Three-dimensional thermo-hydro-mechanical modelling of the full-scale <i>in-situ</i> system test (FISST)

The full-scale in-situ system test (FISST) constitutes one of the most comprehensive and large-scale experimental investigations to date, aimed at advancing the design and understanding of engineered barrier systems within the framework of nuclear waste repository development. Initiated in 2018, the FISST involves the placement of two test canisters within designated deposition holes in the ONKALO ® underground research facility's demonstration area, located in a tunnel approximately 50 m in length. The test represents a full-scale implementation of the KBS-3V disposal design concept, the reference methodology adopted in Finland and Sweden for the final disposal of nuclear waste. A laboratory testing campaign was undertaken to calibrate the thermo-hydro-mechanical (THM) model parameters for the deposition hole buffer and tunnel backfill materials used in FISST. These materials consisted of Wyoming-type bentonite utilized for blocks and pellets within the deposition hole, along with Italian and Bulgarian bentonites employed in the form of blocks and pellets as tunnel backfill materials. Blocks and pellets were produced with these three types of bentonites. This study focuses on the calibration of key material properties associated with the components of FISST, including thermal conductivity, water retention characteristics, permeability and mechanical parameters. The Barcelona basic model (BBM) was used to represent the block materials, while model with double porosities was employed for the pellets. Additionally, a methodology was developed to linearize the BBM for improving computational efficiency in the simulations that followed. Subsequently, the linearized BBM was employed in THM calculations. Following the calibration of the material models, a large-scale 3D thermo-hydraulic (TH) simulation was performed to define TH boundary conditions for a 3D THM model on a reduced scale. This approach validated the feasibility of 3D modelling under defined TH boundary conditions with accurately calibrated THM parameters. All simulations were conducted using CODE_BRIGHT, a finite element method program specifically tailored for advanced THM modelling of complex systems.