Update: As we have noted, there has been a fire and the spent fuel rods of one of Fukushima's reactors are now at risk.

Concerns about a radiation release from the Fukushima Daiichi power facility have focused on its stricken nuclear reactors, but the plants of that design also store highly radioactive spent fuel in pools outside the protective containment structure that surrounds the reactor itself.

Opponents of nuclear power have warned for years that if these pools drain, either by accident or terrorist attack, it could lead to a fire and a catastrophic release of radiation. Now, there have been hydrogen explosions at two of the reactor buildings housing spent fuel pools at Fukushima.

This diagram shows where spent fuel pools are typically located in the 1970s-vintage GE Mark I reactor design in use at Fukushima units 1, 2 and 3, where officials suspect reactor fuel has melted.

The nuclear industry says fears about the storage pools at U.S. plants are overblown because the pools are protected and, even if fuel is exposed to the air, the chance of a fire is incredibly small. And with limited information being released about conditions at Fukushima, the status of spent fuel pools is uncertain.

The fuel that powers a nuclear reactor only works so long. When it is exhausted, it is still radioactive and physically very hot. So it needs to cool in a deep tank of water before it can be stored elsewhere.

At Fukushima, these tanks are attached to the outside of the reactor’s containment structure. The pools are deep – typically the fuel lies under 25 feet of water. Although the concrete-and-steel containment is designed to trap radiation leaks, there is no such protection for pools outside.

When the U.S. reactors were built, everyone assumed the government would open a national storage center to handle the tons of radioactive spent fuel from nuclear plants. The proposed Yucca Mountain site in Nevada never opened, so the old fuel sits at nuclear plants across the country.

Many plants have been operating for 20 years and have tons of used fuel in cooling pools.

The concern is that if the water in the pools ever drops too low, the zirconium cladding that holds the radioactive fuel pellets would begin to heat up and eventually burn. And if it did, the smoke from the fire could carry radiation away from the plant because the pool is outside the containment.

“People should be very concerned because the NRC has acknowledged that spent fuel pools that are not located inside the containment have the potential to cause catastrophic accidents,” said Diane Curran, a lawyer who has represented environmental groups and governments in challenges to fuel storage plans.

“These are not high-probability accidents,” Curran said, “but we have seen how low-probability accidents can happen.”

After the Sept. 11 terrorist attacks, Congress asked the National Academies to study the vulnerability of spent fuel to a terrorist attack.

The resulting 2005 report, “Safety and Security of Commercial Spent Nuclear Fuel Storage,” concluded that “an attack which partially or completely drains a plant's spent fuel pool might be capable of starting a high-temperature fire that could release large quantities of radioactive material into the environment.”

The report found that the vulnerability of the spent fuel to fire depends on how old it is and how it is stored. As the fuel ages, it cools, so it becomes less susceptible to a fire.

“The industry standard is that fuel that is older than five years can be dry-stored,” said Kevin Crowley, director of the nuclear and radiation board for the National Research Council, part of National Academies.

The report recommended that the nuclear industry take steps to decrease the vulnerability of the storage pools to fire. Some of those steps are classified, Crowley said. But he said others, like making sure there were fire hoses or spray systems above the pools, were pretty simple.

Crowley said he does not have enough information on the status of the Japanese plants to say whether the pools are vulnerable.

The Nuclear Regulatory Commission did not reply to an e-mail for this story. The agency says on its website that it is confident the spent fuel pools at U.S. plants are safe.

The nuclear industry disagreed with the national academy about the vulnerability of the spent fuel to a fire. Carl Baab, a spokesman for the Nuclear Energy Institute, said studies show that the risk is low in part because the spent fuel contains relatively little energy compared to the fuel inside the reactor.

“The potential for a fire from damage or loss of water is so remote that we believe it is misleading,” he said.

Baab also said plant workers only need to replace about 25 gallons of water each day to the fuel pool to maintain water levels in the event that primary systems were knocked out.

How hydrogen explosions at Fukushima may have affected the spent fuel pools is unclear.

Some nuclear plants have moved their older fuel into reinforced metal storage casks that are located away from the reactor building. According to information on the website of Tokyo Electric Power Co., which operates the Fukushima facility, more than 200 tons of spent fuel is stored in casks.

Baab said the nuclear power industry has been pushing the federal government to open a long-term storage site for nuclear waste.

“From the beginning it was intended to stay at the sites for a relatively short time,” he said. “It was never intended or designed that it remain at the site.”