Information about the Cassini spacecraft and its risks

Jeff Cuzzi's order-of-magnitude analysis of health risks

Thanks to Bill Higgins for posting this to sci.astro.

From: (Bill Higgins)
Newsgroups: sci.astro,,sci.environment,talk.environment
Subject: J. Cuzzi on Cassini plutonium hazard
Date: 18 Sep 97 14:30:55 -0600
Organization: Fermi National Accelerator Laboratory
Summary: Simple approximate calculation of worst Cassini accident effects
Keywords: Cassini plutonium 238 Saturn NASA

I've just seen a message from Jeff Cuzzi, a prominent planetary
scientist working on the Cassini mission, and I thought I would pass
it along with his permission.  Opinions are his, not mine and not my
employer's. He offers this additional disclaimer: "Remember it is
not an official NASA position but a calculation ANYONE could do!"

Bill Higgins                            Internet: HIGGINS@FNAL.FNAL.GOV 
Fermi National Accelerator Laboratory  

Date: Tue, 16 Sep 1997 22:14:46 -0700
From: (Jeff Cuzzi)
Subject: Plutonium primer

Cassini Plutonium for the technically minded
by Jeff Cuzzi

I'm sure we will all have friends and relatives asking us what's up
with the Cassini Plutonium issue as launch approaches in early
October. Allegations of risk have arisen due to Cassini's onboard
RTG's (Radioisotope Thermal Generators) which derive electricity from
decay of 72 lb (33kg) of Plutonium dioxide fuel.

In anticipation, I wanted to provide some "derived from basic
principles" satisfaction that the Cassini health threat is negligibly
small even in the extremely small chance that anything does go wrong
with the mission (either at launch or at flyby). The Cassini project
has devoted more than a million dollars to a thorough analysis of the
problem, but the back-of-an-envelope analysis below is a little easier
to grasp and serves as a calibration and sanity check.

I am a Cassini scientist, and neither a health expert, nor a nuclear
physicist.  I do care about the health of the people of the world.  I
had several discussions with a physicist at the Nuclear Regulatory
Commission (NRC) concerning decay rates and comparative relationships
to health effects. I also had this reviewed by the President of the
Health Physics Society, a 6500 member national organization (who has
publicly stated that NASA has done a very good job and has, if
anything, OVERestimated the health risks).

For my initial health effect data I relied on Web sites maintained by
the EPA and the Agency for Toxic Substances and Disease Registry
(ATSDR; part of the Center for Disease Control - see references
below); my NRC and Davis contacts confirmed these values and
identified their primary source (FGR-11, 1988).  I suspect anyone can
reproduce the calculations below who can read a simple physics
textbook and the World Wide Web.

238-Pu decays by alpha-particle emission (like the longer-lived
weapons grade isotope 239-Pu, but 250x faster).  The decay rate can be
calculated from the half life (88 yrs) and the number of nuclei per
gram, and is about 6E11 decays/sec/gm, defined as 17 Curie/gm. A Curie
(Ci) of 238-Pu and a Ci of 239-Pu have the same radiation damage
potential (they emit the same alpha particles).  Because 238 decays
faster, it has a higher Ci/g rating by the ratio of half lives (about
250). The convenient unit is pico-Curies (10^-12 Ci = pCi).

Health standards are set by the International Commission on
Radiological Protection (ICRP), and found in FGR-11 and the ATSDR web
page.  The conversion factors between radioactivity (Ci) and potential
tissue damage in rem (Roentgen Equivalent Measure, or more often
millirem (mrem = 10^-3 rem) are from the FGR-11 (note 1). They can be
derived from values on the ATSDR page as well. The ATSDR quoted Annual
Limit on Intake (ALI) is 20000pCi/yr for "workers", and the
corresponding dose limit is 5 rem/yr, giving a conversion factor of
0.25 mrem/pCi (note 1), in good agreement with the standard value of
0.29 mrem/pCi tabulated in FGR-11.

Several expressions can be found for EPA-allowable levels of
radioactivity. The ATSDR web page gives a mixture of recommended
limits for the public and for "occuptional exposure" in rem, Annual
Limits on Intake (ALI) in pCi/yr, and in Derived Air Concentration
(DAC; pCi/m3) levels. These are generally consistent with a 10 times
lower limit for the general public than for workers, but my NRC
contact says the DAC's for the general public are maybe another 10
times smaller than can be inferred from this web page (probably
factors for time off-job as fraction of 24 hr, etc). 

Also, it appears that the 500 mrem annual limit for the public cited
by ATSDR probably includes the unavoidable background level of 360
mrem/yr from Radon gas, cosmic rays, the dentist, etc. My NRC contact
thinks this would be consistent with his knowledge of an ICRP
recommendation for the public of no more than 100 mrem annually above
the annual background.

Presume a worst case scenario involving vaporization of ALL the Pu-238
that is in the RTG's. This 'astrophysical accuracy' calculation makes
no allowance for removal of Pu into the ocean, by rainout, deposition
onto uninhabited terrain, etc.  The 72lb of Cassini fuel is actually
nearly 30% oxygen and less active Pu isotopes, so is only 50 lb Pu-238
= 23 kg = 400,000 Ci (about 17 Ci/g). The volume of air in the
Northern troposphere and stratosphere (which receive 99% of the Pu) =
2 pi X (10 + 40) X 6000^2 km3  = 10^19 m3.

