VIENNA Japan struggled on Sunday to limit the damage at earthquake-crippled nuclear power reactors, but experts said a bigger disaster could be averted.
They said any meltdown of nuclear fuel -- which contains most of the radioactivity -- would not lead to a major escape of potentially dangerous clouds into the air as long as thick walls shielding the reactor cores were not breached.
Japanese authorities "appear to be having enough success to have forestalled a definite core melt accident that's difficult to control," said Mark Hibbs of the Carnegie Endowment for International Peace. "After three days that is very good news."
But he said it was "still a touch and go situation."
Professor Richard Wakeford at the Dalton Nuclear Institute of Britain's University of Manchester said the passage of time may help to reduce the danger.
"The reactor cores were still hot when the reactor shut down, as time goes on that radioactive decay heat will get less and the problem will get less," he said in a statement.
Robin Grimes, director of the Center for Nuclear Engineering at Imperial College London, said: "There's no risk of an extensive radiation leak into the surrounding areas ... the structure of the core is probably still intact."
In a sign of the desperate efforts under way at the stricken Fukushima nuclear plant to prevent fuel rods from overheating, Japan's Jiji news agency said the operator was preparing to put sea water into a third reactor at the site.
Cooling functions at the plant, in northern Japan, were damaged after a massive earthquake and tsunami on Friday.
The fear is that if the fuel rods do not cool, they could melt the container that houses the core, or even explode, releasing radioactive material into the wind.
LIKE THREE MILE ISLAND?
"Injection of sea water into a core is an extreme measure. This is not according to the book," Carnegie's Hibbs said.
"In doing this they are basically accepting the eventuality that these reactors will never operate again."
Japan's Chief Cabinet Secretary Yukio Edano said there might have been a partial meltdown of the fuel rods at the No. 1 reactor at Fukushima.
A German industry expert, however, said any partial meltdown "is not a disaster" and that a complete meltdown was unlikely.
Robert Engel, a structural analyst and senior engineer at Switzerland's Leibstadt nuclear power plant, said he believed Japanese authorities would be able to manage the situation at the damaged Fukushima facility north of Tokyo.
Engel was an external member of a team sent by the International Atomic Energy Agency (IAEA) to Japan after a 2007 earthquake that hit the Kashiwazaki-Kariwa plant, until then the largest to affect a nuclear complex.
"I think nobody can say at this time whether there is a small melting of any fuel elements or something like that. You have to inspect it afterwards," he said.
Normally, Engel said, the water level inside a reactor core is 3 to 4 meters above the fuel. If the rods are not covered by water for a longer time then a core melting is possible.
"I think they will be able to manage it ... when the (reactor) containment is intact only a small amount of radioactivity can go out, like in Three Mile Island," he said referring to the 1979 nuclear accident in the United States.
At Three Mile Island, a cooling fault led to a build-up of pressure in the radioactive core and resulted in a relatively small radiation leak.
But Wakeford, the University of Manchester professor, said the Japanese authorities were doing the right thing by evacuating people in the case the worst happens.
"If the fuel is uncovered by cooling water it could become so hot it begins to melt -- if all the fuel is uncovered you could get a large scale meltdown," he said.