Coupled THMC Simulation Based on Explicit Fracture Representation Using Extrinsic Cohesive Zone Model

Abstract

ABSTRACT: We presented a new coupled thermal-hydraulic-mechanical-chemical (THMC) simulator by combining the numerical models that can explicitly introduce the rock fracture surfaces to reproduce the processes of changing permeability as realistically as possible. Then, the presented simulator was applied to estimate the long-term evolution in the permeability of a rock mass within a geological disposal system of high-level radioactive waste (HLW). The numerical predictions showed that the time-dependent decrease in permeability was observed only within specific fractures where the geochemical creep (i.e., pressure dissolution) had been induced after disposal of HLW into disposal cavity. This permeability drop was caused by the fracture sealing due to pressure dissolution at asperity contacts in the fracture that depended on the characteristics of each fracture (fracture formation, aperture, closing stress and contact area). Overall, it was implied that the presented simulator had a possibility to reveal the detailed spatial distribution of permeability within a natural barrier during a long-term disposal period of HLW. 1. INTRODUCTION When examining the performance of the geological disposal facility of high-level radioactive waste (HLW) for preventing the migration of radionuclides, it is significantly important to numerically simulate the evolution processes of permeability within a rock mass that works as a natural barrier. During the disposal period of HLW, the convolved phenomena such as rock fracturing during excavation of disposal cavity, heat radiation from waste package, fluid-driven transport, and kinetic dissolution/precipitation of the minerals may occur within a natural barrier. The complex interactions between these phenomena, namely, coupled thermal-hydraulic-mechanical-chemical (THMC) interactions, may bring about the change in permeability of a rock mass. Particularly, permeability distribution within a natural barrier is influenced by the transition of a rock mass from continuum to discontinuum due to the short-term rock fracturing during the excavation of the disposal cavity. In addition, subsequent occurrence of mineral reactions (e.g., pressure dissolution, free-face dissolution/precipitation) within rock fractures after disposing HLW may have non-negligible impacts on permeability evolution over the long-term. Pressure dissolution is a mineral dissolution caused at the asperity contacts within rock fractures, which may depend on compressive stress acting on a fracture plane and temperature (Yasuhara et al. (2011)). Previous work by Yasuhara et al. (2011) have shown that pressure dissolution can cause the permeability reduction of rock fractures by several orders magnitude over a long duration.