Regional distribution and isotope ratios of radiocesium from the Fukushima Daiichi Nuclear Power Station and global fallout in Tokai-mura

Shimada, Asako ; Tsukahara, Takehiko*; Nomura, Masao*; Shimada, Taro ; Takeda, Seiji ; Takahashi, Hiroaki*

Due to the accident at the Fukushima Daiichi Nuclear Power Station (FDNPS), radiocesium such as Cs, Cs, and Cs was dispersed over a wide area of eastern Japan and mixed with radiocesium from global fallout. The depth profiles of Cs for samples taken in 2003 before the FDNPS accident and in 2017 after the FDNPS accident in Tokai-mura (about 115 km NE of Tokyo) were both described by exponential equations from the surface up to a depth of 15 cm. Systematic grid sampling of surface soil at a depth of 5 cm was conducted at 3 sites in Tokai-mura in 2019, and distributions of the Cs concentration, Cs/Cs radioactivity ratio, and Cs/Cs isotope ratio were measured. It was found that the Cs concentration varied among sites and within individual sites, while the Cs/Cs radioactivity ratio was constant for all samples collected at 3 sites, 1.010.04 (2). The Cs/Cs isotope ratio for the two sites was constant and comparable to that obtained for soil sampled near FDNPS. On the other hand, the Cs/Cs isotope ratio. For the other site varied and showed higher values (0.355-0.446), suggesting the influence of global fallout. Based on the results, the mixture percentages of radiocesium originating from global fallout and the FDNPS accident were estimated.