A report from the Institute for Policy Studies says that the spent nuclear fuel currently stored in pools at dozens of sites in the U.S. poses a danger and should be moved into dry storage as soon as possible.

The report, authored by Robert Alvarez, who served as a Senior Policy Advisor to the Secretary of Energy during the Clinton administration, says the problem is that too often the spent fuel pools are storing more fuel -- and more highly radioactive fuel -- than they were designed for.

Alvarez also says there have been at least 10 incidents in the last decade in which the spent fuel pool lost a significant amount of water, and there are other cases in which the systems that keep the pools functioning as they should are under strain. Much of this, he says, is simply because most of the pools in the country are at capacity already.

The United States has 65,000 metric tons of spent fuel at various facilities. About 75 percent of it is stored in the pools. Spent fuel rods are, when they are first removed from a reactor, highly radioactive.

Spent fuel pools became a recent concern during the disaster at the Fukushima Daiichi nuclear power plant in Japan. As the generators in the plant were knocked out, the pools lost the ability to replace water that evaporates. That meant the water had to be resupplied via fire hoses.

Nuclear fuel, when spent, still generates decay heat, which comes from its radioactivity. The spent fuel is made up of many radioactive elements, primarily isotopes of plutonium, cesium and strontium. Cesium-137 is of particular concern because it has a long half-life, about 30 years, and is liquid at low temperatures. That means it is easy for it to get into the environment if there is a fire.

A fire can happen if the water in the pool evaporates away, which can if the pumps that supply water lose power, or if the pool itself leaks. When the water levels fall, the cladding around the fuel rods can be exposed to steam. The cladding, which is made of zirconium alloy, will react with the steam when its temperature reaches about 800 degrees Celsius, and combust.

An additional risk, Alvarez says, is that even when there is still a lot of water left in the pool, the radiation levels near it can get too high for workers to approach (and to fix a problem). The water in the pool also functions as a shield against the radiation emitted by the fuel, but if enough is removed then the ambient radioactivity can rise to lethal levels.

The solution, he says, is to make it government policy to move the spent fuel assemblies to dry casks after the fuel has been in the pool for about five years. It would, he says, take about a decade to implement such a policy and cost on the order of $3-$7 billion. The funds could be taken from the $18.1 originally allocated to building a permanent waste storage facility, a project that has been stalled.

While nuclear fuel has to be in the pools for a certain amount of time to allow it to decay (and for its temperature to drop), after a few years it can be moved to dry casks, which are essentially steel and concrete containers that rely on the air to cool them. They are considered much safer as they require much less maintenance.

Not everyone thinks the danger is so acute. Everett Redmond, director of special projects and policy at the Nuclear Energy Institute, says there is no evidence that the spent fuel pools at Fukushima leaked any water or failed - the radioactive material detected outside the plant, he says, likely came from the reactor itself.

In addition, most spent fuel pools are designed to withstand a lot of damage, and the Nuclear Regulatory Commission is looking at what happens in a station blackout - a situation like that at Fukushima in which the power inside the plant is lost and the pumps stop working. Every plant, he says, is required to have some method of getting water into the pools in that situation.

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