The uranium fuel inside the reactor will gradually be used up while the reactor is at power. As such, a refuelling outage, which is usually conducted annually or in every 18 months depending on the enrichment of U-235 in the fuel, will be carried out at the end of a fuel cycle. About one-third of the fuel assemblies will be replaced with new ones during each refuelling outage. The used fuel assemblies (spent fuel) contain highly radioactive nuclides and generate a considerable amount of heat due to radioactive decay. Hence, it would be transported for storage in a "spent fuel pool" inside the Fuel Building situated adjacent to the Containment Building. The process, carried out under water, includes unloading of fuel assemblies from the reactor core and its transportation to the spent fuel pool. The spent fuel pool is filled up with borated cooling water to shield the radiation. In addition, it is equipped with a cooling system to extract the decay heat from the spent fuel by circulation of the pool water. The bottom of the spent fuel pool has over 700 storage cells and allows storage of all spent fuels taken out from the reactor unit up to ten years.