Influence of Hyper-Alkaline pH Leachate on Mineral and Porosity Evolution in the Chemically Disturbed Zone Developed in the Near-Field Host Rock for a Nuclear Waste Repository

This paper evaluates the effect of hyper-alkaline (NaOH/KOH) leachate on the mineralogy and porosity of a generic quartzo-feldspathic host rock for intermediate- and low-level nuclear waste disposal following permeation of the cementitious repository barrier by groundwater. The analysis is made with reference to expected fluid compositions that may develop by contact of groundwater with the cementitious barrier to form a chemically disturbed zone (CDZ) in the adjacent host rock, as informed by relevant natural analogue sites. Theoretical analysis and numerical modelling is used to explore the influence of different host rock mineral assemblages on changes in pore fluid chemistry, multiple mineral dissolution and precipitation reactions and matrix porosity within the CDZ under these conditions. The numerical modelling accounts for kinetic and surface area effects on the mineral transformation and porosity development for periods of up to 10,000 years travel time from the repository and ambient temperature of 20∘C. The analysis shows that dissolution of quartz, feldspar and muscovite in the host rock, by the hyper-alkaline waste leachate, will create relatively high concentrations of dissolved Si and Al in the pore fluid, which migrates as chemical fronts within the CDZ. Precipitation of secondary mineral phases is predicted to occur under these conditions. The increase in matrix porosity that arises from dissolution of primary aluminosilicate minerals is compensated by a reduction in porosity due to precipitation of the secondary phases, but with a net overall increase in matrix porosity. These coupled physical and geochemical processes are most important for contaminant transport in the near-field zone of the CDZ and are eventually buffered by the host rock within 70 m of the repository for the 10,000 year travel time scenario. The predicted changes in matrix porosity may contribute to increased transport of radionuclides in the host rock, in the absence of attenuation by other mechanisms in the CDZ.