar
to that afforded by the NAS recommendation to use a CG. The RMEI model
uses a series of assumptions about the lifestyle of a hypothetical
individual. The desired degree of conservatism can be built into the
model through choices of assumed values of RME parameters. However,
these values would be within certain limits since we are proposing to
require the use of Yucca Mountain-specific characteristics in choosing
those parameters and their values. In subpart B of 40 CFR part 197, we
propose a framework of assumptions for NRC to incorporate into its
implementing regulations.
Our proposed RMEI would be representative of a future population
group termed ``rural-residential.'' The CEDE received by this RMEI
would be calculated by DOE using cautious, but reasonable, exposure
parameters and parameter-value ranges. The projected CEDE would be used
by NRC in the determination of compliance with the proposed standards.
We believe that the results obtained by using this approach would be
similar to those which would be obtained by using the subsistence-
farmer CG approach put forth in Appendix D of the NAS Report. In both
cases, the objective is to determine the magnitude of the potential
exposure using reasonable, not extreme, assumptions. Under the proposed
standards, the RMEI will have food and water intake rates, diet, and
physiology like that of individuals currently living in the
downgradient direction of flow of the ground water passing under Yucca
Mountain. The Department will perform the dose calculation to estimate
exposure resulting from releases from the waste into the accessible
environment based upon the assumption of present-day conditions in the
vicinity of Yucca Mountain. Presently, we expect the ground water
pathway to be the most significant pathway for exposure from
radionuclides that are transported from the repository. Our initial
evaluation of potential exposure pathways from the disposal system to
the RMEI suggests that the dominant fraction of the dose incurred by
the RMEI likely will be from ingestion of food irrigated with
contaminated water (see the BID). It is possible, however, that another
exposure pathway will be determined by DOE and NRC to be more
significant for radiation exposure. Consequently, DOE and NRC must
consider and evaluate all potentially significant exposure pathways in
the performance assessment. As a result of the performance assessment,
there will be a distribution of the highest potential doses incurred by
the RMEI. We are proposing that the mean or median value (whichever is
higher) of that distribution be used by NRC to determine compliance
with the individual-protection standard. We request comments upon this
method of determining compliance with the individual-protection
standard.
We are also requesting comments upon the alternative of adopting
the CG approach rather than the RMEI. Comments supporting the CG
approach should address the level of detail EPA's rule should include
on the parameters of the CG.
Exposure scenario for the RMEI. A major part of the exposure
scenario is the location of the RMEI. In preparing to propose a
location for the RMEI, we collected and evaluated information on the
natural geologic and hydrologic features, such as topography, geologic
structure, aquifer depth, aquifer quality, and the quantity of ground
water, that may preclude drilling for water at a specific location.
Based upon these factors and the current understanding of ground water
flow in the area of Yucca
[[Page 46989]]
Mountain, it appears that an individual could reside anywhere along the
projected radionuclide flow path extending from Forty-Mile Wash,
approximately five kilometers (km) from the proposed repository
location, to the southwestern part of the Town of Amargosa Valley,
Nevada, where the ground water is close to the land surface and where
most of the farming in the area is done. However, an individual's
ability to reside at any particular point along that path depends upon
that individual's purpose and available resources. To explore these
variations, we developed the four scenarios described below. We present
our evaluation of factors associated with these scenarios more fully in
the BID. We welcome comment upon the appropriateness of each of these
scenarios and upon our preferred scenario. In developing scenarios, we
assumed that the level of technology and economic considerations
affecting population distributions and life styles in the future are
the same as today (for more detail, see the What Should Be Assumed
about the Future Biosphere? section below).
The RMEI in the first scenario is a subsistence (low technology)
farmer. Such an individual would have continuous exposure to
radionuclides in water, air, and soil which are arriving through all
exposure pathways. The RMEI's location and habits would be generally
consistent with historical locations of Native Americans and early
settlements in Amargosa Valley and influenced heavily by easy access to
water, that is, where the water table is near the surface
(approximately 30-40 km away from the disposal system). In addition,
all of the RMEI's water and food would come from contaminated sources.
We did not choose this option because we believe that such a scenario
is overly conservative given the site-specific characteristics of the
area and reasonable consideration of the lifestyles of individuals in
that area.
In the second scenario, we considered using a commercial farmer as
the RMEI. We evaluated economic factors and current and potential
future technologies which could be economically viable. There are areas
in the vicinity of Yucca Mountain which are currently being farmed
commercially or could be economically farmed based upon reasonable
assumptions, current technology, and experience in other arid parts of
the western United States. The exposure pathways in this scenario would
be the same as those used for the subsistence-farmer scenario. We did
not choose this as our preferred scenario since we believe that
commercial farming would not be representative of the general
population and would not be likely in areas other than where there is
currently such farming, approximately 30 kilometers from the disposal
system.
The third scenario, selected as our preferred approach, involves a
rural-residential RMEI. We assume that the rural-residential RMEI is
exposed through the same general pathways as the subsistence farmer.
However, this RMEI would not be a full-time farmer but would do
personal gardening and earn income from other sources of work in the
area. We assume further that all of the drinking water (two liters per
day) and some of the food consumed by the RMEI is from the local area.
The consumption of two liters per day of drinking water is a high value
since people consume water from outside sources, such as commercial
products. Similarly, we assume that local food production will use
radioactively contaminated water coming from the disposal system. We
believe this lifestyle is similar to that of most people living in
Amargosa Valley today.
The fourth scenario which we considered is domestic use of an
underground source of drinking water (USDW) by a community living near
the repository site. A USDW is essentially an aquifer which is large
enough to supply or could supply a public water system (the full
definition is in 40 CFR 144.3). Based upon current water usage in the
arid western United States, a public water supply inside of the current
NTS could exist since a community would have greater resources to
access and recover water than would most individuals. Such a community
water supply would have characteristics similar to DOE's water wells J-
12 and J-13. These wells have supplied water needs (including human
consumption) since the early 1960s for the Federal government. While we
consider such a scenario possible, it could be less protective than the
rural-residential scenario because it would not protect individuals
from the ingestion of contaminated home-grown food. Also, we consider
this scenario less representative of current conditions for most people
in the vicinity of Yucca Mountain.
Location of the RMEI. The location of the RMEI is a basic part of
the exposure scenario. We considered locations within a region
occupying an area bordering Forty-Mile Wash, within a few kilometers of
the repository site, to the southwestern border of the Town of Amargosa
Valley. This region, which we believe is hydrologically downgradient
from Yucca Mountain, can be considered as three general subareas.
