MOdelling of GAs behaviour and migration at the REpository scale storage/disposal (MOGARE)

Objective

To provide a benchmark for repository-scale gas transport modelling and the associated sensitivity analysis, to enhance the confidence in the numerical approaches used to simulate gas generation, gas migration, and gas pressure evolution, thereby supporting the assessment of long-term safety and the design and optimisation of the engineered barrier system (EBS), while fostering confidence among waste management organisations (WMOs), regulatory bodies, and the public in fit-for-purpose models.

Description of the WP

Predicting the fate of gases is a crucial element in assessing the long-term safety of a repository. The main challenges are determining which gas transport processes govern repository-scale behaviour, how the repository interacts with the host-rock at the system level, and finally how to reliably transfer the knowledge derived from laboratory and in-situ investigations to repository-scale performance assessment. 
In absence of experiments covering the spatial and time scales required for the performance assessment, the challenge is representing in a coherent numerical model the scales across which the physical processes take place: from millimetres and seconds in case of localised gas flow to the hundreds of meter and thousands of years to capture gas diffusion and advection away from the repository into the host-rock. 
Modelling the fate of relevant gases at the repository scale requires then specific strategies, on one hand considering the complete architecture of the repository and the properties of the different materials and the complex physical processes happening, while on the other hand it must adopt appropriate simplifying assumptions to render the problem computationally tractable, including decisions on process parametrization, mesh generation, numerical methods, and up‑scaling techniques.
The WP will address the research questions regarding:
1.    Process representativity. Identifying which physical processes and which elements of the EBS need representation in repository scale modelling and which can be neglected or captured by simplified constitutive relationships while maintaining an accurate representation of the relevant gas transport processes.
2.    Upscaling. Establishing transparent links between laboratory observations (e.g. localised gas advection), constitutive relationships and repository-scale performance assessment
3.    Design and safety relevance. The methodologies developed will support repository design optimization and improve confidence in long-term safety assessments
In the recent past, two European projects were devoted to the issue of gases in deep underground radioactive waste repositories: FORGE (2009-2013) and EURAD-GAS (2019-2024). However, in these projects, the tasks dedicated to performance assessment at the repository scale were designed to arrive downstream of the experimental results produced by the other tasks. These projects therefore did not allow the development of independent and self-supporting performance evaluation programs at the scale of a complete repository. The gap between the generated physical process understanding on one side and reliable design recommendations on the repository scale on the other side will therefore be closed by this WP.
Strategies and approaches for the representation of small-scale processes at the repository scale will be evaluated, assessing the possible benefits of the latest developments in numerical aspects (surrogate models, digital twins representations). General numerical approaches to optimize spatial and temporal discretization will also be addressed. The aim will be to consolidate the choices and models used to represent gas transport at this scale, which can improve post-closure safety and the EBS design process. 
Additionally, the benchmarking activity will establish a common understanding of model capabilities and limitations, enabling the stakeholders (WMOs, regulatory bodies and society at large) to gain mutual confidence in the consistency and credibility of the different numerical approaches.

Outcomes

Scientific Insight:
-    Assessment of the relevance of processes driving the gas pressure build-up in deep underground repositories in clay host rocks;
-    Evaluation of the relevance of processes driving the gaseous and soluble radionuclides transfer in deep underground repositories;
-    Discussion of the relevance of numerical approaches (geometric and physical simplifications, meshing strategy, homogenisation…) driving the two-phase flow evaluations in deep underground repositories
-    Assessment of sensitivity analysis approaches, both quantitative and rank-based, to repository model.
-    Discussion of transferability of recommendations in relation to different repositories (shallower, low and intermediate level-waste) as well as to the relevant gas-transport properties of different host rocks.
Knowledge Management:
-    Training activities: a joint workshop with WP GEOSCALE, a summer doctoral school and a seminar on previous large scale repository modelling work with a focus on gas are in the planning.
-    Benchmark definition: to gain confidence in the consistency and credibility of the current and future numerical approaches;
-    Determination of elements for a shared strategy on how to manage gas for a deep underground repository at WMO level.
Implementation Safety:
-    Better understanding of gas processes and characteristic time used to define gas linked scenarios at repository scale;
-    Better understanding of the impact of design choices of the repository w.r.t. maximum gas pressures, to support both a robust performance assessment and to allow for performance optimization;
-    Increase the confidence in gas linked scenarios in clay host rocks;