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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."
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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.
There are reactor designs that currently exist that are more resilient to meltdowns. Most notably, thorium molten salt reactors, but there are only a handful of experimental reactors in existence. There is also the CANDU reactor primarily used and designed in Canada which is a uranium heavy water reactor.
I will agree with you that the ancient nuclear technology most reactors use today is not that safe, but more modern reactors have solved that issue. The only problem has been rolling out thorium and CANDU reactors.
And WRT your comment on terrorism, there's a video on Youtube I've seen that debunks the whole "flying a plane into a reactor" myth. Nuclear plants have concrete walls that are like 10 feet thick and the plane collapses on it self and does nothing to the wall.
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.
It started to carry a negative connotation when some people started using junk science to raise false alarms. Look at Helen Caldicott telling everyone that Chernobyl resulted in millions of deaths, and that Fukushima will result in millions of cancers.
She repeatedly appeals to a single source - a Greenpeace "Report" which they somehow managed to get the NYAS to publish without any peer review, which specifically states that it does not use standard scientific analysis methods because those methods don't give the results the report author wants to find.
She ignores all the other science which has been done to determine the results of Chernobyl, decrying it all as a massive "cover up" and "fraud". There's only one report in the world, apparently, which tells "the truth". These people cherry pick their sources to get the alarming results they want to find.
See: Confirmation Bias
That is the sense that most people use when they pejoratively use the term 'alarmist' - someone who spreads FUD which is not based on sound science.