Isotopic ratios of 36Cl/Cl in Japanese surface soil

Abstract

We have measured the 36Cl/Cl ratio of uncultivated surface soil samples collected from 11 areas distributed throughout Japan to determine the undisturbed value of the ratio. The ratio was found to be on the order of 10−13 except for the Tokai-mura area, where four research reactors, two commercial nuclear power plants and a nuclear fuel reprocessing plant have been operated. The observed ratio in the Tokai-mura area was higher than 10−12. Notably, soil samples collected from a site of commercial BWR nuclear power plants in Fukushima prefecture showed no significant increase in 36Cl/Cl ratio.The 36Cl/Cl ratio depth profiles of soil samples collected at both of Makabe-town and Tokai-mura were also measured. Since Makabe-town is located about 50km apart from Tokai-mura, we do not expect it to be affected by the nuclear facilities. No large variations were observed in the Makabe depth profile; the measured ratios ranged from ∼3 to ∼5×10−13. The result obtained for Tokai-mura is significantly different in that from the surface to about 80cm depth, the measured ratios, ∼10−12, are much higher than any at Makabe. At depth below 80cm, the Tokai-mura ratios are lower and become indistinguishable from those at Makabe.The 36Cl/Cl ratio in unaffected areas of Japan is estimated to be 3–4×10−13.