Applied Geochemistry (v.49, #C)

Preface: SI: Geo and risk assessment by Lara Duro; Birgitta Kalinowski (1).

Applying the squeezing technique to highly consolidated clayrocks for pore water characterisation: Lessons learned from experiments at the Mont Terri Rock Laboratory by A.M. Fernández; D.M. Sánchez-Ledesma; C. Tournassat; A. Melón; E.C. Gaucher; J. Astudillo; A. Vinsot (2-21).
Knowledge of the pore water chemistry in clayrock formations plays an important role in determining radionuclide chemical speciation and migration in the context of nuclear waste disposal. Among the different in situ and ex situ techniques for pore water sampling in clay sediments and soils, the squeezing technique dates back 115 years. Although different studies have been conducted on the reliability and representativeness of squeezed pore waters, most of these involved high-porosity, high water content and unconsolidated clay sediments. Very few of them analysed squeezed pore water from low-porosity, low water content and highly consolidated clayrocks.In this paper, a specially designed and fabricated one-dimensional compression, two-directional fluid flow cell was used to extract and analyse the pore water composition of Opalinus Clay core samples from Mont Terri (Switzerland) with water contents between 6.2 and 7.8 wt.%. A study of the influence of the squeezing pressure on the chemistry of the pore water was performed at pressures of up to 200 MPa. To validate and demonstrate the validity of the squeezing technique for obtaining reliable pore water, different tests were performed at various pressures and the squeezed waters were compared with those obtained by other methods, such as in situ collected borehole waters. The reproducibility and quality of the squeezing method was also checked, as well as different artifacts which could affect the reliability of the pore water data.The results show that the reproducibility of the technique is good and no membrane effect (ionic ultrafiltration, chemical fractionation, anion exclusion) was found in Opalinus Clay at Mont Terri in the range of pressures analysed (70–200 MPa). Pore waters extracted in this range of pressures do not decrease in concentration as a function of pressure, which indicates that a dilution of water by mixing of the free pore water and the outer layers of diffuse layer water does not occur and a threshold (safety) squeezing pressure of 175 MPa could be established to avoid membrane effects. Furthermore, a direct comparison against in situ collected borehole waters shows that the pore waters extracted at these pressures are representative of the Opalinus Clay Formation at Mont Terri. Extraction artifacts such as temperature effect, oxidation and degassing can be avoided or minimised by taking special precautions when preparing and handling the core sample and extracting the pore water. Finally, we show that it is possible to obtain insightful information on anion-accessible porosity by combining squeezing data with aqueous leaching data to characterise the pore water composition and solute distribution for different rock porosities.

Characterization of porewater chemistry in low-permeability rocks can provide insight into the origin and residence time of porewater, the history of fluid movement and the nature of transport and reaction processes. However, the measurement of porewater chemistry in low-permeability rocks is challenging because of the small fluid volume and the difficulty of extracting representative samples. Several techniques are available, but the results they provide can be affected by ion exchange and mineral dissolution, and they may require independent porosity measurements. The objective of this work is to develop a method of extracting representative samples of in situ porewater from low-permeability rocks and accurately quantify solute concentrations in the extracted porewater. A preliminary trial demonstrated the feasibility of extracting porewater by absorption into hydrophilic cellulosic membranes from low-permeability shale (Georgian Bay Formation, Michigan Basin, southwest Ontario, Canada). Solute concentrations are calculated from independent measurements of solute mass quantified by Inductively-Coupled-Plasma Mass Spectrometry (ICP-MS) and water-content quantified by Near Infrared (NIR) spectrometry. Sorption experiments indicate that there is no preferential sorption of solutes (Na, Cl, Mg, Ca, K, Sr, and Br) to the cellulosic membrane. The results indicate that the method is capable of determining solute concentrations on absorbed porewater with analytical precision that is within the margins of error recommended by EPA Method 6020a for analysis of saline water samples.

Dissolution of ZrO2 based pyrochlores in the acid pH range: A macroscopic and electron microscopy study by S. Finkeldei; F. Brandt; K. Rozov; A.A. Bukaemskiy; S. Neumeier; D. Bosbach (31-41).
The dissolution kinetics of ZrO2–Nd2O3 polycrystalline pyrochlore and defect fluorite ceramic powders under acidic conditions were observed following a combined macroscopic and electron microscopic (SEM) approach. Dynamic dissolution experiments were carried out with a variation of temperature and pH as well as the chemical composition within the ZrO2–Nd2O3 system. SEM observations indicate a preferential leaching at the grain boundaries for all experiments. A preferential release of Nd during the initial stages of dissolution, which is several orders of magnitude higher than the Zr release, was measured by ICP-MS. At steady state, the normalised Nd-rate approaches the Zr based dissolution rate within one order of magnitude, becoming congruent for most experiments. Zr-based BET surface area normalised steady state rates at c(H+) = 0.1 N and 90 °C are in the range between 4 × 10−7 and 3 × 10−6  g m−2  d−1, indicating no significant influence of the transition pyrochlore to defect fluorite and the chemical composition on the macroscopic dissolution rate. Based on the different numbers of grain boundaries per surface area in the pyrochlore compared to the defect fluorite, a slightly higher “true” dissolution rate could be assumed for the defect fluorite. The influence of pH and temperature variations to the dissolution rates of defect fluorite and pyrochlore are similar and in the range observed for other multioxide materials.

