Through-diffusion Experiments Research Articles

Consistency of the strontium transport parameters in Boom Clay obtained from different types of migration experiments

AbstractThe consistency of migration parameters obtained by different methods is examined for strontium in Boom Clay. In addition to a previous in-situ percolation experiment and electromigration experiments (using an electrical field as driving force), a lab percolation test and two through-diffusion tests (on clay cores of different lengths) have been performed.All experiments lead to a robust value for the apparent diffusion coefficient of strontium between 5 ? 10−12m2/s and 1 ? 10−11m2/s.No reliable value is obtained for the product ηR of the diffusion accessible porosity and theretardation factor R. For one of the through-diffusion experiments, the concentration decrease inthe inlet as a function of time was fitted simultaneously with the strontium profile in the clay,leading to a ηR value around 400. Estimating ηR at the inlet/clay interface as the ratio of thebulk concentration and the pore water concentration leads to a similar value. The valuesobtained from the strontium profiles in the percolation and electromigration experiments are inline as well. However, fitting for both through-diffusion tests the outlet concentrations versustime leads to very low and unrealistic values of ηR (0.15 and 0.5).Modeling shows that the transport of strontium in Boom Clay cannot be satisfactorilydescribed with the current assumptions (like e.g. instantaneous linear sorption equilibrium andboundary conditions), meaning that there is a problem to obtain a reliable value for the product ηR for strontium in Boom Clay.

Ion equilibrium between montmorillonite interlayer space and an external solution—Consequences for diffusional transport

Bentonite clay is proposed as buffer material in several concepts of High Level Radioactive Waste repositories, and a correct description of ion diffusion in this material is of vital importance for any quantification of the chemical evolution of the repository near field. This study investigates the importance of ion equilibrium between montmorillonite interlayer space and an external solution for the diffusional behavior of bentonite. Two distinct and well established mechanisms govern this type of ion equilibrium: Donnan equilibrium and ion exchange equilibrium. Donnan equilibrium influences both cations and anions in a symmetric manner, while ion exchange only is relevant in systems of more than one type of cations. Both mechanisms generate ion concentration discontinuities across the bentonite/external solution interface. A general theoretical framework for describing through-diffusion is developed. An expression for the effective diffusion coefficient, D e , is derived, taking into account also the influence of filters typically present in these types of experiments D e = ϕ c Ξ ς 2 + ς D c , where ϕ c is total clay porosity, D c is the diffusion coefficient in the clay, ς describes the influence of filters and Ξ is a general ion equilibrium coefficient. The above expression is valid for both cations and anions. The theory has been applied to one laboratory study concerning cation (Na +) and two independent laboratory studies concerning anion (Cl −) diffusion in Na-bentonite. The commonly observed different transport behavior for anions and cations in bentonite is principally explained by the concentration discontinuities: Ξ is large for tracer cations at low electrolyte concentrations, but approaches zero for anions in the same concentration limit. The presented theory implies that effective diffusion coefficients evaluated from tracer through-diffusion experiments do not describe diffusive mass transfer in bentonite in general. Furthermore, it shows that ion diffusion in compacted bentonite is principally explained without the commonly used concepts of anion porosity and sorption mechanisms. The striking explanatory power of this basic approach shows the necessity to consider interlayer ion equilibrium in compacted bentonite, and that these considerations must be at the core of any type of modeling (transport, pore water chemistry, mechanics etc.) of bentonite exerting swelling pressure.

Tracer Diffusion in Sintered Stainless Steel Filters: Measurement of Effective Diffusion Coefficients and Implications for Diffusion Studies with Compacted Clays

