Enriched uranium is a type of uranium in which the percent composition of uranium-235 (written 235U) has been increased through the process of isotope separation. Naturally occurring uranium is composed primarily of three isotopes: uranium-238 (238U, 99.2732–99.2752% natural abundance), uranium-235 (235U, 0.7198–0.7210%), and uranium-234 (234U, 0.0049–0.0059%). 235U is the only primordial nuclide present in nature in appreciable quantities that is fissile with thermal neutrons. Enriched uranium is used in both civil nuclear power generation and military nuclear weapons. Low-enriched uranium (LEU), containing less than 20% 235U, is used as fuel in light-water reactors, which make up most nuclear power reactors worldwide. Highly enriched uranium (HEU), containing 20% or more 235U, has been used in nuclear weapons, naval propulsion reactors, some research reactors, and certain specialized reactor designs. There are about 2,000 tonnes of highly enriched uranium in the world. Large-scale enrichment was first developed during the Manhattan Project, which used gaseous diffusion and electromagnetic isotope separation to produce enriched uranium. Historically, gaseous diffusion and the gas centrifuge are the two enrichment methods that have operated on a commercial scale, but gaseous diffusion is now obsolete and has been replaced by centrifuge technology in commercial use. Laser isotope separation has also been developed as a possible third-generation enrichment technology; Global Laser Enrichment has applied to license a laser-based uranium enrichment facility in the United States, but centrifugation remains the enrichment process used commercially. The 238U remaining after enrichment is known as depleted uranium (DU), and is considerably less radioactive than natural uranium, though still very dense. Depleted uranium is used as a radiation shielding material and in armor-penetrating weapons.
Grades Uranium as it is taken directly from the Earth is not suitable as fuel for most nuclear reactors and requires additional processes to make it usable (the CANDU design is a notable exception). Uranium is mined either underground or in an open pit depending on the depth at which it is found. After the uranium ore is mined, it must go through a milling process to extract the uranium from the ore. This is accomplished by a combination of chemical processes with the end product being concentrated uranium oxide, which is known as "yellowcake", contains roughly 80% uranium whereas the original ore typically contains as little as 0.1% uranium. This yellowcake is further processed to obtain the desired form of uranium suitable for nuclear fuel production. After the milling process is complete, the uranium must next undergo a process of conversion, "to either uranium dioxide, which can be used as the fuel for those types of reactors that do not require enriched uranium, or into uranium hexafluoride, which can be enriched to produce fuel for the majority of types of reactors". Naturally occurring uranium is made of a mixture of 235U and 238U. The 235U is fissile, meaning it is easily split with neutrons while the remainder is 238U, but in nature, more than 99% of the extracted ore is 238U. Most nuclear reactors require enriched uranium, which is uranium with higher concentrations of 235U ranging between 3.5% and 4.5% (although a few reactor 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) undergoes a series of chemical and physical treatments to extract usable uranium from spent nuclear fuel. 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. Reprocessed uranium often carries traces of other transuranic elements and fission products, necessitating careful monitoring and management during fuel fabrication and reactor operation.
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) High-assay LEU (HALEU) is enriched between 5% and 20% and is called for in many small modular reactor (SMR) designs. Fresh LEU used in research reactors is usually enriched between 12% and 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. This high enrichment level is essential for nuclear weapons and certain specialized reactor designs. 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.
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