Through-diffusion Experiments Research Articles

HTO and selenate diffusion through compacted Na-, Na–Ca-, and Ca-montmorillonite

Radionuclide transport in smectite clay barrier systems used for nuclear waste disposal is controlled by diffusion, with adsorption significantly retarding transport rates. While a relatively minor component of spent nuclear fuel, 79Se is a major driver of the safety case for spent fuel disposal due to its long half-life (3.3 × 105 yr) and its low adsorption to clay (KD < 10 L/kg), thus a thorough understanding of Se diffusion through clay is critical for understanding the long-term safety of spent fuel disposal systems. Through-diffusion experiments with tritiated water (HTO, conservative tracer) and Se(VI) were conducted with a well-characterized, purified montmorillonite source clay (SWy-2) under a constant ionic strength (0.1 M) and three different electrolyte compositions: Na+, Ca2+, and a Na + -Ca2+ mixture at pH 6.5 in order to probe the effects of electrolyte composition and interlayer cation composition on clay microstructure, Se(VI) aqueous speciation, and ultimately diffusion. The results were modeled using a reactive transport modeling approach to determine values of porosity (ε), De (effective diffusion coefficient), and KD (distribution coefficient for adsorption). HTO diffusive flux was higher in Ca-montmorillonite (De = 1.68 × 10−10 m2 s−1) compared to Na-montmorillonite (De = 7.83 × 10−11 m2 s−1). This increase in flux is likely due to a greater degree of clay layer stacking in the presence of Ca2+ compared to Na+, which leads to larger inter-particle pores. Overall, the Se(VI) flux was much lower than the HTO flux due to anion exclusion, with Se(VI) flux following the order Ca (De = 1.03 × 10−11 m2 s−1) > Na–Ca (De = 2.12 × 10−12 m2 s−1) > Na (De = 1.28 × 10−12 m2 s−1). These differences in Se(VI) flux are due to a combination of factors, including (1) larger accessible porosity in Ca-montmorillonite due to clay layer stacking and smaller electrostatic effects compared to Na-montmorillonite, (2) larger accessible porosity for neutral-charge CaSeO4 species which makes up 32% of aqueous Se(VI) in the pure Ca system, and (3) possibly higher Se(VI) adsorption for Ca-montmorillonite. Through a combination of experimental and modeling work, this study highlights the compounding effects that electrolyte and counterion compositions can have on radionuclide transport through clay. Diffusion models that neglect these effects are not transferable from laboratory experimental conditions to in situ repository conditions.

Influence of salinity gradients on the diffusion of water and ionic species in dual porosity clay samples

Most of the available data on diffusion in natural clayey rocks consider tracer diffusion in the absence of a salinity gradient despite the fact that such gradients are frequently found in natural and engineered subsurface environments. To assess the role of such gradients on the diffusion properties of clayey materials, through-diffusion experiments were carried out in the presence and absence of a salinity gradient using salt-diffusion and radioisotope tracer techniques. The experiments were carried out with vermiculite samples that contained equal proportions of interparticle and interlayer porosities so as to assess also the role played by the two types of porosities on the diffusion of water and ions. Data were interpreted using both a classical Fickian diffusion model and with a reactive transport code, CrunchClay that can handle multi-porosity diffusion processes in the presence of charged surfaces. By combining experimental and simulated data, we demonstrated that (i) the flux of water diffusing through vermiculite interlayer porosity was minor compared to that diffusing through the interparticle porosity, and (ii) a model considering at least three types of porous volumes (interlayer, interparticle diffuse layer, and bulk interparticle) was necessary to reproduce consistently the variations of neutral and charged species diffusion as a function of salinity gradient conditions.

Diffusion of HTO, 36Cl and 22Na in the Mesozoic rocks of Northern Switzerland: III. Cross-lab comparison of diffusion measurements on argillaceous twin samples

