I rradiated MOX fuel contains more plutonium and minor actinides than the standard fuel. However, it has been demonstrated that the PUREX process works with irradiated MOX fuel without major difficulties.

In 1991-2, the CEA (French Atomic Energy Commission) reprocessed, in a pilot plant at Marcoule, about 2.1 tonnes of MOX fuel from the German Grafenrheinfeld nuclear power plant. And in 1992, 4.7 tonnes of MOX fuel from the German Obrigheim-Neckar-Unterweser nuclear power plant were reprocessed at La Hague.

Existing MOX contains 5.3% of plutonium. With irradiation, the concentration of even-numbered isotopes in the fuel increases — mainly plutonium-242. Since these isotopes are non-fissile in a pressurized-water reactor (PWR), their neutronics hinder the chain reaction. There are two possible solutions: an increase in either the plutonium content (taking it beyond 5.3%) or the uranium-235 content. The first solution would limit the number of possible recycles because of the build-up of Pu-240. The second is considered to be more promising. On this basis, several recycles of MOX fuel may be possible. Considering the relatively long time for the radioactive and thermal decay of irradiated MOX, a single cycle is likely to take about 12 years and two or three cycles will take 24 or 36 years. Provided that life extensions are permitted, these could be accommodated by existing reprocessing plants. In coming to such decisions, important points to be considered are the impact of the irradiated MOX composition on waste, effluent and releases and the change in composition of the MOX that might be introduced along with the recycles.