The Japanese Nuclear Incident:
Technical Aspects
Jonathan Medalia
Specialist in Nuclear Weapons Policy
March 31, 2011
Congressional Research Service
7-5700
www.crs.gov
R41728
CRS Report for Congress
P
repared 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.
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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...................................................................................... 7
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
Contacts
Author Contact Information ...................................................................................................... 13
Acknowledgments .................................................................................................................... 14
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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 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.
• Distance: The farther radioactive material travels, the greater the volume of air in
which the material disperses, diluting it.
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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• 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 70-year
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 the atoms in a mass of 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 percent 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
percent of it to decay.12 According to Richard Firestone, staff scientist, Lawrence Berkeley
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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National Laboratory, 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 uranium-
235 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 Reactors16,17
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.
13 Personal communication, March 30, 2011.
14 “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 Mark Holt, Specialist in Energy Policy, Resources, Science, and Industry Division. See also CRS
Report R41694, Fukushima Nuclear Crisis, by Richard J. Campbell and Mark Holt.
17 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/.
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A nuclear reactor cannot explode like an atomic bomb because the fuels and configurations differ.
In nature, uranium is 99.3 percent uranium-238 and 0.7 percent uranium-235. Only the latter is
“fissile,” that 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.”18 An atomic bomb uses uranium enriched to about 90 percent uranium-235
(“highly enriched uranium,” HEU), while nuclear reactor fuel is typically enriched to less than 5
percent (“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.19
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 1-3.20 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.
18 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.
19 “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.
20 Information provided by Nuclear Regulatory Commission, personal communication, March 25, 2011.
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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,
their zirconium cladding can ignite, which may cause the uranium inside to melt and release
radioactive material.21
To alleviate this problem, workers have begun using fresh water instead of seawater.22
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.23
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.
21 Keith Bradsher, “New Problems at Japanese Plant Subdue Optimism and Present a Risky Agenda,” New York Times,
March 24, 2011, p. 11.
22 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.
23 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.
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Health Effects of Ionizing Radiation24
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
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 treatment 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
24 This section was written by Jonathan Medalia, Specialist in Nuclear Weapons Policy, Foreign Affairs, Defense, and
Trade Division, and C. Stephen Redhead, Specialist in Health Policy, Domestic Social Policy Division.
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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).
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).25 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.26 Table 1 shows various doses and their
health consequences or regulatory limits.
25 National Council on Radiation Protection and Measurement, “Ionizing Radiation Exposure of the Population of the
United States,” report no. 160, 2009.
26 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,
Dose,
mSv
rem
Source Comments
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 Dai chi
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.”
1,000-
100-
(3)
For an acute whole-body external dose of ionizing radiation, “Minimal symptoms;
2,000
200
nausea and fatigue with possible vomiting; reduction in [certain white blood cel s],
with delayed recovery.”
2,000-
200-
(3)
For an acute whole-body external dose of ionizing radiation, “Nausea and vomiting
3,000
300
on first day; fol owing 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.”
3,200-
320-
(5)
Half the population exposed to an acute whole-body external dose of ionizing
3,600
360
radiation will die within 60 days despite receiving minimal supportive care.
3,500-
350-
(2)
NRC believes that half the population receiving this dose in a few hours or less
5,000
500
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.
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/fact-
sheets/bio-effects-radiation.html, and 10 CFR 20. (3) Dade Moel er, 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.27
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
27 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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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.”28 Further, “Some general side effects caused by KI may include intestinal upset,
allergic reactions (possibly severe), rashes, and inflammation of the salivary glands.”29
28 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.
29 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 percent
of the population will be diagnosed with cancer in their lifetimes.30 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. 31
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.32 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.
30 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.”
31 Keith Bradsher and Hiroko Tabuchi, “50 Workers Bravely Stay at Troubled Japan Reactors,” New York Times,
March 16, 2011.
32 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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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.33 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.34
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.35
Author Contact Information
Jonathan Medalia
Specialist in Nuclear Weapons Policy
jmedalia@crs.loc.gov, 7-7632
33 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.
34 David Jolly and Denise Grady, “Tokyo Says Radiation in Water Puts Infants at Risk,” New York Times, March 23,
2010.
35 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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Acknowledgments
The Nuclear Regulatory Commission provided technical comments on this report.
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