Abstract:
This report details the progress in evaluating engineered barrier system (EBS) design concepts for the long-term isolation of nuclear waste in geological repositories, with a strong focus on the role of modeling and simulation tools, including finite element analysis. The work addresses the development of EBS design concepts, the assessment of clay phase stability, and the advancement of thermodynamic databases for cement and clay phases. It also describes the ongoing development of the Disposal System Evaluation Framework (DSEF), a tool for formalizing the design and evaluation of repository concepts by integrating information on waste forms, geologic environments, and design constraints. A key part of the research involves modeling coupled thermal-hydrological-mechanical-chemical (THMC) processes in the EBS. The Barcelona Basic Model (BBM) has been implemented into the TOUGH-FLAC simulator to model the THM behavior of bentonite-backfilled tunnels and their interaction with the host rock. These numerical simulations are used to assess the impact of parameters like buffer saturation, thermal conductivity, and canister spacing on peak temperatures and to study rock failure in layered rock environments. The report also highlights the use of computational chemistry methods, such as molecular dynamics simulations, to understand clay mineral properties and swelling behavior at a nano-scale, providing a supplemental approach to traditional experimental methods. In the thermal analysis section, the DSEF incorporates both analytic models and finite element models using TOPAZ3D and the mesh generator TrueGrid. Initial testing of these models shows good agreement between the analytic solutions and the finite element simulations for a simple 3D solid, though the finite element solution's accuracy is highly sensitive to mesh size. This validation work demonstrates the DSEF's capability to perform thermal calculations and to be a central repository for documenting and comparing results from different models and scenarios. The overall effort is aimed at developing a robust, flexible framework for evaluating multiple disposal system options and addressing key knowledge gaps in repository science.
