A commercial nuclear power reactor is loaded with nuclear fuel assemblies, or clusters of metal rods containing typically enriched uranium in oxide form, for reactor operation during which the uranium undergoes nuclear fission and is gradually transformed into fission products and becomes depleted enough not to sustain reactor operation. While it is possible with certain types of reactors to replace the nuclear fuel assemblies or to conduct refuelling while the reactor is operating, most of the reactors operating today need to be shut down before their fuel assemblies can be replaced.

Neutrons are produced and maintained during a continuous chain of nuclear fission in an operating reactor. They are more abundant at the centre of the reactor than at its periphery, where they are more likely to escape. This leads to more intense nuclear fission and faster fuel consumption at the reactor centre than at its periphery. It is possible to calculate the degree of fuel consumption over time in the reactor and devise a plan to identify and replace individual fuel assemblies, so that there will be enough nuclear fuel to keep the reactor critical and operable until the next fuel change.

In the case of a pressurised water reactor, which is employed for most of the commercial nuclear power plants worldwide, the reactor core typically consists of over a hundred fuel assemblies, housed within a sealed pressure vessel during operation, and its fuel replacement can only be performed after the plant is shut down and the pressure vessel is opened. As the extent of consumption of nuclear fuel is non-uniform within the reactor and fuel consumption is higher at the centre than at the periphery, fuel replacement is conducted for the fuel assemblies with the highest consumption that generally represents about a third to over 40% of the fuel assemblies and the remaining fuel assemblies are reloaded into the reactor core for further consumption. The operation is conducted typically every 12 to 18 months, with new fuel assemblies generally dispersed in the middle of the core, and each fuel assembly will experience typically two but sometimes three reloads before reaching its final discharge.

This article is contributed by Ir Richard Fung with the coordination of the Nuclear Division.