The use of uranium as fuel is based on the fission reaction of the atomic nuclei of its isotope 235U. In a fission reaction, a heavy atom splits into two or more lighter atomic nuclei as a free neutron collides with it. The reaction also releases 2-3 neutrons and a large amount of energy that heats the nuclear fuel. The neutrons released in the reaction can cause new fissions, making a chain reaction possible. Electricity production in a nuclear power plant is based on the utilisation of the thermal energy generated by a controlled chain reaction.
Uranium is a heavy, slightly radioactive metal element that is mainly found in nature as isotopes 238U (99.28%) and 235U (0.71%). Uranium is a relatively common element existing everywhere in the bedrock granite. There are about four grams of uranium per tonne of bedrock and about three milligrams of uranium per tonne of sea water. The reactor physical properties needed to maintain the chain reaction in a light water reactor require enriching the uranium in relation to the isotope 235U. The degree of enrichment in a light water reactor is normally 3-5%.