Dispersion of all this vaporized Pu in the northern atmosphere gives a
radiation density of about 0.04 pCi/m3, comparable to the allowable
DAC. The ATSDR numbers imply that you breathe air at about 0.1
liter/sec (plausible) so get 3000 m3/yr, or about 120pCi/yr. the
conversion factor above (0.25) gives a 50 year dose of 30 mrem from
each year of breathing this Plutonium - less than 10% of the annual
background.  You'd need to breathe it for 10 years just to get the
equivalent of one year of natural radiation. Meanwhile, of course, it
is being lost from the system so the real numbers are far smaller. And
this is using ALL the Plutonium.

Looked at another way, all the Pu settles out eventually, providing
2000 pCi/m2, probably over a few years. If a person has a cross
section of 1 m2 and inhales ALL the fallout in this area, he gets a
500 mrem 50 year dose. This is still considerably smaller than the
18000 mrem we naturally receive over the same 50 year period.

For comparison, 500 mrem total dose is about the same as one
mammogram. Of course, most of this settling Pu misses people's noses
and mouths, and if this amount of Plutonium were mixed into the top 1
mm of soil, it could be shipped as non-radioactive material. And this
is using ALL the Plutonium.

No credible indication has ever been found of increased health risk
even to the many people who worked milling Pu in the Hot and Cold War
days. The only documented health effects I have been able to find are
on the ATSDR web site (see references). Dogs (apparently beagles)
inhaled Plutonium at a rate of 1400 - 100,000 pCi per kg body mass in
a day, and suffered lung damage, even cancer, depending on dose, after
several months to years.

Allowing for 20 kg body mass, these dog martyrs consumed, in one DAY,
amounts which would be 14 to 1000 times the average person's share of
the entire Cassini Pu load as overestimated above.  The president of
the Health Physics Society has himself done extensive research on mice
that confirms these dog results.

Vaporization of all the Plutonium is, of course, a gross overestimate.
Forget (for a moment) the one-in-a-million probability that ANY kind
of flyby mishap will even occur which leads to reentry and
vaporization.  Even if a mishap does occur, only a tiny fraction of
the Pu is able to end up in people (this is the analogue of the fact
that there are enough germs in one sneeze to give a billion people a
cold - it's the distribution problem that stops this from happening).

The Cassini project and its consultants have done exhaustive analyses
of this problem. Atmospheric incineration and ground impact have both
been considered. The RTG housing itself probably does come apart under
entry heating, but the triple-protected modules (2 layers of carbon
composite, and an iridium cladding on each Plutonium golf ball) are
extremely durable, and designed to withstand atmospheric deceleration
and heating.  They hit the ground at terminal velocity - only 100-300
feet/second, or one-tenth the speed of a rifle bullet.  Rifle bullets
don't vaporize on impact. Neither do meteorites; they dig a little
hole.  So the units might dent the hood of your car pretty badly, or
make a hole in your yard, but won't spray pulverized plutonium all
over your house.  All this has been tested. 

Factoring in these issues, the projects finds that the average
expected dose (per person) is only 1 mrem over the entire 50 year
lifespan of the at-risk population. Comparing this to the above upper
limit of about 500 mrem/50 yr, one gets a distribution efficiency
factor of about .002. If a sneeze had the same efficiency then each
sneeze would give 2 million people a cold.  So the project's
distribution efficiency factor, which includes the difficulty of
burning through the carbon-composite and Iridium cladding of the fuel,
is hardly unreasonable and actually seems quite conservative.

Given the low distribution efficiency, the "average" person receives
practically no Pu at all. So what's all the fuss about?  There is a
very narrow range of "hot" particle sizes (about 6-10 micron radius)
that is both large enough to have a significant radiation damage
potential (in the range that damaged dogs' lungs) AND small enough to
have any conceivable chance of being inhaled (but only a very, very
small chance - see note 2).

Because of the high density of the Pu (11 g/cm3), the aerodynamic
radius is 11 times the actual radius. That is, cigarette smoke
particles as large as 6-10 microns are inhalable with small
probability (a percent or less), but Pu particles of the same size
behave like 60-100 micron carbon grit. If ALL the Cassini Pu were in
this 6-10 micron size range, there would be 5 E11 particles to
distribute - "100 for each person" is what the critics might say.  But
in reality there are enormous reduction factors that must be

For instance, the fraction of Pu fuel that is actually vaporized is
probably less than 10%.  The fraction of all released particles that
lie in the narrow hazardous size range is perhaps 1%. The fraction of
Pu that ends up landing where people live (say, the 20 largest cities)
is roughly their area fraction or say 0.0001. The fraction of these
grit particles that are actually inhaled, because of their large
aerodynamic size of about 100 microns, is also small  - surely less
than 0.01 (note 2).  There is slop in these estimates, but they are
plausible "delivery inefficiences" and lead to 500 inhaled "hazardous"
particles worldwide, consistent with the Cassini project's far more
careful estimate of 100 additional fatalities over a 50 year period.

