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Japan evacuates thousands from vicinity of two nuclear power plants

An explosion at a nuclear power station tore down the walls of one building Saturday as smoke poured out. Japanese officials said they feared the reactor could melt down following the failure of its cooling system in the earthquake and tsunami. (March 12)

Japanese authorities initially evacuated about 3,000 residents living within 1.9 miles of the Fukushima Daiichi plant, on the east coast about 150 miles north of Tokyo and south of the heavily damaged city of Sendai. Later they widened that evacuation to a six-mile radius and, after the explosion, extended the evacuation area to 12.5 miles.

According to NHK news, the Japanese Health Ministry had dispatched an emergency medical team, including experts on radiation exposure, to the Fukushima Daiichi complex nuclear plant.

The problems at the nuclear plants came in waves, starting with two of the six Daiichi units.

The quake disrupted the electric power the reactors used to run their cooling facilities, which pump water into the reactor core to cool the fuel rods there. The reactors switched to backup diesel generators, but the tsunami then swept in and shut down the generators used for the No. 2 reactor at Fukushima Daiichi. The unit then tapped excess steam in the core to power a turbine and switched to battery power, which would last only a few hours.

"There's a basic cooling system that requires power, which they don't have," said Glenn McCullough, former chairman of the Tennessee Valley Authority, who was tracking the Japan situation.

Japanese utility and government officials raced to get another generator to the site to prevent a possible partial meltdown similar to what took place in 1979 at Three Mile Island in Pennsylvania. By Saturday morning they said they had succeeded. The utility said it had restored power from the grid, but the IAEA said power was restored from "mobile electricity supplies."

But it appeared that some of the fuel rods inside the reactor remained exposed rather than submerged in water.

Later Saturday, an official with Japan's nuclear safety commission said a meltdown was possible because of the overheating, the Associated Press reported. Ryohei Shiomi added that even if there were a meltdown, it wouldn't affect residents outside the evacuated radius.

Meanwhile, Tokyo Electric said it had decided to vent slightly radioactive steam and gas to relieve pressure that had increased sharply in the containment building at unit No. 1. The company said on its Web site that the increase was "assumed to be due to leakage of reactor coolant." It remained unclear where the leak was. The company said it did not think there was leakage of reactor coolant in the containment vessel "at this moment."

The purpose of a containment building, which surrounds the reactor core, is to contain unplanned releases of steam or gases from the core. If there is not enough water in the reactor core, water turns to steam and is released through special valves into the containment building, nuclear experts said.

That could cause an increase in pressure inside the sealed containment building and ultimately force a release of gas and steam through filters meant to keep most, though not all, of the radiation inside the building.

There were also reports of elevated radiation levels inside the control room of that reactor unit. NISA said levels were 1,000 times the norm. The AP later quoted an official from NISA as saying that a measurement of radiation levels outside the plant was eight times as high as normal. Even that level of radiation still posed little danger to residents, nuclear experts said. They also said the release of steam and gas from containment buildings posed little danger.

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