Fukushima: French watchdogs alarmed over fuel-rod pools

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French safety agencies voiced deep worries on Wednesday for a tank holding spent nuclear fuel rods at Fukushima, and one said "the next 48 hours" were critical for keeping the rods safely cooled.

The fuel-rod pool at the plant's No. 4 reactor "is the major concern," the Nuclear Safety Authority (ASN) said in a statement.

"The next 48 hours will be decisive," said Thierry Charles, director for factory, laboratory, transport and waste safety at the Institute for Radiological Protection and Nuclear Safety (IRSN).

"I am pessimistic, because since Sunday I have seen that almost none of the solutions has worked," he said.

He described the situation as "a major risk," but added: "All is not lost, and I hope that the Japanese can find a way."

The tank contains fuel rods which remain extremely radioactive after being used in the reactor.

They are kept immersed in a deep pool of water until they reach a lower and more manageable temperature. But this water has been evaporating, even as plant engineers struggle to replenish it.

Unlike the fuel rods used in the reactor vessel, the spent rods are not surrounded by a steel-and-concrete containment vessel, which is designed to confine leaks of radioactive gas and particles.

Instead, they are housed in the overall building covering the No. 4 unit, and this building has been badly damaged, the French experts said.

As a result, if the tank runs dry, the rods can overheat and the metal sheaths that surround the fuel could be ruptured, boosting the risk of a direct release of radioactivity into the atmosphere, the two agencies said.

The IRSN said in a statement that the No. 4 pool was "boiling."

"Without water replenishment, the fuel-rod assemblies will start to be exposed in a few days. If the pool runs dry, this would eventually lead to the meltdown of the fuel.

"In such an event, the corresponding releases of radioactivity would be far higher than those that have occurred up till now."

The IRSN said that at around 6 a.m. local time on Wednesday (2100 GMT on Tuesday, "responders were unable to approach the pool because of an excessive ambiant dose" of radiation "of around 400 millisieverts per hour."

A single dose of 100 millisieverts is considered by experts to boost the risk of several types of cancer.

Asked about the maximum possible amount of radioactive release, Charles said, "it would be in the same range as Chernobyl."

In the 1986 disaster in Soviet-era Ukraine, 12 billion billion becquerels of radioactivity was released, equivalent to 30,000 times the total emissions from all the world's nuclear facilities in a single year.

Explosions have occurred at the external buildings housing the Nos. 1, 2 and 3 reactors at Fukushima, which shut down automatically after the March 11 mega-quake.

Reactors 4, 5 and 6 had already been shut down for maintenance, but water supply to the fuel-rod pools has been hampered by lack of pumps, which were knocked out by the quake-driven tsunami.

Temperatures in the pools at the No. 5 and 6 reactors "are rising slowly," the IRSN said.

"Without cooling, these pools could start to boil in a few days. According to unconfirmed information, additional diesel generators are being installed to ensure the cooling of these pools."

© 2011 AFP

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