Mixed Oxide (MOX) fuel is a nuclear fuel type that blends plutonium dioxide (PuO2) with depleted or natural uranium dioxide (UO2), enabling the recycling of plutonium recovered from spent fuel reprocessing as fresh reactor fuel. In MOX fuel, plutonium-239 and plutonium-241 serve as the primary fissile isotopes, substituting for the enriched uranium-235 in conventional uranium oxide fuel. France has been the world leader in MOX fuel utilization, with Orano's La Hague facility reprocessing spent fuel and the MELOX plant fabricating MOX assemblies used in approximately 20% of French reactor cores. Japan, the UK (historically), and several other nations have also pursued MOX programs, though the U.S. abandoned its MOX Fuel Fabrication Facility at Savannah River in 2018 after cost overruns.
For the advanced reactor sector, MOX fuel is central to Newcleo's lead-cooled fast reactor strategy. Newcleo's LFR-AS-200 (200 MWe) is specifically designed to use MOX fuel manufactured from reprocessed spent fuel, positioning the company at the intersection of waste management and energy production. This strategic dependency on plutonium availability was the direct cause of Newcleo's relocation from the UK to France in 2025, after the UK government decided to immobilize its civil plutonium stockpile at Sellafield rather than making it available for MOX fabrication. France, with its established reprocessing infrastructure and decades of MOX experience, provides a more supportive policy environment for Newcleo's fuel cycle approach. The company has raised over $755 million since 2021 to advance both its reactor and fuel development programs.
The broader relevance of MOX to the SMR industry lies in the closed fuel cycle concept. Fast-spectrum reactors with breeding ratios near or above 1.0, including designs from TerraPower, Oklo, and ARC Clean Technology, can theoretically transition to a closed cycle where plutonium bred in the reactor is recovered and recycled as MOX or metallic fuel, dramatically extending uranium resource availability and reducing the radiotoxicity and volume of high-level waste requiring geological disposal. However, reprocessing and MOX fabrication add significant cost and complexity to the fuel cycle, and raise proliferation concerns that require robust international safeguards. The economic and political viability of the closed fuel cycle remains one of the most consequential long-term questions for the nuclear industry, with implications for uranium market demand, repository requirements, and the ultimate sustainability of nuclear energy.