Ameer Randolph
Spent fuel pools (SFPs) are used to store spent nuclear fuel from reactors. This spent fuel is highly radioactive and must be cooled and shielded. Water is used to cool the fuel and provide radiological protection from its radiation. The water in the pools is continuously cooled to remove the heat produced by the spent fuel assemblies. The fuel is stored in these pools for up to 20 years until its radioactivity has decreased enough to be moved into dry casks. However, the risk of radiation exposure remains high, as evidenced by an incident in 2010 where a diver was exposed to radiation above the statutory annual dose limit.
| Characteristics | Values |
|---|---|
| Radiation levels in a spent fuel pool | High levels of radiation; a fatal whole-body dose for humans is about 500 rem, while a typical spent fuel assembly can exceed 10,000 rem/hour after 10 years of removal from a reactor |
| Water temperature in normal operating conditions | Below 50 °C (120 °F) |
| Depth of spent fuel pools | Typically 40 feet or more (12 m) deep, with the bottom 14 feet (4.3 m) equipped with storage racks |
| Purpose of spent fuel pools | To cool down spent fuel assemblies and provide radiological protection from radiation |
| Storage duration in spent fuel pools | Typically one year or more, ranging from 5 to 20 years depending on the site |
| Safety concerns | Risk of explosions due to radiolysis; potential for criticality and release of radioactive material in case of terrorist attacks or natural disasters like earthquakes |
| Fuel manipulation in the pool | Automated handling systems or manual systems |
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What You'll Learn
Spent fuel pools cool nuclear fuel rods
Spent fuel pools (SFPs) are storage pools for spent fuel from nuclear reactors. They are typically 40 or more feet deep, with the bottom 14 feet equipped with storage racks designed to hold fuel assemblies removed from reactors. These storage racks are made of metal and keep the fuel in controlled positions for physical protection and for ease of tracking and rearrangement. The racks also incorporate boron-10, often as boron carbide, or other neutron-absorbing materials to ensure subcriticality.
Spent fuel pools are used for the short-term cooling of nuclear fuel rods. When a nuclear reactor has been shut down and the nuclear fission chain reaction has ceased, a significant amount of decay heat will still be produced in the fuel due to the beta decay of fission products. This decay heat slowly decreases over time. Spent fuel that has been removed from a reactor is stored in a water-filled spent fuel pool for a year or more (in some sites 10 to 20 years) to cool it and provide shielding from its radioactivity. The water cools the fuel and provides radiological protection from its radiation.
The water in the spent fuel pool is continuously cooled to remove the heat produced by the spent fuel assemblies. Pumps circulate water from the spent fuel pool to heat exchangers, and then back to the spent fuel pool. The water temperature in normal operating conditions is held below 50 °C (120 °F). 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.
Spent nuclear fuel stays a radiation hazard for extended periods, with half-lives as high as 24,000 years. It is therefore stored in either spent fuel pools or dry casks. In the United States, the majority of Independent Spent Fuel Storage Installations (ISFSIs) store spent fuel in dry casks.
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Water provides radiological protection
Water is good for both radiation shielding and cooling. Spent fuel pools (SFPs) are storage pools for spent fuel from nuclear reactors. The water in these pools cools the fuel and provides radiological protection from radiation. The 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 and then back to the spent fuel pool.
The water in SFPs provides radiological protection by acting as a barrier or shield against radiation. Water is an effective shield because it is a hydrogen-rich material. A 1977 report on the topic prepared for the DoE states that a layer of water 7 centimetres thick reduces the ionizing radiation (rays and particles) transmitted through it by half. This is why certain radioactive materials are stored under water.
Water is also used in other ways to protect against radiation. For example, in the case of a radiation emergency, drinking bottled water and eating food in sealed containers can help protect against radiation exposure.
It is important to note that while water provides radiological protection, it is not always effective against all types of radiation. For instance, radiolysis, the dissociation of molecules by radiation, is a particular concern in wet storage as water may be split by residual radiation, and hydrogen gas may accumulate, increasing the risk of explosions. Additionally, neutron radiation is better absorbed by materials like specially mixed concrete or high-density polyethylene, in addition to water.
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Fuel is stored for decades
The fuel is moved from the reactor to the pool using automated handling systems or, in some cases, manual systems. Metal racks are used to keep the fuel in controlled positions for physical protection, ease of tracking, and rearrangement. High-density racks also incorporate boron-10 or other neutron-absorbing materials to ensure subcriticality. The fuel is stored in the pools for a minimum of five years, with some sites storing fuel for 10 to 20 years or more.
