Reprocessing of fuel gives rise to materials safeguards concerns in terms of proliferation, particularly for MOX fuels because of the possibility of misappropriation of plutonium. Paradoxically, the use of plutonium in MOX fuels is also cited as a valuable method of reducing weapons-grade plutonium stockpiles through the once-through MOX cycle.
Remember: Weapons-grade plutonium has a low Pu-240 content (high Pu-239 content). Reactor-grade plutonium has a high Pu-240 content because longer irradiation leads to more neutron capture in Pu-239.