AbstractThe use of porous filters is indispensable in laboratory- and field-scale diffusion studies, where sample confinement is needed for mechanical reasons. Examples are diffusion studies with compacted swelling clays or brittle clay stones. Knowledge of the diffusion properties of these filters is important in cases where they contribute significantly to the overall diffusive resistance in the experimental setup. In the present study, measurements of effective diffusion coefficients (Db) in porous, stainless steel filter discs are reported for tritiated H2O (HTO), 22Na+, Cs+, and Sr2+ before and after use of the filters in diffusion experiments with different clay minerals. The Db values for used filters were found to be less than those of the as-received filters by ∼30–50%. The Db values measured for the diffusion of HTO, 22Na+, Cs+, and Sr2+ in unused and used stainless steel filter discs correlated fairly well with the respective molecular diffusion coefficients in bulk water. Although such correlations are inherently associated with some uncertainties, they allow reasonable estimates to be made for diffusants for which no Db values are available. For the first time, a procedure is outlined that allows an integrative assessment to be made for the impact of the uncertainties in the filter diffusion properties on the combined standard uncertainties of the diffusion parameters obtained from through-diffusion experiments. This procedure can be used in the design and optimization of through-diffusion experiments in which the diffusive resistance of the porous filters must not be ignored. Shown here, as a general rule of thumb, is that, if the effective diffusion coefficient in the porous filter is at least three times larger than that in the clay, the choice of geometrical boundary conditions is rather uncritical, as long as the thickness of the clay sample is greater than that of the porous filters.

Diffusion of iodide and iodate through Opalinus Clay: Monitoring of the redox state using an anion chromatographic technique

The through- and out-diffusion behaviour of 125I− and 125IO3- were separately studied in an intact sample of Opalinus Clay (OPA) from the Benken drilling site (Switzerland) using purified tracer solutions. The concentrations of the two radioactive I species, 125I− and 125IO3-, were monitored using a high performance anion exchange chromatographic technique. An effective diffusion coefficient, De = (4.5 ± 0.5) × 10−13 m2 s−1, and a rock capacity factor, α = 0.04 ± 0.01, were measured in the through-diffusion experiments with 125I−. The latter result shows that no sorption of I− takes place on OPA because in previous work the same rock capacity factor was measured for the non-sorbing Cl− tracer. The out-diffusion of 125I− was characterised by an unexpectedly high flux from the surface adjacent to the high concentration reservoir in the through-diffusion experiment. The present experiments resolved former inconsistencies with respect to the sorption of I− on OPA, where unpurified tracers were used. However, they confirmed the inconsistencies with respect to the observed De values between through- and out-diffusion experiments. No break-through of 125IO3- could be observed in the through-diffusion experiment with 125IO3- as the source species. Instead a flux of 125I− into the target reservoir was substantiated. This observation, together with the speciation analysis in the feed reservoir, showed that IO3- was rapidly converted to I− upon contact with OPA. The transformation was presumably mediated by unidentified reducing components of OPA because such fast reactions are not observed in homogeneous aqueous solutions. The rapid conversion of IO3- to I− in the OPA sample was corroborated in the profile analysis performed after the through-diffusion experiment, where a strongly one-sided distribution of the tracer ion was observed.

A technique for estimating one-dimensional diffusion coefficients in low-permeability sedimentary rock using X-ray radiography: Comparison with through-diffusion measurements

The measurement of diffusive properties of low-permeability rocks is of interest to the nuclear power industry, which is considering the option of deep geologic repositories for management of radioactive waste. We present a simple, non-destructive, constant source in-diffusion method for estimating one-dimensional pore diffusion coefficients ( D p) in geologic materials based on X-ray radiography. Changes in X-ray absorption coefficient (Δ μ) are used to quantify changes in relative concentration ( C/ C 0) of an X-ray attenuating iodide tracer as the tracer solution diffuses through the rock pores. Estimated values of D p are then obtained by fitting an analytical solution to the measured concentration profiles over time. Measurements on samples before and after saturation with iodide can also be used to determine iodide-accessible porosity (ϕ I). To evaluate the radiography method, results were compared with traditional steady-state through-diffusion measurements on two rock types: shale and limestone. Values of D p of (4.8 ± 2.5) × 10 − 11 m 2·s − 1 (mean ± standard deviation) were measured for samples of Queenston Formation shale and (2.6 ± 1.0) × 10 − 11 m 2·s − 1 for samples of Cobourg Formation limestone using the radiography method. The range of results for each rock type agree well with D p values of (4.6 ± 2.0) × 10 − 11 m 2·s − 1 for shale and (3.5 ± 1.8) × 10 − 11 m 2·s − 1 for limestone, calculated from through-diffusion experiments on adjacent rock samples. Low porosity (0.01 to 0.03) and heterogeneous distribution of porosity in the Cobourg Formation may be responsible for the slightly poorer agreement between radiography and through-diffusion results for limestones. Mean values of ϕ I for shales (0.060) and limestones (0.028) were close to mean porosity measurements made on bulk samples by the independent water loss technique (0.062 and 0.020 for shales and limestones, respectively). Radiography measurements offer the advantage of time-saving for diffusion experiments because the experiment does not require steady-state conditions and also allows for visualization of the small-scale heterogeneities in diffusive properties within rocks at the mm to cm scale.

