More remarkably, Tepco warned on Wednesday: "The possibility of re-criticality is not zero".
It's not zero, but right now it doesn't look likely, and several people more expert than I are confused by that statement. This is, however, a situation that has been as extreme an example of Murphy's Law as you're ever likely to see.
Tepco and the NRC are at odds about the status of the Unit 4 pool. Tepco says it has water; the NRC says it does not. The NRC has people in Japan, but there hasn't been any indication that they're on site. I'd guess they're extrapolating from the radiation readings, but they've gone up and down wildly. Tepco has increased the number of workers on site, which indicates they really don't think the fuel is uncovered. Ultimately, that would be hard to lie about, because there would be dead workers. Tepco says it has temperature readings in all the fuel pools of 60-70 C, and the temperature would have to go up about 2,000 degrees for criticality. I hope Tepco understand that by the time this event is picked over by hundreds of nuclear engineers, no lie will stand. At the same time, the NRC's first instinct in almost any situation is "Ur doing it wrong."
Criticality wouldn't mean it would blow up, but it would become much more radioactive, probably making it impossible to keep working on the site.
If they can just get the offsite power to the site, they can start normal cooling. Apparently they're fighting earthquake debris every inch of the way.
It looks like they're now consulting with the World Association of Nuclear Operators, which would include people in Atlanta, and WANO seems to concur that it's possible that containment has not been breached on Units 1-3, which is the good news.