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

Abstract

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