Advances in the numerical modeling strategy (concept) of a generic nuclear waste repository in crystalline rock

Abstract

Safe deep geological disposal of heat-generating nuclear waste requires an accurate assessment of barrier integrity. Therefore, the evaluation of the coupled mechanical, hydraulic, thermal and chemical processes, occurring in the host rock due to the nuclear waste storage, excavation among others is needed. For this purpose, numerical modeling is an essential and powerful tool. This contribution focuses on a generic repository system that relies on a Containment Providing Rock Zone (CRZ) in a crystalline rock, which acts as the principal containment barrier according to German law. Using the open-source finite element code OpenGeoSys version 6, a closer look at the CRZ regarding the influence of fractures on the local hydraulics and potentially available rock zone volume for repository emplacement is shown. Since fractures and other type of discontinuities usually characterize crystalline rock, they are expected to influence the hydraulic behavior of the system. Hence, their representation in numerical models becomes non-trivial. Here a comparison between different numerical fracture representations and their impact on the hydraulic regime surrounding the CRZ, is presented. Due to the presence of fractures, it cannot be assumed that a sufficiently large area in which to emplace the waste will be found. As a consequence, multiple smaller CRZs [1], each providing undisturbed rock, have to be defined. Typically, it is only possible to characterize fracture networks statistically, which leads to the use of stochastically generated discrete fracture networks. Using a geometrical characterization of the potentially undisturbed CRZs based on a stochastically generated discrete fracture network, a methodology is proposed to evaluate the feasibility of the multiple CRZs concept.