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放射性廃棄物地層処分における緩衝材および岩体中の核種移行に関する研究

A Study on nuclide migration in buffer materials and rocks for geological disposal of radioactive waste

佐藤 治夫

Sato, Haruo

本論文は、放射性廃棄物地層処分における緩衝材候補材料の1つであるNa型ベントナイトおよび代表的岩石について、間隙率および屈曲度などの間隙構造因子ならびに核種拡散係数の測定を行い、基礎的データを蓄積すると共に、長期的な核種移行を定量的に予測できる拡散モデルを構築し、予測手法の基礎的理論を確立することを目的として研究された結果をまとめたものであり、7章から構成される。各章では、人工バリアとしてのNa型ベントナイトと天然バリアとしての花崗閃緑岩、玄武岩と泥岩の物性値に関する測定データと電気二重層理論および分子拡散理論に基づいて構築されたモデルが、地層処分の立案の際、緩衝材と地層構成岩石の特性を考慮することにより、バリア内の核種拡散挙動の予測と処分場性能評価を槻ね定量的に可能にしすることを示している。尚、本論文は、筆者が平成6年4月22日$$sim$$平成9年3月18日にかけて秋田大学大学院鉱山学研究科博士後期課程(地球工学専攻)に在籍し、事業団において得られた成果(投稿論文)を中心に博士論文としてまとめたものである。

This thesis summarizes the results investigated in order to establish a basic theory on the predictive method of diffusion coefficients of nuclides in compacted sodium bentonite which is a candidate buffer material and in representative rocks for the geological disposal of radioactive waste by measuring the pore structural factors of the compacted bentonite and rocks such as porosity and tortuosity, measuring diffusion coefficients of nuclides in the bentonite and rocks, acquiring basic data on diffusion and developing diffusion models which can quantitatively predict nuclide migration in long-term. This consists of 7 chapters. Chapter 1 is the introduction, in which conventional studies on nuclide migration in buffer materials and rocks for the geological disposal of radioactive waste carried out to date are reviewed, and those problems are summarized as well as the objectives of this study are described. Besides, the difinition of geological disposal is explained. In Chapter 2, it is described on non-steady state diffusion of HTO, Sr-90, Tc-99, I-129, Cs-137, Np-237, Am-241 and Pu in purified sodium bentonite, Kunipia-F, in which the rate of constituent Na-smectite was raised approximately 100wt%. In-diffusion experiments were carried out in a range of bentonite densities of 200 $$sim$$ 2000 kg$$cdot$$m$$^{-3}$$ under ambient aerobic conditions at room temperature (20 $$sim$$ 23$$^{circ}$$C), and apparent diffusion coefficients (Da) were obtained. The apparent diffusion coefficients decreased with increasing dry density of bentonite. It was quantitatively indicated from diffusion experiments using HTO that these Da values include the effect of geometric retardation such as the tortuosity factor of compacted bentonite. It was experimentally clarified that Da is not affected by diffusion time based on diffusion experiments for different experimental periods using Sr and Cs. Moreover, it was also experimentally clarified that Da is not affected by tracer ...

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