|The uranium fuel inside the reactor will gradually be used up while the reactor is at power. As such, a refueling outage, which is usually carried out 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) which contain highly radioactive nuclides, will be moved to store in a "spent fuel pool" in the Fuel Building adjacent to the Containment Building. The spent fuel is stored under borated water inside the pool to shield the radiation and the water is circulated for extraction of the residual heat from the spent fuel.