Bentonite buffers at temperatures beyond 100 °C could reduce the amount of high-level radioactive waste in a deep geological repository. However, it is necessary to demonstrate that the buffer surrounding the canisters withstands such elevated temperatures, while maintaining its safety functions (regarding long-term performance). For this reason, an experiment with thermal loading of bentonite powder at 150 °C was arranged. The paper presents changes that the Czech Mg/Ca bentonite underwent during heating for one year. These changes were examined by X-ray diffraction (XRD), thermal analysis with evolved gas analysis (TA-EGA), aqueous leachates, Cs sorption, cation exchange capacity (CEC), specific surface area (SSA), free swelling, saturated hydraulic conductivity, water retention curves (WRC), quantitative polymerase chain reaction (qPCR), and next-generation sequencing (NGS). It was concluded that montmorillonite was partially altered, in terms of the magnitude of the surface charge density of montmorillonite particles, based on the measurement interpretations of CEC, SSA, and Cs sorption. Montmorillonite alteration towards low- or non-swelling clay structures corresponded well to significantly lower swelling ability and water uptake ability, and higher saturated hydraulic conductivity of thermally loaded samples. Microbial survivability decreased with the thermal loading time, but it was not completely diminished, even in samples heated for one year.

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The study deals with sorption and diffusion behaviour of strontium in Czech bentonite B75. The study is a part of a research on reactive transport of radioactive contaminants in barrier materials of a deep geological repository of radioactive waste in the Czech Republic. Series of sorption and diffusion experiments with Sr and non-activated Ca bentonite B75 produced in the Czech Republic were performed in two background solutions (CaCl 2 and NaCl). On the basis of sorption batch experiments the kinetics of strontium sorption on bentonite was assessed and the sorption isotherms for various experimental conditions were obtained. As a result of performed diffusion experiments the parameters of diffusion (i.e. effective diffusion coefficient D e and apparent diffusion coefficient D a ) were determined. The observed discrepancies between sorption characteristics obtained from the sorption and diffusion experiments are discussed.

<p>The EURAD European Joint Project (www.ejp-eurad.eu) is a European project supported by the European Commission via the Horizon 2020 framework programme. WP 5: Fundamental understanding of radionuclide retention (FUTURE) concerns the quantification of the long-term retention of key radionuclides in solid phases aimed at developing models of reactive transport in the host rock in close cooperation with the Development and improvement of numerical methods (DONUT) WP. The Mobility of radionuclides in crystalline rock Task of the WP is aimed at the observation of the retention of DGR-relevant radionuclides in a crystalline host rock fracture filling and the evaluation of its contribution to the safety function of the crystalline host rock. A calcite fracture filling, nickel as the radionuclide of interest and caesium as the reference radionuclide were selected so as to allow for the study of sorption processes in the crystalline fracture environment. Samples of a natural calcite fissure infill and migmatite host rock extracted from the Bukov URF, Czech Republic, were distributed to the various project partners for experimental research purposes. Batch sorption experiments were subsequently performed on both materials using Ni, Cs and other elements in synthetic ground water and CaCl<sub>2</sub> for the calcite. The results of Ni sorption on natural calcite revealed a lower level of retention than that of the surrounding host rock (migmatite).</p>

<p>As the final barrier of the multi-barrier deep geological repository (DGR) for radioactive waste (RAW)  the rock environment fulfils the primary safety function by limiting the transport of radionuclides to the biosphere via the low hydraulic conductivity of the rock mass compared to other rock massifs (1). Moreover, the various properties and characteristics of the rock environment comprise important considerations with respect to the DGR safety assessment.</p><p>Samples of the various types of igneous and metamorphic rocks present in the Bohemian Massif were collected as part of the Research Support for the Safety Assessment of the DGR project (SURAO). The study of the rock materials also included that of the fracture fillings, the characteristics of which supplemented the input data set for the future DGR safety assessment. All the rock samples were subjected to both mineralogical (X-ray analysis) and petrological characterisation (2).</p><p>Fracture fillings (e.g. clay minerals, biotite, Fe oxyhydroxides, calcite) generally evince higher specific surface areas and cation exchange capacities than do the rocks themselves, i.e. properties that are able to significantly influence the sorption of radionuclides</p><p>The sorption experiments performed with radionuclides revealed differing degrees of sorption on the rock and fracture filling samples (e.g. <sup>134</sup>Cs, <sup>85</sup>Sr, U, Se). The initial experiments on the fracture filling materials determined that their presence can to significantly enhance the capture of radionuclides (e.g. <sup>134</sup>Cs) during their migration towards the biosphere, and thus to enhance the safety function of the rock environment (2).</p><p>The diffusion characteristic values were determined experimentally using the through diffusion method (2). With respect to the diffusion characteristics (the effective diffusion coefficient D<sub>e</sub>), although the samples were taken from different parts of the Czech Republic and from differing rock types, the effective diffusion coefficients were found to lie within a relatively narrow range: for <sup>3</sup>H (4–10) · 10<sup>−13</sup> m<sup>2</sup> s<sup>−1</sup>, for <sup>36</sup>Cl (1–10) · 10<sup>−13</sup> m<sup>2</sup> s<sup>−1</sup> and for <sup>125</sup>I (1–4) · 10<sup>−13</sup> m<sup>2</sup> s<sup>−1</sup>. Anionic exclusion was demonstrated for the metamorphic rock samples, which led to the determination of lower D<sub>e</sub> values for <sup>36</sup>Cl and <sup>125</sup>I in comparison to  <sup>3</sup>H (2)</p><p>The experimental results were determined as part of the Research Support for the Safety Assessment of the Deep Geological Repository project, financed by SURAO (SO2014-061-01), and the EURAD WP FUTURE project.</p>

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