Solubility study and point of zero charge of studtite (UO2O2·4H2O) by J. Giménez; J. de Pablo; I. Casas; X. Martínez-Lladó; M. Rovira; A. Martínez Torrents (42-45).
The knowledge of the characteristics of the U(VI)-peroxide chemical system is of importance because of the relevance of the uranyl peroxide solid phases studtite (UO2O2·4H2O) or metastudtite (UO2O2·2H2O) as uranyl secondary solid phases during the leaching of the spent nuclear fuel as well as to the formation in alkaline waters of soluble nanoscale uranyl peroxo cage clusters. In this work, the solubility of studtite was determined for the first time as a function of pH (from 3 to 11) by means of studtite undersaturation experiments. The results obtained showed a V-shaped solubility curve, which was modelled considering the uranyl complexes formation constants (database of the Nuclear Energy Agency, NEA) and recently published formation constants of uranium hydroxo-peroxo-complexes. The best fit of the model to the experimental solubility data was obtained with log  K s0  = −2.7 ± 0.2 for the studtite solubility reaction: UO 2 O 2 · 4 H 2 O ( s ) + 2 H + ⇆ UO 2 2 + + 4 H 2 O + H 2 O 2 On the other hand, the so-called immersion methodology was used in order to determine the pHpzc of studtite, which resulted to be 4.0 ± 0.2. This value would corroborate a sorption mechanism based on electrostatic interactions described for previously published sorption data of cations (Cs+, Sr2+) on studtite.

Dissolution study of tremolite and anthophyllite: pH effect on the reaction kinetics by M. Rozalen; M.E. Ramos; F. Gervilla; T. Kerestedjian; S. Fiore; F.J. Huertas (46-56).
The effect of pH on the kinetics of tremolite and anthophyllite dissolution was investigated at 25 °C in batch reactors over the pH range of 1–13.5, in inorganic buffered solutions. Dissolution rates were obtained based on the release of Si and Mg. Results obtained in this study show different behaviors for both minerals. For tremolite, dissolution rates show a noticeable dependence on pH between 1 and 8, decreasing as pH increases and reaching a minimum around neutral conditions. At basic pH this dependence becomes even stronger, but dissolution takes place together with collateral effects of saturation and carbonation. A preferential release of Ca and Mg is observed in acid media, lowering the Mg/Si ratio to the extent that Mg solubility decreases with pH. For anthophyllite, dissolution rates also show a strong dependence on pH, between 1 and 9.5. At the same pH, anthophyllite dissolves up to 8 times faster than tremolite. For pH > 9.5 this dependence is smooth, and it is probably associated with effects of saturation and carbonation. Dissolution is also non-stoichiometric with a faster release of Mg with respect to Si in acid media. SEM observations show differences in the breakage mechanism of the fibers. The anthophyllite particle breakage during dissolution consists of the splitting of bundle fibers parallel to the fiber longitudinal direction. However, for tremolite, other than fiber splitting, particles shorten induced by coalescence of etch pits developed perpendicular to c axe.

The linkage between the iron and the carbon cycles is of paramount importance to understand and quantify the effect of increased CO2 concentrations in natural waters on the mobility of iron and associated trace elements. In this context, we have quantified the thermodynamic stability of mixed Fe(III) hydroxo-carbonate complexes and their effect on the solubility of Fe(III) oxihydroxides. We present the results of carefully performed solubility measurements of 2-line ferrihydrite in the slightly acidic to neutral–alkaline pH ranges (3.8–8.7) under constant pCO2 varying between (0.982–98.154 kPa) at 25 °C.The outcome of the work indicates the predominance of two Fe(III) hydroxo carbonate complexes FeOHCO3 and Fe(CO3)3 3−, with formation constants log* β°1,1,1  = 10.76 ± 0.38 and log  β°1,0,3  = 24.24 ± 0.42, respectively.The solubility constant for the ferrihydrite used in this study was determined in acid conditions (pH: 1.8–3.2) in the absence of CO2 and at T  = (25 ± 1) °C, as log* Ks ,0  = 1.19 ± 0.41.The relative stability of the Fe(III)-carbonate complexes in alkaline pH conditions has implications for the solubility of Fe(III) in CO2-rich environments and the subsequent mobilisation of associated trace metals that will be explored in subsequent papers.

