[논문]공학적방벽재로서 벤토나이트 거동의 X선 단층촬영 기반 비파괴 특성화 현황

멜빈; 김주연; 김광염; 이창수; 김진섭

doi:10.7474/tus.2021.31.6.400

[국내논문] 공학적방벽재로서 벤토나이트 거동의 X선 단층촬영 기반 비파괴 특성화 현황
Current Status of X-ray CT Based Non Destructive Characterization of Bentonite as an Engineered Barrier Material 원문보기

터널과 지하공간: 한국암반공학회지 = Tunnel and underground space, v.31 no.6, 2021년, pp.400 - 414

멜빈 (한국해양대학교 해양에너지자원공학과) , 김주연 (한국해양대학교 해양에너지자원공학과) , 김광염 (한국해양대학교 해양에너지자원공학과) , 이창수 (한국원자력연구원) , 김진섭 (한국원자력연구원)

초록
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고준위방사성폐기물 처분장에서 벤토나이트는 공학적방벽재로서 주로 사용되어지는 재료로서 열-수리-역학-화학적 복합적 거동을 겪게 된다. 본 보고에서는 이러한 벤토나이트에 대한 X선 단층촬영 기반의 분석 및 특성화와 관련된 최근 연구 및 기술동향을 고찰하였다. X선 단층촬영 기반 벤토나이트의 평가는 분말형태와 펠렛형태에 대해 적용된 내용을 다루었다. X선 이미징을 통해 마이크로스케일에서 입자의 정보를 추출할 수 있으며 벤토나이트의 불균질성을 야기할 수 있는 펠렛 내부의 균열을 검출할 수 있다. 수화조건하에서 분말과 펠렛이 혼합된 벤토나이트에 대한 X선 분석을 통해 실험과정에서 발생하는 불균질 영역을 특정하고 모니터링이 가능하다. 펠렛으로만 구성된 벤토나이트가 펠렛과 파우더의 혼합으로 이루어진 벤토나이트보다 더 빨리 팽윤되는 특성이 보고되기도 하였다. 벤토나이트의 입자와 블록에 존재하는 작은 균열들이 건조-수화 조건하에서 각각 균열의 닫힘과 열림이 발생하는 것도 확인되었다. 전문 소프트웨어를 이용하여 시공간 단층 이미지로부터 변형률분포를 추출한 경우도 있었다. 최근의 연구들에서는 X선 단층촬영 기술을 이용하여 시간경과에 따른 벤토나이트의 건조밀도, 함수비, 입자의 이동 등을 평가하기도 하였다. 또한, 수화과정에 온도 조건을 고려하여 시간에 따른 재료의 전체 밀도 및 국부적 밀도 변화를 관찰하는 연구도 진행되고 있다.

Abstract ▼ AI-Helper

Under high-level radioactive waste repository conditions, bentonite as an engineered barrier material undergoes thermal, hydrological, mechanical, and chemical processes. We report the applications of X-ray Computed Tomography (CT) imaging technique on the characterization and analysis of bentonite over the past decade to provide a reference of the utilization of this technique and the recent research trends. This overview of the X-ray CT technique applications includes the characterization of the bentonite either in pellets or powder form. X-ray imaging has provided a means to extract grain information at the microscale and identify crack networks responsible for the pellets' heterogeneity. Regarding samples of pellets-powder mixtures under hydration, X-ray CT allowed the identification and monitoring of heterogeneous zones throughout the test. Some results showed how zones with pellets only swell faster compared to others composed of pellets and powder. Moreover, the behavior of fissures between grains and bentonite matrix was observed to change under drying and hydrating conditions, tending to close during the former and open during the latter. The development of specializing software has allowed obtaining strain fields from a sequence of images. In more recent works, X-ray CT technique has served to estimate the dry density, water content, and particle displacement at different testing times. Also, when temperature was added to the hydration process of a sample, CT technology offered a way to observe localized and global density changes over time.

