The worst situation (that I can think of) and unfortunately a likely one is that reactor No. 2 which has breached its containment somewhere at the base of the containment vessel, will meltdown and the super heated steam will shatter the remainder of the containment dome. This will creat an intense local zone of high radiation while scattering further radiation into the atmosphere.
But it gets worse. If that happens they will be unable to maintain a human presence at reactors No. 1 and No. 3 to keep the water level in those reactors. Once they are abandoned, each of those reactors will melt down destroying their containment domes.
Thus all three plants are likely to melt down in a brief period of time and are likely to all be spewing radiation at the same time.
Now, Reactor No. 4 appears to be a storage area for spent nuclear fuel rods. There are conflicting reports that a fire is either ongoing or put out at that location. Considering the accuracy of what we have told so far, I am assuming that it is still burning. It seems that the Japanese store significant quantities of nuclear waste on site. (Yes, I know, it took me a while to wrap my mind around the idea that someone would want to do that.)
It would seem likely to me that after three meltdown explosions destroy the containment vessels, a new fire will be started at the spent fuel rods in Reactor No. 4.
The big question after this is “How long will these meltdown reactors spew radiation into the air and in what quantities?
I have heard repeatedly that these reactors are not as bad as the one at Chernobyl. I’m not convinced. It seems to me that the Russian reactor did not melt into the earth for any real depth. I think that these Japanese reactor cores could travel some distance into the earth and that area has a high water table.
After you ask the question about how much radiation and for how long, you get to ask a new question, “How do we stop it?”
Probably, you will want to drop some kind of neutralizing agent into the reactors to slow and eventually stop the nuclear reaction. That’s all very well and good, if the containment vessels are breached upward. But what if they breach the reactor walls sideways? How will you get a neutralizing agent into the building under those circumstances?
This problem could be ongoing for months and cleanup could last decades.
These are just my thoughts. Please criticize them or add to them. This is what I think is likely.
James Pilant
Pilant's Business Ethics Blog 
The spent fuel rods are very hot and still very radioactive when they are pulled from the primary reactors. Because of this, they cannot be transported to a long term storage facility until they’ve undergone a cool down phase, which can take years. This has to be done on site and is done within the containment area, but separate of the primary reactors of course. Once they’ve “cooled down” enough to be transported, then they are moved to a long term facility.
Unfortunately, there is no good known way of fixing a problem like this once it has begun. All you can really do is evacuate and hope. Thats why “over-engineering” really is the name of the game when it comes to nuclear engineering. I guess the moral of the story is that mother nature can always seem to make a bigger storm.
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