But even in their most dire assessments, some experts had not expected even bursts of re-criticality to occur, because it was unlikely that the fuel would melt in just the right way — and that another ingredient, water, would be present in just the right amounts — to allow for any nuclear reaction. If episodes of fission at Fukushima were confirmed, Mr. Koide said, “our entire understanding of nuclear safety would be turned on its head.”
This view does not seem to be supported by some experts. A report in Nature News Blog notes that
... experts say that small amounts of fission in the reactor core would not be that surprising, and there seems no danger of either a self-sustaining critical chain reaction or significant release of fission products into the environment.
"This does not look like a major release of radiation from the plant, but it is worth noting that even if the fuel is cooled, there is still a small amount of residual natural fission of the large amount of uranium fuel in the core," said Paddy Regan, a nuclear expert at the University of Surrey in the United Kingdom, adding that the amounts released would be far less than were the fuel to go critical. The detected xenon, he said, "does not appear to show any new radiological hazard from the disaster.”
Any one else want to chime in?