Mixed oxide fuel

Mixed oxide fuel, or MOX, is nuclear fuel made by blending plutonium dioxide with uranium dioxide. It recycles plutonium recovered from spent reactor fuel into new fuel assemblies.

Nuclear fission: The splitting of an atomic nucleus into smaller nuclei, releasing energy and neutrons. Fission in MOX’s plutonium and uranium isotopes supplies reactor heat and further neutrons.
Pressurized water reactor: A light-water reactor that keeps water under pressure to transfer heat without boiling in the core. Many commercial MOX assemblies have been irradiated in pressurized water reactors.
Nuclear reprocessing: The chemical separation of reusable uranium and plutonium from spent nuclear fuel. Reprocessing supplies separated plutonium for the oxide blend, although MOX can also use other plutonium sources.
Low-enriched uranium: Uranium in which the proportion of uranium-235 has been raised above its natural abundance but remains below 20 percent. Most light-water reactors use low-enriched uranium fuel instead of plutonium-bearing MOX.
Nuclear proliferation: The spread of nuclear weapons, weapons-usable materials, or the capacity to produce them. Separating plutonium for MOX fabrication raises security and safeguards requirements.
Plutonium-239: A fissile isotope of plutonium produced in reactors from uranium-238. It is a principal fissile isotope in the plutonium oxide blended into MOX.
Light-water reactor: A nuclear reactor that uses ordinary water as coolant and neutron moderator. Commercial MOX use has chiefly taken place in this broad reactor family.
Spent nuclear fuel: Reactor fuel removed from a core after irradiation, containing fission products and remaining uranium and plutonium. Its plutonium content became a feedstock for reprocessed MOX fuel.
Fast-neutron reactor: A nuclear reactor that sustains fission mainly with high-energy neutrons rather than moderating them to thermal energies. Some fast reactors can use plutonium fuel while offering different fuel-cycle performance from MOX in light-water reactors.
International Atomic Energy Agency safeguards: Measures used by the IAEA to verify that nuclear material is not diverted from peaceful activities. MOX’s plutonium content requires accounting and verification throughout fabrication and use.

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