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."
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