In the past decades, there has been a high interest in evaluations of argillaceous rock formations to host repositories for spent fuel or radioactive waste material. One of the main reasons for this interest is that argillaceous rocks are dominated by clay minerals, resulting in a fine-grained sediment matrix with micropores, low pore connectivity and thus low permeability, and good self-sealing properties. This entails that diffusion is the prime mechanism for transport of gas, fission products and radionuclides in clay-rich rocks. The determination of diffusion parameters is therefore key to evaluate the quality of the rock from long-term safety perspectives. Cross-lab comparison of diffusion data is however often challenging as various methods, concepts and models are utilised in the different laboratories across the globe. Here, a direct cross-lab comparison study of through-diffusion experiments was performed to compare and assess the effect of experimental method (through-diffusion) and modelling uncertainties of the parameters by comparing results obtained by two independent laboratories. The R&D group ‘Disposal’ at SCK CEN (Belgium) and the PSI-LES group (Switzerland) both performed through-diffusion experiments on (nearly) the same sample material using their in-house experimental and modelling methodologies for through-diffusion experiments. Adjacent (twin) samples at five depths between 870 and 940 m in the Trüllikon1-1 borehole (Switzerland) were selected and each lab subjected these five clay rock samples to diffusion in synthetic pore water with three different tracers, HTO, 36Cl− and 22Na+, representative for neutral, anionic and cationic transport behaviour. The two labs used a similar design of diffusion cell and worked with similar experimental conditions, but there were small differences in the experimental set-up/conditions and in the modelling approach. The independently determined diffusion parameters from SCK CEN and PSI-LES for all three tracers confirmed previously observed uncertainties. For all three radionuclides, the variability of the effective diffusion coefficients estimated independently by both institutes was less than a factor 2 and in general much lower (deviations ranging between 0 and 73%). Besides, the parameter estimations of the capacity factor (or accessible porosity in case of HTO and 36Cl−) agreed well. Moreover, the experimental datasets of HTO and 36Cl− were also cross-fitted. The evaluation revealed that the minor deviations can be attributed predominantly to variations in temperature (experimental conditions) and to a lesser extent to minor distinctions in the modelling approach. It is important to acknowledge that local heterogeneity might also contribute to these differences.

Chloride accessible porosity fractions across the Jurassic sedimentary rocks of northern Switzerland

The Jurassic clay-rich sedimentary sequence in northern Switzerland was investigated in the context of nuclear waste disposal. Tracer profiles across this sequence play an important role when assessing transport processes and long-term evolution of the environment. Negatively charged clay-mineral surfaces cause a depletion of anions in the porewater close to the surfaces compared to free porewater further away. The chloride accessible porosity fraction (fCl) is an important parameter to scale Cl bulk porewater concentrations from aqueous extraction to free porewater concentrations, which are required to establish anion profiles used for transport modelling of anionic tracers. A vast set of fCl data from advective displacement (AD), squeezing (SQ) and through-diffusion (DI) experiments allowed for method comparison and yielded insight into controlling factors of fCl across the sedimentary sequence. Two observations were made: (1) fCl varies strongly depending on lithology and partly on ionic strength; (2) at high ionic strength, the three methods yield consistent results, but at low ionic strength, fCl from DI are lower than those from AD and SQ. The observations are discussed in terms of influencing factors, including structural charge, ionic strength, rock texture, pore sizes and the fact that the different methods sample different parts of the pore space. The stronger variation of fCl from DI experiments with the ionic strength of the experimental porewater is in agreement with theoretical concepts. The volume fraction of small pores (r < 2 nm) inversely correlates with fCl; it accounts for ∼50% of the anion depleted pore volume. Two empirical approaches were tested to derive functions for fCl valid for the lithologically diverse sequence: the clay-content model uses a constant fCl at a clay-mineral content >30% and extrapolates to a fCl of 1 at zero clay content. The formation model applies formation- or member-specific average values of experimental fCl for each unit. Both approaches work reasonably well, with the formation model producing less apparent outliers of free porewater concentrations than the clay-content model. Still some scatter remains, particularly in heterogeneous and low-porosity formations. This work results in a comprehensive and coherent fCl dataset for all studied Jurassic formations and serves as a basis to interpret the profiles of anions with regard to transport across the entire low-permeability sequence.

Diffusion of HTO, 36Cl and 22Na in the Mesozoic rocks of northern Switzerland: I. Effective diffusion coefficients and capacity factors across the heterogeneous sediment sequences

