[en] The deep geological repository concept for spent nuclear fuel considers many safety elements. Among them, compacted bentonite has been selected as the primary engineering barrier between the encapsulated radioactive waste and the host rock. Thermo-hydro-mechanical behaviour of this material has been studied in detail from an experimental and numerical point of view. Furthermore, the study of chemical behaviour has become very important, both for the evaluation of the transport of species through the clay matrix and for the evaluation of their coupling to other physico-chemical phenomena. Generally, to conduct these types of studies, infiltration tests through compacted bentonite columns are carried out using an experimental setup composed of common parts: (i) porewater and infiltration water reservoirs, (ii) pumping devices and (iii) a confined sample of bentonite. The infiltration/output solutions are injected/extracted through filters positioned in the top/bottom of the sample. The results obtained in these tests are strongly influenced by the properties of these filters. For this reason, it is very important how the chemical species and the fluid are transported in the filters to correctly interpret the experimental observations. The study presented in this work is framed in this context, in which a numerical sensitivity analysis of the transport properties and size of the filters has been conducted. For this purpose, a reactive transport model for bentonites (assuming this material as a double porosity media) formulated by the authors and fully implemented in the multiphysics platform, COMSOL, has been used to simulate a cation exchange-infiltration test in MX-80 bentonite defined in the “Chemical session” of the Task Force on Engineered Barrier Systems (EBS) organised by SKB AB. The results obtained depend on the tortuosity, porosity and thickness of the filters. These parameters have been estimated for the correct interpretation of the selected test.

[en] Published in summary form only

[en] The authors report an analysis of results obtained on 304 patients who have been treated by curie-therapy for a non-metastatic choroidal melanoma. They address the treatment (exclusive curie-therapy, local relapse, and adjuvant surgery), the treatment type (iodine-125 plate, ruthenium plate, and dose level), the survival, metastatic evolutions, toxicity occurrence, and vision quality. In most of the cases, curie-therapy results in ocular conservation and in an acceptable vision. Short communication

[en] Document available in extended abstract form only. The aim of this work is to develop a Hydro-Chemo-Mechanical (HcM) model able to consistently reproduce the whole swelling process of both unsaturated and saturated bentonites. The Barcelona Expansive Model (BExM) was taken as a starting point, as it has been satisfactorily applied to model the behaviour of compacted bentonites. However, its suitability for the analysis of free swelling has not been proved, namely for the case when porosities reach values close to and over 0.9 and the soil becomes disarranged. These conditions mean pulling BExM further away from the domain for which it was initially conceived. For this reason, a modified formulation of BExM has been developed. It has been named m/BExM. In order to explain the high swelling ability of bentonites, it is assumed that the distortion of the water structure induced by the clay sheets begins to have a significant effect also in macro water structure when the micro void ratio goes beyond a certain threshold value and the confining forces do not exceed the repulsive forces. Accordingly, macro water will experience a decrease in its chemical potential, and the voids in which this phenomenon takes part will act as sinks, increasing their volume and causing an important raise of macro-porosity (disarrangement of the soil macro skeleton). When this phenomenon starts, the micro strain rate is greater than that of the macro disarrangement. Nonetheless, when the micro void ratio reaches high values, the disarrangement becomes more significant. The variation of the micro void ratio with the swelling pressure is described with an exponential law. The developed formulation takes into account the geochemical effects by using a modified swelling pressure, in keeping with the proposal of Karnland et al. (2005) when working with a saline solution of relevant concentration. The presence of sodium and calcium cations, as well as that of a generic polyvalent anion (by default, Cl^-), have been considered. The transport of species is modelled considering different diffusion coefficient for macro and microstructures. The hypothesis of equilibrium between macro and micro water may be too restrictive, as the dehydration processes of microstructural water are not immediate. Consequently, this work includes the transient nature of the mass exchange between macro and microstructural water. It has been verified that the process can be satisfactorily simulated using the formulation presented by Navarro and Alonso (2001) and Alonso and Navarro (2005). When macro-micro water equilibrium is not assumed, the chemical potential of micro water becomes a new state variable of the problem. The micro water mass balance equation must be solved in order to get its value. Besides the mentioned, m/BExM presents two additional features with respect to BExM. First, the microstructural strain is not necessarily taken as isotropic. The possibility of assuming that the orientation of the aggregates strains depends on the orientation of the stress field is introduced. Second, several definitions of the macro plastic strains induced by micro strains are considered. The developed model has been proved to promisingly reproduce the swelling recorded in two laboratory tests carried out by B+TECH using natural MX-80 bentonite samples hydrated with deionised water. Although other existing models, namely BBM and BExM, can also show the swelling trend, m/BExM performs significantly better