Recall that the probability of this happening in the first place is
one in a million; another type of celestial mishap with the same
probability, impact of a mile-wide asteroid, would kill over a billion
people. Also recall that a billion people will die from cancer
unrelated to Cassini during this same 50 years.

The health hazard numbers are even smaller for a launch-related
accident (even while it is perhaps 1000 times more "probable" at
1/1500 chance of Pu-release related to launch accident), because a far
smaller amount of Pu is vaporized and fewer people are exposed. The
RTG's have been exhaustively tested under conditions comparable to
such accidents; their Carbon-Iridium protection scheme is incredibly

Overall, I think the above simple arguments make the more exhaustive
analysis done by the Cassini project very easy to understand and
accept. The health hazard due to Cassini Plutonium really is
negligible.  Statistics in the World Almanac verify that a person's
risk of dying from Cassini is a million times smaller than his or her
risk of a fatal auto accident while driving one mile.


1) For the cognoscenti, all doses given here are effective (whole
body), equivalent (radiation type independent), committed (50-year)
doses (unless specified as annual). This is necessary to compare
different sources of radioactivity. There are factor-of-2 or 3
differences depending on how soluble the Plutonium is; the values on
the web page are appropriate for "insoluble" Plutonium such as the
Cassini ceramic form.  The basic constants are thus the 50-year
integrated effective (whole body) damage-causing dose in mrem from a
certain quantity of radioactivity in pCi.

2) The human nose is 100% effective at filtering particles that are 10
microns or greater and 95% effective at filtering particles over 5
microns. These particles can then be excreted easily. The critical
size for deposition in lung cells is 1-2 microns. Once inhaled, the
material is subject to removal processes involving incoproration in
mucous suspension and being swept out by the action of the cillia
which line the portions of the lung which are exposed to air
(Glasstone and Dolan 1977).


FGR-11 (1988), or Federal Guidance Report-11: "Limiting values of
radionuclide intake and air concentration and dose conversion factors
for inhalation, submersion, and ingestion"; K. F. Eckerman et al, EPA
Report EPA-520/1-88-020. This is based on standards developed by the
International Commission on Radiological Protection, and is endorsed
by the President of the United States.

Glasstone and Dolan (1977), Department of Defense Publication, "The
Effect of Nuclear Weapons"

ASTDR Web site:

JPL Cassini Home Page:

Excerpt from "Florida Today" article on testing of the plutonium cannisters

Thanks to Bill Higgins for posting this to sci.astro.

From Sat Sep 20 18:25:58 1997
From: (Bill Higgins)
Subject: Testing Cassini plutonium containers
Date: 18 Sep 97 15:26:00 -0600
Organization: Fermi National Accelerator Laboratory
Summary: Newspaper describes tests; see Los Alamos for technical details
Keywords: Cassini plutonium testing NASA Saturn

*Florida Today*, the newspaper local to Cape Canaveral, is devoting
much coverage to the Cassini mission and to the controversy over its
safety.  See .

One of the articles you'll find there,  "Plutonium protection: Tests
show containers are durable" by Todd Halvorson, appeared on 14
September. It gives a good qualitative description of the testing
program for the plutonium canisters Cassini uses.  I'll quote an


> The canisters have been subjected to a range of extensive tests for
>nearly four decades. They include: 
>Rocket explosions
>Fuel pellets without their protective casings have been blown up with
>the high-grade explosive C-4, which is commonly used by demolition
>The tests exposed the iridium-covered pellets to blasts 18 times
>greater than the biggest explosion that could occur during the launch
>of a Titan4 rocket. The result: No plutonium release.
>Rocket fuel fires
>Fuel pellets also have been placed in raging fires of liquid and solid
>rocket propellants.
>The liquid rocket fuel fires don't burn hot enough to breach the
>metal-covered pellets. Fires ignited with solid rocket fuel burn
>hotter - up to 4,352 degrees Fahrenheit - but the pellets survived
>those tests, too. 
>Flying shrapnel
>Specially designed .30- and .50-caliber guns were used to shoot
>aluminum and titanium bullets at the canisters to simulate small
>pieces of debris - such as nuts and bolts - that could be hurled at
>them in a launch explosion. 
>The tests showed the fuel pellets will remain intact even if hit by
>shrapnel traveling as much as 900 mph - a velocity 42 percent greater
>than expected in a launch accident. 
>Larger chunks of wreckage
>A rocket-powered sled was used to propel large pieces of steel rocket
>booster casing into the canisters at speeds up to 474mph.
>The tests showed only small amounts of plutonium fuel would be
>released if a canister was struck by the sharp edge of a large piece
>of debris. Wreckage striking a canister head-on would not trigger a
>fuel release, the tests showed. 
>Other tests have shown that in certain scenarios plutonium could be
>released during an inadvertent plunge back through the atmosphere.
>Still others prove small amounts of fuel could escape if they slammed
>into a hard surface - such as steel or concrete - rather than water or


Higgins here again.  The newspaper story, of course, doesn't give
references to the various tests it describe.  If you really want to
dig into the technical details, I recommend you look at the Los Alamos
National Laboratory library  .  Search the
LANL publications catalogue for the phrase "General Purpose Heat
Source," which is the name of the plutonium capsule assembly
incorporated into Cassini's (and Galileo's and Ulysses's) Radioisotope
Thermoelectric Generators.

