Assessment of groundwater chemical evolution for a spent nuclear fuel repository under prolonged temperate conditions: an application of efficient coupled groundwater flow and reactive transport simulation

Abstract:

SKB has submitted a license application for a spent nuclear fuel repository at Forsmark sited in crystalline rocks of the Fennoscandian shield. In support of this application various quantitative assessments were made to demonstrate the long-term safety of the proposed repository. One such assessment involved simulation of groundwater chemical evolution to quantify impacts on safety functions for the disposal system related to the geochemical conditions, particularly salinity, pH and redox conditions. In the reference case the current temperate period lasts until 12,000 AD. A case of prolonged meteoric infiltration to 60,000 AD is also considered resulting from e.g. global warming. This is to fulfil a regulatory request to assess whether extended dilute water infiltration might lead to a rise in redox potential and also to an increase in erosion of the bentonite barrier due to formation of colloids. In order to perform long transient simulations of groundwater flow and solute transport with water-solute-rock interactions, new tools have been developed to closely couple geochemical, groundwater flow and transport calculations, and perform these efficiently using parallel computing techniques. In assessing this case, sensitivities are tested to the geochemical reaction schemes appropriate to the site. The results of this work predict that the chemical environment at repository depth stabilises at around 20,000 AD and shows little change beyond that. The salinity of the groundwater is governed by the low permeability (c. 10^-19 m²) of the bedrock and by rock matrix diffusion, resulting in relatively shallow and slow circulation of groundwater. The chemical reactions influence concentrations of reactive species, the calculated pH and redox potential. In particular, the redox reactions thought to be relevant for the Forsmark site maintain reducing conditions at repository depth, even with infiltration at the ground surface of meteoric water with relatively high redox potential.