Argillaceous rocks are currently being examined worldwide as potential host rocks for repositories for radioactive waste and spent fuel due to their favourable characteristics such as the self-sealing ability or limited diffusive transport of fluids, solutes or gases. In this study, through-diffusion experiments focused on rock samples with different mineralogical compositions taken from deep boreholes in the Mesozoic rock sequence of northern Switzerland, to determine diffusion parameters of HTO, 36Cl− and 22Na+ perpendicular to the bedding plane. Both effective diffusion coefficients and accessible porosities were calculated from the data obtained. In the Opalinus Clay, the primary selected host rock of the repository, the effective diffusion coefficients (De) and accessible porosity values (ε) of the neutral species HTO within and across all study areas are relatively consistent (De = 8.8 ± 1.9 × 10−12 m2 s−1; ε = 0.12 ± 0.02), indicating that the material can be considered homogeneous in terms of diffusion properties. Importantly, the chemical composition of the pore water has no effect on the values of the effective diffusion coefficients of HTO. On the other hand, the rocks above and below the Opalinus Clay, up to the next aquifer, consist of tight sedimentary units (carbonates, siliciclastics including argillaceous rocks) with a wider range of diffusion coefficients, indicating a higher level of heterogeneity. The diffusion coefficients of anions, such as 36Cl−, tend to be smaller due to anion exclusion effects, while those of cations, such as 22Na+, tend to be larger due to surface enhanced diffusion. The extent of these effects is largely determined by the composition of the pore water, and – in the case of cations – by the extent of sorption which depends also on the sorption capacity – indicated by the cation exchange capacity - of the solid. As a result, variations in the effective diffusion coefficient of 36Cl− and 22Na+ can be observed firstly between different rock types (i.e. composition, fabric), and secondly between the different study areas due to differences in the chemical composition of the pore waters in contact with the solid phase.Based on the analysis of around 130 samples, it has been observed that the diffusion coefficients of the Mesozoic units in northern Switzerland exhibit a significant correlation with the total clay content of the rocks. These findings pave the way for the establishment of a diffusion database for the relevant elements under consideration in the Swiss radioactive waste programme and the Safety Case.

Diffusion of dissolved gases: from lab scale to in situ scale

Over the last decade, SCK CEN obtained a large set of diffusion coefficients for different gases in different Members of the Boom Clay. These diffusion experiments were performed in the lab on Boom Clay samples with diameter 80 mm and length 30 mm [1, 2]. When performing safety calculations, diffusion of gas is assumed to take place within an approximate domain over 100’s of meters. This led to the question: is the lab-scale diffusion coefficient also valid on a larger (meter) scale? In the past, a similar question was raised for more classical tracers such as HTO and iodide, and several in situ diffusion experiments lead to the confirmation of lab results. Hence, SCK CEN, ESV Euridice and ONDRAF/NIRAS will perform a new in situ diffusion experiment with dissolved neon in the HADES URL with main objective to confirm/improve the current knowledge on diffusion of dissolved gas at a large scale. To perform an experiment under the most relevant conditions for the Boom Clay, i.e. without being perturbed by the presence of an excavation-damaged zone, the MEGAS setup (drilled in 1992) is re-used. The MEGAS setup consists of four piezometers in a 3D configuration. The most suitable location to inject dissolved gas is filter 17. Monitoring will take place in filters 18, 9 and 22, which allows assessing the anisotropy of diffusion (Figure 1 left).&#x0D; In a geological disposal facility, hydrogen is the main gas that will be produced. Performing experiments with hydrogen is complicated due to microbial conversion of hydrogen into methane. Therefore, this experiment will make use of neon, the best proxy for hydrogen. The principle of this in situ gas diffusion experiment is similar to the experiments in the lab: gas (neon) is dissolved in water, and the water with dissolved neon is circulated over a filter that is in contact with the clay. After some time, dissolved neon will be present in the water in the monitoring filters. By circulating this water with dissolved neon through a vessel with a non-interfering gas phase (e.g. helium), an equilibrium reaction will occur and neon will be present in the gas phase of the monitoring vessel. The monitoring vessels are connected to a gas analyser that performs on line analysis of the neon concentration. By measuring the concentration increase of neon over time in the monitoring vessels at the different filters, the diffusion parameters can be determined. Note that the initial pressure in each vessel will be equal to the in situ pore water pressure, to minimize disturbing the in situ hydraulic field. A summary of the experiment is available from [3].&#x0D; Prior to the start of the diffusion experiment with neon, and in order to better understand the performance of the setup and the processes in the host formation during the experiment, a trial in-diffusion experiment with helium was performed in filters 8 and 21. Both filters will not be involved in the neon through-diffusion experiment, but the processes and conditions around these filters during in-diffusion tests are considered relevant for the through-diffusion experiment. Two circuits are filled with 50% synthetic pore water and 50% helium at an initial pressure equal to the in situ pore water pressure at respective filters. Due to in-diffusion of helium in the host rock, the pressure in the circuits decreases over time, which in turn induces water inflow to the circuits and a subsequent increase of circuit pressure. The gas and water phase balance in the circuit is therefore dynamically adjusted by the whole system, until a steady state is reached.&#x0D; A numerical model was set up in COMSOL to predict pressure variations in both circuits. The diffusion coefficient of helium measured from the lab-scale diffusion test was used. The hydraulic conductivity of the clay comes from in situ permeability tests around these filters. If water flow is not considered in the model, the circuit pressure decrease of filter 21 follows the blue curve as shown in figure 1-right. The red curve gives the circuit pressure decrease considering diffusion of dissolved neon in clay as well as water flow under water pressure gradient. The three-month measurements follow well the red curve, which means that water inflow to the circuit plays a significant role in adjusting the circuit pressure. The good agreement between model results and field measurements confirms our lab scale test results. It also implies the good understanding of the in situ experimental system, which lays a good foundation for the long-term neon through-diffusion experiment afterwards.