Analysis of anion adsorption effects on alumina nanoparticles stability by Tiziana Missana; Ana Benedicto; Natalia Mayordomo; Ursula Alonso (68-76).
Nanoparticles and colloids can be relevant in contaminant migration if the contaminant is strongly adsorbed onto the particles and they are stable and mobile. The main conditions required for a colloidal system to be considered as “stable” are: (1) low ionic strength of the groundwater (⩽1 × 10−3  M) and (2) pH far from the point of zero charge (pHPZC). These conditions however, are too simplified to describe the colloidal behaviour in real cases; in fact, specific adsorption of different ions may also have an important impact on colloid stability.In particular, in this study we analyse the effects of the adsorption of inorganic anions on Al2O3 oxide nanoparticles (alumina NPs) stability, combining batch adsorption studies with electrophoretic and dynamic light scattering measurements. Selenite adsorption was studied in a wide range of pHs (3–11), ionic strengths (5 × 10−4 – 1 × 10−1  M) and selenite concentration. Different electrolytes were used to understand the competitive effects for selenite sorption of different anions in solution (ClO4 , NO3 , HCO3 , SO4 2−) and especially their overall influence on alumina NPs stability.The positive charge of alumina, under acid and neutral-alkaline pHs, favours anion adsorption which, in turn, may result in a decrease of the net surface charge, promoting particle aggregation and the destabilisation of the system. Results showed that, the higher the anion affinity for alumina surface and sites occupancy, the higher the destabilisation of particles. The sorption selectivity observed in our study was: SeO3 2−  > SO4 2−  > HCO3  > NO3  > ClO4 .Upon anion adsorption, particles aggregation was evident, but a clear change in ζ-potential, was only observed with very high surface occupancies. Surface complexation modelling has been shown to be useful supporting tool for stability studies.

Microbial communities in bentonite formations and their interactions with uranium by Margarita López-Fernández; Omar Fernández-Sanfrancisco; Alberto Moreno-García; Inés Martín-Sánchez; Iván Sánchez-Castro; Mohamed Larbi Merroun (77-86).
A reliable performance assessment of deep geological disposal of nuclear waste depends on better knowledge of radionuclide interactions with natural microbes of geological formations (granitic rock, clay, salts) used to host these disposal systems. In Spain, clay deposits from Cabo de Gata region, Almeria, are investigated for this purpose. The present work characterizes the culture-dependent microbial diversity of two bentonite samples (BI and BII) recovered from Spanish clay deposits. The evaluation of aerobe and facultative anaerobe microbial populations shows the presence of a high number of cultivable bacteria (e.g. Stenotrophomonas, Micrococcus, Arthrobacter, Kocuria, Sphingomonas, Bacillus, Pseudomonas, etc.) affiliated to three phyla Proteobacteria, Actinobacteria, and Firmicutes. In addition, a pigmented yeast strain BII-R8 related to Rhodotorula mucilaginosa was also recovered from these formations. The minimal inhibitory concentrations of uranium for the growth of these natural isolates were found to range from 4 to 10.0 mM. For instance, strain R. mucilaginosa BII-R8 was shown to tolerate up to 8 mM of U. Flow cytometry studies indicated that the high U tolerance of this yeast isolate is a biologically mediated process. Microscopically dense intracellular and cell wall-bound precipitates were observed by Scanning Transmission Electron Microscopy-High-Angle Annular Dark-Field (STEM-HAADF). Energy Dispersive X-ray (EDX) element-distribution maps showed the presence of U and P within these accumulates, indicating the ability of cells to precipitate U as U(VI) phosphate minerals. Fundamental understanding of the microbial diversity of clays and microbial interaction with radionuclides will be useful in predicting the microbial impacts on the performance of the waste repositories, as well as in the development of bioremediation strategies for U contaminated sites.

Se(IV) uptake by Äspö diorite: Micro-scale distribution by Ursula Alonso; Tiziana Missana; Alessandro Patelli; Daniele Ceccato; Miguel Garcia-Gutierrez; Valentino Rigato (87-94).
Risk assessment of deep geological repositories (DGR) for high level radioactive waste demands radionuclide (RN) sorption data, with minimised uncertainties, on repository barriers. Sorption data in crystalline rocks, are principally obtained on crushed material and reported as distribution coefficients, relative to the mass of solid (K d in m3/kg). Main sources of uncertainty on available sorption data are due to differences in experimental approach (comparison of crushed and intact material), groundwater and rock composition, redox conditions or the high heterogeneity of crystalline rock.In this study, the effect of rock mineral heterogeneity on selenium(IV) surface distribution on diorite crystalline rock was analysed under oxic and anoxic conditions. The micro-Particle Induced X-ray Emission (μPIXE) ion beam technique was selected because it allows quantifying tracer distribution directly on intact rock samples, at mineral micro-scale.Diorite samples were extracted from the Äspö underground research laboratory (Sweden), handled and transported under anoxic conditions. Maintaining controlled redox conditions during the whole experiment is considered particularly relevant to preserve real repository conditions.Selenite distribution on Äspö diorite surface was heterogeneous, its retention being higher under anoxic conditions. By μPIXE analyses the main Se retentive regions were identified, equivalent in both oxic/anoxic conditions, and surface distribution coefficients (K a) were determined on main diorite minerals. Sorption values ranged from near zero, on quartz or K-feldspars, to higher values on Fe-bearing minerals like biotite (K a  ∼ 7 × 10−5  m) under anoxic conditions. Experimental K a values determined here are compared to reported distribution coefficients (K d).

