Almost all uranium found in nature contains about 0.7 percent by weight of uranium-235 (U-235) and about 99.3 percent by weight uranium-238 (U-238) along with minor amounts of other uranium isotopes, for example, uranium-234. Enrichment is a process used to increase the concentration of the U-235 isotope relative to U-238. HEU is defined as uranium enriched to concentrations greater than or equal to 20 percent by weight in U-235. Uranium enriched to concentrations less than 20 percent by weight in U-235 is LEU.
Uranium is enriched by exploiting the small (three-neutron) mass difference between U-235 and U-238. Two enrichment processes are in commercial use today: an older and less efficient gaseous diffusion process that was developed during World War II and is still being used in the United States; and a more efficient gas centrifuge process that is being used in Europe, Russia, and other countries. Two centrifuge facilities are currently being constructed in the United States. A third enrichment process (laser enrichment) has been developed but is not used commercially.
Enriched uranium is used to fuel the majority of today’s research and commercial nuclear reactors. Ordinary water is used as a coolant and moderator for light-water reactors (LWRs) that typically use LEU fuel enriched in U-235 up to about 5 percent by weight. The majority of commercial nuclear reactors that produce about 16 percent of the world’s electrical power are LWRs. Most existing research and test reactors were designed to use HEU fuel, but many of these have been or are being converted to LEU fuel (see Chapter 11).
Most of the world’s production of Mo-99 is carried out by irradiating HEU targets in research and test reactors that are fueled with LEU. With one exception, the United States is currently the world’s primary supplier of HEU for Mo-99 production, either directly through DOE or indirectly through the European organization Euratom Supply Agency (ESA). The U.S.-origin HEU that is used for Mo-99 production has an enrichment of about 93 percent U-235 and was originally produced for use in nuclear weapons. The exception is South Africa, which uses its own HEU (which is 45 percent enriched) to