The folks at Los Alamos have put lots of their unclassified reports on
the Web in Acrobat (*.PDF) format, so you can download 'em if you

[Opinions are mine, and not those of Fermilab or the U.S. Department
of Energy.]

Bill Higgins                            Internet: HIGGINS@FNAL.FNAL.GOV 
Fermi National Accelerator Laboratory  

Lawrence Livermore National Laboratory paper on the dangers of plutonium

Copied from
Go to the original article for all the equations and references.


A Perspective on the Dangers of Plutonium

W. G. Sutcliffe, R. H. Condit, W. G. Mansfield, D. S. Myers, D. W. Layton,
and P. W. Murphy

Lawrence Livermore National Laboratory

April 14, 1995


Following the seizure of 10 ounces of plutonium at the Munich airport in
August 1994, some press accounts stated that terrorists could kill "hundreds
of thousands of people" by introducing plutonium into a municipal water
supply. In response to such incorrect and misleading statements, we describe
the acute and long-term health effects that can arise from ingesting or
inhaling various amounts of plutonium. Our estimates indicate that plutonium
introduced into drinking water supplies would produce a radiation dose much
less than normal background, and could kill only a very few people (by
inducing cancers that might take years to appear). We also estimate the
(considerably greater) risks associated with the inhalation of plutonium,
clarifying press claims that "a tiny speck ... can cause lung cancer." We
estimate the number of people that might die of cancer if terrorists were to
introduce plutonium into the atmosphere in a large city. This paper provides
a scientific perspective for evaluating possible terrorist threats.


Since the breakup of the Soviet Union, television and print news media have
widely reported that plutonium from that part of the world is available on
the black market. The primary concern aroused by this fact is that, if
obtained in sufficient quantities, such plutonium might be made into a
nuclear explosive. However, The New York Times and other newspapers have
reported that terrorists might also use black-market plutonium to
contaminate the air or drinking water of a large city. Specifically on
August 16, 1994, The New York Times claimed[1] that "A tiny speck of the
fine powder can cause lung cancer in anyone who inhales it, and a small
amount in the water supply of a large city like Munich could kill hundreds
of thousands of people." Other newspapers made similar claims.[2],[3] The
first of these claims is misleading; the second is false. This note provides
a scientific perspective on this perceived danger.

Although the popular myth that "plutonium is the most hazardous substance
known to man" has been refuted many times, the misconception persists that
even a small amount of plutonium taken into the body will be fatal.
Plutonium is hazardous, but it is not as immediately hazardous to health as
many more common chemicals. This is not to say that plutonium is not a
dangerous, toxic material. Chronic exposure to even small amounts should be
a matter of concern. But dispersal by terrorists as described in the press
could not produce the drastic health effects that are popularly imagined,
and that is the issue addressed here.

Toxic Effects of Plutonium

Plutonium is a dense, metallic element that (in the contexts dealt with
here) is normally found in the form of an oxide, [Image].[4] To understand
the toxicity of plutonium, it is important to understand the mechanisms by
which it can produce detrimental health effects.[5] Plutonium is primarily a
radiological hazard, whose danger arises from the radiation dose delivered
to various internal organs if it is taken into the body. Plutonium delivers
a negligible radiation dose to human skin, because it emits alpha particles,
which do not in general have enough energy to penetrate the skin. The
chemical toxicity of plutonium (a heavy metal) is inconsequential alongside
the radiation effects.

The severity of the radiation dose, and the organs that are irradiated,
depend primarily on the quantity of plutonium taken into the body and on the
route by which it enters the body. In general, plutonium that is inhaled is
far more hazardous than plutonium that is ingested, because it is more
readily absorbed into the blood stream via the lungs than via the
gastrointestinal (GI) tract. Inhaled plutonium will deliver a radiation dose
to the lungs; ingested plutonium will deliver a radiation dose to the walls
of the GI tract. From either of these entry points, plutonium may migrate
via the blood stream to selectively concentrate in the bones and liver.

Plutonium exposure may produce acute health effects (e.g., inhalation may
lead to pulmonary edema, and ingestion to damage to GI tract walls), or
long-term effects, such as increased risk of cancer mortality. Relatively
high doses are required to produce acute effects. Ingestion of about 0.5
grams of plutonium would be necessary to deliver an acutely lethal dose.[6]
(For comparison, ingestion of less than 0.1 gram of cyanide can cause sudden
death.[7]) Inhalation of about 20 milligrams of plutonium dust of optimal
size would be necessary to cause death within roughly a month from pulmonary
fibrosis or pulmonary edema.[8] As we explain below, it is hard to imagine
scenarios in which a person would ingest or inhale such quantities of

People inhaling less than acutely lethal quantities of plutonium will still
have an increased probability of getting cancer. The lungs are exposed to
alpha-particle radiation, increasing the risk of lung cancer, until the
plutonium is (eventually) carried to other organs, primarily the bones and
liver, where the radiation causes cell damage and increases the likelihood
of cancer at those sites.