Modelling of Cs sorption in natural mixed-clays and the effects of ion competition by Tiziana Missana; Miguel García-Gutiérrez; Ana Benedicto; Carlos Ayora; Katrien De-Pourcq (95-102).
Cs migration in the environment is mainly controlled by sorption onto mineral surfaces, in particular on clay minerals. With the objective of designing a geochemical reactive barrier to treat 137Cs accidental pollution in an industrial waste repository, different natural clayrocks were studied to analyse their capacity to retain Cs.The simple semi-empiric Kd -approach for experimental data analysis, is unsatisfactory to describe the variability of sorption upon chemical changes. Indeed, due to the high salinity of the site, the effects of competitive ions must be evaluated and quantified. Thus, the development of sorption models, capable of reproducing experimental data obtained under conditions representative of the contaminated site, and applicable to reactive transport studies, is needed.In this study, a model for Cs sorption, which takes into account the main mineralogy of the sorbent, the composition of the natural water (and ion competition) was successfully applied to interpret the non-linear Cs sorption under natural conditions.The selectivity coefficients of Cs with respect to the most important cations present in the site water (Na, K, NH4, Ca) were derived by means of experiments in single clay minerals and synthetic mono-component solutions. Then, these parameters were tested in systems of increasing complexity.Considering the mineralogical composition of raw materials, it was shown that the principal contribution to Cs sorption is given by the mineral illite, while smectite starts to be relevant only at very high Cs loadings. Kaolinite, even in concentrations around 10 wt% of the clayey fraction, played only a minor role.With respect to the solution composition, the model was able to predict Cs sorption in electrolyte concentrations up to twice than that of seawater and up to 500 mg/L NH4 +. The effect of highly competing ions, especially NH4 + and K+, on Cs retention is more important at low ionic strengths and low Cs loadings, where adsorption is dominated by illite selective frayed edge sites, FES. Divalent cations are not especially relevant as competing cations for Cs.

A generic modelling approach has been used to estimate the influence of the stable inventory, i.e. stable isotopes in a radioactive waste repository on the migration of radionuclides from waste canisters into the surrounding bentonite or Opalinus Clay. The model radionuclide chosen was bivalent 59Ni(II); the stable isotopes Ni(II), Fe(II), Mn(II), Zn(II) and Cu(II) are considered to be competitive for the same sorption sites on bentonite or Opalinus Clay. A simplified one-dimensional modelling approach in space was used for reactive transport calculations using the MCOTAC code incorporating the 2SPNE SC/CE sorption model. Calculated 59Ni(II) breakthrough curves in bentonite and Opalinus Clay are compared to estimates of the influences of the individual competing metals present in the porewaters.Generally, faster migration, i.e. a reduced sorption, for 59Ni(II) was calculated – up to two orders of magnitude in arrival time at specified locations in the bentonite or Opalinus Clay. This influence is a maximum for highest specified stable isotope concentrations.Fe, Zn and Mn have approximately the same effect on the migration of 59Ni(II); Cu has the potential for a much stronger effect. However, their individual effects at reducing the retardation of 59Ni(II) through sorption competition do not sum up linearly. In the various scenarios calculated, an upper limit for the reduction of the retardation of 59Ni(II) has been assessed for the combined sorption competition influence of all the stable isotopes.Although the calculated scenarios include several simplifications, they cover a wide range of combinations of sorption competition effects of stable isotopes present in a high-level waste repository on the migration of radioactive 59Ni(II). More detailed scenario calculations would be possible if a more detailed “geochemical inventory” of radionuclides and stable isotopes were to become available. Nevertheless, upper limits for the effects of sorption competition of bivalent stable isotopes on the migration of 59Ni in the vicinity of a high-level nuclear waste repository were assessed.

Interaction of U(VI) with Äspö diorite: A batch and in situ ATR FT-IR sorption study by K. Schmeide; S. Gürtler; K. Müller; R. Steudtner; C. Joseph; F. Bok; V. Brendler (116-125).
Pristine diorite drill cores, obtained from the Äspö Hard Rock Laboratory (HRL, Sweden), were used to study the retention properties of fresh, anoxic crystalline rock material towards the redox-sensitive uranium. Batch sorption experiments and spectroscopic methods were applied for this study. The impact of various parameters, such as solid-to-liquid ratio (2–200 g/L), grain size (0.063–0.2 mm, 0.5–1 mm, 1–2 mm), temperature (room temperature and 10 °C), contact time (5–108 days), initial U(VI) concentration (3 × 10−9  to 6 × 10−5  M), and background electrolyte (synthetic Äspö groundwater and 0.1 M NaClO4) on the U(VI) sorption onto anoxic diorite was studied under anoxic conditions (N2). Comparatively, U(VI) sorption onto oxidized diorite material was studied under ambient atmosphere (pCO2  = 10−3.5  atm). Conventional distribution coefficients, K d, and surface area normalized distribution coefficients, K a, were determined. The K d value for the U(VI) sorption onto anoxic diorite in synthetic Äspö groundwater under anoxic conditions by investigating the sorption isotherm amounts to 3.8 ± 0.6 L/kg which corresponds to K a  = 0.0030 ± 0.0005 cm (grain size 1–2 mm). This indicates a weak U sorption onto diorite which can be attributed to the occurrence of the neutral complex Ca2UO2(CO3)3(aq) in solution. This complex was verified as predominating U species in synthetic Äspö groundwater by time-resolved laser-induced fluorescence spectroscopy (TRLFS). Compared to U sorption at room temperature under anoxic conditions, U sorption is further reduced at decreased temperature (10 °C) and under ambient atmosphere. The U species in aqueous solution as well as sorbed on diorite were studied by in situ time-resolved attenuated total reflection Fourier-transform infrared (ATR FT-IR) spectroscopy. A predominant sorbing species containing a UO2(CO3)3 4− moiety was identified. The extent of U sorption onto diorite was found to depend more on the low sorption affinity of the Ca2UO2(CO3)3(aq) complex than on reduction processes of uranium.