Application of machine learning to study the effective diffusion coefficient of Re(VII) in compacted bentonite

Machine learning was used to predict the effective diffusion coefficient of radionuclides in compacted bentonites to reduce the cost of experimental methods. Through-diffusion experiments were conducted to determine the effective diffusion coefficient of Re(VII), which was used as a surrogate for 99Tc(VII), in compacted Anji bentonite. Five parameters (the external surface area, the ionic strength, the mass ratio of montmorillonite, the compacted dry density, and the accessible porosity) that affect the effective diffusion coefficient were calculated by a multi-porosity model to generate data for the analysis of the machine learning models to overcome the limited experimental data. The effective diffusion coefficient was predicted using two popular machine learning models, the Light Gradient Boosting Machine and Artificial Neural Network models, where the former exhibited higher sensitivity and accuracy in the prediction than the latter. The performance of the machine learning models was validated by comparing the experimental effective diffusion coefficients between this study and previous studies. The present work revealed that the machine learning method can be a powerful tool and may offer a new means of studying the effective diffusion coefficient.

Experimental and modeling study of the diffusion path of Ce(III)-EDTA in compacted bentonite

The impact of the diffuse double-layer on the diffusion of radionuclide anions remains unclear, impeding the application of Archie's law for adsorptive anions. Through-diffusion experiments were carried out to measure the effective diffusion coefficient and distribution coefficient of Ce(III) in the presence of EDTA. A multi-porosity model, in combination with Archie's law, was used to estimate the effective diffusion coefficient. The predominant species, CeEDTA−, exhibited an effective diffusion coefficient of (0.88–1.92) × 10−11 m2/s and the distribution coefficient of (0.81–1.18) × 10−3 m3/kg at the compacted dry density of 1300–1800 kg/m3. The accessible porosity, the diffuse double-layer porosity, and the interlayer porosity were analyzed using the multi-porosity model, based on the characterization of bentonite via Scanning electron microscopy with energy dispersive spectrometry, X-ray diffractometer, and Nitrogen-Brunner-Emmett-Teller method. Due to the low external surface area of Zhisin bentonite (27.7–35.3 m2/g), the impact of the diffuse double-layer was insignificant on the total and accessible porosities. Additionally, the diffuse double-layer porosity had insignificant impact on the effective diffusion coefficient of CeEDTA− in compacted bentonite, as its value was one order magnitude lower than that of the interlayer porosity and the accessible porosity.

Bentonite barrier performance affected by precipitation of secondary minerals: Diffusivities of water and iodide ions in the presence of calcium carbonate

Precipitation of secondary minerals potentially affects the long-term barrier performance of bentonite, including hydraulic conductivity and diffusivity. In this regard, diffusion pathways of anions are expected to be preferentially clogged with secondary minerals, that may contribute to the containment of anionic radionuclides. However, experimental evidence is lacking because of the slow transport of reactants and complex alterations in the compacted bentonite. In this study, bentonite sample clogged with CaCO3 was prepared by an electrochemical technique. Then, through-diffusion experiments of HDO (deuterium water) and I− ions were conducted to elucidate the effect of CaCO3 on their effective diffusivities. Overall effective diffusivities were determined from through-diffusion experiments, and local effective diffusivities at the CaCO3 layer were determined from concentration gradients of tracers inside bentonite after reaching steady states of through-diffusion. A CaCO3 layer consisting of vaterite and calcite was precipitated in Na-bentonite; correspondingly, total porosity was maximally reduced from 0.57 to 0.35 at the CaCO3 layer. The overall effective diffusivity slightly decreased in the presence of CaCO3, while the local effective diffusivity at the CaCO3 layer considerably decreased for both HDO and I− ions. The impact of CaCO3 on the local effective diffusivity of I− ions was greater than that of HDO. The results suggest that long-term precipitation of secondary minerals originating from alterations of bentonite may decrease the diffusivity of radionuclides, particularly anions.

