Fukushima: has meltdown already happened?

Radiation levels well above those that are harmful to human health have been recorded at Fukushima Daiichi nuclear power plant. The news comes after an explosion at a third reactor at the tsunami-hit facility and a fire at the spent fuel pond of reactor 4.

The International Atomic Energy Agency said today that it had detected radiation up to 400 millisieverts per hour at Fukushima Daiichi. To put that in context, exposure to over 100 millisieverts a year can lead to effects such as cancer, radiation sickness, and temporary sterilisation.

Nuclear analyst John Large told The First Post today that despite all the contradictory information coming out of Japan, the crisis-hit country in all probability has a significant nuclear disaster on its hands.

The explosion at reactor 2 - after similar explosions at reactors 1 and 3 - was more or less inevitable, and has been put down to the same cause: a chemical reaction producing an explosive mix of hydrogen and oxygen from seawater pumped in to cool the fuel rods in the reactors.

A meltdown happens when fuel rods in a nuclear reactor are allowed to overheat. The problems at Fukushima's reactors have arisen because back-up cooling systems failed in the wake of Friday's tsunami. Luckily the reactors automatically shut down following the earthquake that preceded the massive tidal wave.

After the explosions at reactors 1 and 3, officials insisted that the containment vessels had not been breached. However, they have given no such assurance following this morning's explosion at reactor 2 - and say that there could be a crack in the reactor's suppression pool that would allow radioactive steam to escape continuously.

Meanwhile, news of a fire in the spent fuel pond at reactor 4 brings a whole new dimension to the Fukushima crisis. This is where old fuel rods are stored; they are still hot because of radioactive decay and need to be cooled. The fire probably started when the fuel rods became exposed when water leaked out. A meltdown here is unlikely, but spent fuel at reactors 5 and 6 is also reportedly heating up.

A spokesman for Tepco, which operates Fukushima Daiichi, has admitted that reactors 1, 2 and 3 are all likely to have suffered a meltdown to some degree, which means a full-scale meltdown cannot be ruled out.

Meanwhile, French nuclear safety agency ASN has rated Fukushima a 5 or 6 on the international scale of nuclear disasters, which runs from 1 to 7. Three Mile Island was a 5 and Chernobyl was a 7.

All of this bears out the prediction of nuclear analyst John Large, who at the weekend told The First Post that he thought a "significant nuclear event" had occurred at Fukushima.

He said that the "jellyfish" shape of Saturday's explosion and the decision to vent the reactor's secondary containment - releasing radioactive vapour and necessitating the evacuation of local people - all suggest fuel rods had melted and leaked from the primary containment.

Speaking to The First Post today, Large said that speculation regarding the events at Fukushima had spiralled out of control - a situation not helped by the Japanese government, who he suspects may be playing down the true extent of the disaster in order to avoid "startling the horses".

Large talks about inconsistencies between official pronouncements and what is happening on the ground. "I've looked at crystal clear satellite images of the site and they show after the extremely violent explosion at reactor 3 that there are no people and no fire engines on site - but they are supposed to be using fire engines to pump seawater into the reactor."

All the indications are that something serious is afoot. Large points out that increasing the evacuation zone, as the Japanese government has done over the past few days, is "a tremendous undertaking when half your country has been wrecked by a tsunami".

Large is at pains to point out that coming to a concrete conclusion is impossible given the lack of information coming out of Japan. However, he believes there is a serious disagreement between the scientists in charge of the Fukushima operation and the government minister overseeing them.  

He bemoans the lack of data on radiation levels, but points that that the American aircraft carrier the USS Ronald Reagan, which is bristling with nuclear detection instruments, has moved away from Fukushima.

Large also had harsh words for representatives of the UK nuclear industry who have been popping up in the media to play down the seriousness of the Fukushima crisis.

He was particularly amused by one nuclear power advocate who said that Fukushima was designed to explode in the way it did and, referring to Japanese civilians lining up to be tested for radiation levels, claimed: "I wouldn't be worried if I was in that queue." ·