Fissionable Materials

Fissionable isotopes are either fissile or fertile, as discussed in Chapter 1. Incident neutrons of any energy cause fission in a fissile material. Figure 2.9 depicts the fission cross section of urainium- 235, the only fissile material that occurs naturally. Uranium-238, which makes up 99.3% of natural uranium, fissions only from neutrons with an incident energy of a MeV or more as illustrated by the threshold in its fission cross section plotted in Fig. 2.8. It is, however, fertile, for following neutron capture it decays according to Eq. (1.28) to plutonium-239, which is fissile. Figure 2.10 shows the fission cross section of plutonium-239. If plutonium-239 captures an additional neutron instead of fissioning, it becomes plutonium- 240, which is also a fertile isotope, for and if it captures an additional neutron it becomes plutonium-241, which is fissile. In addition to uranium-238, thorium-232 is a naturally occurring fertile isotope, for following neutron capture it undergoes radioactive decay to become uranium-233, which is fissile. The fission cross section of uranium-233 appears similar to those plotted in Figs. 2.9 and 2.10.

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