The Japanese Nuclear Incident: Technical Aspects

The Japanese Nuclear Incident:

Technical Aspects

Jonathan Medalia

Specialist in Nuclear Weapons Policy

April 5, 2011

Congressional Research Service

7-....

www.crs.gov

R41728

CRS Report for Congress

Prepared for Members and Committees of Congress

The Japanese Nuclear Incident: Technical Aspects

Summary

Japan’s nuclear incident has engendered much public and congressional concern about the

possible impact of radiation on the Japanese public, as well as possible fallout on U.S. citizens.

This report provides information on technical aspects of the nuclear incident, with reference to

human health.

While some radioactive material from the Japanese incident may reach the United States, it

appears most unlikely that this material will result in harmful levels of radiation. In traveling

thousands of miles between the two countries, some radioactive material will decay, rain will

wash some out of the air, and its concentration will diminish as it disperses.

Many atoms are stable; they remain in their current form indefinitely. Other atoms are unstable,

or radioactive. They “decay” or “disintegrate,” emitting energy through various forms of

radiation. Each form has its own characteristics and potential for human health effects.

Nuclear reactors use uranium or mixed oxides (uranium oxide and plutonium oxide, or MOX) for

fuel. Uranium and plutonium atoms fission, or split, releasing neutrons that cause additional

fissions in a chain reaction, and also releasing energy. A nuclear reactor’s core consists of fuel

rods made of uranium or MOX encased in zirconium, and neutron-absorbing control rods that are

removed or inserted to start or stop the chain reaction. This assembly is placed underwater to

carry off excess heat. The incident at the Fukushima Daiichi Nuclear Power Plant prevented water

from circulating in the core of several reactors, causing water to evaporate and temperature to

rise. High heat could melt the fuel rods and lead to a release of radioactive material into the air.

When uranium and plutonium fission, they split into smaller atoms that are highly radioactive and

generate much heat; indeed, fuel rods that have just been removed from a reactor are much more

radioactive, and hotter, than fuel rods before they have been inserted into a reactor. After fuel rods

can no longer efficiently produce energy, they are considered “spent” and are placed in cooling

pools of water for several years to keep them from overheating while the most radioactive

materials decay. A concern about the spent fuel pool at reactor 4 is that it may have lost most or

all of its water, yet it has more fuel rods than pools at the other five reactors, as it contains all the

active fuel rods that were temporarily removed from the reactor core in November 2010 to permit

plant maintenance in addition to spent fuel rods.

A nuclear reactor cannot explode like an atomic bomb because the concentration of the type of

uranium or plutonium that fissions easily is too low to support a runaway chain reaction, and a

nuclear weapon requires one of two configurations, neither of which is present in a reactor.

Some types of radiation have enough energy to knock electrons off atoms, creating “ions” that are

electrically charged and highly reactive. Ionizing radiation is thus harmful to living cells. It

strikes people constantly, but in doses low enough to have negligible effect. A concern about the

reactor incident is that it will release radioactive materials that pose a danger to human health. For

example, cesium-137 emits gamma rays powerful enough to penetrate the body and damage cells.

Ingesting iodine-131 increases the risk of thyroid cancer. Potassium iodide tablets protect the

thyroid, but there is no need to take them absent an expectation of ingesting iodine-131.

This update adds a section on useful links.

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The Japanese Nuclear Incident: Technical Aspects

Contents

Introduction ..........................................................................................................................1

Could Harmful Levels of Fallout Reach the United States?....................................................1

What Is Radiation?................................................................................................................3

Radioactivity and Nuclear Reactors.......................................................................................4

Health Effects of Ionizing Radiation......................................................................................6

Sources of Radiation Exposure........................................................................................7

Measuring Exposure: Absorbed Dose v. Equivalent Dose ................................................7

External v. Internal Exposure: Effective Dose................................................................ 10

Acute Health Effects v. Long-Term Cancer Risk............................................................ 11

Potassium Iodide........................................................................................................... 11

The Japanese Situation.................................................................................................. 12

Tables

Table 1. Radiation Dose Levels ...................................................................................................9

Appendixes

Appendix. Useful Links ............................................................................................................ 14

Contacts

Author Contact Information ...................................................................................................... 18

Acknowledgments .................................................................................................................... 18

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The Japanese Nuclear Incident: Technical Aspects

Introduction

The Japanese earthquake and tsunami of March 2011 caused extensive damage to the Fukushima

Daiichi Nuclear Power Plant (NPP). This damage has released some radioactive materials, and

there are widespread fears about the health effects of current and possible future releases. These

fears, and public concern about radiation in general, have attracted the world’s attention. This

report presents scientific and technical aspects of these issues in order to provide a basis for

understanding the risks associated with this event.

Could Harmful Levels of Fallout Reach the United States?1

To monitor radiation in the United States, the Environmental Protection Agency (EPA) operates

RadNet, which “is a national network of monitoring stations that regularly collect air,

precipitation, drinking water, and milk samples for analysis of radioactivity. The RadNet network,

which has stations in each state, has been used to track environmental releases of radioactivity

from nuclear weapons tests and nuclear accidents.”2 EPA has an online map of these stations,3 and

provides updates on the results of its air monitoring as it relates to the Japanese nuclear incident.4

Whether harmful levels of radioactive material from the incident reach the United States depends

on many factors:

•

Particle size: Tiny particles are more readily carried by the wind and can travel

farther than large particles, which fall to Earth more rapidly.

•

Wind patterns.

•

Amount of material released: The more material released, the more likely some

of it is to travel long distances.

•

Melt vs. burn: If nuclear fuel rods (fresh or spent) melt and form a pool of very

hot, highly radioactive liquid, that liquid might be contained by a containment

structure. If it melts through that structure, it might contaminate groundwater. If

the fuel rods burn, the fire would loft radioactive material into the air. The larger

and hotter the fire, and the longer it burns, the more material would be injected

into the air.

•

Travel time: The longer radioactive material is in the air, the more of it will

decay.

1

This section was written by Jonathan Medalia, Specialist in Nuclear Weapons Policy, Foreign Affairs, Defense, and

Trade Division.

2

U.S. Environmental Protection Agency. “RadNet—Tracking Environmental Radiation Nationwide,”

http://www.epa.gov/narel/radnet/.

3

U.S. Environmental Protection Agency. “RadNet Map View,” https://cdxnode64.epa.gov/radnet-public/showMap.do.

4

U.S. Environmental Protection Agency. “Japanese Nuclear Emergency: EPA’s Radiation Air Monitoring,”

http://www.epa.gov/japan2011/.

