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Spent fuel pool
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Spent fuel pool
Spent fuel pools (SFP) are storage pools (or "ponds" in the United Kingdom) for spent fuel from nuclear reactors. They are typically 40 or more feet (12 m) deep, with the bottom 14 feet (4.3 m) equipped with storage racks designed to hold fuel assemblies removed from reactors. A reactor's local pool is specially designed for the reactor in which the fuel was used and is situated at the reactor site. Such pools are used for short-term cooling of the fuel rods. This allows short-lived isotopes to decay and thus reduces the ionizing radiation and decay heat emanating from the rods. The water cools the fuel and provides radiological protection from its radiation.
Pools also exist on sites remote from reactors, for longer-term storage such as the Independent Spent Fuel Storage Installation (ISFSI), located at the Morris Operation, or as a production buffer for 10 to 20 years before being sent for reprocessing or dry cask storage.
While only about 20 feet (about 6 m) of water are 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 a quarter to a third of the total fuel load of a reactor is removed from the core every 12 to 24 months and replaced with fresh fuel. Spent fuel rods generate heat and 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. In the SFP at a pressurized water reactor (PWR), the fuel bundles fresh from the core are normally segregated for several months for initial cooling before being sorted into other parts of the pool to wait for final disposal. Metal racks keep the fuel in controlled positions for physical protection and for ease of tracking and rearrangement. High-density racks also incorporate boron-10, often as boron carbide (Metamic, Boraflex, Boral, Tetrabor and Carborundum) or other neutron-absorbing material to ensure subcriticality. Water quality is tightly controlled to prevent the fuel or its cladding from degrading. This can include monitoring the water for contamination by actinides, which could indicate a leaking fuel rod. In addition, boric acid may be added to the water in PWR spent fuel pools to further increase the margin to criticality. 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 two and four years, and less from four to six 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.
Rather than manage the pool's inventory to minimize the possibility of continued fission activity, China is building a 200 MW 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 swimming pool 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.
The neutron absorbing materials in spent fuel pools have been observed to degrade over time, reducing the safety margins of maintaining subcriticality; in addition, it has been shown that the in-site measurement technique used to evaluate these neutron absorbers (Boron Areal Density Gauge for Evaluating Racks, or BADGER) has an unknown degree of uncertainty.
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Spent fuel pool
Spent fuel pools (SFP) are storage pools (or "ponds" in the United Kingdom) for spent fuel from nuclear reactors. They are typically 40 or more feet (12 m) deep, with the bottom 14 feet (4.3 m) equipped with storage racks designed to hold fuel assemblies removed from reactors. A reactor's local pool is specially designed for the reactor in which the fuel was used and is situated at the reactor site. Such pools are used for short-term cooling of the fuel rods. This allows short-lived isotopes to decay and thus reduces the ionizing radiation and decay heat emanating from the rods. The water cools the fuel and provides radiological protection from its radiation.
Pools also exist on sites remote from reactors, for longer-term storage such as the Independent Spent Fuel Storage Installation (ISFSI), located at the Morris Operation, or as a production buffer for 10 to 20 years before being sent for reprocessing or dry cask storage.
While only about 20 feet (about 6 m) of water are 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 a quarter to a third of the total fuel load of a reactor is removed from the core every 12 to 24 months and replaced with fresh fuel. Spent fuel rods generate heat and 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. In the SFP at a pressurized water reactor (PWR), the fuel bundles fresh from the core are normally segregated for several months for initial cooling before being sorted into other parts of the pool to wait for final disposal. Metal racks keep the fuel in controlled positions for physical protection and for ease of tracking and rearrangement. High-density racks also incorporate boron-10, often as boron carbide (Metamic, Boraflex, Boral, Tetrabor and Carborundum) or other neutron-absorbing material to ensure subcriticality. Water quality is tightly controlled to prevent the fuel or its cladding from degrading. This can include monitoring the water for contamination by actinides, which could indicate a leaking fuel rod. In addition, boric acid may be added to the water in PWR spent fuel pools to further increase the margin to criticality. 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 two and four years, and less from four to six 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.
Rather than manage the pool's inventory to minimize the possibility of continued fission activity, China is building a 200 MW 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 swimming pool 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.
The neutron absorbing materials in spent fuel pools have been observed to degrade over time, reducing the safety margins of maintaining subcriticality; in addition, it has been shown that the in-site measurement technique used to evaluate these neutron absorbers (Boron Areal Density Gauge for Evaluating Racks, or BADGER) has an unknown degree of uncertainty.
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