Potential impact of cementitious leachates on the buffer porewater chemistry in the Finnish repository for spent nuclear fuel – A reactive transport modelling assessment

Cementitious materials will be used during the construction and operation of a geological repository for spent nuclear fuel at Olkiluoto in Finland. Upon contacting water, cement dissolution generate high-pH leachates, which might eventually reach proximity canister deposition hole through an interconnected network fractures crystalline host rock. Highly-alkaline conditions near bentonite buffer surrounding could affect performance safety functions. In this study, we evaluate potential impact such leachates have on chemical composition porewater over time-frames relevant assessment repository. Although comprehensive mechanistic these interactions is not possible due to their complexity, demonstrate that key processes can bounded. To end apply reactive transport modelling based analysis parameter values. The model considers 3D geometry including: canister, buffer, discrete fracture rock intersecting hole. Cement flow around hole, while solutes exchange with via advective diffusive mass transfer. Modelling results indicate combination restricted water within fracture, slow solute reactions act together minimise extent perturbation, should vicinity pessimistically estimates maximum pH variation below 0.1 unit, concentration change about factor 3 (relative initial) reacting components most volume. This only double perturbation values predicted “natural” evolution porewater, absence leachates. Model uncertainties are evaluated by series sensitivity cases. These calculations suggest additional processes, directly accounted base (such as leachate-groundwater mixing dilution network, porosity reduction mineral precipitation), significantly contribute further magnitude perturbation.