Safer Storage of Spent Nuclear Fuel

What are spent fuel pools?

When fuel rods in a nuclear reactor are “spent,” or no longer usable, they are removed from the reactor core and replaced with fresh fuel rods. The spent fuel rods are still highly radioactive and  continue to generate significant heat for decades. The fuel assemblies, which consist of dozens to hundreds of fuel rods each, are moved to pools of water to cool. They are kept on racks in the pool, submerged in more than twenty feet of water, and water is continuously circulated to draw heat away from the rods and keep them at a safe temperature.

Because no permanent repository for spent fuel exists in the United States, reactor owners have kept spent fuel at the reactor sites. As the amount of spent fuel has increased, the Nuclear Regulatory Commission has authorized many power plant owners to increase the amount in their storage pools to as much as five times what they were designed to hold. As a result, virtually all U.S. spent fuel pools have been “re-racked” to hold spent fuel assemblies at densities that approach those in reactor cores. In order to prevent the spent fuel from going critical, the spent fuel assemblies are placed in metal boxes whose walls contain neutron-absorbing boron. 

What are the risks and vulnerabilities?

If a malfunction, a natural disaster, or a terrorist attack causes the water to leak from the pool or the cooling system to stop working, the rods will begin to heat the remaining water in the pool, eventually causing it to boil and evaporate. If the water that leaks or boils away cannot be replenished quickly enough, the water level will drop, exposing the fuel rods. 

Once the fuel is uncovered, it could become hot enough to cause the metal cladding encasing the uranium fuel to rupture and catch fire, which in turn could further heat up the fuel until it suffers damage.  Such an event could release large amounts of radioactive substances, such as cesium-137, into the environment.  This would start in more recently discharged spent fuel, which is hotter than fuel that has been in the pool for a longer time.  A typical spent fuel pool in the United States holds several hundred tons of fuel, so if a fire were to propagate from the hotter to the colder fuel a radioactive release could be very large.

The spent fuel pools in boiling-water reactors are located only within the secondary containment of the reactor—the reactor building—and not within the more robust primary containment that is designed to keep radiation released from the reactor vessel during an emergency event from escaping into the environment. Thus, any radiation released from a spent fuel pool is more likely to reach the outside environment than is radiation released from the reactor core. Moreover, because it is outside the primary containment, the spent fuel pool is more vulnerable than the reactor core to certain terrorist attacks like deliberate aircraft crashes.

Continuing to add spent fuel to these pools compounds this problem by increasing the amount of radioactive material that could be released into the environment. A large radiation release from a spent fuel pool could release more cesium-137 than the Chernobyl disaster, resulting in thousands of cancer deaths and hundreds of billions of dollars in decontamination costs and economic damage.

Advantages of dry cask storage

The risks from spent fuel in storage pools can be reduced by moving some of it to dry casks. Typical dry casks are made of steel and concrete, with the concrete providing radiation shielding , and are stored at U.S. reactors outdoors on concrete pads. To become cool enough to be placed in the dry casks currently licensed and used in the United States, the spent fuel must first spend five years in a spent fuel pool. By then it is cool enough that further cooling can be accomplished by natural convection—air flow driven by the decay heat of the spent fuel itself.

By transferring fuel from spent fuel pools to dry casks, plants can lower the risk from spent fuel in several ways:

1. With less spent fuel remaining in the pools, workers will have more time to cope with a loss of cooling or loss of water from the pool, because the amount of heat released by the spent fuel is lower. With less heat, it takes longer for the water to heat up and boil away.
2. If there is less fuel in the pool, it can be spread out more, making it easier for the fuel to be cooled by water, or even air if the pool is rapidly drained after an accident.
3. Because there is less fuel in the pool, if workers are unable to prevent an accident, the amount of radioactive material emitted from the pool will be much lower than it would be otherwise.

After the 9/11 attacks, the NRC imposed new requirements on reactor owners to reduce the risks of aircraft attacks on both reactors and spent fuel pools.  In particular, it required that hotter spent fuel should be dispersed throughout the pool instead of being concentrated in one spot.  It also required the development of strategies for providing backup water supplies to the pools in the event of an aircraft attack.  However, these measures do not go far enough to ensure the safety of the pools under a wide range of accident and attack scenarios.

The combination of reducing the likelihood of an event and reducing the consequences of an event significantly reduces the risk from a spent-fuel accident. In contrast to spent fuel pools, dry casks are not vulnerable to loss of coolant because their cooling is passive.

While dry casks are still vulnerable to safety and security hazards, those risks are reduced. In contrast to the large amount of fuel in a single spent fuel pool, each dry cask only holds 10 to 15 tons of spent fuel, or only a few percent of a typical spent fuel pool. Thus, it would require safety failures at many dry casks to produce the scale of radiological release that could result from a safety failure at one spent fuel pool. Likewise, terrorists would have to break open many dry casks to release as much radioactivity as a single spent fuel pool could release. Therefore, an attack on a dry cask storage area would, in most circumstances, result in a much smaller release of radioactivity than an attack on a storage pool.

UCS recommendations

• All spent fuel should be transferred from wet to dry storage within five years of discharge from the reactor core. This can be achieved with existing technologies.
• The NRC should upgrade existing regulations to require that dry cask storage sites be made more secure against a terrorist attack.