Migration Behaviour of Strontium in Czech Bentonite Clay

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 (CaCl2 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 De and apparent diffusion coefficient Da) were determined. The observed discrepancies between sorption characteristics obtained from the sorption and diffusion experiments are discussed

Modelling of the LTDE-SD radionuclide diffusion experiment in crystalline rock at the Äspö Hard Rock Laboratory (Sweden)

Acknowledgement. The comments from Dr. Kersti Nilsson, the analytical work by VKTA (Dresden, Germany) for some of the rock samples, and the initial review by Dr. Anna-Maria Jakobsson are gratefully acknowledged. The constructive comments and suggestions by Dr. Jordi Cama and an anonymous reviewer contributed to a significant improvement of the manuscript. Funding was provided through the Task Force partner organisations participating in this modelling exercise (SKB, Sweden; POSIVA OY, Finland; SÚRAO, Czech Republic; BMWi, Germany; KAERI, Republic of Korea; NUMO and JAEA, Japan). IDAEA-CSIC is a Severo Ochoa Centre of Research Excellence (Spanish Ministry of Science and Innovation, Project CEX2018-000794-S). The contributions of TUL, CTU and PROGEO are the result of the SÚRAO project "Research support for Safety Evaluation of Deep Geological Repository". JAEA's modelling was performed as a part of "The project for validating assessment methodology in geological disposal system" funded by the METI of Japan. A.I., P.T., M.V., G.D., and D.B. gratefully acknowledge the computing time granted by the JARA Vergabegremium and provided on the JARA Partition part of the supercomputer JURECA at Forschungszentrum Jülich.This study shows a comparison and analysis of results from a modelling exercise concerning a field experiment involving the transport and retention of different radionuclide tracers in crystalline rock. This exercise was performed within the Swedish Nuclear Fuel and Waste Management Company (SKB) Task Force on Modelling of Groundwater Flow and Transport of Solutes (Task Force GWFTS).Task 9B of the Task Force GWFTS was the second subtask within Task 9 and focused on the modelling of experimental results from the Long Term Sorption Diffusion Experiment in situ tracer test. The test had been performed at a depth of about 410m in the Äspö Hard Rock Laboratory. Synthetic groundwater containing a cocktail of radionuclide tracers was circulated for 198 days on the natural surface of a fracture and in a narrow slim hole drilled in unaltered rock matrix. Overcoring of the rock after the end of the test allowed for the measurement of tracer distribution profiles in the rock from the fracture surface (A cores) and also from the slim hole (D cores). The measured tracer activities in the rock samples showed long profiles (several cm) for non- or weakly-sorbing tracers (Cl-36, Na-22), but also for many of the more strongly-sorbing radionuclides. The understanding of this unexpected feature was one of the main motivations for this modelling exercise. However, re-evaluation and revision of the data during the course of Task 9B provided evidence that the anomalous long tails at low activities for strongly sorbing tracers were artefacts due to cross-contamination during rock sample preparation. A few data points remained for Cs-137, Ba-133, Ni-63 and Cd-109, but most measurements at long distances from the tracer source (>10mm) were now below the reported detection limits.Ten different modelling teams provided results for this exercise, using different concepts and codes. The tracers that were finally considered were Na-22, Cl-36, Co-57, Ni-63, Ba-133, Cs-137, Cd-109, Ra-226 and Np-237. Three main types of models were used: i) analytical solutions to the transport-retention equations, ii) continuum-porous-medium numerical models, and iii) microstructure-based models accounting for small-scale heterogeneity (i.e. mineral grains, porosities and/or microfracture distributions) and potential centimetre-scale fractures. The modelling by the different teams led to some important conclusions, concerning for instance the presence of a disturbed zone (a few mm in thickness) next to the fracture surface and to the wall of the slim hole and the role of micro-fractures and cm-scale fractures in the transport of weakly sorbing tracers. These conclusions could be reached after the re-evaluation and revision of the experimental data (tracer profiles in the rock) and the analysis of the different sets of model results provided by the different team

