Local surface and vertical distribution and isotope ratios for radiocesium

Shimada, Asako ; Tsukahara, Takehiko*; Nomura, Masao*; Takeda, Seiji 

Because of the accident at the Fukushima Daiichi Nuclear Power Station (1F), a large area of eastern Japan was contaminated by radiocesium. There are many old nuclear facilities in the area. When the old nuclear facilities will be decommissioned in three decades, the radioactive level of radiocesium from 1F would be more than clearance level. When radiocesium would be detected at the completion of their decommissioning in the site, estimation of the origin of which is from background originated 1F and fallout of atmospheric nuclear test or from decommissioned facilities will be important. Therefore, soils of 3 sites were collected by using U8 containers and by core boring, and Cs radioactivity, Cs/Cs radioactivity ratios, Cs/Cs isotope ratios were determined. Separation method of Cs from soil materials were developed based on the reported methods. The Cs radioactivity concentrations were varied in not only between sites but also in the same site, however, Cs/Cs radioactivity ratio was approximately constant in all sites. In case of highly contaminated soils, Cs/Cs isotope ratio was almost comparable to that for the soil collected at near the 1F. On the other hand, in case of lower contaminated soils, Cs/Cs isotope ratio was slightly higher than that for soil collected at near the 1F. It is possibly that fallout before the 1F accident contained Cs to increase the Cs/Cs isotope ratio. In the presentation, vertical distribution will also be discussed.