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Japan nuclear crisis: Is massive water dump making any difference?

Japan pours tons of water into a reactor building where the water level in a cooling pool for spent fuel rods was dangerously low. The nuclear crisis is now rated as severe as Three Mile Island.

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Hydrogen buildup has caused at least two reactor buildings to explode.

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“The priority would be the spent fuel pools because they are more exposed and it is more likely radioactivity could get out,” says David Lochbaum, director of the nuclear safety program at the Union of Concerned Scientists.

The fuel pools were designed they way they are because when the reactor complex was constructed decades ago engineers thought spent fuel would spend only a few weeks on-site before being shipped to a reprocessing plant elsewhere. But around the world the problems of finding safe sites, adequate transportation, and local populations willing to tolerate them have made large reprocessing facilities difficult to construct.

The good news is that at least some of the spent fuel rods from Fukushima had already been shipped elsewhere for safer dry cask storage.

Thus the vulnerable pools “weren’t filled to capacity,” says Lochbaum.

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The bad news is that enough fuel rods remain in the pools that, if exposed to air for a long enough period of time, it is theoretically possible that they could burn and melt into a pile of fissile rubble dense enough to restart a nuclear chain reaction.

Tokyo Electric Power said earlier this week that the possibility of such “recriticality” occurring in the pools is not zero. If it happened the pools would then in essence have turned into running mini-reactors, and begin emitting much more heat and dangerous radiation.

More boron is needed

Pouring water laced with boron into a pool would help stop such a reaction, as boron absorbs neutrons which are emitted during a nuclear reaction. Japan on Thursday announced plans to import 150 tons of boron from South Korea and France to replenish its stocks.

However, it is possible that water has leaked from the pools via cracks instead of boiled off or evaporated due to heat. If that is the case some of the heroic efforts by Japanese workers to spray water into the damaged containment buildings could be in vain.

Burying the spent fuel pools with sand and earth could prevent them from releasing any further radiation. But the act of shoveling stuff on top of the rods could break them up and force them together into a denser mass more prone to recriticality.

“You don’t want to solve one problem and cause another,” says David Lochbaum.

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