But even at a reactor that does not fare as well in a large earthquake and is not immune to the loss of off-site power, there is "essentially zero risk of early fatalities," according to the NRC worst-case modeling. Even when a release of radioactive material reaches the environment, "it's small enough and takes so long to reach the community that people have already been evacuated or otherwise protected," NRC's Burnell argues. "The public avoids any short-term dose large enough to kill." And that is exactly what happened at Fukushima.
The dangers of spent fuel
The multiple explosions at Fukushima Daiichi revealed another safety risk, one that is, if anything, of more concern in the U.S. The explosions tore open reactor buildings, damaging the 12-meter-deep pools where used nuclear fuel is kept cool, potentially setting off another meltdown in the fuel there as the surrounding water drained away or boiled off. Densely packed spent fuel without water can heat enough to burst its zirconium cladding and, ultimately, set the cladding ablaze. Without walls, which had been blown out by previous explosions, there was nothing left to keep the cesium 137 and other radioisotopes in the nuclear fuel from escaping in such an event at the Fukushima reactors.
In fact, the plume of contamination spreading to the northwest of the stricken nuclear power plant may have come from such spent fuel. Despite having been shut down for refueling, Unit 4 also suffered an explosion and what remains identified by Japanese authorities as a "lube oil fire." (NRC experts, though, disagreed, at least at the time: "We know it wasn't a lube oil fire," argued Larry Camper, director of the NRC's Division of Waste Management and Environmental Protection, on March 20, 2011, according to the transcripts.) The fire, whatever kind it was, appears to have carried radioactive particles into the surrounding countryside to the northwest as it coincided in time with the wind blowing in that direction.
In the U.S., because of a lack of a long-term plan for dealing with such nuclear waste, spent-fuel pools are even more densely packed, making it easier for a meltdown to occur in the event of a loss of water. Such pools at the nation's 104 nuclear reactors hold more than 45,000 metric tons of the nation's approximately 65,000 metric tons of such used nuclear fuel. That said, the nuclear industry's FLEX approach, would also include additional pumps and hoses to get water to the spent-fuel pools, as well as instruments to monitor their condition.
Ironically, the loss of walls and roofs may have been the key to preventing a worse accident at Fukushima. By ripping off the walls and roofs, the explosions enabled emergency workers to spray cooling water into the pools directly—if inefficiently—via water cannons and other devices. "What would have happened had those explosions not occurred?" asks nuclear engineer David Lochbaum of the Union of Concerned Scientists. "The radiation levels in the buildings were too high to allow access, even if workers had equipment with which to add water to the pools." In that case, the spent nuclear fuel likely would have begun melting down and there would have been few ways of restoring cooling water.
In fact, throughout the first week of the Fukushima crisis, emergency workers tried to figure out a way to open up a larger hole in the Unit 2 reactor building, which had not suffered an explosion, to allow better access to inject cooling water without creating the kind of spark that might cause another hydrogen blast.