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Re: Alternative isotopes in RTGs?
I surmise that Pu-238 emerged as the RTG isotope of choice based on
consideration of the following information regarding many potential
candidate isotopes:
ISOTOPE Half-life (y) SpA (Ci/g) Energy Output Daughter How Produced
(watts/g)
H-3 12.33 9665 0.325 He-3 (stable) fission
reactor prod
Co-60 5.2714 1130 17.45 Ni-60 (stable) Co-59 (n,gamma)
reactor produced
Kr-85 10.72 392.26 0.59 Rb-85 (stable) fission prod
reactor prod
Sr-90 28.78 138 0.916 Y-90 which decay fission prod
to stable Zr-90 reactor prod
Ru-106 1.023 3297 31.8 Rh-106 decays fission prod
to stable Pd-106 reactor prod
Cs-137 30.07 86.78 0.427 Ba-137 (stable) fission prod
Ce-144 0.78 3183 25.5 Pr-144 decays fission prod
to virtually stable
Nd-144
Tm-170 0.352 5973 11.86 Yb-170 (stable) Tm-169 (n,gamma)
reactor produced
Po-209 102 16.77 0.477 Pb-205 accelerator prod
(long-lived) rare reactions on
Bi-209 (d,2n; p,n)
Po-210 0.379 76.34 141.3 Pb-206 (stable) Bi-209 (n,gamma)
Ac-227 21.773 72.33 28.8 Th-227 then U-238 to
many daughters Ra-226 (n,gamma) &
eventually to beta decay
Pb-207 (stable)
U-232 68.9 22.37 0.70 Th-228 and U-235 decay, then
& radon gas Pa-231 (n,gamma)
then beta decay of
Pa-232
Pu-238 87.7 17.13 0.558 U-234 reactor prod from
(long-lived) U-235
Pu-239 24,110 0.062 0.00187 U-235 U-238 (n,gamma) to
(long-lived) U-239 beta decay to
Np-239 beta decay to
Pu-239
Cm-242 0.446 3314 120 Pu-238 reactor prod
from U-238
Cm-243 29.1 50.56 1.75 Pu-239 reactor prod
from U-238
Cm-244 18.10 80.95 2.78 Pu-240 reactor prod
Cf-250 13.08 109.3 3.9 Cm-246 reactor prod
(long-lived)
For a power consuming, multi-year space mission, in order to maintain a
stable level of heat, I would tend to select those isotopes of half-lives
approximately 100 y, then of those, select the one with the highest heat
output. This small group includes Po-209, U-232, Pu-238, and Pu-239.
Eliminate Po-209 due to difficulty to produce and eliminate U-232 due to
eventual Rn gas buildup which might cause RTG rupture. Pu-239 has too low
a heat output. Having said all this, it should be remembered that RTGs are
not comprised of 100% Pu-238. On a MASS basis, a Cassini RTG is about 82%
Pu-238; 15% Pu-239; 2% Pu-240; 2.5% other actinides; 0.15% impurities; and
13% oxygen.
At 12:53 PM 14-10-97 -0500, you wrote:
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>> Date: Tue, 14 Oct 1997 11:36:23 -0500 (CDT)
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>> From: Bernard L Cohen <blc+@pitt.edu>
>> To: Multiple recipients of list <radsafe@romulus.ehs.uiuc.edu>
>> Subject: Re: Alternative isotopes in RTGs?
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>>
>> On Mon, 13 Oct 1997, Donivan Porterfield, LANL CST-3 wrote:
>>
>> >
>> > Given all the attention to the Pu-238 in the Cassini RTGs
>> > I was wondering what other isotopes might have been suitable
>> > for this application. Below are those alpha emitters with
>> > half-lifes of 10 to 150 years. Presumably beta emitters,
>> > despite possibly having a higher energy density (MeV/Z),
>> > would not be desirable due to bremsstrahlung.
>> >
>> > Po- 209 103 a
>> > Ac- 227 21.8 a
>> > U - 232 72.0 a
>> > Pu- 238 87.7 a
>> > Cm- 243 28.5 a
>> > Cm- 244 18.1 a
>> > Cf- 250 13.1 a
>> >
>> > For example, in retrospect would U-232 have been a less
>> > controversial isotope to use in RTGs? This given that this
>> > or the others are feasible, e.g. cost of of production and
>> > other technical issues such as gamma emmissions or physical
>> > properties.
>>
>> --There is no reason to believe that these would be less dangerous
>> than Pu-238
>>
>U-232 is very rare and would be extremely expensive to produce.
>Further, ingrowth of its daughters, particularly Tl-208, would
>require serious shielding of the RTG until launch.
> Jim Thompson>
>
>
>
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David W. Lee
Los Alamos National Laboratory
Radiation Protection Services Group (ESH-12)
PO Box 1663, MS K483
Los Alamos, NM 87545
PH: (505) 667-8085
FAX: (505) 667-9726
lee_david_w@lanl.gov