||Effect of Tritium and Radioactive Caesium Released from the Accident of Fukushima Daiichi Nuclear Power Plants on the Environment and the Construction Method of Evaluation of Tritium Internal Exposure by Applying Hydrogen Isotope Exchange Reaction
Kataoka, Noriaki片岡, 憲昭
76 , 2015-03-23 , 新潟大学
学位の種類: 博士（工学）. 報告番号: 甲第4038号. 学位記番号: 新大院博（工）甲第420号. 学位授与年月日: 平成27年3月23日
When tritium (T) in precipitation was produced by the interaction between ^<14>N (or ^<16>O) and ^1n, the specific activity of T in environment is 0.5～1.0Bq/kg in Niigata. On the other hand, T is also produced by nuclear bomb test. When atmospheric nuclear-bomb test were performed (1960s), the maximum value of T was over 200Bq/kg in Niigata city. After that, the specific activity of T continuously decreased and attained the environmental revel (0.5～1.0Bq/kg). However, after the accident of Fukushima Daiichi Nuclear Power Plants, the specific activity of T in precipitation was increased in Niigata city. There are two processes of T production in a nuclear reactor. In this study, precipitation in Niigata city was collected monthly, and the evaluation of the specific activity of T was performed. In addition, we also collected the precipitation hourly (short precipitation) and spring waters of several mountains in and around Fukushima prefecture. Furthermore, radioactive caesium and T in some lake sediments in Fukushima and Niigata was investigated. T also causes a hydrogen isotope exchange reaction (T-for-H exchange reaction), and can be divided into exchangeable organically bound tritium (OBT) and non-exchangeable OBT. Exchangeable OBT acts as Free Water Tritium (FWT) in the body, and the dose coefficient of T in FWT is unity. On the other hand, the dose coefficient of T in non-exchangeable OBT is 2.3. At present, distinction of exchangeable OBT and non-exchangeable OBT are very vague. Thus, the study on exchangeable OBT and non-exchangeable OBT is more important. In this study, the reactivity of hydrogen isotope exchange reaction for Exchangeable OBT and non-exchangeable OBT was observed. Consequently, the evaluation of internal exposure of T was performed. From the above-mentioned, the following matters can be found. (1) The tritium concentrations in the samples in March and April 2011 were twice or three times higher than in March and April in annual years. In other words, it is considered that the thus high level concentration of tritium attributes the accident of the Fukushima Daiichi Nuclear Power Plants on Niigata city. (2) All of the activities of T in spring-water sample are less than 1.0Bq/kg and similar to the level of the monthly precipitation in Niigata city. (3) The specific activity of T in spring-water was not increased 3.5 years after 3.5 years from the accident of Fukushima Nuclear Power Plants. (4) Transfer coefficient of T from FWT to non-exchangeable OBT is 0.000070. (5) When internal exposure of T is calculated by applying the transfer coefficient, the amount of increase in dose is only 1.3×10^<-13>μSv. The increased does not affect human body. As the results, the method used in this work can quantitatively clarify the reactivity of the T-for-H exchange reaction, and can be useful to estimate the internal exposure of exchangeable and non-exchangeable OBT.