Some 30 million such rods are stored in spent-fuel pools at 51 sites around the country that "contain some of the largest concentrations of radioactivity on the planet," the study said. The rods are usually kept in tightly-packed racks submerged in pool water, which requires a steady flow of electricity to keep from overheating. If water drains from a spent-fuel pool, it can lead to a catastrophic fire that emits dangerous radioactive elements like Cesium 137.
Typically, spent-fuel pools are rectangular, about 40 to 50 feet deep, and made of reinforced concrete walls four to five feet thick, with stainless steel liners. Those without liners may crack or corrode more often, the report says.
The National Academy of Sciences in 2004 cited the pools as vulnerable to terrorist attack and fires. While the Nuclear Regulatory Commission has said the pools are safe, the Fukushima reactors' core meltdowns and spent-fuel pool fire prompted a new study of the possible impact of an earthquake or electrical blackout on US sites.
Spent-fuel pools need electricity to pump water to cool the fuel rods, as well as to maintain a high water level to prevent radiation from escaping. At present, however, there is no federal requirement for back-up power supplies for spent-fuel pools, to ensure the circulation of water continues in a blackout situation, the report notes.