Josep M. Soler

Comparison between in situ and laboratory diffusion studies of HTO and halides in Opalinus Clay from the Mont terri

Summary A long-term single-borehole diffusion experiment (DI) using tritiated water (HTO) and stable iodide (127I-) was carried out In the Opalinus Clay of the Mont Terri Underground Rock Laboratory (URL). Diffusion coefficients DL and accessible porosity for HTO, 36Cl- and 125I- were also measured on centimetric Opalinus clay samples using the through diffusion technique. The evolution of tritium and iodide concentration in the injection system over time and in situ profiles were interpreted with a 3-D numerical simulation. A detailed analysis of the results pointed out the effect of a disturbed zone around the borehole with higher diffusion coefficients. The best estimate values for HTO and iodide in the undisturbed rock are DL = 5×10-11 m2/s and DL = 1.5×10-11 m2/s respectively. For the laboratory tests, DL values for HTO are in the range of 5×10-11 m2/s to 8.5×10-11 m2/s. For 125I- and 36Cl- the measured values are DL=νmber1.4×10-11 and DL=1.6×10-11 m2/s respectively. All HTO results obtained with a through diffusion technique are within the same range as those obtained in the in situ tests. The DL values obtained in diffusion cells with 125I- and 36Cl- and the value drawn from the interpretation of stable 127I- concentration profiles from the in situ tests are very close. In fact, some significant uncertainties could be identified (i.e. a likely chemical retention of iodide on argillites, effect of the disturbed zone).

Comparative modeling of an in situ diffusion experiment in granite at the Grimsel Test Site

An in situ diffusion experiment was performed at the Grimsel Test Site (Switzerland). Several tracers ((3)H as HTO, (22)Na(+), (134)Cs(+), (131)I(-) with stable I(-) as carrier) were continuously circulated through a packed-off borehole and the decrease in tracer concentrations in the liquid phase was monitored for a period of about 2years. Subsequently, the borehole section was overcored and the tracer profiles in the rock analyzed ((3)H, (22)Na(+), (134)Cs(+)). (3)H and (22)Na(+) showed a similar decrease in activity in the circulation system (slightly larger drop for (3)H). The drop in activity for (134)Cs(+) was much more pronounced. Transport distances in the rock were about 20cm for (3)H, 10cm for (22)Na(+), and 1cm for (134)Cs(+). The dataset (except for (131)I(-) because of complete decay at the end of the experiment) was analyzed with different diffusion-sorption models by different teams (IDAEA-CSIC, UJV-Rez, JAEA) using different codes, with the goal of obtaining effective diffusion coefficients (De) and porosity (ϕ) or rock capacity (α) values. From the activity measurements in the rock, it was observed that it was not possible to recover the full tracer activity in the rock (no activity balance when adding the activities in the rock and in the fluid circulation system). A Borehole Disturbed Zone (BDZ) had to be taken into account to fit the experimental observations. The extension of the BDZ (1-2mm) is about the same magnitude than the mean grain size of the quartz and feldspar grains. IDAEA-CSIC and UJV-Rez tried directly to match the results of the in situ experiment, without forcing any laboratory-based parameter values into the models. JAEA conducted a predictive modeling based on laboratory diffusion data and their scaling to in situ conditions. The results from the different codes have been compared, also with results from small-scale laboratory experiments. Outstanding issues to be resolved are the need for a very large capacity factor in the BDZ for (3)H and the difference between apparent diffusion coefficients (Da) from the in situ experiment and out-leaching laboratory tests.

