This work presents the conceptual approach and preliminary results obtained from modelling sulfide cycling and its impact on the chemical stability of the copper waste canister in the near-field of a geological repository for high-level radioactive waste. Reactive transport modelling allows the study of a complex system of geo-bio-chemical reactions, including relevant feedback among them as well as their coupling to transport processes. The presented approach enables quantitative understanding of system controlling processes, allows canister corrosion rates from sulfide fluxes to be estimated, and may be used as a tool in decision making regarding the design of the engineered barrier system of a waste repository.