Long-term storage of plutonium separated from fission products is not a good solution according to the current non-proliferation criteria as well as from an economic point of view. This material has thus to be converted to the equivalent of the “spent fuel standard.” Only one technique has so far reached the industrial maturity necessary to convert the important existing plutonium stockpiles: it is the use of plutonium to manufacture and irradiate mixed-oxide (MOX) fuel. ©2000 American Institute of Physics.