低放射性廢棄物最終處置場工程障壁材料於未飽和/飽和環境下之長期穩定性研究

張皓鈞(Hao-chun Chang)

指導教授 ： 黃偉慶

國立中央大學/工學院/土木工程學系/博士(2015年)

摘要

低放射性廢棄物最終處置採多重障壁設施，由於處置場設計年限極長，處置場從施工階段至封閉後之服務期間，其近場環境的演化可能使障壁材料處於未飽和/飽和之狀態下，導致障壁材料性質發生變化，因此本研究藉由各種模擬試驗獲取本土化工程障壁於不同狀態下之參數，做為最終處置場設計之參考。
本研究以美國懷俄明州膨潤土混合不同比例台東硬頁岩，運用壓製方式製作純膨潤土或碎石與膨潤土混合物障壁材料，分別與依照ACI配比設計法設計的傳統混凝土（參考混凝土），以及添加鋼纖維的活性粉混凝土接觸，進行電滲加速試驗，以模擬處置場服務期間未飽和狀態下障壁材料之間的長期交互作用反應。接著對於處置場在未來是否遭遇到地下水入侵的情況，分別設計相關試驗，包括乾燥環境試驗、潮濕環境試驗、乾溼循環試驗和緩衝材料保水性試驗，模擬處置場長久未飽和狀態；而地下水入侵處置場，導致處置場達到飽和狀態的情形則是透過地下水滲透試驗來進行模擬，之後將各試驗完成後的混凝土試體以及緩衝材料試體切片分層進行分析比較，以探討處置場之近場環境對障壁材料性質的影響。
研究結果顯示，兩種未飽和工程障壁材料相互接觸經過電滲加速試驗後，混凝土中鈣離子將會釋出，這使得與其接觸的緩衝材料由接觸面開始，其鈣/鈉比升高、pH值降低；此外，障壁材料於長久未飽和狀況下若有相對濕度變化，則障壁材料內水份的散失或吸收將會造成重量、體積的變化，使材料的角隅剝落或表面開裂，添加台東硬頁岩碎石粒料有助於減少緩衝材料於乾溼循環中所造成的重量、體積變化量，但是台東硬頁岩為一具有鹼質粒料反應潛勢之礦物，於長久潮濕的環境下會產生鹼質粒料反應，造成材料表面凸起剝落。
處置場遭遇飽和環境後，地下水於障壁材料中的滲流會造成混凝土持續溶出失鈣，以及含有高鹼性物質的滲出液與緩衝材料接觸，這將導致膨潤土從原本的鈉型膨潤土轉變為鈣型膨潤土，並產生不具回脹性質的NaAlSi2O6無機物而損失原本應具有的回脹能力。
綜合實驗分析結果發現，於緩衝材料中添加碎石不但可以降低障壁材料之間交互作用對緩衝材料所造成的影響，並且在乾溼循環的環境中，重量與體積的變化量也較少；另一方面，混凝土障壁於不同環境中發生鈣離子釋出或溶出失鈣現象時，就混凝土障壁本身而言，以活性粉混凝土所受到的影響較傳統配比的參考混凝土小，同時，活性粉混凝土對緩衝材料所造成的功能影響也較小。

關鍵字

緩衝材料 ； 未飽和/飽和處置環境 ； 混凝土障壁材料

並列摘要

Both concrete and buffer serve as engineered barriers for isolation of low-level radioactive wastes in a repository. As the disposal site is expected to serve a very long time, the interactions between the two barriers need to be evaluated under the potential unsaturated/saturated situations. The proposed study aims at simulating the long-term scenario of engineered barrier materials and the corresponding effects of the scenario on the expected function of the barrier materials in a final disposal site for low-level radioactive wastes.
In this research, a migration technique was applied to accelerate the migration of calcium ions from the pore solution of concrete so as to investigate the alteration of compacted bentonite in contact with the concrete. The buffer material, was made using Black Hills bentonite from Wyoming mixed with Taitung area argillite to produce different ratios of sand-bentonite mixture as buffer. And the barrier concrete mixes were proportioned according to traditional American Concrete Institute (ACI) mix design method and Reactive Powder Concrete (RPC) with steel fiber.
Also, concrete barrier and buffer are both subjected to extended unsaturated situation as well as saturated situation for further evaluation. High-low relative humidity cycling test was designed to investigate the extended unsaturated situation. Finally, the buffer was treated with water permeated through the concrete after the migration test so as to simulate the sequence of unsaturated and saturated scenarios to be experienced. And the buffer was then tested for the change accompanied by the saturation process.
It was found from the accelerated migration test that the release of calcium from concrete in unsaturated situation results in reduction of swelling capacity of the contacting buffer. And the shorter the distance to the interface, the more the increases in the ratio of calcium to sodium content of buffer material.
In case that the disposal vault remains unsaturated for extended periods, the absorption/loss of moisture causes changes in volume and weight of buffer material, which in turn result in some cracking and localized scaling off at the outer edge of compacted buffer materials. The use of crushed Taitung area argillite will decrease the change in volume and weight of buffer during the wet-dry cycling. However, due to the reactivity nature of Taitung area argillite, traces of alkali-aggregate reaction were noticed on surfaces of buffer mixed with crushed argillite as a result of long-term exposure to wet environment.
Upon saturation of the disposal vault, the leaching of hydroxyl ions from concrete to the buffer may cause (1) dissolution of montmorillonite, and (2) precipitation of mineral such as NaAlSi2O6. Both tend to decrease the swelling capacity of the buffer.
The addition of crushed Taitung argillite aggregate to bentonite showed less damaging effects, resulting from interactions between the contacting barriers, than pure bentonite on the buffer itself. Also, buffer material with 30% crushed aggregate can reduce the change in volume and weight of buffer subjected to high-low relative humidity cycling. On the other hand, the effect of leaching from reactive powder concrete (RPC) on the buffer is less detrimental than that from traditional concrete. Thus, RPC is considered more appropriate as barrier concrete for the final disposal of low-level radioactive wastes.

並列關鍵字

final disposal of low-level radioactive wastes ； buffer material ； unsaturated/saturated situations ； barriers

參考文獻

江彥霆，（2012），「酸蝕環境對於輕質骨材混凝土耐久性影響」， 朝陽科技大學，碩士論文，台中市。

張皓鈞，（2011），「高放廢棄物最終處置場緩衝材料與混凝土障壁的交互作用」，碩士論文，國立中央大學土木工程研究所，中壢。

王俊堯，（2011），「低放射性廢棄物最終處置回填材料於近場環境下之長期穩定性研究」，碩士論文，國立中央大學土木工程研究所，中壢。

莊怡芳，（2008），「未飽和緩衝材料吸力與水力傳導度推求及再飽和行為」，碩士論文，國立中央大學土木工程研究所，中壢。

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