FACTBOX-Experts on Explosion at Japan Nuclear Plant

LONDON, March 12 (Reuters) - Radiation was leaking from an unstable nuclear reactor north of Tokyo on Saturday, the Japanese government said, after an explosion blew the roof off the facility following a massive earthquake.

The development has led to fears of a disastrous meltdown. Here are comments from experts about what might have happened.

ROBIN GRIMES, PROFESSOR OF MATERIALS PHYSICS AT IMPERIAL COLLEGE LONDON

"It does seem as if the back-up generators although they started initially to work, then failed," Grimes, an expert in radiation damage told BBC TV.

"So it means slowly the heat and the pressure built up in this reactor. One of the things that might just have happened is a large release of that pressure. If it's that then we're not in such bad circumstances.

"Despite the damage to the outer structure, as long as that steel inner vessel remains intact, then the vast majority of the radiation will be contained.

"At the moment it does seem that they are still contained and it's a release of significant steam pressure that's caused this explosion. The key will be the monitoring of those radiation levels."

PROFESSOR PADDY REGAN, NUCLEAR PHYSICIST FROM BRITAIN'S SURREY UNIVERSITY

"What is important is where that explosion is," Regan told Sky News.

"It's not clear what has exploded. The big problem would be if the pressure vessel has exploded but that does not look as though that's what's happened.

"If the pressure vessel, which is the thing that actually holds all the nuclear fuel ... if that was to explode -- that's basically what happened at Chernobyl -- you get an enormous release of radioactive material.

"It doesn't look from the television pictures ... as though it's the vessel itself.

He said media reports suggested that a small fraction of the nuclear fuel might have melted at the core of the reactor which would not be surprising.

NUCLEAR EXPERT MARK HIBBS OF THE CARNEGIE ENDOWMENT FOR INTERNATIONAL PEACE

"We don't have any information from inside the plant. That is the problem in this case.

"If it melts down the probability that there would be a breach or that radiation would get outside of the plant because of weakness of the structure of the plant ... is much greater." (Reporting by Michael Holden and Fredrik Dahl in Vienna)