* 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"
"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
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
"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
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)