TERRAPUB Geochemical Journal

Geochemical Journal, Vol. 53 (No. 2), pp. 103-118, 2019

Radioactive 35S emitted from the Fukushima Nuclear Power Plant and its re-suspension from the contaminated area

Sebastian Oscar Danielache,1,2* Chisato Yoshikawa,3 Mizuo Kajino,4,5 Satoshi Itou,1 Wataru Kakeya,6 Naohiro Yoshida6,7 and Yasuhito Igarashi8,9

1Dep. of Material and Life Sciences, Faculty of Sci. & Tech., Sophia University, Chiyoda-ku, Tokyo 102-8554, Japan
2Earth and Life Sci. Inst., Tokyo Inst. of Tech., Meguro-ku, Tokyo 152-8551, Japan
3Department of Biogeochemistry, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2-15 Natsushima-cho, Yokosuka 237-0061, Japan
4Meteorological Research Institute, Japan Meteorological Agency, 1-1 Nagamine, Tsukuba, Ibaraki 305-0052, Japan
5Faculty of Life and Environmental Sciences, University of Tsukuba, 1-1-1, Ten-noudai, Tsukuba, Ibaraki 305-8572, Japan
6Department of Chemical Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of Technology, G1-17, 4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8502, Japan
7Earth-Life Science Institute, Tokyo Institute of Technology, 2-12-1-IE-1 Ookayama, Meguro-ku, Tokyo 152-8550, Japan
8College of Science, Ibaraki University, 2-1-1 Bunkyo, Mito, Ibaraki 310-8512, Japan
9Center for Research in Isotopes and Environmental Dynamics, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8577, Japan

(Received January 24, 2018; Accepted October 4, 2018)

Abstract: We present a numerical study conducted using a regional Lagrangian model to account for the transport, deposition and radioactive decay of 35S in sulfur dioxide and sulfate aerosols emitted into the atmosphere during the Fukushima Dai-ichi Nuclear Power Plant incident. The model is a Eulerian-Lagrangian hybrid system that accounts for chemical conversion of SO2 into SO42- in a Eulerian manner. The simulations were compared to field measurements of atmospheric 35S in sulfate collected at Kawamata, Tsukuba, Kashiwa, Fuchu and Yokohama, Japan. The 35S emission scenario that best replicated the field measurements followed the same temporal variation pattern as the 134/137Cs emissions. These results suggest that 35S and 134/137Cs follow a similar release pattern. Among the considered emission scenarios, a maximum flux of emitted chemical compounds was assumed to be either 100% 35SO42- or 100% 35SO2, with values of 4.0 × 1019 molecules/hour and 4.0 × 1020 molecules/hour, respectively on March 14th. These emission scenarios reflect the findings reported in the literature, where traces of 35SO2 were measured along with 35SO42-, so the actual emission is expected to be a combination of both chemical forms. The Kawamata measurements (Figs. 5 and 6) presented a large concentration in the July-August period, several months after emissions decreased by more than an order of magnitude. To explain this anomaly, re-suspension ratios were calculated for the Kawamata site, which ranged between 0.1 and 1.5% and partially, but not fully, explain the large measured concentrations. Furthermore, they show large discrepancies with 134/137Cs re-suspension values for measurements at the town of Namie. This situation indicates a lack of understanding of the transformations of 35S that occurs after deposition and the mechanisms involved in the 35S re-suspension process.
Key words: Fukushima NPP, radioactive isotopes, 35-sulfur, sulfur chemistry, Regional Lagrangian model

*Corresponding author E-mail: sebastian.d@sophia.ac.jp

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