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•

Distance: The farther radioactive material travels, the greater the volume of air in

which the material disperses, diluting it.

•

Rain and snow: Precipitation washes some particles out of the air.

The first four of these factors depend on circumstances; the other three would reduce the amount

of material reaching the United States under any circumstances.

According to U.S. nuclear authorities, the reactor incident does not appear to pose an immediate

threat to the United States. On March 13, the Nuclear Regulatory Commission (NRC) stated,

“Given the thousands of miles between the two countries [United States and Japan], Hawaii,

Alaska, the U.S. Territories and the U.S. West Coast are not expected to experience any harmful

levels of radioactivity.”5 On March 18, EPA and the Department of Energy stated that a

monitoring station in Sacramento “today … detected minuscule quantities of iodine isotopes and

other radioactive particles that pose no health concern at the detected levels,” and that between

March 16 and 17, a detector in Washington State detected “trace amounts of Xenon-133, which is

a radioactive noble gas produced during nuclear fission that poses no concern at the detected

level.”6 In a briefing to the Nuclear Regulatory Commission on March 21, Bill Borchardt, NRC

Executive Director for Operations, said, “natural background from things like … rocks, sun,

buildings, is 100,000 times more than any level that has been detected to date. We feel confident

in our conclusion that there is no reason for concern in the United States regarding radioactive

releases from Japan.”7 A press report of March 22 stated that equipment in Charlottesville, VA,

detected radiation from the reactor incident, but that “health experts said that the plume’s

radiation had been diluted enormously in its journey of thousands of miles and that—at least for

now, with concentrations so low—its presence will have no health consequences in the United

States.”8

It is useful to put these doses in perspective. Using the figure that natural sources provide 100,000

times the dose recorded in California and Washington state, it is possible to calculate a rough

approximation of the dose from the Japanese incident, using the improbable assumption that the

dose persists at the detected rate for an entire year. As discussed later, a report estimates that the

average American receives a dose of 310 millirem (mrem) per year from natural sources. (Units

of radiation dose are discussed under “Health Effects of Ionizing Radiation.”) NRC requires its

licensees to “limit maximum radiation exposure to individual members of the public” to 100

mrem per year. One-one-hundred-thousandth of 310 mrem per year is a dose of 0.00310 mrem

per year. At that rate, it would take 32,258 years to accumulate a dose of 100 mrem; over a 70year lifespan, the cumulative dose at this rate would amount to 0.22 mrem.

5

U.S. Nuclear Regulatory Commission. “NRC Sees No Radiation at Harmful Levels Reaching U.S. from Damaged

Japanese Nuclear Power Plants,” press release no. 11-046, March 13, 2011, http://pbadupws.nrc.gov/docs/ML1107/

ML110720002.pdf.

6

U.S. Department of Energy and Environmental Protection Agency. “Joint EPA/DOE Statement: Radiation Monitors

Confirm That No Radiation Levels of Concern Have Reached the United States,” press release, March 18, 2011,

http://www.energy.gov/news/10190.htm.

7

U.S. Nuclear Regulatory Commission. “Briefing on NRC Response to Recent Nuclear Events in Japan,” public

meeting, March 21, 2011, p. 13, http://www.nrc.gov/reading-rm/doc-collections/commission/tr/2011/20110321.pdf.

8

William Broad, “Radiation over U.S. Is Harmless, Officials Say,” New York Times, March 22, 2011, p. 6.

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What Is Radiation?9

Many atoms are stable: they will remain in their current form indefinitely. Some atoms are

unstable, or radioactive. They “decay” or “disintegrate,” often transforming into atoms of a

different element, such as through emission of radiation, which permits the atom to reach a more

stable state.10 The most common types of radiation emitted in decay, and their characteristics, are:

•

Alpha particles are two protons plus two neutrons. They are electrically charged

and massive by subatomic standards, and travel relatively slowly, so they lose

energy quickly in matter. They travel only an inch in air, and are stopped by a

sheet of paper or the dead outer layers of skin.

•

Beta particles (an electron or positron11) are electrically charged, so are readily

absorbed by matter, but are much less massive than alpha particles or neutrons.

Depending on their energy, some are stopped by outer layers of skin, while others

can penetrate several millimeters. They can travel up to several feet in air.

•

Neutrons are typically emitted by heavy atoms like uranium and plutonium. They

have no electrical charge and may be highly penetrating, depending on their

speed. They can travel tens of meters in air; energetic neutrons can penetrate the

body. They can be slowed down by hydrogen-containing material like water.

•

Gamma rays are photons released during radioactive decay. Photons may be

thought of as packets of electromagnetic energy; radio waves, light, and x-rays

are less-energetic photons. Gamma ray energies vary widely. Those of medium to

high energies are highly penetrating and can travel hundreds of meters in air.

Stopping them requires a thick layer of a dense material like lead.

Several measurements are useful in discussing radioactivity. Radioactivity is measured in units of

curies (Ci), where 1 Ci = 3.7 x 1010 disintegrations per second, or becquerels (Bq), where 1 Bq =

1 disintegration per second. (The curie is widely used in the United States; the Becquerel is more

widely used internationally.) Specific activity—curies per gram—measures how radioactive a

material is. Half-life is the time for half of the atoms in a mass of a particular type of radioactive

material to decay. Specific activity is inversely related to half-life. For example, radioactive

iodine-131 is intensely radioactive. It has a specific activity of 124,000 curies per gram and a

half-life of 8 days; in 10 half-lives (80 days), 99.9% of the iodine-131 created at a given time will

have decayed. In contrast, uranium-235 has a specific activity of 0.000002 curies per gram and a

half-life of 700 million years; it would take 7 billion years (10 half-lives) for 99.9% of it to

decay.12 According to Richard Firestone, staff scientist, Lawrence Berkeley National Laboratory,

9

This section was written by Jonathan Medalia, Specialist in Nuclear Weapons Policy, Foreign Affairs, Defense, and

Trade Division.

10

For descriptions of radiation, see Roger Eckhardt, “Ionizing Radiation—It’s Everywhere,” Los Alamos Science, no.

23, 1995, http://www.fas.org/sgp/othergov/doe/lanl/00326627.pdf, and U.S. Environmental Protection Agency,

“Radiation: Ionizing and Non-Ionizing,” http://www.epa.gov/radiation/understand/index.html.

11

A positron is a positively-charged electron.

