The geotechnical properties of GMZ buffer/backfill material used in high- level radioactive nuclear waste geological repository: a review

Buffer/backfill material in engineered barrier plays an important role in preventing leakage and migrating high-level radioactive nuclear waste (HLW) in geological repository. GMZ bentonite has been proposed as prior buffer/ backfill material to be applied in HLW geological repository in China. It is crucial to have a better understanding of the geotechnical properties of this material for optimizing the design and ensuring the long-term stability of the repository. In this paper, the research progress on the geotechnical properties of GMZ bentonite was summarized, including hydraulic conductivity, water retention capacity, thermal physical properties, swelling properties, microstructure, compressibility, multi-fields coupling behavior and long-term characteristics. The following findings are obtained: (1) The basic geotechnical properties of GMZ bentonite are significantly related to the thermal, hydraulic and mechanical boundary conditions, which can be characterized by complex coupling of multi-phase and multi-field; (2) on the premise of meeting the basic requirements of engineered barrier, adding quartz sand and other additives toGMZ bentonite is a currently available and feasible way to improve the geotechnical properties ofGMZ bentonite; (3) the observed volumetric deformation and mechanical behavior of GMZ bentonite and mixture under thermal, hydraulic, mechanical and chemical boundary conditions significantly depend on its compaction conditions, relative montmorillonite content and microstructure characteristics; (4) the current researches on GMZ bentonite are mainly limited by traditional laboratory small-scale tests under single-field and single boundary conditions in obtaining single parameter. However, there is still a gap between the simulated environmental conditions in the tests and the real environmental conditions in geological repository. Based on the above understanding and current research shortages in this field, some important research topics were proposed for future work, including multi-field/phase/component coupling characteristics and the mechanisms, longterm performance, large-scale model tests, underground in situ tests, numerical modeling and the constitutive modeling ofGMZ bentonite under complex boundary conditions.