TOKYO, April 15 (UPI) -- Nuclear fuel in the damaged reactors of Japan's Fukushima Daiichi nuclear plant has partially melted and settled in granular form, an analysis said Friday.
The grains have collected at the bottom of the pressure vessels of the No. 1, 2 and 3 reactors in the six-reactor plant damaged by a March 11 earthquake and tsunami, but do not pose any threat of a chain reaction, Kyodo News reported, quoting the Atomic Energy Society of Japan, an academic panel on nuclear energy safety.
An analysis by the panel said the pressure vessels have been kept at relatively low temperatures, thereby preventing any large buildup of the melted fuel grains, which could become hot and cause damage that could trigger radioactive emissions.
The panel's deputy chairman was quoted as saying it could take up to three months for the reactor fuel to be stabilized.
The Japanese government this week raised the severity rating of the nuclear crisis from level 5 to the highest level of 7 because of the radiation emissions from the facility. The new rating puts the crisis at the Fukushima plant on the same level as the April 1986 Chernobyl accident in Ukraine although its emissions are only 10 percent of the radioactivity at Chernobyl.
Tokyo Electric Power, operator of the Fukushima plant, said traces of plutonium, which may have escaped during the crisis, were detected in soil samples at the facility, Kyodo reported. The utility said the levels were about the same as noticed in earlier tests.
Utility workers continued with injection of nitrogen, an inert gas, into the No. 1 reactor to avert a hydrogen explosion while putting up more steel sheets near the No. 2 reactor to check any radioactive leaks into the sea. Workers also planned to stack sandbags containing a mineral to absorb radioactive material as added protection against seawater contamination.
Crews also were engaged in pumping out the thousands of tons of highly radioactive water that has flooded the basement of the turbine buildings of the three reactors. This is a major effort to restore the critical cooling systems of the reactors that were knocked out March 11. The flooding has been caused by the pumping of water to keep the reactors cool.
The International Atomic Energy Agency in Vienna said on its Web site that that the overall situation at the Fukushima Daiichi plant remains "very serious" but noted "early signs of recovery in functions such as electrical power and instrumentation."
The U.S. State Department reportedly had lifted its March 16 advisory to families of U.S. embassy staff in Tokyo, asking them to leave the country voluntarily. The change came, apparently, because efforts to stabilize the plant were going well. The Americans, however, were still urged to stay outside a radius of about 50 miles from the Fukushima plant.