Interferometric study of pyrite surface reactivity in acidic conditions

Abstract Production of acid mine drainage (AMD) is in large part due to pyrite oxidation. The tie-in between environmental remediation and pyrite oxidation requires understanding pyrite oxidation in aqueous systems. In this study, ex-situ measurements using vertical scanning interferometry (VSI) were utilized to investigate pyrite surface reactivity under AMD conditions, such as pH 1 (HCl), an O2-saturated atmosphere, and room temperature, including (1) ex-situ measurements using vertical scanning interferometry (VSI) and (2) solution chemistry measurements using a flow reactor. In the former, two fragments were immersed in the acidic solution for 27 days at undersaturation with respect to pyrite. Weathered surfaces of pyrite that showed surface history (e.g., existence of terraces, steps, etch pits, and non-uniform surface roughness) were selected to examine surface topography changes with time. Based on the VSI measurements, the reaction mechanism includes the formation and coalescence of etch pits leading to overall surface retreat. This result is consistent with the stepwave model that predicts that, under sufficiently high undersaturation (i.e., ΔG < ΔGcrit), pit opening and generation of stepwaves are mechanisms that control mineral dissolution. Surface reactivity was not uniform over the entire surface, yielding surface regions with a considerable variety of dissolution rates that ranged from 1.7 × 10−7 mol/(m2·s) to lower than 2 × 10−11 mol/(m2·s). The overall pyrite dissolution rate calculated over the explored surface was 2.8 × 10−9 mol/(m2·s), which agrees very well with the absolute rate measured [3.1 × 10−9 mol/(m2·s)]. Based on the release of Fe in the flow experiment and normalizing with the geometric surface area, the steady-state pyrite dissolution rate obtained was 7.2 ± 1.5 × 10−9 mol/(m2·s), i.e., a factor of about 2 higher than that determined by VSI. Collectively, these rates also agree with pyrite dissolution rates obtained by bulk dissolution measurements. These results highlight (1) “local” dissolution rates vary widely over mineral surfaces at the short range scale from nano- to millimeter lengths; (2) inferred dissolution rates that are usually based on topographic changes on rather small (micrometer-scale size) surface regions are not necessarily representative of rates under field conditions; and (3) BET surface area and geometric surface area associated with reactive surface area and used to normalize the pyrite dissolution rates yield a variation in rate that ranges from 6.3 × 10−11 to 7.2 ± 1.5 × 10−9 mol/(m2·s).

Interaction between hyperalkaline fluids and rocks hosting repositories for radioactive waste : Reactive transport simulations

Abstract Reactive transport calculations simulating the interaction between hyperalkaline solutions derived from the degradation of cement and potential host rocks for repositories for low- and intermediate-level radioactive waste have been performed. Two different cases are shown: (a) The example of the planned repository at Wellenberg and (b) the modeling of the GTS-HPF experiment at the Grimsel Test Site. The GIMRT code has been used for the simulations. Mineral reactions are described by kinetic rate laws. The reaction rates for the primary minerals are based on experimentally determined rates published in the literature and geometric considerations combined with measurements regarding mineral surface areas. Relatively fast rates for the secondary minerals have been used, so the results resemble the local equilibrium solution for these minerals. In both cases, the alteration of the rock and the precipitation of secondary phases cause a reduction in the permeability of the system, which would actually be beneficial for the performance of a repository. Mineral surface area controls, to a large extent, the amount of mineral alteration and the change in permeability.

Cement-rock interaction: infiltration of a high-pH solution into a fractured granite core

Within the framework of the HPF project (Hyperalkaline Plume in Fractured Rock) at the Grimsel Test Site (Switzerland), a small scale core infiltration experiment was performed at the University of Bern. A high-pH solution was continuously injected, under a constant pressure gradient, into a cylindrical core of granite containing a fracture. This high-pH solution was a synthetic version of solutions characteristic of early stages in the degradation of cement. The interaction between the rock and the solutions was reflected by significant changes in the composition of the injected solution, despite the negligible pH-buffering capacity, and a decrease in the permeability of the rock. Changes in the mineralogy and porosity of the fault gouge filling the fracture were only minor. Within the new LCS (Long-Term Cement Studies) project at Grimsel, new one-dimensional reactive transport modeling using CrunchFlow has been used to improve the interpretation of the experimental results. Dispersive and advective solute transport, adsorption processes and mineral reaction kinetics have been taken into account. The evolution of solution composition is mainly controlled by dissolution/precipitation reactions. Adsorption processes (cation exchange, surface complexation) only play a role in the very early stages of the experiment.

Ni ENRICHMENT AND STABILITY OF Al-FREE GARNIERITE SOLID-SOLUTIONS: A THERMODYNAMIC APPROACH

Garnierites represent significant Ni ore minerals in the many Ni-laterite deposits worldwide. The occurrence of a variety of garnierite minerals with variable Ni content poses questions about the conditions of their formation. From an aqueous-solution equilibrium thermodynamic point of view, the present study examines the conditions that favor the precipitation of a particular garnierite phase and the mechanism of Ni-enrichment, and gives an explanation to the temporal and spatial succession of different garnierite minerals in Ni-laterite deposits. The chemical and structural characterization of garnierite minerals from many nickel laterite deposits around the world show that this group of minerals is formed essentially by an intimate intermixing of three Mg-Ni phyllosilicate solid solutions: serpentine-népouite, kerolite-pimelite, and sepiolite-falcondoite, without or with very small amounts of Al in their composition. The present study deals with garnierites which are essentially Al-free. The published experimental dissolution constants for Mg end-members of the above solid solutions and the calculated constants for pure Ni end-members were used to calculate Lippmann diagrams for the three solid solutions, on the assumption that they are ideal. With the help of these diagrams, congruent dissolution of Ni-poor primary minerals, followed by equilibrium precipitation of Ni-rich secondary phyllosilicates, is proposed as an efficient mechanism for Ni supergene enrichment in the laterite profile. The stability fields of the solid solutions were constructed using [{iexxxsbap19011700254003}] (predominance) diagrams. These, combined with Lippmann diagrams, give an almost complete chemical characterization of the solution and the precipitating phase(s) in equilibrium. The temporal and spatial succession of hydrous Mg- Ni phyllosilicates encountered in Ni-laterite deposits is explained by the small mobility of silica and the increase in its activity.

