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Standard nuclear reactors use uranium that has been enriched to 3% U-235.
When the uranium inside a reactor is bombarded with neutrons, the U-235 nuclei split apart, releasing energy and giving off 2 or 3 more neutrons per split. These neutrons go on to collide with other U-235 atoms and cause their nuclei to split, and so on. This process continues as a self-sustaining chain reaction.
(Technically speaking, the U-235 absorbs neutrons and transforms to the highly excitable U-236, whose nuclei readily split apart, emitting neutrons.)
What about all the U-238 that does not fission?
A U-238 atom can absorb a neutron and convert to Pu-239 (plutonium). This conversion happens less readily than the fission of U-235. But over time, the Pu-239 builds up in the reactor and starts to fission, contributing a significant amount of energy near the end of the fuel's life.