The spent fuel pool is located in the reactor building well above ground level. This diagram is for a BWR with a reinforced concrete superstructure roof. The superstructures and pools were not, however, specifically designed to resist terrorist attacks.
The typical spent fuel pool is about 40 feet 12 meters deep and can be 40 or more feet 12 meters in each horizontal dimension. The pool walls are constructed of reinforced concrete typically having a thickness between 4 and 8 feet 1. The pools also contain vertical storage racks for holding spent and fresh fuel assemblies, and some pools have a gated compartment to hold a spent fuel storage cask while it is being loaded and sealed see Chapter 4.
The storage racks are about 13 feet 4 meters in height and are installed near the bottom of the spent fuel pool. The racks have feet to provide space between their bottoms and the pool floor. This provides substantial radiation shielding even when an assembly is being moved above the rack. Transfers of spent fuel from the reactor core to the spent fuel pool or from the pool to storage casks are carried out underwater to provide shielding and cooling. The general elevation of the spent fuel pool matches that of the vessel containing the reactor core.
Pressurized water reactor designs use comparatively shorter reactor. The spent fuel pool is located in the fuel-handling building next to the domed reactor containment building at or slightly below ground level, SOURCE: Modified from Duderstadt and Hamilton , Figure 3—4. The design shown in this figure is typical of the fuel pool arrangement for PWRs, Nuclear power plant sites that contain two reactors are usually arranged in a mirror-image fashion, with the two spent fuel pools or a shared pool located in a common area adjoining both reactor buildings.
For single-plant or two-plant arrangements, the building covering the spent fuel pool and crane structures is typically an ordinary steel industrial building. In contrast, in boiling water reactor designs, the reactor vessel is at a higher elevation, and the BWR vessels are somewhat taller than PWR vessels, 7 Consequently, BWRs have more elevated spent fuel pools, generally well above grade.
Nuclear Regulatory Commission staff is conducting a survey of the plants to obtain a better understanding of the variations in design of spent fuel pools across the nation.
The following information was provided to the committee from that survey:. The higher elevation accommodates control mechanisms that sit under the reactor, and the extra height accommodates steam separation and drying equipment at the top of the vessel.
The fuel is about the same length as PWR fuel. The pool is located to the right in the figure; the support equipment to the left. Some pools are positioned such that their spent fuel is below grade. As shown in Figure 3.
Some plants have structures surrounding the spent fuel pool building that would provide some shielding of the pools from low-angle line-of-sight attacks. A more complete plant survey would be needed to establish the extent of pool exposure to such attacks.
Some pools are also shielded by the reactor buildings. The vulnerability of a spent fuel pool to terrorist attack depends in part on its location with respect to ground level as well as its construction. Pools are potentially susceptible to attacks from above or from the sides depending on their elevation with respect to grade and the presence of surrounding shielding structures.
As noted in Chapter 1 , nearly all pools contain high-density spent fuel racks. These racks allow approximately five times as many assemblies to be stored in the pool as would have been possible with the original racks, which had open lateral channels between the fuel assemblies to enhance water circulation. In response to a request from Congress, the Nuclear Regulatory Commission and the Department of Homeland Security sponsored a National Academies study to assess the safety and security risks of spent nuclear fuel stored in cooling pools and dry casks at commercial nuclear power plants.
The information provided in this book examines the risks of terrorist attacks using these materials for a radiological dispersal device. Safety and Security of Commercial Spent Nuclear Fuel is an unclassified public summary of a more detailed classified book. The book finds that successful terrorist attacks on spent fuel pools, though difficult, are possible.
A propagating fire in a pool could release large amounts of radioactive material, but rearranging spent fuel in the pool during storage and providing emergency water spray systems would reduce the likelihood of a propagating fire even under severe damage conditions. The book suggests that additional studies are needed to better understand these risks. Although dry casks have advantages over cooling pools, pools are necessary at all operating nuclear power plants to store at least the recently discharged fuel.
The book explains it would be difficult for terrorists to steal enough spent fuel to construct a significant radiological dispersal device. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.
