[en] This report summarizes the results of integrated calculations on the near-field evolution in the VHLW/steel/bentonite/clay system. The calculations of the near-field evolution include different components: the vitrified waste packages, the steel container, the bentonite-based EBS (optional), the EDZ and the geological medium. Coupled reaction-transport (X-T) is used to simulate the corrosion of the steel canister and the glass alteration phase in presence of corrosion products (CPs), looking at mass transfer for chemical elements, especially iron and silica, pH, and porosity change. Calculations as performed give actual parameters for PA calculations: rate of glass alteration (through the calculated pH) as a function of time, extension of altered zone for iron-clay interactions with their own transport parameters, nature of CPs, effect on porosity distribution. According to the operational model currently used at the CEA and the calculations performed on the glass-iron-clay system, the alteration rate of glass and the evolution of the system strongly depend on the timing of CPs saturation with respect to silica sorption. The fate of silica which can be sorbed or precipitate is crucial to the lifetime of glass and to the overall evolution of the system. The other process that might influence the glass is the porosity decrease due to the precipitation of CPs and silica rich phases. However, it is difficult to assign a safety functions to clogging. It is scarcely observed in experiments, either because the conditions are not met for clogging or because the timescale of experiments does not allow for observable clogging. Moreover, the effect of mechanical stress in the NF has to be accounted for in the assessment of the effect of porosity changes. (author)

[en] The author gives an overview of his research and teaching activities. His researches first dealt with the development of a simulation of the chemistry/transport coupling and of the retroactive effects on transport parameters, then with the chemistry/transport modelling and its coupling with mechanics, and finally with the multi-scale investigation of porous materials. Perspectives are discussed and publications are indicated

[en] This paper presents the results of a numerical investigation of transient redox conditions in argillaceous material in the context of HLW deep geological disposal. The redox state is a crucial parameter not only for the lifetime of the canisters but also for the migration of radionuclides after canister failure. Since reducing redox conditions prevail in deep underground porous media, the digging of excavations to build the repository will induce an oxidising perturbation in the vicinity of the galleries and tunnels. After emplacement of waste packages and closure of galleries, other processes will control the evolution of the redox conditions such as the corrosion of steel canisters and structures, or the radiolysis of water close to the waste packages. Since reducing conditions are considered favourable for the confining function of the repository, the purpose of this study is to quantify the persistence in time and space of the initial oxidising atmospheric perturbation. (authors)

[en] Natural or engineered clay-rich materials are ubiquitous when it comes to achieving sequestration of acid gas, confinement of pollutants or high level radioactive waste (HLRW), and trapping hydrocarbon oil and gas in geological settings. The sequestration, confinement, and trapping functions rely on properties such as low permeability, high sorption and ion exchange capacity, and, in some cases, on swelling abilities. Clay-rich materials contain specific clay minerals possessing these properties due to the small size and high tortuosity of the pores as well as the very high specific surface area and the surface charge of these minerals (especially smectites). For performance and safety purposes, the persistence of this initial sealing function has to be ensured over time, as the clay minerals of interest and the foreign anthropogenic materials (concrete, steel, and other clay materials in situ) will undergo physicochemical interactions and may lead to irreversible transformations. The clay minerals will also be subjected to perturbations due to the heat release of waste packages in the case of HLRW repository, and liquid water and vapour transfers. To tackle the complexity of these phenomena, we combine multi-scale and multi-technique characterisation (middle and far Fourier transform infrared, X-ray diffraction, scanning electron microscopy (SEM) and transmission electron microscopy (TEM)) on samples coming from laboratory experiments and natural analogues, and integrate the results through reactive transport modelling. As an example, the characterisation methodology is used to establish the illitisation of clay-stones due to a basaltic dyke intrusion. The approach is compared with classical ones and the application to diagenetic clay sequences for petroleum exploration is discussed. We also explore the high sensitivity of smectic (gel phase)/smectite properties as a function of water content/composition and temperature by investigating the interactions between metallic iron and smectitic clays. This comprehensive study reveals an iron/clay mass ratio threshold above which the smectites tend to be altered into 7 Angstroms Fe-rich clay minerals with much lower swelling and cation exchange capacity. With a comprehensive description and understanding, the prediction of the long-term evolution of such systems seems to be at hand. However, modelling the overall behaviour of clay-rich materials remains a difficult task because of the strong, multi-scale coupling between chemical, mechanical and transport phenomena, potentially mediated by a smectitic gel phase. (authors)

