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Uplifting fractured granitic rocks occur in substantial areas of countries such as Japan. Some of these areas might be considered when siting a deep geological repository for radioactive wastes. A repository site would be selected in such an area only if it is possible to make a safety case, accounting for the changing conditions during uplift. The safety case must include robust arguments that chemical processes in the rocks around the repository will contribute sufficiently to minimise radiological doses to biosphere receptors. Numerical modelling is an important aspect of making these arguments. To provide confidence in the safety arguments, numerical models need to be sufficiently realistic, but also parameterised conservatively (pessimistically). However, model development is challenging because uplift involves many complex couplings between groundwater flow, chemical reactions between water and rock, and changing rock properties. The couplings would affect radionuclide mobilisation and retardation, by influencing diffusive radionuclide fluxes between groundwater flowing in fractures and effectively immobile porewater in the rock matrix (rock matrix diffusion, RMD) and radionuclide partitioning between water and solid phases, via: (i) mineral precipitation/dissolution; (ii) mineral alteration; and (iii) sorption/desorption. It is difficult to represent all this complexity in numerical models while showing that they are parameterised conservatively. Here we present a modelling approach, illustrated by simulation cases for some exemplar radioelements, to identify realistically conservative process conceptualisations and model parameterisations., Competing Interests: Declaration of competing interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: All authors report financial support was provided by Japan Ministry of Economy Trade and Industry, as part of “The project for validating near-field system assessment methodology in geological disposal (FY2019-2022, JPJ007597)”., (Copyright © 2024 Elsevier B.V. All rights reserved.)

The multi-barrier deep geological repository system is currently considered as one of the safest option for the disposal of high-level radioactive wastes. Indigenous microorganisms of bentonites may affect the structure and stability of these clays through Fe-containing minerals biotransformation and radionuclides mobilization. The present work aimed to investigate the behavior of bentonite and its bacterial community in the case of a uranium leakage from the waste containers. Hence, bentonite microcosms were amended with uranyl nitrate (U) and glycerol-2-phosphate (G2P) and incubated aerobically for 6 months. Next generation 16S rRNA gene sequencing revealed that the bacterial populations of all treated microcosms were dominated by Actinobacteria and Proteobacteria, accounting for >50% of the community. Additionally, G2P and nitrate had a remarkable effect on the bacterial diversity of bentonites by the enrichment of bacteria involved in the nitrogen and carbon biogeochemical cycles (e.g. Azotobacter). A significant presence of sulfate-reducing bacteria such as Desulfonauticus and Desulfomicrobium were detected in the U-treated microcosms. The actinobacteria Amycolatopsis was enriched in G2P‑uranium amended bentonites. High-Angle Annular Dark-Field Scanning Transmission Electron Microscopy analyses showed the capacity of Amycolatopsis and a bentonite consortium formed by Bradyrhizobium-Rhizobium and Pseudomonas to precipitate U as U phosphate mineral phases, probably due to the phosphatase activity. The different amendments did not affect the mineralogy of the bentonite pointing to a high structural stability. These results would help to predict the impact of microbial processes on the biogeochemical cycles of elements (N and U) within the bentonite barrier under repository relevant conditions and to determine the changes in the microbial community induced by a uranium release. Unlabelled Image • U and G2P exposure shaped the bentonite bacterial community under DGR concept. • Stability of bentonite mineralogy was showed. • Bacteria with impact on the biogeochemical cycle of U and N were enriched. • U phosphate biomineralization by bentonite isolates was demonstrated. [ABSTRACT FROM AUTHOR]

Deep geological repository is considered the internationally accepted method for spent fuel (SF) disposal. In countries where salt, clay, tuff and granite are unavailable at geologically suitable area, other rock types may come into consideration. In Israel, carbonate rocks make up a significant portion of the surface and subsurface lithologies, thus, low permeability carbonates were evaluated as possible host rocks for a repository, and for an interim storage facility. Sorption and retardation capacity of SF components to low permeability carbonate rocks were evaluated using their chemical simulants. Strontium and Cs represent components that may leach during interim storage, while U and Ce (as a simulant for redox-active actinides) represent components that may leach under repository conditions. Rocks from the Upper Cretaceous Mount Scopus Group were sampled from boreholes at the Yamin Plateau, Israel. Single point batch experiments were conducted with synthetic rainwater spiked with tracers and interacted with five rock types of various particle sizes at 25 °C. Results were evaluated using the LeachXS™-ORCHESTRA geochemical speciation and data management program. Cerium removal was found to be related to the HCO 3 – concentration in solution, where Ce precipitated as Ce 2 (CO 3) 3 ·XH 2 O and as an amorphous carbonate phase. Removal of Cs and Sr was controlled by clays. No Sr co-precipitation as carbonate species was observed. Uranium was removed mainly by sorption onto solid organic matter, whereas clays had no significant role in U sorption. Iron-(hydr) oxides may have also played a role in U removal. Calculated partition coefficients for U, Cs, and Sr were in the order of 101–102 mL/g. Grain size had no significant effect on the retention capacity of the studied rocks due to similar effective surface area. The current study indicates that a repository or an interim storage facility within carbonate rocks, would provide only partial isolation of radionuclides from the environment, hence, additional engineered barriers may be required. Unlabelled Image • Low permeability carbonates evaluated for Spent Fuel repository & interim storage • Batch experiments of synthetic rainwater & 5 rock types doped with Ce, Cs, Sr & U • Cerium precipitated as Ce 2 (CO 3) 3 ·XH 2 O and as an amorphous carbonate • Cs and Sr removed by clays, U sorbed onto organic matter and Fe-(hydr)oxides • Spent Fuel storage within carbonate rocks may require accessory engineered barriers. [ABSTRACT FROM AUTHOR]