Cementitious materials will be used for the construction of the engineered barrier of the planned repositories for radioactive waste in Switzerland. Superplasticizers (SPs) are commonly used to improve the workability of concretes and, along with a set accelerator (Acc), to produce shotcrete. In this study the influence of a polycarboxylate- (PCE) and a polynaphthalene-sulphonate-based (PNS) SP on the hydration process, mineral composition and the sorption behaviour of metal cations has been investigated using an ordinary Portland cement (OPC), a low-alkali cement mix (LAC) consisting of CEM III-type cement and nanosilica, and a shotcrete-type cement mix (ESDRED) consisting of a CEM I-type cement and silica fume prepared in the presence of an alkali-free set accelerator.Both the PCE and PNS SP do not significantly influence the amount and quantity of hydrates formed during hydration. The concentration of both SPs decreased rapidly in the early stage of the hydration process for all cements due to sorption onto cement phases. After 28 days of hydration and longer, the concentration of the PNS SP in the pore fluids of all cements was generally lower than that of the PCE SP, indicating stronger uptake of the PNS SP. The formate present in the Acc sorbs only weakly onto the cement phases, which led to higher aqueous concentration of organics in the ESDRED cement than in OPC and LAC.Sorption experiments with 63Ni, 152Eu and 228Th on a cation exchange resin indicate that, at concentrations above 0.1 g L−1, the two SPs could reduce sorption of metal cations. Thermodynamic modelling further indicates that radionuclide complexation by formate at the concentration level in Acc can be excluded, suggesting that the apparent effect of Acc in the sorption measurements on the resin can be attributed to colloids formed owing to the high concentrations of Al and S in Acc. Sorption studies with the same radionuclides on hardened cement paste (HCP) in the presence of concrete admixture solutions and pore fluids squeezed from cement pastes further revealed no significant effect on sorption by either the concrete admixtures or their degradation products that were potentially present in the pore fluids. This finding suggests that the investigated concrete admixtures (PNS, PCE, Acc) and their degradation products have no significant impact on radionuclide mobilisation.

Near-surface cement-based disposal systems for hazardous materials such as radioactive waste will undergo chemical alterations due to interaction with the surrounding environment. One of the most relevant long-term geochemical alteration processes is decalcification or leaching of the cement phases by percolating water. Consequently, the cementitious components of the disposal system will evolve through different chemical degradation states, also altering physical material parameters such as porosity and bulk density and chemical parameter relevant to solute migration such as the solid–liquid partition coefficient or distribution ratio. This paper presents a novel approach in which geochemical modelling serves as a fundamental basis for assessing the evolution of geochemical conditions within a cement-based near-surface disposal facility. On one hand, geochemical modelling is used to quantify uncertainties related to the infiltrating water composition and C–S–H degradation model, both of which allow for various conceptualisations of the evolution of retardation factor. On the other hand, the concept of mixed tank reactor is used to represent cement degradation within the entire disposal system. This paves the way to establish a link between the evolution of the geochemical conditions and the evolution of the retardation factor via the knowledge of amount of percolated water through the system. The usefulness of the approach is demonstrated via a number of case studies concerning leaching of radionuclides (14C and 94Nb) from a cementitious near-surface disposal facility. The studies reveal that there is a large effect of the conceptualisation on calculated fluxes from the disposal facility, depending on the type of radionuclide. A crucial factor is the amount of radionuclide mass present in the disposal system when large changes in the retardation factor occur, for instance, when different retardation factors exist in different chemical degradation states.

