This project aims to develop a scientific methodology for evaluating the capabilities of mathematical models and computer codes used in Performance Assessment and applied to the design, construction, operation, performance and safety assessment, and post-closure monitoring of geological nuclear waste repositories, based on the scientific principles governing coupled thermo-hydro-mechanical and chemical (THMC) processes in geological systems and geo-materials. The project work will be concentrated on the most essential issues for PA, with focus on rock salt, buffer materials and the buffer-rock interface, as guided by the tool of Issue Evaluation Tables (IET). The evaluation will be carried out by scientific/technical auditing (TA) procedures. The evaluation will examine the participating teams’ codes and approaches in terms of system characterisation (conceptualisation, simplification, mathematical representation and parameterisation), flexibility in handing realistic in-situ geological conditions, capacities in uncertainty source identification and treatment, and computational aspects. The evaluation will be organised around numerical simulations of small-to-moderate scale laboratory tests, specifically defined generic Bench-Mark Tests (BMT), and large-scale in-situ experiments (Test Cases – TC) involving coupled THM, HMC or THMC processes in rock salt and buffer material, so that simulations and evaluations can be conducted within a realistic physical context. This proposed project represents, for the first time, such a coherent and logical effort based on sound scientific principles, and with potentially far-reaching consequences in the field of nuclear waste disposal.