A mixture of uranium and plutonium oxide used as a nuclear fuel in nuclear reactors. Plutonium usually comes from reprocessing, or weapon-grade plutonium extracted from retired nuclear weapons may be used. A mixture of depleted or natural uranium with plutonium can replace fuel with enriched uranium (3—5 percent of ²³⁵U), i.e., a save on costly enrichment. MOX can be used after minor modifications in existing reactors (mostly PWR), as the use of up to about 50% of MOX does not change the operating characteristics of a reactor.