Markus Olin

Evaluation of factors affecting diffusion in compacted bentonite

The information available from the open literature and our studies on exclusion, sorption and diffusion mechanisms of ionic and neutral species in bentonite has been compiled and re-examined in relation to the microstructure of bentonite. The emphasis is placed on a more thorough understanding of the diffusion processes taking place in compacted bentonite. Despite the scarcity of experiments performed with neutral diffusants, these imply that virtually all the pores in compacted bentonite are accessible to neutral species. Anion exclusion, induced by the overlap of electrical double layers, may render the accessible porosity for anions considerably less than the porosity obtained from the water content of the clay. On the basis of the compiled data, it is highly probable that surface diffusion plays a significant role in the transport of cations in bentonite clays. Moreover, easily soluble compounds in bentonite can affect the ionic strength of porewater and, consequently, exclusion, equilibrium between cations, and surface diffusion.

Effect of layer charge on the crystalline swelling of Na+, K+ and Ca2+ montmorillonites: DFT and molecular dynamics studies

Abstract The swelling and cation exchange properties of montmorillonite are fundamental in a wide range of applications ranging from nanocomposites to catalytic cracking of hydrocarbons. The swelling results from several factors and, though widely studied, information on the effects of a single factor at a time is lacking. In this study, density functional theory (DFT) calculations were used to obtain atomic-level information on the swelling of montmorillonite. Molecular dynamics (MD) was used to investigate the swelling properties of montmorillonites with different layer charges and interlayer cationic compositions. Molecular dynamics calculations, with CLAYFF force field, consider three layer charges (-1.0, -0.66 and -0.5 e per unit cell) arising from octahedral substitutions and interlayer counterions of Na, K and Ca. The swelling curves obtained showed that smaller layer charge results in greater swelling but the type of the interlayer cation also has an effect. The DFT calculations were also seen to predict larger d values than MD. The formation of 1, 2 and 3 water molecular layers in the interlayer spaces was observed. Finally, the data from MD calculations were used to predict the selfdiffusion coefficients of interlayer water and cations in different montmorillonites and in general the coefficient increased with increasing water content and with decreasing layer charge.

Chemical Evolution of Bentonite Buffer in a Final Repository of Spent Nuclear Fuel During the Thermal Phase

Abstract Finnish spent nuclear fuel final disposal is planned to be based on the Kärnbränslesäkerhet 3-Vertical concept, which was originally planned for fractured crystalline bedrock. Within this concept, the role of the bentonite buffer is considered central. The aim of the study was to model the evolution of the final repository during the thermal phase (heat-generating period of spent fuel) when the bentonite is initially only partially saturated. There is an essential need to determine how temperature influences saturation and how both of these factors affect the chemistry of bentonite. In this study the Long-Term Test of Buffer Materials A2 parcel test at the Äspö hard rock laboratory in Sweden was modeled using TOUGHREACT code. The results focused on the following phenomena occurring in the bentonite: cation exchange, changes of bentonite pore water, mineral alterations, saturation, and pressure changes in bentonite buffer. The results show similarity with experimental data. However, the results are open to questions, and further study is needed to confirm the validity of the results. Differences between modeled and experimental results can be explained, for example, so that the experimental results are not from the fracture position as our one-dimensional model assumes.

Interaction of Water and Compacted Sodium-Bentonite in Simulated Nuclear Waste Disposal Conditions

Alteration of compacted sodium-bentonite (Volclay MX-80) caused by groundwater in simulated repository conditions for high level radioactive waste, was studied in an experiment where bentonite (wrapped by a copper cylinder) was let to react with two types (A,B) of synthetic granitic groundwater that are distinguished by their initial concentration of potassium and chloride. The reaction took place in ambient conditions at a temperature of 75 °C and proceeded during several time intervals up to 36 months. At the end of each time interval the water was chemically analysed for determination of possible changes in composition. Chemical and mineralogical changes in the bentonite were investigated by using NH 4 Cl extractions, XRD and microprobe (SEM, EDS) analyses and were studied as a function of the reaction time (months) as well as of the distance (mm) from the contact front with water. Sodium ions were found to migrate out from the bentonite while being replaced by other cations such as calcium, magnesium and to some extent, particularly during the reaction of the bentonite with water B, by potassium. No clear evidence was found for the fixation of potassium ions in the interlayer position of montmorillonite clay and the transformation to illite. The main mineralogical change in the bentonite was from Na- to Ca-rich montmorillonite. Secondary processes were the dissolution-precipitation of sulphur compounds, dissolution of gypsum and carbonates and the dissolution-precipitation of copper compounds.

Microstructural investigation of calcium montmorillonite

Abstract Bentonite clay is planned to form a part of deep-geological repositories of spent nuclear fuel in several countries. The extremely long operation time of the repository requires an indepth understanding of the structure and properties of used materials. In this work the microstructure of a simplified system of Ca-montmorillonite is investigated using a set of complementary methods: X-ray diffraction, small angle X-ray scattering, nuclear magnetic resonance, transmission electron microscopy and ion exclusion. The paper presents experimental results obtained from compacted, water saturated samples in the dry density range 0.6-1.5 g/cm3. It can be observed that different methods yield similar quantification of water present in the interlamellar space. Combined results support the multiple porosity concept of the bentonite structure.

