– Japanese Researcher: Recriticality in July and August? (UPDATED) (EX-SKF, Sep. 17, 2011):
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.
「私は福島第一原発1～3号機 のいずれかで、メルトダウンした核燃料が連鎖的に核分裂する『再臨界』が発生し、4月以降では最大量の放射性物質が施設外へ漏れ出したと考えています。時 期は7月28日から31日頃と、8月19日から21日頃の2回。放射物質の大量流出は、発表数値が実際よりも低めではないかと疑われている東京都や横浜市 の線量測定データからもはっきりと読み取れる科学的事実です」
“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.
「さらに詳しくデータを分析してみると、7月よりもよりはっきりと数値が上昇 している8月のほうでは、規模が大きく継続時間が長い『即発臨界』という分裂反応が起きたと推定されます。これによって大量発生した放射性のセシウムやヨ ウ素などが首都圏にも達し、線量を増大させたと考えていいでしょう」（日沼氏）
“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.
振り返ってみれ ば、東京都と神奈川県の「下水脱水汚泥」から高い数値の放射性ヨウ素131が検出され始めたのも7月後半から。ヨウ素131の半減期は8日間なので、3月 の臨界で発生したものが7月以降になってから検出されることはありえない。また8月には、都内各区の数十ヶ所の砂場で安全基準値を大幅に上回る放射線量が 測定され、砂を全交換するまで使用禁止の措置がとられた。だが、その砂場のほとんどが7月以前の計測では基準値以下だったのだ。
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