NHK is reporting that TEPCO has measured 135Iodine, with a half life of just 9 hours, at Fukushima Daiichi unit 2. Moreover 133Iodine (half life 5.2 days) has also been measured.
Measurement of Xenon isotopes is proof positive of active nuclear fission.
TEPCO is adding boron to water inserted into unit 2 to try to stop the nuclear fission.
Because TEPCO has not previously reported Xenon measurements, there is no evidence of how long "recriticality" has been taking place.
Tepco has starting injecting boric-acid solution into the (broken) reactor.
At reactor 2,Tepco finally started analyzing air filter of the container vessel on10/28/2011. As the result, they detected xenon 133 and 135. It may have been fissioning since months ago.
In multiple posts here last summer I showed graphs of spikes in radioactive iodine levels in water near unit 2 that appeared to indicate that "recriticality" had occurred. However, there were alternative explanations for the spikes, so those data weren't conclusive. TEPCO needed to measure and report xenon levels at that time so that they could get the situation under control if recriticality had occurred.
This new information indicates that the situation at unit 2 has likely been unstable for many months. Moreover, it would tend to indicate that a larger number radioisotopes may have been released in larger amounts to the environment than we were previously led to believe. Recent estimates of releases by international experts have been far higher than earlier Japanese reports indicated. These data indicate that even more activity may have been released into air and groundwater near the reactor units than anyone has estimated because less volatile isotopes may have entered the environment.
FINAL LATE UPDATE
The absolute amounts of xenon detected are extremely low according to data released by TEPCO
Updates after the Kos croissant.
Update 2
On June 3 I noted that Iodine isotope spikes in water near reactor unit 2 might be evidence of recriticality.
Moreover, radiation levels in water inside the silt fence near reactor 2 are high and rising, despite large amounts of dilution. Continued very high levels of Iodine 131 with a half life of 8 days are very hard to explain for a reactor that has been "shut down". Normally Iodine levels would drop several orders of magnitude below cesium activity levels over the sixty day period shown in the graph, but instead they continue to track each other. The level of 10,000 Bq/liter I-131 is very problematic. It is much higher than would be expected for a reactor in cold shut down for 2 1/2 months.
And I reported this figure released by TEPCO that showed the iodine radioisotope spikes.
The conditions at unit 2 are clearly unstable. Multiple recriticality events may have occurred. Because TEPCO has reported so little information about the situation at unit 2, the nature of the situation is highly uncertain.
Update 1
Blogger ex-skf has translated NHK's Japanese to English He notes that Unit 2 was where TEPCO reported elevated levels of hydrogen gas several days ago.
From NHK News (6:38AM JST 11/2/2011):
東京電力福島第一原子力発電所の2号機の格納容器から、核分裂反応が起きた時にできる放射性物質のキセノンが検出され、東京電力は核分裂反応が起きている可能性が否定できないとして、原子炉に核分裂反応を抑えるホウ酸水を注入しました。
Xenon has been detected in the Containment Vessel of Reactor 2 at Fukushima I Nuclear Power Plant. Xenon is created when there is nuclear fission.
福島第一原発の2号機では、先月28日から格納容器の中から気体を吸い出し、フィルターを通して放射性物質を取り除く装置の運転を始めていて、この装置の出口付近で放射性物質の種類や濃度を測定して分析しています。その結果、1日に行った分析で、核燃料のウラン235が核分裂してできる放射性物質のキセノン133と、キセノン135が検出されたということです。
The gas management system has been operating since October 28 in Reactor 2 to suck out the gas from the Containment Vessel and filter out radioactive materials. TEPCO has been analyzing the gas to identify the nuclides and density. The analysis done on November 1 found xenon-133 and xenon-135, which are created when uranium-235 undergoes nuclear fission.
キセノン133は放射性物質の量が半分になる半減期が5日と短いため、東京電力は、再び核分裂反応が起きている可能性が否定できないとして、午前3時前から1時間にわたって原子炉に核分裂反応を抑えるホウ酸水の注入を行いました。
Xenon-133's half life is 5 days. TEPCO says it could not rule out the possibility of nuclear chain reaction happening again, so the company poured water with boric acid into the reactor to suppress the nuclear chain reaction for one hour starting 3AM [on November 2].
東京電力によりますと、原子炉の温度や圧力、それに敷地周辺の放射線量を計測しているモニタリングポストの値に大きな変動はないため、核分裂反応が起きていても規模は小さいとしています。経済産業省の原子力安全・保安院によりますと、検出されたキセノンの濃度は低く、原子炉の温度などに変化がないことから、今のところ燃料が再び溶融しているとは考えにくいとしていますが、ホウ酸水を注入したあともキセノンの検出が継続されないかどうか、状況を見極めるとしています。
According to TEPCO, there is not much change in the reactor temperature or pressure, or at the monitoring posts that measure radiation levels in and around the plant compound. Therefore, if the nuclear chain reaction is happening, it is small-scale, says the company. The Nuclear and Industrial Safety Agency says since the density of xenon detected is low and there is no change in the reactor temperature and other parameters, it is hard to imagine the fuel is melting again. But the Agency says it will monitor the situation to see if xenon continues to be detected after pouring in the water with boric acid.