Distribution of I-129 in the terrestrial environment after the Fukushima Daiichi Nuclear Power Plant accident

松村 万寿美*; 笹 公和*; 松中 哲也*; 富田 涼平; 高橋 努*; 松崎 浩之*; 末木 啓介*

no journal, , 

A significant amount of radioactive material was released into the environment after the accident at the Fukushima Daiichi Nuclear Power Plant (FDNPP) on March 11, 2011, with atmospheric emissions commencing on March 12 and peaking during March 1516 and 2022 (TEPCO, 2012). Anthropogenic sources such as nuclear-fuel reprocessing plants had already increased the I-129 level in the environment above its natural background (I-129/I-127 = 1.510; e.g.). Total amounts of radionuclides discharged into the atmosphere were estimated to be 8.1 GBq for I-129 (T1/2=1.57107 y) and 120200 PBq for I-131 (T1/2=8.02 d) released from the FDNPP accident. Although the accident-derived I-131 in soil extinguished in a few months, the long-lived I-129 can be used as a tracer to retrospectively infer the level of I-131. We mentioned at AMS-15, the I-129/ I-131 atomic ratio in 5-cm-long surface soils of Fukushima area for reconstruction of the I-131 deposition using the long-lived I-129, average ratio of I-129/ I-131 was estimated to be 26.05.7 corrected at 14:45, March 11, 2011. Several years were passed, I-129 released into the environment exists through repeated deposition and resuspension. We investigated the concentration of I-129 and isotopic ratio of I-129/ I-127 in river water in Fukushima Prefecture, the sampling was conducted at five sites on the Niida-River, once per year, from 2014 to 2020. Sample measurements were performed using AMS at the MALT, The University of Tokyo until 2015 and at UTTAC, The University of Tsukuba thereafter. As a results, the I-129 concentrations were in the range (0.352.8)10 atoms L0 and the I-129/ I-127 ratio was (0.514.6)10. Generally, values are high at the earliest sampling time and appear to have stabilized since then.