designs using a graphite or heavy water moderator, such as the RBMK and CANDU, are capable of operating with natural uranium as fuel). There are two commercial enrichment processes: gaseous diffusion and gas centrifugation. Both enrichment processes involve the use of uranium hexafluoride and produce enriched uranium oxide. Reprocessed uranium (RepU) Reprocessed uranium (RepU) is a product of nuclear fuel cycles involving nuclear reprocessing of spent fuel. RepU recovered from light water reactor (LWR) spent fuel typically contains slightly more 235U than natural uranium, and therefore could be used to fuel reactors that customarily use natural uranium as fuel, such as CANDU reactors. It also contains the undesirable isotope uranium-236, which undergoes neutron capture, wasting neutrons (and requiring higher 235U enrichment) and creating neptunium-237, which would be one of the more mobile and troublesome radionuclides in deep geological repository disposal of nuclear waste. Low-enriched uranium (LEU) Low-enriched uranium (LEU) has a lower than 20% concentration of 235U; for instance, in commercial LWR, the most prevalent power reactors in the world, uranium is enriched to 3 to 5% 235U. Slightly enriched uranium (SEU) has a concentration of under 2% 235U. High-assay LEU (HALEU) is enriched from 5–20%. Fresh LEU used in research reactors is usually enriched 12 to 19.75% 235U, the latter concentration is used to replace HEU fuels when converting to LEU. Highly enriched uranium (HEU) Highly enriched uranium (HEU) has a 20% or higher concentration of 235U. The fissile uranium in nuclear weapon primaries usually contains 85% or more of 235U known as weapons-grade, though theoretically for an implosion design, a minimum of 20% could be sufficient (called weapon-usable) although it would require hundreds of kilograms of material and "would not be practical to design"; even lower enrichment is hypothetically possible, but as the enrichment percentage decreases the critical mass for unmoderated fast neutrons rapidly increases, with for example, an infinite mass of 5.4% 235U being required. For criticality experiments, enrichment of uranium to over 97% has been accomplished. The very first uranium bomb, Little Boy, dropped by the United States on Hiroshima in 1945, used 64 kilograms of 80% enriched uranium. Wrapping the weapon's fissile core in a neutron reflector (which is standard on all nuclear explosives) can dramatically reduce the critical mass. Because the core was surrounded by a good neutron reflector, at explosion it comprised almost 2.5 critical masses. Neutron reflectors, compressing the fissile core via