Implication of spatial heterogeneity in geological environment to development of disposal concept

Multi-barrier concept utilizes both engineered barrier system (EBS) and natural barrier system (NBS) to guarantee long-term safety of geological disposal. For this purpose an appropriate disposal concept has to be adopted based on the information obtained in the site characterization at each stage, which includes enhancement or modification of some components of the EBS. Although properties of geological environment often exhibit significant spatial heterogeneity, in most of the existing studies, nuclide release from a single waste package surrounded by an “average” host rock is calculated and the result is regarded as a representation of the whole heterogeneous repository region. However, spatial heterogeneity in the repository region implies that there exist sub-regions that are more favorable from the safety perspective and the ones that are less favorable. In other words safety performance of an entire repository cannot be determined without knowing how many wastes can be accommodated within the more favorable sub-regions and how thoroughly hydrological connection between favorable and less favorable sub-regions can be mitigated by design and engineering solutions. In this study, as an alternative to the strategy based on homogeneous approximation, an approach to optimizing repository layout by maximizing number of waste packages to be accommodated in the favorable sub-regions in a heterogeneous geological environment and to evaluate its contribution to improving safety performance of the entire system is proposed.