Feasibility Assessment on the Application of X-ray Computed Tomography on the Characterization of Bentonite under Hydration

Melvin B. Diaz; Gyung Won Lee; Seohyeon Yun; Kwang Yeom Kim; Chang-soo Lee; Minseop Kim; Jin-Seop Kim

doi:10.7474/TUS.2022.32.6.491

All Issue

2022 Vol.32, Issue 6 Preview Page Next Page

Technical Note

Feasibility Assessment on the Application of X-ray Computed Tomography on the Characterization of Bentonite under Hydration 벤토나이트 수화반응 특성화를 위한 X선 단층촬영 기술 적용성 평가

31 December 2022. pp. 491-501

PDF XML

Abstract

Bentonite has been proposed as a buffer and backfill material for high-level radioactive waste repository. Under such repository environment conditions, bentonite is subjected to combined thermal, hydrological, mechanical, and chemical processes. This study evaluates the feasibility of applying X-ray CT technology on the characterization of bentonite under hydration conditions using a newly developed testing cell. The cylindrical cell is made of platic material, with a removable cap to place the sample, enabling to apply vertical pressure on the sample and to measure swelling pressure. The hydration test was carried out with a sample made of Gyeonju bentonite, with a dry density of 1.4 g/cm³, and a water content of 20%. The sample had a diameter of 27.5 mm and a height of 34 mm. During the test, water was injected at a constant pressure of 0.207 MPa, and lasted for 7 days. After one day of hydration, bentonite swelled and filled out the space inside the cell. Moreover, CT histograms showed how the hydration process induced an initial increase and later progressive decrease on the density of the sample. Detailed profiles of the mean CT value, CT standard deviation, and CT gradient provided more details on the hydration process of the sample and showed how the bottom and top regions exhibited a decrease on density while the middle region showed an increase, especially during the first two days of hydration. Later, the differences in CT values with respect to the initial state decreased, and were small at the end of testing. The formation and later reduction of cracks was also characterized through CT scanning.

Keywords

Bentonite

Hydration test

X-ray CT

High-level radioactive waste

벤토나이트는 고준위 방사성 폐기물 처분장의 완충재 및 뒷채움재의 주재료로 고려되고 있다. 처분환경에서 벤토나이트는 열-수리-역학-화학적 복합적 거동을 겪게 된다. 본 연구는 제작된 수화거동 실험용 셀을 사용하여 수화 조건에서 벤토나이트의 거동 특성을 X선 단층촬영 기술을 이용하여 평가하고자 하였다. 플라스틱재료로 만들어진 원통형 셀은 상부의 탈착식 캡을 이용하여 시료 상부에 수직응력을 가하거나 팽윤압을 측정할 수 있도록 제작하였다. 수화실험은 건조밀도 1.4 g/cm³, 함수율 20%의 조건으로 제작된 경주 벤토나이트 블록시료로 수행되었다. 샘플의 직경은 27.5 mm, 높이는 34 mm 이며, 수화 실험 중 0.207 MPa의 일정한 압력으로 물을 주입하였으며, 7일 동안 수화실험을 지속하였다. 하루 동안 수화 과정을 거치면서 벤토나이트가 팽창하여 셀 내부의 공간을 채우는 것을 확인하였다. 또한, 샘플의 X선 CT값의 히스토그램 분석을 통해 수화 과정 초기의 샘플 밀도 증가와 이후 점진적인 밀도 감소가 발생함을 평가할 수 있었다. 평균 CT 값, CT값의 표준 편차, CT값 변화량에 대한 분석을 통해 샘플의 수화 과정에 대한 자세한 정보를 확인할 수 있었다. 즉, 수화 시작 후 2일 동안 시료 하부 및 상부 영역은 밀도가 감소하고 중간 영역은 밀도가 증가하였다. 그 후 수화가 진행되면서 샘플의 각 위치에서의 밀도 변화는 초기 샘플의 밀도와 비교할 때 그 차이가 점차 감소함을 확인하였다. 샘플 내 균열의 형성과정과 이후 감소되는 현상도 X선 단층촬영에 의해 확인되었다.