[en] A thorough understanding of the energy sources used by microbial systems in the deep terrestrial subsurface is essential since the extreme conditions for life in deep biospheres may serve as a model for possible life in a nuclear waste repository. In this respect, H₂ is known as one of the most energetic substrates for deep terrestrial subsurface environments. This hydrogen is produced from abiotic and biotic processes but its concentration in natural systems is usually maintained at very low levels due to hydrogen-consuming bacteria. A significant amount of H₂ gas will be produced within deep nuclear waste repositories, essentially from the corrosion of metallic components. This will consequently improve the conditions for microbial activity in this specific environment. This paper discusses different study cases with experimental results to illustrate the fact that microorganisms are able to use hydrogen for redox processes (reduction of O₂, NO^3-, Fe III) in several waste disposal conditions. Consequences of microbial activity include: alteration of groundwater chemistry and shift in geochemical equilibria, gas production or consumption, bio-corrosion, and potential modifications of confinement properties. In order to quantify the impact of hydrogen bacteria, the next step will be to determine the kinetic rate of the reactions in realistic conditions. (authors)

[en] Porosity evolution at reactive interfaces is a key process that governs the evolution and performances of many engineered systems that have important applications in earth and environmental sciences. This is the case, for example, at the interface between cement structures and clays in deep geological nuclear waste disposals. Although in a different transport regime, similar questions arise for permeable reactive barriers used for biogeochemical remediation in surface environments. The COMEDIE project aims at investigating the coupling between transport, hydrodynamics and chemistry when significant variations of porosity occur. The present work focuses on a numerical benchmark used as a design exercise for the future COMEDIE-2D experiment. The use of reactive transport simulation tools like Hytec and Crunch provides predictions of the physico-chemical evolutions that are expected during the future experiments in laboratory. Focus is given in this paper on the evolution during the simulated experiment of precipitate, permeability and porosity fields. A first case is considered in which the porosity is constant. Results obtained with Crunch and Hytec are in relatively good agreement. Differences are attributable to the models of reactive surface area taken into account for dissolution/precipitation processes. Crunch and Hytec simulations taking into account porosity variations are then presented and compared. Results given by the two codes are in qualitative agreement, with differences attributable in part to the models of reactive surface area for dissolution/precipitation processes. As a consequence, the localization of secondary precipitates predicted by Crunch leads to lower local porosities than for predictions obtained by Hytec and thus to a stronger coupling between flow and chemistry. This benchmark highlights the importance of the surface area model employed to describe systems in which strong porosity variations occur as a result of dissolution/precipitation. The simulation of highly non-linear reactive transport systems is also shown to be partly dependent on specific numerical approaches

[en] Full text of publication follows: The interactions between glass and clay are investigated using a modified version of the reaction transport code Crunch [1], especially looking at pH changes and possible cementation at the interface with the clayey materials (here the argillite of Bure). These perturbations may indeed affect the lifetime of glass matrix in deep repositories since high pH enhances the rate of glass alteration while porosity could prevent it. The glass material used in the calculations is a simplified glass containing silica, boron, and sodium. The calculations were performed at 50 deg. C with constant parameters for the glass alteration (irreversible rate constant and surface area). The results show that the glass alteration is imposing a high pH value in the vicinity of the interface: up to a value of 9.2, compared to 7.3 which is the initial pH value in the argillite. Experimentally, the rate of glass alteration is much higher in such pH conditions (the rate is ca. 5 times higher for R7T7-type glass at pH 9 compared to the value at pH 7). This pH perturbation migrates through the clayey domain following mobile elements such as boron and sodium, despite the existence of strong pH buffers in the argillite (carbonates and ion exchange). The argillite does not seem to be much destabilized except for some dissolution of kaolinite and K-feldspar and some adjustments of carbonate minerals (calcite, dolomite, and siderite). The cementation of porosity at the interface between glass and clay is predicted by the model due to the massive precipitation of silica minerals (essentially amorphous silica, but also chalcedony) while kaolinite is strongly destabilized at this location. As porosity drops to zero, the migration of the alteration products of glass is reduced leading to their accumulation in solution at the interface. At this point, the pH starts to decrease in the whole domain and the alteration of the glass could be significantly reduced. This porosity clogging effect has yet to be confirmed by experiments since existing data tend to show a pervasive precipitation of silica in the domain instead of a localized precipitation at the interface. This model was developed as a template for future developments on the Alliances platform and will be used to help testing and qualifying such developments. [1] CRUNCH: Software for modeling multicomponent, multidimensional reactive transport. User's Guide, UCRL-MA-143182. Livermore, California. Steefel, C.I. (2001). (authors)

