Japanese nuclear plants' operator scrambles to avert meltdowns
Sunday, March 13, 2011
Japanese authorities said Sunday that efforts to restart the cooling system at one of the reactors damaged by Friday's earthquake had failed, a major setback in the struggle to contain what has become the most serious nuclear power crisis in a quarter century.
Officials said utility workers released "air containing radioactive materials" in an effort to relieve pressure inside the reactor, even as they raced to bring several other imperiled reactors under control.
Chief Cabinet Secretary Yukio Edano said officials were acting on the assumption that a meltdown could be underway at that reactor, Fukushima Daiichi's unit 3, and that it was "highly possible" that a meltdown was underway at Fukushima Daiichi's unit 1 reactor, where an explosion destroyed a building a day earlier.
Hours before he spoke, authorities began evacuating more than 200,000 residents from a 12.5-mile radius around Fukushima Daiichi and another nuclear power complex, made preparations to distribute potassium iodide pills, and warned people in the vicinity to stay inside and cover their mouths if they ventured outdoors.
Federal safety agency officials said that as many as 160 people had been exposed to radiation from the plants. "Only the gravest danger would justify an evacuation at such a moment," said Peter Bradford, a former commissioner at the U.S. Nuclear Regulatory Commission.
Tokyo Electric Power Co., owner of the two heavily damaged complexes, took the unprecedented step of pumping seawater mixed with boric acid into Fukushima Daiichi's unit 1 reactor to tame ultra-high temperatures from fuel rods that had been partially exposed. In keeping with the natural as well as mechanical challenges of the week, the company had to delay the plan briefly after another, more mild, earthquake rocked the area and led to another tsunami warning.
Tokyo Electric said it had also vented or planned to vent steam and gas containing small amounts of radioactivity from six of its other reactor units. One worker died after being trapped in an exhaust stack, the company said, and another was hospitalized for radiation exposure.
The explosion inside Fukushima Daiichi unit 1 destroyed a building that housed both the reactor vessel and its containment structure. It was caused by hydrogen, which nuclear experts said could only have been produced from inside the reactor vessel by the exposure of zirconium cladding that surrounds the fuel rods. Those rods are supposed to be covered by water, but at very high temperatures, steam reacts with the zirconium and produces hydrogen.
When pressure rose in the reactor vessel, it vented the gas into the containment structure and then into the outer building. Experts believe devices designed to ignite the hydrogen before it reached dangerous levels were not working because of power failures.
Those power failures helped start the crisis at the nuclear plants. After grid power was knocked out by the quake, the tsunami flooded and disabled backup diesel generators, and battery power ran out. Margaret Harding, a U.S. nuclear safety consultant in touch with experts in Japan, said that the entire complex was blacked out for a period of time before new backup generators arrived.
Another indication that the fuel rods in Fukushima Daiichi unit 1 were exposed is that Japan's Nuclear and Industrial Safety Agency (NISA) said Saturday that the reactor could be nearing a meltdown and that two radioactive substances, cesium and radioactive iodine, had already been detected nearby.
The explosion also rattled public confidence, sparking a run on bottled water in Tokyo.