12

For data on half-lives and other characteristics of radionuclides, see Lawrence Berkeley National Laboratory,

“Exploring the Table of Isotopes,” http://ie.lbl.gov/education/isotopes.htm, and U.S. Department of Energy. Office of

Environmental Management. “Table B.1. Characteristics of important radionuclides,” http://www.orau.org/ptp/

PTP%20Library/library/DOE/Misc/Table%20B_1_%20Characteristics%20of%20Important%20Radionuclides.htm.

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uranium-235 emits so little radiation that “holding a piece in the hand would cause negligible

radiation exposure.”13

Energy released per decay is measured differently. A standard measure is the electron volt or,

more commonly, thousands of electron volts (keV). 14 The penetrating power of gamma rays, and

thus their threat to human health, increases as their energy increases.

Each radioactive atom, or “radionuclide,” decays in a specific way. For example, when uranium235 decays,15 it emits gamma rays, most of which are of 186 keV (a low energy) or less, and

alpha particles; cesium-137 emits gamma rays, virtually all of which are of 662 keV, a medium

energy, and beta particles. Each radionuclide that emits gamma rays does so in a unique pattern,

or “spectrum,” of energies that is the primary characteristic used to identify many radionuclides.

Radioactivity and Nuclear Reactors16

Some heavy atoms, such as uranium-235 and plutonium-239, “fission” when struck by a neutron.

In fission, an atom typically (1) splits into two lighter atoms, called “fission products”; (2)

releases two or three neutrons; and (3) emits vast quantities of radiation. Fission products are

often highly radioactive, such as cesium-137, iodine-131, and strontium-90.

Uranium-235 and plutonium-239 can support a nuclear chain reaction: to oversimplify, one

neutron fissions one atom, which releases two neutrons that fission two atoms, releasing four

neutrons that fission four atoms, and so on. Neutrons thus drive chain reactions; this is a key

concept for understanding nuclear reactors. A supercritical mass supports an increasing rate of

fission; fission diminishes in a subcritical mass; and fission proceeds at a constant rate in a critical

mass. In an atomic bomb, a supercritical mass of uranium or plutonium supports a chain reaction

that proceeds in a tiny fraction of a second, releasing vast quantities of energy. A nuclear reactor

is designed to maintain a constant rate of fission. If fission proceeds too quickly, it gets out of

control, in which case the fuel rods generate so much heat that they melt. When control rods are

inserted into the reactor core, individual atoms continue to fission but the chain reaction stops.

Control rods typically contain boron or cadmium because they are efficient neutron absorbers.

(Because boron absorbs neutrons, it was added to cooling water in the Fukushima Daiichi NPP

incident to prevent inadvertent criticality.) Fission that proceeds at the desired rate releases energy

over several years from one load of fuel. The energy heats water to generate steam that spins

turbines to generate electricity.

A nuclear reactor cannot explode like an atomic bomb because the fuels and configurations differ.

In nature, uranium is 99.3% uranium-238 and 0.7% uranium-235. Only the latter is “fissile,” that

13

Personal communication, March 30, 2011.

“An electron volt is a measure of energy. An electron volt is the kinetic energy gained by an electron passing through

a potential difference of one volt.” Fermi National Accelerator Laboratory, “How Big Is an Electron Volt?,”

http://www-bd.fnal.gov/public/electronvolt.html.

15

The number following the name of an element is the number of protons plus neutrons in the nucleus.

16

This section was written by Jonathan Medalia, Specialist in Nuclear Weapons Policy, Foreign Affairs, Defense, and

Trade Division, and (name redacted), Specialist in Energy Policy, Resources, Science, and Industry Division. See also CRS

Report R41694, Fukushima Nuclear Crisis, by (name redacted) and (name redacted). For the status of each reactor, see

“Status of the Nuclear Reactors at the Fukushima Daiichi Power Plant,” New York Times, http://www.nytimes.com/

interactive/2011/03/16/world/asia/reactors-status.html., and Japan, Nuclear and Industrial Safety Agency,

http://www.nisa.meti.go.jp/english/.

14

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is, it will fission when struck by neutrons moving at relatively slow speeds. To make fuel for a

bomb or a reactor, the fraction of uranium-235 must be increased through “enrichment.”17 An

atomic bomb uses uranium enriched to about 90% uranium-235 (“highly enriched uranium,”

HEU), while nuclear reactor fuel is typically enriched to less than 5% (“low enriched uranium,”

LEU). LEU does not have enough uranium-235 to support a chain reaction of the sort found in an

atomic bomb. In addition, a bomb must be configured in one of two ways to create a large enough

mass to support a runaway chain reaction; reactors are arranged in an entirely different

configuration.

A nuclear reactor uses pellets of LEU or mixed oxides (MOX, i.e., uranium oxide and plutonium

oxide) for fuel. Fuel rods—thin zirconium tubes typically between 12 and 15 feet long—hold the

fuel. According to one report,

Zirconium is the metal of choice in this application because it absorbs relatively few of the

neutrons produced in a fission reaction and because the metal is highly resistant to both heat

and chemical corrosion.

Low neutron absorption is vital to any structural material used in a nuclear reactor because

large numbers of neutrons produced by the reaction must be free to interact simultaneously

with all the nuclear fuel confined inside hundreds of fuel rods. This interaction sustains the

necessary chain reaction throughout the reactor’s core.18

Even with control rods fully inserted to halt the nuclear chain reaction, the radioactive decay of

the fuel rods (primarily from fission products) generates heat, which must be dissipated. At the

Fukushima Daiichi NPP, cooling was done by pumping cool water into the reactor. If the heat is

not dissipated, the rods become so hot that they melt or burn. A fire would loft particles of

radioactive material into the air. If fuel rods become too hot, their zirconium cladding may also

react with water and produce hydrogen. The Fukushima Daiichi NPP primary containments used

inert nitrogen gas to preclude hydrogen ignition. However, the operators had to vent the primary

containment to relieve pressure, introducing hydrogen into the secondary containment, which is

believed to have caused the explosions at reactor units 1-3.19 This explains the urgency of the

efforts to keep the fuel rods cool, and why the reactors suffered major damage when backup

cooling systems failed.

In order to cool the fuel rods, personnel have been spraying huge amounts of seawater into the

reactors and spent fuel pools. However, when seawater boils away from the heat of the fuel rods,

it leaves behind large quantities of salt.

The big question is how much of that salt is still mixed with water, and how much now

forms a crust on the reactors’ uranium fuel rods. Chemical crusts on uranium fuel rods have

been a problem for years at nuclear plants.

