Tunnel and Underground Space (터널과지하공간)

Korean Society for Rock Mechanics and Rock Engineering (한국암반공학회)
권호 표지
DOI QR코드

DOI QR Code

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

  • Melvin B., Diaz (Department of Ocean Energy & Resources Engineering, Korea Maritime and Ocean University (KMOU)) ;
  • Gyung Won, Lee (Department of Ocean Energy & Resources Engineering, Korea Maritime and Ocean University (KMOU)) ;
  • Seohyeon, Yun (Department of Ocean Energy & Resources Engineering, Korea Maritime and Ocean University (KMOU)) ;
  • Kwang Yeom, Kim (Department of Ocean Energy & Resources Engineering, Korea Maritime and Ocean University (KMOU)) ;
  • Chang-soo, Lee (Korea Atomic Energy Research Institute (KAERI)) ;
  • Minseop, Kim (Korea Atomic Energy Research Institute (KAERI)) ;
  • Jin-Seop, Kim (Korea Atomic Energy Research Institute (KAERI))
  • 멜빈 (한국해양대학교 해양에너지자원공학과) ;
  • 이경원 (한국해양대학교 해양에너지자원공학과) ;
  • 윤서현 (한국해양대학교 해양에너지자원공학과) ;
  • 김광염 (한국해양대학교 해양에너지자원공학과) ;
  • 이창수 (한국원자력연구원) ;
  • 김민섭 (한국원자력연구원) ;
  • 김진섭 (한국원자력연구원)
  • Received : 2022.12.15
  • Accepted : 2022.12.21
  • Published : 2022.12.31
Download PDF
⟨ Previous Next ⟩

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.

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.

Keywords

Acknowledgement

This work was supported by the Institute for Korea Spent Nuclear Fuel (iKSNF) and National Research Foundation of Korea (NRF) grant funded by the Korea government (Ministry of Science and ICT, MSIT) (No. 2021M2E1A1085197 & No. 2021R1A2C2011634).

References

  1. Booker, J.R., Brachman, R., Quigley, R.M., and Rowe, R.K., 2004, Barrier Systems for Waste Disposal Facilities. Crc Press.
  2. 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.
  3. 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. https://doi.org/10.7474/TUS.2021.31.6.400
  4. Ewing, R.C., Whittleston, R.A., and Yardley, B.W., 2016, Geological disposal of nuclear waste: a primer, Elements, 12(4), 233-237. https://doi.org/10.2113/gselements.12.4.233
  5. 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. https://doi.org/10.1016/j.conbuildmat.2021.124080
  6. 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. https://doi.org/10.1016/j.enggeo.2022.106718
  7. 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. https://doi.org/10.1016/j.clay.2016.10.030
  8. 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. https://doi.org/10.1016/j.jrmge.2014.01.006
  9. 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. https://doi.org/10.1016/j.pnucene.2021.103759
  10. 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. https://doi.org/10.1016/j.enggeo.2016.08.015
  11. 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. https://doi.org/10.7733/JNFCWT.2016.14.2.135
  12. 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.