The first subarea occupies the land south from near Yucca Mountain
to the vicinity of U.S. Route 95. This subarea has deep ground water
(up to about 300 meters) which is accessed by Federally owned wells
used for DOE activities associated with Yucca Mountain and the NTS.
This land is currently under government control and ownership. In
addition, the likelihood of small or economically viable agricultural
activities in this area is questionable when the depth to the water
table is taken into consideration.
The next subarea borders the first and extends several kilometers
south of U.S. Route 95. The northern portion of the Town of Amargosa
Valley, including the businesses at the intersection of U.S. Route 95
and Nevada State Route 373 (Lathrop Wells), is included in this
subarea. This subarea currently includes about 15 residents and no
agricultural activities, although abandoned irrigation wells exist (see
the BID). The depth to water in this area ranges from slightly more
than 100 to about 60 meters. The U.S. Natural Resource Conservation
Service has designated the types of soils in this area as suitable for
rangeland and wildlife habitat.
The third subarea borders the second and covers the remainder of
the Town of Amargosa Valley. This subarea is the closest downgradient
location to Yucca Mountain with perennial agricultural activity. The
depth to ground water is relatively shallow--approximately 50 to 15
meters. The agriculture consists of both personal gardens and
commercial activities. The commercial agriculture is a mainstay of the
local economy. Commercial farms produce crops, livestock, and dairy
products for either local consumption or for transport out of the
region. Most of the residents of the Town of Amargosa Valley are within
this subarea, as are the community center, school, clinic, library,
post office, and sheriff's office. The population consists of all age
groups.
Based upon these considerations of the subareas, we propose that
the intersection of U.S. Route 95 and Nevada State Route 373, known as
Lathrop Wells, is a likely location for the RMEI. In this example, we
do not consider it probable that the rural-residential RMEI would
occupy locations significantly north of U.S. Route 95. We make this
assumption mainly because the rough terrain and increasing depth to
ground water nearer to Yucca Mountain would likely discourage
settlement by individuals because access to water is more difficult
than it would be a few kilometers
[[Page 46990]]
farther south. Also, there are currently several residents and
businesses near this location whose source of water is the underlying
aquifer (which we understand flows from under Yucca Mountain).
Therefore, we believe that it is reasonable to assume that individuals
could reside near this intersection in the future.
Farming occurs today farther south, in the southwestern portion of
the Town of Amargosa Valley in an area near the California border and
west of Nevada State Route 373. However, soil conditions in the
vicinity of Lathrop Wells are similar to those in southwestern Amargosa
Valley. Therefore, it should be feasible for the RMEI to grow some of
his or her own food, including a grazing cow, using a fraction of the
water recovered but not used for household purposes. Larger-scale food
production at Lathrop Wells is unlikely because of the cost of
recovering sufficient water. To supplement the gardening and grazing,
we propose that it is also reasonable to assume that the RMEI would
obtain much of his or her food from the local area.
Finally, we believe that a rural-residential RMEI near Lathrop
Wells would be among the most highly exposed individuals in the
downgradient direction from Yucca Mountain. We believe that this is
true even though individuals residing closer to the repository (where
the ground water is at a greater depth) could be consuming higher
concentrations of radionuclides in their drinking water. Because of the
significant cost of finding and withdrawing the ground water, we
further believe that individuals living nearer the repository are
unlikely to withdraw water from the significantly greater depth and in
the much larger quantities needed for farming activities. Based upon
our analyses of potential pathways of exposure, discussed above, we
believe that irrigation would be the most likely pathway for most of
the dose from the most soluble, least retarded radionuclides (such as
technetium-99 and iodine-129). The percentage of the dose that results
from irrigation would depend upon the assumptions about the fraction of
all food assumed to be consumed by the RMEI from gardening or other
crops grown using contaminated water. We also are proposing that
protection of a rural-residential RMEI would be protective of the
general population (see the How Will the General Population Be
Protected? section below).
Our identification of Lathrop Wells as a potential location of the
RMEI is based upon a review of available, site-specific information. Of
course, DOE and NRC must consider other, more appropriate locations
based upon additional data which DOE or others may develop later, but
the selection of that other location must be based upon the same
considerations used for this example. For example, if DOE subsequently
determines that the direction of ground water flow is different than we
have assumed, DOE and NRC must choose the location, at the same
distance from the center of the repository footprint as the original
point of compliance, where the highest radionuclide concentrations
occur.
As stated earlier, the method of calculating the RME is to select
average values for most parameters except one or a few which are set at
their maximum, that is, high-end, values. We believe that the Lathrop
Wells location and a consumption rate of two liters per day of drinking
water from the plume of contamination represent high-end values for two
of these factors. The Commission may identify additional parameters for
which to assign high-end values in projecting the dose to the RMEI. To
the extent possible, NRC should use site-specific information for any
remaining factors. For example, NRC should use the most accurate
projections of the amount of contaminated food that would be ingested
in the future. Projections might be based upon surveys which indicate
the percentage of the total diet of Amargosa Valley residents which is
from food grown in the Amargosa Valley area.
We particularly request comment upon whether:
(1) Based upon the above criteria, there is now sufficient
information for us to adequately support a choice for the RMEI location
in the final rule or should we leave that determination to NRC in their
licensing process based upon our criteria;
(2) Another location in one of the three subareas identified
previously should be the location of the RMEI; and
(3) Lathrop Wells and an ingestion rate of two liters per day of
drinking water are appropriate high-end values for parameters to be
used to project the RME. We also request comment upon the potential
approaches and assumptions for the exposure scenario to be used for
calculating the dose incurred by the RMEI.
III.B.5. How Will the General Population be Protected?
In section 801(a)(2)(A) of the EnPA, Congress asked whether an
individual-protection standard could also protect the general
population. In response, the NAS concluded that an individual-
protection standard could provide such protection for the case of the
proposed Yucca Mountain repository. The NAS premised this conclusion
upon the condition that the public and policymakers would accept the
idea that extremely small individual radiation doses spread out over
large populations pose a risk that is negligible (NAS Report p. 57).
The NAS refers to this concept as ``negligible incremental risk'' (NAS
Report p. 59). Earlier, we described our proposed individual-protection
standard for the RMEI which would establish the highest allowable
radiation dose. This section of the notice raises another question--
should we also adopt a standard to limit the possible widespread
exposure of whole populations to extremely small individual doses?
