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

Predictive and Inverse Modeling of a Radionuclide Diffusion Experiment in Crystalline Rock at ONKALO (Finland)

The REPRO-TDE test was performed at a depth of about 400 m in the ONKALO underground research facility in Finland. Synthetic groundwater containing radionuclide tracers [tritiated water tracer (HTO), 36Cl, 22Na, 133Ba, and 134Cs] was circulated for about 4 years in a packed-off interval of the injection borehole. Tracer activities were additionally monitored in two observation boreholes. The test was the subject of a modeling exercise by the SKB GroundWater Flow and Transport of Solutes Task Force. Eleven teams participated in the exercise, using different model concepts and approaches. Predictive model calculations were based on laboratory-based information concerning porosities, diffusion coefficients, and sorption partition coefficients. After the experimental results were made available, the teams were able to revise their models to reproduce the observations. General conclusions from these back-analysis calculations include the need for reduced effective diffusion coefficients for 36Cl compared to those applicable to HTO (anion exclusion), the need to implement weaker sorption for 22Na compared to results from laboratory batch sorption experiments, and the observation of large differences between the theoretical initial concentrations for the strongly sorbing 133Ba and 134Cs, and the first measured values a few hours after tracer injection. Different teams applied different concepts, concerning mainly the implementation of isotropic versus anisotropic diffusion, or the possible existence of borehole disturbed zones around the different boreholes. The role of microstructure was also addressed in two of the models.</p

Evaluation report of Task 9B based on comparisons and analyses of modelling results for the Äspö HRL LTDE-SD experiments. Task 9 of SKB Task Force GWFTS – Increasing the realism in solute transport modelling based on the field experiments REPRO and LTDE-SD

Task 9B of the SKB Task Force on Modelling of Groundwater Flow and Transport of Solutes (Task Force GWFTS) was the second subtask within Task 9 and focused on the modelling of experimental results from the LTDE-SD in situ tracer test. The test had been performed at a depth of about 410 m 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 an artefact 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 (> 10 mm) were now below the reported detection limits.Ten different modelling teams provided results for this exercise, using different concepts and codes. One additional team provided results related to conceptual development. 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: (1) analytical solutions to the transport-retention equations, (2) continuum-porous-medium numerical models, and (3) 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 summarised below.Concerning Na-22 and Cl-36, which showed long penetration profiles, tracer profiles within ca 30 mm from the tracer source could be interpreted with transport and retention parameters consistent with those obtained from laboratory-scale experiments. A disturbed zone, with a thickness of about 5 mm, could also be identified. This disturbed zone was especially evident in the Cl-36 data. However, some of the measured cores showed rather flat end tails for Na-22, which could not be reproduced by the homogeneous (i.e. constant transport and retention properties) or the continuum-porous-medium models using parameters consistent with those derived from laboratory-scale experiments. Reproduction of those tails by some of the microstructure-based models was performed by implementing fast transport along microfractures and cm-scale fractures.For the rest of the tracers, which were more strongly sorbing, the profiles did not in general extend beyond 10 mm from the tracer source, with only some data points showing measurable activities further into the rock. Overall, the best fits to the measured profiles within a few mm from the tracer source were achieved by the homogeneous models (constant transport and retention parameters with distance), with apparent diffusion coefficients consistent with laboratory-derived experimental results. Good fits were also achieved by models assuming the presence of a disturbed zone with gradually changing parameters. The fact that most data points above detection limits fell within 5 to 8 mm from the tracer source, and the observations from the Cl-36 data, suggest the existence of a disturbed zone with a thickness of a few mm and characterised by rather constant transport and retention parameters. The longer profiles for Cl-36 and Na-22 advocate for an undisturbed rock matrix (unaffected by borehole drilling or alteration zones next to fractures) beyond these 5 to 8 mm from the tracer source. Additionally, the flat profile tails observed for some of the Na-22 profiles may point to the effect of microfractures and cm-scale fractures on radionuclide transport.These conclusions could be reached after (1) the re-evaluation and revision of the experimental data (tracer profiles in the rock), and (2) the analysis of the different sets of model results performed by the different teams.(1) The revision of the experimental data led to the dismissal of most of the measurements showing anomalously high activities far from the tracer source (long flat profile tail ends) for the strongly sorbing tracers, as possible contamination mechanisms during sample preparation were identified. As an additional consequence, this discovery highlighted the importance of using blank samples in future tracer transport experiments. Using blank samples together with samples from the test rock sections during preparation and analysis will aid in the detection of potential contamination or background effects.(2) The work performed by the different modelling teams allowed the comparison of many different model concepts, especially in terms of potential zonations of rock properties, such as the presence of a disturbed zone close to the tracer source, the potential effects of micro- and cm-scale fractures, or the implementation of microstructure-based models. An added value was the motivation provided by these exercises to advance in conceptual and numerical model development, which is a key goal of Task 9