A partial meltdown at two nuclear reactors is likely under way, a Japanese official said today, as authorities frantically tried to head off the danger following a catastrophic earthquake and tsunami. A complete meltdown would release uranium and dangerous byproducts into the environment that could pose serious health risks.
Chief Cabinet Secretary Yukio Edano said operators released slightly radioactive air from Unit 3 at the Fukushima Daiichi nuclear plant near Iwaki today, while injecting water into it as an effort to reduce pressure and temperature to save the reactor from a possible meltdown.
Still, a partial meltdown in the unit is "highly possible," he told reporters.
The Japanese government said radiation emanating from the plant appeared to have decreased after a blast at the Unit 1 reactor on Saturday, which produced a cloud of white smoke that obscured the complex. But the danger was grave enough that officials pumped seawater into the reactor to avoid disaster and ordered 170,000 people out of the area within 12 miles of the complex.
The quake and tsunami damaged three reactors at the Fukushima plant, which lost their cooling functions necessary to keep the fuel rods functioning properly. At first the Unit 1 reactor was in trouble, with the explosion destroying the walls of the room in which it is placed. Later, Unit 3 also began to experience problems.
Edano said radiation levels at Fukushima briefly rose above legal limits, but that it has since declined significantly.
Also, fuel rods were exposed briefly, he said, indicating that coolant water didn't cover the rods for some time. That would contribute further to raising the temperature in the reactor vessel.
Tokyo Electric Power Co., owner of the troubled nuclear power complexes, said it had also vented or planned to vent steam and gas containing small amounts of radioactivity from seven of its reactor units, the Washington Post reported.
The explosion at the nuclear plant, Fukushima Daiichi, 170 miles northeast of Tokyo, appeared to be a consequence of steps taken to prevent a meltdown after the quake and tsunami knocked out power to the plant, crippling the system used to cool fuel rods there.
The blast destroyed the building housing the reactor, but not the reactor itself, which is enveloped by stainless steel 6 inches thick.
Inside that superheated steel vessel, water being poured over the fuel rods to cool them formed hydrogen. When officials released some of the hydrogen gas to relieve pressure inside the reactor, the hydrogen apparently reacted with oxygen, either in the air or the cooling water, and caused the explosion.
"They are working furiously to find a solution to cool the core," said Mark Hibbs, a senior associate at the Nuclear Policy Program for the Carnegie Endowment for International Peace.
Nuclear agency officials said Japan was injecting seawater into the core — an indication, Hibbs said, of "how serious the problem is and how the Japanese had to resort to unusual and improvised solutions to cool the reactor core."
Officials declined to say what the temperature was inside Unit 1. At 2,200 degrees Fahrenheit, the zirconium casings of the fuel rods can react with the cooling water and create hydrogen. At 4,000 F, the uranium fuel pellets inside the rods start to melt, the beginning of a meltdown.
Although the government played down fears of radiation leaks, Japanese nuclear agency spokesman Shinji Kinjo acknowledged there were still fears of a collapse of a power plant's systems.
Yaroslov Shtrombakh, a Russian nuclear expert, said it was unlikely that the Japanese plant would suffer a meltdown like the one in 1986 at Chernobyl, when a reactor exploded and sent a cloud of radiation over much of Europe. That reactor, unlike the reactor at Fukushima, was not housed in a sealed container.