The committed effective doses[9] and the increased probability of cancer
death resulting from them have been studied extensively, as outlined in
Appendix A. The estimated cancer fatality risk associated with exposure to
weapons-grade plutonium is 12 cancer deaths per milligram inhaled, or 1 per
0.08 milligrams inhaled; and it is 0.0021 cancer deaths per milligram
ingested,[10] or 1 per 480 milligrams ingested.[11] For perspective, an
inhaled mass of about 0.0001 milligram would increase the cancer mortality
from about 200 in 1000 (the risk of cancer mortality from all causes) to
about 201.2 in 1000. This risk increase corresponds to a decrease in life
expectancy of about 15 days; for comparison, smoking a pack of cigarettes a
day reduces life expectancy by about 2250 days (more than six years).[12]

Plutonium in the Atmosphere

It is important to understand the claims made in the press concerning
particles of plutonium in the air. The New York Times[1] says that "A tiny
speck of the fine powder can cause lung cancer in anyone who inhales it."
The largest speck of plutonium that can be readily inhaled is about 3
micrometers in diameter and has a mass of about 0.14 millionths of a
milligram. The risk of dying of cancer as a result of inhaling that amount
of plutonium is about 0.0000017 (12 cancers per milligram x 0.00000014
milligrams = 0.0000017 cancers, or 0.00017% additional risk); that is not
zero risk, but it is very small.

The Los Angeles Times[2] says that one ten-thousandth of a gram (0.1
milligram) inhaled can cause cancer. This is correct: we have already
estimated that 0.08 milligrams inhaled will have 100% probability of causing
a fatal cancer. To inhale 0.1 milligram of plutonium, however, a person
would have to inhale more than seven hundred thousand particles. (A single
0.1-milligram particle would have a diameter of over 260 micrometers, about
90 times too big to be readily inhaled.) Although a single respirable
particle is unlikely to harm an individual,[13] there is still cause for
concern if plutonium were to be dispersed in the atmosphere.

The Herald (Glasgow, Scotland)[3] says that one millionth of a gram (0.001
milligram) can kill: the actual additional risk of cancer death resulting
from the inhalation of 0.001 milligram of plutonium is 0.012 (12 cancers per
milligram x 0.001 milligram = 0.012, or 1.2% additional risk).

The public health impact of the illicit dispersal of plutonium into the
atmosphere would depend strongly on the circumstances and mechanisms of
dispersal. People very near the dispersal site could experience serious
acute health effects or significant increased cancer risks, but it is
inconceivable that large numbers of people would suffer grave health
effects, as implied by the news media. In particular, only someone quite
near the source would have a significant risk of being exposed to an acutely
lethal amount of plutonium, and that person would as likely be injured by
the explosion or fire that dispersed the plutonium.

For this discussion, the dispersal of plutonium in the atmosphere has two
important aspects: (1) the amount of plutonium converted into particles of
respirable size, and (2) dispersal into the air, fallout (including rainout)
onto the ground, and possible resuspension of those particles into the air.
We discuss these aspects in turn.

The primary danger from plutonium is that small particles will become
airborne and be inhaled. Particles that are too large fall to the ground,
and only the smallest particles are carried very far from the source.
Moreover, unless the particles are "respirable" (smaller than about 3
micrometers in diameter), they are not inhaled into the depths of the lung,
where they can be absorbed. An explosion would be of greater concern than a
fire in this regard. As much as 50% of the plutonium dispersed by an
explosion might be respirable[14]; 20% may be a better estimate.[15] In a
fire, by contrast, it is likely that no more than about 0.05% of the
oxidized plutonium would be respirable.[15]

For this discussion, we assume that one kilogram of plutonium is available
to a terrorist group. It is unlikely that dispersal of this plutonium would
kill many people outright, i.e., by subjecting them to an acutely lethal
dose (20 milligrams of plutonium inhaled). A person engaged in light
activity breathes about 10 to 20 liters of air per minute, or about 1 cubic
meter per hour. To inhale 20 milligrams of plutonium, a person would have to
breathe air containing 20 milligrams of respirable particles per cubic meter
for at least an hour, or 40 milligrams per cubic meter for at least half an
hour, etc. At an average concentration of 20 milligrams of respirable
particles per cubic meter, air containing 200 grams (20% of a kilogram) of
plutonium would occupy a cube only about 22 meters on a side. It is highly
unlikely that there would be no movement of air to disperse the plutonium
and that anyone would remain and continue breathing the contaminated air for
an hour. There are obviously all sorts of variants on this calculation, but
the conclusion will remain the same: it is unlikely that a large number of
people will receive an acutely lethal dose from a plutonium dispersal in the

A simple illustrative estimate of the less acute effects of dispersion can
also be made without knowing all the details of a population distribution
and the meteorological conditions. Suppose, for example, that 200 grams of
respirable plutonium particles are uniformly dispersed through a cube of air
one kilometer on a side. This gives a concentration of 0.0002 milligrams per
cubic meter. A person breathing this air for an hour would sustain an
additional 0.24% risk of death from cancer (12 cancers per milligram x
0.0002 milligrams = 0.0024 cancers, or 0.24% additional risk). If the
plutonium was dispersed over a city, such as Munich, many people would be
exposed, and the total risk of cancer would be increased. Munich's average
population density is about 4300 people per square kilometer. (The actual
density of people outdoors and exposed to the plutonium-contaminated air
would likely be significantly less.) If exposed for one hour, 4300 people
(under the cube of contaminated air) would inhale 0.86 milligrams of
plutonium (4300 people x 0.0002 milligrams per cubic meter x 1 cubic meter
per hour), resulting in the expectation of about 10 additional deaths due to
cancer (12 cancers per milligram x 0.86 milligram = 10.3).