During this time, the short-lived isotopes in the fuel decay, reducing the ionizing radiation and decay heat emanating from the rods. The water also helps to cool the fuel, as the fuel rods generate heat and radiation that must be contained. The pools are designed to prevent criticality by dispersing the fuel assemblies and including neutron absorbers in the storage racks.
While the fuel is stored in the pools, the water must be continually monitored and treated to prevent radiolysis, which can lead to the accumulation of hydrogen gas and increase the risk of explosions. After the fuel has cooled and the radioactivity has decreased sufficiently, it is moved to large storage casks for final disposal. However, there is still ongoing research into the future permanent disposal of this high-level radioactive waste.
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Safety concerns and risks
Spent fuel pools are a prime source of high-level radioactive waste and pose significant safety and health risks. Nuclear fuel rods become progressively more radioactive as they are fissioned in the reactor. These rods are then stored in spent fuel pools for a year or more, and in some sites, for up to 10 to 20 years. The water in these pools provides shielding from radiation and helps cool the fuel. However, if the water evaporates or there is an interruption in the active cooling system, the water may boil off, potentially resulting in the release of radioactive elements into the atmosphere. This raises concerns about the possibility of radiation leaks in the event of accidents, natural disasters like earthquakes, or terrorist attacks.
The safety of spent fuel pools has been questioned by scientists and organizations like the National Academies of Sciences, Engineering, and Medicine. They warn that a leak or sabotage at a densely filled pool could lead to a catastrophic release of radiation, endangering public health, and resulting in large-scale evacuations and economic costs. The Fukushima Daiichi power plant incident in 2011 highlighted the potential consequences, where three spent fuel pools were damaged and emitting water vapour, raising the risk of radioactive release.
Additionally, the degradation of neutron-absorbing materials in the pools over time reduces the safety margins for maintaining subcriticality. There is also a chance of fuel failure during normal operation, where the primary coolant can enter the element. The accumulation of spent fuel in pools contributes to the ongoing challenge of permanent waste disposal.
While some argue that the risks associated with spent fuel pools are minimal, others emphasize the need for a comprehensive understanding of the safety and financial risks. This includes evaluating the possibility of terrorism, insider sabotage, and the benefits of alternative storage methods like dry cask storage. The Nuclear Regulatory Commission has implemented rules mandating that all fuel pools be impervious to natural disasters and terrorist attacks, but concerns remain about the potential impact on public safety and the environment.
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Long-term radioactive decay
Spent fuel pools (SFP) are storage pools for spent fuel from nuclear reactors. The spent fuel is highly radioactive and needs to be cooled and stored for a couple of decades until it is inert enough to be moved into dry casks. The water in the pools cools the fuel and provides radiological protection from its radiation.
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 and then back to the spent fuel pool. The water temperature in normal operating conditions is held below 50 °C (120 °F). The water also shields the radiation emitted by the spent fuel.
When looking at long-term radioactive decay, the actinides in the spent nuclear fuel (SNF) have a significant influence due to their characteristically long half-lives. The presence of 233U, for example, will affect the long-term radioactive decay of the spent fuel. The use of nuclear fuels with thorium will result in the production of fissile U-233, which has a half-life of 159,200 years. This will impact the long-term activity curve of the SNF around a million years.
The radioactivity of the SNF decays over time, and the decay heat production rate will continue to slowly decrease over time. This provides an incentive to store high-level waste for about 50 years before disposal. The storage of used fuel allows for the decay of radioactivity and heat, making handling much safer.
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Frequently asked questions
Spent fuel pools contain highly radioactive material, with water providing radiological protection and cooling. The water is actively pumped through heat exchangers to maintain a temperature below 50°C.
Spent nuclear fuel is typically stored in a spent fuel pool for at least five years, with some sites storing it for 10 to 20 years.
After being cooled in the spent fuel pool, the spent nuclear fuel is moved to large storage casks for final disposal or dry cask storage.
About a quarter to a third of the fuel in a reactor is replaced every 12 to 24 months.
On August 31, 2010, a diver at the Leibstadt Nuclear Power Plant in Switzerland was exposed to a hand dose of about 1,000 mSv, twice the statutory limit of 500 mSv.
