They are placed inside long metal fuel rods. These rods are grouped together into fuel assemblies, which are placed inside the reactor. This pictures shows how the fission process takes place. Then begins a process called fission. Uranium atoms, one after another, begin to split in a chain reaction.
Fission yields a tremendous amount of energy in the form of heat.
This heat is used to boil water into steam, which drives a turbine-generator to produce electricity. Certain changes take place in the fuel during the fission process. Over time, these trapped fission fragments reduce the efficiency of the chain reaction. So, about every 18 months, the oldest fuel assemblies, which have already released their energy, are removed and replaced with fresh fuel. Small A typical nuclear power plant produces about 30 tons of used fuel each year.