Natural uranium consists of two different types of uranium, U-235 and U-238. U-238 has three extra neutrons. At this stage in the fuel cycle, the uranium will consist of 99.3% U-238, and 0.7% U-235.
U-235 is more useful in nuclear fuel, so enrichment will increase the percentage of U-235 to 3.5%.
After this, the uranium will be turned back into an oxide (UO2).
