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Japanese nuclear plants' operator scrambles to avert meltdowns

A massive 8.9-magnitude quake Friday triggered a tsunami along the coastline north of Tokyo. A nuclear reactor was left unstable by the aftershocks, stoking fears of a meltdown.

Authorities made preparations to distribute potassium iodide pills and warned people in the vicinity to stay inside and cover their mouths if they ventured outdoors.

Tokyo Electric Power, 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 think 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 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.

Japan has an ambivalent relationship with nuclear issues. As victim of the only wartime nuclear bombings, it opposes such weapons. But as a resource-scarce country, it has turned to nuclear power to help fuel its economy.

Japan's dependence on nuclear energy soared after 1973 in response to skyrocketing oil prices that year. In 2002, the country mapped a future that sought to decrease the country's greenhouse gas emissions by further increasing its reliance on nuclear power. Plans call for 50 percent of the country's electricity to come from nuclear plants by 2017, up from about 30 percent today. The country has 54 nuclear reactors.

Fukushima Daiichi unit 1 is one of the oldest operating nuclear plants in Japan, turning 40 years old on March 26. NISA, the country's regulatory authority for the sector, licenses reactors to operate for 40 years - meaning that unit 1 was scheduled to be taken offline this month. It is unclear whether NISA had planned to extend the reactor's license.

There are 23 reactors in the United States with the same design as Fukushima Daiichi unit 1.

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