Thursday, 8 September 2005

This presentation is part of: Poster Session II

Isotopic ratios of 36Cl/Cl in Japanese surface soil

Riki Seki1, Takeshi Matsuhiro1, Yasuo Nagashima1, Tsutomu Takahashi1, Kimikazu Sasa1, Keisuke Sueki1, Yuki Tosaki1, Kotaro Bessho2, Hiroshi Matsumura2, and Taichi Miura2. (1) AMS Group, University of Tsukuba, Tennoudai 1-1-1, Tsukuba, Japan, (2) Radiation Science Center, High Energy Accelerator Research Organization, Oho 1-1, Tsukuba, Japan

We have previously reported that the 36Cl/Cl ratios of surface soil samples collected near the JCO criticality accident site, Tokai-mura, Ibaraki Prefecture, showed somewhat high values of the range of 10−11. In order to discuss this abnormality, the natural background values of 36Cl in Japan were essentially requested. Therefore, we measured the 36Cl/Cl ratios of uncultivated surface soil samples, which were expected to give undisturbed value, collected from 11 areas distributed over Japan. The ratios were found to be in the range of 10−13 except in Tokai-mura area, where 4 research Reactors and two commercial nuclear power plants have been operated. One of two power plants was categorized in a gas cooled reactor. The observed ratios in Tokai-mura area were higher than 10−12. On the other hand, soil samples collected from a site of commercial BWR nuclear power plants in Fukushima Prefecture showed no significant increase of the 36Cl/Cl ratios. It is comparatively easy to say that the gas cooled reactor released some amount of 36Cl to the environment because of weakness of air isolation from the reactor building and consequently caused the high 36Cl/Cl ratios in Tokai-mura. We measured the depth profiles of the 36Cl/Cl ratios in soil at Tokai-mura and Makabe-town, Ibaraki Prefecture. The Makabe-town is about 50 km apart from JCO in Tokai-mura. Therefore, we can expect there to be undisturbed area from the long term nuclear activities. No variation was observed in the depth profile of the Makabe sample, ~10−13. On the other side, the result of the Tokai-mura was somehow different from the Makabe one. From the surface to about 80 cm depth, the higher ratios, ~10−12, were obtained than those of the Makabe sample. However, the ratios became almost the same as the Makabe ones at the depth more than 80 cm deep. In this report, we will discuss the influence of the nuclear activities to the environment through the 36Cl measurement of surface soil in Japan.

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