In NTB 24‑22 Rev. 1 (Nagra 2024u), numerical simulations are used to judge the performance of the repository using predefined performance targets. Critical aspects considered in these simulations include, for example, the integrity of the multi-barrier system, scenarios for the potential release of radionuclides, thermal and gas-related factors, and geological attributes. The simulations rely on conceptualisations of geological and hydrogeological conditions. Based on the geological characterisation described in the geosynthesis report NTB 24‑17 (Nagra 2024i) and references therein, it is assumed in these conceptualisations that diffusion is the dominant transport mechanism in the CRZ, with advective transport along steeply dipping faults not being relevant. Sedimentary features, such as hard beds in the upper and lower confining units, do not act as lateral release paths.

Critical aspects noted above are screened for their relevance to repository performance with a model-based assessment that uses a code capable of coupling thermal and hydraulic processes (e.g., TOUGH). Parametric uncertainty is considered probabilistically through Monte Carlo simulations. This necessitates a model setup with simplified representations of complex coupled processes to ensure that the demands on computational resources are manageable.

Findings from extensive 3D simulations confirm decoupling between HLW and L/ILW repository sections, allowing independent analysis. Consequently, separate model setups are employed for each repository section. Performance indicators include temperature (T) and pore pressure (P) at specific points, as well as the fluxes of tracers (F) representing conservative dissolved and volatile species, which are crucial for assessing safety functions related to radionuclide immobilisation and barrier integrity.