Reactivity of chlorite and gneiss from ONKALO (Western Finland): Laboratory experiments and reactive transport modeling

The Finnish geological repository for spent nuclear fuel is being constructed at the ONKALO® underground facility in Olkiluoto Island. The bedrock is mainly migmatitic gneiss, including brittle fault zones where groundwater flows. An important process is the dissolution of chlorite, which releases Fe(II) that can lead to FeS precipitates and regulate dissolved HS − concentrations. Elevated HS − concentrations could induce corrosion of the copper canisters containing the spent fuel. Two powder chlorite samples from faults in the gneiss and provided by Posiva Oy (Finnish nuclear waste management company) were characterized. They contained both muscovite and chlorite. Laboratory batch and flow-through experiments were performed to study the solubilities and dissolution reaction rates of the chlorite (and muscovite) samples. The results were consistent with previous results reported in the literature. Two core-infiltration experiments were also performed, using a gneiss sample containing a fracture (also provided by Posiva Oy). The results were interpreted by means of one- and two-dimensional reactive transport models. The 1D model included flow, solute transport and reaction only along the fracture. It needed unrealistically large mineral surface areas to reproduce the experimental results. In addition to the fracture, the 2D model included transport by diffusion in the rock matrix, with mineral reactions in both fracture and matrix. With this approach the large surface areas in the fracture were no longer needed, highlighting the reactivity of the rock matrix despite the small porosities (about 1 %) and diffusivities. The calculated dissolution of chlorite could indeed release Fe(II) under repository conditions. • Chlorite and muscovite from fractures at ONKALO were characterized. • Solubilities and dissolution rates are consistent with previous literature. • Fractured-core infiltration experiments were also performed. • Modeling results highlight matrix reactivity (despite the small porosity).