* Breach of a core pressure vessel most feared scenario
* No signs that has happened
* Latest exposure highlights risk to workers
By Gerard Wynn
LONDON, March 25 (Reuters) - Higher radiation in water seeping from Japan's stricken nuclear reactor is not likely to be a sign of damage to the central container of uranium fuel, experts said on Friday.
Damage to a central pressure vessel has always been the most feared development since the crisis caused by a massive earthquake and tsunami on March 11, as these vessels contain the hottest, most radioactive fuel in the power plant's reactors.
Workers were burned trying to lay power cables at the plant in Fukushima prefecture and exposed to radiation levels 10,000 times higher than normal, but officials on Friday played down suggestions that a reactor core was leaking. [ID:nL3E7EO2UX]
The exposure could be dangerous for the workers involved but probably did not threaten a wider radioactive release, European nuclear experts said.
Most importantly, there have yet to be any signs of damage to a pressure vessel, which would take the form of higher levels around the site of radioactive caesium and iodine and falling pressure within reactor cores. "If a pressure vessel had in any way failed I'd think you'd know about it by now," said Laurence Williams, professor of nuclear safety at the John Tyndall Institute for Nuclear Research in Britain and a former chief nuclear power plant inspector.
Richard Wakeford of the Dalton Nuclear Institute at the University of Manchester agreed if there had been a major release of radioactivity it would likely have been picked up.
COULD BE POSITIVE
Experts said the exposure on Thursday was likely due to the continuing venting of steam from reactor vessels to relieve pressure and heat. They said such activity had no serious health consequences outside the 20-km (12-mile) evacuation zone.
"It isn't too surprising given the problems they have had," said Ole Reistad, a senior expert at the Norwegian Radiation Protection Authority.
"Getting water through the primary circuit is the fundamental function they need to maintain. But they need to be able to control the workers' exposure."
He said the release could even be seen as positive if it meant workers had restored water circulation through the reactor pressure vessels after pumps were knocked out.
Uranium fuel rods within some reactor pressure vessels are damaged, the operating company TEPCO has said, increasing the chance that cooling water passing through them and then vented would contain radioactive materials.
"Obviously the fuel is damaged and caesium and iodine will come out," Williams said.
"10,000 times sounds horrendous but it's probably not really. But you wouldn't want to wash your hands in it."
Experts stressed that it was difficult to say exactly what was going on without full information about reactor pressure levels and normal baseline water radioactivity.
"Normally in a water reactor like this you keep the water very clean," said Tony Roulstone, nuclear energy expert at Cambridge University and a former industry adviser.
"We've seen nothing in the reports there to indicate that a pressure vessel is breached."
(Additional reporting by Alister Doyle in Oslo and Kate Kelland in London, Editing by Sonya Hepinstall)
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