Coupled chemical and diffusion model for compacted bentonite

A chemical equilibrium model has been developed for ion-exchange and to a limited extent for other reactions, such as precipitation or dissolution of calcite or gypsum, in compacted bentonite water systems. The model was successfully applied to some bentonite experiments, especially as far as monovalent ions were concerned. The fitted log-binding constants for the exchange of sodium for potassium, magnesium, and calcium were 0.27, 1.50, and 2.10, respectively. In addition, a coupled chemical and diffusion model has been developed to take account of diffusion in pore water, surface diffusion and ion-exchange.d the model was applied to the same experiments as the chemical equilibrium model, and its validation was found partly successful. The above values for binding constants were used also in the coupled model. The apparent (both for anions and cations) and surface diffusion (only for cations) constants yielding the best agreement between calculated and experimental data were 3.0 {times} 10{sup {minus}11} m{sup 2}/s and 6.0 {times} 10{sup {minus}12} m{sup 2}/s, respectively. These values are questionable, however, as experimental results good enough for fitting are currently not available.

Influence of sample preparation on MX-80 bentonite microstructure

Abstract Compacted bentonite is to be used as a buffer material between waste canisters and the bedrock in the deep geological disposal of high-level nuclear waste in several countries. In spite of the fact that such large bentonite systems have long equilibration times, estimation of the material properties and performance in repository conditions is often based on short-term, laboratory-scale experiments. Sample-preparation procedures in these experiments may differ from the natural evolution of the bentonite in the repository, however, affecting the bentonite properties. The present study reports the influence on the structure of clay tactoids of four different preparation procedures of water-saturated, compacted MX-80 bentonite samples using four target dry bulk densities. Small-angle X-ray scattering was used to illustrate the differences between the samples. The different treatments of the bentonite samples may lead to different structural features. Clear differences between low-density samples prepared using different procedures were observed. The influence of the preparation methods was less, but still noticeable, for the high-density samples.

Sorption Aspects for in Situ Matrix Diffusion Modelling at Palmottu Natural Analogue Site, Sw Finland

Concentration profiles in rock matrix around water-carrying fissures were measured at Palmottu U deposit. The profiles were interpreted by the classical matrix diffusion concept. Site-specific sorption studies were performed for U using standard batch experiments and surface complexation modeling; the response of sorption isotherms was also tested. Site-specific matrix properties as well as initial and boundary conditions were used in simulations. The results indicate that matrix diffusion alone cannot explain the observed enrichment of U and its daughters in the rock matrix.

Diffusivity and Porosity in Rock Matrix—Laboratory Methods Using Artificial and Natural Tracers

The nature of diffusivity and porosity in crystalline rock was studied by electrical conductivity measurements, steady-state diffusion experiments, saturation-leaching of tracers with cylindrical rock samples and analysis of the concentrations of different elements from core samples or pore water near fractures. The phenomena of main interest were dead-end porosity, ion-exclusion, sorption, and the continuity of pore networks. The modelling of experimental results was based on a modified Fick`s second law for diffusion, which was solved either by analytical or numerical methods. The measured D{sub e} and {epsilon} were found to statistically follow an exponential presentation: Archie`s law. The existence of ion-exclusion for anions was confirmed. The connectivity of the pore network extended in the laboratory experiments at least six centimetres, in coarse of the pore network extended in the laboratory experiments at least six centimetres, in coarse-grained granite in nature several metres but in fine-grained rock samples of a uranium deposit the element mobilization effects could be seen only to the depth of 2-3 centimetres.

Diffusivity and Porosity in Rock Matrix Related to the Ionic Strength in the Solution

The nature of diffusivity and porosity in rock was studied as a function of various parameters. The phenomena of main interest were dead-end porosity, ion-exclusion and sorption. The rock types studied were rapakivi granite, granite and gneiss, and tracer techniques with 36 Cl, 22 Na + and 3 H (HTO) were used as a research method. A mathematical solution for outdiffusion from a porous cylinder was developed by applying a corrected form of Ficks second law for a case where part of the pores are so-called dead-end pores. With this model the theoretical curve could be closely fitted to the measured values. It was found that the rock-capacity factor is an increasing function of the ionic concentration of the solution in the case of Cl indicating ion-exclusion, while the opposite is true in the case of Na + indicating ion-exchange type sorption. The effective diffusion coefficient was also found to vary as a function of the salinity in the case of 36 Cl. In the case of 22 Na, the effect was opposite and weaker. The diffusion of tritium through the rock samples was clearly higher than the diffusion of 36 Cl. Part of the difference is explained by the smaller effective porosity for 36 Cl. The rest can probably be explained by the steric effects on the chloride ion caused by the negatively charged pore surfaces in the narrow pores.