Factbox - Experts on explosion at Japan nuclear plant

(Reuters) - Japan was fighting to stop fuel rods in three earthquake-damaged reactors at the Fukushima Daiichi plant from overheating after some controlled leaks of radioactive steam into the air to relieve pressure.

The fear is that if the fuel rods do not cool, they could melt the container that houses the core, or cause an explosion, and in a worst case scenario, release radioactive material into the wind.

Here are comments from experts:

PROFESSOR RICHARD WAKEFORD, DALTON NUCLEAR INSTITUTE,

UNIVERSITY OF MANCHESTER, BRITAIN

"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.

"In this case, the diesel generators that ran the cooling systems were flooded by the tsunami, so sea water is now being used to cool the core.

"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. So Japanese authorities are doing all they can at the moment to keep the core cooled with seawater.

"This is a well known potential accident scenario, so the regulatory authorities ensure that there are appropriate plans in place to ensure that cooling water is kept pumping."

"There will be intense monitoring around the site of the reactors for release of radiation. We will know immediately if there had been a major release -- and this clearly hasn't happened at the moment.

"The Japanese authorities are doing the right thing by evacuating people just in case the worst happens. If it did, there are measures that will be taken -- for example ... people would be given stable iodine tablets that would block the uptake of radioactive iodine by the thyroid."

"This approach is very different to Chernobyl where the authorities were in denial and did not act to protect the population (they did not issue iodine tablets or stop people eating local food) -- many of the thyroid cancers seen could have been prevented by taking action immediately."

ROBERT ENGEL, STRUCTURAL ANALYST AND SENIOR ENGINEER AT

SWITZERLAND'S LEIBSTADT NUCLEAR POWER PLANT

"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."

But a partial meltdown "is not a disaster" and a complete meltdown is not likely, he told Reuters by phone.

"I only see they are trying to cool the reactor, that is the main task, and they are trying to get cooling water from the sea." Normally, he said, the water level inside a reactor core is 3 to 4 metres 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.

ROBIN GRIMES, DIRECTOR OF THE Centre FOR NUCLEAR ENGINEERING

AT IMPERIAL COLLEGE LONDON

"There's no risk of an extensive radiation leak into the surrounding areas. The worst-case scenario is it's just going to be more difficult to clean up," he told Reuters.

"The bad thing about Three Mile Island is that because of the way the core was destroyed it meant that even after it cooled down they couldn't take the fuel out of the reactor."

"They had a load of rubble at the bottom of a metal pot."

"With these reactors at the moment we're still in a position that after it's all cooled down it may well still be possible to simply remove the fuel and dispose of it in a relatively normal procedure," said Grimes.

"What's clear because of the incidental radiation being released at the moment, which is significant but not overwhelming, is that the structure of the core is probably still intact. So it's not as bad as Three Mile Island."

MARCO RICOTTI, NUCLEAR PLANT SPECIALIST AND PROFESSOR AT THE

POLYTECHNIC INSTITUTE OF MILAN

"At Chernobyl the reactor had reached a huge increase in power, there was no safety container and there was not enough time to evacuate people," he told Italian newspaper Corriere Della Sera.

"At Fukushima the reactor was turned off, there was a safety container and there was enough time to move away the surrounding population."

Pressure in Fukushima had doubled the normal levels.

"To lower it, technicians have let out vapour in the air twice. In simple terms you can think of the vent of a pressure cooker. The aim is to maintain the integrity of the safety container."

KEIJO VALTONEN, ASSISTANT DIRECTOR OF STUK, FINNISH

RADIATION AND NUCLEAR SAFETY AUTHORITY

"Tokyo Electronic Power (TEPCO) announced they had some problems with pumping seawater, so they will apparently need to install new pipes. But as much as we know, the fuel rod is covered by water in both of the reactors, and the speed of radiation doses are relatively low outside, less than 50 millisieverts per hour," he said Reuters.

"I can't see any acute problem at this stage, but of course it can develop, if they fail in the installing work."

"The problem is that worldwide there are lots of operating reactors built in early 1970's. I think that (after this catastrophe) the oldest plants will simply be closed and new compensatory ones will be built, unless political situation turns dramatically and nothing gets built."

"In the United States, there are 100 plants, and half of those are built in 1970's," he said. "It was a disservice for humankind that no new plants have been built in last 20 years."

(Reporting by Michael Holden in LONDON, Catherine Hornby in ROME, Fredrik Dahl in VIENNA and Gerard Wynn in LONDON)