After extraction from ore, a uranium concentrate (“yellowcake”) is manu­factured and shipped to facilities where it is enriched (the proportion of 235U is increased from the natural 0.72 atom%) if required and fabricated into fuel. Enrichment is needed for modern reactor fuels which are made from UO2, and generally requires conversion of the uranium into UF6, a relatively volatile compound which is attractive for enrichment because fluorine is monoisotopic, followed by multiple stages of membrane diffusion or centrifugation. The enrichment process, as used for fuel production, creates two uranium streams, one enriched, typically to 3-5 atom% 235U (referred to as low enriched ura­nium), and one depleted to around 0.2 atom% 235U. Typically, therefore, pro­duction of 1 kg of low enriched uranium creates 5 or 6 kg of depleted uranium, for which there is little current use. In total, about 1.2 M tonnes of depleted uranium exist worldwide. A significant proportion of the global depleted ura­nium inventory is still in the form of UF6, a reactive, corrosive material which is not suitable for long term storage or disposal.

For use in current reactors, enriched uranium is “deconverted” from UF6 into UO3, reduced to UO2, a durable ceramic, and formed into pellets. The pellets are loaded into metal tubes, generally of stainless steel or zircaloy (a range of zirconium-based alloys, often containing tin or niobium), depending on reactor type, and are then suitable for loading into a reactor. Early reactors, such as the first Hanford production reactors in the USA, and the UK Magnox reactors, were designed to use fuel of natural isotopic composition, which obviously avoids the difficulty and cost of enrichment. However, such reactors cannot tolerate the dilution of fissile isotopes which occurs in UO2, and have to use uranium metal, whose properties limit reactor operating temperatures and efficiency.