Criticality is not some sort of horrible, terrible, destructive catastrophe. In fact, criticality is the normal, intentional operational state of a nuclear fission reactor. Ideally, criticality is the state that your nuclear power reactor is actually supposed to be kept in all of (or most of) the time.
Criticality does not mean explosion.
Criticality just means that it emits a large amount of ionising radiation (even larger than what you'd get from the fission products in the used fuel) and heat.
Anyway, let's suppose you've got a power reactor; a GE Mk. 3 BWR, or whatever.
Let's suppose that you completely melt the hell out of its core.
So, what do you have then? You have a semi-molten mass of used nuclear fuel (with its 235U content already partially burned up, because it's not fresh fuel), mixed with molten Zircaloy cladding and various bits of steel and/or Zircaloy from the core fuel assembly mounting and supporting hardware in the core around the fuel assemblies, which is mixed with the control rods which were fully inserted prior to core damage, so you've got a full set of control rods now molten and mixed through the molten mixture, plus you've got the stable fission products which accumulated in the used fuel prior to the damage, such as 149Sm, which are also contributing to negative reactivity in the mixture. You've also got whatever boron or boric acid or gadolinium has been injected by the operators in the emergency situation and/or by the Standby Liquid Control System, so that also contributes substantial negative reactivity to the system (enough to keep a full water-covered core subcritical without any control rod insertion if control rod insertion fails).
If you've got a core damage accident, that also means that you've got little or no water covering the core, because you can't have a meltdown unless the water has first been lost or has boiled away. Less water covering the core means less reactivity, and removing most of the water from the core means it will be subcritical even without the control rods and/or soluble poison injection present. You can't have criticality in LEU without some moderator, no matter how much LEU you've got, and even if there were no other negative reactivity contributions from these other factors as well.
So to get criticality following a core damage accident, you would have to wait until the damaged melted fuel has cooled down a bit and then re-cover it with water, water that doesn't have soluble poisons such as B or Gd added, and then maybe if you've got enough LEU in a compact enough geometry and you've got no other control rods melted into the mixture then you might, maybe, get recriticality.
Basically, given the negative reactivity contributed by all these different factors, I don't think it's credible to consider any realistic possibility of any re-criticality in molten damaged fuel sitting in the bottom of the reactor pressure vessel following a Fukushima like situation.
...suffer from the computer disease that anybody who works with computers now knows about. It's a very serious disease and it interferes completely with the work. The trouble with computers is you play with them. They are so wonderful. - Richard Feynman