Transient modelling of sulphide diffusion under conditions typical of a deep geological repository

Abstract

ABSTRACTThe Canadian Nuclear Waste Management Organization (NWMO) has chosen a used fuel container (UFC) design that consists of an inner steel core and outer copper coating that resists corrosion in the anaerobic underground. However, in these anaerobic conditions, microbiologically influenced corrosion (MIC) can contribute to UFC corrosion if sulphides are present in the repository. Therefore, an engineered barrier system (EBS) consisting of bentonite blocks and pellets has been designed to inhibit production of sulphide by microorganisms and impede the movement of sulphides from the host rock to the UFC. Initially, the bentonite within the deep geological repository will contain no sulphides and it will take time for sulphide from the host rock to saturate the repository. To examine the transient effect of sulphide transport and the effectiveness of the EBS, a 3D numerical model was developed capable of simulating the diffusive transport of sulphide within the NWMO repository. The model was developed using COMSOL Multiphysics, a finite element software package. The developed model included the NWMO placement room, as well as a stand-alone UFC package. Conservative assumptions such as constant-concentration boundary condition and transport-limited corrosion were assumed. The time-dependent analysis predicted the inflow of sulphide into the EBS over time, and the results showed that a steady-state condition would be reached in over 2,000 years in the placement room. The model also showed that the highest sulphide flux occurred at the semi-spherical end caps of the UFC and consequently, that corrosion from sulphide diffusion would not be uniform over the container surface.This paper is part of a supplement on the 6th International Workshop on Long-Term Prediction of Corrosion Damage in Nuclear Waste Systems.