The lifetime of the cloud of contaminated air depends on the height of the
cloud and the rate at which the particles fall out. Although variable, a
settling or fallout rate of 0.3 centimeters per second is a reasonable
estimate for 3-micrometer-sized particles. Larger particles will fall out
faster.[16] Although smaller particles will remain in the air longer, the
concentration of plutonium will be decreased. Rain or moisture would cause
the plutonium to fall out more rapidly. Falling at 0.3 centimeters per
second, particles from the top of a one-kilometer-high cloud would take
almost 93 hours to reach the ground. It is hard to imagine that the
contaminated air would remain over a city for so long. Even a light breeze
(5 km/hr) would carry the cloud beyond a city the size of Munich (20 km x 20
km) in a few hours.

If there were no evacuation, no filtering of air by being inside, and if the
cloud did not migrate beyond the city, the population could inhale 80
milligrams of plutonium (0.0002 milligrams per cubic meter x 1 cubic meter
per hour x 93 hours x 4300) in 93 hours, which would result in about 960
cancer deaths (12 cancers per milligram x 80 milligrams = 960), in addition
to the 860 cancer deaths one expects (20% of the population), from other
causes, among 4300 people.

Of course, an actual plume or cloud of particles is unlikely to be a cube,
but we argue that, in this very simple model, the number of additional
cancer deaths does not depend on the height or extent of the cloud
containing plutonium particles. Fetter and von Hippel[17] give a more
rigorous derivation of this result. If the cloud were half as high (500
meters), the concentration would be twice as great (0.0004 milligrams per
cubic meter), but the cloud would last only half as long (46.5 hours), so
that 4300 people could inhale 80 milligrams of plutonium as before.

The number of additional cancer deaths is also independent of the shape or
extent of the cloud, as long as the cloud remains over the city. If the
cloud were twice as long (2 kilometers) (or twice as broad), twice the
number of people (8600) would potentially be exposed, but the average
concentration would only be half as great (0.0001 milligrams per cubic
meter), resulting again in 80 milligrams for the amount of plutonium that
could be inhaled.

Finally, no error arises as a result of assuming a uniform (average)
population distribution and a uniform (average) distribution of plutonium
particles in the cloud. This is because, as stated above, (at less than
acute levels) the risk of additional cancer death due to the inhalation of
plutonium depends only on the total amount of plutonium inhaled, and not on
the number of people who inhale the plutonium. Also, because of this, we do
not have to assume that the cloud remains motionless, or that it has any
particular height or lateral extent; the resultant number of additional
cancer deaths is the same as long as the cloud is somewhere over the city.
The foregoing arguments are described quantitatively in Appendix B.

It should be pointed out that our simple estimate of 960 additional cancer
deaths is pessimistic to the point of not being credible. Certainly the
amount of plutonium inhaled would be greatly reduced by evacuation (or at
least a retreat indoors) if a cloud of plutonium particles persisted over a
city for many hours. Even a light breeze (5 km/hr) would carry the cloud
beyond a city the size of Munich (20 km x 20 km) in a few hours. In any
case, it is unbelievable that the total population, 4300 people per square
kilometer, under the cloud would be outside breathing contaminated air for
almost four days. As stated above, a light breeze would carry the
contamination beyond a city in a few hours. Thus a better estimate of the
exposure time might be made by assuming that the diurnal variation causes
such a breeze and the cloud remains over the city for only 12 hours. In this
case the amount of plutonium inhaled would be about 10 milligrams, leading
to the still pessimistic expectation of about 120 additional deaths due to
cancer. As a matter of fact, it is unlikely that the dispersion of 200 grams
of plutonium would result in any observable increase in the number of
expected deaths (860) due to cancer.

Plutonium remains a health concern even after it has settled to the ground.
Disturbances such as pedestrian traffic or wind can pick up a (small)
fraction of the particles, like any other dust, and resuspend them in the
air. Although the concentration of resuspended particles would be much less
than the concentration in the original cloud, continued exposure would be
hazardous, and deposited plutonium would therefore have to be cleaned up to
prevent long-term exposures and possible spreading to neighboring areas.
Fetter and von Hippel[17] take resuspension into account in a quantitative

In the above estimates we have assumed that one kilogram of plutonium was
available to the terrorist. Clearly our estimates of health effects would be
doubled if the terrorists had two kilograms, etc. If the terrorists had
several kilograms of plutonium, however, they would more likely try to
construct a nuclear explosive than simply to disperse the plutonium.

Given details about the source of plutonium particles and the atmospheric
conditions, it is possible to use more sophisticated models to estimate the
downwind concentrations and the consequent health effects.[18] However, our
conclusions would remain the same: although the dispersal of 200 grams of
plutonium in the atmosphere could cause a significant increase in the risk
of cancer, and would therefore have to be taken very seriously, it would be
unlikely to cause a large number of deaths.