Jump up to the previous page or down to the next one. As a result, under the right conditions, the spent-fuel pools at the plant potentially pose as large a threat of environmental contamination — if breached — as the multiply-shielded reactor cores themselves. In particular, two of the reactors — No. Many nuclear power plants in the United States have a beefier final layer of protection — containment domes of very thick concrete. Each of the six reactor buildings at the Fukushima I plant has its own spent-fuel pool, which holds fuel rods that no longer produce enough energy to be useful, but are still too hot — and too radioactive — for safe disposal.
Ordinarily, pumps circulate water through each spent-fuel pool to carry the heat away. But if the water level in the pool drops too low, the decay heat can begin boiling off the water. If the water level drops enough to expose the metal housing containing the fuel assemblies, the assemblies can heat to the point where the fuel-rod casings catch fire, releasing radioactive gases and particles to find their way into the environment.
Since the magnitude 9. Despite workers' best efforts, temperatures Tuesday were rising in the spent-fuel pools of reactors No. Each "fuel assembly," roughly 15 feet long, is a unit containing 82 fuel rods full of the reactor's fuel: uranium oxide pellets.
During periodic refueling shutdowns, workers typically replace 20 to 30 percent of the fuel assemblies. Once extracted from the reactor, the used assemblies are housed in close-fitting steel containers that are treated with boron, to ensure they don't resume the chain reactions necessary to generate electricity.
The used assemblies are then submerged in 45 feet of water in a spent-fuel pool. The water acts as a radiation barrier, in addition to serving as a coolant. Spent fuel releases its most intense heat during the first to days after it is removed from the core, says Glenn Sjoden , a professor of nuclear engineering at Georgia Tech in Atlanta.
Indeed, as workers try to cope with three reactor cores and at least four troubled spent-fuel pools, "we're looking at having to manage this heat over the next three months.
After that first few months, a fuel assembly can be removed from the pool for reprocessing, he adds. At the stricken Fukushima I plant, much of the spent fuel in the six pools had been moved to a reprocessing facility. But No. After the explosion and fire Tuesday morning, radiation measurements at the plant's front gate spiked up to millisieverts mSv per hour, but that rate came down within hours. The radiation's most likely sources include radioactive gases that build up inside the fuel rods — a byproduct of splitting uranium atoms while the rods are in the reactors, says Professor Sjoden.
In a boiling-water reactor such as those used at the Fukushima I plant, the gases build to sizable pressures inside the fuel rods, as the reactor uses the fuel. But the gas is held in check by the extraordinary water pressures inside the reactor — at least 1, pounds per square inch.
Pressures are far lower in the spent-fuel pool, Sjoden continues, allowing the gases — radioactive krypton 85 and xenon — to percolate out of the fuel assembly, up through the water, and into the interior of the reactor building.
Ordinarily, these gases are vented through a tall stack and its attendant filters. But without power to operate the filtering systems, these non-explosive gases build up, adding to the inventory of radioactivity already released by the explosions in reactors No.
The explosions earlier this week that destroyed the roofs of reactor buildings No. He refers to satellite images showing the striking absence of the heavy crane, used for fuel changes and reactor maintenance, from the roof of reactor No.
The images suggest that the crane collapsed into the building reactor building — where it may have cracked the spent-fuel pool, increasing the rate at which water could leak out, he says. Even so, the open roofs in reactors No. Helicopters could transport large volumes of sea water and drop it into the spent-fuel pool through the open roof.
Even before Tuesday's explosion, plant workers' list of challenges included coolant loss and at least partial exposure of fuel in the reactor cores of units 1, 2, and 3 over the past few days. To prevent the water in the nuclear cores or the spent-fuel pools from falling to dangerously low levels, workers must keep adding water, whether via hose or helicopter.
Workers pour in water, the water boils off, the interior temperature rises, workers vent the steam and replace the water that was lost via boiling — and the process begins again. Even if nothing else untoward happens at the plant, "they're going to have to do this valving and feeding of water over the next several months" to keep the reactors and pools stable, Sjoden says. Already a subscriber? Monitor journalism changes lives because we open that too-small box that most people think they live in.
We believe news can and should expand a sense of identity and possibility beyond narrow conventional expectations. Your subscription to The Christian Science Monitor has expired.
You can renew your subscription or continue to use the site without a subscription. If you have questions about your account, please contact customer service or call us at This message will appear once per week unless you renew or log out. Skip to main content Skip to main menu Skip to search Skip to footer.
0コメント