Two months on

Two months on: "

The Economist has a report from the International Conference on Advances in Nuclear Power Plants of a plenary session discussing the Fukushima Daiichi accident: it's well worth reading.

The main highlights seem to be:

* The accident wasn't the result of a single disaster, but of two, and arguably three: earthquake, tsunami, and subsequent hydrogen explosions.

* The plan survived the earthquake (which exceeded its design requirements) quite well, and the reactors scrammed correctly. However, scrammed reactors continue to need power to run their cooling systems. The earthquake tore down the cables connecting the plant to the rest of the grid, forcing them onto backup power.

* The tsunami struck 15 minutes later, and was roughly five times higher than the plant had been designed for. A review of disaster preparedness in 2002 recommended raising "the average wave height they needed to be designed to cope with to about double the height of the biggest waves in the historical record" — 5.7 metres, for the FD plant. In the event, the tsunami that struck had 15 metre waves. It washed right over the plant and wrecked the seawater intakes, electrical switchgear, backup generators, and on-site diesel storage.

* The 2002 severe accident review that increased the tsunami wave height estimates recommended installing hardened hydrogen release vents, to prevent a build-up of hydrogen in event of a similar accident. These are standard on American and other reactors, but had not been retrofitted to the FD BWRs. Were such vents fitted, the explosions would not have occurred. (The explosions compounded the difficulty of bringing the plant under control.)

* Despite all this there appears to have been no public health impact due to radiation (stress and fear are another matter), and no plant workers were exposed to more than 250 millisieverts — the raised limit for emergency nuclear responders, equal to five years' regular working exposure, but insufficient to cause a serious helath risk.

So: serious accident, yes — but it's no Chernobyl. (Go read the article. It's good.) The main take-away seems to be that, like a plane crash, it takes more than one thing going wrong to cause an accident — in this case, two major natural disasters, each of which exceeded the plant's design spec, occurring within the space of an hour, compounded by failure to implement a safety system that is standard elsewhere. Despite which, they managed to dodge the bullet (for the most part: it's still going to take billions of dollars and several years to clean up the plant).

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