Mixed oxide, or MOX fuel, is a blend of plutonium and natural or depleted uranium which behaves similarly (though not identically) to the enriched uranium feed for which most nuclear reactors were designed. MOX fuel is an alternative to low enriched uranium (LEU) fuel used in the light water reactors which predominate nuclear power generation.
Some concern has been expressed that used MOX cores will introduce new disposal challenges, though MOX is itself a means to dispose of surplus plutonium by transmutation.
Reprocessing of commercial nuclear fuel to make MOX was done in the Sellafield MOX Plant (England). As of 2015, MOX fuel is made in France (see Marcoule Nuclear Site), and to a lesser extent in Russia (see Mining and Chemical Combine), India and Japan. China plans to develop fast breeder reactors (see CEFR) and reprocessing.
The Global Nuclear Energy Partnership, was a U.S. proposal in the George W. Bush administration to form an international partnership to see spent nuclear fuel reprocessed in a way that renders the plutonium in it usable for nuclear fuel but not for nuclear weapons. Reprocessing of spent commercial-reactor nuclear fuel has not been permitted in the United States due to nonproliferation considerations. All of the other reprocessing nations have long had nuclear weapons from military-focused "research"-reactor fuels except for Japan.  Normally, with the fuel being changed every three years or so, about half of the  is burned in the reactor, providing about one third of the total energy.  It behaves like  and its fission releases a similar amount of energy. The higher the burn-up, the more plutonium in the spent fuel, but the lower the fraction of fissile plutonium. Typically about one percent of the used fuel discharged from a reactor is plutonium, and some two thirds of this is fissile (c. 50% , 15% ). Worldwide, some 70 tonnes of plutonium contained in used fuel is removed when refueling reactors each year.