Plutonium (atomic number 94) and its alloys can be used as nuclear fuels in nuclear reactors and space batteries. Notably Pu239 is the major fissile isotope of plutonium. Among them, plutonium (Pu239) serves as a fissile fuel as its fission cross section is high (742.5 b) with thermal neutrons. Pu241 isotope also has signifi­cant fission cross section in the thermal spectrum (1009 b), whereas Pu240 can act as burnable poison allowing reactor to have constant reactivity throughout reactor lifetime. Plutonium is found in natural uranium only in trace quantity. It is mainly produced artificially by the transmutation reaction of U238 isotope (fertile fuel) with a neutron as described in Eq. (1.2). Radioisotopic thermoelectric generators also use plutonium (Pu238) to power spacecrafts. Plutonium appears originally as a bright silvery-white metal, but soon loses its bright color when oxidized in air. Smaller critical mass of plutonium (almost one-third of that of uranium), its high toxicity, and pyrophoricity warrant safe handling of this metal. Plutonium can be recovered from the spent fuel of a thermal reactor through chemical treatment. However, depleted uranium can be kept together with plutonium for fuels used in fast breeder reactors such as LMFBRs. In other cases, separated plutonium can be used in plutonium-burning reactors.

Hecker [5] noted the following in his paper about plutonium:

“PLUTONIUM is a notoriously unstable metal — with little provocation, it can change its density by as much as 25 pct; it can be as brittle as glass or as malleable as alumi­num; it expands when it solidifies; and its silvery freshly machined surface will tar­nish in minutes, producing nearly every color in the rainbow. In addition, plutonium’s continuous radioactive decay causes self-irradiation damage that can fundamentally change its properties over time.”

As we present in the next several sections various characteristics of plutonium, we must bear in mind this fickle nature of plutonium.