用過核子燃料衰變熱對緩衝材料傳輸行為之影響

Abstract

高放射性廢棄物地質處置的概念是使用多重障壁系統來隔絕輻射可能對人類的影響，範圍涵蓋處置設施周圍環境提供的天然地質屏障，以及人造工程障壁系統，其中包括廢棄物包件、廢棄物罐、緩衝材料和回填材料。 多重障壁系統可在廢棄物包件失效時發揮其所需功能，延遲廢棄物罐中的核種流出至地質圈中。 由於放射性核種衰變熱與處置設施中的水力和水-礦物反應會有交互作用，溫度與壓力導致的膨潤土製作成的緩衝材料功能降低。國際上較少討論熱-水-化對與蒙脫石脫水影響的相關研究。因此，本研究採用蒙脫石脫水化學動力學模型來計算蒙脫石黏土礦物受到用過核子燃料衰變熱影響而排出的水量。本研究發現，蒙脫石黏土會因層間水的損失而導致體積收縮。了解衰變熱是否會引起膨潤土孔隙率變化，並評估了孔隙率變化對放射性核種通過緩衝材料的影響。 本研究進行熱傳計算得知處置設施的溫度峰值約會出現在10年左右。大約2萬年後，用過核子燃料釋放產生的熱量已經逐漸消散，溫度降低到接近地溫的背景水平。緩衝材料中因溫度改變而造成孔隙率變化，陽離子的修正孔隙率為0.177、0.321、0.435，陰離子的修正孔隙率為0.174、0.128、0.07。 在近場放射性核種傳輸模擬結果中，本研究不同過往近場核種傳輸模型，使用3維模式建模，進行熱與傳輸的耦合模型計算，耦合模型考慮溫度變化下的修正孔隙率，並使用地化模式計算受不同溫度下的吸附係數，本研究發現修正後的有效孔隙率可以獲得更保守的濃度，此結果可用於處置設施的安全評估，黏土礦物的脫水概念可以進階分析高放射性廢棄物處置設施的受到用過核子燃料衰變熱的性能影響。

The concept of geological disposal for high-level radioactive waste (HLW) is based on a multi-barrier system comprising the natural geological barrier that repository host rock provides and its surroundings, as well as an engineered barrier system (EBS) that includes waste form, waste canisters, buffer materials, and backfill. The multi-barrier system is expected to perform its desired functions to isolate waste from the biosphere and achieve HLW disposal. Because of the interaction processes of radionuclide decay heat with hydraulic and water–mineral reactions in the radioactive waste repository, the bentonite buffer degradation resulting from the external temperature and pressure warrants investigation. However, seldom addressed the influence of thermo-hydro-chemical (T-H-C) processes on buffer material degradation in the EBS of HLW disposal repositories as related to smectite clay dehydration. Therefore, we adopted the chemical kinetic model of smectite dehydration to calculate the amount of water expelled from smectite clay minerals caused by higher temperatures of waste decay heat. We discovered that smectite clay could cause volume shrinkage because of a loss of interlayer water, which can lead to bentonite buffer compression. We investigated whether the decay heat caused the bentonite porosity change and evaluated the influence of porosity change on the radionuclide transport through the buffer material. We found that the temperature peak occurred about 10 years. After ap-proximately 20,000 years, the thermal caused by the release of the canister had dispersed and the temperature had reduced close to the geothermal background level. The modified cation porosity of buffer due to the temperature evolution was equal to 0.177, 0.321, 0,435. The modified anion porosity of buffer was equal to 0.174, 0.128, and 0.071. In the simulation results of near-field radionuclides transport, we found that the modified effective porosity can obtain a more conservative concentration which can be used for the safety assessment of the repository. The results of this study may be used in advanced research on the evolution of bentonite degradation for both performance assessments and safety analyses of final HLW disposal.