Yoyo Hinuma, currently at University of California San Diego, says recriticality may have happened in one of the three reactors at Fukushima I Nuclear Power Plant in late July and again in mid August, releasing the largest amount of radioactive materials since April.
I cannot find much about the researcher, other than he was at MIT before, and that he has published papers in peer-reviewed magazines.
The article below says there was a criticality in March, but I don't know if the reporter is referring to the explosion events in March or the recriticality in one of the broken reactors as some researchers have alleged and hypothesized.
(UPDATE: I totally forgot about the report by the UC San Diego researchers that radioactive sulphur of Fukushima origin was detected in California in late March, indicating on-going chain reaction. So the Playboy magazine reporter may be right. To recap, chlorine in the seawater being injected back then captured neutron, turning to radioactive sulphur.)
(Of all magazines) the article appeared in the Weekly Playboy magazine in Japan.
Part from Shukan Playboy Japan (9/12/2011):
... Indeed, there are some inexplicable numbers reported on the air radiation levels by the government agencies. That is, a big rise in the radiation levels from late July to mid August. Dr. Yoyo Hinuma, who has published papers on the net about the seriousness of the situation since the beginning of the accident in March, explains.
"I think there was a recriticality in which the melted fuel started a chain reaction again in one of the reactors at Fukushima I Nuclear Power Plant, and that the largest amount of radioactive materials since April was released from the plant. It may have happened twice. The first was between July 28 and 31. The second was between August 19 and 21. That the large amount of radioactive materials were released can be clearly seen in the radiation survey data in Tokyo and Yokohama City, where it is suspected that the announced radiation levels are lower than the actual levels. So it's a scientific fact."
For example, according to the data released by the Tokyo Metropolitan government, the air radiation level on August 19 was 0.0865 microsievert/hour (maximum), 1.4 times as high as the previous day. On the same day, Yokohama City measured 0.051 microsievert/hour, the level last recorded in late March.
"Further analyzing the data, we find that August saw a more definite rise than July. It may have been caused by the so-called "prompt criticality" which is bigger and lasts longer. The large amount of radioactive cesium and radioactive iodine released as the result may have reached the Tokyo Metropolitan area, raising the air radiation level", says Dr. Hinuma.
Looking back, it is since the end of July that the high level of iodine-131 have started to show up in the "dehydrated sewer sludge" in Tokyo and Kanagawa Prefecture. Half life of iodine-131 is 8 days, so it couldn't have come from the criticality happened in March. Also in August, radiation that exceeded the safety standard was detected in tens of sandboxes in the parks where children play, and the use of the sandboxes was prohibited until the sand was replaced. But before July, air radiation at most of these sandboxes measured lower than the safety standard.
We'd have to admit that there is a high possibility that the recriticality happened this summer at Fukushima I Nuclear Power Plant, at least twice, and fresh radioactive materials are being released.
I checked the Tokyo Metropolitan government's official measurement of air radiation in Shinjuku. The maximum levels recorded on July 30 and August 19 are indeed elevated:
July 29: 0.0697 microsievert/hour
July 30: 0.0735 microsievert/hour
July 31: 0.0610 microsievert/hour
August 18: 0.0621 microsievert/hour
August 19: 0.0865 microsievert/hour
August 20: 0.0605 microsievert/hour
For July and August, the maximum air radiation level recorded in Tokyo is in 0.06 microsievert/hour range, and does not exceed 0.07 microsievert/hour, except for these two days.
Iodine-131 detected in sewer sludge in mid August in Tokyo and in Oshu City in Iwate Prefecture (among others) is attributed to the medical use, because there was no corresponding increase in radioactive cesium. Also in the case of Tokyo, radioactive iodine has been continuously detected in sewer sludge since they started to take measurements in May.
However, iodine-131 detection in sewer sludge is NOT the reason why Dr. Hinuma thinks there were two recriticality events at Fukushima I; it is what the Playboy reporter lists as a supporting evidence.
I do want to point out though that when Fukushima Prefecture detected tellurium-132 in the early morning of March 12 - that is, before the explosion of Reactor 1 building - in Namie-machi and other locations outside the plant including Minami Soma City, 25 kilometers from the plant (and they didn't bother to tell anyone for nearly 3 months), there was no concurrent, significant detection of iodine-131 and radioactive cesium even though this was clearly a precursor to the explosive event that took place that afternoon