模擬低放射性廢棄物於工程障壁之水文地質化學傳輸

Abstract

台灣自從1972年開始發展核能發電，核能電廠能提供社會民眾穩定且價廉的電力，不過卻也產生放射性廢棄物待處理的議題，因此妥善處置放射性廢棄物成為核能發電廠能永續發展的關鍵。台東縣達仁鄉為台灣低放射性廢棄物最終處置之建議候選場址之一，該地區面積約為30,664.5公頃，場址地形為丘陵地形，高程介於50公尺至200公尺間，台灣電力公司依達仁場址之地形特性，初步規劃達仁場址為坑道處置方式。本研究以水文地質化學觀點，模擬分析低放射性核種於工程障壁之傳輸，應用地化模式PHREEQC，選取關鍵核種之反應式並模擬達仁地區地下水水質之平衡分布，並將模擬之結果作為地化傳輸模式HYDROGEOCHEM 5.0之邊界條件。選取核種鈷-60、鍶-90、碘-129、銫-137、鋂-241，並假設核種洩漏於固化桶中，模擬核種於工程障壁之傳輸歷程並分析工程障壁之阻絕功能。HYDROGEOCHEM 5.0模擬結果顯示出混凝土處置窖及其周遭區域皆能保持穩定，即混凝土處置窖於模凝時間300年後能保持其阻絕功用。混凝土處置窖之外圍因地下水之入侵，產生Ettringite、Friedel`s salt與Thaumasite等礦物，造成混凝土之體積膨脹使混凝土破裂，由於混凝土裂縫的增加，將造成滲透性增加，裂縫可加速混凝土退化的速率。此外模擬區域之水流速度也因礦物之產生而改變其速度與方向。核種傳輸部分，核種考慮其衰變反應，故半衰期小於30年之核種，如鈷-60、鍶-90、銫-137於核種開始洩漏後其濃度隨模擬時間增加而減少，長半衰期之核種碘-129、鋂-241其濃度皆保持穩定並無隨著模擬時間增加而減少。模擬結果顯示核種皆侷限於混凝土處置窖中，由於地下水流相對很小僅少量濃度因擴散作用傳輸至坑道回填材料當中。

Taiwan has developed nuclear power since 1972. The nuclear power plants provided sufficient electricity but it brought the radioactive waste disposal problem in Taiwan. Thus the successful construction of the final disposal site of nuclear waste is the key to sustainable development for nuclear power. A proposed site for final disposal of low-level radioactive waste (LLRW) located in Daren Township of Taitung County along the southeastern coast has been on the selected list in Taiwan. The Daren Township is hilly terrain and the area is approximate 30,664.5 hectare. Taiwan Power Company proposed to build a tunnel disposal plant according to terrain features of Daren. This study aims to simulate the hydrogeochemical transport of low-level nuclides through engineered barrier and analyze the durability of engineered barrier. The geochemical equilibrium model PHREEQC was applied to simulate the quality of groundwater and selected the major reactions of low-level nuclides. The Simulated results by PHREEQC were used for the input data of the hydrogeochemical transport model HYDROGEOCHEM 5.0. Radionuclides of Cobalt-60, Strontium-90, Iodine-129, Cesium-137, Americium-241 were selected and assumed that the nuclides leaked from the waste container after 100 years of disposal. The simulated results by HYDROGEOCHEM 5.0 show that the contents of concrete engineered barrier remained essentially stability after 300 years. Formation of Ettringite, Friedel`s salt and Thaumasite observed outside concrete engineered barrier due to groundwater intrusion causing the swollen concrete and destructing the engineered barrier. The increased permeability accompanied with increase of concrete crack. Concrete cracks accelerated the rate of degradation. In addition, the flow rate and direction were altered due to the formation of Ettringite and Thaumasite in simulation area. The simulated results of radionuclides transport show that the concentration of nuclides of which the half-life is less than 30 years such as Cobalt-60, Strontium-90 and Cesium-137 decreased with time. The concentration of radionuclides with long half-life such as Iodine-129 and Americium-241 were stable. Because of the rate of groundwater flow was relatively small, the radionuclides were mostly confined within concrete engineered barrier, only a small amount of concentration diffused to the backfill of tunnel system.