[en] FEBEX (Full-scale Engineered Barrier Experiment) is a demonstration and research project dealing with the bentonite engineered barrier designed for sealing and containment of waste in a high level radioactive waste repository (HLWR). It includes two main experiments: an situ full-scale test performed at Grimsel (GTS) and a mock-up test operating since February 1997 at CIEMAT facilities in Madrid (Spain). One of the objectives of FEBEX is the development and testing of conceptual and numerical models for the thermal, hydrodynamic, and geochemical (THG) processes expected to take place in engineered clay barriers. A significant improvement in coupled THG modeling of the clay barrier has been achieved both in terms of a better understanding of THG processes and more sophisticated THG computer codes. The ability of these models to reproduce the observed THG patterns in a wide range of THG conditions enhances the confidence in their prediction capabilities. Numerical THG models of heating and hydration experiments performed on small-scale lab cells provide excellent results for temperatures, water inflow and final water content in the cells. Calculated concentrations at the end of the experiments reproduce most of the patterns of measured data. In general, the fit of concentrations of dissolved species is better than that of exchanged cations. These models were later used to simulate the evolution of the large-scale experiments (in situ and mock-up). Some thermo-hydrodynamic hypotheses and bentonite parameters were slightly revised during TH calibration of the mock-up test. The results of the reference model reproduce simultaneously the observed water inflows and bentonite temperatures and relative humidities. Although the model is highly sensitive to one-at-a-time variations in model parameters, the possibility of parameter combinations leading to similar fits cannot be precluded. The TH model of the 'in situ' test is based on the same bentonite TH parameters and assumptions as for the 'mock-up' test. Granite parameters were slightly modified during the calibration process in order to reproduce the observed thermal and hydrodynamic evolution. The reference model captures properly relative humidities and temperatures in the bentonite. It also reproduces the observed spatial distribution of water pressures and temperatures in the granite. Once calibrated the TH aspects of the model, predictions of the THG evolution of both tests were performed. Data from the dismantling of the in situ test, which is planned for the summer of 2001, will provide a unique opportunity to test and validate current THG models of the EBS. Copyright (2001) Material Research Society

[en] Document available in extended abstract form only. The use of numerical methods, especially the Finite Element Method (FEM), for solving boundary problems in Unsaturated Soil Mechanics has experienced significant progress. Several codes, both built mainly for research purposes and commercial software, are now available. In the last years, Multi-physic Partial Differentiation Equation Solvers (MPDES) have turned out to be an interesting proposal. In this family of solvers, the user defines the governing equations and the behaviour models, generally using a computer algebra environment. The code automatically assembles and solves the equation systems, saving the user having to redefine the structures of memory storage or to implement solver algorithms. The user can focus on the definition of the physics of the problem, while it is possible to couple virtually any physical or chemical process that can be described by a PDE. This can be done, for instance, in COMSOL Multiphysics (CM). Nonetheless, the versatility of CM is compromised by the impossibility to implement models with variables defined by implicit functions. Elasto-plastic models involve an implicit coupling among stress increments, plastic strains and plastic variables increments. For this reason, they cannot be implemented in CM in a straightforward way. This means a very relevant limitation for the use of this tool in the analysis of geomechanical boundary value problems. In this work, a strategy to overcome this problem using the multi-physics concept is presented. A mixed method is proposed, considering the constitutive stresses, the pre-consolidation pressure and the plastic variables as main unknowns of the model. Mixed methods usually present stability problems. However, the algorithmics present in CM include several numerical strategies to minimise this kind of problems. Besides, CM is based on the application of the FEM with Lagrange multipliers, an approach that significantly contributes stability when using the mixed method. The constitutive formulation is implemented as a balance equation. This way, the user can freely implement models involving implicit relationships between variables. To illustrate the application of this method, we have analysed the implementation of a modified formulation of the Barcelona Expansive Model, a critical state model (CSM) of reference for expansive clays, as it is the case for bentonite clays for engineered barriers for radioactive nuclear spent fuel confinement. The tool developed was used to satisfactorily model the coupled hydro-mechanical problem of the free-swelling of a MX-80 bentonite disc. The bentonite sample had an initial dry density of 1600 kg/m³ and a water content of 10% in mass. The initial dimensions of the disk were a height of 15.85 mm and a diameter of 100 mm. The hydration with deionised water applied on the top face of the sample was modelled with a saturation boundary condition. Figure 1 illustrates the case modelled and results obtained on pore water pressure and swelling of the sample at an intermediate time in the simulation. In addition, the modelling results have been compared with the experimental results obtained in the laboratory test carried out by B+TECH, as shown in Figure 2. In conclusion, the present proposal means a useful approach for the introduction of advanced Soil Mechanics models to the modelling of bentonite clays in multi-physics partial differential solvers. The use of it enables to overcome the limitations of some MPDES to integrate state functions defined implicitly. This makes it possible to combine the versatility of MPDES with powerful constitutive grounds. (authors)