In discussing the feasibility of protecting the general population
from releases of radionuclides from Yucca Mountain, NAS considered the
potential for the release of gaseous radionuclides. The NAS Report
explained how the release of carbon dioxide gas containing \14\ C from
the Yucca Mountain disposal system might expose a large population:
Global populations might be affected because radionuclide
releases from a repository can in theory be diffused throughout a
very large and dispersed population. In the case of Yucca Mountain,
the likely pathway leading to widely dispersed radionuclides is via
the atmosphere beginning with release of carbon dioxide gas
containing the carbon-14 (\14\ C) radioactive isotope which might
escape from the waste canisters. (NAS Report p. 7)
On page 61 of its Report, NAS estimated that the average dose to
members of the global population, based upon this scenario, to be 0.003
Sv/year (0.0003 mrem/yr) and equated that to an annual risk of
fatal cancer of 1.5 in 10 billion (1.5 x 10-10).
The NAS relied upon the recommendations of the NCRP in its report
titled ``Limitation of Exposure to Ionizing Radiation'' (NCRP Report
No. 116) to support their claim that such doses are negligibly small.
In this report, the NCRP stated that a radiation dose of less than 10
Sv (1 mrem)/yr for any source or practice would represent a
``negligible incremental dose.'' The NCRP endorsed the assumption that
there is some radiation risk for every radiation exposure. Further,
they explained that there are great uncertainties in trying to
understand the meaning of radiation effects upon populations,
especially when these effects are calculated by summing extremely small
individual doses among huge populations. Agreeing with this
[[Page 46991]]
concept, the NAS preferred to use risk instead of dose. The NAS then
estimated the risk level associated with the NCRP's NID level of 10
Sv/yr and adopted the term ``negligible incremental risk.''
The NAS then proposed this NIR level as the starting point for a
process to establish a risk level for individuals that would be
``negligible.''
For different reasons, we provisionally agree with the NAS that an
individual-risk standard can adequately protect the general population
near Yucca Mountain. Our agreement is based upon the particular
characteristics of the Yucca Mountain site. We emphasize that our view
relates to the specific circumstances associated only with Yucca
Mountain. We are not proposing to adopt either an NID or NIR level. We
are concerned that such an approach is not appropriate in all
circumstances. Again, our proposed determination that an individual-
risk standard is adequate to protect both the local and general
population is based upon considerations unique to the Yucca Mountain
site--it is not a general policy judgment by us upon other uses of the
concept of NID or NIR.
We considered the NAS suggestion to adopt a general NIR level but
have not done so because of reservations regarding the reasoning and
analysis employed by NAS. As noted above, NAS referred to the NID level
of 10 Sv (1 mrem)/yr per source or practice recommended by the
NCRP. The International Atomic Energy Agency (IAEA) has made similar
recommendations regarding exemptions in its Safety Series No. 89,
``Principles for the Exemption of Radiation Sources and Practices from
Regulatory Control.'' The IAEA has recommended that individual doses
not exceed 10 Sv (1 mrem)/yr from each exempt practice. The
IAEA's recommendations relate to criteria for exempting whole sources
or practices, such as waste disposal or recycling generally, not
whether radiation doses from a portion of a given practice, such as the
release of gases from a specific geologic repository, may be considered
negligible. Finally, the IAEA's recommendations intend their exemption
to be for sources and practices ``which are inherently safe.'' It is
not clear that the low individual doses or risks projected from gaseous
releases from the Yucca Mountain repository should be considered on
their own as a ``source'' or ``practice'' or that such a source or
practice should be considered inherently safe. Also, we believe it to
be inappropriate to not calculate a radiation dose merely because the
dose rate from a particular source is small.
Further, we are not sure it is appropriate to apply the NIR concept
to consideration of population dose. A recent NCRP report questions the
application of the negligible incremental dose (NID) concept to
consideration of population doses. According to NCRP Report No. 121:
``A concept such as the NID (Negligible Incremental Dose) provides a
legitimate lower limit below which action to further reduce individual
dose is unwarranted, but it is not necessarily a legitimate cut-off
dose level for the calculation of collective dose. Collective dose
addresses societal risk while the NID and related concepts address
individual risk.'' Based upon this, we think it would be inappropriate
to use the negligible incremental dose or risk concept to evaluate
whether an individual-protection standard adequately protects the
general population.
Although we do not advocate use of the NID concept, we acknowledge
that the extremely low levels of individual risk and dose cited by NAS
as being associated with the release of \14\ C from Yucca Mountain are
many orders of magnitude below the levels at which we have regulated in
other circumstances. For example, we used the following policies under
the pre-1990 Clean Air Act (CAA) hazardous air pollution control
program: (1) provide public health protection for the greatest number
of persons possible based upon a lifetime (70 years) risk level no
higher than approximately 1 x 10-6 for an individual, and
(2) limit the maximum, individual-lifetime, estimated risk to no higher
than 1 in 10,000 (1 x 10-4) (54 FR 51654, 51655, December
15, 1989). Even though we adopted this approach in a different policy
context, it provides insight into how we have dealt with similar risk-
management issues in a regulatory context. In 1990, Congress amended
the CAA to require us to develop technology-based standards to reduce
emissions. At the same time, Congress authorized us to delete
categories of sources from regulation if no source in that category
could cause a lifetime risk of cancer exceeding 1 x 10-6 for
the most-exposed individual in the population. The risk over an
individual's lifetime from exposure to gaseous \14\ C released from the
Yucca Mountain repository, as estimated by NAS, would be about 100
times lower than 10-6. This particular risk level is
extremely low and well below the risk level that we generally regulate.
The disposal standards in 40 CFR part 191 include release limits
(or containment requirements) to protect populations and an individual-
protection standard. We rejected adopting only an individual-protection
standard in those standards because of a concern that an individual-
dose limitation alone might encourage selection of disposal sites that
relied upon dilution of radionuclides at the expense of increased
overall population exposures. Specifically, we were concerned that, in
the absence of release limits, ``disposal sites near bodies of surface
water or large sources of ground water might be preferred--which the
Agency believes is an inappropriate policy that would usually increase
overall population exposures'' (50 FR 38066, 38078, September 19,
1985). For example, it is possible to have a site that could meet the
150 Sv (15 mrem)-CEDE/yr individual-protection standard while
still having large numbers of people being exposed to radiation levels
just below the standard. This scenario could result in significant
numbers of calculated health effects for each generation exposed and
very large numbers of calculated health effects over the regulatory
period. We believe that the policy embodied in the generic 40 CFR part
191 disposal standards is sound. The provisions in 40 CFR part 191,
which could apply to a variety of potential disposal sites, should
discourage reliance upon dilution of radionuclides in the general
environment as a disposal method.
