The latest blast at Japan’s most crippled nuclear reactor is the worst yet, and raises the likelihood of a nuclear catastrophe to uncomfortable levels, according to worried industry experts. The explosion, at the Fukushima Daiichi plant 150 miles from Tokyo, damaged the integrity of the reactor’s steel containment structure, and nearby radiation levels spiked afterward. Emergency workers were evacuated due to the threat of radiation exposure, meaning that they will not be around to help avert the possible nuclear meltdown. Such a meltdown would be the worst since Chernobyl 25 years ago.

While Japan scrambles to repair its plants, another danger lurks: radiation from the spent fuel resting in nearby pools—a storage approach some scientists want banned in the U.S. Plus, see our full coverage of Japan's quake.

To the growing list of worries at the Fukushima Daiichi nuclear-power plant, where last week’s earthquake and tsunami have led to explosions, several releases of radioactive steam, and the threat of a core meltdown at the three nuclear reactors, add this: could the spent nuclear fuel sitting in a nearby storage pool pose an even bigger threat to people and the environment? The spent fuel produced by reactors has been a challenge since the dawn of the nuclear industry, with most reactor operators opting to store it in pools of cooling water on site. At the 40-year-old Fukushima plant, which was built by General Electric, the fuel rods are stored at a pool about three stories up, next to the reactor (a schematic is here). Satellite photos raise concerns that the roof of the building housing the pool has been blown off, says Robert Alvarez, a senior scholar at the Institute for Policy Studies and a senior policy adviser to the secretary of energy and deputy assistant secretary for national security and the environment from 1993 to 1999. He and other experts are now warning that any release of radioactivity from the spent-fuel pool could make the releases from the reactors themselves pale in comparison.

The spent-fuel pools are rectangular basins about 40 feet deep, made of four- to five-foot-thick reinforced concrete lined with stainless steel. That was thought to be sufficient to prevent a breach. But the disastrous combination of an earthquake (which knocked out power form the electricity grid) and a tsunami (which swamped the diesel generators serving as backup power) forced the power-plant operators to turn to batteries for core cooling.

When battery-powered cooling failed, hydrogen in two of the units exploded, damaging the reactor buildings—and, apparently, the spent-fuel area as well. Satellite photos appear to show that two cranes used to move spent fuel into the pool “are both gone,” Alvarez told a press conference organized by Friends of the Earth, a nonprofit environmental group that opposes nuclear power. “There has definitely been damage to the pool area.”

Fuel-storage pools 'contain very large concentrations of radioactivity, can catch fire, and are in much more vulnerable buildings' than the reactors.

NEWSWEEK/The Daily Beast's Lennox Samuels reports from Japan

The pools “contain very large concentrations of radioactivity, can catch fire, and are in much more vulnerable buildings,” he warns. If the pools lose their inflow of circulating cooling water, the water in the pools will evaporate. If the level of water drops to five or six feet above the spent fuel, Alvarez calculates, the release of radioactivity “could be life-threatening near the reactor building.” Since the total amount of long-lived radioactivity in the pool is at least five times that in the reactor core, a catastrophic release would mean “all bets are off,” he says.

Of particular concern: cesium-137 in the pool, at levels Alvarez estimates at 20 million to 50 million curies. The 1986 Chernobyl accident released about 40 percent of the reactor core’s 6 million curies. In a 1997 report for the Nuclear Regulatory Commission, scientists at Brookhaven National Laboratory estimated that a severe pool fire—made possible by the loss of cooling water—could leave about 188 square miles uninhabitable and cause up to 28,000 cancer deaths.

The new concern at Fukushima Daiichi highlights an ongoing controversy about the way spent fuel gets stored: what if Tokyo Electric Power had heeded the growing scientific consensus and moved the spent fuel out of the storage pool and into dry, hardened casks for storage? Germany did this 25 years ago. The NRC has rejected this recommendation, but a 2006 analysis by the National Academy of Sciences warned that “breaches in spent fuel pools could be much harder to plug [than those in dry casks], especially if high radiation fields or the collapse of the overlying building prevented workers from reaching the pool. Complete cleanup from a zirconium cladding fire would be extraordinarily expensive, and even after cleanup was completed large areas downwind of the site might remain contaminated to levels that prevented reoccupation.”

The NAS report focused on the risk that terrorists would attack a spent-fuel pool (by, for instance, crashing a plane into one), and concluded that “recovery from an attack on a dry cask would be much easier than the recovery from an attack on a spent fuel pool. Breaches in dry casks could be temporarily plugged with radiation-absorbing materials until permanent fixes or replacements could be made ... It is the potential for zirconium cladding fires in spent fuel pools that gives dry cask storage most of its comparative safety and security advantages.”

The NRC counts almost 100 spent-fuel pools in the United States.

Sharon Begley is the science columnist and science editor of NEWSWEEK. She is a coauthor of the 2002 book The Mind and the Brain and author of the 2007 book Train Your Mind, Change Your Brain.