(nuclear physics, military) Enriched uranium containing at least 20% uranium-235 (some applications, such as fission weapons and naval reactors, use uranium enriched to as high as 80-95% uranium-235), and therefore capable of sustaining a nuclear fission chain reaction without needing a moderator; used in nuclear weapons and in a few types of nuclear reactors requiring very high power density or high neutron flux.

(nuclear physics) Enriched uranium with 0.9-2% uranium-235, requiring a special moderator with very low neutron absorption (such as heavy water or high-purity graphite) in order to be able to sustain a nuclear fission chain reaction. Used to increase power output and efficiency in a few types of nuclear reactors capable of running on natural uranium, and for downblending of highly-enriched uranium from decommissioned nuclear weapons into low-enriched uranium suitable for use in commercial power reactors.

(archaic) second-person singular simple present form of enrich