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Friday, April 15, 2011

Fuel rod fragments at bottom of vessels

No meltdown risk if cooling efforts continue


Staff writer

Melted fuel rod fragments have sunk to the bottoms of three reactors at the Fukushima No. 1 power plant and could theoretically burn through the pressure vessels if emergency water-pumping operations are seriously disrupted, the Atomic Energy Society of Japan said Friday.

If too many of the melted fuel fragments puddle at the bottom, they can generate enough concentrated heat to bore a hole in the pressure vessel, which would result in a massive radioactive release to the environment..

"It will take at least two or three months ... until the situation of fuel rods is stabilized" said Takashi Sawada, vice chairman of the nuclear body.

The fuel rods are being cooled by tons of water that is being manually injected into the reactors and their spent-fuel pools by truck. But if those operations are interrupted for two or three days by an aftershock or other unforeseen event, the reactors' cores will again be at risk of melting down further, he said.

The nuclear safety committee, consisting of nuclear engineers and academics, compiled the analysis based on the data disclosed by Tokyo Electric Power Co. and the Nuclear and Industrial Safety Agency, the government's nuclear watchdog. Earlier reports quoting experts giving the same explanation have been circulating in western media for a week.

The fuel rods in reactors 1, 2 and 3 are heavily damaged. Pieces of the rods melted into fragments about 1 cm wide or smaller and drifted to the bottom of the pressure vessels, the committee said.

The fuel rods in reactors 1 and 2 still seem to be partially exposed, but the entire fuel assembly at reactor 3 is believed to be safely submerged, it said.

Meanwhile, radioactive substances have been detected in groundwater and soil on the plant's premises, giving Tepco even more problems..

Tepco said Thursday night that the concentration of radioactive materials detected in groundwater extracted near the turbine buildings of reactors 1 and 2 jumped 17-fold in a week.

Radioactive iodine-131 was 610 becquerels per cubic centimeter, 17 times the amount detected April 6, while cesium-134 was 7.9 becquerels, eight times the amount detected previously. Normally none would be detected.

Tepco said toxic water may be seeping from the nearby reactor 2 turbine building, which is filled with high-level radioactive water. The utility will increase its inspections from once to three times a week.

Tepco also said it detected a small amount of plutonium at two locations 500 meters from the No. 1 reactor. Some 0.16 becquerels per kg and 0.12 becquerels per kg of plutonium-238 were detected in soil collected between March 31 to April 4.

The amount is the same level as those normally detected, Tepco said, suggesting there is no immediate health risk.

The earth also contains radioactive substances that have been falling from the sky since the United States, Russia and other nuclear states carried out atmospheric atomic tests in past decades.

As for the storage pool for spent fuel rods in reactor 4, radioactive iodine and cesium were detected in a water sample extracted Tuesday, but the amount indicated damage to the rods was "not big," NISA said.

According to the results, 220 becquerels per cubic cm of radioactive iodine-131, 88 becquerels of cesium-134 and 93 becquerels of cesium-137 were detected, Tepco said.

On Friday, workers continued efforts to bring the reactors under control and stop radioactive leaks, injecting more nitrogen gas into the No. 1 reactor.

Melted fuel rod fragments have sunk to the bottoms of three reactors at the Fukushima No. 1 power plant and could theoretically burn through the pressure vessels if emergency water-pumping operations are seriously disrupted, the Atomic Energy Society of Japan said Friday.



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