Something that some have been anxiously awaiting to see at Fukushima is "re-criticality." It's also referred to as "localized criticality" or "episodic fissioning." I've read descriptions that make it sound as though the fuel elements are dripping pieces with sparks of neutrons, light, and other scary things.
So far none of those proclaiming it have provided persuasive explanations, with numbers.
In order to have criticality or fission (with one exception, which I'll explain later), there must be a critical mass. The water being injected into the reactors has boron in it, and the fuel elements are spaced to allow water flow around them. The boron absorbs neutrons and prevents fission. So "re-criticality" isn't happening in fuel elements that retain their original configuration.
But the tops of the fuel elements are exposed in the reactors. So they can heat up (although the partial immersion will help to remove heat from them) and deform or break open. At worst, if they are suspended from the top, they may break off and fall to the bottom of the reactor vessel. Or small pieces may fall to the bottom of the vessel. The fuel pellets within the fuel elements are centimeter-sized. They are oxides and have very high melting points.
If fuel pellets come out of the fuel elements, each contains a small amount of uranium or plutonium and is incapable of becoming critical. Some large number would have to accumulate in the proper configuration without borated water between them to form a critical mass. And if that were the case, criticality would continue; it wouldn't be a flash.
If the fuel elements break off and fall to the bottom of the reactor, it seems unlikely that they would form critical masses. They would have to fall into a very orderly pile, and, again, criticality would continue.
It may be a lack of imagination, but I just don't see how you could get small bursts of criticality, and I haven't seen the proponents of "re-criticality" provide as much thinking-through as I've given here, let alone numbers for what the critical masses might be (they depend on configuration) or how critical configurations might occur.
The one exception is whatever plutonium-240 might be in the reactor. Plutonium-240 spontaneously fissions, but it is present in very small quantities. I'm wondering if the alleged activation products result from plutonium-240 neutrons, but, like those making that claim, I haven't looked up the numbers. The next things I want to do will require numbers, so that may take a little time. And I'll try to check this as well.