After manufacture, nuclear fuel is transported from the production facility to a nuclear power plant for use in a reactor. Specialized transport companies transport nuclear fuel assemblies which release little radioactivity and do not require special shielding.
In a typical nuclear reactor, sets of fuel rods called cells surround a control rod which can be inserted or withdrawn to control the neutron flux and thus, the rate of the chain reaction.
U-235 atoms are bombarded by neutrons and fission which produces heat and more neutrons. Some of the U-235 transmutes into plutonium which also undergoes fission producing about one third of the heat in the reactor core. The heat from the core is used to produce steam which drives the turbines that produce the electricity.
As the nuclear fuel in the rods fissions, the heat generated causes thermal expansion which can cause cracking. The nuclear fuel reacts with cladding materials such zirconium alloy which forms the shell of the fuel rod. The chemical composition of the fuel near the edge of the pellet changes as does its thermal conductivity. The purer uranium oxide in the center of the pellet will reach higher temperatures than the fuel near the outer edge of the pellet.
One ton of natural uranium can generate about fourty four million kilowatt hours. It would require over twenty thousand tons of coal or eight million cubic meters of natural gas to generate the same amount of electricity.
The rate at which the fuel is consumed is measured in gigawatt-days per ton of fuel and it is proportional to the level of concentration of U-235 in the nuclear fuel contained in the rods. The level of heat generation that can be safely handled by the current reactors limits the enrichment to about four percent which will yield a burn up rate of fourty gigawatt-days per ton. With improvements in materials and design, enrichment as high as five percent can be utilized, ultimately producing seventy gigawatt-days per ton.
Only a third of the heat produced by the core is captured in steam production. The other two thirds of the heat is passed to the water of the cooling system and either released in into a body of water such as a large river or the ocean. Alternatively, the water may be sent into cooling towers for evaporative cooling. Normally, a small amount of radioactivity is released into the cooling water.
As the fission fragments build up in the nuclear fuel rods, the heat generated by the fuel declines. Typically, nuclear fuel is depleted in eighteen to thirty six months and is no longer useful. The total amount of energy produced by a given amount of nuclear fuel depends on the configuration of the core and the overall design of the reactor.
Periodically, new sets of fuel rod assemblies must be inserted into the core. About one third of the rods in a core are replaced at one time. Different rods burn up at different rates which is partially dependant on the exact location of a given assembly of rods in the core. The exact placement of the new set of rods is referred to as the optimal fuel reloading problem.