Calculation of thermal-hydraulic-mechanical response of a deep geological repository for radioactive used fuel in granite

Abstract

The NWMO is considering siting a deep geological repository (DGR) for used fuel in a crystalline or sedimentary rock setting. One aspect of the long-term performance is the Thermal-Hydraulic-Mechanical (THM) response of the rock caused by the heat released from the high-level radioactive used fuel placed in the repository. In this paper, a method was proposed by building an integrated coupled THM model (Integrated model) to explore the importance of scale in modelling the host rock THM response in a crystalline rock setting. This integrated model included a site-scale domain of rock, with embedded large fracture zones (FZ). The full DGR geometry was represented by using large flat blocks to represent the bulk of the repository panels, with five placement rooms modelled explicitly. In one of these five placement rooms, a 9.24-m-long placement room section with 14 containers was modelled in detail. A preliminary study of the THM response of the rock mass around the proposed repository was performed using this model and its results were validated by comparing with accurate thermal response. The results from this integrated model were compared with the results from a near-field coupled THM model; and the comparison proves that the near-field model cannot give the correct THM response in a repository. To accurately model the THM response in a DGR, the proposed integrated model approach can be used to avoid the influence of the thermal, hydraulic and mechanical boundary conditions applied on near-field models.