Overview of the DECOVALEX Initiative - Building Confidence Via Model Comparison

Here we provide an overview of an international research collaboration for advancing the understanding and modeling of coupled thermo-hydro-mechanical-chemical (THMC) processes in geological systems. The creation of the international DECOVALEX Initiative, now running for more than 30 years, was motivated by the recognition that prediction of these coupled effects is an essential part of demonstrating the performance and safety of radioactive waste disposal and other subsurface engineering applications. DECOVALEX emphasizes joint analysis and comparative modeling of state-of-the-art field and laboratory experiments, across a range of host rock options and repository designs. Participating research teams are from radioactive waste management and other organizations, national research institutes, regulatory agencies, universities, as well as industry and consulting groups, providing a wide range of perspectives and solutions to these complex problems. The most recent phase of the initiative, referred to as DECOVALEX-2023, started in 2020 and ended in December 2023. Modeling teams from 17 international partner organizations participated in the comparative evaluation of seven modeling tasks involving complex experimental and modeling challenges. The presentation provides examples of research contributions made within DECOVALEX-2023 and illustrates how these have helped building confidence in long-term performance predictions. These examples range from the modeling of large-scale in situ heater tests representing mock-ups of nuclear waste disposal tunnels, the analysis of gas transport tests in clay-based materials, the prediction thermal and gas pressure rock fracturing, to the comparison of performance assessment models. The main characteristic of DECOVALEX is the close collaboration on analyzing and simulating state-of-the-art field and laboratory experiments, which provides a wide range of experiences, perspectives and solutions to these complex problems and allows for detailed comparison of analysis and modeling results. Much insight can be gained through this cooperative comparison of results from different research teams using different model approaches, not only on the effects of complex THMC processes, but also on the strengths, weaknesses, and adequacies of the various approaches and predictive models used by these research teams. We make the case in this presentation that DECOVALEX has contributed, and continues to contribute, to enhancing confidence in the technical adequacy of radioactive waste disposal, by improving our collective understanding of complex subsurface perturbations and coupled processes, by developing and comparing predictive models for these processes, by evaluating their uncertainties, by recognizing areas for additional research, and by emphasizing means of learning from each other and knowledge sharing. The insight and scientific knowledge gained would not have been possible if individual groups had studied these data alone rather than within a truly collaborative setting.