OpenWorkFlow - Open source software for the optimisation of the numerical process simulation of safety analyses in the process of site selection

The siting process of a deep geological nuclear repository is a complex long-term endeavour involving many different stakeholders. Assessing the suitability of a site for a nuclear waste repository requires, among others, robust simulation models of the relevant underground thermal, hydrological, mechanical, and chemical (THMC) processes. Screening such sites for an entire country involves running these simulation models for various parameter sets, on various scales, with various degrees of simplification. The data integration from different sources and post-processing and visualization of results are of equal importance as the models themselves.

The OpenWorkFlow project, funded by the Bundesgesellschaft für Endlagerung (BGE), aims at developing open source, automated, robust, quality assured simulation workflows to support the nuclear waste repository siting process in Germany. During the first project phase from 2021 to 2024 several key ingredients allowing to run such workflows and visualize their results have been developed. The workflows were developed for simulations at the scale of the geological barrier as well as for components of the repository system on the scale of disposal galleries. The simulations can be extended by methods for scenario analyses and uncertainty quantification. Both workflows and process models were tested in international code comparison projects as well as against experimental data, like for instance, from the Mt. Terri rock laboratory. As a specific example for analyses as part of representative preliminary safety investigations, a fully automated workflow for the thermal dimensioning of a nuclear waste repository has been developed. This workflow determines the required area for a repository for various parameter combinations. It has already been used in the siting process in Germany in practice. The second phase of the OpenWorkFlow project has started in January 2025. Until the end of 2029 the existing simulation workflows will be heavily extended to support the next phase of preliminary safety investigations and the analyses of the siting regions to be identified before the start of phase II of the site selection process.

The OpenWorkFlow project is closely linked to other international repository research projects, such as DECOVALEX 2027 and EURAD-2. As part of DECOVALEX, OWF is developing numerical methods and implementing benchmarking concepts for the quality-assured assessment of the barrier integrity of crystalline (see Mollaali et al. 2025) and clay rock (Buchwald et al. 2025). Within EURAD, models are being developed and tested for use in licensing procedures (Grunwald et al. 2025).

References:

Buchwald et al. (2025): The relevance of two-phase flow in the thermo-hydro-mechanical evolution of clay formations exposed to high temperatures by heat-emitting waste. Appl. Therm. Eng. 264 , art. 125379 10.1016/j.applthermaleng.2024.125379

Grunwald et al. (2025): Benchmarking strategy for the evaluation high gas pressure processes in clay host host rocks. submitted to SafeND

Mollaali et al. (2025): Numerical benchmarking of GREAT cell experiments: Poly-axial stress effects on fluid flow in fractured rock using smeared and discrete methods. Journal of Rock Mechanics and Geotechnical Engineering, submitted