高放處置場環境下低密度緩衝材料潛變行為研究;Study on Creep Behavior of Low Density Buffer Materials in High-level Radioactive Waste Disposal Environment

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Title:	高放處置場環境下低密度緩衝材料潛變行為研究;Study on Creep Behavior of Low Density Buffer Materials in High-level Radioactive Waste Disposal Environment
Authors:	任國亮;REN, GUO-LIANG
Contributors:	土木工程學系
Keywords:	緩衝材料;MX80;潛變;數值模擬;buffer material;MX80;creep;numerical simulation
Date:	2022-08-25
Issue Date:	2022-10-04 10:48:10 (UTC+8)
Publisher:	國立中央大學
Abstract:	針對用過核子燃料之最終處置場建置，國際間目前一致採用『深層地質處置』的方式，以人工障壁與天然障壁相結合，使之與人類生活環境完全隔離。在處置設施封閉後數百年乃至十萬年的複雜環境中，緩衝材料之潛變將影響人工障壁的長期穩定性。由於施工建造、吸水膨脹以及沖蝕等原因，緩衝材料局部範圍內密度可能降低。因此，本研究採用美國懷俄明州MX80膨潤土為實驗材料，運用壓製方式製作試體，量測低密度膨潤土之回脹壓力、水力傳導係數，隨後進行等速率直剪與等應力直剪實驗以求取潛變參數，從而完善低密度情形下緩衝材料之潛變模型。同時引入有限元素法，並進行數值模擬來探討低密度緩衝材料長期之潛變行為，為最終處置場設計提供參考。實驗研究結果顯示：在較低的乾密度情形下，緩衝材料性能有所降低。隨著乾密度的減小，最大回脹壓力呈指數型衰減，而水力傳導係數呈現指數型增長，且未飽和試體以及飽和試體之抗剪強度均折減較多。因此在處置孔長期安全考量下，需要限制緩衝材料密度降低的幅度。另一方面，在預膨脹模式下，緩衝材料的性能較之定體積模式有明顯差異。在預膨脹模式下，由於膨潤土經歷的發展變化不同，其回脹壓力之時程曲線呈現三段式的特點，有别於定體積回脹之雙峰曲線，且預膨脹模式下最大回脹壓力衰減更為明顯。水力傳導係數的量測結果顯示，在預膨脹模式下水力傳導係數增長趨勢亦更為明顯，阻水能力愈發弱化。同時，預膨脹模式下，膨潤土試體之抗剪強度亦出現較大幅度的折減，且局部低密度的區域將會較早產生潛變，並持續產生潛變之行為，這一現象在處置孔長期安全評估中應特別關注。數值模擬分析發現：透過潛變參數建立潛變模型預估潛變之長期行為，潛變位移與飽和歷程相關，在不同點位趨勢相似但細節有所差異。緩衝材料在前50年飽和歷程發展較快，隨後減緩，在第100年左右達到完全飽和。而在緩衝材料與上方回填材料交界處，其垂直位移不斷增大，並在第十萬年達到最大值。低密度與常密度1600 kg/m³之緩衝材料在飽和歷程方面較為類似，最終變形亦差異較小，僅在最大垂直位移數值上略低。 ;For the construction of a repository for spent nuclear fuel, the deep geological disposal method is currently accepted internationally. The multi-barrier design is used by combining engineered barriers and natural barriers for isolation of the wastes from the human living environment. In the complex environment of hundreds or even 100,000 years after the closure of the disposal facility, the creep characteristics of the buffer material will significantly affect the long-term stability of the engineered barrier. During the process of saturation, due to construction, expansion and erosion, reduced density of buffer may occur in local area. In this study, MX80 bentonite from Wyoming, USA was used as the test material, and buffer material was made by static loading. Through measurement of hydro-mechanical parameters including swelling pressure and hydraulic conductivity, and shear experiments including direct shear test and constant-stress shear test, the creep parameters were finally obtained and can be used in the creep model of the buffer material at varying densities. In addition, numerical simulation was used to explore the long-term creep behavior of buffer in a deposition hole by the finite element method. The experimental results show that the performance of the buffer material is degraded at low dry densities. With the decrease of dry density, the maximum swelling pressure decreases exponentially while the hydraulic conductivity increases exponentially, and the shear strength of both unsaturated and saturated specimens decreases significantly. Thus, it is necessary to limit the magnitude of the reduction in the density in the long-term consideration. Meanwhile, in the pre-swell mode the performance of buffer is found to be different from that in the constant volume mode. In the former, due to the swelling process experienced by the bentonite, the time-history curve presents a three-stage characteristic, which is different from the bimodal curve exhibited in the later mode. The measurement of hydraulic conductivity shows that the increasing trend is also more obvious for the pre-swelled specimens, indicating reduced water resistance. Moreover, the shear strength in the pre-swell mode is greatly reduced. The creep in the low-density area will take place earlier and continue to develop progressively, which should draw special attention in the long-term safety assessment for the disposal. The numerical simulation results show that the creep behavior of the deposition hole can be predicted using the creep parameters obtained and the creep model established. The displacement is related to the saturation history with similar trends but different details for different parts. The saturation process will develop rapidly in the first 50 years, then slow down and reach full saturation at around 100 years. At the interface of the buffer material and the backfill material, the vertical displacement increases continuously and reaches its maximum value at the 100,000th year. The saturation time history of the low-density buffer material is very similar to that of the constant density buffer of 1600 kg/m³, with slightly smaller maximum vertical displacement at the interface of buffer and backfill.
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