Monday, March 14, 2011
The heavily damaged nuclear power plant in Fukushima, Japan, has thrown a new problem at operators struggling to prevent the release of more radiation from the facility's six reactors.
The International Atomic Energy Agency reports that the plant's No. 4 reactor was partly destroyed by a fire in an area where spent fuel rods were stored.
Experts speculate that the fire was touched off when water that is supposed to keep the spent fuel rods cool had drained away. That reactor wasn't even in service, so officials hadn't expected a problem there. Now they have to worry about the spent fuel in two other out-of-service reactors.
France's nuclear safety agency, ASN, has upgraded the disaster at the Fukushima plant to level six on an international scale of one to seven. The only event that has ever received a level seven rating was the 1986 explosion at the Chernobyl nuclear plant in Ukraine.
Meanwhile, Japanese officials say a second explosion on Tuesday in the plant's No. 2 reactor probably has caused a breach in the heavy containment vessel, called a suppression pool or torus. That potentially allows radioactive steam or water to escape from a new route. Previously, radioactivity emanated from controlled releases of steam in an attempt to de-pressurize the reactors and allow cooling seawater to be pumped in.
The new problems means there are now two sources of radioactive hazard at the damaged plant – and potentially beyond. Winds that had been blowing radioactivity toward the sea have now shifted, potentially pushing radioactivity southward down the coast of Honshu island. Tokyo is 170 miles south of the power plant.
In response to the new problems, Japanese Prime Minister Naoto Kan warned of "a very high risk" of further radiation leakage, and asked 140,000 residents within 19 miles of the Fukushima plant to stay indoors and seal their doors and windows. It was not clear how long that precaution will be necessary.
Spiking Radiation Levels
Radiation levels at the Fukushima plant's gate spiked as high as 400 milliSieverts per hour during Tuesday's fire and explosion. That's potentially dangerous, and even fatal, for people in the immediate vicinity if the level is sustained. But officials say radiation levels dropped soon after the explosion and fire.
According to the International Atomic Energy Agency, in the hours after the fire and explosion, dose rates at the plant's main gate fell to 11.9 milliSieverts per hour and then to 0.6 milliSieverts.
The peak radiation dose of 400 milliSieverts per hour is equivalent to 40 abdominal CT scans, according to NPR's Jon Hamilton – if, that is, someone actually experienced an hour's worth of a 400 milliSievert dose, which currently seems unlikely.
Still, Tokyo Electric Power, which operates the Fukushima plant, evacuated all but 50 of the facility's 800 workers for their own safety. Those who remained are presumably necessary to maintain the pumping and venting maneuvers that officials hope will bring the situation under control.
It is not clear how much radiation exposures increased in the 12-mile evacuation zone around the plant or beyond. Radiation levels went up transiently in Tokyo, but not to levels that pose any public health concern, officials say.
Scramble to Prevent A Meltdown
The chief worry at the moment continues to be getting enough seawater pumped into the beleaguered reactors to prevent further melting of fuel rods, which can disperse radioactive elements into the environment. The root cause of Tuesday's explosion may have been a stuck valve that prevented workers from venting steam; a buildup of steam blocks the injection of water, so water levels drop and fuel rods are exposed, so they begin melting.
At some points prior to Tuesday's explosion, almost nine feet of reactor No. 2's fuel rods were exposed – half their length.
A reaction between the rods' zirconium cladding and steam produces hydrogen, a highly inflammable gas.
Officials suspect it was the hydrogen buildup that caused the latest explosion. That "tells you there was a lot more hydrogen available than there should have been," Dale Klein of the University of Texas at Austin told NPR.
The worst-case scenario is a total melting of a reactor's fuel rods, which conceivably could burn through the thick stainless steel reactor chamber.
A Breach Possible?
Experts differ on what could happen next. Klein, a former chairman of the Nuclear Regulatory Commission, believes the molten fuel could breach the surrounding concrete containment vessel.
"That containment would not survive very long and would result in a very large release of radioactivity ... and probably result in more evacuation," Klein says.
But other experts doubt that the lava-like nuclear fuel could breach the surrounding cement containment. That is one feature of the Fukushima reactors that was missing in the Chernobyl power plant catastrophe a quarter-century ago.
"The chances of a catastrophic release of the kind that occurred in Chernobyl are very, very low," says Richard Lester, chairman of nuclear science and engineering at MIT. If there is a full meltdown of fuel rods at Fukushima, Lester told NPR, "the most likely scenario is that the molten fuel will collect at the bottom of the pressure vessel and stay there."
Lester also points out that the longer operators of the Fukushima plant can stave off further melting of fuel rods, the lower the risk, because the rods are no longer undergoing nuclear fission – unlike the situation at Chernobyl. So the heat they generate will gradually diminish.
"Each day it becomes lower, because the fuel is decaying," Lester says.
Lester also says he can't foresee any scenario in which radiation release from Fukushima could reach North America in amounts that would pose a public health threat.