Nuclear Energy and Nuclear Disasters.Uranium enriched to at least 20 percent uranium-235 (a higher concentration than exists in natural uranium ore). For detail, See: Uranium Enrichment
U.S. Nuclear Regulatory Commission (NRC), Full-Text Glossary, Washington, DC, USA 2014. http://www.nrc.gov/reading-rm/basic-ref/glossary/f ull-text.html, (Dzstr/ Disabl Mass Care - Amer) {826}