FASTREACT – An efficient numerical framework for the solution of reactive transport problems by Paolo Trinchero; Jorge Molinero; Gabriela Román-Ross; Sten Berglund; Jan-Olof Selroos (159-167).
In the framework of safety assessment studies for geological disposal, large scale reactive transport models are powerful inter-disciplinary tools aiming at supporting regulatory decision making as well as providing input to repository engineering activities. Important aspects of these kinds of models are their often very large temporal and spatial modelling scales and the need to integrate different non-linear processes (e.g., mineral dissolution and precipitation, adsorption and desorption, microbial reactions and redox transformations). It turns out that these types of models may be computationally highly demanding. In this work, we present a Lagrangian-based framework, denoted as FASTREACT, that aims at solving multi-component-reactive transport problems with a computationally efficient approach allowing complex modelling problems to be solved in large spatial and temporal scales. The tool has been applied to simulate radionuclide migration in a synthetic heterogeneous transmissivity field and the results have been successfully compared with those obtained using a standard Eulerian approach. Finally, the same geochemical model has been coupled to an ensemble of realistic three-dimensional transport pathways to simulate the migration of a set of radionuclides from a hypothetical repository for spent nuclear fuel to the surface. The results of this modelling exercise, which includes key processes such as the exchange of mass between the conductive fractures and the matrix, show that FASTREACT can efficiently solve large-scale reactive transport models.

Hydrogen uptake and diffusion in Callovo-Oxfordian clay rock for nuclear waste disposal technology by Fabrizio Bardelli; Claudia Mondelli; Mathilde Didier; Jenny G. Vitillo; Demetrio R. Cavicchia; Jean-Charles Robinet; Laura Leone; Laurent Charlet (168-177).
The Callovo-Oxfordian clay-rich rock formation is currently considered as host rock barrier in the French geological repository facility for radioactive waste (Meuse/Haute-Marne). After the closure of the facility, hydrogen gas is expected to develop mainly from anaerobic corrosion of steel containers and other iron-containing structures. Gas pressure build-up could impact the safety of the repository. It is therefore important to acquire in-depth knowledge on the interaction between hydrogen gas and surrounding clay rock in terms of uptake ability and diffusion. Hydrogen uptake capacity was evaluated on dried clay rock samples: (i) at 20 K, to allow for hydrogen liquefaction and determine the maximal H2 uptake of the clay, and (ii) at typical pressure and temperature conditions expected to develop in the repository (up to 363 K and a hydrogen pressure of 40–60 bar). H2 absorption on the dried raw Callovo-Oxfordian start to saturate at about 30–40 bar, and the average adsorption above 40 bar is about 0.1% in weight. Quasi-elastic neutron scattering spectroscopy was used to study the diffusion mechanism of hydrogen gas in the clay rock at the microscopic scale and to determine hydrogen self-diffusion coefficients in the dry samples in the temperature range 25–300 K. Neutron data suggested that hydrogen diffuses in the dry clay rock according to Fick’s law. The findings reported in this work can help to better understand the behavior of H2 in clay rock samples.

The present paradigm on UO2 spent fuel stability under anoxic conditions assumes that the potential oxidative alteration of the matrix is suppressed in the presence of the hydrogen generated by the anoxic corrosion of iron by water. The observations from the Cigar Lake Natural Analogue project indicated the long-term stability of the uraninite ore under anoxic conditions and with substantial hydrogen generation. The radiolytic models developed in the analogue project have been used to test some of the hypothesis concerning the activation of hydrogen on the uranium(IV) oxide surface. Suggestions to pathways of radiolytic oxidant consumption by other processes than uranium dioxide or sulphide oxidation are presented. The stability of the ore body for billions of year indicates the presence of processes which neutralise radiolytic oxidants and one major factor may be the presence of dissolved hydrogen in the groundwaters contacting the ore body. The results from this test would indicate that hydrogen is activated on the surface of the Cigar Lake uraninites by alpha radiation consuming the generated radiolytic oxidants.

Method development for evaluating the redox state of Callovo-Oxfordian clayrock and synthetic montmorillonite for nuclear waste management by Mathilde Didier; Antoine Géhin; Jean-Marc Grenèche; Laurent Charlet; Eric Giffaut (184-191).
Understanding the redox characteristic of the host geological layer is vital for radioactive waste management. In order to predict the radionuclides behavior during their release it must be thoroughly evaluated. This redox property could be affected by hydrogen gas which arises from the anaerobic corrosion of the stainless steel container. In this study, reduction methods using hydrogen gas or sodium dithionite as a reductive agent were tested on reference synthetic montmorillonites with various Fe(III) contents. The reduced samples were systematically studied with 57Fe transmission Mössbauer spectrometry. After reduction with H2(g) in dry conditions, the Mössbauer spectra are characterized by hyperfine parameters located between those for Fe(III) and Fe(II), compared to reduction in water suspension with Na2S2O4(aq) and H2(g) which gives standard Fe(II) hyperfine parameters. The former results with dry H2(g) highlight an incomplete reduction and the possibility to have a Fe(III)–Fe(II) system with one-electron sharing. A natural clayrock sample, the Callovo-Oxfordian (COx), was also considered. The results above allowed its reduction to be evaluated. Much attention has also been focused on the modeling of the hyperfine spectra because of COx structure complexity. In addition, a hyperfine parameter data base was developed for a variety of Fe components based on an extensive literature review. This database provides additional statistical order to the study. This study highlights also that Mössbauer spectrometry remains a useful and non-destructive method to determine the reduction process and the reduction capacity of a reactant. Therefore we could estimate the redox property of the rock to evaluate and predict radionucleide behavior for nuclear waste management.