Crusts insulate the rods from the water and allow them to heat up. If the crusts are thick

enough, they can block water from circulating between the fuel rods. As the rods heat up,

17

For information on the enrichment process, see U.S. Nuclear Regulatory Commission. “Fact Sheet on Uranium

Enrichment,” May 15, 2009, http://www.nrc.gov/reading-rm/doc-collections/fact-sheets/enrichment.html.

18

“Zirconium: Covering for Fuel Rods,” New York Times, June 9, 1995, http://www.nytimes.com/1995/06/09/

nyregion/zirconium-covering-for-fuel-rods.html.

19

Information provided by Nuclear Regulatory Commission, personal communication, March 25, 2011.

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their zirconium cladding can ignite, which may cause the uranium inside to melt and release

radioactive material.20

To alleviate this problem, workers have begun using fresh water instead of seawater. 21

As the fuel fissions in a reactor, the fraction of fission products in fuel rods increases. When the

ratio of fission products to fissile material rises to the point at which a fuel rod can no longer

efficiently maintain a chain reaction, it is referred to as spent fuel. “Spent” seems to imply that

the fuel has been used up, and is therefore less dangerous, than fresh uranium fuel, but this is not

necessarily the case. When fuel rods are first removed from a nuclear reactor, they have a high

level of short-lived radionuclides, unlike new fuel rods, so they are intensely radioactive. This

radioactivity generates intense heat, so spent fuel rods are placed in pools of water to cool them,

typically for several years, until most of the short-lived radionuclides decay. The water also

provides shielding against any radioactive release into the air, and the spent fuel pools have no

hardened containment structure that would protect against radiation release. If a pool is drained,

the fuel rods would heat up, melt, and perhaps burn. This possibility led to concern about the

spent fuel rods at Fukushima Daiichi NPP reactor 4:

The spent fuel pools can be even more dangerous than the active fuel rods, as they are not

contained in thick steel containers like the reactor core. As they are exposed to air, the

zirconium metal cladding on the rods can catch fire, and a deadly mix of radioactive

elements can spew into the atmosphere.…

According to Tokyo Electric [Power Company]’s data, the spent fuel pool at the No. 4

reactor contains 548 fuel assemblies that were in use at the reactor until last November, when

they were moved to the storage pool on the site. That means that the fuel rods were only

recently taken out of active use and that their potential to burn and release radioactivity is

higher than spent fuel in storage for a longer period.22

Another danger comes from the potential release of plutonium from the MOX fuel used at reactor

3. Even very small amounts of plutonium, if inhaled, can potentially cause lung cancer. This

explains the concern about that reactor, as it is the only one that uses MOX fuel, although

irradiation of uranium fuel also creates plutonium. Water is being pumped into the spent fuel

pools at the Fukushima Daiichi NPP reactors as well to cool the fuel rods and prevent additional

radiation release.

Health Effects of Ionizing Radiation23

Humans are continuously exposed to significant amounts of ionizing radiation from various

naturally occurring and manmade sources. Because of its relatively high energy level, ionizing

radiation is capable of producing significant biological change. Ionizing radiation gets its name

20

Keith Bradsher, “New Problems at Japanese Plant Subdue Optimism and Present a Risky Agenda,” New York Times,

March 24, 2011, p. 11.

21

David Nakamura and Steven Mufson, “Japan Urges More to Evacuate,” Washington Post, March 26, 2011, p. 1, and

“Nuclear Energy—Crisis in Japan,” New York Times, update of March 30, 2011.

22

David Sanger, Matthew Wald, and Hiroko Tabuchi, “U.S. Sees ‘Extremely High’ Radiation Level at Plant, Focusing

on Spent Fuel’s Impact,” New York Times, March 17, 2011, p. 13.

23

This section was written by Jonathan Medalia, Specialist in Nuclear Weapons Policy, Foreign Affairs, Defense, and

Trade Division, and (name redacted), Specialist in Health Policy, Domestic Social Policy Division.

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from the fact that it causes ionization—ejection of electrons—when it interacts with atoms in the

molecules that constitute cells and tissue. This process creates charged, often unstable, and highly

reactive entities. The ensuing reactions may result in permanent molecular damage. Radiation

disrupts cell division, which is why the most sensitive tissues are those in which cells frequently

divide, such as skin, hair, bone marrow (where precursor cells give rise to new blood cells), and

the cells that line the stomach and small intestine. Ionizing radiation may also damage DNA in

chromosomes, resulting in mutations that are responsible for long-term effects such as the

development of cancer.

Sources of Radiation Exposure

Naturally occurring sources of ionizing radiation to which all humans are exposed include cosmic

radiation from outer space and terrestrial radiation from radioactive materials in rock deposits and

soil. The Earth’s atmosphere acts as a shield against cosmic radiation, so exposure levels increase

with altitude (especially when flying). The most important source of terrestrial exposure is the

inhalation of radon, which is produced by the radioactive decay of naturally occurring uranium.

In the United States, radiation exposure as a result of medical practice has increased significantly

over the past 25 years as a result of the growing use of CT scans and nuclear medicine procedures

to diagnose and treat disease. Other manmade sources of radiation account for a relatively small

fraction of the U.S. population’s total exposure. Those sources include consumer products (e.g.,

cigarettes, building materials, appliances); industrial, security, educational, and research

activities, including nuclear power generation; and various types of occupational exposure.

Measuring Exposure: Absorbed Dose v. Equivalent Dose

Human exposure is measured by the amount of energy that ionizing radiation deposits in a unit

mass of tissue. This is called the absorbed dose. The international unit for the absorbed dose is

the gray (Gy), which replaced an earlier unit of dose, the rad (short for “radiation absorbed

dose”). One gray equals 100 rad. The biological impact of ionizing radiation, however, depends

not just on the absorbed dose (i.e., the amount of energy absorbed) but on the type of radiation.

For example, an alpha particle is more damaging to biological tissue than a beta particle or

gamma radiation because of its mass, electrical charge, and slow speed. Alpha particles lose their

energy much more densely along the relatively short path they travel though biological tissue.

Thus, 1 Gy of alpha radiation is more harmful than 1 Gy of beta or gamma radiation.

Radiation scientists use another quantity, called equivalent dose, which allows them to measure

all types of exposure on an equal basis. Equivalent dose is equal to the absorbed dose multiplied

by a factor that takes into account the relative effectiveness of each type of radiation to cause

harm. For beta particles and gamma radiation, the factor is set at 1; that is, the absorbed dose

equals the equivalent dose. For alpha particles the factor is set at 20, which means that the

equivalent dose is 20 times the absorbed dose. This reflects the fact that alpha radiation is more

harmful than beta and gamma radiation. The international unit for the equivalent dose is the

sievert (Sv). So, 1 Sv of alpha radiation to the lung would create the same risk of lung cancer as 1

Sv of beta radiation. The sievert is a large unit relative to common exposures, so the more

common unit is the millisievert (mSv), which is one-thousandth of a sievert. The sievert replaced

an earlier unit of equivalent dose, the rem, which is still widely used in the United States. One

sievert = 100 rem; 1 mSv = 100 millirem (mrem).

