A British nuclear expert, Professor Paddy Regan, offers this analysis of what happened at Fukushima.
Regan, professor of nuclear physics at the University of Surrey, said:
"It looks as if the coolant pumps had initially stopped working. They shut down automatically when the reactor shuts down, but there is a backup system running off a diesel generator - it looks as though that's the bit that failed.
"As a result there is no way of pumping heat out of the reactor, so it has to cool naturally. If the reactor gets too hot, in principle this means the fuel rods can melt - but it looks unlikely this has happened to any great extent in this case.
"To reduce the pressure, you would have to release some steam into the atmosphere from the system. In that steam, there will be small but measurable amounts of radioactive nitrogen - nitrogen 16 (produced when neutrons hit water). This remains radioactive for only about 5 seconds, after which it decays to natural oxygen.
"But if any of the fuel rods have been compromised, there would be evidence of a small amount of other radioisotopes in the atmosphere called fission fragments (radio-caesium and radio-iodine). The amount that you measure would tell you to what degree the fuel rods have been compromised. Scientists in Japan should be able to establish this very quickly using gamma ray spectroscopy as the isotopes have characteristic decay signatures. Current reports seem consistent with a small leak to relieve pressure.
"But we still need to establish the cause and exact location of the explosion, which is a separate issue. So far it looks like it's not the reactor core that's affected which would be good news."
12.02pm: More from my colleague Terry Macalister on the situation at Fukushima No 1:
"Getting information out of the nuclear industry is never easy (a legacy perhaps of its Cold War origins) and the Japanese political system is also notorious for keeping a tight lid on unwelcome news. This all means it will be hard to know what is exactly happening at any one time at Fukushima.
"But I have been talking to a top British nuclear engineer who visited that plant in Japan and he says it was built in the 1970s and is not as earthquake-proof as later models. He also said there had long been speculation about how strong was the containment dome over the top of the reactor - the final barrier for any radioactive emissions to be released into the environment.
"It is also not the first time there have been problems at Fukushima. There have been reports of a loss-of-power incident in June last year. I have also seen suggestions that one reactor at the complex began using MOX (mixed plutonium-uranium) fuel starting in September."
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