The process of replacing spent nuclear fuel in a nuclear reactor with fresh fuel. Usually about a quarter of the spent fuel is changed, with the core rearranged to optimise the utilisation of the fissile material. The fuel can be refuelled during a total shutdown, but some reactors (e.g. CANDU) allow for so-called online refuelling, changing the fuel while the reactor is in operation. If the fuel is in the form of fuel assemblies, the exchange is usually done by a remotely operated fuel loader picking up a fuel bundle from the core, placing it in underwater storage, and placing a fresh fuel bundle in its place. In the case of pebble bed reactors, fuel balls are continuously removed from the bottom of the core and fresh ones are added. Molten salts reactors, in which the fuel is dissolved in molten salts, remove the decay products and add fresh fuel continuously at the chemical processing plant.