. "Appendix A Physics and Technology of Nuclear-Explosive Materials." Monitoring Nuclear Weapons and Nuclear-Explosive Materials: An Assessment of Methods and Capabilities. Washington, DC: The National Academies Press, 2005.
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Monitoring Nuclear Weapons and Nuclear-Explosive Materials
Reactivity, Critical Mass, and Explosive Yield
TABLE A-1 Properties of Nuclear-Explosive Nuclides
Isotope or Mixture
Critical Mass (kg)
Half Life (years)
Decay Heat (watts/kg)
Neutron Production From Spontaneous Fission (per kg-sec)
Main Gamma Energies (MeV)
2.6 from Tl-208
0.66 from Am-241
The critical masses given are for a bare sphere of metal at normal density. Plutonium metal can exist in six different forms corresponding to different crystalline configurations, with different densities. The two of these that are most germane for nuclear weapons are alpha phase (density 19.6 grams per cubic centimeter) and delta phase (density 15.7 grams per cubic centimeter). The indicated critical masses are for alpha-phase plutonium. For delta-phase plutonium, the critical masses would be about 60 percent larger. In the case of Pu-239, neutron production is 22/kg-sec from spontaneous fission but 630/kg-sec from alpha-n reactions. In Pu-238, alpha-n reactions add 200,000/kg-sec to the 2,700,000/g-sec produced by spontaneous fission. In the other cases, augmentation by alpha-n reactions is not significant.
Adapted from: Nuclear Energy Research Advisory Committee, Attributes of Proliferation Resistance for Civilian Nuclear Power Systems, U.S. Department of Energy, October 2000; General Electric, Nuclides and Isotopes, 14th ed., 1989.
The nuclear reactivity of any nuclear-explosive nuclide or mixture of such nuclides depends on the cross sections (reaction probabilities) of the relevant nuclides for induced fission by incident neutrons of various energies and, alternatively, for absorbing such neutrons without fissioning. The reactivity also depends on the geometries, densities, and chemical forms in which the nuclear-explosive nuclides are present, and whether and to what extent the elements or compounds containing the nuclear-explosive isotopes are diluted or contaminated with other nuclides and compounds that can slow or absorb neutrons.
A nuclear explosion is achieved by the rapid assembly, in a suitable geometry, of NEM embodying sufficient nuclear reactivity