Study Maps Cesium-137 Fallout in Japan

Japanese and American scientists recently simulated the contamination caused by the damage to the Fukushima Daiichi Nuclear Power Plant following the earthquake and tsunami that hit Japan on March 11, 2011. The study focused on modeling the dispersion of Cesium-137 (137Cs) over all of Japan and produced a series of maps showing predicted levels of 137Cs deposited in soils throughout the whole country. Environmental 137Cs contamination was one of the major concerns following the March 11 event, due to its serious negative effects on agriculture and livestock.

According to the article, published in this month’s Proceedings of the National Academy of Sciences, a number of nearby prefectures, along with Fukushima prefecture itself, have likely exceeded the legally allowed limit of 137Cs contamination in surface soils. This limit, set by the Japanese government, is indicative of the threshold at which contamination begins to affect humans and the point at which regulations need to be imposed on use to protect the general public.

The group claims that it is most likely areas such as Miyaki, Tochigi, and Ibaraki prefectures will have the highest levels of contamination as a result of their proximity to the event. Much of eastern Japan was sheltered from the airborne contaminant because of the mountains which lie between those prefectures and Fukushima. As a result, the researchers believe that most of the nation’s detailed survey work should be focused on the western portions of the nation.

The study’s findings are unique in their use of a particle dispersion model called FLEXPART, designed by the Norwegian Institute for Air Research, which uses meteorological data to drive a quantitative projection of the airborne chemical’s distribution. This allowed researchers to assume a constant rate of emission from the power plant itself and subsequently to model the dispersion over all of Japan, rather than simply over the immediate outlying areas.

The only real limitation this model has is its timeframe. Because no deposition data was taken from March 12 to March 19, the model begins its simulation of leakage and contamination on March 20. The group does include a relative contribution map though, that serves to fill this period in with a predicted contribution to the overall deposit of 137Cs. According to this map, this period when no observations were taken was characterized by the majority of airborne contaminants being carried out over the Pacific Ocean, away from populated areas.

These findings will help to shape the contamination removal and control efforts the government will undertake, such as removal of contaminated soils when possible or the imposition of soil use limitations when contaminated soil cannot be removed. Yet the Fukushima plant is still facing serious issues, as this study has now emerged shortly after it was discovered that highly contaminated water was still being released from the plant and that a dysfunctional misting component had sprayed workers with contaminated water. Reports like these can only ever be helpful when the active threat has already been addressed, which it seems in this case, it has not.

Just for a bit of context, the two principal radioactive isotopes being examined in the wake of the Fukushima incident, cesium-137 and cesium-134, were also the principal threats following the Chernobyl incident in 1986. According to the Chernobyl Forum’s 2005 report, cesium-137 was the principal source of radiation in the zone of exclusion as of 2005, meaning the presence of such high levels of cesium in Japan’s soil could continue to present problems well into the future.

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