SIDEBAR 1.2Uranium Enrichment and Use

Most uranium found in nature contains about 99.3 percent by weight of uranium-238 and about 0.7 percent by weight of uranium-235 along with minor amounts of other uranium isotopes. Enrichment refers to processes used to increase the weight percentage of uranium-235 relative to uranium-238. Uranium that is enriched to less than 20 percent uranium-235 by weight is referred to as low enriched uranium (LEU). Highly enriched uranium (HEU) contains 20 percent or greater weight percent of uranium-235. HEU enriched to 90 percent or above is referred to as weapons-grade HEU.

The processes used to enrich uranium exploit the small (three-neutron) mass difference between uranium-235 and uranium-238. Two enrichment processes, gaseous diffusion and gas centrifuge, have been used to produce HEU in the United States and several other countries for defense and civilian applications.

As noted in the text, the primary concern with civilian utilization of HEU is its potential diversion by terrorists to make nuclear explosive devices. The International Atomic Energy Agency (IAEA) defines a significant quantity of HEU to be the approximate quantity of HEU from which the possibility of manufacturing a nuclear explosive device cannot be excluded (see https://www.iaea.org/sites/default/files/iaea_safeguards_glossary.pdf). The IAEA significant quantity for HEU is 25 kilograms (kg). HEU from unirradiated targets or target processing waste could potentially be used to make a nuclear explosive device if sufficient quantities could be stolen by terrorists.