However, the potential for large-scale dilution of radionuclides,
through ground water and into surface water, as modeled in the
supporting analyses for 40 CFR part 191, does not exist at Yucca
Mountain, thereby minimizing the need for the kind of population-
protection requirements found in 40 CFR part 191. Rather, DOE plans to
locate the Yucca Mountain repository in an unsaturated rock formation
with limited amounts of infiltrating water passing through it and into
the underlying tuff aquifer. (``Unsaturated'' means that the rock could
absorb more water than it is holding.) That aquifer is, in turn, within
a ground water system which discharges into arid areas having high
evaporation rates and very little surface water. In other words, we
believe that the characteristics of the saturated zone under Yucca
Mountain are such that dilution from other sources will be limited and
the aquifer does not discharge into any large bodies of surface water.
Therefore, our basis for inclusion of a population-protection
requirement in 40 CFR part 191 does not appear to apply to the
development of site-specific standards for Yucca Mountain.
[[Page 46992]]
In addition, we based the release limits in 40 CFR part 191 partly
upon technology and partly upon risk levels which we believed to be
acceptably small. The technology basis for the release limits was based
upon assessments of repository performance of several generic disposal
systems, including one located in tuff. In finalizing 40 CFR part 191,
we stated:
[T]he rule cannot be interpreted as setting precedents for
``acceptable risk'' levels to future generations that should not be
exceeded regardless of the circumstances. Instead, because of a
number of unique circumstances, the Agency has been able to develop
standards for the management and disposal of these wastes that are
both reasonably achievable . . . and that limit risks to levels that
the Agency believes are clearly acceptably small. (50 FR 38066,
38070, September 19, 1985)
We developed these standards during the siting process mandated by the
NWPA in the 1980s. The inclusion of release limits pointed to the
importance of considering population doses during site selection. We
established the standards at a level that appeared to be reasonably
achievable for several types of rocks or geologic media and which would
keep risks to future populations acceptably small. The assessments we
performed in support of these generally applicable standards, however,
did not include a gaseous-release pathway similar to that described by
NAS for \14\ C because no one foresaw the potential importance of that
pathway at that time. In fact, according to the generic analyses we
performed in support of 40 CFR part 191, the unsaturated site in tuff
was generally more protective, in terms of limiting total releases,
than the other geologic media we evaluated.
For these reasons, we do not believe that these generic analyses
and conclusions supporting the development of release limits in 40 CFR
part 191 are appropriate for judging the need for population-risk
limits or the acceptability of population risks from releases from
wastes in the Yucca Mountain disposal system. We are proposing to find
that the individual-protection standard is sufficient to protect public
health based upon the unique characteristics of the area around the
Yucca Mountain site.
In summary, we are proposing to adopt an individual-protection
standard for Yucca Mountain that will limit the annual radiation dose
incurred by the RMEI to 150 Sv (15 mrem) CEDE. At the same
time, we are not proposing to adopt a separate limit on radiation
releases for the purpose of protecting the general population, but we
are recommending that collective dose be estimated and considered (see
the following paragraph). We based this decision upon several factors.
The first factor is the NAS projection of extremely small doses to
individuals resulting from air releases from Yucca Mountain. That dose
level is well below the risk corresponding to our proposed individual-
protection standard for Yucca Mountain. It is also well below the level
that we have regulated in the past through other regulations. Further,
while we decline to establish a general NIR level, we do agree with NAS
that estimating the number of health effects resulting from a 0.0003
mrem/yr dose rate, in addition to the dose rate from background
radiation, in the general population is uncertain and controversial.
The second major factor is that, based upon current and site-specific
conditions near Yucca Mountain, there is not likely to be great
dilution resulting in exposure of a large population. In addition, we
are proposing additional ground water protection standards that would
establish specific limits to protect users of ground water and ground
water as a resource. Finally, we are still proposing to require that
all of the pathways, including air and ground water, would be analyzed
by DOE and considered by NRC under the individual-protection standard.
We request comment upon this approach. Commenters who disagree with
this approach should specifically address why it is inappropriate for
the Yucca Mountain disposal system and make suggestions about how we
might reasonably address this issue.
While we are not proposing to adopt additional regulatory
requirements for collective exposures of the general population from
releases from the Yucca Mountain disposal system, we urge DOE to
examine design alternatives for the disposal system, for the purpose of
reducing potential risk to the general population, in the National
Environmental Policy Act (NEPA) process for Yucca Mountain. We received
public comments, in response to our request for comments regarding the
NAS Report, noting that DOE had already proposed, in its Notice of
Intent to prepare a NEPA-prescribed environmental impact statement
(EIS) for Yucca Mountain, to evaluate technical alternatives (60 FR
40167, August 7, 1995). In other words, DOE has previously proposed to
evaluate technical alternatives as part of its waste containment and
isolation strategy for Yucca Mountain (DOE, ``Strategy for Waste
Containment and Isolation for the Yucca Mountain Site,'' Preliminary
Review Draft, October 9, 1995). Thus, we recommend that DOE incorporate
these or similar considerations into its NEPA process to assess the
effectiveness of design alternatives to mitigate population exposures.
The following language provides context to the approach we consider
appropriate for calculating population exposure in the NEPA process. We
recommend that DOE calculate the collective dose without truncation and
with full consideration of the appropriate factors. This recommendation
is supported by a recent NCRP report upon the principles and
application of a collective dose in radiation protection (NCRP Report
No. 121). The NCRP advocated the use of collective dose for
optimization of protection and provided guidance on future exposures
from long-lived radionuclides, the situation that will likely exist at
Yucca Mountain:
The most reasonable risk assessment that can be made for such
situations is to calculate potential individual doses for a range of
scenarios in order to: (1) evaluate protective measures and (2) to
try to place some boundaries on estimates of future individual
risks. For the few very long-lived radionuclides that are
metabolically regulated in the body and more or less uniformly
distributed within the biosphere (e.g., \14\ C and \129\ I), future
average individual doses may be estimated from total quantities in
the environment. . . . (NCRP Report No. 121, pp. 57-58)
III.B.6. What Should Be Assumed About the Future Biosphere?
We propose to require DOE and NRC to use the biosphere assumptions
described in this section in all analyses of repository performance,
including the performance assessment for determining compliance with
the individual-protection standard, the assessment for determining
compliance with the ground water standards, and the human-intrusion
analysis. Projecting biosphere conditions necessitates making
assumptions, many of which are very uncertain and may not be boundable.
