While only about 8 feet of water is needed to keep radiation levels below acceptable levels, the extra depth provides a safety margin and allows fuel assemblies to be manipulated without special shielding to protect the operators.
About one-fourth to one-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 normally are 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 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. 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 permanently be monitored and treated.
Rather than manage the pool’s inventory to minimise the possibility of continued fission activity, the Chinese are building a 200 MWt nuclear reactor to run on used fuel from nuclear power stations to generate process heat for district heating and desalination. Essentially an SFP operated as a deep pool-type reactor; it will operate at atmospheric pressure, which will reduce the engineering requirements for safety.
Other research envisions a similar low-power reactor using spent fuel where instead of limiting the production of hydrogen by radiolysis, it is encouraged by the addition of catalysts and ion scavengers to the cooling water. This hydrogen would then be removed to use as fuel.
Without cooling, the fuel pool water will heat up and boil. If the water boils or drains away, the spent fuel assemblies will overheat and either melt or catch on fire. Fear has been expressed that sabotage, an accident, or an attack which partially or completely drains a plant's spent fuel pool or disables its cooling, might be capable of causing a high-temperature fire that could release large quantities of radioactive material into the environment. Since there is no standard design, most SFPs are housed in far less robust structures than reactor containment vessels and moreover, an SFP often contains much more radioactive material than the reactor core, this is not a misplaced concern.
It is estimated that by 2014, all of the nuclear power plants in the United States will be out of room in their spent fuel pools, most likely requiring the use of temporary storage of some kind. Yucca Mountain is expected to open in 2017 at the earliest.
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