NEW YORK (Reuters) - A fire and blast at the building protecting a massive pool holding spent atomic fuel has taken Japan's nuclear crisis to a more critical level.
After days of Tokyo Electric Power Co's (TEPCO) struggle to regain control of overheating reactors, an explosion blasted holes at the outer building of the No. 4 reactor.
Japanese media said the pools may be boiling and experts worried that they might be exposed to the outside air.
TEPCO may pour water into the overheating fuel pool in the No. 4 reactor of the Fukushima Daiichi nuclear-power plant within two or three days, possibly through holes in the building or dropping it via helicopter.
The pools of spent fuel are dangerous for two reasons: they are more easily exposed to the atmosphere because they don't have the feet-thick containment wall that protects the nuclear core; the building housing the pool has already suffered hydrogen gas explosions and is open to the sky in places.
They also hold far more radioactive elements that could quickly heat up again if water burns off. Experts worry that this could expose the used nuclear fuel and start a fire that would release more radioactivity.
"There is more radiation in the spent fuel pool than in the reactor," said Arnie Gundersen, a 39-year veteran of the nuclear industry, now chief engineer at Fairwinds Associates Inc, who worked on reactor designs similar to Daiichi plant.
"They need to keep water in those pools because the roof over the building housing the pools is already damaged and radiation will escape."
The rods contain radioactive cesium, strontium and plutonium. When a rod is exposed to the air the zirconium metal on the rods will set on fire, which could release the radiation, said Gundersen.
If the spent fuel rods burn, which is what is believed to have occurred at Unit 4 at Daiichi, those radioactive elements can get in into the air. Keeping them cool is crucial.
"All they need to do with the spent fuel pools is make up for the amount of water evaporating or boiling away," said David Lochbaum, director of the nuclear safety project at the Union of Concerned Scientists, said at a press conference Tuesday. "They should be able to do that, which should give them time to get cooling restored."
Typically 40 or more feet deep, the pools contain storage racks designed to hold fuel assemblies removed from the reactor, keeping the fuel cool until it quits generating any heat after a year or more. The water also helps prevent radioactive elements from escaping into the air.
The fuel pool at the Daiichi reactors is about 10 stories up in the air outside of the primary containment in the buildings that have already been rocked by hydrogen explosions.
Because the rods continue to produce heat, burning off the coolant, operators must constantly add water to the pool to keep the level over the fuel assemblies to prevent radiation from escaping and the rods from being exposed to the air.
Experts noted that even after a reactor is in cold shutdown, the fuel continues to produce heat at about 5 percent its normal rate. To cool that last 5 percent, the fuel must be cooled for a year or more. After that it must sit in the spent fuel pool for at least another five years. Usually, it sits there much longer to await reprocessing or dry cask storage.
The water should remain about eight feet over the spent fuel to maintain acceptable radiation levels but the level usually is kept much higher. The International Atomic Energy Agency said it had been told by Japanese authorities that radioactivity was being released directly into the air at the No. 4 pool.
The uranium fuel is burned in the reactor for three to six years before being placed into the pool. About one third of the fuel is removed from the reactor core to the pool every 18 to 24 months during refueling outages.
Many environmentalists are as concerned about the spent fuel as they are about the three reactors that were active prior to the quake.
"Because of power failures to reactor cooling systems and now the explosions, we are increasingly concerned about approximately 200 tons of nuclear waste stored outside of containment in rooftop cooling ponds at these three reactors," said Paul Gunter, director of Reactor Oversight Project at the Beyond Nuclear environmental group.
Gundersen said the 50 or so people left at the plant cannot babysit six nuclear plants, and that the greatest danger remained a meltdown at the Unit 2 reactor, which could release deadly radioactive elements if it breaches both the steel sheathe and the already damaged containment dome.
"That evacuation is a sign they may be throwing in the towel."
Reporting by Scott DiSavino and David Sheppard; Editing by David Gregorio