Assessment of the evolution of the redox conditions in a low and intermediate level nuclear waste repository (SFR1, Sweden) by Lara Duro; Cristina Domènech; Mireia Grivé; Gabriela Roman-Ross; Jordi Bruno; Klas Källström (192-205).
Display OmittedThe evaluation of the redox conditions in an intermediate and low level radioactive waste repository such as SFR1 (Sweden) is of high relevance in the assessment of its future performance.The SFR1 repository contains heterogeneous types of wastes, of different activity levels and with very different materials, both in the waste itself and as immobilisation matrices and packaging. The level of complexity also applies to the different reactivity of the materials, so that an assessment of the uncertainties in the study of how the redox conditions would evolve must consider different processes, materials and parameters.This paper provides an assessment of the evolution of the redox conditions in the SFR1. The approach followed is based on the evaluation of the evolution of the redox conditions and the reducing capacity in 15 individual waste package types, selected as being representative of most of the different waste package types present or planned to be deposited in the SFR1. The model considers different geochemical processes of redox relevance in the system. The assessment of the redox evolution of the different vaults of the repository is obtained by combining the results of the modelled individual waste package types.According to the model results, corrosion of the steel-based material present in the repository keeps the system under reducing conditions for long time periods. The simulations have considered both the presence and the absence of microbial activity. In the initial step after the repository closure, the microbial mediated oxidation of organic matter rapidly causes the depletion of oxygen in the system. The system is afterwards kept under reducing conditions, and hydrogen is generated due to the anoxic corrosion of steel. The times for exhaustion of the steel contained in the vaults vary from 5 ky to more than 60 ky in the different vaults, depending on the amount and the surface area of steel. After the complete corrosion of steel, the system still keeps a high reducing capacity, due to the magnetite formed as steel corrosion product.The redox potential in the vaults is calculated to evolve from oxidising at very short times, due the initial oxygen content, to very reducing at times shorter than 5 years after repository closure. The redox potential imposed by the anoxic corrosion of steel and hydrogen production is on the order of −0.75 V at pH 12.5. In case of assuming that the system responds to the Fe(III)/magnetite system, and considering the uncertainty in the pH due to the degradation of the concrete barriers, the redox potential would be in the range −0.7 to −0.01 V.A Monte-Carlo probabilistic analysis on the rate of corrosion of steel shows that the reducing capacity of the system provided by magnetite is not exhausted at the end of the assessment period, even assuming the highest corrosion rates for steel.Simulations assuming presence of oxic water due to glacial melting, intruding the system 60 ky after repository closure, indicate that magnetite is progressively oxidised, forming Fe(III) oxides. The time at which magnetite is completely oxidised varies depending on the amount of steel initially present in the waste package.The behaviour of Np, Pu, Tc and Se under the conditions foreseen for this repository is discussed.

Redox processes influence key geochemical characteristics controlling radionuclide behaviour in the near and far field of a nuclear waste repository. A sound understanding of redox related processes is therefore of high importance for developing a Safety Case, the collection of scientific, technical, administrative and managerial arguments and evidence in support of the safety of a disposal facility. This manuscript presents the contribution of the specific research on redox processes achieved within the EURATOM Collaborative Project RECOSY (REdox phenomena COntrolling SYstems) to the Safety Case of nuclear waste disposal facilities. Main objectives of RECOSY were related to the improved understanding of redox phenomena controlling the long-term release or retention of radionuclides in nuclear waste disposal and providing tools to apply the results to Performance Assessment and the Safety Case. The research developed during the project covered aspects of the near-field and the far-field aspects of the repository, including studies relevant for the rock formations considered in Europe as suitable for hosting an underground repository for radioactive wastes. It is the intention of this paper to highlight in which way the results obtained from RECOSY can feed the scientific process understanding needed for the stepwise development of the Safety Case associated with deep geological disposal of radioactive wastes.