Diffusion Behavior of Organic Carbon and Iodine in Low-heat Portland Cement Containing Fly Ash

AbstractThe diffusion of radionuclides in cementitious materials used as an engineered barrier is an important parameter in the performance assessment of the sub-surface repository system used for low-level radioactive waste disposal in Japan. In particular, organic carbon-14 and iodine-129 would provide large contributions to the dose evaluation, because of their low ability to be adsorbed on cementitious materials. In this study, the diffusion of acetate and iodide in hardened cement pastes was examined by through-diffusion experiments. Low-heat Portland cement containing 30 wt% fly ash (FAC), which is a candidate cement material for the construction of the sub-surface repository, was prepared for the diffusion experiments. The effective diffusion coefficients, De, of the trace ions for hardened FAC cement pastes were estimated to be on the order of 10-13 m2 s-1 at the beginning of the diffusion experiments. Then, the rate of diffusion of the trace ions decreased over the experimental period of 1-15 months. This is probably due to the change in the microstructure of the FAC as the result of a pozzolanic reaction. After a few months, the values of De were estimated to be on the order of 10-14 m2 s-1. These results suggest that an engineered barrier made of FAC can act as an effective barrier inhibiting the diffusion of trace ions such as organic carbon and iodine.

Diffusion of anionic species in Callovo-Oxfordian argillites and Oxfordian limestones (Meuse/Haute–Marne, France)

Abstract Diffusive parameters of tritiated water (HTO) and several anionic tracers (36Cl−, 125I−, 35 SO 4 2 - and 75 SeO 3 2 - ) were determined in the Callovo-Oxfordian argillite formation (Meuse/Haute–Marne, France), and also in the Oxfordian limestone formation, located just above. Twenty two drillcore samples, from depths of 150–480 m and from almost each lithofacies along the same borehole EST205, were tested. Three distinct and complementary experimental set-ups were used (batch, column filled with crushed argillite and through-diffusion experiments) associated with a thorough characterisation of the rock, care being taken to maintain redox conditions. Tritiated water behaviour is similar to water which diffuses in the total porosity of the rock. Tritiated water and anionic tracers displayed analogous diffusive behaviour in calcareous samples with ea(anion)/ea(HTO) = 1 and De(anion)/De(HTO) close to 0.85, i.e. the ratio of their diffusion coefficients in free water. Lower and scattered values were observed in the lower part of the Oxfordian limestones which is characterized by the presence of mineralogical heterogeneities and dolomitic diagenesis. In the Callovo-Oxfordian argillite formation, diffusive fluxes of 36Cl−, 125I− and 35 SO 4 2 - are similar but lower than that of HTO. The diffusive behaviour analogy between I− and Cl− was verified, as De(halide)/De(HTO) and ea(halide)/ea(HTO) ratios were always lower than 1. This reduction in diffusivity is a consequence of the anionic exclusion, which limits the diffusion-accessible porosity to a value lower than the total porosity for anions. A weak sorption of I− and SO 4 2 - on argillite samples was always observed whatever the technique used. In the case of I−, this sorption appears to be partly reversible and kinetically controlled. Very low values of diffusive parameters were obtained for 75 SeO 3 2 - and are still not well explained. Last, diffusive parameters of each radioactive tracer tested can be plotted according to Archie’s law, which therefore allows an estimation of De(non-reactive species) from the knowledge of both the sample total porosity and the corresponding exponent m = 2 for argillite samples.

