Re: Cassini Mission and Plutonium RTG's

[Bernard L Cohen <blc+@pitt.edu>]

On Wed, 27 Aug 1997, Otto G. Raabe wrote:

> 
> 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,

 --I noticed these numbers in UNSCEAR 1982, but they do not agree
with the numbers I gave in my message -- about 3.6 million Curies of
Pu239 evaporated in bomb tests. This is seven times the amount in the 
Cassini probe, rather than 1/2 the amount as Otto states.I don't see how
my calculation can be substantially in error, unless I am going "off
the deep end". Can someone help me with this, by confirming my calculation
or by showing where I went wrong? [My calculation is 150,000 Ktons of
fission with 1.7 Ktons/lb = 90,000 lb of Pu used. 80% of this not consumed
but evaporated = 72,000 lb. Bomb Pu is .11 Curie/gram x 454 gram/lb x
72,000 lb = 3,600,000 Curies.]
	I noted that the UNSCEAR numbers were for Pu "produced in bomb
tests" in Table 21, which may not include Pu evaporated, and "Pu
deposited" in Table 22 (UNSCEAR 1982 page 2380 which would not include Pu
still suspended in the atmosphere), so maybe there is no conflict. Clearly
the number to be compared with the Cassini probe is the amount evaporated,
rather than the amount produced or deposited.

> 
> 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).] 

	--Helen Caldicot never calculated anything; she just used the old
Nader and Gofman statements from 20 years ago, quoted word for word
without noting the difference between Pu238 and Pu239 (I doubt if she
knows the difference), and based on cancer risk according to Gofman.


Bernard L. Cohen
Physics Dept.
University of Pittsburgh
Pittsburgh, PA 15260
Tel: (412)624-9245
Fax: (412)624-9163
e-mail: blc+@pitt.edu