Estimation of the Amount of I-129 in the Environment Generated Due to the Decay of Te-129m Discharged by the Fukushima NPS Accident

Abstract

Abstract The accident at the Fukushima Daiichi Nuclear Power Station (1F-NPS) occurred following the Great East Japan Earthquake in March 2011, and led to the release of volatile radionuclides, which were deposited on the environment such as soils, forests, residential land, etc. in the Fukushima prefecture and the neighbouring areas. From immediately after the accident, air dose rate monitoring and analyses of radionuclides in soil, etc. were conducted in a wide range of area. Consequently, volatile radionuclides such as I-131, Cs-134, Cs-137, Te-129m, and Ag-110m were detected in a wide range of area, where was distant over 60km from 1F-NPS. Since I-131 is short-lived, it approximately disappeared after a few months. After the decay of short-lived I-131, Radiocaesium (Cs-134 and Cs-137) are the main contributors to radiation dose rate. On the other hand, Te-129m, of which amount was approximately the same level as both Radiocaesium, was also detected although its half-life is short about 1 month. Since very long-lived I-129 is generated due to the decay of Te-129m, the accumulation of I-129 in the environment by accident is worried. Te-129m decays to I-129 by the following 2 decay series; Te-129m → Te-129 (half-life: 69.6min) → I-129 (64%), Te-129m → I-129 (36%). Since the half-lives of Te-129m and Te-129 are 33.6d and 69.6min, respectively, radioactive equilibrium is established between both nuclides, but it is not established between Te-129 and I-129 and between Te-129m and I-129. In this case, the amount of I-129 increases due to the decay of Te-129m, but also the maximum value of the amount theoretically exists. In this study, the amount of I-129 in the environment generated due to the decay of Te-129m discharged by the 1F-NPS accident was estimated based on the analysed data of Te-129m in soil. The analysis was conducted for 3 investigation locations (Okuma Town, ca. 2.7km from 1F-NPS, Naraha Town, ca. 19km from 1F-NPS, and Fukushima City, ca. 60km from 1F-NPS) among the analysed data (as of 14 June 2011) of soil samples obtained at about 2,200 investigation locations in the Fukushima prefecture and the neighbouring prefectures. The maximum amount of I-129 was estimated 1.03 × 10−1 (Bq/m2) after about 1,000d even for the maximum deposition amount of Te-129m (2.7 (MBq/m2) as of 14 June 2011). This is approximately the same level as radioactivity concentration in the environment (5.75 × 10−3 ∼ 2.96 × 10−1 (Bq/m2)). Additionally, the results of field investigation around 1F-NPS also support this conclusion.