Spent fuel pools (SFP) are storage pools 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 the reactor. A reactor's pool is specially designed for 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 (2.4 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.
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]
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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.[2]
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)[1]. 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, 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.[3]
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.[4]
If there is a prolonged interruption of cooling, including natural disasters or terrorist attacks, the water in the spent fuel pools may boil off, resulting in large amounts of radioactive elements being released into the atmosphere.[5]
In the magnitude 9 earthquake which struck the Fukushima nuclear plants in March 2011, one of the spent fuel pools lost its roof and was reported to be emitting steam so was possibly boiling. According to The Nation, "Spent fuel pools at Fukushima are not equipped with backup water-circulation systems or backup generators for the water-circulation system they do have." [6] Later, there was some disagreement among sources as to whether the pool had boiled dry.[7][8][9]
TEPCO, the plant owner, announced that if the rods were exposed, there was a small chance they would reach criticality, setting off a nuclear chain reaction (not an explosion).[10] According to nuclear plant safety specialists, the chances of criticality in a spent fuel pool are very small, usually avoided by the disposal of the fuel assemblies, inclusion of a neutron absorber in the storage racks and overall by the fact that the spent fuel has a too low enrichment level to self-sustain a fission reaction. They also state that if the water covering the spent fuel evaporates, there is no element to moderate the chain reaction.[11][12][13] On April 1, 2011, United States Energy Secretary Steven Chu said that after efforts by workers to pour water on the Fukushima pools, these were “now under control.” [14]
Spent fuel pools lack the "4-ft.-thick (1.2 m) concrete cocoons" of operating reactors but are "housed in more conventional buildings that are conceivably more susceptible to aircraft strikes or explosives".[15]
According to Dr. Kevin Crowley of the Nuclear and Radiation Studies Board, "successful terrorist attacks on spent fuel pools, though difficult, are possible. If an attack leads to a propagating zirconium cladding fire, it could result in the release of large amounts of radioactive material." [16] The Nuclear Regulatory Commission after the September 11, 2001 attacks required American nuclear plants "to protect with high assurance" against specific threats involving certain numbers and capabilities of assailants. Plants were also required to "enhance the number of security officers" and to improve "access controls to the facilities".[17]
In 1997, the Brookhaven National Laboratory estimated that a "massive calamity at one spent-fuel pool could ultimately lead to 138,000 deaths and contaminate 2,000 sq. mi. (5,200 sq km) of land".[18]