The life cycle of nuclear fuel starts with the mining of uranium ore, the extraction of uranium from the ore and its initial processing. Natural uranium contains only about 0.7% of the fissile isotope ²³⁵U and, for most commercial power reactors, it must therefore be enriched before nuclear fuel is manufactured. Today, uranium enrichment is carried out predominantly using gas centrifuges. The next step is the fabrication of nuclear fuel in a form appropriate for the type of reactor in which it will be used. The fuel typically remains in the reactor for several years, during which some of its fissile material is consumed by nuclear fission and energy is released. After removal from the reactor, the spent fuel is initially stored and cooled in a spent fuel pool. It may subsequently be stored for a longer period, reprocessed to recover reusable materials, or eventually disposed of in a deep geological repository.