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The National Council on Radiation Protection and Measurement (NCRP) estimates that the

average annual equivalent dose to an individual in the United States is 6.2 mSv (620 mrem).24 Of

that amount, 3.1 mSv (310 mrem) is from natural background sources, primarily inhalation of

radon and its decay products, and 3.0 mSv (300 mrem) is from diagnostic and therapeutic medical

procedures. The remaining 0.1 mSv (10 mrem) is from consumer products, industrial activities,

and occupational exposure, among other sources. For comparison, the radiation dose from a jet

airplane flight is 0.5 millirems (mrem) per hour in the air; from a chest x-ray, 6 mrem; and from

living at an altitude of one mile, about 50 mrem/year.25 Table 1 shows various doses and their

health consequences or regulatory limits.

24

National Council on Radiation Protection and Measurement, “Ionizing Radiation Exposure of the Population of the

United States,” report no. 160, 2009.

25

American Nuclear Society, “Radiation Dose Chart,” http://www.ans.org/pi/resources/dosechart/. This interactive

chart permits the user to adjust values to find an approximation of his or her total annual dose.

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Table 1. Radiation Dose Levels

Dose,

mSv

Dose,

rem

Source

1/yr

0.1/yr

(2)

NRC requires its licensees to “limit maximum radiation exposure to individual

members of the public” to this level.

6.2/yr

0.62/yr

(1)

Average U.S. individual’s total effective radiation dose in 2006; half is from natural

background and half is from medical uses and other human activities.

20

2

(7)

Federal Emergency Management Agency and Environmental Protection Agency

recommend relocating the public from an area if the expected dose in the first

year after a radiological incident is above this level.

50/yr

5/yr

(2)

NRC requires its licensees to “limit occupational radiation exposure to adults

working with radioactive materials” to this level.

100

10

(6)

A National Research Council committee defines “low dose” of certain types of

ionizing radiation, such as gamma rays, as this level or below.

0-250

0-25

(3)

For an “acute” (i.e., received over a short time) whole-body external dose of

ionizing radiation, “No detectable clinical effects; small increase in risk of delayed

cancer and genetic effects.”

250

25

(4)

Japan raised the permitted dose for emergency workers at the Fukushima Daiichi

NPP from 100 mSv/10 rem to this level.

500

50

(5)

For an acute whole-body external dose of ionizing radiation, “blood count

changes.”

1,0002,000

100200

(3)

For an acute whole-body external dose of ionizing radiation, “Minimal symptoms;

nausea and fatigue with possible vomiting; reduction in [certain white blood cells],

with delayed recovery.”

2,0003,000

200300

(3)

For an acute whole-body external dose of ionizing radiation, “Nausea and vomiting

on first day; following latent period of up to 2 weeks, symptoms (loss of appetite

and general malaise) appear but are not severe; recovery likely in about 3 months

unless complicated by previous poor health.”

3,2003,600

320360

(5)

Half the population exposed to an acute whole-body external dose of ionizing

radiation will die within 60 days despite receiving minimal supportive care.

3,5005,000

350500

(2)

NRC believes that half the population receiving this dose in a few hours or less

would die within 30 days.

8,000

800

(5)

100% mortality, despite best available treatment, for people receiving this external

dose of whole-body ionizing radiation.

Comments

Sources: (1) National Council on Radiation Protection and Measurement, “Ionizing Radiation Exposure of the

Population of the United States,” report no. 160, 2009, p. 11. (2) U.S. Nuclear Regulatory Commission. “Fact

Sheet on Biological Effects of Radiation,” January 2011, http://www.nrc.gov/reading-rm/doc-collections/factsheets/bio-effects-radiation.html, and 10 CFR 20. (3) Dade Moeller, Environmental Health, revised edition,

Cambridge, Harvard University Press, 1997, p. 250. (4) Keith Bradsher and Hiroko Tabuchi, “50 Workers

Bravely Stay at Troubled Japan Reactors,” New York Times, March 16, 2011. (5) Princeton University,

Environmental Health and Safety. “Open Source Radiation Safety Training, Module 3: Biological Effects,”

http://web.princeton.edu/sites/ehs/osradtraining/biologicaleffects/page.htm, adapted from National Council on

Radiation Protection and Measurements, Report No. 98, “Guidance on Radiation Received in Space Activities,”

Bethesda, MD, 1989. (6) National Research Council, Committee to Assess Health Risks from Exposure to Low

Levels of Ionizing Radiation, “Health Risks from Exposure to Low Levels of Ionizing Radiation,” BEIR [Biological

Effects of Ionizing Radiation] VII Phase 2, p. 2, http://www.nap.edu/openbook.php?record_id=11340&page=1 and

click on PDF Summary. (7) U.S. Environmental Protection Agency. Office of Radiation Programs. Manual of

Protective Action Guides and Protective Actions for Nuclear Incidents, revised 1991 (second printing, May 1992), p. 4-4,

http://www.epa.gov/radiation/docs/er/400-r-92-001.pdf, and Federal Emergency Management Agency, “Planning

Guidance for Protection and Recovery Following Radiological Dispersal Device (RDD) and Improvised Nuclear

Device (IND) Incidents,” 73 Federal Register 45034, August 1, 2008.

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External v. Internal Exposure: Effective Dose

The health risks of ionizing radiation can occur as a result of both external and internal exposure.

External exposure is almost exclusively from radioactive material that emits gamma radiation,

which is very penetrating and, at higher energies, can only be stopped by a thick layer of lead or

concrete. External sources of gamma radiation produce a whole-body exposure. Importantly, the

level of exposure to gamma radiation falls off sharply with distance from the source. Cesium-137

(137Cs), which has a half-life of 30 years, is the most common source of gamma radiation from

nuclear weapons tests and reactor accidents.

Alpha and beta particles outside the body are typically not a source of external exposure. Alpha

particles travel only a few centimeters through the air and cannot penetrate clothing or the

outermost dead layer of skin. Beta particles, composed of electrons or positrons, can travel at

most several feet through the air and penetrate to the live layer of skin causing burns (as

happened to workers at Chernobyl). But they too are blocked by radiation suits.