Intercomparison of determining diffusion coefficients of I− in compacted bentonite using various mathematical models of through-diffusion experiments in the laboratory

Intercomparison of determining diffusion coefficients of I− in compacted bentonite using various mathematical models of through-diffusion experiments in the laboratory

Molecule Diffusion Behavior of Tritium and Selenium in Mongolia Clay Rock by Numerical Analysis of the Spatial and Temporal Variation

Clay rock in the Tamusu (TMS) area in the Inner Mongolia province is one candidate as a geological disposal site for highly radioactive waste in China. The diffusion behavior of HTO and Se(IV) in Tamusu clay rock was studied by through-diffusion (TD) experiments and numerical analysis to determine the spatial and temporal variation. A minimum error analysis was conducted to determine the HTO and Se(IV) diffusion coefficients in compacted TMS clay. The TD experimental results and numerical analysis showed that the diffusion of HTO and Se reached a steady state within 7 and 50 days, respectively, and the apparent diffusion coefficients (Da) decreased with the increases in the compacted density. In fact, there was retardation of Se diffusion in intact TMS clay rock. A two-site sorption model for Se was applied to simulate fast and slow sorption behavior quantitatively.

Measurement on Diffusion Coefficients and Isotope Fractionation Factors by a Through-Diffusion Experiment

For radioactive waste disposal, it is important that local groundwater flow is slow as groundwater flow is the main transport medium for radioactive nuclides in geological formations. When the groundwater flow is very slow, diffusion is the dominant transport mechanism (diffusion-dominant domain). Key pieces of evidence indicating a diffusion-dominant domain are the separation of components and the fractionation of isotopes by diffusion. To prove this, it is necessary to investigate the different diffusion coefficients for each component and the related stable isotope fractionation factors. Thus, in this study, through-diffusion and effective-porosity experiments were conducted on selected artificial materials and natural rocks. We also undertook measurements relating to the isotope fractionation factors of Cl and Br isotopes for natural samples. For natural rock samples, the diffusion coefficients of water isotopes (HDO and H218O) were three to four times higher than those of monovalent anions (Cl−, Br- and NO3−), and the isotope fractionation factor of 37Cl (1.0017–1.0021) was slightly higher than that of free water. It was experimentally confirmed that the isotope fractionation factor of 81Br was approximately 1.0007–1.0010, which is equivalent to that of free water. The enrichment factor of 81Br was almost half that of 37Cl. The effective porosity ratios of HDO and Cl were slightly different, but the difference was not significant compared to the ratio of their diffusion coefficients. As a result, component separation was dominated by diffusion. For artificial samples, the diffusion coefficients and effective porosities of HDO and Cl were almost the same; it was thus difficult to assess the component separation by diffusion. However, isotope fractionation of Cl and Br was confirmed using a through-diffusion experiment. The results show that HDO and Cl separation and isotope fractionation of Cl and Br can be expected in diffusion-dominant domains in geological formations.

Evolution of HTO and 36Cl− diffusion through a reacting cement-clay interface (OPC paste-Na montmorillonite) over a time of six years

