Mixed Oxide, or MOX fuel, is an alternate to lightly enriched uranium fuel in the light water reactors which predominate nuclear power generation. Mixed oxide fuel is a blend of plutonium-239 into natural or depleted uranium which behaves similarly (though not identically) to the enriched uranium feed for which most reactors were designed. Relicensing will need to precede the introduction of MOX fuel into existing reactors. 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.

Plutonium is a natural or intended by-product of nuclear power generation, and reprocessing operations, initially developed for the Manhattan project for recovery of weapon-grade plutonium, can recover it for further use. The United States has abandoned the commercial recovery of plutonium in favor of direct disposal of spent nuclear fuel, but plutonium recovery and use as MOX fuel is being practiced or contemplated in Europe and Japan.

A Mox is a famous and expensive card in the collectible card game Magic the Gathering (see power nine ).