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Thursday 17 March 2011

Meltdown Shock Horror – Updated

One might have thought that things were bad, if the normally reclusive Emperor Akihito had to appear on TV to urge his subjects not to give up. Maybe things are bad, but that does not justify some of the panic inducing headlines of the past few days. But events have moved on from last weekend, so another look at the current state of play at Japan’s Fukushima Daiichi nuclear power complex is overdue.

At the outset, it has to be stressed that there are two nuclear power plants in the area: Fukushima Daini, which has had little media attention, has been shut down without any serious problems. Like its stricken neighbour, it uses Boiling Water Reactor (BWR) technology, although its four reactors are more recent and also more powerful.

The Fukushima Daiichi complex has six reactors, the oldest of which first “went critical” back in 1970. The others were added between 1973 and 1979. At the time of the recent earthquake and subsequent tsunami, the three most recently commissioned reactors had been shut down for maintenance, and these are not causing any problem.

The oldest three reactors, as I’ve previously noted, had the shutdown process disrupted by the tsunami, and there were subsequent hydrogen detonations following release of excess pressure caused by overheating. Fortunately, the reactor containments appear to have held any serious leak of radioactivity, although one may have been cracked.

So thus far, even if some of the fuel within the reactors has melted, the effect is, as I previously predicted, that those reactors are no longer a commercial proposition and the consequences are merely economic. The problem now – and it could result in a serious release of radiation – is with spent fuel “ponds” attached to reactors 3 and 4.

Spent nuclear fuel assemblies are held in either “ponds” – like large swimming pools – or in dry stores. The last completed UK Magnox plant, Wylfa, uses a dry store. In both cases the spent fuel needs to be kept cool: this means water or carbon dioxide gas being regularly circulated. Not only is that circulation apparently not happening at reactors 3 and 4, but the water level seems to have dropped, exposing the assemblies to the atmosphere.

And that means an uncontrolled and possibly serious radiation leak. How serious? That depends partly on how long the spent fuel has been exposed to the atmosphere, and of course how much of it is on site. How the situation got to this point is not known, but it’s entirely possible that teams were so occupied with keeping the reactors cooled that the fuel ponds slipped from view. It could yet get worse: I’ll check back in the near future.

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