good one, davefitz. According to the long and detailed Wikipedia article "Radiation effects from the Fukushima Daiichi nuclear disaster", it seems that no authorities on the subject are very alarmed yet about the long term effects of the initial and continuing contamination. But the article does not discuss the probability that three of the cores have melted into the ground. I don't believe anyone has any good idea how to deal with this problem, or how bad it could be.
The greatest danger otherwise is the spent fuel rods in the pool at Unit 4. TEPCO and the Japanese government, with the help as reported today of the French and maybe the Russians, intend to start removing them in November. The potential for a catastrophe during this operation cannot be dismissed:
"...A lot depends on what blows up, if anything. If only Unit 4 blows up, Japan is at risk, including Tokyo, and the nuclear dust will pass across the Pacific to the U.S. People on the West Coast will be warned to keep their windows closed for a while.
If the whole facility blows up, one scientist is talking about moving her family to the southern hemisphere. From the article quoted above:
Chernobyl’s first 1986 fallout reached California within ten days. Fukushima’s in 2011 arrived in less than a week. A new fuel fire at Unit 4 would pour out a continuous stream of lethal radioactive poisons for centuries..."
I wonder whether it would be possible and effective to build a large structure under the spent fuel pool at Unit 4, to anchor it in place against any earthquake, ground liquifaction, tsunami or other threat from nature. Then the fuel rods could be left in place with cooling as needed, instead of risking the enormous danger of removing them.