Internal radiation exposure occurs through the inhalation of airborne radioactive material or the

ingestion of contaminated food and drink. The potential for harm depends on the type and

quantities of radioactive material taken in and the length of time they remain in the body. As

already noted, isotopes that emit alpha particles present a greater hazard than those that emit beta

particles and gamma radiation. In addition, the fate of the radioactive material depends on its

chemical identity. For example, Strontium-90 (90Sr), which is chemically similar to calcium and

emits beta particles, accumulates in bone and can cause leukemia and bone cancer.

Iodine-131 (131I), another beta emitter, tends to accumulate in the thyroid gland, where it is used

in the synthesis of thyroid hormones. Beta radiation from iodine-131 damages the surrounding

cells and increase the risk of non-malignant thyroid disease and thyroid cancer. Iodine-131 from

radioactive fallout accumulates on grass and leafy crops and becomes concentrated in the milk of

cows and goats that feed on the contaminated vegetation. Children who drink the contaminated

milk are especially at risk because they are still growing and their thyroid glands are very active.

However, iodine-131 has a half-life of only 8 days, so it decays relatively quickly on the ground,

in the food chain, and in the body.

Iodine-131 posed the most important health risk following the incident at the Chernobyl nuclear

power plant in 1986. According to the International Atomic Energy Agency:

The main consequence of the Chernobyl accident is thyroid cancer in children, some of

whom were not yet born at the time of the accident. Following the vapour [sic] explosion and

fire at the Chernobyl reactor, radioactive iodine was released and spread in the surrounding

area. Despite measures taken, children in southern Belarus and northern Ukraine, were

exposed to radiation in the weeks following the accident, particularly by consuming milk

from pastured cows and leafy vegetables that had been contaminated with radioactive

iodine.26

Unlike whole-body external exposures, the exposure from ingested or inhaled radioactive

material is often limited to certain parts of the body or even specific organs. Radiation scientists

26

International Atomic Energy Agency, “Thyroid Cancer Effects in Children,” staff report, August 2005,

http://www.iaea.org/newscenter/features/chernobyl-15/thyroid.shtml.

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The Japanese Nuclear Incident: Technical Aspects

are able to calculate a whole-body equivalent dose, or effective dose, for partial-body exposures.

These amounts can be summed with external exposure to calculate a total dose.

Acute Health Effects v. Long-Term Cancer Risk

The health effects of ionizing radiation exposure depend on the total dose and dose rate.

Radiation health experts distinguish between (1) acute, or short-term, effects such as radiation

sickness that are associated with relatively high doses over a short period; and (2) long-term

effects such as increased lifetime cancer risk that result from chronic exposure to low levels of

radiation. Short-term health effects are typically seen in workers and others in close proximity to

nuclear weapons tests and accidents, while the long-term cancer risks apply to the general

population. Scientists calculate the cancer risk from radiation exposure using data from

epidemiological and other studies, such as those following the health outcomes of the Japanese

atomic bomb survivors. According to the International Commission on Radiological Protection

(ICRP), the lifetime risk of contracting a fatal cancer from chronic exposure to low-level

radiation exposure is 0.05 per sievert, or 1 in 20 per sievert (i.e., 1 in 2,000 per rem). The ICRP

and NCRP both recommend an annual exposure limit of 1 mSv (100 mrem) for members of the

general population. An individual that received that much annual exposure over a 70-year lifetime

(a total of 70 mSv, or 7 rem) would, as a result, have an increased risk of cancer death of

approximately 1 in 300.

Table 1 summarizes the health effects of exposure to various acute doses of ionizing radiation.

For comparison, the table also includes the current exposure standards for the general public and

workers, and the average background radiation exposure in the United States.

Potassium Iodide

There is considerable interest in potassium iodide (also referred to by its chemical formula, KI)

tablets to protect against thyroid cancer. These tablets contain non-radioactive iodine-127, the

same type used in iodized table salt, to saturate the thyroid with iodine. Once the thyroid is

saturated, it cannot absorb more of any isotope of iodine, including iodine-131. As a result,

potassium iodide tablets, taken shortly before exposure to iodine-131, offer protection from

thyroid cancer. The protection is of limited duration, however, and potassium iodide protects only

the thyroid only against radioactive iodine. It does not protect against any other radioactive

material or against radiation in general. Nor is there value in taking potassium iodide as a

precautionary measure unless iodine-131 is expected to be present. As the next section of this

report discusses, the amount of radioactive material that has reached the United States from the

Japanese nuclear reactor incident is minuscule. Accordingly, the website of the Centers for

Disease Control and Prevention, accessed on March 22, said, “At this time, CDC does not

recommend that people in the United States take KI or iodine supplements in response to the

nuclear power plant explosions in Japan. You should only take KI on the advice of emergency

management officials, public health officials, or your doctor. There are health risks associated

with taking KI.”27 Further, “Some general side effects caused by KI may include intestinal upset,

allergic reactions (possibly severe), rashes, and inflammation of the salivary glands.”28

27

U.S. Department of Health and Human Services. Centers for Disease Control and Prevention. “Emergency

Preparedness and Response: Radiation and Potassium Iodide (KI),” http://www.bt.cdc.gov/radiation/japan/ki.asp.

28

U.S. Department of Health and Human Services. Centers for Disease Control and Prevention. “Emergency

(continued...)

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The Japanese Situation

Understanding dose and its health effects casts light on the Japanese situation. The (U.S.)

Committee to Assess Health Risks from Exposure to Low Levels of Ionizing Radiation of the

National Research Council reported on the health risks from a certain type of radiation that

includes gamma rays and x-rays. It considered doses below about 100 mSv (10 rem) to be low

doses. The committee found that many factors “make it difficult to characterize the effects of

ionizing radiation at low levels,” and that “at doses less than 40 times the average yearly

background exposure (100 mSv), statistical limitations make it difficult to evaluate cancer risk in

humans.” To develop an estimate of risk, the committee constructed a “lifetime risk model [that]

predicts that approximately 1 person in 100 would be expected to develop cancer (solid cancer or

leukemia) from a dose of 0.1 Sv [10 rem] above background.” For comparison, about 42% of the

population will be diagnosed with cancer in their lifetimes.29 At Fukushima Daiichi NPP,

The workers are being asked to make escalating—and perhaps existential—sacrifices that so

far are being only implicitly acknowledged: Japan’s Health Ministry said Tuesday that it was

raising the legal limit on the amount of radiation exposure to which each worker could be

exposed, to 250 millisieverts from 100 millisieverts, five times the maximum exposure

permitted for nuclear plant workers in the United States.