Modelling of the LTDE-SD radionuclide diffusion experiment in crystalline rock at the Äspö Hard Rock Laboratory (Sweden)

This study shows a comparison and analysis of results from a modelling exercise concerning a field experiment involving the transport and retention of different radionuclide tracers in crystalline rock. This exercise was performed within the Swedish Nuclear Fuel and Waste Management Company (SKB) Task Force on Modelling of Groundwater Flow and Transport of Solutes (Task Force GWFTS). Task 9B of the Task Force GWFTS was the second subtask within Task 9 and focused on the modelling of experimental results from the Long Term Sorption Diffusion Experiment in situ tracer test. The test had been performed at a depth of about 410m in the Äspö Hard Rock Laboratory. Synthetic groundwater containing a cocktail of radionuclide tracers was circulated for 198 days on the natural surface of a fracture and in a narrow slim hole drilled in unaltered rock matrix. Overcoring of the rock after the end of the test allowed for the measurement of tracer distribution profiles in the rock from the fracture surface (A cores) and also from the slim hole (D cores). The measured tracer activities in the rock samples showed long profiles (several cm) for non-or weakly-sorbing tracers (Cl-36, Na-22), but also for many of the more strongly-sorbing radionuclides. The understanding of this unexpected feature was one of the main motivations for this modelling exercise. However, re-evaluation and revision of the data during the course of Task 9B provided evidence that the anomalous long tails at low activities for strongly sorbing tracers were artefacts due to cross-contamination during rock sample preparation. A few data points remained for Cs-137, Ba-133, Ni-63 and Cd-109, but most measurements at long distances from the tracer source (>10mm) were now below the reported detection limits. Ten different modelling teams provided results for this exercise, using different concepts and codes. The tracers that were finally considered were Na-22, Cl-36, Co-57, Ni-63, Ba-133, Cs-137, Cd-109, Ra-226 and Np-237. Three main types of models were used: i) analytical solutions to the transport-retention equations, ii) continuum-porous-medium numerical models, and iii) microstructure-based models accounting for small-scale heterogeneity (i.e. mineral grains, porosities and/or microfracture distributions) and potential centimetre-scale fractures. The modelling by the different teams led to some important conclusions, concerning for instance the presence of a disturbed zone (a few mm in thickness) next to the fracture surface and to the wall of the slim hole and the role of micro-fractures and cm-scale fractures in the transport of weakly sorbing tracers. These conclusions could be reached after the re-evaluation and revision of the experimental data (tracer profiles in the rock) and the analysis of the different sets of model results provided by the different teams.The comments from Dr. Kersti Nilsson, the analytical work by VKTA (Dresden, Germany) for some of the rock samples, and the initial review by Dr. Anna-Maria Jakobsson are gratefully acknowledged. The constructive comments and suggestions by Dr. Jordi Cama and an anonymous reviewer contributed to a significant improvement of the manuscript. Funding was provided through the Task Force partner organisations participating in this modelling exercise (SKB, Sweden; POSIVA OY, Finland; SÚRAO, Czech Republic; BMWi, Germany; KAERI, Republic of Korea; NUMO and JAEA, Japan). IDAEA-CSIC is a Severo Ochoa Centre of Research Excellence (Spanish Ministry of Science and Innovation, Project CEX2018-000794-S). The contributions of TUL, CTU and PROGEO are the result of the SÚRAO project “Research support for Safety Evaluation of Deep Geological Repository”. JAEA’s modelling was performed as a part of “The project for validating assessment methodology in geological disposal system” funded by the METI of Japan. A.I., P.T., M.V., G.D., and D.B. gratefully acknowledge the computing time granted by the JARA Vergabegremium and provided on the JARA Partition part of the supercomputer JURECA at Forschungszentrum Jülich.Peer reviewe

Cesium uptake from aqueous solutions by bentonite: A comparison of multicomponent sorption with ion-exchange models

