Thursday, November 10, 2011

FEARS OF FISSION RISE AT STRICKEN JAPANESE PLANT

Hiroko Tabuchi reports in The New York Times November 3, 2011 that “…the plant’s owner admitted for the first time that fuel deep inside three stricken reactors was probably continuing to experience bursts of fission. … The disclosures raise startling questions about how much remains uncertain at the plant… The presence of xenon 135 in particular, which has a half-life of just nine hours, seemed to indicate that fission took place very recently. … The jumble of material and conditions in the damaged reactors seem very unlikely to be able to produce sustained fission, but some experts have long suspected intermittent criticalities. In other accidents, nuclear material has burst into fission, but the released energy then re-arranges the damaged fuel into a configuration that no longer support (sic) fission. Gradually, the material re-forms in a way to support another burst. …Junichi Matsumoto, a Tokyo Electric spokesman, acknowledged episodes of fission, saying… ‘There is a possibility that certain conditions came together temporarily that were conducive to re-criticality,’ and that the measurements indicated a burst that occurred at a slightly higher rate than prior cases. ‘It’s not that we’ve had zero fission until now,’ Mr. Matsumoto said. ‘But at this point, we do not think there is a large-scale and self-sustained re-criticality.’ … Hiroaki Koide, assistant professor at Kyoto University’s Research Reactor Institute… [said] if episodes of fission at Fukushima were confirmed… ‘our entire understanding of nuclear safety would be turned on its head.’”

The IAEA issued a follow-up status report the day after the Times article appeared, indicating a re-criticality event was highly unlikely (http://www.iaea.org/newscenter/news/2011/fukushimareport01.html). The IAEA update notes the following:
“Based on further analysis, Japanese authorities have concluded that the xenon concentrations are not due to a criticality event but rather from the spontaneous fission of curium-242 and 244. (Spontaneous fission is a form of radioactive decay that does not involve chain reactions and is characteristic of very heavy isotopes. Spontaneous fission occurs in low levels in all nuclear reactors.)
This conclusion is based on three key factors outlined and discussed in the report:
• The inventory of Cm-242 and Cm-244 was calculated as was the concentration of Xe-135, resulting from the spontaneous fission of Cm-242 and Cm-244. If nuclear fission of the reactor's uranium fuel were occurring, at the lowest possible level, the levels of xenon detected would be several orders of magnitude higher than those measured. Current levels of xenon are consistent with those that would be generated by spontaneous fission of Cm-242 and Cm-244;
• If the core had been experiencing a criticality event, the injection of boron water should have stopped the criticality and terminated the generation of xenon. However, the xenon levels were not influenced by injection of boron water into the core; and
• If the core was undergoing a criticality event the temperature and pressure readings would be expected to rise as the event would increase heat production within the core. However, the temperature and pressure levels have not undergone any significant increases either before or after xenon were detected, indicating that no criticality event is occurring.”
The issue of intermittent criticalities could certainly be clarified if TEPCO and NISA would install real time monitors at each of the four reactors that would provide minute by minute, and thus hourly data on airborne emissions. Any re-criticality incidents would create enough heat to cause increased steam emissions, which would be much more obvious and easy to monitor than the micro-emissions in the spontaneous emission of curium-242 and -244. The IAEA status report is probably correct in that no re-criticality has occurred, but it’s certainly interesting that there remains a potential for re-criticality until such time as the molten fuel actually is removed from the reactor vessels.

Further information on the re-criticality issue can be accessed at: http://fukushimaupdate.com/tepco-to-redefine-its-idea-of-criticality.

Also in the news is the new estimate of the time period needed to decommission the four reactors at Fukushima Daiichi. A CNN report by Junko Ogura (Tuesday November 1, 2011) quotes the Japanese Atomic Energy Commission as stating “Decommissioning… will likely take more than 30 years to complete [following cold shutdown, which is expected by the end of the year.]… Removal of debris – or nuclear fuel – should begin by the end of 2021.”

Also of note is a recent headline in Yahoo! News (October 24, 2011; http://news.yahoo.com/blogs/envoy/20-million-tons-debris-japan-tsunami-moving-toward-143640503.html) “Up to 20 million tons of debris from Japan’s tsunami moving toward Hawaii”. While the Yahoo! News article provides a graphic illustration of when the debris is expected to reach the west coast, within three years or sooner, there was no mention that this debris field is likely to be highly contaminated with fallout resulting from the prevailing winds depositing Fukushima-derived radioactive contamination on the tsunami debris. Also of continuing interest is that significant hot particle fallout would accompany any contamination of the tsunami debris. For information on the nature of hot particle nuclear accident-derived contamination and its presence in the Chernobyl plumes please refer to our publication Fukushima Daiichi: Nuclear Information Handbook. See page 77 for the definition as well as the bibliographic citations of the literature pertaining to hot particle fallout from the Chernobyl accident on pages 229-232.

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