Spent Fuel Pool Information
Spent fuel pool (SFP) are storage pools for spent fuel from nuclear reactors. Typically 40 or more feet (12,19+ m) deep, with the bottom 14 feet (4,26 m) equipped with storage racks designed to hold fuel assemblies removed from the reactor. These fuel pools are specially designed at the reactor in which the fuel was used and situated at the reactor site. In many countries, the fuel assemblies, after being in the reactor for 3 to 6 years, are stored underwater for 10 to 20 years before being sent for reprocessing or dry cask storage. The water cools the fuel and provides shielding from radiation.
While only about 8 feet (2,43 m) 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.
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Operation
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.
Other possible configurations
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. [1]
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. [2]
Status
Combustion of the nuclear fuel is not possible with American or Canadian electrical power reactor fuel rods since these consist of very stable uranium dioxide pellets. Burning of the cladding is unlikely since storage methods in the United States require fuel to be stored such that in the event of a catastrophic loss of coolant the steady state temperature of the fuel assemblies in air does not exceed the cladding melt temperature.
The Nuclear Regulatory Commission estimates that many of the nuclear power plants in the United States will be out of room in their spent fuel pools by 2015, most likely requiring the use of temporary storage of some kind.[1]
See also
- Deep geological repository
- Dry cask storage
- Lists of nuclear disasters and radioactive incidents
- Nuclear fuel cycle
- Radioactive waste
- Spent nuclear fuel shipping cask
References
External links
- Radiological Terrorism: Sabotage of Spent Fuel Pool
- Storage of Spent Nuclear Fuel U.S. Nuclear Regulatory Commission (NRC)
Categories: Nuclear power plant components | Radioactive waste | Waste treatment technology
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