[en] Full text of publication follows: In deep geological nuclear waste disposal, stainless steel canisters will be put in physical contact with clay-rich materials from the host-rock and/or the engineered barrier systems (EBS). After closure of the repository, the site will progressively re-saturate and the corrosion of the canisters will start to release large amounts of iron. The interactions between iron and clay-rich materials may lead to adverse transformations of clay minerals with a potential loss of confining properties such as swelling and capacity to exchange cations. Such transformations have been experimentally observed at temperatures starting at ca. 80 deg. C, where smectites contained in a mixture of bentonite and iron powder is transformed into a 7 Angstrom iron-rich serpentine mineral. The reaction-transport code CRUNCH [1] is used to investigate the iron-clay interactions at 80 deg. C over a period of 10 000 y, which are the conditions considered here to represent the mean temperature value and expected timescale for the corrosion stage. The system is a 10 m long, purely diffusive 1D domain containing a zero-valent iron interface, a 80 cm thick MX80 bentonite EBS in contact with an argillaceous site (the argillite of Bure). The results of the calculations show that the source of aluminium and, to a lesser extent of silica is key to the evolution of the system, along with the release of iron in solution. Without destabilization of the initial clay mineral, montmorillonite in the EBS, the iron is immobilized by the precipitation of iron oxides (essentially magnetite) and small amounts of siderite. With the destabilization of montmorillonite, which requires a higher value for the dissolution kinetic constant in the code than those found in the literature, all the elements are provided for the precipitation of the iron-rich serpentine mineral observed in the experiments. In this case, siderite is the most important by-product of the corrosion of steel. The results also show that the interactions between iron and clay may lead to a significant reduction of the porosity in the EBS, at the interface with the steel canister, due to the precipitation of carbonates (siderite, calcite), despite the fact that the molar volume of the secondary iron-rich clay mineral is lower than the initial montmorillonite. [1] CRUNCH: Software for modeling multicomponent, multidimensional reactive transport. User's Guide, UCRL-MA-143182. Livermore, California. Steefel, C.I. (2001). (authors)

[en] Methodologies based on secondary ion mass spectrometry (SIMS) for isotopic measurements in nuclear forensic applications relevant to the age determination of Pu particles and isotopic composition of oxygen for geolocation assignment are described. For the age determination of Pu particles, a relative sensitivity factor (RSF) to correct for the different ionization efficiencies of U and Pu, was obtained by analyzing standard Pu materials with known ages. A RSF of 2.41±0.05 was obtained for PuO₂ from measurements on samples with different Pu/U ratios. In a sample of known origin, using this RSF value, the age calculated from the ²³⁸Pu/²³⁴U and ²⁴⁰Pu/²³⁶U ratios agreed well with the reported age of 2.3 years. For geolocation assignment, a new approach based on the measurement of differences in the natural abundance of ¹⁸O and ¹⁶O isotopes and their ratio was developed. The instrumental mass discrimination of the ¹⁸O/¹⁶O ratio was determined using three O-isotope samples of different chemical composition. The measured precision (the standard error of 100 cycles/analysis) obtained for the oxygen isotopic measurement an the samples was typically ±1.1 %o. (author)

[en] Full text of publication follows: Simulations of atmospheric carbonation of concrete intermediate-low level waste (ILLW) cell components are conducted in order to evaluate the potential chemical degradation during the operating period (with ventilation up to 100 years). Atmospheric carbonation of concrete in unsaturated conditions is a complex process that involves intricate couplings between transport of water and CO₂ in both gas and liquid phases, capillary flow during drying of the concrete and chemical reactions involving cement hydrates with CO₂ dissolved in the liquid water phase. In a first approach, the drying process of concrete is simulated with liquid water-air flow and diffusion of gas components. Preliminary results were obtained in 1-dimensional geometry with a reactive-transport numerical tool (ToughReact EOS4). These simulations consider a complex mineralogical composition of cementitious and neo-formed phases, as well as porosity / permeability evolutions due to precipitation / dissolution phenomena. The results show a strong coupling between the carbonation and the drying processes affecting both the extent and the amplitude of the mineral transformations. Carbonation depths in the order of 0.6 to 1.0*10^-3 m/y are predicted for cementitious components. However, these values are slightly overestimated compared to experimental data and the computing time is quite long (several months), which call for some improvements of the drying model as well as comparison with other approaches. In this context, a series of simulations have been designed to explore the influence of the coupling between CO₂ gaseous/aqueous diffusion and the flow of water. Results are presented which compare the output of different numerical codes associated with different processes: reactive diffusion of CO₂ gas in stationary saturation conditions (Crunch), reactive diffusion of aqueous CO₂ with water flow governed by Richards' equation (ToughReact EOS9, Hytec). Additional improvements of the modelling with respect to experimental results concern the kinetic model of mineral reactivity, including the influence of liquid saturation and the protective effect of secondary carbonates. (authors)