While many fissionable isotopes exist in nature, one useful fissile isotope found in viable quantities is U. About 0.7% of the uranium in most ores is the 235 isotope, and about 99.3% is the non-fissile 238 isotope. For most uses as a nuclear fuel, uranium must be enriched - purified so that it contains a higher percentage of U. Because U absorbs fast neutrons, the critical mass needed to sustain a chain reaction increases as the U content increases, reaching infinity at 94% U (6% U).