Tetsuo Matsui is a professor at University of Tokyo's Institute of Physics. On May 2, he submitted his paper titled "Deciphering the measured ratios of Iodine-131 to Cesium-137 at the Fukushima reactors" to arXiv.org, in which he says "The data of the water samples from the unit-4 cooling pool and from the sub-drain near the unit-2 reactor show anomaly which may indicate, if they are correct, that some of these fission products were produced by chain nuclear reactions reignited after the earthquake." (quote taken from his paper)
Chain nuclear reactions reignited = recriticality
That is what some of the non-mainstream researchers like Japan's Hiroaki Koide of Kyoto University and the US's Arnie Gundersen of Fairewind Associates have been saying all along, that there may have been recriticality in some of the reactors at Fukushima I Nuclear Power Plant.
Now the Japanese mainstream Tokyo University professor is saying it. TEPCO's and the Japanese government's acknowledgement can't be that far away. (And no doubt it will be presented as "it's nothing, it's safe.")
From The Physics arXiv Blog at Technology Review at MIT (5/9/2011):
Chain Reactions Reignited At Fukushima After Tsunami, Says New Study: Radioactive byproducts indicate that nuclear chain reactions must have been burning at the damaged nuclear reactors long after the disaster unfolded
..... Today, 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. He has calculated what the starting ratio must have been by assuming the reactors had been operating for between 7 and 12 months.
He says the ratios from drains at reactors 1 and 3 at Fukushima are consistent with the nuclear reactions having terminated at the time of the earthquake.
However, the data from the drain near reactor 2 and from the cooling pond at reactor 4, where spent fuel rods are stored, indicate that the reactions must have been burning much later.
"The data of the water samples from the unit-4 cooling pool and from the sub-drain near the unit-2 reactor show anomaly which may indicate, if they are correct, that some of these ﬁssion products were produced by chain nuclear reactions reignited after the earthquake," he says.
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.
The blogpost above includes some imprecise information that does not appear in the original paper. For those of you who want to read Matsui's original paper (9 pages with calculations), the link is this: http://arxiv.org/PS_cache/arxiv/pdf/1105/1105.0242v1.pdf
ArXiv.org was originally hosted at the Los Alamos National Laboratory and is currently hosted by Cornell University. Researchers submit electronic preprints in the fields of mathematics, physics, astronomy, computer science, quantitative biology and statistics, which can be accessed via the world wide web. In many fields of mathematics and physics, almost all scientific papers are self-archived on the arXiv. Though it is not peer-reviewed, there are moderators in each field who review the submissions. (from Wikipedia.)