The site was divided into 20 areas based on their air dose rates, and samples for analysis were collected from the areas to evaluate the distribution of radioactive nuclides within the site. Standing trees were selected as samples for analysis in FY2013 because they have been in the area since the accident, whereas rubble might have been moved from other areas after the accident.

Three pine trees in each area were selected and branches at four meters above the ground were collected.  Thirty samples with high dose rates at their surface were selected and sent to the JAEA Nuclear Science Research Institute in Tokai for analysis.  The findings revealed that the standing tree samples showed a similar distribution trend of the measured Cesium-137 (Cs-137), Strontium-90 (Sr-90), and Tritium (H-3) highly concentrated around the reactor buildings and it was also similar to that of air dose rate. 　Therefore, the main cause of the contamination can be presumed to be radioactive nuclides spread by the accident.  Analyzing the correlations among radioactive nuclide concentrations revealed a proportional relationship between the　concentrations of Cs-137 and Sr-90. The ratios of Cs-137 and Sr-90 are similar to those found in soil outside the site, rubble within the site and cut trees.

We will continue conducting radiochemical analysis including analysis of other radioactive nuclides and accident analysis, and the results of those studies will support planning of radioactive waste processing and disposal.

　　　

※Progress Status and Future Challenges of the Mid-and-Long-Term Roadmap towards the Decommissioning of TEPCO’s Fukushima Daiichi Nuclear Power Station Units 1-4 (Outline) (February 27, 2014)　p.4
http://www.meti.go.jp/english/earthquake/nuclear/decommissioning/pdf/20140227-e.pdf (English)

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