Cement and clays are proposed as sealing materials in underground repositories for radioactive waste. When cement and clay come into contact, chemical gradients between their very different porewater compositions lead to diffusive exchange of solutes, which can result in mineral transformations and alterations of transport properties at the materials interface. Small samples of cement-clay interfaces were prepared and let react over a period of six years. During this time, the changes in transport properties of the samples were periodically monitored by means of through-diffusion experiments. This technique allows studying the evolution of the diffusive flux across the reacting interface and characterizing the variation of the corresponding effective diffusion coefficient (De) over time. All experiments were performed on samples consisting of a hardened high porosity OPC paste and Na-montmorillonite. These model materials were chosen in order to simplify the mineralogy of the system. The experiments allowed obtaining relevant information regarding the development of the diffusive properties and the reactivity of such a cement-clay interface system. HTO and 36Cl− were used as tracers to study the evolution of both the total and the anion accessible porosity. After six years of reaction a considerable reduction of the flux for both HTO and 36Cl− was observed. The flux of HTO did, however, not approach zero, which means that connected porosity for diffusive transport of water is still available. The periodic monitoring of the sample evolution showed a strong reduction of the effective diffusion coefficient De of the samples within the first 1.5 years of the experiment and a less prominent decrease in the period between 1.5 and 6 years. The De of 36Cl− showed a stronger reduction compared to that of HTO; for some cells no chloride flux at all could be measured anymore at t > ~4 a. Using additional information on the extension of porosity changes from a neutron imaging study performed in parallel on the same samples, the diffusive properties of each component of the interface, or of a clay zone with reduced porosity could be estimated. For HTO the relation between the evolution of De and of the porosity in the clay part can be well described with Archie's empirical law. For chloride large uncertainties regarding the accessible porosity do not allow a precise correlation. Whether a complete porosity clogging will take place or some fraction of connected pore space will persist in the sample over reaction times ≫ than 6 years remains an open question.

Through-diffusion study of Se(IV) in γ-irradiated bentonite and bentonite–magnetite

Through-diffusion experiments with HSeO3− and SeO32− (as a surrogate of 79Se(IV)) in bentonite samples have been performed. The effect of density, magnetite and irradiation on the experimental results have been investigated. It could be shown that the effect diffusion coefficient decreased with the increasing density. In the γ-radiation bentonite and bentonite–magnetite, the diffusion coefficient for SeO32− and HSeO3− were (0.45–4.11) × 10−11 m2/s and (0.95–3.74) × 10−11 m2/s, respectively. While the Kd values were (1.19–4.00) × 10−4 m3/kg and (1.83–4.86) × 10−4 m3/kg, respectively. Moreover, the relationship of De and eacc could be described by Archie’s law. This study reveals that the magnetite has large influence on the diffusion, while the effect of γ radiation on the bentonite is similar.

Use of quantitative digital autoradiography technique to investigate the chlorine-36-labelled radiotracer transport in concrete

This work presents the capability of digital autoradiography, a non-destructive technique, to improve diffusion parameters determination by mean of high resolution radionuclide mapping. The results show a good agreement between Cl-36 diffusion profiles acquired by digital autoradiography compared to the classic protocol based onto multi-step-abrasive-surface-peeling procedure. The digital autoradiography technique has been successfully used to localize and quantify Cl-36-labelled radiotracer on the surface of a concrete sample and into the contaminated depth. This study shows how optimized in-diffusion investigations can be performed in only 100 days to assess the diffusion parameters of Cl-36 in a low diffusive concrete sample, prepared from a composite cement (CEM V/A), while a through-diffusion experiment would take several years to reach diffusion steady state for tritiated water, considered as the reference tracer, and consequently, to allow the estimation of the diffusion parameters for Cl-36.

Diffusion of organic anions in clay-rich media: Retardation and effect of anion exclusion

The transport of emerging organic contaminants through the geosphere is often an environmental issue. The sorption of organic compounds slows their transport in soils and porous rocks and retardation is often assessed by extrapolation of batch experiments. However, transport experiments are preferable to strengthen migration data and modelling. In this context, we evaluated the adsorption of various organic acids by means of through-diffusion experiments in a sedimentary clay-rich rock (Callovo-Oxfordian, East of Paris Basin, France). A low diffusivity of organic anions was quantified with effective diffusion coefficients, De, ranged between 0.5 and 7 10−12 m2 s−1. These values indicated an organic anion exclusion. As for chloride, the porosity accessible to organic anions was lower than that of water: εa(organic anions) < ε(water). The partial exclusion of organic anions from rock porosity was linked to both charge and size effects. A significant retardation was observed for organic anions such as oxalate, citrate or α-isosaccharinate. Yet, retardation measured by diffusion experiments was significantly lower than expected from batch experiments on crushed samples. An empirical correction factor is proposed to account for a possible decrease of retardation with accessible porosity of diffusing solute. This feature has significant implications for the estimation of migration parameters of organic compounds in the environment.

