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DOE-STD-3013-2004
B.4.
Plutonium Radioactive Decay and Heat Generation
This portion of Appendix B is intended to provide basic information about the various
radionuclides of interest to the Standard, and to illustrate the behavior of the specific heat
generation rate as a function of time for a variety of isotopic mixes. This material is not
intended to replace methods of determining heat generation rates that the sites may decide to
use in conjunction with this Standard.
B.4.1
Expected Isotopic Compositions
Plutonium is produced in a nuclear reactor, and the vast majority of plutonium in the Complex
was produced either in Hanford production reactors or Savannah River production reactors. It is
produced by irradiating uranium, and in these production reactors the uranium has a low
238
U accepts a neutron
enrichment and is in a metallic form often referred to as a "target." The
239
239
Pu after beta decay through neptunium. The
Pu thus produced is
and is converted to
exposed to the neutron flux as the target remains in the reactor. Most, but not all neutron
239
240
Pu cause fission, but some produce
Pu. That isotope will accept a neutron to
absorptions in
241
242
241
Pu, which, in turn, will accept another to produce
Pu, provided the
Pu does not
produce
235
U in the target will be
fission. In addition, through a similar chain of neutron absorptions,
236
237
238
U and
Np to produce
Pu. Consequently, plutonium can be expected to
converted through
have isotopes from 238 to 242 in noticeable quantities. The exact mix of isotopes will depend on
the irradiation time and the target and reactor characteristics, with longer irradiation times
238
Pu, and
producing more of the higher isotopes, higher target enrichment producing more
238
241
Pu and
Pu
reactor characteristics having effects that are less easily described. Note that
both require three neutron absorptions, and, as a consequence, their concentrations as a
function of time will behave in a similar fashion.
Once the target is discharged from the reactor, production of these isotopes stops, and any
241
further changes are the result of radioactive decay. With the exception of
Pu, all the
plutonium isotopes decay by alpha decay. For the time spans of interest here, their progeny are
not important in terms of heat generation or helium production with the exception, again, of
241
241
Pu. That isotope decays fairly rapidly by beta decay into
Am, which then decays somewhat
more slowly by alpha decay. Table B-5 contains pertinent data for these isotopes, and for some
uranium isotopes of interest.
64


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