Sorption and diffusion of HTO and cesium in crushed granite compacted to different lengths

In this study, batch and through-diffusion experiments have been performed in order to determine the distribution coefficients (Kd), apparent diffusion coefficients (Da) and retardation factor (Rf), respectively. Both apparent and effective diffusion coefficient (Da and De) of Cs were obtained by accumulative concentration method developed by Crank (1975). In addition, a non-reactive radionuclide, HTO, was initially conducted in through-diffusion experiment for assessing the ability of radionuclide retardation. The distribution coefficients (Kd) obtained by batch tests in 14 days under aerobic and anaerobic systems were 2.06 and 3.52 ml/g. Moreover, it is found in through-diffusion test that Rf = 4.12 and 4.40 and Kd = 0.97 and 1.06 of Cs did not have an obvious discrepancy in a length/diameter/ (L/D) ratio of 0.44 and 1.78. However, Rf and Kd revealed a larger difference in an L/D ratio closing to 1 due to the geometric change of one-dimension diffusive hypothesis. Therefore, it demonstrates that Rf and Kd obtained by through-diffusion experiments only could be achieved at a lower or larger L/D ratio and would be reliable for long-term performance assessment.

Evaluation of tritiated water diffusion through the Toarcian clayey formation of the Tournemire experimental site (France)

Through-diffusion experiments with tritiated water were performed on argillaceous samples from various zones of the Tournemire test site. It was intended to evaluate the homogeneity of the transport property of unfracturated samples and the influence of the orientation and the nature of the samples (presence of an opened fracture or a pre-existing tectonic fracture filled with calcite and pyrite). Homogeneous values of the tritiated water (HTO) effective diffusion coefficients were deduced from experiments carried out when diffusion occurred parallel to the stratigraphic bedding, with an apparent sensitivity to experimental conditions. Anisotropy was significant, De(HTO) perpendicular to the bedding being 1/3 lower than that parallel to the bedding. The observed fractures of the samples created by mechanical stress and partial dehydration during sawing and the presence of a pre-existing opened fracture did not affect the effective diffusion coefficients of tritiated water, which is probably due to the healing ability of the clayey medium during the re-saturation phases of the equilibrium steps performed prior to the diffusion experiments. On the contrary, a significant decrease of this transport parameter was induced by the occurrence of a pre-existing tectonic fracture, which was assigned to the dense structure of the filling phases.

Study of Sorption and Diffusion of 137Cs in Compacted Bentonite Saturated with Saline Water at 60°C

The effect of compaction of bentonite on the sorption behavior of 137Cs was studied for the safety assessment of the high level radioactive waste. The diffusion coefficients (effective De and apparent Da ) and the distribution coefficient for sorption Kd for 137Cs in compacted and dispersed bentonite saturated with saline water were investigated at 60°C by four different sorption and diffusion experiments: the in-diffusion, through-diffusion, reservoir-depletion and batch sorption experiments. The system of the through-diffusion experiment was carefully designed to maintain the boundary conditions of constant concentration at each end of the specimen. De and Da were found to be reproducible and showed good consistency among three of the diffusion experiments (through-diffusion, in-diffusion and reservoir depletion). Kd of 137Cs in compacted bentonite determined from the three types of diffusion experiments was in good accordance with that determined by the batch sorption experiment for dispersed bentonite.

Study of Sorption and Diffusion of 137Cs in Compacted Bentonite Saturated with Saline Water at 60.DEG.C

The effect of compaction of bentonite on the sorption behavior of 137Cs was studied for the safety assessment of the high level radioactive waste. The diffusion coefficients (effective De and apparent Da ) and the distribution coefficient for sorption Kd for 137Cs in compacted and dispersed bentonite saturated with saline water were investigated at 60°C by four different sorption and diffusion experiments: the in-diffusion, through-diffusion, reservoir-depletion and batch sorption experiments. The system of the through-diffusion experiment was carefully designed to maintain the boundary conditions of constant concentration at each end of the specimen. De and Da were found to be reproducible and showed good consistency among three of the diffusion experiments (through-diffusion, in-diffusion and reservoir depletion). Kd of 137Cs in compacted bentonite determined from the three types of diffusion experiments was in good accordance with that determined by the batch sorption experiment for dispersed bentonite.

