The potential utility of plutonium as a component of MOX fuel depends on its isotopic composition. Thermal irradiation can produce isotopes from 239Pu to 242Pu inclusively (see Table 1). As irradiation time increases, so does the proportion of heavier isotopes, with mass number >239. Only Pu and Pu are fissile in a thermal reactor, so plutonium separated from high burnup fuels will have a lower fissile content than that separated from low burnup fuels. In addition, 241Pu has a relatively short half life so, after storage periods of years to decades, a significant proportion will have decayed to 241Am. This has

two effects. First, the fissile content of the plutonium has decreased and, second, the decay of 241Am is accompanied by a 59.5 keV gamma emission, which makes handling much more difficult because shielded facilities are needed to handle plutonium with a significant americium content. Thus, the Melox plant at Cadarache, which produces MOX fuel, is limited to using plutonium which is less than five years old.

The isotope 238Pu is also formed in irradiated uranium. This is formed by neutron capture in 237Np, itself produced through either an n,2n reaction in 238U, or by successive neutron captures in 235U.