Sorption behavior of radionuclides in lamprophyre and granodiorite: implications for colloid-mediated transport in crystalline host rocks of Grimsel Test Site

The safe long-term disposal of high-level radioactive waste in deep geological repositories requires a thorough understanding of radionuclide (RN) migration at interfaces between host rocks and geoengineered barriers, such as bentonite, particularly under geochemically relevant conditions. As part of the international Colloid Formation and Migration (CFM) project [1], laboratory-scale experiments are being conducted to investigate interactions between RNs, bentonite colloids, and host rock materials under the geochemical conditions (pH = 9.6, Eh = -220 mV) of the Grimsel groundwater (GGW) system. Colloid-mediated RN migration must be assessed in the context of repository scenarios in crystalline host rock, where bentonite erosion and container failure are considered, and advective groundwater flow is assumed.

To our knowledge, this is the first attempt to study the sorption properties of lamprophyre dykes from the Grimsel Test Site (GTS), NAGRA’s generic underground research laboratory in crystalline rock, and to compare them with granodiorite [2] to assess their capacity to immobilize key RNs relevant to colloid-mediated transport, including U, Np, Pu, Am, and Tc.

A two-phase experimental approach is being implemented in parallel to assess various sorption properties of the rocks. In Phase 1, batch sorption experiments using powdered lamprophyre and granodiorite samples (<63 µm) are being conducted under controlled conditions. Characterization techniques, including XRD, XRF, BET, SEM-EDS imaging, and ICP-OES/MS, have been employed to determine the composition and surface properties of the materials. An essential aspect of this phase is the selection of an optimal method to maintain reducing in-situ conditions, which is crucial for studying the behavior of redox-sensitive RNs. Rongalite and hydrazine are currently being tested in batch experiments without RNs, with Eh, pH, and elemental composition monitored over time.

Phase 2 focuses on RN surface sorption behavior onto bulk rock material using autoradiography after equilibration with GGW in the presence and absence of bentonite colloids. Prior to these sorption experiments, micro-CT imaging was conducted to examine mineralogical differences between the two rock types, revealing a layered structure in lamprophyre and an isotropic mineral distribution in granodiorite. Following imaging, the rock samples were cut into representative sections, and SEM-EDS analyses are being used to further characterize their mineralogical composition. Sorption experiments and subsequent autoradiography are planned, with ICP-MS analysis being used to quantify residual RN concentrations in solution post-sorption.

Preliminary mineralogical and sorption results will be presented. The outcomes of this study will support the design and execution of an upcoming in-situ RN tracer test in a region of the GTS where granodiorite contains fractured lamprophyre intrusions. This will provide valuable data to quantify radionuclide and colloid retention/retardation properties on fractured granitic rock surfaces. In turn, these data will help parameterize models for assessing radionuclide migration in crystalline host rock systems.

Acknowledgements:

This study is supported by the BMUV funded EVIDENT Project “Erosion von Bentonit unter In-situ Bedingungen durch Einwirkung natürlicher Wässer in geologischen Tiefenlagern”. Förderkennzeichen: 02 E 12153B.

References:

[1] https://www.grimsel.com/gts-projects/cfm-section/cfm-introduction

[2] Huber, F., Kunze, P., Geckeis, H., Schäfer, T. Sorption reversibility kinetics in the ternary system radionuclide-bentonite colloids/nanoparticles-granite fracture filling material. Applied Geochemistry, 2011, 26(12), pp. 2226–2237