The NAS stated:
In view of the almost unlimited possible future states of
society and of the significance of these states to future risk and
dose, . . . we have recommended that a particular set of assumptions
be used about the biosphere (including, for example, how and where
people get their food and water) for compliance calculations . . .
we recommend the use of assumptions that reflect current
technologies and living patterns. (NAS Report p. 122)
The NAS also stated:
. . . unlike our conclusion about the earth science and geologic
. . . factors described [earlier], we believe that it is not
possible to
[[Page 46993]]
predict on the basis of scientific analyses the societal factors
that must be specified in a far-future exposure scenario. . . . Any
particular scenario about the future of human society near Yucca
Mountain . . . should not be interpreted as reflecting conditions
that eventually will occur. Although we recognize the burden on
regulators to avoid regulations that are arbitrary, we know of no
scientific method for identifying these [exposure] scenarios. (NAS
Report p. 96)
We agree with the NAS on this point and propose that speculation
concerning some characteristics of the future should not be the focus
of the compliance determination process. Instead, we believe that it
would be more appropriate to assume that those characteristics will be
the same as they are today. No one should interpret this assumption so
literally that only current residences and lifestyles of individuals
living in the area on the day of promulgation of this part can be
considered. Rather, we intend that, based upon current knowledge, DOE
and NRC may use those characteristics in combinations in a cautious,
but reasonable, manner as input into the Yucca Mountain performance
projections. Future characteristics which NRC and DOE may assume to be
the same as they are today include the level of human knowledge and
technical capability (including medical), human physiology and
nutritional needs, general lifestyles of the population, and potential
pathways through the biosphere leading to radiation exposure of humans.
Also, we propose that it is inappropriate to speculate upon extreme
changes in the number of residents, but that consideration should be
given to changes in population near the location of the RMEI.
In concert with the NAS Report, we also propose not to allow the
assumption that conditions in the future will be the same as present
conditions for geologic, hydrologic, and climatic conditions. We are
proposing this because we believe the parameter values in the
performance assessment which relate to these conditions can be
reasonably bounded. We propose to require that these conditions be
varied within reasonable bounds over the compliance period and request
comment upon this proposed approach.
III.B.7. How Far Into the Future Is It Reasonable To Project Disposal
System Performance?
The NAS recommended that the time over which compliance should be
assessed, that is, the compliance period, should be ``the time when the
greatest risk occurs, within the limits imposed by long-term stability
of the geologic environment'' (NAS Report p. 7). The NAS stated that it
based this recommendation upon technical, not policy, considerations.
However, we believe the selection of the compliance period necessarily
involves both technical and policy considerations. For example, NAS
stated that we might choose to establish similar policies for managing
risks ``from disposal of both long-lived hazardous nonradioactive
materials and radioactive materials'' (NAS Report p. 56). As NAS
recognized, we must consider, in this rulemaking, both the technical
and policy issues associated with establishing the appropriate
compliance period for the performance assessment of the Yucca Mountain
disposal system.
We request public comment upon two alternatives for the compliance
period for the individual-protection standard. One alternative is to
adopt a compliance period that is the time to peak dose within the
period of geologic stability. The second alternative is to adopt a time
period during which the repository must meet the disposal standards.
For the reasons described below, we believe that the second alternative
is preferable. Therefore, we are proposing that the peak dose within
10,000 years after disposal must comply with the individual-protection
standard. Also, the EPA-preferred approach would require calculation of
the peak dose within the period of geologic stability. It does not,
however, apply a quantitative limit after 10,000 years. The intent of
examining disposal system performance after 10,000 years is to estimate
the long-term performance of the disposal system to see if dramatic
changes in the performance of the disposal system could be anticipated.
We would require DOE to include the results and bases of the additional
analysis in the EIS for Yucca Mountain as an indicator of the future
performance of the disposal system. This analysis also would serve as
another source of information for decisionmakers in making both design
and licensing decisions. However, NRC is not to use the additional
analysis in determining compliance with proposed Sec. 197.20.
The principal tool used to assess compliance with the individual-
protection standard is a quantitative performance assessment. This
method relies upon modeling of the potential processes and events
leading to releases of radionuclides from the disposal system,
subsequent radionuclide transport, and consequences upon health. To
consider compliance for any length of time, several facets of knowledge
and technical capability are necessary. First, the scientific
understanding of the relevant, potential processes and events leading
to releases must be sufficient to allow a quantitative estimate of
projected repository performance. Second, adequate analytical methods
and numerical tools must exist to incorporate this understanding into a
quantitative assessment of compliance. Third, scientific understanding,
data, and analytical methods must be adequately developed to allow
evaluation of performance with sufficient robustness to judge
compliance with reasonable expectation over the regulatory period.
Finally, the analyses must be able to produce estimated results in a
form capable of comparison with the standards.
The NAS evaluated these requirements for Yucca Mountain and
concluded that those aspects of disposal system and waste behavior that
depend upon physical and geologic properties can be estimated within
reasonable limits of uncertainty. Also, NAS believed that these
properties and processes are sufficiently understood and boundable over
the long periods at issue to make such calculations possible and
meaningful. The NAS acknowledged that these factors cannot be
calculated precisely, but concluded that there is a substantial
scientific basis for making such calculations. The NAS concluded that
by taking uncertainties and natural variabilities into account, it
would be possible to estimate, for example, the concentration of
radionuclides in ground water at different locations and the times of
gaseous releases. Second, NAS concluded that the mathematical and
numerical tools necessary to evaluate repository performance are
available or could be developed as part of the standard-setting or
compliance-determination processes. Third, NAS concluded that: ``So
long as the geologic regime remains relatively stable, it should be
possible to assess the maximum risks with reasonable assurance'' (NAS
Report p. 69). The NAS used the term ``geologic stability'' to describe
the situation where geologic processes, such as earthquakes and
erosion, that could affect the performance assessment of the Yucca
Mountain site are active (not static) and are expected to occur. Based
upon the use of the terms ``stable'' and ``boundable'' throughout the
NAS Report, one can infer that NAS applied the term ``geologic
stability'' or ``stable'' to the situation where the rate of processes
and numeric range of individual physical properties could be bounded
with reasonable certainty. The
[[Page 46994]]
subsequent use of the term ``stable'' will not imply static conditions
or processes. Rather, it will describe the properties and processes
that can be bounded. Finally, NAS found that the established procedures
of risk analysis should enable the results of each performance
simulation of the disposal system to be combined into a single estimate
for comparison with the standard.
Time to peak dose within the period of geologic stability. The NAS
recommended that the compliance period for the Yucca Mountain disposal
system be the time to peak risk within the long-term stability of the
geologic environment. Since the time to peak risk is generally the time
to peak dose, subsequent discussion of the NAS findings will refer to
the time to peak dose. The ``peak dose'' is the mean value of the range
of the highest potential annual doses, as determined by the performance
assessment, incurred by the RMEI within the compliance period. The NAS
based its recommendation to use the time to peak dose upon its review
of:
(1) The technical analyses supporting 40 CFR part 191;
(2) Information derived from current performance assessments of the
Yucca Mountain disposal system; and (3) The geologic and physical
processes that could affect the release and transport of radionuclides
to the biosphere.
