* Stricken nuclear cores cooling naturally
* Little chance of big leak, three reactors written off
By Gerard Wynn LONDON, March 14 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.
For a factbox on the stricken cores: [ID:nLDE72D0BM]
For graphics click here - r.reuters.com/fyh58r
"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.
At the Chernobyl nuclear plant in Ukraine in 1986, control rods failed to limit the production of heat -- the process in which high-energy neutrons smash atoms into pieces in a chain reaction -- leading to blasts which blew the reactor apart, spreading contamination across Ukraine, Russia and Europe.
The core of a nuclear reactor consists of metal rods containing pellets of uranium fuel bundled into fuel assemblies, all contained within a steel shell.
The fuel in the Japanese reactors may now not melt through the steel container, but the individual rods were probably damaged.
"I'd be amazed if one or two hadn't split," said Grimes.
A more serious meltdown, where the rods congealed into a mess which was hard to remove, would increase the cost of clean-up. That happened at Three Mile Island in Pennsylvania in 1979, where the site had to be sealed.
"It becomes much more complicated, you need much more radiological protection, you'll probably need to develop new bespoke techniques and that's where the expense piles up," said Grimston.
The three flooded Japanese reactors have been written off because of the effects of seawater, experts said.
The fate of ponds of water which contained canisters of spent radioactive fuel is uncertain, but damage to the ponds would also increase clean-up costs without adding seriously to radiation levels, said Grimston.
Engineers have vented hydrogen gas produced from overheated fuel rods, contributing to two explosions and some increase in local radiation. That venting will continue for several days as reactors cool, experts said. (Additional reporting by Scott DiSavino; editing by Tim Pearce)