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Chain Reactions Reignited At Fukushima
mdsolar writes "Radioactive byproducts indicate that nuclear chain reactions must have been burning at the damaged nuclear reactors long after the disaster unfolded. Tetsuo Matsui at the University of Tokyo, says the limited data from Fukushima indicates that nuclear chain reactions must have reignited at Fuksuhima up to 12 days after the accident. Matsui says the evidence comes from measurements of the ratio of cesium-137 and iodine-131 at several points around the facility and in the seawater nearby."
It did scram completely. The decay heat, which is 7% of 1000 MW boiled away all the water they lost the ability to pump, and then melted the zircalloy fuel rods into a pile of molten slag in places. That slag then has the geometrical configuration to do some more fission. Ironically, they may have had no problems if they didn't scram, as the reactor could then drive power to the cooling pumps, as opposed to relying on diesel generators.
They didn't initially use seawater. They still had normal water in the pile and as far as I know hadn't triggered the systems to release boron in it.
These would be tiny little areas that would have an accelerated fission rate over just the fuel sitting in the elements. I'm not even sure you could truly call it a criticality in that it wouldn't be self sustaining. You'd get a momentary spike that would tail off. It's pretty insignificant as far as a source of heat or radiation compared to the decay heat and radiation from the fission products.
Thing is, using a mass spectrometer, you can measure truly tiny amounts of isotopes. You could expect some of the shorter life isotopes from just from occasionaly fissions without criticality. What this study was saying was that the observed ratio of isotopes was such that the particular researcher felt that it would require more than just the expected rate of fissions to get to that ratio.
That really doesn't surprise me. Nor is it terribly significant.