From the Guardian's March 12 Earthquake live blog on the blast, including that it was the walls, not the reactor container, that exploded, and that the radiation is going down.

Note the 12:57 item in my quote is unrelated to the blast, it is included because it is incredible (they are headlining that item on The Guardian's home page:)

4.28pm: The incident at the Fukushima No 1 nuclear plant is less serious than both the Three Mile Island accident in 1979 and the 1986 Chernobyl nuclear disaster, according to Japan's nuclear safety agency.

An official at the agency said it has given the incident a rating of 4 on the International Nuclear and Radiological Event Scale (INES), which equates to an accident with local consequences. Three Mile Island was rated 5 while Chernobyl received the highest rating of 7, he added.

3.23pm: More than 300,000 people have now been evacuated from their homes in northern Japan and that number will rise as the exclusion zone around the Fukushima nuclear power plant is widened, Kyodo News reports.

1.49pm: Here's a lunchtime update on events so far on Saturday in Japan in the aftermath of Friday's devastating 8.9 magnitute earthquake and tsunami.

•There has been an explosion at the Fukushima No 1 nuclear power plant in north-eastern Japan, close to the epicentre of the quake. Officials say the blast is not a meltdown but exterior walls of one of the reactor buildings have been destroyed. There are also concerns about Fukushima No 2 plant....
 
12.57pm: Japan's chief cabinet secretary Yukio Edano has said that radiation levels at Fukushima No 1 nuclear power plant have dropped slightly since the blast, NHK reports.

The blast destroyed the exterior walls of the building, the plant operator, Tokyo Electric Power Co, has confirmed there is no damage to the steel container housing the reactor, he added.

"We have confirmed that the walls of this building were what exploded, and it was not the reactor's container that exploded," said Edano.

12.57pm: Japanese TV is reporting that about 10,000 people are missing in the town of Minamisanrikucho in Miyagi prefecture.

12.03pm: 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."

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."