A nuclear power plant uses uranium as fuel. Natural uranium primarily consists of U-238 atoms with a small percentage of U-235 atoms. It is the U-235 atoms that undergo fission. When one atom of U-235 is bombarded with a neutron, the neutron is absorbed and the uranium atom is broken into an atom of barium, an atom of krypton with the release of 3 nucleons in the process. As the mass of the resultant particles is less than that of the uranium atom and the neutron there is a large amount of energy released.
For a nuclear reactor to function the number of neutrons being released when the U-235 is split should be sufficient to sustain a chain reaction; when this condition is achieved it is called criticality. Light water fission reactors use normal water to transfer heat from the fission reaction to produce electricity. As the mass of a nucleon is very close to that of a water molecule, the water absorbs many of the nucleons. This prevents the chain reaction from continuing unless the percentage of U-235 in the uranium fuel is high enough.
As natural uranium has only 0.72% of U-235, it has to be processed or enriched to increase the percentage of U-235. The enrichment process is necessary for the functioning of light water fission reactors.