Diffusion of 22Na and 85Sr in Montmorillonite: Evidence of Interlayer Diffusion Being the Dominant Pathway at High Compaction

A mechanistic understanding of transport phenomena in compacted clays is essential for the use of such materials as engineered barrier systems for the safe geological disposal of radioactive wastes. The present contribution is a first step in the development of an integrative treatment of the properties of tracer cations in compacted bentonites with respect to diffusion and sorption. The diffusion of 22Na and 85Sr in highly compacted montmorillonite and kaolinite is investigated as a function of the "external salt concentration" (NaClO4), i.e., of the solution in equilibrium with the clay. Consistent results were obtained from through-diffusion experiments and tracer profile analysis. Knowledge of genuine diffusion coefficients of the filter plates turned out to be crucial in cases where the diffusive resistance of the filter plates was similar to that of the clay. Diffusion coefficients formally calculated on the basis of the tracer concentration gradient in the external aqueous phase, and the sorption distribution ratios were found to decrease with increasing external salt concentration in the case of montmorillonite. In a logarithmic representation of these data, a slope of -1 was obtained for the monovalent 22Na, whereas the slope was -2 for the divalent 85Sr. In the case of kaolinite, diffusion coefficients were independent of the external salt concentration. It is postulated that the diffusion of the tracer cation through the interlayer water is the dominant pathway in compacted swelling clays under the experimental conditions tested. Effective diffusion coefficients, based on a tracer concentration gradient in the interlayer water of the clay, were found to be independent of the composition of the external aqueous phase. The latter gradient is assumed to be a function of the external salt concentration, according to a calculated distribution of the tracer cation between free pore water and the interlayer water via cation exchange.

Evaluation of the reversibility of iodide uptake by argillaceous rocks by the radial diffusion method

Laboratory radial in- and out-diffusion experiments were performed to investigate the reversibility of the iodide (I-) uptake by argillaceous rocks from the Tournemire site (France). At first, the suitability of the method was demonstrated by means of deuterium depleted water (DDW) diffusion experiments. The values for the DDW effective diffusion coefficient (1.7 to 2.7×10-11m2s-1) are indeed very close to those obtained from previous through-diffusion experiments carried out on Tournemire samples with tritiated water. The diffusion of chloride and bromide led to the determination of halide-accessible porosities, which are necessary to calculate the retardation factor (R) and the distribution ratio (RD). The calculated values for the halide-accessible porosity (2 to 5%) clearly indicated the effect of anionic exclusion and are consistent with previous data. On the contrary, the in-diffusion experiments performed with iodide clearly showed its uptake by argillite, with rock capacity factor values ranging from 14% to 25%. The corresponding values ofRD(0.035 to 0.08 L kg-1) are one order of magnitude lower than those previously derived from batch methods. At last, the experiments of iodide out-diffusion revealed that only iodide located in the halide-accessible porosity diffused out of the rock samples, suggesting that the uptake of iodide by argillite would not be reversible or that the kinetics of desorption would be low (>70 days).

Characterization of sulphate sorption on Callovo-Oxfordian argillites by batch, column and through-diffusion experiments

Callovo-Oxfordian argillites are under investigation as a potential host rock for a high-level radioactive waste repository. The purpose of this study was to investigate sulphate sorption on two borehole samples (EST205 K119c and K100c), using three different and complementary laboratory techniques. Batch experiments revealed a fast, weak and reversible sorption of sulphate ions on Callovo-Oxfordian argillites. Quantification was difficult with this method due to uncertainties on low K d values obtained. Column and through-diffusion experiments confirmed this retention and allowed a better quantification. A clear dependence of sulphate sorption on both mineralogical composition and sulphate content was shown. For the K119c argillite, the sulphate isotherm was fitted with a Langmuir isotherm. K d values are ranging from 0.15 to 0.37 mL g −1, depending on total sulphate content.