Field experiments support the conclusions drawn above. To study the
formation and dispersal of plutonium particles, 200 grams of plutonium was
burned in open desert in Australia in 1959. Analysis of the data, normalized
to 1 kilogram, shows that at 200 meters from the burning plutonium source,
no person would have inhaled more than 0.0001 milligram of plutonium.[19]
Data from points closer to the source are not available, but extrapolation
of the field data suggests that one would have to be closer than 100 meters
directly downwind from the source to have a greater than 1% chance of
inhaling 0.001 milligram, an amount that would increase the risk of cancer
mortality from about 200 in 1000 to about 212 in 1000. Recent analysis
suggests that about 0.05% of the plutonium oxidized was converted to
respirable particles in this experiment.[20]

Explosive dispersal was studied in operations Plumbob and Roller Coaster at
the Nevada Test Site.[21] In none of these experiments would a person
standing 300 meters directly downwind from the detonation have inhaled more
than about 0.0001 milligrams of plutonium, an amount that would have
increased the risk of death by cancer by 0.12%.

Although people close to a dispersal point could receive serious radiation
doses, atmospheric dispersal of plutonium is unlikely to be an effective
means for creating major health effects in a large number of people.
However, the dispersal of plutonium in air could result in painful
disruption of normal activity and onerous measures to avoid the hazard
presented by plutonium. Burdensome cleanup operations would also be
required. The cleanup following the 1966 plutonium-dispersal accident near
Palomares, Spain, indicates that such operations can be long and

Plutonium in Drinking Water

It is equally important to understand the claims made in the press
concerning plutonium in drinking water. The Los Angeles Times[2] says that a
small amount in the water supply of a large city like Munich could kill
hundreds of thousands of people. The Herald (Glasgow, Scotland)[3] says that
10 ounces (283,000 milligrams) would be enough to contaminate all Germany's
drinking water. But if 10 ounces of plutonium were introduced into a
reservoir, only about 3 milligrams (one part in 100,000) would be dissolved
and suspended[23]; the rest would be immobilized in sediments. At 0.0021
cancers per milligram ingested (see Appendix A), if all that dissolved
plutonium were ingested (an unlikely occurrence), by whatever number of
people, one would expect 0.006 additional cancer deaths. The actual
occurrence of even one additional cancer death would be remarkable.[24]

Plutonium is much less of a hazard in water than in air. Even if a kilogram
of plutonium were introduced into a reservoir, it would be unlikely to reach
concentrations that could cause acute health effects or even significantly
increase the risk of death from cancer. Three factors diminish the dangers
from reservoir contamination:

1. Most of the plutonium would settle out.

2. The plutonium remaining in solution would be greatly diluted in the large
volumes of water available.

3. The ingestion pathways to man (drinking the water, or ingesting aquatic
organisms) discriminate strongly against plutonium.[25]

Ingestion of small amounts of plutonium would increase the cancer mortality
risk. Ingestion of 1 milligram of weapons-grade plutonium oxide (which would
contain about 94% plutonium-239) would produce a dose of almost 5 rem (see
Appendix A), the occupational regulatory limit for one year. A 5-rem dose
would increase the cancer mortality risk by about 2.5 in 1000.[26] For
comparison, everyone on earth receives an average dose of 0.3 rem each year
from natural background radiation.[27]

Plutonium is much less soluble in water than ordinary sand
(quartz).[23],[28] Plutonium introduced into a water system tends to settle
out and become trapped in sediment, rather than remaining in the water
itself; about one part of plutonium remains in solution for each hundred
thousand parts trapped in sediments.[27] Fish and vegetation in the water
might redistribute the plutonium to a very limited extent, but most of the
plutonium would remain in the sediment, rather than being taken up in any
animal or vegetable material.

Any plutonium suspended or dissolved in water would be greatly diluted in
the volume of the reservoir (city reservoirs may contain roughly a billion
cubic meters of water). If one kilogram of plutonium were entirely dissolved
(that is, with none settling out) in a billion-cubic-meter reservoir, the
resulting concentration would be about 0.001 microgram (one millionth of a
milligram) per liter; because of settling out, the actual concentration to
be expected would be a small fraction of this very small value. Someone who
drank this water for a lifetime (say, two liters per day for 70 years, for a
total intake of 0.05 milligrams of plutonium) would have an 0.01% additional
risk (0.05 x 0.0021) of dying of cancer.[29] If all residents of a city of
one million people did the same, one would expect additional 105 cancer
deaths--additional, that is, to the 200,000 cancer deaths to be expected in
that population from other causes.

In cities having somewhat smaller reservoirs or retention tanks (with
capacities comparable to the daily or weekly volume consumed), dilution
would still bring the concentration far below an acutely toxic level. For
example, Munich uses retention tanks with a total volume of about 300,000
cubic meters, about the amount consumed in a day. In this case, one kilogram
of entirely dissolved plutonium would be diluted to 0.003 milligrams per
liter. A resident of Munich drinking 3 liters of water per day might
therefore expect to ingest a little less than 0.01 milligrams of plutonium
during that day when the water was contaminated. This intake would result in
a dose of about 0.04 rem (see Appendix A), much less than the 0.3-rem annual
dose from natural background radiation. If all 1.3 million residents of
Munich did the same, one would expect 27 additional cancer deaths (0.0021
cancer per milligram x 0.01 milligram x 1,300,000 people = 27.3) among them.