키워드

벤토나이트

수화 실험

X선 단층촬영

고준위방사성폐기물

References

Booker, J.R., Brachman, R., Quigley, R.M., and Rowe, R.K., 2004, Barrier Systems for Waste Disposal Facilities. Crc Press. 10.1201/9781482271935

Chang, C., Borglin, S., Chou, C., Kneafsey, T., Wu, Y., Zheng, L., Nakagawa, S., Xu, H., Peruzzo, L., and Birkholzer, J., 2021, Experimental Study of coupled thmc processes in bentonite buffer for geologic disposal of radioactive waste. In 55th US Rock Mech./Geomech. Symposium, virtual.

Diaz, M.B., Kim, J.Y., Kim, K.Y., Lee, C., and Kim, J.S., 2021, Current status of x-ray ct based non destructive characterization of bentonite as an engineered barrier material. Tunnel and Underground Space, 31(6), 400-414.

Ewing, R.C., Whittleston, R.A., and Yardley, B.W., 2016, Geological disposal of nuclear waste: a primer, Elements, 12(4), 233-237. 10.2113/gselements.12.4.233

Liu, Z.R., Ye, W.M., Cui, Y.J., Zhu, H.H., Wang, Q., and Chen, Y.G., 2021, Development of swelling pressure for pellet mixture and compacted block of GMZ bentonite, Construction and Building Materials, 301, 124080. 10.1016/j.conbuildmat.2021.124080

Liu, Z.R., Ye, W.M., Cui, Y.J., Zhu, H.H., and Wang, Q., 2022, Water infiltration and swelling pressure development in GMZ bentonite pellet mixtures with consideration of temperature effects, Engineering Geology, 305, 106718. 10.1016/j.enggeo.2022.106718

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, 538-546. 10.1016/j.clay.2016.10.030

Saba, S., Barnichon, J.D., Cui, Y.J., Tang, A.M., and Delage, P., 2014, Microstructure and anisotropic swelling behaviour of compacted bentonite/sand mixture, J. of Rock Mech. and Geotech. Eng., 6(2), 126-132. 10.1016/j.jrmge.2014.01.006

Park, S., Yoon, S., Kwon, S., Lee, M.S., and Kim, G.Y., 2021, Temperature effect on the thermal and hydraulic conductivity of Korean bentonite buffer material, Progress in Nuclear Energy, 137, 103759. 10.1016/j.pnucene.2021.103759

Xu, L., Ye, W.M., Chen, B., Chen, Y.G., and Cui, Y.J., 2016, Experimental investigations on thermo-hydro-mechanical properties of compacted GMZ01 bentonite-sand mixture using as buffer materials, Engineering Geology, 213, 46-54. 10.1016/j.enggeo.2016.08.015

Yoo, M., Choi, H.J., Lee, M.S., and Lee, S.Y., 2016, Measurement of Properties of Domestic Bentonite for a Buffer of an HLW Repository, J. of Nuclear Fuel Cycle and Waste Tech., 14(2), 135-147. 10.7733/jnfcwt.2016.14.2.135

Yoon, S., Cho, W., Lee, C., and Kim, G., 2018, Thermal conductivity investigation of korean bentonite buffer materials, Proceedings of the Korean Radioactive Waste Society Conference, Busan, Republic of Korea, May.

Information

Publisher :Korean Society for Rock Mechanics and Rock Engineering
Publisher(Ko) :한국암반공학회
Journal Title :Tunnel and Underground Space
Journal Title(Ko) :터널과 지하공간
Volume : 32
No :6
Pages :491-501
Received Date : 2022-12-15
Revised Date : 2022-12-19
Accepted Date : 2022-12-21
DOI :https://doi.org/10.7474/TUS.2022.32.6.491

Tunnel and Underground SpaceISSN:1225-1275(Print) 2287-1748(Online)한국암반공학회

All Issue