Re: Cassini Mission and Plutonium RTG's

["Otto G. Raabe" <ograabe@ucdavis.edu>]

August 26, 1997
Davis, CA

The main question relates to complete burn up of the space vehicle on
fly-by, if that is even possible. 72 pound of pure Pu-238 metal with a
specific activity of 17.1 Ci/g would represent about 500,000 Ci of Pu-238.

The United Nations Scientific Committee on the Effects of Atomic Radiation
(UNSCEAR, 1982) provides estimates of the quantities of plutonium that have
already been released to the atmosphere of the earth and that have
deposited on the surface of the earth. The total deposition in the north
temperate zone of the earth is about 7,700 Ci (41 pCi/m2 of 238Pu), 256,000
Ci (1600 pCi/m2) of 239+240Pu. Clearly, vaporization of the Pu-238 in the
Cassini probe could exceed that amount by about a factor of two. However,
the typical dose to people in the northern hemisphere who were alive when
the principle releases occurred is less than 1 mrem per year effective dose
equivalent. Therefore, the dose that might occur to people on the earth
from the total burn-up of the whole 72 kg Pu-238 in the atmosphere would be
only one or two mrem/year effective dose equivalent. The lowest risk would
occur if the Pu-238 is widely dispersed around the earth rather than
concentrated in one location.

The extreme analysis of the "Physicians for Social Responsibility" is based
on inhalation of massive amounts of Pu-238 that lead to acute respiratory
distress and death within a matter of days or weeks. They assume that all
of the plutonium in the space vehicle will end up in the lungs of people in
a short time after an imagined atmospheric re-entry. [The approximate doses
required can be found my paper: O.G. Raabe and M. Goldman. "A predictive
model of early mortality following acute inhalation of PuO2 aerosols."
Radiation Research 78: 264-277 (1979).] 
 
As noted earlier, the problem is delivery. They pose the argument to the
press, that if this plutonium is all converted to inhalable particles and
inhaled by people, the results will be devastating.

Earlier I used the drowning analogy for the people of Chicago. I thank
William Nabor for pointing out to me the better analogy: "If properly
distributed (one gallon to each lung), the great lakes contain enough water
to drown every human being on earth.  There is a similar probability of
this happening as there is the Pu being "properly" distributed." 

Otto

		*****************************************************
		Prof. Otto G. Raabe, Ph.D., CHP
                [President, Health Physics Society, 1997-1998]
		Institute of Toxicology & Environmental Health (ITEH)
		     (Street address: Old Davis Road)
		University of California, Davis, CA 95616
		Phone: 916-752-7754     FAX: 916-758-6140
		E-Mail: ograabe@ucdavis.edu
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