Naturally uranium consists of 99.284% uranium-238, with U-235 contributing only 0.711% of the weight. Only the U-235 is fissile, meaning that when it splits in a way the produces a high-energy, high radiation chain reaction.
Slightly enrich uranium contains up to 2% of U-235 and is used in heavy water nuclear reactors to generate electricity. Low-enriched uranium contains less than 20% U-235. When the concentraion is about 3% it is used in light water reactors; when it between 3% and 19.9% it is used for research.
Highly enriched uranium contains more than 20% U-235 and is used for nuclear weapons. Serious security and diplomatic concerns revolve around its production, storage, disposal and smuggling.
Uranium Enrichment - Stories, Links and Resources
about Uranium Enrichment
Feds pull plug on Ky. uranium plant at Paducah and 1,000 jobs
Businessweek - 2013-05-24
Urenco, Powerhouse of the Uranium Enrichment Industry, Wants Out
NYTimes.com - 2013-05-28