The fuel load in a conventional pressurised water reactor (PWR) lasts around three years. During this period, the fissile elements in the fuel are depleted and the fuel is enriched with fission products, some of which play a poisoning role. After three years, the spent fuel must be replaced with fresh fuel. Rather than replacing all the fuel at the same time, one third is replaced each year.
Fuel is unloaded with the reactor shut down, with the reactor cooling system depressurised and the reactor vessel open. The operation takes place underwater. Fuel assemblies are transferred from the reactor core to the reactor cavity and then to the spent fuel pit. The role of the water is to protect operators and cool the fuel. During the operation, any fuel assemblies that leak as a result of cladding rupture are detected. Defective fuel assemblies are stored in sealed casks pending specialised processing.
When the fuel is unloaded, it contains extremely radioactive short-lived radioelements (radioactivity is inversely proportional to half-life) that release large quantities of heat. Conversely, these more highly radioactive products disappear during storage. For example, an element with a half-life of 10 days will have decayed by a factor of 200,000 after 6 months.
Irradiated assemblies are stored for at least a year in the spent fuel pit, which is generally located adjacent to the reactor and is also used when loading new fuel. Water has a high heat capacity and readily absorbs released heat. It also provides effective radiation shielding. Radiation breaks down water molecules but does not render the hydrogen and oxygen atoms radioactive. The water in the pool has a characteristic blue colour: certain beta electrons travel faster than light through water, emitting photons with a bluish tinge. This effect is known as Cherenkov light.
After an adequate period in storage, the radioactivity in the waste decays sufficiently to allow subsequent operations. After a year, the fuel's activity level falls to around 2 million curies per tonne. Although still very high, this activity level is compatible with transport to a reprocessing plant. Spent fuel assembly handling operations are robotised.
Non-reprocessed fuel assemblies may remain in storage in the spent fuel pit for decades or more, pending final decisions regarding appropriate final disposal.
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