The 40 CFR part 191 standards contain a compliance period of 10,000
years. There were three reasons that we set this time frame:
(1) After that time, there is concern that the uncertainties in
compliance assessment become unacceptably large (50 FR 38066, 38076,
September 19, 1985);
(2) There are likely to be no exceptionally large geologic changes
during that time (47 FR 58196, 58199, December 29, 1982); and
(3) Using time frames of less than 10,000 years does not allow for
valid comparisons among potential sites. For example, for 1,000 years,
all of the generic sites analyzed appeared to contain the waste
approximately equally because of long ground water travel times at
well-selected sites (47 FR 58196, 58199, December 29, 1982).
One purpose of geologic disposal is to provide long-term barriers
to the movement of radionuclides into the biosphere (NAS Report p. 19).
As described earlier, the Department plans to locate the Yucca Mountain
repository in tuff about 300 meters above the local water table. When
nongaseous radionuclides are released from the waste packages, they
most likely will be transported by rain water that moves from the
surface both horizontally within individual tuff layers and vertically
downward, through fractures in the tuff layers, toward the underlying
aquifer. Once the radionuclides reach the aquifer, they will be carried
away from the repository in the direction of ground water flow. The
most probable route for exposing humans to radiation resulting from
releases from the Yucca Mountain disposal system is via withdrawal of
contaminated water for local use. In the case of Yucca Mountain, DOE
estimates that most radionuclides would not reach currently populated
areas within 10,000 years (see the BID).
While this finding alone seems to indicate that the compliance
period for Yucca Mountain should be longer than 10,000 years to be
protective, NAS concluded that the need to consider the exposures when
they are calculated to occur must be weighed against the problem of
cumulative uncertainty. As noted above, exposures could occur over
tens-to hundreds-of-thousands of years. However, as the compliance
period is extended to such lengths, uncertainty increases and the
resulting projected doses are increasingly meaningless from a policy
perspective. The NAS stated that there are significant uncertainties in
a performance assessment and that the overall uncertainty increases
with time. Even so, NAS found that, ``. . . there is no scientific
basis for limiting the time period of the individual-risk standard to
10,000 years or any other value'' (NAS Report p. 55). Estimates by NRC
and DOE related to the Yucca Mountain disposal system have indicated
wide differences in estimates of the time that radionuclides may take
to reach the biosphere and cause the peak dose to occur (see the BID).
However, while the results have indicated that the time to peak dose
may vary anywhere from a few tens-of-thousands to hundreds-of-thousands
of years, the estimated values of the peak doses, while separated in
time, are similar in magnitude (see the BID). These estimates differ
because the analysts used different assumptions and conceptual models
for flow and transport of radionuclides through the Yucca Mountain
unsaturated zone. We believe that this situation will exist
independently of the compliance-period issue. The NAS also stated that
data and analyses of some of the factors that are uncertain at one time
might be more certain at a later time. For example, there is
uncertainty as to how many waste packages might fail in the near term.
However, at some later time in the distant future, the uncertainty is
very small because when enough time has passed, all of the packages
will fail (NAS Report p. 72). Also, NAS stated that many of the
uncertainties in parameter values describing the geologic system are
not due to the length of time but rather to the difficulty in
estimating values of site characteristics which vary across the site.
We believe that these difficulties are always present and that analysts
must consider them in the compliance assessment for any period chosen
(NAS Report p.72).
As NAS noted, evaluating compliance with the 40 CFR part 197
standards depends upon being able to:
(1) Understand and model radionuclide-transport processes and the
processes and events that might lead to transport;
(2) Use appropriate analytical methods to determine the levels of
human exposure;
(3) Quantify or bound the probabilities of the processes and
events, including the related uncertainties; and
(4) State the results in a form capable of being compared with the
standards.
The NAS reviewed how radionuclides might enter the biosphere in
order to determine the feasibility of evaluating them in a compliance
assessment. In addition, to determine whether the modifying processes
should also be evaluated in a compliance assessment, NAS analyzed the
geologic and physical processes that could modify the properties of the
contaminant-containing media and processes by which radionuclides are
moved.
The radionuclide-transport processes evaluated by NAS included:
(1) Release from the waste form;
(2) Transport from canisters into the near-field (near the waste
canisters) unsaturated zone;
(3) Gas-phase transport from the unsaturated zone into the
atmosphere around Yucca Mountain;
(4) Atmospheric circulation leading to dispersal of gaseous
radionuclides in the global atmosphere;
(5) Aqueous-phase transport from the unsaturated zone to the water
table; and
(6) Transport of radionuclides through the saturated zone beneath
the repository to other locations from which water may be extracted by
humans or ultimately reach the surface at a discharge area (NAS Report
pp. 85-90).
The NAS concluded that these processes are ``sufficiently
quantifiable and the uncertainties are sufficiently boundable that they
can be included in performance assessments that extend over time frames
corresponding to those over which the geologic system is relatively
stable or varies in a boundable
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manner'' (NAS Report p. 85). The NAS concluded that the ``geologic
record suggests that this time frame is on the order of about one
million years'' (NAS Report pp. 9 and 85). Likewise, NAS concluded that
the probabilities and consequences of these processes and events that
could modify the way in which radionuclides are moved in the vicinity
of Yucca Mountain, including climate change, seismic activity, and
volcanic eruptions, ``are sufficiently boundable so that these factors
can be included in performance assessments that extend over periods on
the order of about one million years'' (NAS Report p. 91).
Thus, NAS recommended, on a technical basis, that the compliance
period for the protection of the individual should extend to the time
of the peak dose during the period in which geologic processes are
stable or boundable. This would require determining compliance and
licensing the disposal system on the basis of projections of
performance over tens- to hundreds-of-thousands of years into the
future. We believe that such an approach is not practical for Yucca
Mountain.
As noted earlier, NAS concluded that ``there is no scientific basis
for limiting the time period of the individual-risk standard to 10,000
years or any other value.'' Nevertheless, there is still considerable
uncertainty as to whether current modeling capability allows
development and validation of computer models that will provide
sufficiently meaningful projections over a time frame up to tens-of-
thousands to hundreds-of-thousands of years. Simply because such models
can provide projections for those time periods does not mean those
projections are either meaningful for decisionmakers or accurate.
