Resources & Stats

May 2011

Key Facts

• When nuclear fuel is removed from a reactor, it is transferred into steel-lined concrete pools of water for cooling and storage. Pools are a safe way to store radioactive fuel from a reactor.

• Water is an efficient natural shield that protects workers and the environment from radioactivity and heat produced by used nuclear fuel. Typically, the total depth of the water in a fuel pool is 40 feet with 20 feet of water above the fuel assemblies.

• 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. Once the radioactivity and heat have decreased sufficiently, the fuel may be relocated to concrete storage containers.

• The spent fuel pools that hold the used fuel are robust, thick steel re-enforced and stainless steel-lined concrete structures. The NRC requires that used nuclear fuel stored in pools be protected against all natural phenomena such as earthquakes, tornados, hurricanes, floods, tsunamis and seiches.

• If electrical power is interrupted at a nuclear power station during a severe event, a series of backup systems and procedures is in place to ensure power and water are supplied to vital safety functions, including maintaining water levels in the fuel pool.

• If these systems are unable to function, the heat generated by the used fuel would result in a slow increase in the temperature of the water, giving engineers days to put alternative measures into place. Rapid evaporation would not occur.

• Utilities have safety procedures that could be used in an extreme situation (such as the loss of all electrical power on site) to continue to provide water to fuel pools. Highly trained operators would initiate these procedures.

An Important Way to Manage Used Nuclear Fuel
To generate electricity, nuclear power plants use uranium fuel in the form of small ceramic pellets inside metal fuel rods. The rods are grouped into bundles called assemblies. Every 18 to 24 months, the plant is shut down and about one-fifth to one-third of the fuel in the reactor core, consisting of the oldest fuel assemblies, is removed and replaced. It is placed into large pools of water, which cool the spent fuel and contain radiation.

Generally, when a pool reaches capacity at U.S. nuclear power plants, some of the used fuel is transferred to large containers made of steel or steel-reinforced concrete. The containers are located on site, but away from the main reactor buildings in a safe and secure area that is monitored 24 hours a day. (For more on dry storage, see the NEI fact sheet “Safely Managing Used Nuclear Fuel.”)

Fuel pools are also used for temporary storage of nuclear fuel during refueling outages. This gives workers the opportunity to perform needed maintenance, and to load new fuel or reconfigure existing fuel in the reactor core.

What Are Fuel Pools?
Water is an efficient natural shield that protects workers and the environment from the radioactivity and heat produced by used nuclear fuel. U.S. Nuclear Regulatory Commission regulations for radiation exposure typically result in a layer of water 20 feet from the top of the fuel assemblies to the surface. In addition to containing radiation, water also cools the fuel.

Fuel pools are robust concrete and steel structures. A small one might measure approximately 20 feet wide by 30 feet long by 40 feet deep. A fuel pool can hold hundreds of fuel assemblies.

The location of fuel pools varies with plant design. Some plants have the fuel pool in the reactor building; at others, the fuel pool is in a separate building. The advantage of having the fuel pool near the reactor is that it takes less time to move used fuel from the reactor to the pool. The advantage of having the fuel pool in a separate building is that the pool can be larger.

The water in fuel pools is filtered and purified and continuously cooled. The water is circulated through a heat exchanger to cool it, and the cooled water is returned to the top of the pool. Fuel pools also rely on evaporation for cooling. The temperature of the pools is typically maintained at approximately 70 to 100 degrees Fahrenheit.

Safety Procedures for Fuel Pools During Severe Events
The spent fuel pools that hold the used fuel are robust, thick steel re-enforced and stainless steel-lined concrete structures. The NRC requires that used nuclear fuel stored in pools be protected against all natural phenomena such as earthquakes, tornados, hurricanes, floods, tsunamis and seiches. They are designed without drains in their sides or floor so water cannot drain as a result of damage to piping or cooling systems.

Some plants store used fuel in fuel pools in a “checkerboard” arrangement as a safety precaution. In this layout, the newest fuel assemblies are distributed among older, cooler assemblies. If external cooling is lost, the cooler assemblies will draw the heat away from the hotter ones. In addition, one empty space in the pool’s rack is usually left for every two or three fuel assemblies in a repeating pattern.

If, despite backup safety procedures, the systems that cool and maintain water levels in the fuel pool are disabled due to a severe event, the heat generated by the used fuel would cause a slow increase in the temperature of the water in the fuel pool and increase evaporation.

Rapid evaporation would not occur. Rather, the water level would decrease by only a small percentage of the total volume each day. At that rate, operators would have days to implement backup safety methods to add water to the pools.

Utilities have emergency procedures and equipment—monitored and inspected by the NRC—that can be used in an extreme situation to continue to provide water to fuel pools. One procedure might involve using a fire hose to supply necessary cooling water to a fuel pool. In addition, the element boron can be manually injected into the water to assure that a chain reaction does not occur.
Even if the fuel assemblies were exposed to the air, it is highly unlikely their temperatures could reach the point where fuel melting could occur, although some damage to the cladding that encases the assemblies cannot be ruled out. As the assemblies cool in the pool, the likelihood of damage from exposure decreases substantially.

At the surface of the fuel pool, the dose rate from radiation is typically less than 2 millirem per hour. If the water level were to decline to very low levels, radiation would increase enough to be detected by the plant’s electronic monitors. Notified by the alarms, operators would immediately initiate procedures to re-establish normal water levels.

Reracking
Fuel pools were never intended to be used for long-term storage of used fuel assemblies. The Nuclear Waste Policy Act of 1982 requires the U.S. Department of Energy to remove used fuel from commercial nuclear power plants—starting in 1998. DOE has yet to act, and utilities have been forced to develop strategies to manage more used fuel on their sites than originally planned.

One strategy is to store assemblies closer to each other, called reracking. Reracked pools must meet the same stringent NRC requirements for safety that apply to the original pool racks. Once the pools are reracked to maximum capacity, the next alternative is storage in concrete and steel containers.