The change means that workers can now remain on site longer, the ministry said. “It would

be unthinkable to raise it further than that, considering the health of the workers,” the health

minister, Yoko Komiyama, said at a news conference. 30

An acute dose of 250 mSv (25 rem) is the upper threshold at which dose is unlikely to cause

noticeable health effects, but it increases the risk of cancer. Based on the National Research

Council report, 25 of 1,000 people would be expected to develop solid cancers or leukemia as a

result of receiving this dose. Workers exposed to this dose will probably not be allowed to be

exposed to additional radiation above background for at least a year to give their bodies time to

repair cell damage.

Beyond the Fukushima Daiichi NPP, the external doses reported fall far below the low-dose

threshold of the U.S. Nuclear Regulatory Commission (NRC). Japan’s Ministry of Education,

Culture, Sports, Science and Technology reported dose readings from 80 monitoring stations

between 25 and 60 km from the Fukushima Daiichi NPP.31 On March 20, almost all the readings

were less than 15 microsieverts per hour. (One millisievert = 1,000 microsieverts; 1 microsievert

= 0.1 millirem.) At a rate of 15 microsieverts per hour, it would take 278 days to accumulate a

dose of 10 rem. At the highest rate reported, 110 microsieverts per hour, it would take 38 days to

accumulate that dose. Staying inside an uncontaminated building would reduce exposure

(...continued)

Preparedness and Response: Potassium Iodide (KI),” http://emergency.cdc.gov/radiation/ki.asp#med.

29

National Research Council. Committee to Assess Health Risks from Exposure to Low Levels of Ionizing Radiation.

Health Risks from Exposure to Low Levels of Ionizing Radiation, Washington, National Academies Press, 2006, pp. 1,

2, 7, 8, http://www.nap.edu/openbook.php?record_id=11340&page=1, and click on “pdf summary.”

30

Keith Bradsher and Hiroko Tabuchi, “50 Workers Bravely Stay at Troubled Japan Reactors,” New York Times,

March 16, 2011.

31

Japan. Ministry of Education, Sports, Culture, Science and Technology (MEXT), “Readings at Monitoring Post out

of 20 Km Zone of Fukushima Dai-ichi NPP [Nuclear Power Plant],” news release, as of 19:00 March 20, 2011,

http://www.mext.go.jp/component/english/__icsFiles/afieldfile/2011/03/20/1303972_2019.pdf.

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The Japanese Nuclear Incident: Technical Aspects

considerably, and short-lived radionuclides like iodine-131 (half-life, 8 days) would decay

significantly during a month or more, sharply reducing the dose they produce. On the other hand,

a larger release of radionuclides would be expected to increase dose, and cesium-137 (half-life,

30 years) decays much more slowly than iodine-131, so it would contribute to dose for many

decades.

Given the increase in thyroid cancer as a result of the Chernobyl disaster, a major concern in

Japan is minimizing the risk of thyroid cancer. This is especially important for children. At

Chernobyl, as noted earlier, ingestion of radioactive iodine-131 resulted mainly from drinking

milk from cows that ate contaminated feed, and from eating leafy greens. Accordingly, Japanese

authorities have tested spinach, other vegetables, and milk for iodine-131, and found elevated

levels. In response, on March 23 Prime Minister Naoto Kan restricted the distribution and

consumption of spinach, cabbage, broccoli, and other vegetables in Fukushima Prefecture, and

restricted the distribution of fresh raw milk and parsley produced in Ibaraki Prefecture.32 In

addition, authorities have reportedly found traces of radioactive iodine in drinking water in

Tokyo. On March 23,

Ei Yoshida, head of water purification for the Tokyo water department, said … that infants

in Tokyo and surrounding areas should not drink tap water. He said iodine-131 had been

detected in water samples at a level of 210 becquerels per liter, about a quart. The

recommended limit for infants is 100 becquerels per liter. For adults, the recommended limit

is 300 becquerels.… The Health Ministry said in a statement that it was unlikely that there

would be negative consequences to infants who did drink the water, but that it should be

avoided if possible and not be used to make infant formula.33

However, by March 24 the level was reported to be 79 becquerels per liter, and by March 27 had

diminished to the point where two readings showed no radiation and one showed 27 becquerels

per liter. 34

32

Japan. Policy Planning and Communication Division. Inspection and Safety Division. Department of Food Safety.

“Restriction of Distribution and/or Consumption of Foods Concerned in Fukushima and Ibaraki Prefectures (in

Relation to the Accident at Fukushima Nuclear Power Plant),” March 23, 2011, http://www.mhlw.go.jp/stf/houdou/

2r98520000015wun-att/2r98520000015xym.pdf.

33

David Jolly and Denise Grady, “Tokyo Says Radiation in Water Puts Infants at Risk,” New York Times, March 23,

2010.

34

David Jolly, “Radiation in Tokyo’s Water Has Dropped, Japan Says,” New York Times, March 24, 2011, and David

Jolly, Hiroko Tabuchi, and Keith Bradsher, “High Radiation Found in Water at Japan Plant,” New York Times, March

28, 2011, p. 11.

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Appendix. Useful Links35

U.S. Government

Centers for Disease Control and Prevention

“2011 Earthquake, Tsunami, and Radiation Release in Japan: Travel Information”

http://wwwnc.cdc.gov/travel/content/id/2511.aspx

“CDC Situation Awareness - 2011 Japan Earthquake”: includes some maps of Japan relevant to

the nuclear incident

http://emergency.cdc.gov/situationawareness/japan/

“Radiation Emergencies”: includes fact sheets on Japan: radiation and health

http://emergency.cdc.gov/Radiation/?s_cid=emergency_001

Congressional Research Service

CRS Report R41694, Fukushima Nuclear Crisis, by (name redacted) and (name redacted):

describes the crisis, presents status of reactors, includes a map and diagrams

Department of Agriculture

“USDA’s Radiation Safety Questions and Answers”: includes information on food imports from

Japan

http://www.usda.gov/wps/portal/usda/usdahome?contentidonly=true&contentid=

radiation_safety_qa.html

Department of Energy

“The Situation in Japan”: includes statements, congressional testimony, photos, and detailed

radiological assessments based on DOE’s Aerial Measuring System

http://blog.energy.gov/content/situation-japan

Department of State

“Embassy of the United States, Tokyo, Japan”: includes information for Americans in Japan,

updates on Tokyo water, availability of potassium iodide, travel warnings, press releases, etc.

http://japan.usembassy.gov/

35

This appendix was prepared by Jonathan Medalia, Specialist in Nuclear Weapons Policy, Foreign Affairs, Defense,

and Trade Division.

