Operation
About a quarter to a third of the total fuel load of a reactor is removed from the core every 12 to 18 months and replaced with fresh fuel. Spent fuel rods generate intense heat and dangerous radiation that must be contained. Fuel is moved from the reactor and manipulated in the pool generally by automated handling systems, although some manual systems are still in use. The fuel bundles fresh from the core are normally segregated for several months for initial cooling before being sorted in to other parts of the pool to wait for final disposal. Metal racks keep the fuel in safe positions to avoid the possibility of a "criticality" — a nuclear chain reaction occurring. Water quality is tightly controlled to prevent the fuel or its cladding from degrading. Current regulations in the United States permit re-arranging of the spent rods so that maximum efficiency of storage can be achieved.
The maximum temperature of the spent fuel bundles decreases significantly between 2 and 4 years, and less from 4 to 6 years. The fuel pool water is continuously cooled to remove the heat produced by the spent fuel assemblies. Pumps circulate water from the spent fuel pool to heat exchangers, then back to the spent fuel pool. The water temperature in normal operating conditions is held below 50°C (120°F). Radiolysis, the dissociation of molecules by radiation, is of particular concern in wet storage, as water may be split by residual radiation and hydrogen gas may accumulate increasing the risk of explosions. For this reason the air in the room of the pools, as well as the water, must be continually monitored and treated.
Read more about this topic: Spent Fuel Pool
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