Risks are incredibly large

Quoting from an irreverent student of nuclear energy, this is a few days old.
http://motls.blogspot.com/2011/03/radioactivity-sieverts-and-other-units.html?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed%3A+LuboMotlsReferenceFrame+%28Lubos+Motl%27s+reference+frame%29

Back to Japan

Today, near the worst reactor building in Fukushima, they detected 400 millisieverts per hour: this figure was ultimately confirmed by IAEA (which was, until very recently, trying to downplay all radiation risks in Japan - a fact that may be related to the current Japanese leader of IAEA, Yukiya Amano). I want you - including all fellow big fans of nuclear energy - to understand that this is just a huge number. We have quantified one death to be 5 sieverts above: and the kids playing next to the reactor receive 0.4 sieverts per hour. Thank you, you're welcome.

If you spend twelve hours by playing in the vicinity of the worst reactor of the Fukushima power plant, you will probably die. And if you die, who will continue to fight against the meltdown threats? Between the reactor buildings 2 and 3, the equivalent dose is 0.03 Sievert per hour. That will give you 150 hours of life over there - unless you are protected in some way.

Of course, it's much more important what the radiation levels will be in the nearby large towns - and I don't even want to use the word Tokyo in this paragraph. But be sure that if the radiation level in Tokyo or another city managed to jump to something like a millisievert per hour, or even per day (and it would be sustained for a day), that would mean that 1/5,000 of the population of the city would ultimately die as a consequence of the exposure during the hour (except for those who would manage to die earlier because of another reason) unless they were successfully kept indoors all the time.

These are not negligible doses - the kind of events that Greenpeace loves to hype. These are genuinely dangerous doses for the people who work for the nuclear power plant, to say the least. Nuclear energy was sensibly calculated to be a low-risk source of energy, given the expected number of dangerous earthquakes etc. However and sadly, those old probabilities have to be replaced by the conditional probabilities right now: we already know that a very damaging earthquake has taken place near such power plants...

Just to end up with some relatively good news: a millisievert per hour is (so far?) insanely far in Tokyo. They measured 0.8 microsieverts per hour. I defined one death per person to be 5 Sv, so 0.8 microsieverts per hour means 0.16 ppm (parts per million) death per person and per hour. Multiply it by 37 million people in the Tokyo metro area and you get 6 deaths in the city per hour (or 150 deaths per day or so, if the radiation remains elevated). That's nonzero but won't be measurable statistically and will remain hugely smaller than the casualties of other lethal threats.