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(Reuters) - The Japanese Nuclear Safety Agency on Saturday rated the nuclear accident at Tokyo Electric Power Co's (TEPCO) Fukushima Daiichi plant in Japan at a four on a scale of one to seven, which is not quite as bad as the Three Mile Island accident in the United States in 1979, which was rated a five.
The agency hasn't provided an update of its assessment since Saturday despite further problems at the Daiichi plant.
So how were the events different?
* Three Mile Island is the worst nuclear power accident in U.S. history even though it resulted in no injuries.
* About half the reactor core in one unit at the Three Mile Island plant in Pennsylvania melted due to the loss of coolant. The other unit suffered no damage and still operates today.
* The plant did vent a small amount of radiation to release pressure but nuclear experts said that release did not result in radiation levels beyond what environmental regulations allow.
* State authorities recommended a voluntary evacuation of pregnant women and pre-school age children from within five miles of the plant. Within days, about 140,000 people had left the area.
* Operators at Three Mile Island stopped the meltdown by restoring cooling water to the reactor core.
* The Three Mile Island accident was caused by a combination of personnel error, design deficiencies and component failures, according to the U.S. Nuclear Regulatory Commission (NRC).
* After Three Mile Island, the NRC strengthened safety standards for all U.S. reactors, delaying the construction of many reactors already under construction. The industry did not start building a new reactor in the United States for about 30 years, and only in the past few years have power companies sought permission to construct new reactors
* Three units at the Daiichi nuclear plant suffered a loss of coolant to their reactors following a massive earthquake and a devastating tsunami. The fuel rods in at least one unit were damaged due to the loss of coolant and suffered a partial meltdown.
* The plant vented a small amount of radiation to release pressure in the units' containment, raising the levels in the immediate vicinity of the plant above the legal limit. The plant also suffered an explosion caused by a buildup of hydrogen that collapsed the roof on at least one of the units.
* The government ordered the evacuation of tens of thousands of people from around the plant and by Saturday a total of 140,000 had been evacuated from around Daiichi and another nearby plant Daini.
* Operators at Daiichi are pouring seawater into at least two reactors at the plant to prevent a further meltdown.
* As for the future, nuclear experts said it was too soon to say what affect the Daiichi accident would have on the so-called nuclear renaissance, but noted it would certainly be a lot harder to build a new reactor for a while.
Reporting by Scott DiSavino in New York and Bernie Woodall in Detroit. Edited by Martin Howell