Enriched uranium
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 of three major isotopes: uranium-238 (238U with 99.2739–99.2752% natural abundance), uranium-235 (235U, 0.7198–0.7202%), and uranium-234 (234U, 0.0050–0.0059%). 235U is the only nuclide existing in nature (in any appreciable amount) that is fissile with thermal neutrons.[1]
Enriched uranium is a critical component for both civil nuclear power generation and military nuclear weapons.
There are about 2,000 tonnes of highly enriched uranium in the world,[2] produced mostly for nuclear power, nuclear weapons, naval propulsion, and smaller quantities for research reactors.
The 238U remaining after enrichment is known as depleted uranium (DU), and is considerably less radioactive than even natural uranium, though still very dense. Depleted uranium is used as a radiation shielding material and for armor-penetrating weapons.
"Separative work"—the amount of separation done by an enrichment process—is a function of the concentrations of the feedstock, the enriched output, and the depleted tailings; and is expressed in units that are so calculated as to be proportional to the total input (energy / machine operation time) and to the mass processed. Separative work is not energy. The same amount of separative work will require different amounts of energy depending on the efficiency of the separation technology.[20] Separative work is measured in Separative work units SWU, kg SW, or kg UTA (from the German Urantrennarbeit – literally uranium separation work). Efficient utilization of separative work is crucial for optimizing the economic and operational performance of uranium enrichment facilities.
Cost issues[edit]
In addition to the separative work units provided by an enrichment facility, the other important parameter to be considered is the mass of natural uranium (NU) that is needed to yield a desired mass of enriched uranium. As with the number of SWUs, the amount of feed material required will also depend on the level of enrichment desired and upon the amount of 235U that ends up in the depleted uranium. However, unlike the number of SWUs required during enrichment, which increases with decreasing levels of 235U in the depleted stream, the amount of NU needed will decrease with decreasing levels of 235U that end up in the DU.
For example, in the enrichment of LEU for use in a light water reactor it is typical for the enriched stream to contain 3.6% 235U (as compared to 0.7% in NU) while the depleted stream contains 0.2% to 0.3% 235U. In order to produce one kilogram of this LEU it would require approximately 8 kilograms of NU and 4.5 SWU if the DU stream was allowed to have 0.3% 235U. On the other hand, if the depleted stream had only 0.2% 235U, then it would require just 6.7 kilograms of NU, but nearly 5.7 SWU of enrichment. Because the amount of NU required and the number of SWUs required during enrichment change in opposite directions, if NU is cheap and enrichment services are more expensive, then the operators will typically choose to allow more 235U to be left in the DU stream whereas if NU is more expensive and enrichment is less so, then they would choose the opposite.
When converting uranium (hexafluoride, hex for short) to metal, 0.3% is lost during manufacturing.[30][31]
Codename[edit]
During the Manhattan Project, weapons-grade highly enriched uranium was given the codename oralloy, a shortened version of Oak Ridge alloy, after the location of the plants where the uranium was enriched.[40] This covert terminology underscores the secrecy and sensitivity surrounding the production of highly enriched uranium during World War II, highlighting the strategic importance of the Manhattan Project and its role in the development of nuclear weapons. The term oralloy is still occasionally used to refer to enriched uranium. Its continued usage serves as a historical reminder of the pivotal role of enriched uranium in shaping the course of modern history and its ongoing significance in various nuclear applications, including energy production, medicine, and scientific research.