Plutonium Radioactive Decay and Heat Generation

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

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

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

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.