Electricity generation from nuclear power reactors is an internationally proven technology based on the nuclear chain reaction induced by uranium burning. When a uranium nucleus absorbs a neutron, the nucleus splits in two fission products and releases some energy in the form of heat. Two or three additional neutrons are thrown off during this process. These released neutrons progressively transfer their kinetic energy through successive collisions with other uranium nuclei releasing further neutrons and creating a fission 'chain reaction’. When this happens over and over again, many millions of times, a very large amount of heat is produced from a relatively small amount of uranium. It is this process, in effect "burning" uranium, which occurs in a nuclear reactor.
In a nuclear reactor the heat created by a chain reaction is used to make steam which spins a turbine to drive a generator, producing electricity. This chain reaction is triggered, developed, maintained or stopped through controlling the population of neutrons with the help of rods, which can be inserted or withdrawn to set the reactor at the required power level.
The fuel elements are surrounded by a substance called a moderator to slow the speed of the neutrons and thus enable the chain reaction to continue. Water, graphite and heavy water are used as moderators in different types of reactors. 435 nuclear power reactors are today in operation in 30 countries. Most of them are light water type reactors.