博士学位論文内容の要旨

博士学位論文内容の要旨

氏 名

Siriprapaシ リ ッ プ ラ パ ー

Somboon^{ソ ム ブ ー ン}

所 属 人間健康科学研究科 人間健康科学専攻 学 位 の 種 類 博士（放射線学）

学 位 記 番 号 健博 第

158

号 学位授与の日付 平成

30

年

9

月

30

日 課程・論文の別 学位規則第４条第１項該当

学 位 論 文 題 名

Distribution of Radiocesium and Uranium in Rivers near Fukushima Daiichi Nuclear Power Plant

（福島第一原子力発電所周辺の河川における放射性セシウムおよび ウランの分布に関する研究）

論 文 審 査 委 員 主査 准教授 井上 一雅 委員 教 授 福士 政広

委員 教 授 大谷 浩樹(帝京大学)

委員 上席研究員

Sarata Kumar Sahoo(放射線医学総合研究所)

【論文の内容の要旨】

On 11 March 2011 tsunami followed by earthquake seriously caused Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident that emitted a large amount of radionuclides in the atmosphere. The total amount of ¹³⁷Cs released in Fukushima Prefecture due to the accident is estimated to be approximately 15 PBq. Radiocesium is of major concern from radiological safety point of view, because of long half-lives (¹³⁷Cs, t1/2~ 30.07 y and ¹³⁴Cs, t1/2~ 2.06 y). The main objective of this study is to measure ¹³⁷Cs activity in five rivers namely Ukedogawa, Takasegawa, Maedagawa, Kumagawa and Tomiokagawa within 20 km from Fukushima Daiichi Nuclear Power Plant (FDNPP).

Distribution of ¹³⁷Cs was carried out on river sediment as well as river bed sediments in five rivers to ascertain the contamination levels. Radiocesium activity concentration in river sediments was determined using high purity Germanium (HPGe) gamma spectrometry. The radiocesium activity concentration in riverbed sediments was found to be (103-22,000) Bq kg^-1 and floodplain deposits(92-117,000) Bq kg^-1. The decay corrected (as to 15 March, 2011) ratio of ¹³⁴Cs/¹³⁷Cs was 1.02 in all samples.

Radiocesium activity concentration in 45 μm to 2 mm particle size fractions of samples revealed that radiocesium was primarily attached to fine clay particles and would be

博士学位論文内容の要旨

expected to remain for a long time in river environments.

In nuclear power plants, ²³⁵U has the main role in energy production since it is a fissionable isotope suitable to maintain nuclear chain reactions in neutron reactions.

Moreover, uranium contamination is a possible scenario at the time of nuclear accidents.

As a result of artificial contamination or natural occurrence, uranium enters to aquatic environment and air due to leaching and re-suspension processes. Therefore, uranium concentration in five-river surface waters was measured using inductively coupled plasma mass spectrometry (ICP-MS) as a base line study. The uranium concentration was in the range of 1 to 2300 ng L^-1 with a mean value of 136 ng L^-1. The measured uranium content was found to be less than the drinking water limit of 30 μg L^-1 recommended by the World Health Organization (WHO).

Research of this study provides radiocesium and uranium concentration in river samples surrounding Fukushima Daiichi nuclear power plant. The study is useful primarily for environmental radioactivity monitoring and also for radiological risk assessment.