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Pool holding spent nuclear fuel now at risk

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.
/ Source: 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 trying to regain control of three overheating reactors, an explosion blasted holes at a fourth reactor's outer building, where the pool is located 10 stories above ground.

Japanese officials acnowledged some radiation has escaped from the pool, and there is speculation that the water there might be boiling.

Officials intend to pour water into the overheating fuel pool at the Fukushima Dai-ichi nuclear-power plant within two or three days, possibly through holes in the building.

The pool of spent fuel is dangerous for several reasons:

  • It is more easily exposed to the atmosphere because it doesn't have the thicker containment wall that protects the core of a nuclear reactor;
  • The building housing the pool has already suffered hydrogen gas explosions and is open to the sky in places.
  • Such pools also hold far more radioactive elements than reactors that could quickly heat up again if water burns off.

Experts worry this latter consideration 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 industry veteran and now chief engineer at Fairwinds Associates who has worked on reactor designs similar to Dai-ichi.

"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 Dai-ichi, 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."

Here's how the process from fuel to pool works:

  • Uranium fuel is burned in a reactor for three to six years before being placed into a pool. About one third of the fuel is removed from the reactor core to the pool every 18 to 24 months during refueling outages.
  • The water should remain about eight feet over the spent fuel to maintain acceptable radiation levels but the level usually is kept much higher.
  • 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 several years. The water also helps prevent radioactive elements from escaping into the air.
  • After several years in a pool, the spent fuel is sent to dry cask storage or reprocessed.