LONDON/OSLO (Reuters) - Spent fuel rods and fuel ponds pose the main radiation risk at Japan's stricken nuclear plant, European experts said Thursday.
Japanese military helicopters were dumping water on the Fukushima nuclear plant Thursday. Authorities said the pressure vessels at the core of the plant were stable for now, throwing the focus on the ponds that store spent uranium.
The ponds, rather than the reactors, had become the "main problem," said Ole Reistad, of the Norwegian Radiation Protection Authority.
"The ponds remain a serious issue which isn't resolved yet," said Andrew Sherry, director of the Dalton Nuclear Institute at the University of Manchester.
As long as the pressure vessels stayed intact, then the leakage risk was "low level," said Laurence Williams, nuclear safety expert at the University of Central Lancashire and former UK chief inspector of nuclear installations.
"You've had six days now and we haven't had any sustained release which is a good thing," he added.
Most experts say that the problems at the Fukushima reactor are far less severe than the 1986 catastrophe at Chernobyl, which exploded and spewed radioactivity over Europe.
Experts were unsure how far dangerous radioactive materials and especially caesium would spread if the ponds over-heated.
"The caesium is an issue," said Robin Grimes, head of the centre for nuclear engineering at Imperial College London.
Williams said he did not expect the ponds to get hot enough, even if boiled dry, to release large amounts.
"It might be 100, 200 degrees Celsius but so long as it doesn't go to 500, 600, 1000 degrees, which I can't believe it would do, it should be manageable."
A major issue in the ponds was whether small-scale nuclear heat generation could re-start if the fuel ponds boiled dry, meaning enough neutrons from the old uranium fuel would collect and start bashing other atoms in a chain reaction, as they do when nuclear power plants are producing electricity.
"The effect is to dump energy into the system," said Grimes, of the risk, called criticality.
"Criticality alarms will go off and they'll have to evacuate, just in case there's a big burst (of radiation), that will be the thing, it will limit people's ability to work.
Experts said a restoration of electricity could be a breakthrough, allowing pumps to bring in cooling water.
"The one thing that I would be looking for is this business of re-establishing the electrical power. The worst case is they can't re-establish it," said Grimes.
"They seem to be taking a lot longer to get there than they originally intended. The other thing is that just because you established the power then the question is are the pumps still working," he said.
Overall, the direction was positive, some experts said.
"We're not out of the woods, but my sense is that we're moving in the right direction," said Sherry. "With the reactor cores, while there may be some damage to those cores, they are contained."
(With additional reporting by Gerard Wynn and Alissa de Carbonnel; editing by Janet McBride)
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