Computational thermo-hydro-mechanics of crystalline rock salt for nuclear waste disposal

SeonHong Na, WaiChing Sun

Abstract

Rock salt is one of the major materials used for nuclear waste disposal. The desired characteristics of rock salt, i.e. the high thermal conductivity, low permeability and self-healing is highly related to the crystalline microstructure. Conventionally, this microstructural effect is often incorporated phenomenologically in macroscopic models. Nevertheless, Rock salt is a crystalline material of which the thermo-mechanical behavior is dictated by the nature of crystal lattice and mcriomechanics the slip system. This paper present a model proposed to examine these fundamental mechanisms at the grain scale level. We employ the single-crystal plasticity framework on salt and idealized it as an FCC crystal lattice with a pair of Na+ and Cl- ions as basis. Utilizing an viso-elasto-plastic framework, we capture the existence of elastic region in the stress space and the sequence of slip system activation of salt under different temperature ranges. To capture the intragranular fracture, we introduce an anisotropic phase-field based model to capture the anisotropy of critical energy release rate of a single crystal. Numerical examples demonstrated that the proposed model is able to capture the brittle-ductile transition under various of loading rate, temperature and confining pressure. 

Abstract:​

A combined phase field and crystal plasticity approach for capturing thermo-mechanical behavior of polycrystalline rock salt
 
SeonHong Na, WaiChing Sun
Department of Civil Engineering and Engineering Mechanics, Fu Foundation School of Engineering and Applied Science, Columbia University, New York
 
Rock salt or halite is one of the major materials used for nuclear waste disposal. The desired characteristics of rock salt, i.e. the high thermal conductivity, low permeability and self-healing are highly related to the crystalline microstructure. Conventionally, this microstructural effect is often incorporated phenomenologically in macroscopic models. Nevertheless, effort to directly simulate the interplays among micro-mechanical mechanisms, such as micro-cracking, inelastic dilatancy, grain boundary sliding and dislocation creeping, which may bring more reliable and robust forward predictions for the long-term thermo-mechanical behavior of salt, are lacking. The goal of this research is to fulfill this knowledge gap. In particular, we first formulate a computational framework that may predict multi-slip system crystal plasticity of single crystal in rate-independent and rate-dependent regimes. Using this as a starting point, we introduce an anisotropic phase-field based model in which displacement and temperature jumps are regularized to model the grain boundary where the temperature-dependent plastic flow is parallel to the ground boundaries. The texture of the polycrystalline salt is assumed to be random by assigning different sets of orientations to each grain. The grain boundary is approximated using the crystal plasticity constitutive model with single slip system to include the inelastic behavior, while the fracture response is captured via the evolving phase field. Numerical examples demonstrate that the proposed model is able to capture the brittle-ductile transition under various of loading rate, temperature and confining pressure with a minimal set of material parameters.
 
Keywords: salt, halite, crystal plasticity, slip system, thermo-mechanical analysis, constitutive model