Human Health Risk Assessment of a landfill based on volatile organic compounds emission, immission and soil gas concentration measurements by Vicenç Martí; Irene Jubany; Consol Pérez; Xavier Rubio; Joan De Pablo; Javier Giménez (218-224).
A Human Health Risk Assessment (HHRA) was required for a closed landfill located in Cerdanyola del Vallès (Barcelona, Spain). The HHRA had two objectives, to evaluate the present risk of the identified receptors in the area and to safely develop the future urban planning of the area, therefore 3 scenarios for the current situation and 4 for the future situation were developed.After reviewing the existing data and exploring the needs of information, the assessment in this study was focused on the measurement of volatile organic compounds (VOCs) fluxes from the subsoil (emission from the landfill at 5 points), concentrations of VOCs in the air (immission in 4 urban sites) and concentration of VOCs in soil–gas (measurements at 5 m below ground surface outside the landfill at 8 sites). Around 70 VOCs were analyzed by using multi-sorbent tubes and Thermal Desorption Gas Chromatography (TD–GC–MS). The VOCs that were detected and quantified include alkanes, aromatic hydrocarbons, alcohols, ketones, halocarbons, aldehydes, esters, terpenoids, ethers and some nitrogenated and sulfur compounds, furans and carboxylic acids. Specific mercury flux measurements were performed in a hot spot by using carulite tubes, that were also analyzed by using Thermal Decomposition, Amalgamation, and Atomic Absorption Spectrophotometry.Results showed average values of volatile emission fluxes ranging from non-detected to 331 μg m−2  day−1 (dichlorodifluoromethane). In the case of immission, the concentration of VOCs measured in the air of populated area surrounding the landfill ranged values from non-detected to 42.0 μg m−3 (acetic acid). The soil–gas measurements in piezometers around the landfill showed individual VOC values with a maximum 830 μg m−3 for dichlorodifluoromethane.With the obtained fluxes and concentrations in air and soil–gas, USEPA methodology and modeling was used to evaluate equivalent concentration in the scenarios considered. Toxicity values from IRIS database were used to finally obtain chemical risk indicators. Admissible risk indicators were obtained in all scenarios. The VOCs that contributed more to risk indexes in RH2 were trichloroethylene, trimethylbenzene, chloroform, 1,2-dichloroethane and carbon tetrachloride. The carcinogenic risk in RH7 was linked to the presence of benzene and chloroform. The comparison of the measurements of the present work with other landfills evidence that HHRA in ambient air would be needed in order to perform a correct landfill management.

Andra thermodynamic database for performance assessment: ThermoChimie by E. Giffaut; M. Grivé; Ph. Blanc; Ph. Vieillard; E. Colàs; H. Gailhanou; S. Gaboreau; N. Marty; B. Madé; L. Duro (225-236).

Nuclear waste disposal: I. Laboratory simulation of repository properties by Bernd Grambow; Catherine Landesman; Solange Ribet (237-246).
After more than 30 years of research and development, there is a broad technical consensus that geologic disposal will provide for the safety of humankind, now and far into the future. Safety analyses have demonstrated that the risk, measured as the exposure to radiation, will be of little consequence. Still, there is not yet an operating geologic repository for highly radioactive waste, and there remains substantial public concern about the long-term safety of geologic disposal. In the two linked papers we argue for a stronger connection between the scientific data (this paper I) and the safety analysis, particularly in the context of societal expectations (paper II). In the present paper I, we use new experimental data on the properties of clay formations simulating geological disposal conditions to illustrate how one can understand the ability of clay to isolate radionuclides. The data include percolation tests on various intact clay–rock cores with different calcite contents. For the first time, hydrodynamic parameters (anion and cation accessible porosities, permeability, dispersion and diffusion coefficients), as well as retention parameters (sorption behavior of iodine, cesium) and materials interaction parameters (glass dissolution rates, etc.) have been obtained for a series of clay–rock samples of varying mineralogy. Increased calcite content leads to lower permeability and porosity, but the difference between anion and cation accessible porosity diminished. The data confirm very slow radionuclide migration, and a direct extrapolation to repository geometry yields isolation times, for a 70 m clay–rock formation, of many hundreds of thousands of years, even for the most mobile radionuclides such as iodine-129 and chlorine 36 and complete retention for the more radiotoxic, less mobile radionuclides such as the actinides or cesium-137.In order to assess the meaning of the technical results and derived models for long-term safety, paper II addresses model validity and credibility not only from a technical perspective, but in a much broader historical, epistemological and societal context. Safety analysis is treated in its social and temporal dimensions. This approach provides new insights into the societal dimension of scenarios and risk, and it shows that there is certainly no direct link between increased scientific understanding and a public position for or against different strategies of nuclear waste disposal.

After more than 30 years of international research and development, there is a broad technical consensus that geologic disposal of highly-radioactive waste will provide for the safety of humankind and the environment, now, and far into the future. Safety analyses have demonstrated that the risk, as measured by exposure to radiation, will be of little consequence. Still, there is not yet an operating geologic repository for highly-radioactive waste, and there remains substantial public concern about the long-term safety of geologic disposal. In these two linked papers, we argue for a stronger connection between the scientific data (paper I, Grambow et al., 2014) and the safety analysis, particularly in the context of societal expectations (paper II). In this paper (II), we assess the meaning of the technical results and derived models (paper I) for the determination of the long-term safety of a repository. We consider issues of model validity and their credibility in the context of a much broader historical, epistemological and societal context. Safety analysis is treated in its social and temporal dimensions. This perspective provides new insights into the societal dimension of scenarios and risk analysis. Surprisingly, there is certainly no direct link between increased scientific understanding and a public position for or against different strategies of nuclear waste disposal. This is not due to the public being poorly informed, but rather due to cultural cognition of expertise and historical and cultural perception of hazards to regions selected to host a geologic repository. The societal and cultural dimension does not diminish the role of science, as scientific results become even more important in distinguishing between the conflicting views of the risk of geologic disposal of nuclear waste.