A typical pool is 40 feet deep with the used fuel at least 20 feet below the water’s surface. It is standard practice to store used fuel in a pool for at least five years, usually longer, after it has been removed from the reactor core. Once the radioactivity and heat have decreased sufficiently, the older fuel may be removed from the pool and placed in dry storage containers.
Despite the strongest earthquake in modern Japanese history, a tsunami estimated at 45 feet in height, loss of all off-site electric power for weeks, and explosions resulting from hydrogen buildup in containment structures, all seven fuel pools at the Fukushima Daiichi power plant in Japan remained intact and the used fuel in the pools remained safely covered with cooling water.
If electrical power is lost at a nuclear power station during an extreme event, a series of systems and procedures are in place to assure backup power and water are supplied to vital safety functions, including maintaining water levels in the fuel storage pool. Even if those systems were to fail and there was no cooling water available, the water in the pool would heat and evaporate slowly, giving plant personnel days or weeks to restore cooling.
Fuel Storage Pools Are Robust Structures
Used fuel storage pools are robust concrete and steel structures designed to withstand extreme events such as earthquakes, floods, hurricanes and tornadoes. To prevent water from escaping because of damage to piping or cooling systems, the pools do not have drains in their floors.
The storage pools vary in size, with a small pool measuring approximately 20 feet wide by 30 feet long. Most pools are about 40 feet deep. A pool can hold hundreds of used fuel assemblies. Depending on the facility, the storage pool can be inside the reactor containment building or in a separate structure.
Water is an effective natural shield that protects workers and the environment from the radioactivity and heat produced by used fuel. To maintain radiation levels near the pool below the limits specified in stringent NRC regulations, the water level above the used fuel assemblies is typically 20 feet deep.
Water in the pools is filtered and purified. It circulates through a heat exchanger for cooling and then is returned to the top of the pool. The temperature of the pools is maintained at about 70 to 100 degrees Fahrenheit, less than the typical temperature of bath water.
Safely Managing Used Nuclear Fuel
It is standard practice to store used fuel in a pool for at least five years—and usually longer—after it has been removed from the reactor. After many years, when the pools reach capacity and radioactivity and heat have decreased in the older assemblies, operators move the fuel into containers made of steel or steel-reinforced concrete. The design of the containers, which are built to withstand even the most sever events, must be approved by the NRC. The containers are located on site but away from the main reactor buildings in a secure, monitored area.
If off-site electrical power is lost at a nuclear energy facility during an extreme event, systems and procedures are in place to ensure backup power and water are supplied to vital safety functions, including maintaining cooling and water levels in the used fuel storage pool.
For example, backup water supply systems or hoses can provide cooling water. Emergency response equipment that that does not need off-site electricity was added at U.S. facilities after the terrorist attacks of Sept. 11, 2001. More portable safety equipment is being added at all nuclear power plant sites as the industry addresses the lessons learned from the nuclear accident at Fukushima.
Even if there was no cooling water available, the water in the pool would heat and evaporate slowly, giving plant personnel days or weeks to restore cooling and keep the water at a safe level.
Fukushima Fuel Pools Protected Safety Throughout Accident
In the early days of the 2011 Fukushima Daiichi accident, there was speculation that used fuel storage pools at the site had been seriously damaged or that the water had drained out of some of them. Later observations and data showed the speculation to be incorrect. Still, the misperception led to discussion in Congress of accelerating the transfer of used fuel rods at U.S. nuclear energy facilities from pools to storage containers.
The independent NRC says that pools and containers are equally safe methods for storing used fuel and that both protect public health and the environment. The agency says “there is no pressing safety or security reason to mandate earlier transfer of fuel from pool to [storage containers].”
Recent NRC Report Affirms Pool Safety
In 2013, the NRC released a report, “Consequence Study of a Beyond-Design-Basis Earthquake Affecting the Spent Fuel Pool for a U.S. Mark I Boiling Water Reactor,” which considered whether faster removal of older used nuclear fuel from on-site storage pools to containers, or dry casks, would significantly reduce the risks to public health and safety. It concluded that, “for the scenarios examined, faster fuel transfer to casks would not provide a significant safety benefit.”
The NRC’s analysis shows that even a powerful earthquake is unlikely to damage a used fuel storage pool to the extent that it would lose water, and even if it did, the used fuel could be safely cooled “in all but a few exceptional [extremely low probability] circumstances.” Even in those cases, the NRC said “existing emergency procedures would keep the population around the plant safe.”
The study considered the used fuel storage pool of a GE Type 4 boiling water reactor with a Mark I containment, the reactor design used at Fukushima Daiichi and in 23 other reactors in the United States.