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Table 2.2 Uses and Availabilities of Plutonium Isotopes - doe-std-1128-98_ch10018
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DOE Standard Guide of Good Practices for Occupational Radiological Protection In Plutonium Facilities
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Table 2.3. Radioactive Decay Properties of Selected


DOE-STD-1128-98
National Council on Radiation Protection and Measurements (NCRP) have
recommended increasing quality factors to a value of 20 for fast neutrons (ICRP,
1985; NCRP, 1987a). Thus, it may be desirable to reduce neutron exposures.
Neutrons arise primarily from even-numbered plutonium isotopes (mostly 238Pu and
240Pu) as a result of spontaneous fission and alpha-neutron reactions with low-atomic-
number impurities in the plutonium. The 239Pu -enriched product of LIS will have
reduced concentration of these isotopes, resulting in lower intrinsic neutron
exposures. The LIS process can also result in significant reductions in gamma-ray
exposures for the product enriched in 239Pu. Much of the whole-body and most of the
extremity exposure is a result of surface contamination on the gloves and the interior
of the glovebox. The 241Am decay product, which results from the beta decay of 241Pu,
is a major contributor. Thus, the reduction of 241Pu can significantly reduce exposures
to hands and arms, as well as reduce the radiation streaming through glove ports in
shielded gloveboxes.
2.2
NUCLEAR PROPERTIES
Of the 15 plutonium isotopes, the two that have proven most useful are masses 239
and 238. Plutonium-239 is fissile, i.e., atoms of plutonium split upon exposure to
thermal or fast neutrons. Chemical reactions can release a few electron volts of
energy per atom; however, when a plutonium nucleus splits, it releases about 200
MeV of energy and two or three neutrons. This release of energy makes 239Pu useful
for nuclear weapons and reactor fuel. In fact, in light water reactors (LWRs) much of
the power originates from the fission of 239Pu, which is produced by neutron capture
in 238U. Because of its higher specific activity, 238Pu is used as long-lived heat sources
for powering planetary space missions where adequate solar energy is not available.
As mentioned before, all plutonium isotopes are radioactive. Isotopes with even mass
numbers (except mass number 246) are primarily alpha emitters. Isotopes of mass
numbers 232, 233, 234, 235, and 237 also decay by electron capture; isotopes of
mass numbers 241, 243, 245, and 246 decay by beta emission. Many of the alpha-
emitting isotopes, such as 238Pu and 240Pu, also fission spontaneously and emit
neutrons. All of the particle emissions are accompanied by X-ray and gamma-ray
emissions over a wide range of energies.
A review of the nuclear properties of plutonium (e.g., cross-sections, nuclear levels,
half-lives, and fission yields) can be found in Volume 1 of the Plutonium Handbook:
A Guide to the Technology (Wick, 1967) and in American National Standards
Institute (ANSI) Standard N317, Performance Criteria for Instrumentation Used for
In-Plant Plutonium Monitoring (ANSI, 1980a). Plutonium decay schemes, neutron
yields, and neutron energy spectra are described in the following sections.
2.2.1 Decay Schemes
The decay modes of some important plutonium and other isotopes and decay
products are shown in Table 2.3. For brevity, only the most abundant radiations
have been included in the table; more detailed information can be found in
papers by Gunnink and Morrow (1967) and Klein (1971), in ICRP Publication
38 (ICRP, 1983), and from the National Nuclear Data Center. Most of the
isotopes are strong alpha-emitters, making alpha heating a problem for the
storage and handling of large amounts of plutonium. The specific activities
2-5


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