TERRAPUB Geochemical Journal

Geochemical Journal, Vol. 52 (No. 2), pp. 211-217, 2018


Direct tritium emissions to the ocean from the Fukushima Dai-ichi nuclear accident

Naoto Takahata,1* Yama Tomonaga,1,2,5 Yuichiro Kumamoto,3 Masatoshi Yamada4 and Yuji Sano1

1Atmosphere and Ocean Research Institute, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8564, Japan
2Eawag, Swiss Federal Institute of Aquatic Science and Technology, Überlandstrasse 133, CH-8600 Dübendorf, Switzerland
3Research and Development Center for Global Change, Japan Agency for Marine-Earth Science Technology, 2-15 Natushima-cho, Yokosuka, Kanagawa 237-0061, Japan
4Institute of Radiation Emergency Medicine, Hirosaki University, 66-1 Hon-cho, Hirosaki, Aomori 036-8564, Japan
5Institute of Geological Sciences, University of Bern, Baltzerstrasse 1+3, CH-3012 Bern, Switzerland

(Received February 3, 2017; Accepted July 28, 2017)

Abstract: In this work we report tritium concentrations determined in seawater samples collected offshore of Fukushima in the northwestern Pacific Ocean, immediately after the Fukushima Dai-ichi Nuclear Power Plant (F1-NPP) accident. We found surface seawater to have high concentrations of tritium (3H) and cesium-137 (137Cs) with respect to the concentrations expected for the investigated region. Tritium concentrations were up to six times the pre-accident level. However, these concentrations were found to be relatively low compared to those of 137Cs and 129I. This is most likely because of the very low production ratio of tritium to 137Cs in the F1-NPP, and the inherently high background concentration of tritium in these environmental waters (mainly ascribed to global fallout from nuclear weapons tests conducted during the 1960s). The tritium distribution in surface seawater showed higher concentrations close to the shore and lower concentrations offshore. Higher tritium concentration areas had spread both northward and southward from the F1-NPP along the coast, indicating that large amounts of tritium were carried by coastal currents. A positive correlation was found to exist between 3H and 137Cs concentrations in the seawater. The calculated 3H/137Cs ratio was similar to the production ratio of these isotopes reported for the broken reactors in the F1-NPP, which indicates that both radionuclides might have originated from the F1-NPP. Direct emission of tritium to the ocean from the F1-NPP was estimated to be approximately 0.05 PBq immediately after the accident, which is much smaller than the total inventory in the environment.
Key words: tritium, radiocesium, Fukushima accident, ocean, direct emission

*Corresponding author E-mail: ntaka@aori.u-tokyo.ac.jp

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