A power reactor is usually designed to operate continuously for up to a year between refuelling shutdowns, at each of which up to half the core fuel, which has reached the end of its irradiation life, is removed and replaced by new.

New Fuel. New ceramic fuel subassemblies are usually delivered from the manufacturing plant in an atmosphere of air and can be kept in an air-filled store until required. Radioactive heating of new ceramic fuel is normally very slight so there is little requirement for cooling the store, but it has to be configured to prevent criticality. Before the fuel is committed to the reactor the store has to be purged with an appropriate gas. In the case of a gas-cooled reactor this would be the cooling gas, carbon dioxide or helium. For a metal-cooled reactor it would be the primary circuit cover gas — usually argon in the case of sodium coolant. The subassembly can then be transferred from the store to its required location in the reactor vessel where it is immersed in the primary coolant.

In the case of sodium coolant this is an irrevocable step, because once it has been wetted the subassembly cannot be withdrawn into an air environment until it has been thoroughly cleaned. This is because any residual sodium would react with atmospheric moisture to form caustic sodium hydroxide that might damage the cladding. If a new subassembly had for some reason to be withdrawn before it had been

irradiated it would have to be inspected and requalified before it could be returned to use.

Recycled metal fuel in the IFR system (see section 2.5.6) contains fission products, the radioactive decay of which generates a signific­ant amount of heat. New IFR fuel subassemblies have to be cooled continuously from manufacture until loaded into the reactor.

Irradiated Fuel. The route for withdrawing irradiated fuel from the reactor and dispatching it either for reprocessing or disposal as waste has to provide cooling to remove the decay heat produced by the fission products. It is important to prevent overheating which might cause failure of the cladding and release of radioactive fission products, fuel material or higher actinides to the environment.

The decay heat from a typical irradiated subassembly is shown in Figure 4.8. It depends slightly on the irradiation history, the fuel com­position and the burnup. The high initial decay heat rating implies that an irradiated core subassembly has to be kept under forced-convection cooling for a period of around a day. In practice this means that the movement of irradiated fuel cannot start for a day after the reactor has been shut down. During the refuelling period the primary coolant is circulated, usually at around 10% of the full-power rate.

After this initial period the decay heat is low enough for the sub­assembly from a liquid-cooled reactor to be cooled by natural con­vection of the primary coolant. It can be removed from its position in the reactor core and placed in a storage position, but throughout the move it has to be kept immersed in the coolant. Only after a period of several months can it be withdrawn into a gas atmosphere.

The Fuel Transfer Route. The irradiated fuel store has to have the capacity to hold some 200 subassemblies. It can be located either within the primary vessel or in a separate vessel. In some older pool reactors it took the form of a rotating carousel in the primary vessel outside the neutron shield, but a more compact arrangement, which allows the main vessel to be smaller and therefore less costly, is to store the irradiated fuel in a ring around the core outside the neutron shield (see Figure 4.2). The alternative, which allows the main vessel to be even more compact, is to place the store in a separate vessel. The disadvantage of this is that it requires complex transfer equipment to lift the subassemblies out of the main vessel and lower them into the storage vessel while keeping them immersed in the coolant.

The transfer procedure for a sodium-cooled reactor is as follows. Fuel-handling machines are mounted on the eccentric rotating shields in the reactor roof (see Figure 4.3). The shields are manoeuvred to bring a handling machine over the subassembly to be moved. An arm is lowered from the machine and attached to the lifting ring at the top of the subassembly (see Figure 3.20). It then lifts the subassembly out of the core, keeping it below the surface of the sodium. The shields are then rotated to bring the subassembly over a transfer position into which it is lowered. A second move involving a second handling machine may be required to bring it to a position from which it can be removed from the reactor vessel.

At the removal position it is received into a cylindrical container, sometimes called a “bucket”. The bucket full of sodium, with the subassembly in it, is then lifted up out of the reactor vessel and lowered down into the separate fuel storage vessel, where it is retained until it has cooled sufficiently to be removed from sodium. It is then lifted into an inert gas atmosphere, taken to a cleaning facility where the residual sodium is removed, and despatched to reprocessing or long­term storage.