Charlie's Diary: The Economist has a report from the International Conference on Advances in Nuclear Power Plants of a plenary session discussing the Fukushima Daiichi accident.... 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 plant survived the earthquake (which exceeded its design requirements) quite well.... However, scrammed reactors continue to need power to run their cooling systems.... The tsunami struck 15 minutes later, and was roughly five times higher than the plant had been designed for....
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....
So: serious accident, yes — but it's no Chernobyl.... [T]wo 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).