Diffusion of humic colloids in compacted bentonite

Through-diffusion experiments in bentonite with humic colloids in the size range of 1–10 nm were carried out. Bentonite was compacted to 0.6–1.8 g/cm 3 dry density and equilibrated with 0.01 and 0.1 M NaClO 4 solutions. Experiments with Eu(III) diffusing in the absence and presence of humics were run in parallel, as well as Co(II) diffusing through compacted bentonite in the presence of humics. The humic colloid diffusion experiments were run for 60 days and the humic concentration in the outlet solutions measured at time intervals. The experimental breakthrough curves for humic substances (HS) as well as the HS, Co(II) and Eu(III) profiles in the bentonite were simulated using the finite difference based computer code ANADIFF. Regardless of the compaction and ionic strength of solutions, humic colloids diffused through the compacted bentonite. The effects of humic colloids on both Co(II) and Eu(III) sorption as well as on diffusion were significant. The apparent diffusivity ( D a) increased significantly for both Co(II) and Eu(III) when humic colloids were present and the distribution coefficient ( K d) values decreased.

Characterization of iodide retention on Callovo-Oxfordian argillites and its influence on iodide migration

Iodine-129 is commonly considered to belong to the largest contributors to the calculated impact on health associated with the long-term nuclear underground waste disposal. Its behaviour in Callovo-Oxfordian argillites, an argillaceous host rock studied by ANDRA, therefore must be fully characterized. Investigated cores have been extracted from a place outside the Meuse/Haute Marne underground laboratory. Thus, batch and through-diffusion experiments were performed to investigate iodide sorption and transport, on two different test samples. Batch experiments give evidence of a kinetically limited slight retention of iodide at low total iodine concentration. This sorption, dependent on the oxic/anoxic conditions, has been modelled with an initial quick reaction and a long-term steady loss of iodide from the solution. Through-diffusion experiments have confirmed this retention but to a lower extent than observed in batch experiments and differences were found between both samples. These differences have been explained by anionic exclusion, which potentially blocks sorption site access.

Effects of Compaction and Temperature on Sorption and Diffusion of Cs and HTO in Compacted Bentonite Saturated with Saline Water

AbstractThe sorption and diffusion of Cs and HTO in compacted bentonite was investigated at temperatures from 30 to 60°C. The apparent (Da) and effective (De) diffusion coefficients were determined by in-diffusion and through-diffusion experiments with a constant boundary concentration maintained. The temperature dependence of De and Da obeyed an Arrhenius-type equation, allowing determination of the activation energy for diffusion of Cs and HTO. The De value of Cs was three times the De of HTO, which is considered to be a result of surface-excess diffusion. Cs may be concentrated near the surface of the negatively charged clay, thus giving a large diffusive flux. The activation energies for Cs diffusion were 21.4×2.8 kJ/mol and 37.3×1.5 kJ/mol as determined based on De and Da, respectively. This difference was due to the temperature dependence of the distribution coefficient Kd of Cs.

The Diffusion of Radionuclides through Waste Encapsulation Grouts

ABSTRACTUnited Kingdom Nirex Limited develops and advises on safe, environmentally sound and publicly acceptable options for the long-term management of radioactive waste. One option Nirex has developed is a phased geological repository concept for intermediate level waste and some low level wastes that makes use of a combination of engineered and natural barriers. Physical containment of radionuclides would be achieved by immobilisation and packaging of wastes (mostly) in stainless steel containers.Existing models of the migration of dissolved radionuclides from packaged wastes suggest that radionuclide release is determined largely by the rate of diffusion through the encapsulation grout used to immobilise the waste. The use of such models requires diffusion coefficient data for radionuclides in waste encapsulation grouts. This paper describes a programme of through-diffusion experiments, and modelling interpretation, aimed at deriving diffusion coefficients for some radionuclides in two types of encapsulation grout.An intrinsic diffusion coefficient of HTO of around 1×10−13 to 2×10−13 m2s−1 was determined for a 3:1 mix of blast furnace slag to ordinary Portland cement, compared to around 4×10−13 to 5×10−13 m2s−1 for a 3:1 mix of pulverised fuel ash to ordinary Portland cement. These values are lower than that assumed for a non-sorbing radionuclide in an earlier modelling exercise. Porosity values around 0.3 were obtained in each case. For 36Cl as chloride, the experiments showed no significant breakthrough over the experimental timescale of about one year, suggesting an intrinsic diffusion coefficient below 5×10−13 m2s−1. One possibility is that chlorine-containing solids are precipitating in the cement. An intrinsic diffusion coefficient for 137Cs in the 3:1 mix of pulverised fuel ash to ordinary Portland cement of 4×10−15 m2s−1 was estimated, significantly lower than that determined for HTO.The results from three of the sixteen experiments could not be fitted with a simple diffusion model, and for a further five experiments there was some doubt as to whether simple diffusion behaviour had been observed. It is suggested that this may have been due to cracks in the grouts that were sufficiently large to affect the diffusion properties of the grouts, although none was visible to the naked eye. Cracking of the waste encapsulation grouts could provide a mechanism for enhanced migration of radionuclides from waste packages, compared with diffusion in a homogeneous porous medium alone.

