Chain Reactions Reignited At Fukushima After Tsunami, Says New Study

Source: MIT Technology Review

Radioactive byproducts indicate that nuclear chain reactions must have been burning at the damaged nuclear reactors long after the disaster unfolded

<snip>

These chain reactions must have occurred a significant time after the accident. "It would be difficult to understand the observed anomaly near the unit-2 reactor without assuming that a significant amount of fission products were produced at least 10 - 15 days after X-day," says Matsui.

So things in reactor 2 must have been extremely dangerous right up to the end of March.

<snip>

But in the meantime, Matsui's analysis gives us one of the best insights so far into the nature of the disaster that unfolded after the tsunami hit.

Read more: http://www.technologyreview.com/blog/arxiv/26738/

"The production of significant concentration of iodine-131 depends on the power level. To be comparable to the concentration left from normal operation one would need thermal power production in the order of a gigawatts for a few hours.
There is no way a molten core can achieve that. Any such critical configuration would fizzle out very quickly."

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I cannot pretend to understand it or be able to judge, but it sounds as if opinions may differ.

The comment you quote may make a good point (I don't know whether the time and power estimates are accurate), but as Matsui notes the analysis doesn't try to take into account the effect of differing chemical and physical transport properties between the iodine and cesium. It could also be the case that, because of these differences, any I-131 produced found its way into the samples more efficiently than the cesium. The comment as well as Matsui's analysis assume the samples are representative of the overall isotope mix, which may not be guaranteed.

A number of stories have falsely attributed some level of certainty ("must have occured" etc.) to Matsui, when his paper presents no such thing.

any I-131 produced found its way into the samples more efficiently than the cesium

Yep. One obvious possibility would be if a core spent a significant amount of time at a temperature above the boiling point of cesium but below that if Iodine. Which does appear to be the case.

Also - since this was written, we've seen pretty solid video evidence that no such thing occured in the spent fuel pool of #4 (one of the sites with cesium/iodine proportions that caused some to think this might have happened).

I guess they were wrong

yup

And that which you were told was next to impossible, has not in fact happened.

yup

... And deleted the post I'm sure you cited.