How Vulnerable are the Pools that Store Spent Nuclear Fuel at U.S. Reactors?

Ask a Scientist - March 2013

M. Daley from Point Richmond, CA, asks “The nuclear accident in Fukushima raised concerns about the safety of spent fuel storage pools in nuclear reactors. How vulnerable are the pools at U.S. reactors, especially in light of the threat of a possible terrorist attack, and what, if anything, can be done to make spent fuel storage safer?” and is answered by UCS Global Security Program Co-Director David Wright, Ph.D.

The highly radioactive fuel rods in nuclear power plants are moved from the reactor core to cooling pools when they are “spent” and are replaced by fresh fuel to drive the nuclear chain reaction that generates electricity. You are right that spent fuel pools have raised safety concerns because an event like a severe accident or terrorist attack that caused a loss of water from the pool could result in a self-sustaining fire that would damage the fuel and could cause a massive radiological release.

In the United States, because there is no permanent storage site for the spent fuel from the nation’s nuclear power plants, spent fuel has been kept at the reactor sites, with the vast majority of it stored in spent fuel pools. In the past, most analysis focused on the safety of spent fuel pools rather than their security against terrorist attack. That changed after September 11, 2001, when studies began to look at the possibility of terrorism. A 2006 National Academy of Sciences study, for example, concluded that “under some conditions, a terrorist attack that partially or completely drained a spent fuel pool could lead to a propagating zirconium cladding fire and the release of large quantities of radioactive materials to the environment.”

Over the past decade, the Nuclear Regulatory Commission (NRC) has continued to assess safety and security issues and has taken steps to address some of the vulnerabilities of spent fuel pools it has identified. However, the NRC and industry continue to rely on active measures—such as pumps to replace water loss in an accident —when responding  to concerns about fires in spent fuel pools and  to oppose operational changes in the way spent fuel is managed and stored at reactor sites. At UCS, we believe more can and should be done to enhance passive safety measures and reduce reliance on active measures. This would be especially desirable for situations such as Fukushima-type events when the prompt and effective implementation of active measures to mitigate problems cannot be guaranteed.

One major problem is that today’s U.S. nuclear operators routinely store spent fuel in pools at a much higher density than they were designed to hold. That increases the risk in two ways. First, it increases the chance that the spent fuel will overheat, burn, and release radiation into the environment if cooling is lost due to a terrorist attack or accident. More spent fuel in a pool results in closer spacing between spent fuel bundles, reducing cooling and increasing the chance of fire spreading between the bundles. Second, high density of fuel rods in cooling pools will increase the consequences of an accident if the fuel overheats. More fuel in the pool means it could release more radioactivity.

The most effective way to lower these risks is simply to reduce the density of fuel in the pools by transferring it out of the pools and into dry casks. Dry casks are by far the safest and most secure way to store spent nuclear fuel at the reactors while it is waiting to be moved offsite to interim or permanent storage. Unlike cooling pools, dry casks are air-cooled “passive” systems. They are less vulnerable to technical or human error and to terrorist attacks, reducing the risk of a radiological release. They also are less vulnerable than pools to the potential impact of earthquakes, hurricanes, and other natural disasters.

The NRC claims that storage of spent fuel in pools provides “adequate protection,” but storage in dry casks is known to be safer. UCS believes that the uncertainties in the available analyses of pool fires are so large that they demand substantial safety margins; these margins can be achieved only by reducing the pool inventory well below the densely packed configuration maintained in many plants today.

Because spent nuclear fuel is cool enough to transfer to dry casks after five years, more than 80 percent of spent fuel that is currently in cooling pools is now eligible for dry cask storage. Considering that the spent fuel will need to be transferred to dry casks at some point so it can be shipped to an interim or permanent waste site, it makes sense to do that sooner rather than later to better protect communities surrounding nuclear power plants. Plus, today’s nuclear plants already have the necessary equipment (such as heavy-duty cranes) and the personnel to get the job done now.

Congress is currently developing legislation to improve the management of spent fuel in the United States. At UCS, we are working hard to ensure that the legislation includes a requirement for plant owners to reduce the density of spent fuel in pools by transferring it to dry casks.


David Wright is a nationally known expert on the technical aspects of missile defense systems, missile proliferation and space weapons. Before joining UCS in 1992, Dr. Wright was a senior research analyst with the Federation of American Scientists and served as an SSRC-MacArthur fellow at Harvard’s Kennedy School of Government. He received his doctorate degree in physics from Cornell University in 1983 and worked as a research physicist from 1983 to 1988.


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