Kyodo

April 15, 2011

Nuclear fuel inside the crippled reactors at the Fukushima Daiichi power plant has partially melted and settled at the bottom of pressure vessels in the shape of grains, according to an analysis by the Atomic Energy Society of Japan made public by Friday.

The academic body’s panel on nuclear energy safety has said the melted fuel at the No. 1 to 3 reactors has been kept at a relatively low temperature, discounting the possibility that a large amount of melted fuel has already built up at the bottom of their reactor vessels given the temperature readings there.

A large buildup of melted nuclear fuel at the bottom could become a molten mass so hot that it could damage the critical containers and eventually leak huge amounts of radioactive material.

The panel has also said that the fuel grains with a diameter of between several millimeters and 1 centimeter are believed to have settled flatly at the bottom of the vessels, leaving almost no possibility of a nuclear chain reaction called ”recriticality.”

Takashi Sawada, deputy chairman of the group, gave the assessment that even if the current stabilization efforts proceed smoothly, it would take at least two to three months for the fuel to be stabilized with few if any radioactive emissions.

The panel also found that the fuel rods in the No. 1 to 3 reactors have been damaged after analyzing information made public by the plant operator Tokyo Electric Poser Co. and the Nuclear and Industrial Safety Agency under the Ministry of Economy, Trade and Industry.

The panel has presumed that the fuel has slowly melted and become grain shaped as it was quenched when it fell into the cooling water and then settled down at the bottom of the reactor pressure vessels.

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Parts of the fuel rods in the No. 1 and 2 reactors have apparently been exposed, while those in the No. 3 reactor have been completely submerged in water, according to the panel.

Meanwhile, small amounts of plutonium believed to have been released as a result of the ongoing disaster have been detected in soil samples taken at the nuclear complex in Fukushima Prefecture, the plant operator known as TEPCO said.

It is the third time that traces of plutonium have been found in soil samples taken at the plant. The latest samples were taken on March 31 and April 4. The levels of plutonium in them were about the same levels observed in Japan following previous nuclear tests elsewhere, according to the utility.

On Friday, workers continued their efforts to bring the reactors under control and stop radioactive leaks from the seaside plant, injecting more nitrogen gas into the No. 1 reactor and installing more steel sheets near a seawater intake for the No. 2 reactor.

TEPCO said it will throw sandbags containing zeolite, a mineral that absorbs radioactive material, into the sea near the plant to reduce the levels of contamination in the seawater.

The nitrogen injection is aimed at preventing a hydrogen explosion at the reactor. At a news conference on Friday, nuclear safety agency spokesman Hidehiko Nishiyama said the agency is also considering injecting nitrogen into the other two troubled reactors soon.

TEPCO has pumped out around 660 tons of highly radioactive water from a tunnel connected to the No. 2 reactor’s turbine building into a container inside the building.

The operation resulted in a lower water level in the vertical part of the tunnel, but the agency said that as of Friday morning the level had risen back to the same level as before the water transfer started on Tuesday.

Removing the highly contaminated water that has flooded the basements of the No. 1 to 3 reactor turbine buildings and adjacent tunnels is seen as key to restoring critical cooling systems for the damaged reactors, which were lost in the March 11 earthquake and tsunami.

The flooding water is believed to be an unintended side effect of TEPCO’s stopgap measure of injecting water into the reactors and their spent nuclear fuel pools to prevent them from overheating.

This article was posted: Friday, April 15, 2011 at 4:31 am

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