MOx

MOX, short for mixed oxide fuel, is a type of nuclear fuel that contains more than one oxide of fissile or fertile materials. The most common form blends plutonium dioxide (PuO2) with uranium dioxide (UO2) to create MOX pellets that are loaded into standard fuel rods for light-water reactors. The plutonium used in MOX is typically recovered from spent nuclear fuel through reprocessing, then refined and mixed with natural or depleted uranium oxides before fabrication.

MOX fuel is fabricated by blending PuO2 with UO2 powder, pressing into pellets, and sintering to form robust ceramic fuel. The pellets are then loaded into fuel rods and assembled into standard fuel assemblies. MOX fabrication requires stringent safeguards because of the radiological and proliferation risks associated with plutonium.

In reactors, MOX can replace a portion of conventional uranium fuel, most often as a partial core insert rather than a full core replacement. Compatibility depends on reactor design and regulatory approval; some reactors permit MOX in limited fractions (for example, up to a third of the core in certain configurations). MOX has different neutronic and thermal characteristics compared with UO2 fuel, due to the presence of plutonium isotopes such as Pu-239 and Pu-241, which affect reactivity, burnup behavior, and heat generation.

Advantages of MOX include recycling plutonium and reducing stockpiles, making use of existing materials and potentially reducing the need for separation of fresh uranium. Challenges include higher fabrication costs, different fuel behavior under irradiation, altered safety analyses, and proliferation concerns requiring strong safeguards and regulatory oversight.

Global use of MOX varies; France and several other countries have operated MOX programs, while the extent of utilization and plant licensing differ by nation and reactor type.