Mon Mar 14, 2011 2:27pm EDT
* Stricken nuclear cores cooling naturally
* Little chance of big leak, three reactors written off
By Gerard Wynn LONDON, March 14 (Reuters) - The risk of a major radiation leak in Japan is subsiding as stricken nuclear reactors cool, but there will be major clean-up costs and three reactors will probably be written off, experts said on Monday.
A massive earthquake and tsunami on Friday knocked out cooling systems at a nuclear plant in Fukushima, eastern Japan, triggering a race to flood reactor cores with seawater and stop radioactive uranium fuel from melting and leaking out.
A natural decaying process means that the amount of heat the fuel produces has fallen dramatically, by more than 90 percent, experts said on Monday.
That reduced the chance of serious damage, especially after workers flooded the three worst-affected reactors with seawater.
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For a factbox on the stricken cores: [ID:nLDE72D0BM]
For graphics click here - r.reuters.com/fyh58r
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"The longer it goes on, the better the situation," said Robin Grimes, director of the Centre for Nuclear Engineering at Imperial College London, asked about the chances of a breach of the steel core, or "pressure vessel", that contains the fuel.
All affected power plants, including the one in Fukushima, stopped generating power automatically when the quake struck.
That left a hot fuel mixture of radioactive materials, such as uranium, plutonium, strontium and caesium, to cool or "decay" over time.
"It's entirely credible that the worst-case now does not involve break-out of a pressure vessel," said Malcolm Grimston, nuclear policy and technology expert at the British think-tank Chatham House.
A breach of the core would increase local radiation but it was unclear to what levels. Japan could not repeat the Chernobyl disaster because the Japanese reactors successfully shut down.