Plutonium ingested in water does not transfer easily from the GI tract
through the intestinal wall into the blood stream. The fraction [Image] of
plutonium absorbed from the GI tract into the bloodstream varies strongly
with the chemical form of the plutonium, and ranges from about one part in a
thousand to one part in a hundred thousand. This uptake fraction is low
because plutonium is not very soluble at the acidity characteristic of body
fluids.[27] In Appendix A, we have used [Image], the value appropriate for
plutonium oxide, to calculate the risk and dose from the uptake of

Plutonium in colloidal suspension can be carried in water to a greater
extent than indicated above, but plutonium is absorbed by the body even less
readily in that form than in chemical solution, so the biological hazards
are not significantly increased.[30]

Although not an immediate hazard, plutonium in a reservoir could present a
significant environmental cleanup problem. Because it would remain in the
sediment for a long time, and because there would be significant uncertainty
in the public's mind about the possibility of migration and concentration in
plants and animals, consideration would have to be given to its removal.


In summary, the claims of dire health consequences from the introduction of
plutonium into the air or into a municipal water supply are greatly
exaggerated. The combination of rapid and almost complete sedimentation,
dilution in large volumes of water, and minimal uptake of plutonium from the
GI tract would all act to preclude serious health consequences to the public
from the latter scenario. And although the dispersal of plutonium in air (as
the result of a fire or explosion, for example) would cause immense concern
and cleanup problems, it would not result in widespread deaths or dire
health consequences, as terrorists might hope. Dissipation due to wind and
air turbulence would rapidly dilute any respirable aerosol. Only people
within a few meters of the source could receive a prompt lethal dose.
Delayed effects in the form of fatal cancers outside this region would
probably not appear in affected individuals until years later. For a vast
majority of the population of any city, the increase in cancer risk arising
from exposure to plutonium aerosol would be a fraction of that arising from
other, more common health hazards.

None of the above discussion should be taken to mean that the diversion and
illicit use of plutonium is not a serious international problem. Such
illicit use does have the potential for serious physical and psychological
impacts on the public. We are concerned, however, that erroneous and
exaggerated statements in the media may actually promote a market for stolen
and smuggled nuclear material for the purpose of nuclear terrorism.
Ignorance and fear should not play major roles in deciding how to deal with
such potential threats.


Work performed under the auspices of the U.S. Department of Energy by
Lawrence Livermore National Laboratory under Contract W-7405-ENG-48.

The authors wish to thank our colleagues for many valuable suggestions. We
wish especially to thank Robert E. Luna of Sandia National Laboratories,
Steve Fetter of the University of Maryland, and Cheryll Faust of Los Alamos
National Laboratory.

Appendix A. Risk and Dose vs Plutonium Intake

The cancer risk associated with the inhalation or ingestion of a given
amount of plutonium can be determined as the product of three quantities:
(1) the activity (activity is measured in curies) of plutonium per
milligram, (2) the dose (measured in rem) delivered per unit of plutonium
activity taken in, and (3) the risk of cancer per unit dose of radiation
delivered to the body by that plutonium. The calculations below follow that

For inhalation, we have


which corresponds to 0.08 mg/cancer.

For ingestion, we have


which corresponds to 480 mg/cancer.

References for the quantities given in the expressions above:

   * 0.08 mCi/mg: Homann, S. G., HOTSPOT Health Physics Codes for the PC,
     Lawrence Livermore National Laboratory, Livermore, CA, UCRL-MA-106315

   * [Image] rem/mCi (inhalation), and 52 rem/mCi (ingestion; we have used
     [Image], the value appropriate for plutonium oxide, for the fraction of
     plutonium absorbed from the GI tract into the bloodstream): Limiting
     Values of Radionuclide Intake and Air Concentration and Dose Conversion
     Factors for Inhalation, Submersion and Ingestion, U.S. Environmental
     Protection Agency, Washington, DC, Federal Guidance Report No. 11

   * [Image] cancer/rem: ICRP 60 (Ref. 25).

The dose associated with a given plutonium intake can be calculated by
dropping the final term in the expressions above and multiplying the first
two terms:

For inhalation, we have


For ingestion, we have


Appendix B.

The following argument indicates why the amount of plutonium that could be
inhaled is independent of the height and extent of the cloud. This amount
(I, in milligrams) is given by concentration (C, in milligrams per cubic
meter) times breathing rate (b, in cubic meters per hour) times breathing
time (t, in hours) times number of people exposed (N). Using the symbols
just given, we can write this as


For a cloud of height h, length l, and width w, containing a total mass Q of
respirable particles, the concentration is given by C = Q/lwh. The breathing
time is just the lifetime of the cloud, which we estimate as the time
required for a particle to fall to the ground from the top of the cloud
(height h) at speed v, which is t = h/v. The number N of people exposed,
given an average population density  [[rho]]  and a cloud whose "footprint"
area on the ground is lw, is just N =  [[rho]] lw.

Making these substitutions, we have



which is just Eq. (6) of Fetter and von Hippel (Ref. 17).

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