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.
TIMOTHY ABRAM, PROFESSOR OF NUCLEAR FUEL TECHNOLOGY AT BRITAIN'S MANCHESTER UNIVERSITY
"By sampling the air around the station, you'd be able to tell how much radioactivity has been released. The thing they'll be looking for more than anything is whether there's any evidence of the fuel actually degrading," he told Reuters.
"If the fuel is substantially intact, then there'll be a much, much lower release of radioactivity and the explosion that's happened might be just due to a build-up of steam in the reactor circuit.
"The most probable (cause of the explosion) is the coolant, particularly if it's water, can overheat and turn to steam more rapidly than it was designed to cope with."
He said it was unlikely it would develop into anything more serious, but this would depend on the integrity of the fuel, which contains nearly all the radioactivity of the plant. He said he thought it would be "pretty unlikely" that the fuel itself had been significantly damaged.
He said if this did occur, some radioactive material might be released into the primary circuit, which in turn might be vented into the containment building to release the pressure.
"Even the worse case scenario from there is the pressure in the containment building itself builds up to dangerous levels and has to be released," he said.
"Consequently you are releasing pressure from in the containment building, some of which contains radioactivity, out into the environment. There are a lot of ifs in that chain of events."
VALERIY HLYHALO, DEPUTY DIRECTOR-GENERAL OF THE CHERNOBYL NUCLEAR SAFETY CENTRE
"The explosion at No. 1 generating set of the Fukushima nuclear plant in Japan, which took place today, will not be a repetition of the Chernobyl nuclear disaster," Interfax quoted the Ukrainian expert as saying.
He said that the Japanese nuclear power plants use reactors of a totally different design to Chernobyl's.
"Japan has modern-type reactors. All fission products should be isolated by the confinement (the reactor's protection shell). Only gas emission is possible."
Hlyhalo said that Japanese nuclear power plants are earthquake resistant.
"Apart from that, these reactors are designed to work at a high seismicity zone, although what has happened is beyond the impact the plants were designed to withstand. Therefore, the consequences should not be as serious as after the Chernobyl nuclear disaster."
IAN HORE-LACY OF THE WORLD NUCLEAR ASSOCIATION, AN INDUSTRY BODY REPRESENTING 180 COMPANIES IN THE NUCLEAR SECTOR
"It is obviously an hydrogen explosion ... due to hydrogen igniting. If the hydrogen has ignited, then it is gone, it doesn't pose any further threat."
"The whole situation is quite serious but the actual hydrogen explosion doesn't add a great deal to it."
He said it was "most unlikely to be a major disaster" and he also did not believe there would be a full fuel meltdown.
"That would have been much more likely early yesterday in the European time. We are now 24 hours into the situatiuon and the fuel has cooled a lot in that time and the likelihood of meltdown at this stage I would think would be very, very small."
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."