Furthermore, we are not aware of a policy basis that we could use to
determine the level of proof or confidence necessary to determine
compliance based upon projections of hundreds-of-thousands of years
into the future. While NAS indicated that analyses of the performance
of the Yucca Mountain disposal system dealing with the far future can
be bounded, a large and cumulative amount of uncertainty is associated
with those numerical projections. Setting a strict numerical standard
at a level of risk acceptable today for the period of geologic
stability would tend to ignore this cumulative uncertainty. For
example, if the performance assessment indicates that the peak dose
occurs 600,000 years in the future at an annual CEDE that has an
uncertainty range of 0.1 mrem to 10,000 mrem, does that indicate that
the disposal system is safe or unsafe and should NRC license it or not?
In light of the cumulative uncertainty for calculations over an
extremely long time, it may be more appropriate to consider, in a
regulatory decisionmaking, assessments of disposal system performance
over such time in a qualitative manner. We request comments upon the
reasonableness of adopting the NAS-recommended compliance period or
some other approach in lieu of the 10,000-year compliance period which
we favor and describe below. We also seek comment upon whether the NAS-
recommended compliance period can be implemented in a reasonable manner
and how that could be done.
A 10,000-year compliance period (proposed Sec. 197.20). As noted
earlier, the selection of the compliance period for the individual-
protection standard involves both technical and policy considerations.
It is our responsibility to weigh both during this rulemaking. In
addition to the technical guidance provided in the NAS Report, we have
considered several policy and technical factors that NAS did not fully
address.
First, as suggested by NAS, we evaluated the policies for managing
risks from the disposal of both long-lived, hazardous, nonradioactive
materials and radioactive materials. Second, we evaluated consistency
with both 40 CFR part 191 and the issue of consistent time periods for
the protection of ground water resources and public health. Third, we
considered the issue of uncertainty in predicting dose over the very
long periods contemplated in the alternative of peak dose within the
period of geologic stability. Finally, we reviewed the feasibility of
implementing the alternative of peak risk within the period of geologic
stability, as recommended by NAS. As a result of these considerations,
we are proposing a 10,000-year compliance period with a quantitative
limit and a requirement to calculate the peak dose, using performance
assessments, if the peak dose occurs after 10,000 years. Under our
proposal, the performance assessment results for the post-10,000-year
period must be made part of the public record by DOE including it in
the EIS for Yucca Mountain.
In its discussion of the policy issues associated with the
selection of the time period for compliance, NAS suggested that we
might choose to establish consistent risk-management policies for long-
lived, hazardous, nonradioactive materials and radioactive materials.
We previously addressed the 10,000-year compliance period in the
regulation of hazardous waste subject to land-disposal restrictions.
Land disposal, as defined in 40 CFR 268.2(c), includes, but is not
limited to, any placement of hazardous waste in land-based units such
as landfills, surface impoundments, and injection wells. Facilities may
seek an exemption by demonstrating that there will be no migration of
hazardous constituents from the disposal unit for as long as the waste
remains hazardous (40 CFR 268.6). We have interpreted the phrase ``for
as long as the waste remains hazardous'' to mean that the no-migration
demonstration shows that hazardous constituents will not exceed
acceptable concentration levels for as long as the constituents retain
the potential to harm human health and the environment. This period may
include not only the operating phase of the facility, but also what may
be an extensive period after facility closure. With respect to
injection wells, we have specifically required a demonstration that the
injected fluid will not migrate within 10,000 years (40 CFR 148.20(a)).
We chose the 10,000-year performance period referenced in our guidance
upon no-migration petitions, in part, to be equal to time periods cited
in draft or final DOE, NRC, and EPA regulations (10 CFR 960, 10 CFR 60,
or 40 CFR 191, respectively) governing siting, licensing, and releases
from HLW disposal systems. With respect to other land-based units
regulated under the Resource Conservation and Recovery Act (RCRA)
hazardous-waste regulations, we concluded that the compliance period is
specific to the waste and site under consideration. For example, for
the WIPP no-migration petition, we found that ``it is not particularly
useful to extend this model beyond 10,000 years into the future.* * *
[However, t]he agency does believe * * * that modeling over a 10,000-
year period provides a useful tool in assessing the long-term stability
of the repository and the potential for migration of hazardous
constituents'' (55 FR 13068, 13073, April 6, 1990).
Second, the individual-protection requirements in 40 CFR part 191
(58 FR 66398, 66414, December 20, 1993) have a compliance period of
10,000 years. The part 191 standards apply to the same types of waste
and type of disposal system as proposed for Yucca Mountain. However, as
we explained in the What Led up to Today's Action? section earlier in
this notice, by statute the part 191 requirements do not apply to Yucca
Mountain. If we finally adopt the 10,000-year compliance period, it
would require the same compliance period for
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the Yucca Mountain disposal system as for other disposal systems
subject to 40 CFR part 191. Such a requirement would be consistent with
40 CFR part 191, which we deem appropriate since both sets of standards
apply to the same types of waste.
Third, we are concerned that there might be large uncertainty in
projecting human exposure due to releases from the repository over
extremely long periods. We agree with the NAS conclusion that it is
possible to evaluate the performance of the Yucca Mountain disposal
system and the lithosphere within certain bounds for relatively long
periods. However, we believe that NAS might not have fully addressed
two aspects of uncertainty.
One of the aspects of uncertainty relates to the impact of long-
term natural changes in climate and its effect upon choosing an
appropriate RMEI. For extremely long periods, major changes in the
global climate, for example, a transition to a glacial climate, could
occur (see the BID). However, over the next 10,000 years, the biosphere
in the Yucca Mountain area will probably remain, in general, similar to
present-day conditions due to the rain-shadow effect of the Sierra
Nevada Mountains, which lie to the west of Yucca Mountain (see the
BID). For the longer periods contemplated for the alternative of time
to peak dose, the global climate regime is virtually certain to pass
through several glacial-interglacial cycles, with the majority of time
spent in the glacial state (NAS Report p. 91). These longer periods
would require the specification of exposure scenarios that would not be
based upon current knowledge or cautious, but reasonable, assumptions,
but rather upon potentially arbitrary assumptions. The NAS indicated
that it knew of no scientific basis for identifying such scenarios (NAS
Report p. 96). It is for these reasons that such extremely long-term
calculations are useful only as indicators, rather than accurate
predictors, of the long-term performance of the Yucca Mountain disposal
system (IAEA TECDOC-767, 1994).
The other aspect of uncertainty concerns the range of possible
biosphere conditions and human behavior. It is necessary to make
certain assumptions regarding the biosphere, even for the 10,000-year
alternative, because the period of 10,000 years represents a very long
compliance period for current-day assessments to project performance.