[en] We review the mechanical properties of defective roughened surfaces with the major emphasis on nanoindentation work. We also report novel results in which force versus penetration curves and AFM images of the nanoindented surface are compared for a flat surface of Au(001) and an Ar⁺ bombarded one, both with a high and a low flux of ions. We have found that bombarded surfaces yield at a lower stress than untreated flat ones. Surfaces bombarded at high flux show a large roughness and their yield point, marking the onset of surface plasticity, decreases with respect to that of the flat surface or of the surface bombarded with a low flux. The present results are compared with earlier work on nanoindented vicinal surfaces in which the sole surface modifications with respect to the flat surface were the presence of a high density of steps. It is concluded that a softening effect due to the bombardment-induced nanostructure of the surface dominates over the hardening one due to defect creation and interaction in the surface neighbourhood.

[en] Cerebral toxoplasmosis is frequent among patients with the acquired immunodeficiency syndrome. The common computed tomography appearance of this complication is multiple low density area with ring enhancement. We describe a very rare picture of cerebral toxoplasmosis, this is multiple hemorrhagic toxoplasmic abscess. (orig.)

[en] Surface defects have a profound influence on many attributes of materials, therefore experimental techniques and specific studies focused on their controlled generation and properties are mandatory. We have carried out a thorough study of the role of surface defects on a variety of physico-chemical properties of metals and oxides, using different experimental techniques and molecular dynamics simulations. In particular, we have studied the defects formed upon bombardment with Ar⁺ ions in a reconstructed Au(100) surface at very low ion doses. At room temperature, the pristine defects are mainly single vacancies, which diffuse by collective atomic motions, then cluster and collapse, resulting in 2D dislocation dipoles. These dislocations exhibit an enhanced chemical reactivity due to the elastic stress of their cores. We have also performed indentation tests of flat and stepped Au(111) samples with an atomic force microscope, revealing noticeable differences in their mechanical behavior when probed at the nanoscale. Thus, the stepped sample has a 20% smaller Young’s modulus, 40% smaller yield point and 50% smaller shear stress. These differences, as well as reversible, quasiplastic behavior of the stepped sample up to a critical load, are due to the active role of steps as dislocation nucleation centers. In contrast, a TiO₂(110) surface, modified with ion bombardment, does not show noticeable changes in its nanomechanical properties, which is an indication of the very different mechanical responses of oxides compared to simple metals at the nanoscale. Finally, we show how surface defects affect the chemical activity of a Pt(111) surface when exposed to methanol. The nature of the adsorbed species and the dynamics of the surface reactions are modified in the presence of surface defects, rendering the defective surface into a more robust state against catalytic poisoning. (paper)

[en] The trophic transfer of pyrene metabolites was studied using Gammarus setosus as a predator and the invertebrates Lumbriculus variegatus and Chironomus riparius as prey. The results obtained by liquid scintillation counting confirmed that the pyrene metabolites produced by the aquatic invertebrates L. variegatus and C. riparius were transferred to G. setosus through the diet. More detailed analyses by liquid chromatography discovered that two of the metabolites produced by C. riparius appeared in the chromatograms of G. setosus tissue extracts, proving their trophic transfer. These metabolites were not present in chromatograms of G. setosus exclusively exposed to pyrene. The present study supports the trophic transfer of PAH metabolites between benthic macroinvertebrates and common species of an arctic amphipod. As some PAH metabolites are more toxic than the parent compounds, the present study raises concerns about the consequences of their trophic transfer and the fate and effects of PAHs in natural environments. - Highlights: ► The trophic transfer of pyrene metabolites between invertebrates was evaluated. ► Biotransformation of pyrene by L. variegatus and C. riparius is different. ► Metabolites produced by L. variegatus and C. riparius are transferred to G. setosus. ► Specifically, two metabolites produced by C. riparius were transferred. - Some of the pyrene metabolites produced by the model invertebrates L. variegatus and C. riparius are transferred to G. setosus through the diet, proving their trophic transfer.