An integrated multiscale experimental-numerical analysis on reconsolidation of salt-clay mixture for disposal of heat-generating waste. Final report
The possibility of permanent disposal of nuclear waste in salt has been investigated for decades. This interest is primarily due to the availability of the stable salt formations and the desirable thermal-hydro-mechanical behaviors of salt (e.g. high thermal conductivity, low permeability, self-healing). To ensure the long-term safety of the disposal, sealing is one of the primary concerns \citep{hansen2011salt}. One promising approach is to reconsolidating the crushed salt readily available in the repository with clay (Bentonite) additive \citep{hansen2015salt}. This treatment is favorable, because clay exhibits low permeability and sorbs radionuclides and water during the early stage of the repository \citep{hansen2011salt}. While reconsolidated salt with clay additive may act as an excellent flow barrier for the sealing system, loss of tightness of salt may occur due to the creation of connected pathways of the grain boundaries of salt and the deviatoric-stress-induced crack growth across and along grain boundaries of salt. This microscale deformation mechanisms play a significant role in the integrity of the repository and the sealing system in rock salt formations, while the grain boundary healing process could prevent active deformation. However, deformation mechanisms at micro-scale and their linkage to the bulk behaviors of salt with additives (i.e., clay and moisture) are still not fully understood. The difficulty of predicting the influence of moisture content and clay additives is particularly due to the lack of the essential physical underpinnings to replicate the clay-salt interaction at the grain scale. Furthermore, a lack of insight on the distribution of fluids and its role in salt-clay consolidation processes makes it difficult to optimize clay and moisture additives for enhancing reconsolidation under various operational conditions and how to incorporate the microscopic interaction of salt and clay into macroscopic predictions. Reconsolidation of granular (e.g., crushed) salt is one of the most critical processes crucial to plugging, sealing, testing, and modeling for sealing a repository \citep{hansen2014granular}. Granular salt is often used as buffer or backfill material where void space created by excavation is reduced over time. The combination of high thermal conductivity and low permeability, the self-healing properties, as well as the fact that crushed salt is readily available in a repository, all make the re-consolidation of crushed salt an attractive option for the salt repository environment. While it is now well-known that elevating temperature may accelerate reconsolidation of the crushed salt, recent experimental work (e.g. \cite{broome2014reconsolidation,hansen2014granular}) has pointed to a new promising approach in which a small amount of moisture and clay additives may be sufficient to alter the coupled TMHC mechanism to meet the performance criteria for backfills. Hence, understanding the reconsolidation process of salt with moisture and clay additives is instrumental for designing high-performance backfilling or sealing nuclear waste repositories in salt. This work provides comprehensive experimental data under a variety of conditions (e.g., temperature, stress, moisture contents) expected for waste disposal sites, which allows us to improve the viability of disposal concepts with a salt-clay mixture. The overall purpose of this research is to improve understanding of THMC coupling effect on the reconsolidation of granular (or crushed) salt-clay mixture used for seal systems of shafts and drifts in salt repositories. This proposed work is partially motivated by the recent work on the Waste Isolation Pilot Plant (WIPP) that shows the promising sealing capability of clay-salt mixture compared to crushed salt. In particular, the primary emphasis is to develop a fully integrated multiscale experiment-numerical study to determine and explain what leads to the superior sealing ability of the clay-salt mixture. These research activities are designed to seek further understanding of (1) why clay additives may enhance the fluid trapping and (2) whether this flow barrier effect may prevail under different combinations of temperature, confining pressure, deviatoric stress and other foreseeable environmental factors. If successful, this enhanced flow trapping ability of the seal provides significant improvement to the seal and repository performance and therefore make the repository safer in the long-term. The experiment component includes microstructural investigation and macroscopic tests on a reconsolidated salt-clay mixture. In the microstructural study, the goal is to (1) characterize microscopic distributions of distinct phases (e.g., clay, salt crystal boundaries, trapped brine, and pore ) to examine the connectivity of the pore network inside the salt-clay mixture with different amounts of clay additive and moisture content and (2) analyze multiscale imaging data to reconstruct the polycrystalline microstructures for numerical simulations. Meanwhile, macroscopic tests are performed to analyze how clay alters the failure/creep mechanisms in the salt-clay mixture. Microscopic and macroscopic experimental observations will both be used to calibrate and validate a multiscale model that explicitly simulates the capillary and multiphase flow in the connected pores and the deformation due to the presence of intra-crystalline brine at the pore scale via a new polyhedral discrete element–lattice Boltzmann method (DEM-LBM) coupling model. The pore-scale simulations are homogenized via an upscaling procedure that converts pore-scale information (e.g. force exerted on grain boundary, sliding, pressure-solution) to continuum measures (e.g. Cauchy stress, Darcy’s flow) at each integration point in the macroscopic multiphase TMHC model. This multiscale scheme will allow coupling between high-fidelity simulations of brine-salt-clay interaction and the macroscopic TMHC model. The multiscale model helps the understanding of how the trapped brine inclusion affects the pressure-solution mechanism with the presence of clay and moisture. This work brings new insight into the sealing capacity of salt-clay mixture under elevated temperature over a long period of time — a key to evaluating the potential of salt-clay mixture usage for salt repositories.
- Research Organization:
- Columbia Univ., New York, NY (United States)
- Sponsoring Organization:
- USDOE Office of Nuclear Energy (NE)
- DOE Contract Number:
- NE0008534
- OSTI ID:
- 1604128
- Report Number(s):
- DOE-Columbia-16-10058-1; DOE/NEUP-16-10058-1; 16-10058; TRN: US2103800
- Country of Publication:
- United States
- Language:
- English
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