For example, it is twice as long as recorded human history (see the
What Should Be Assumed About the Future Biosphere? section earlier in
this notice). For periods approaching the 1,000,000 years that NAS
contemplated under the peak-dose alternative, even human evolutionary
changes become possible. Thus, reliable modeling of human exposure may
be untenable and regulation to the time of peak dose within the period
of geologic stability could become arbitrary.
Fourth, many international geologic disposal programs use a 10,000-
year regulatory compliance period as a requirement.
Finally, an additional complication associated with the time to
peak dose within the period of geologic stability is that it could lead
to a period of regulation that has never been implemented in a national
or international radiation regulatory program. Focusing upon a 10,000-
year compliance period forces more emphasis upon those features over
which man can exert some control, such as repository design and
engineered barriers. It is unlikely that over much longer time frames
that any engineered barrier will be effective. Those features, the
geologic barriers, and their interactions define the waste isolation
capability of the disposal system. By focusing upon an analysis of the
features that man can influence or dictate at the site, it may be
possible to influence the timing and magnitude of the peak dose, even
over times longer than 10,000 years.
Thus, we request comment upon our proposal of a 10,000-year
compliance period to judge compliance with proposed Sec. 197.20 and our
proposal to require consideration of the peak dose, using performance
assessments, if it occurs after 10,000 years. Again, after 10,000
years, we would not require the calculated level to comply with a
specific numerical standard but we would require its consideration as
an indicator of longer-term performance and be included in the EIS for
Yucca Mountain.
We also request comment upon the appropriateness of a 10,000-year
compliance period for the individual-protection standard. Commenters
should address the issues that we should consider in determining the
appropriate compliance period. We also specifically request comments
upon whether the NAS' recommendation of the time to peak dose within
the period of geologic stability can be implemented reasonably and, if
so, how that could be done.
III.C. What Are the Requirements for Performance Assessments and
Determinations of Compliance? (Proposed Secs. 197.20, 197.25, and
197.35)
III.C.1. What Limits Are there on Factors Included in the Performance
Assessments?
The Commission is responsible for deciding whether or not to
license the Yucca Mountain disposal system. It must make that decision
based largely upon whether DOE has demonstrated compliance with our
standards in 40 CFR part 197. Under the proposed 40 CFR part 197, the
quantitative analysis underlying that decision will be a performance
assessment (the proposed definition of ``performance assessment'' is in
Sec. 197.12). We are proposing that performance assessments be a
requirement of licensing. The EnPA requires that the Commission modify
its technical requirements for licensing the disposal system to be
consistent with our final 40 CFR part 197 standards. Therefore, our
standards would require DOE to complete a performance assessment prior
to applying for a license and would require NRC to determine, taking
into consideration that performance assessment, whether the disposal
system's projected performance complies with Sec. 197.20.
We also are proposing, consistent with the performance assessment
requirements in 40 CFR part 191:
(1) To exclude from performance assessments those natural processes
and events whose likelihood of occurrence is so small that they are
very unlikely;
(2) That such performance assessments need not include categories
of processes or events that DOE and NRC estimate to have less than a 1
in 10,000 (1 x 10-4) chance of occurring during the 10,000
years after disposal. Probabilities below this level are associated
with events such as the appearance of new volcanoes outside of known
areas of volcanic activity or a cataclysmic meteor impact in the area
of the repository. We believe there is little or no benefit to public
health or the environment from trying to regulate the effects of such
very unlikely events; and
(3) That the performance assessment need not evaluate, in detail,
the releases from processes, events, and sequences of processes and
events estimated to have a likelihood of occurrence greater than 1 x
10-4 of occurring during the 10,000 years following
disposal, if there is a reasonable expectation that the time to, or the
magnitude of, the peak dose would not be changed significantly by such
omissions. As necessary, the Commission may provide specific
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guidance upon scenario selection and characterization to assure that
processes or events are not excluded inappropriately.
A related issue upon which we request comment is if there is a
period of the geologic record which we should require DOE and NRC to
use to calculate the probability of processes and events occurring. The
probability of a geologic event, such as an earthquake, occurring in
the future typically comes from evidence of previous events which is
preserved in, and can be dated by using, the geologic record. We
believe that the geologic record is best preserved in the relatively
recent past.
We are also proposing to require that DOE and NRC use quantitative
assessments to determine compliance with the human-intrusion and ground
water protection standards (see the What Is the Standard for Human
Intrusion? and How Will Ground Water Be Protected? Sections later in
this notice). The human-intrusion analysis would require a separate
assessment of the effects of human intrusion upon the resilience of the
Yucca Mountain disposal system. Following the recommendation of NAS, we
intend the analysis to be an assessment of the disposal system's
isolation capability following a single, stylized, human intrusion. The
analysis required to determine compliance with the ground water
protection standards applies only to undisturbed performance.
We are proposing to allow the exclusion of unlikely natural events
from both the ground water and human-intrusion assessments. The
approach for the ground water protection requirements is consistent
with subpart C of 40 CFR part 191, ``Environmental Standards for
Ground-Water Protection'' while the approach for the human-intrusion
assessment is consistent with the NAS recommendation (see the What Is
the Standard for Human Intrusion? section later in this notice). We
request public comment upon whether this approach is appropriate for
Yucca Mountain.
III.C.2. Is Expert Opinion Allowed?
The quantitative requirements in proposed subpart B of part 197
require:
(1) Evaluation of processes, events, and sequences of processes and
events leading to radionuclide releases from the disposal system;
(2) Estimation of the resulting doses or radionuclide
concentrations; and
(3) Estimation of the likelihood of the resulting doses or
radionuclide concentrations.
The likelihood of the processes, events, and sequences of processes
and events occurring should be estimated by DOE and NRC based upon
current scientific knowledge of previous occurrences. However, it is
likely that there will be processes, events, and sequences of processes
and events which have not occurred or occurred too infrequently to be
statistically significant. This situation will require the use of
expert opinion, for example, scientific and engineering expertise, to
arrive at cautious, but reasonable, estimates of the probability of
future occurrence. Also, there likely will be many other areas where
DOE could use expert opinion, for example, when there are multiple
models applicable to the performance assessment or human-intrusion
analysis, or significant uncertainties in the variation of parameter
values.
There are two commonly used methods for the gathering of expert
opinion, namely, expert judgment and expert elicitation. Expert
judgment is typically obtained informally from one or more individuals
and is noted by the person(s) seeking the judgment in documentation
used to support the activity. In contrast, expert elicitation is a
formal, structured, and thoroughly documented process. Whether it is
appropriate to conduct an expert elicitation depends upon the issue
under consideration.
We have considered setting guidelines for the
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