Here's what is dangerous about the Japanese situation
according to a WSJ graph, there are 6 nuclear plants units all next to each other, so if one of them blows up the radiation levels will be so high that perhaps work on the other units will not be able to be done(I'm not sure this is correct, its a speculation, the engineers might have the equipment to work in really awful conditions).
If that is the case the probability of a problem there instead of being independent(like 5-10% of a meltdown in each unit) are actually additive(with each meltdown, a prob of a meltdown in other unit increases)
If the meltdown occurs people will fee Tokyo by the droves(And this is a speculation in psychology I'm more comfortable making), regardless of where the wind is blowing. This could tip Japan over into a depression(-10% of GDP) as the disruptions in finance, trade and electricity prove too great for them to bear
If you want to beat the markets you will need special information that most don't understand or have(that is public), in this case I would like to know if the engineers can try to repair the units if there was a meltdown in the area