熱-水-応力連成モデルを用いたニアフィールド解析評価

Coupled thermal hydraulic and mechanical analysis in the near field for geological disposal of high-level radioactive waste

千々松 正和*; 谷口 航 

Chijimatsu, Masakazu*; Taniguchi, Wataru

高レベル放射性廃棄物の地層処分における廃棄体定置後のニアフィールドでは、廃棄体からの放熱、周辺岩盤から人工バリアへの地下水の侵入、地下水の侵入による緩衝材の膨潤圧の発生、周辺岩盤の地下変化などの現象が相互に影響することが予想される。このような、熱-水-応力連成現象を評価することは、ニアフィールド環境の明確化の観点から重要な課題の一つである。熱-水-応力連成現象を明らかにするためには、まず個々の現象に関わるメカニズムを明らかにする必要がある。そのため、不飽和ベントナイトの伝熱特性、浸潤特性、膨潤特性等に関する各種要素試験が実施されている。本論では、熱-水-応力連成モデルを用い、これらの基本特性試験の解析評価を行うとともに、モデル上のパラメータを実験結果から逆解析的に算定した。また、各種要素試験の結果得られた物性値を入力データとしてニアフィールドの熱-水連成解析を実施し熱解析と連成解析の比較を行うとともに、連成解析による緩衝材の再冠水時間の評価を実施した。解析の結果得られた知見は以下の通りである。(1)室内実験から同定された温度勾配水分拡散係数Drを用い連成解析を実施した結果、緩衝材の初期含水比が7%、17%のいずれのケースも緩衝材内の最高温度は熱解析で得られた値より小さい結果となった。

Geological disposal of high-level radioactive waste (HLW)in Japan is based on a multibarrier system composed of engineered and natural barriers. The engineered barriers are composed of vitrified waste confined within a canister, overpack and buffer material. Highly compacted bentonite clay is considered one of the most promising candidate buffer material mainly because of its low hydraulic conductivity and high adsorption capacity of radionuclides. In a repository for HLM, complex thermal, hydraulic and mechanical (T-H-M) phenomena will take place, involving the interactive processes between radioactive decay heat from the vitrified waste, infiltration of ground water and stress generation due to the earth pressure, the thermal loading and the swelling pressure of the buffer material. In order to evaluate the performance of the buffer material, the coupled T-H-M behaviors within the compacted bentonite have to be modelled. Before establishing a fully coupled T-H-M model, the mechanism of each single phenomenon or partially coupled phenomena should be identified and modelled physically and numerically. Under the unsaturated condition, the water movement within the buffer material has often been expressed as a simple diffusion model with the constant apparent water diffusivity. However, the water movement in the low permeable and unsaturated porous medium has been known as a transfer process in both vapor and liquid phases. Therefore, it is necessary to incorporate the two-phase contribution into the physical model. In this study, the water diffusivity of compacted bentonite is obtained as a function of water content and temperature. The proposed water movement model is constructed by applying the Philip & de Vries' model and Darcy's law. While the water retention curve is measured by the thermocouple psychrometer, van Genuchten model is applied as the water retention curve because the smooth derivative of the water potential with respect to water content is ...