Direct nanoscale observations of the coupled dissolution of calcite and dolomite and the precipitation of gypsum

Summary In-situ atomic force microscopy (AFM) experiments were performed to study the overall process of dissolution of common carbonate minerals (calcite and dolomite) and precipitation of gypsum in Na2SO4 and CaSO4 solutions with pH values ranging from 2 to 6 at room temperature (23 ± 1 °C). The dissolution of the carbonate minerals took place at the (104) cleavage surfaces in sulfate-rich solutions undersaturated with respect to gypsum, by the formation of characteristic rhombohedral-shaped etch pits. Rounding of the etch pit corners was observed as solutions approached close-to-equilibrium conditions with respect to calcite. The calculated dissolution rates of calcite at pH 4.8 and 5.6 agreed with the values reported in the literature. When using solutions previously equilibrated with respect to gypsum, gypsum precipitation coupled with calcite dissolution showed short gypsum nucleation induction times. The gypsum precipitate quickly coated the calcite surface, forming arrow-like forms parallel to the crystallographic orientations of the calcite etch pits. Gypsum precipitation coupled with dolomite dissolution was slower than that of calcite, indicating the dissolution rate to be the rate-controlling step. The resulting gypsum coating partially covered the surface during the experimental duration of a few hours.

Reactive transport modeling of concrete-clay interaction during 15 years at the Tournemire Underground Rock Laboratory

Hydrated cement and concrete are major components of the engineered barrier system in proposed underground repositories for radioactive waste. Concrete was in contact with a clay-rich rock during 15 years in a borehole at the Tournemire Underground Rock Laboratory in France. Overcoring of the borehole and mineralogical analyses have shown a reduction of porosity at the interface due to the precipitation of ettringite, C-S-H/C-A-S-H and calcium carbonate, together with dissolution of portlandite in the cement. In the framework of the GTS-LCS project (JAEA, Japan; NAGRA, Switzerland; NDA, UK; POSIVA, Finland; SKB, Sweden), new reactive transport modeling (solute diffusion + mineral reaction) has been performed, including a sensitivity analysis with respect to several compositional and kinetic parameters. Results using the CrunchFlow code show sealing of porosity at the rock side of the interface (mm scale) due to the precipitation of C-A-S-H (calcium aluminum silicate hydrate), calcite and ettringite, together with some clay dissolution. The location of sealing is influenced by cation exchange. Inclusion of cation exchange results in sealing at the rock side of the interface. Without cation exchange, sealing is at the concrete side of the interface. Calculated alteration profiles along cm-scale fractures show increased alteration distances. Sealing of porosity is at the concrete side of the interface, due to the smaller effect of cation exchange.

Calcite interaction with acidic sulphate solutions: a vertical scanning interferometry and energy-dispersive XRF study

This work was financially supported by the CYCIT Project CGL2010-20984-C02-01, by Fundacio´n Ciudad de la Energi´a (Spanish Government) (project ALM11/009) and by the European Union through the ‘‘European Energy Programme for Recovery’’ and the Compostilla OXYCFB300 project and by the PANACEA project (European Community’s Seventh framework Programme FP7/2007-2013 under grant agreement number 282900). The first author is grateful to the National Science Fund of the Bulgarian Ministry of Education and Science (NZMU-1503 and DO1-904).

Modelling of Matrix Diffusion in a Tracer Test in Concrete

A laboratory-scale tracer test has been carried out to improve the characterization of the transport properties of the concrete from the radioactive waste disposal facility at El Cabril (Spain). High entry pressure was employed in order to perform the experiment in a reasonable time span. Lithium, bromide and deuterium were used as tracers. The conceptual model considered matrix diffusion between a mobile pore domain, where water can flow, and an immobile zone without any advective transport. Three geometries have been compared, considering the immobile zone as slabs, spheres or tubes. Porosity of the mobile zone and characteristic time was estimated by calibrating the model results to the measured breakthrough curves of deuterium and bromide. The calculated values showed that the characteristic time depends on the geometry, and similar porosity of the mobile zone was estimated for all geometries. The double-porosity conservative transport model could reproduce the deuterium breakthrough curve. However, the bromide behaviour could not be reproduced even when linear retardation was applied.