10.6. Since natural uranium contains only about 0.7 percent of the ura­nium-235 isotope and commercial LWRs require fuel containing about 3 percent uranium-235, an enrichment step is necessary. Enrichment is dif­
ficult because the effectiveness of separation using possible physical meth­ods depends on the difference in molecular weights of isotopic UF6 com­pounds, which is slight. During World War II, the development of processes yielding uranium enriched to the weapons-grade level of about 93 percent uranium-235 was one of the major challenges of the overall effort. From this activity, a complex of plants using the gaseous diffusion process evolved which were used for many years for reactor fuel enrichment as well as to meet weapons requirements. More recently, gas centrifuge processes have become practical to meet reactor needs. From the viewpoint of the fuel manager, process details are secondary in importance to the materials and work requirements for the separation which are common to any process.