A simulator for modeling of porosity and permeability changes in near field sedimentary host rocks for nuclear waste under climate change influences

A new simulation tool is developed to model coupled thermo-hydro-mechanical-geochemical (THM-GeoC) processes that would occur in the near field of deep geological repositories (DGRs) for nuclear wastes, and their impacts on the evolution of the rock porosity and permeability. First, a coupled thermo-hydro-mechanical-chemical (THMC) model, in which, the chemical (C) process is limited to solute transport, is developed and then implemented into COMSOL Multiphysics finite element code. Then, two types of numerical software are coupled; the first is COMSOL Multiphysics code and the second is the PHREEQC geochemical code. The coupling of the two types of software is performed by developing a special code that has been written by using MATLAB. COMSOL Multiphysics is used to solve the coupled THMC processes (the C process is limited to solute transport) and PHREEQC is used to solve the geochemical reactions resultant of the transport of chemical species. Simulation results obtained by using the THM-GeoC simulator are compared with experimental data and data from modeling reactive transport, with good agreement in the results. The developed simulator is applied to investigate the coupled effect of climate changes and water enriched with carbon dioxide gas, which would be generated from low and intermediate nuclear wastes, on the dissolution of the limestone host rock in Ontario (Canada) for nuclear wastes, and porosity and permeability changes within the near field rock. The results show that the maximum change in porosity is approximately 3.5%, with a gradual decrease to approximately zero. The zone affected by the dissolution process is mainly located on the first 10 m within the host rock and does not cause a significant increase in permeability. From safety and environmental assessment perspectives, the impact of dissolution is not significant. However, parametric studies and experimental investigations need to be implemented to support the predicted results.