Fracture smectites in bedrock at the Kivetty and Romuvaara sites, Finland, as natural analogues for radioactive waste repository buffers

Abstract

Bentonite is used in various geological repository designs for highly radioactive and low- and intermediate-level waste. Stability of such materials has been evaluated in long-term safety assessments, with recent emphasis on potential chemical erosion of these clays in fresh groundwater conditions. The fracture-filling mineralogy at two investigation sites, Kivetty and Romuvaara, in Finland shows the existence of different types of smectites characterized by their respective dominant elements, such as Mg-, Fe- and Na–Ca-rich smectites. Of these, Ca- and Na-smectites are likely to be montmorillonites that provide potential natural analogues for the bentonite buffers and their long-term performance in fractured bedrock environments. The smectite minerals observed reflect the local parent rock mineral composition, suggesting in situ formation during hydrothermal alteration, well predating the current freshwater conditions in the fractured bedrock. These observations support the stability of smectites, including Na- and Ca-montmorillonites, in the fractured bedrock of glaciated terrain with fresh continental groundwater extending to repository-relevant depths. Further, no erosion or sedimentation processes have been observed to have taken place in the fracture systems studied. However, uncertainties remain related to detailed exchangeable cation compositions of the smectite. Current groundwater systems at both sites would alter the composition of montmorillonite towards the Ca-rich form, and more detailed investigations are required to assess the stability of Na-montmorillonite in connection to open fractures. In general, bentonite alteration in fractures to Ca-montmorillonite would be beneficial in repository scenarios that consider potential chemical erosion by dilute groundwater. Supplementary material: Supplementary material 1 (XCT data for fracture smectite samples from Kivetty and Romuvaara) are available at https://doi.org/10.23729/1ce75faa-d52a-43cd-af40-3961b3081f70 [last accessed 5 July 2023] and Supplementary material 2 (XRD diffractogram data) and Supplementary material 3 (SEM-EDS data) are available at https://doi.org/10.6084/m9.figshare.c.7126477 Thematic collection: This article is part of the Sustainable geological disposal and containment of radioactive waste collection available at: https://www.lyellcollection.org/topic/collections/radioactive