Evaluation of Radioactive Contamination Caused by Each Plant of Fukushima Daiichi Nuclear Power Station Using 134Cs/137Cs Activity Ratio as an Index

We estimated the extent of radioactive contamination from each reactor unit (1 to 3) of Fukushima Daiichi Nuclear Power Station by using the Cs-134/Cs-137 activity ratio as an index. Although the activity ratio of Cs-134/Cs-137 emitted by the accident has been reported to be about 1 : 1, variations in the activity ratio has arisen in environmental samples, since the ratio differs slightly for every nuclear reactor. Therefore, it is considered that the ratio is useful for a index for evaluating the contamination from each reactor unit. We collected soils and plant pieces in eastern Japan, measured gamma rays and calculated the Cs-134/Cs-137 activity ratio. We also calculated the Cs-134/Cs-137 activity ratio in contaminated water accumulated in each reactor building (R/B) and turbine building (T/B), and compared it with the ratio of environmental samples. The data of the Cs-134 and Cs-137 activity concentration are presented by Tokyo Electric Power Company. As a result, it turned out that the ratio of a sample in Oshika Peninsula was about 0.91, which was lower than that of other samples, and similar to the ratio of reactor unit 1 of Fukushima Daiichi Nuclear Power Station. It is suggested that Oshika Peninsula is contaminated mainly from reactor unit 1. We also evaluated the extent of activity contamination from each reactor unit at other points.