Numbers Game
How serious is Japan nuclear crisis?
As it was almost bound to do at some point, Japan's nuclear safety agency has uprated its assessment of the Fukushima power station incident from a level four to a level five. The level five rating applies specifically to the nuclear reactors in buildings 2 and 3 at Fukushima, rather than to the spent fuel cooling ponds that have lost water and where the stored fuel is heating up. That implies that the regulators believe the main source of radioactivity coming from the plant has been the reactors. Certainly, one of the the spikes in readings earlier in the week appeared to co-incide with damage to reactor number 2, believed to be a crack in the containment system - the symptoms being a sharp release of steam and an abrupt drop in pressure. On Thursday and Friday, radiation levels around the plant appeared much more stable. And although elevated readings have been noted in some locations 30km from Fukushima, there has been nothing outside the 30km protection zone that has appeared to pose a danger to health. Despite this, a number of governments have advised their citizens to stay much further away - or in the case of the UK, to consider doing so. However, when the UK's chief scientific adviser explained the reasoning to BBC News on Thursday, he was still painting a worst-case scenario that appeared some way short of apocalyptic. "The worst-case scenario would see the ponds starting to emit serious amounts of radiation, with some of the reactors going into a meltdown phase," he said. "We put that together with [a possible scenario of] extremely unfavourable weather conditions - wind in the direction of Tokyo, for example. "Even in that situation, the radiation that we believe could come into the Tokyo area is such that you could mitigate it with relatively straightforward measures, for example staying indoors and keeping the windows closed." Fukushima now becomes the third level five incident in half a century of nuclear power. The first was the Windscale reactor fire in the UK in 1957 - the second, the partial meltdown of a reactor at Three Mile Island in the US in 1979. Richard Wakeford from the Dalton Nuclear Institute, a visiting professor in epidemiology at the University of Manchester, recently re-assessed the effect of radiation released at Windscale. Using data and computer models, his scientific paper concluded that the release could have caused about 240 cases of cancer, half of them fatal. However, inquiries into Three Mile Island concluded it probably caused no deaths. That raises the question of why both are in the same INES category, given that Three Mile Island did not, in the end, have more than a local impact. "The reason why Three Mile Island was rated a five is that there was major damage to the reactor core and there was potential for a widespread release of radioactive material - it didn't happen, but that potential is built into the event scale," said Professor Wakeford. In terms of material released, he said: "Fukushima is somewhere between the two - clearly there have been releases, and you have a possible breach of the containment system - no-one really knows."
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