The removal of cesium from concentrated aqueous solutions into Ca/Mg-bentonite for a wide range of bentonite-to-water (m/V) ratios was studied experimentally and theoretically. Using the batch technique, the equilibrium of Cs uptake was measured. The nonlinear character of cesium sorption substantially influenced by the m/V ratio was observed. The experimental data were evaluated using the multicomponent Langmuir isotherm and an ion-exchange model based on the ion-exchange reaction between Cs+ and M2+ (Ca2+/Mg2+) initially sorbed on bentonite. Constants k(1,Cs)= 0.521 mmol·g(-1), k(2,Cs)= 968 L·mol(-1), and k(2,M)= 592 L·mol(-1) were obtained for Cs uptake described by multicomponent Langmuir isotherm. For the ion-exchange model, the thermodynamic equilibrium constant K = 75.5 mL·g(-1) with a standard deviation of s(K)= 17.4 mL·g(-1) was determined. Using the t test, the calculated data of multicomponent Langmuir and ion-exchange isotherms were fit to experimental data, and the best agreement was obtained for the ion-exchange model. The results show that Cs uptake by bentonite could be substantially decreased in systems with a high bentonite-to-water (m/V) ratio as a consequence of the presence of desorbed divalent cations in the liquid phase

Modelling of acid-base titration curves of mineral assemblages

The modelling of acid-base titration curves of mineral assemblages was studied with respect to basic parameters of their surface sites to be obtained. The known modelling approaches, component additivity (CA) and generalized composite (GC), and three types of different assemblages (fucoidic sandstones, sedimentary rock-clay and bentonite-magnetite samples) were used. In contrary to GC-approach, application of which was without difficulties, the problem of CA-one consisted in the credibility and accessibility of the parameters characterizing the individual mineralogical components

Cesium uptake by Ca/Mg bentonite: evaluation of sorption experiments by a multicomponent two-site ion-exchange model

Cesium sorption on Czech Ca/Mg bentonite was studied using the batch technique for a wide range of both bentonite-to-water ratios (m/V) and initial concentrations of Cs. The experiments revealed non-linear pattern of cesium sorption, which was substantially influenced by the m/V. The results of the exchange reaction between added Cs+ and Mg2+, Ca2+, Na+ and K+ ions present naturally on the bentonite sorption sites enabled to determine the appropriate selectivity coefficients. The capacity of sites with higher selectivity to cesium, which is believed to be associated with the frayed-edge sites, was measured using the AgTU method.Web of Science304143442

Comparative study of radium and strontium behaviour in contact with cementitious materials

A comparative study of the sorption behaviour of radium and strontium was performed on various cementitiousmaterials including crushed hardened cement pastes (HCP) and concretes as well as a synthesised calcium silicatehydrate (CSH) phase.Rd values obtained for the Ra and Sr uptake on commercial cement materials were in the range of50–380 L kg-1 and 10–30 L kg-1, respectively. No significant difference between the distribution ratios of theisotopes 226Ra and 223Ra was observed in the studied liquid to solid (L/S) ratio range, although different isothermswere determined. The Rd values for Ra were found to increase with increasing L/S ratio. The cause of thiseffect is obviously the non-linearity of the sorption isotherm, here of the convex type. In contrast, Sr uptakeseemed to be largely unaffected by variation of L/S ratios; this indicates an isotherm of almost linear type.Sorption experiments with the CSH phase confirmed the distinctive differences in the sorption behaviour betweenRa and Sr as expected, with Rd values significantly higher for Ra. Similarly, the difference between realcementitious materials and the pure CSH phase was confirmed, indicating that the sorption of alkaline earthelements is mainly due to uptake by CSH.The kinetics of Ra and Sr uptake on cementitious materials were evaluated by a set of models describing thesorption in heterogeneous systems based on different rate-controlling processes. The FD (film diffusion) model inthe case of Ra, and the ID (diffusion in inert layer) model in the case of Sr provided the best fits.The influence of temperature on the kinetics of radium sorption was studied, suggesting change in the shape ofisotherm with increasing temperature. Evaluation of sorption kinetic data yielded values of the apparent activationenergy of the uptake process.Complementary through diffusion experiments using compacted crushed HCP confirmed and extended thefindings obtained by evaluation of the batch sorption experiments performed with Ra and Sr