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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.
Chernobyl had grapite rods which added to the problems since they burned.
The Fukushima reactors have boron control rods.
Hopefully there won't be additional fuel damage. There apparently was some in unit 1 a week ago. Although they reported things as stable, they interruptted cooling for an hour or two to set up more permanent power connections. Later the temperature at the bottom of the reactor went from 110C to 143C. They increased the rate of adding water some. I think they're in a hurry to get better cooling with actual recycling, finned radiators, filtering, and good control of the boron levels going. They got air filtering going recently and made the building safe to enter. Last I heard they were about to remove some contaminated material and start checking the original circulating pump. It's good to see them finally making some progress. For a while it seemed like they were hopelessly kept away by the highly contominated water all over. Hopefully they'll get whatever cleans/processes that working well before they run out of space to put the water. Starting to recycle would really help that mess. It sounded like much of the water being pumped out was from turbine areas or tunnels nearby. Without actually sealing up the leak, whatever water does come out will tend to build up more and more contamination.
I believe they concluded that that mess is all coming from the unit 2 suppression tank. In the drawing it looks like a tire around the bottom (old GE Mark I design). But it's huge. A during-construction photo I saw with someone standing nearby made that suppression pool look maybe 30 feet tall. They'd have to pump in an awful lot of concrete or something to seal that leak...don't know if that;d work while wet and many tons of water and hour going through.