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Environmental Protection Agency

“Japanese Nuclear Emergency: EPA’s Radiation Air Monitoring”: home page

http://www.epa.gov/japan2011/

“Japanese Nuclear Emergency Radiation Monitoring: Daily Data Summary”

http://www.epa.gov/japan2011/data-updates.html

“Radiation Doses in Perspective”

http://www.epa.gov/radiation/understand/perspective.html

Food and Drug Administration

“Radiation Safety”: includes FDA actions to assess the situation in Japan and to ensure the safety

of products imported from Japan

http://www.fda.gov/NewsEvents/PublicHealthFocus/ucm247403.htm

Lawrence Berkeley National Laboratory

“Exploring the Table of Isotopes”: detailed data on isotopes of each element

http://ie.lbl.gov/education/isotopes.htm

Los Alamos National Laboratory

“Ionizing Radiation—It’s Everywhere,” by Roger Echkardt, Los Alamos Science, no. 23, 1995: a

detailed primer on ionizing radiation, written in a light style

http://www.fas.org/sgp/othergov/doe/lanl/00326627.pdf

Nuclear Regulatory Commission

“NRC Actions on Japan’s Emergency”: statements and congressional testimony by

commissioners, news releases, FAQs

http://www.nrc.gov/japan/japan-info.html

Japanese Sources

Government of Japan

“Countermeasures for Great East Japan Earthquake”: messages from the Prime Minister, press

conferences by the Chief Cabinet Secretary, links to ministries and agencies

http://www.kantei.go.jp/foreign/incident/index.html

Japan Atomic Industrial Forum, Inc.

Home page: includes “Information Update on Fukushima Nuclear Power Station” with updates

several times a day, such as on reactor status and major events, trend of radiation in the

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The Japanese Nuclear Incident: Technical Aspects

environment, other press releases

http://www.jaif.or.jp/english/

Kyodo News

Home page: news feed, including reports on the nuclear incident

http://english.kyodonews.jp/

Ministry of Education, Culture, Sports, Science and Technology

“Reading of Environmental Radioactivity Level”: includes daily updates of radiation levels at

many radiation detection posts

http://www.mext.go.jp/english/radioactivity_level/index.htm

Ministry of Health, Labour and Welfare

“Information about the Great East Japan Earthquake”: announcements on radiation in food and

water

http://www.mhlw.go.jp/english/topics/2011eq/index.html

NHK World

Home page of Japan Broadcasting Corporation: includes many news stories on the Fukushima

Daiichi incident. Note: Members of the Board of Directors are appointed by the Prime Minister

and approved by both Houses of the Diet.

http://www3.nhk.or.jp/daily/english/01_33.html

Nuclear and Industrial Safety Agency

“Countermeasures for Tohoku—Pacific Ocean Earthquake”: includes updates of information on

radiation levels

http://www.nisa.meti.go.jp/english/

Prime Minister of Japan and His Cabinet

“The Prime Minister in Action”: includes messages, statements, and a blog by the Prime Minister

http://www.kantei.go.jp/foreign/index-e.html

Tokyo Electric Power Company

Home page of the company that owns the Fukushima Daiichi nuclear power plant: includes press

releases and monitoring data

http://www.tepco.co.jp/en/index-e.html

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Other

American Nuclear Society

“Fukushima”: Links to many news sources, updates on reactor status, an archive of updates and

news

http://ansnuclearcafe.org/fukushima/

Areva

“The Fukushima Daiichi Incident,” by Matthias Braun: provides a detailed description, with

photos and diagrams, of plant design, accident progression, radiological releases, spent fuel pools,

and sources of information

http://www.seyth.com/ressources/quake/AREVA-Document.pdf

Health Physics Society

“Fukushima Nuclear Plant Update”: health aspects of radiation exposure in general and from the

Fukushima Daiichi incident

http://www.hps.org/fukushima/

International Atomic Energy Agency

“Fukushima Nuclear Accident Update Log”: daily detailed updates of the situation

http://www.iaea.org/newscenter/news/tsunamiupdate01.html

New York Times

“Status of the Nuclear Reactors at the Fukushima Daiichi Power Plant”: for each reactor, daily

updates of the situation along with recent developments

http://www.nytimes.com/interactive/2011/03/16/world/asia/reactors-status.html?ref=asia

“Assessing the Radiation Danger, Near and Far,” by Joe Burgess et al., April 2, 2011: Combines

data from multiple sources to show the potential danger at different distances and in various

environments resulting from the Fukushima Daiichi incident

http://www.nytimes.com/interactive/2011/04/02/world/asia/assessing-the-radiation-danger.html?

ref=asia

Nuclear Energy Institute

“Japan Earthquake: Additional NEI Updates”: includes daily updates and an archive of these

updates, a timeline of events in Japan, and latest news and events

http://www.nei.org/newsandevents/information-on-the-japanese-earthquake-and-reactors-in-thatregion/japan-earthquake-additional-nei-updates/

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Union of Concerned Scientists

“UCS Spotlight: Nuclear Reactor Crisis in Japan”: daily press briefings on the incident, FAQs,

the organization’s position on nuclear power, and a report on NRC and nuclear power plant safety

http://www.ucsusa.org/

World Health Organization

“FAQs: Japan Nuclear Concerns”: Q&A from a United Nations Agency, including current risk,

human exposure to ionizing radiation, WHO travel advice for Japan

http://www.who.int/hac/crises/jpn/faqs/en/index.html

World Institute for Nuclear Security

“Emerging Lessons from the Fukushima Daiichi Accident: Maintaining Nuclear Security in a

Complex Crisis”: focuses on lessons learned from the incident for maintaining and improving

security, as distinct from safety

http://wins.org/fileitem.aspx?id=249

World Nuclear Organization

“Nuclear Power in Japan”: detailed background on many aspects of this topic including history;

tables and descriptions of Japan’s nuclear power reactors built, under construction, and planned;

fuels used and spent fuel; and R&D. Does not deal with the Fukushima Daiichi incident.

http://www.world-nuclear.org/info/inf79.html

“Fukushima Accident 2011”: technical description of the accident, with diagrams

http://www.world-nuclear.org/info/fukushima_accident_inf129.html

Author Contact Information

Jonathan Medalia

Specialist in Nuclear Weapons Policy

[redacted]@crs.loc.g

ov, 7-....

Acknowledgments

The Nuclear Regulatory Commission provided technical comments on this report.

Congressional Research Service

18

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