주제어

표/그림 (13)

그림 Fig. 1. Schematic of the thermo-hydraulic and transport processes that can occur during the saturation period of the bentonite buffer in nuclear waste repository (Sena et al., 2010).
그림 Fig. 2. a) Photograph of the micro X-ray CT equipment at Korea Maritime and Ocean University (KMOU). b) Schematic of a typical micro-CT scanning setup with a conical X-ray beam. Cases A and B indicate samples placed at different source-object-distances (SOD) from the source and their resultant image resolution.
그림 Fig. 3. Typical X-ray CT slice of a dry montmorillonite sample along with subregions to exemplify the process of grain boundary determination based on a computer code (Tomioka et al., 2010).
그림 Fig. 4. Micro X-ray CT cross-sections of MX80 bentonite pellet in its original condition (Molinero-Guerra et al., 2017).
그림 Fig. 5. Process of porosity calculation based on X-ray CT images of the bentonite sample using subregions. CT thresholding was used to estimate the porosity within each subregion. The resultant porosity data was shown as a function of distance along the vertical and horizontal axis (Saba et al., 2014).
그림 Fig. 6. Vertical and horizontal X-ray CT cross-sections of the sample at different testing times. The horizontal sections at the right were taken at 60 mm from the bottom (Molinero-Guerra et al., 2018).
그림 Fig. 7. (Left) Vertical cross-sections of a second sample prepared with two distinctive halves. The bottom half was composed of a mixture of pellets and powder, while the upper was pellets only. (Right) Horizontal CT cross-sections 10 mm below the top of the sample, showing the swelling process of pellet-only part which is faster than that of the pellet-powder mixture part. Red subregions represent denser areas in the pellets (Molinero-Guerra et al., 2020).
그림 Fig. 8. (Left) Horizontal and vertical sections of a bentonite sample during dehydration indicating strain fields, with positive values representing dilation and negative contraction. (Right) CT images of a smaller region during dehydration and hydration, indicating the relative humidity (RH). Red highlighted regions are included to follow the behavior of the crack during hydration and dehydration (Chen et al., 2020).
그림 Fig. 10. Linear relation between the average CT number and the density of bentonite-sand mixtures, along with the CT number as well as density against the distance from the inner (I) to the outer (O) diameter. (Tan et al., 2021).
그림 Fig. 9. (Left) Profiles of water content and dry density measured directly after testing. (Right) Vertical profiles of fours samples showing normalized gray values along with an example of X-ray CT profile of the smaller cores obtained from a larger sample (Bernachy-Barbe et al., 2020).
그림 Fig. 11. Results on dry density, water content, and vertical displacement of a bentonite powder sample at different time intervals. The injection flow direction is upwards (Bernachy-Barbe 2021).
그림 Fig. 12. CT slices showing the thermocouple sensor's movement and the dynamic CT density changes on Wyoming bentonite for a non-heated and a heated sample. The arrows show the position of the thermocouple sensor (Chang et al., 2021).
그림 Fig. 13. Comparison of the effects of hydration in samples CW1 and CW2. These cuboid samples were tested using a transparent cell under unidirectional hydration from the bottom (Villar et al., 2021).

참고문헌 (20)

Bernachy-Barbe, F., 2021. Homogenization of bentonite upon saturation: Density and pressure fields. Applied Clay Science, 209, pp.106122.

상세보기
Bernachy-Barbe, F., Conil, N., Guillot, W. and Talandier, J., 2020. Observed heterogeneities after hydration of MX-80 bentonite under pellet/powder form. Applied Clay Science, 189, pp.105542.

상세보기
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상세보기
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상세보기
Molinero-Guerra, A., Aimedieu, P., Bornert, M., Cui, Y.J., Tang, A.M., Sun, Z., Mokni, N., Delage, P. and Bernier, F., 2018. Analysis of the structural changes of a pellet/powder bentonite mixture upon wetting by X-ray computed microtomography. Applied Clay Science, 165, pp.164-169.

상세보기
Molinero-Guerra, A., Mokni, N., Delage, P., Cui, Y.J., Tang, A.M., Aimedieu, P., Bernier, F. and Bornert, M., 2017. In-depth characterisation of a mixture composed of powder/pellets MX80 bentonite. Applied Clay Science, 135, pp.538-546.

상세보기
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상세보기
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상세보기
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상세보기
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