| 摘要: | 低放射性廢棄物處置場設施之工程障壁材料由混凝土所組成，不同於一般結構物，服務年限需長達百年以上，因此對障壁混凝土材料可能產生之劣化及耐久性影響必須加以評估。 本研究主要針對工程障壁材料以實驗室模擬可能遭受之劣化狀況，製作符合ACI 349-97核能安全混凝土結構物材料規定配比及完整性容器可能採用之配比等試體，進行氯離子入侵、硫酸鹽侵蝕、溶出失鈣效應等劣化試驗，瞭解並分析其劣化之情況。氯離子入侵及硫酸鹽侵蝕試驗結果顯示，低水膠比以及添加較多卜作嵐礦物摻料之混凝土內部結構較緻密，導致離子擴散係數較小，但離子會在混凝土表層持續累積，造成表層離子濃度較高。溶出失鈣效應試驗以浸泡硝酸銨溶液之加速試驗來判斷配比的優劣，浸泡純水及人工海水進行溶出失鈣效果可不考慮其劣化影響。 為推估氯離子引致之腐蝕劣化，依氯離子入侵濃度預估鋼筋開始腐蝕之時間，主要影響因素為擴散係數與時間因子。本文針對不同歷時時間濃度剖面進行殘差分析，利用信賴度之概念建立其擴散係數與時間m因子之分佈，在氯離子引致腐蝕濃度門檻值與保護層厚度條件下，推估鋼筋開始腐蝕之時間，以建立障壁混凝土服務年限之推估模式。 劣化試驗結果顯示，完整性容器可能採用之配比在抵抗各種侵蝕能力都優於ACI 349-97配比，主要因為完整性容器配比低水膠比，內部結構較緻密，致使有害離子在表面累積而無法進入混凝土內部。而在溶出失鈣方面也比ACI 349-97配比有較佳之抗溶出失鈣之能力。Engineering barrier for the final disposal of low-level radioactive wastes serves to isolate the wastes from human biosphere for a very long design life. Concrete has been widely accepted as engineering barrier material due to its longevity, which provides good structural integrity for prolonged service life. The half life of 137Cs and 90Sr, the 2 major radio nuclides in low-level wastes, is about 30 years. It is estimated that the radioactivity of these nuclides would decay to a level that is comparable to the background in 10 half-life cycles. Hence, concrete material is expected to serve at least 300 years in the final disposal site. However, the adverse environmental conditions at the disposal site could attack concrete barrier material and results in degradation of the material.This study focuses on the effect of the chloride ingress, sulfate attack and leaching on the degradation rate of the long-term durability of the concrete. Test and analysis on concrete mixtures include: (1) high integrity container of H mixture, and (2) ACI 349-94 mixture designs. The results show that concrete internal structures with a low water-binder ratio are more compact, resulting in a lower ion diffusion coefficient. In terms of the diffusion coefficient, H mixture is better than ACI 349-94 mixture. To estimate the time to initial corrosion caused by chloride, two main factors are taken into consideration: the diffusion coefficient and the time factor. Concentration profiles of chloride at different times are used for residual analysis. To define the acceptable range of the diffusion coefficient, we looked into the threshold of chloride concentration causing corrosion and the cover thickness of concrete, and further estimate the time to initial corrosion of steel in concrete.The study attained the following findings and conclusions:(1) The diffusion coefficient of chloride and sulfate into concrete decreases as the age and exposure period increases. The use of pozzolanic materials in concrete mixes reduces the diffusion coefficient by providing a fine internal pore structure, which, in turn, improves the resistance of concrete to chloride ingress and sulfate attack.(2) The leaching degradation in concentrated ammonium nitrate solution is faster than that obtained with water as medium and is useful for accelerating leaching degradation. Leaching of concrete in pure water and synthetic sea water is very limited, and is not considered to cause significant porosity increase and promote the chloride ingress and sulfate attack much.(3) A reliability-based scheme for estimating the service life of concrete barrier has been developed based on a large amount of chloride ingress profiles obtained from various concrete specimens with different water/binder ratios and pozzolanic admixtures subjected to exposure of chloride ions for as long as 3 years. The predicted service life is found to be sensitive to not only the diffusion coefficient of concrete, but also the time factor on the reduction of diffusion coefficient, which requires long-term testing of concrete for determination. |
