I am absolutely not an expert on nuclear disasters, so take this for what it's worth.
It appears to me that the situation at the Fukushima-Daiichi nuclear plant is slipping out of control. After multiple explosions and fires, radiation levels are getting high enough that it's becoming increasingly unsafe to work in the area:
The radiation levels at Fukushima Daiichi are now too high for staff from the Tokyo Electric Power Company, which operates the nuclear power plant, to stay in control rooms thereMeanwhile, radiation levels on site have been up to 400miSv/hr in the worst spots. Here's a helpful scale from the BBC:
In consequence, the Tokyo electric company has withdrawn all but 50 workers from the site. However, there are at least three reactors that were operating, as well as at least six pools containing spent fuel that need to be maintained. All have the potential to boil off their cooling water and melt down or explode if not maintained.
That motivates me to wonder about the worst case here: suppose the company cannot contain the situation and it devolves into a Chernobyl-style release of radioactivity. I found this map of the closed and contaminated areas at Chernobyl at the Wikipedia:
The pink/purple areas are the worst hit.
Then I superimposed this map on a map of Japan, with the scales matched, and the Chernobyl point matched to Fukushima. That looks like this:
Obviously, this is an extremely rough exercise: not only do we not know the scale of the Japanese disaster yet, but prevailing winds will be different, and carry the contamination in an unknown direction. Still, I think this map makes it clear that a Chernobyl scale disaster could cause huge problems for a sizeable fraction of northern Japan.
I note also that there's one way in which Chernobyl might not be a worst case. The Soviet authorities were willing and able to sacrifice hundreds of emergency workers who got acute radiation sickness while putting out the fires and building the emergency sarcophagus over the site of Chernobyl reactor 4. Is a democratic country like Japan going to do the same? And if not, then what?
I sincerely hope the Japanese authorities are able to bring this situation under control in the next few days. If not, it may be many decades before anyone builds any more nuclear power plants.