Re: Public Comment - DOE - Pu for Space Use RADSAFER

[Ray Russ <rruss@stanford.edu>]

>A Notice Of Intent has been published.  The DOE intends to prepare
>an Environmental Impact Statment (EIS) for the proposed production
>of Pu-238 for use in Advanced Radisotope Power Systems for future
>pace missions (DOE/EIS-299).
>
>The public comment period began 5 October and will continue
>untill 4 November.
>
>DOE contact is:
>
>Colette Brown, Office of Nuclear Energy, Science and Technology
>(NE-50)
>USDOE
>19901 Germantown Road
>Germantown, MD 20874
>
>Telephone: 301-903-6924
>FAX: 301-903-1510
>
>Email: Colette.Brown@HQ.DOE.GOV
>
>You may call 1-800-708-2680 and leave a verbal message.
>
>
>Jesse Coleman
>RADSHOALS@AIRNET.NET

Thanks for the message Jesse. Attched below is a copy of a message I
received from Jeff Cuzzi a NASA scientist involved with the Cassini mission
which was successfully launched last year. I've included the full text of
Jeff's message so that those who might be unfamiliar with some of the
issues surrounding RTGs might quickly get up to speed. I believe Jeff has
done a superb job of countering many of the outrageous claims made by some
of the more vocal and misinformed critics of radioisotopic power systems
used in some of NASAs space missions. The information included below might
also be used by any RADSAFE reader interested in contributing to the public
comment record.

Ray Russ
SLAC OHP


----begin attached message----


I am the cassini interdisciplinary scientist for rings, and was involved in its
planning. Thanks very much to you and your people for helping out with the
Plutonium hysteria. Below I attach a "do-it-yourself" document that hopes to
expose the fallacy of certain doom; it was reviewed by Otto Raabe. This little
paper has been posted on a few web sites (our own will be rings.arc.nasa.gov,
starting tomorrow we hope). I'm not sure I can join you but I might be able
to; I am still not comfortable with how things are going. Please give me a
call at 650-604-6343 and let's talk about what's going on.

>>>>>>>>>>>>>>>>>>

          	Cassini Plutonium for the technically minded
				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 ten 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 Agency (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 considered. 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 robust.

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.

Notes:

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).

References:

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 US.

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

ASTDR Web site: http://atsdr1.atsdr.cdc.gov:8080/ToxProfiles/phs9021.html.

JPL Cassini Home Page: http://www.jpl.nasa.gov/cassini/
and http://www.jpl.nasa.gov/cassini/MoreInfo/rtginfo/riskframes.htm


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