Enriched uranium
Enriched uranium is a type of uranium in which the percent composition of uranium-235 has been increased through the process of isotope separation. Naturally occurring uranium is composed of three major isotopes: uranium-238, uranium-235, and uranium-234. 235U is the only nuclide existing in nature that is fissile with thermal neutrons.
Enriched uranium is a critical component for both civil nuclear power generation and military nuclear weapons. Low-enriched uranium is necessary to operate light water reactors, which make up almost 90% of nuclear electricity generation. Highly enriched uranium is used for the cores of many nuclear weapons, as well as compact reactors for naval propulsion and research, as well as breeder reactors. There are about 2,000 tonnes of highly enriched uranium in the world.
Enrichment methods were first developed on a large scale by the Manhattan Project. Its gaseous diffusion method was used in the 1940s and 1950s, when the gas centrifuge method was developed in the Soviet Union, and became widespread.
The 238U remaining after enrichment is known as depleted uranium, and is considerably less radioactive than natural uranium, though still very dense. Depleted uranium is used as a radiation shielding material and for 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. 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%. 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 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 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 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 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 has a concentration of under 2% 235U.High-assay LEU (HALEU)
High-assay LEU is enriched between 5% and 20% and is called for in many small modular reactor 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 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 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 first uranium bomb, Little Boy, dropped by the United States on Hiroshima in 1945, used of 80% enriched uranium. Wrapping the weapon's fissile core in a neutron reflector can dramatically reduce the critical mass. Because the core was surrounded by a neutron reflector, at explosion it comprised almost 2.5 critical masses. Neutron reflectors, compressing the fissile core via implosion, fusion boosting, and "tamping", which slows the expansion of the fissioning core with inertia, allow nuclear weapon designs that use less than what would be one bare-sphere critical mass at normal density. The presence of too much of the 238U isotope inhibits the runaway nuclear chain reaction that is responsible for the weapon's power. The critical mass for 85% highly enriched uranium is about, which at normal density would be a sphere about in diameter. The highly enriched uranium core used in the first Chinese nuclear test, Project 596, weighed about 15 kilograms, corresponding to a diameter of 11.4 cm.
Later U.S. nuclear weapons usually use plutonium-239 in the primary stage, but the jacket or tamper secondary stage, which is compressed by the primary nuclear explosion, often uses HEU with enrichment between 40% and 80% along with the fusion fuel lithium deuteride. This multi-stage design enhances the efficiency and effectiveness of nuclear weapons, allowing for greater control over the release of energy during detonation. For the secondary of a large nuclear weapon, the higher critical mass of less-enriched uranium can be an advantage as it allows the core at explosion time to contain a larger amount of fuel. This design strategy optimizes the explosive yield and performance of advanced nuclear weapons systems. The 238U is not said to be fissile but still is fissionable by fast neutrons such as the ones produced during D–T fusion.
HEU is also used in fast neutron reactors, whose cores require about 20% or more of fissile material, as well as in naval reactors, where it often contains at least 50% 235U, but typically does not exceed 90%. These specialized reactor systems rely on highly enriched uranium for their unique operational requirements, including high neutron flux and precise control over reactor dynamics. The Fermi-1 commercial fast reactor prototype used HEU with 26.5% 235U. Significant quantities of HEU are used in the production of medical isotopes, for example molybdenum-99 for technetium-99m generators. The medical industry benefits from the unique properties of highly enriched uranium, which enable the efficient production of critical isotopes essential for diagnostic imaging and therapeutic applications.
Enrichment methods
is difficult because two isotopes of the same element have nearly identical chemical properties, and can only be separated gradually using small mass differences. This problem is compounded because uranium is rarely separated in its atomic form, but instead as a compound.A cascade of identical stages produces successively higher concentrations of 235U. Each stage passes a slightly more concentrated product to the next stage and returns a slightly less concentrated residue to the previous stage.
There are currently two commercial methods employed internationally for enrichment: gaseous diffusion and gas centrifuge, which consumes only 2% to 2.5% as much energy as gaseous diffusion. Some work is being done that would use nuclear resonance; however, there is no reliable evidence that any nuclear resonance processes have been scaled up to production.