Uranium 235

One of the three natural isotopes of uranium. Natural uranium contains approximately 99.3% U-238, 0.7% U-235 and traces of U-234. U-235 is naturally fissile and the only isotope that can be used as a natural nuclear fuel in nuclear power plants. To be used as a fuel, the uranium must be enriched with U-235 (3 to 5% for light water reactors and up to 19.75% for research reactors).