Diffusivity anisotropy in a rhyolite and its relation to pore structure

The diffusivity anisotropy of ions through rock pore water was evaluated quantitatively by through-diffusion experiments on a rhyolite rock having anisotropic pore structure. The rhyolite has planar flow structure with elongated pore shapes to the flow direction ( X– Y plane). Diffusion coefficients of K + ions for the direction perpendicular to this flow structure ( Z-direction) were about 5–9 times smaller than those for the orthogonal parallel directions ( X- and Y-direction). Pore geometrical analyses on backscattered electron microscopic images indicated that the pore length ratios among the X-, Y-, and Z-direction were roughly 2 : 2 : 1. This shorter pore length for the Z-direction appears to reduce pore connectivity, causing larger tortuosity and smaller diffusivity for this direction. This diffusivity anisotropy needs to be taken into account for precise modeling of diffusion-limited geological processes such as contaminant transport and rock deformation.

A high-resolution abrasive method for determining diffusion profiles of sorbing radionuclides in dense argillaceous rocks

The diffusion of 134Cs + and 22Na + in Opalinus Clay (OPA) was studied by in-diffusion laboratory experiments. The diffusive tracer profiles in the rock were determined using a high-resolution abrasive peeling method. The radionuclide activities in the grinding swarf were measured directly via γ -spectrometry. By choosing the appropriate abrasive paper, a resolution down to 15 μm can be achieved. This is important when analysing strongly sorbing radionuclides such as tri- and tetravalent actinides that show steep, shallow diffusion profiles. In this study, a resolution between 20 and 90 μm was obtained which was sufficient for a good spatial resolution of the diffusion profiles. Both the effective diffusion coefficients and the distribution coefficients of the radionuclides could be determined by applying a single reservoir with decreasing source concentration analysis for a semi-infinite case. In the case of 22Na +, effective diffusion coefficients of D e = 2.0 × 1 0 - 11 m 2 s - 1 and D e = 1.5 × 1 0 - 11 m 2 s - 1 for Benken (Zürcher Weinland) OPA and Mont Terri OPA, respectively, were derived. The distribution coefficients were K d = 3.1 × 1 0 - 4 and 0.9×10 −4 m 3 kg −1, respectively. For 134Cs + the effective diffusion coefficients were higher, i.e. D e = 3.1 × 1 0 - 11 m 2 s - 1 for OPA from Benken and D e = 3.0 × 1 0 - 11 m 2 s - 1 for OPA from Mont Terri. The distribution coefficients determined were K d = 0.16 m 3 kg - 1 for Benken and 0.23 m 3 kg −1 for Mont Terri. Comparison of the data obtained for the weakly sorbing 22Na + with those from earlier through-diffusion experiments showed that there is good agreement between the two methods. In the case of 134Cs + such a comparison was not possible because through-diffusion data are not available. Because through-diffusion methods cannot be applied to strongly sorbing tracers in reasonable time periods, in-diffusion combined with high-resolution abrasive peeling offers an excellent alternative for measuring the diffusion properties of strongly sorbing tracers in dense argillaceous rocks.