They are made from enriched uranium oxide . This contains approximately 3-4% U-235. The fuel is then converted into small pellets, packed into tubes called pins. The pins are bundled into larger tubes and are ready to be put into the reactor.
Moderators slow down the neutrons. Materials used for moderators include graphite and heavy water, containing the isotope deuterium (H-2).
The core also has neutron-absorbing control rods to control the neutrons released. While the moderator promotes stronger interaction of the neutrons with the uranium nuclei, the control rods are there to reduce the concentration of neutrons whenever necessary.
The material used for these control rods is cadmium whose nuclei have a large cross-section for the capture of neutrons. The cadmium rods are inserted into the nuclear reactor to slow down or shut down the chain reaction whereas withdrawing the rods restarts or speeds up the reaction.
The temperature of the reactor is very high although the actual temperature depends on the reactor type. Some reactors operate at temperatures as high as 1500 °C. The coolant flows around the core and carries heat energy to a steam generator. The coolant materials vary between reactors. They may be water or carbon dioxide at high pressures. Some reactors such as fast breeders use liquid sodium as coolant.
Generation of electricity
The coolant carries the heat energy to a heat exchanger (or steam generator), where it is used to make steam. The steam then drives the turbine, and the turbine drives the generator that produces electricity. Waste steam goes to the cooling towers to be condensed back into water.