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Fukushima emissions double estimates - new international study

Nuclear Monitor Issue: 
WISE Amsterdam

A new study by an international team of researchers estimates that the emissions from the power plant started earlier, lasted longer and are therefore higher than assumed in most studies conducted before. The study estimates the emissions of the radioactive noble gas Xenon-133 and the aerosol-bound nuclide Caesium-137 from the Japanese Fukushima Daiichi nuclear power plant until April 20 (!) by combining a large set of measurements from Japan and worldwide, atmospheric transport model calculations, and available information and reasonable approximations on radionuclide inventories and accident events at Fukushima Daiichi.

The study led by Andreas Stohl, an atmospheric scientist at the Norwegian Institute for Air Research, was released on the website of Atmospheric Chemistry and Physics Discussions. The calculations are based on about 1000 measurements of activity concentrations and deposition conducted in Japan, USA and Europe. This is the most comprehensive investigation so far. There is no doubt that the Fukushima accident is, at least in terms of the isotopes Xenon-133 and Caesium-137,  the most significant event after the catastrophe in Chernobyl 25 years ago, says Dr. Andreas Stohl from NILU - Norwegian Institute for Air Research, lead author of the study.

Regarding the radioactive noble gas Xenon-133, the results indicate an emission of 16.7 million terabecquerel (1 Becquerel is one radioactive decay per second, 1 terabecquerel equals one million times one million becquerels). This is the largest civilian noble gas release in history, exceeding the Chernobyl noble gas release by a factor of 2.5. Xenon-133 is neither ingested nor retained in the inhalation process and therefore of less health concern, but it is important for understanding the accident events.

This study confirms there is strong evidence that emissions started already on 11 March 2011 at 6:00 UTC, which is immediately after the big earthquake. So contrary to official assumptions (Japan’s Nuclear and Industrial Safety Agency remains convinced the quake didn’t cause significant damage to the plant, Tadashige Koitabashi, a NISA spokesman, said by phone to Bloomberg) it becomes more and more clear that the reactors and fuel pool were already severely damaged by the earthquake before the tsunami hit. And that is despite the fact that the earthquake "did not exceed design base values significantly", according to Jan Leen Kloosterman a Dutch scientist and important nuclear advocate from the Technical University Delft. But it was a big earthquake (magnitude 9.0) out at sea but not so big 130 km from the epicentre at Fukushima. NISA and Tepco blame the tsunami, which swamped backup generators, causing a loss of cooling and the meltdowns of the three reactors operating at the time of the disaster. Explosions at the plant sent radiation into the atmosphere.

Regarding Cesium-137, which is of high relevance for human health due to its physical properties and the long half-life time of 30 years, the new estimate shows that emissions started earlier and ended later than assumed in most studies so far. The total release amounts to 36 petabecquerel (1 p-Bq is 1000t-Bq), which equals 42% of the Chernobyl emissions. 19% of the cesium was deposited on Japanese territory, while about 80% was deposited in the water.

While the winds transported most of the Fukushima emissions toward the Pacific Ocean, the plume headed inland during and following March 14-15, the period of highest cesium emissions, although “the situation could have been even much worse, as fortunately no rain occurred at the time.” During a second episode March 20-22, even larger areas of Honshu were covered by the plume, from Osaka in the south to areas north of the Fukushima Daiichi plant, and heavy rains “nearly completely cleansed the atmosphere of 137Cs  and again produced strong deposition of this radionuclide over Honshu, including Tokyo,” the study said. “This episode again followed a period of high (though fortunately not as high as on 14–15 March) 137Cs emission fluxes on 19 March, which were transported to Japan on 20 March.” There were “a few other periods” when the plume went over land, “but the areas affected were smaller and the emissions lower.”

The study also suggests that, contrary to government claims, pools used to store spent nuclear fuel played a significant part in the release of the long-lived environmental contaminant caesium-137, which could have been prevented by prompt action. The levels of cesium-137 emissions “suddenly dropped” after Tepco started spraying water on the spent fuel pool of the No. 4 reactor, they said. Reactor 4 was idle before the quake and the fuel assemblies in the core had been placed in the spent fuel pool of the unit. The radioactivity released into the atmosphere represented “nearly 2% of the available inventory of the reactor cores in units 1–3,” the study said, “and the spent-fuel pool [radioactive content] in unit 4 was discharged into the atmosphere.” Indeed, it was the spent fuel pools at Fukushima that contained the bulk of the offending material, according to the study, which looked only at the aerosol-bound cesium-137 and the noble gas xenon-133

Sit-in outside Ministry of Economy
On October 28, close to two hundred women from Fukushima began a three-day sit-in outside the Tokyo office of Japan's Ministry of Economy calling for the evacuation of children from areas with high radiation levels and the permanent shut down of nuclear reactors in Japan currently switched off. Their peaceful protest is a powerful – almost radical - act in a country where standing up for something can often mean ostracism from one's community. These are not women who regularly participate in civil protest. These are mothers who fear for their children's safety and future. These are grandmothers separated from their families. The fact that they have put their own lives and families on hold for these three days reflects the harrowing situation these women and their families have found themselves in since the nuclear disaster.
Greenpeace International, 28 October 2011


Sources: Report "Xenon-133 and caesium-137 releases into the atmosphere from the Fukushima Daiichi nuclear power plant: determination of the source term, atmospheric dispersion, and deposition" by A. Stohl, P. Seibert, G. Wotawa, D. Arnold, J. F. Burkhart, S. Eckhardt, C. Tapia, A. Vargas, and T. J. Yasunari / Nuclear Monitor, 727, 27 May 2011 / Bloomberg, 27 October 2011 / Nuclear Intelligence Weekly, 13 October 2011 / Press release NILU, 21 October 2011

The full report is available at:
Contact: Dr. Andreas Stohl, NILU - Norwegian Institute for Air Research, PO Box 100, 2027 Kjeller, Norway