highly-enriched uranium

Short definition from Wiktionary

Noun

1. 1
   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.