Comparative study of HTO diffusion on individual and coupled systems of compacted bentonite and fresh ordinary Portland cement paste

The radionuclide transport on the interface of compacted bentonite and cementitious materials has been rarely investigated. The goal of our project is to understand how processes at the interface of bentonite and cementitious materials affect the transport properties of bentonite. The paper describes methodology and evaluation procedure applied on HTO through-diffusion experiments on compacted bentonite/fresh hardened cement paste interface (coupled system). Under the same experimental conditions, HTO through-diffusion experiments were performed on individual materials and compared to experiments in the coupled system. To compare these two types of experiments, samples were saturated separately and coupled before the start of the through-diffusion experiments. An original method of modelling and evaluation of through-diffusion experiments performed in coupled system was applied. The effective diffusion coefficients of HTO in the compacted bentonite in both experimental arrangements, individual and coupled, were found to be in a very good agreement. Based on these finding, the prepared procedure of evaluation of through-diffusion experiments on the coupled system of two porous materials can be considered applicable on further planned studies of bentonite interfaces. A less good agreement, but still satisfactory, was obtained for the fresh hardened cement paste.

The diffusion of SO42− in Opalinus Clay: Measurements of effective diffusion coefficients and evaluation of their importance in view of microbial mediated reactions in the near field of radioactive waste repositories

Through-diffusion experiments with 36Cl−, 35SO42− and HTO in Opalinus Clay (OPA) samples from a deep borehole in North-East Switzerland (Benken; BE) have been performed. The effect of burial depth on the experimental results has been investigated.It could be shown that the effective diffusion coefficients decrease with sample depth for all three tracers. Moreover, there was a good correlation with the texture of the samples. The diffusion coefficients for HTO are the largest (De = 5.4–8.8 × 10−12 m2 s−1), followed by those for 36Cl− (De = 0.7–1.9 × 10−12 m2 s−1), and finally 35SO42− (De = 0.2–0.6 × 10−12 m2 s−1). 36Cl− was partially excluded from the total porosity resulting in an accessible porosity smaller than the total porosity (εCl = 0.041–0.064). 35SO42−, on the other hand, showed interaction with OPA resulting in a capacity factor (α) larger than the total porosity (εtot = 0.13–0.16). Using extended Archie's law the accessible porosity for 35SO42− was estimated between 0.013 and 0.030. This enabled to evaluate the sorption coefficient of 35SO42− from the measured capacity factor, resulting in values of Kd between 6 × 10−5 and 9 × 10−5 m3 kg−1.

Effect of the pore water composition on the diffusive anion transport in argillaceous, low permeability sedimentary rocks

The effect of the pore water composition on the diffusive anion transport was studied for two different argillaceous, low permeability sedimentary rocks, Opalinus Clay (OPA) and Helvetic Marl (HM). The samples were saturated with different solutions with varying molar concentration and different main cations in the solution: NaCl based pore solutions and CaCl2 based pore solutions. The total porosity was measured by through-diffusion experiments with the neutral tracer HTO. Experiments performed in NaCl solutions resulted in a porosity of 0.12 for OPA and 0.03 for HM, and are consistent with results of the experiments in CaCl2 solutions. The total porosity was independent of the molar concentration, in contrast to the measured anion porosity, which increased with increasing molar concentration. It could further be observed that the pore solution based on the bivalent cation calcium shielded the negative surface charge stronger than the monovalent cation sodium, resulting in a larger measureable anion-accessible porosity in the case of CaCl2 solutions.The data was modelled based on an adapted Donnan approach of Birgersson and Karnland (2009). The model had to be adjusted with a permanent free, uncharged porosity, as well as with structural information on the permanent anion exclusion because of so-called bottleneck pores. Both parameters can only be evaluated from experiments. Nevertheless, taking these two adaptions into account, the effect of varying pore water compositions on the anion-accessible porosity of the investigated argillaceous rocks could be satisfactorily described.

How mobile is tritiated water through unsaturated cement-based materials? New insights from two complementary approaches

This work presents two complementary approaches (for low and high desaturation) to study tritiated water (HTO) diffusion through unsaturated cement-based materials. The first approach was based on through-diffusion experiments where suction was controlled by osmosis. In the second approach, diffusion experiments were performed in humidity chambers controlled by under-saturated saline solutions. Results revealed a decrease of effective diffusion coefficient by a factor of 10 from 100% to 23% of saturation degree. Comparison with gaseous H2 suggests that HTO diffuses through unsaturated cement-based materials at rates 4 orders of magnitude lower.