Transfert de l'iode dans l'argilite de Tournemire

Dans un site potentiel de stockage de déchets radioactifs en milieu géologique profond, l'iode-129 constitue un des éléments les plus pénalisants en terme de sûreté, du fait de sa longue durée de vie et de sa faible affinité vis-à-vis de l'encaissant argileux. L'objectif de ce travail est donc d'étudier le transfert de l'iode à travers l'argilite de Tournemire (Aveyron, France), et ce, afin d'approfondir la connaissance de son comportement diffusif à travers une matrice argileuse indurée. Un programme d'étude, basé sur le croisement de plusieurs méthodes expérimentales (expériences en réacteurs fermés, de diffusion radiale et de through diffusion), a été appliqué pour trois traceurs différents : les iodures, les bromures et l'eau tritiée. Dans un premier temps, la comparaison des paramètres de diffusion de l'eau tritiée, avec ceux relatifs aux deux traceurs anioniques, met en lumière l'existence d'un phénomène d'exclusion anionique qui va limiter l'accès des anions à une porosité accessible inférieure à la porosité totale de la roche. Dans un deuxième temps, les résultats de nos expériences de rétention et de diffusion montrent qu'il existe une affinité significative entre les iodures et l'argilite de Tournemire. Cette rétention des iodures par la roche semble être à caractère irréversible, ou à cinétique de relarguage trop lent des anions au regard de la durée de nos expériences. D'autre part, l'intensité de cette rétention apparaît être fonction de différents paramètres expérimentaux comme les teneurs initiales en iodure ou la force ionique de la solution synthétique utilisée au cours de nos étudesIn the framework of an underground radioactive waste disposal, 129I constitutes for the argillaceous media one of the most penalizing elements owing to its very long life and its very low affinity. The objective of this work is to characterize the iodide transfer through the Tournemire argillite (Aveyron, France), in order to study thoroughly the iodide diffusive behaviour through an argillaceous matrix. An experimental methodology, which was based on a multi experimental method approach (batch, radial diffusion and through diffusion experiments), was applied for three different tracers: iodide, bromide, HTO. In the first part of this work, we show that the comparison between the HTO diffusion parameters and the anionic species highlights the existence of an anionic exclusion phenomenon which restricts the access of the anionic species to an accessible porosity inferior to the total rock porosity. In the second part of this work, we demonstrate that our retention and diffusion experiments indicate the existence of a significant affinity between iodide and the Tournemire argillite. The iodide retention appears to be irreversible, or with a kinetically very slow desorption within the duration of our experiments. Moreover, this retention intensity seems to be dependent on different experimental parameters such as the initial concentration of iodide or the ionic strength of the synthetic pore-water solutionMONTPELLIER-BU Sciences (341722106) / SudocSudocFranceF

Influence of initial iodide concentration on the iodide uptake by the argillite of Tournemire

Among the radionuclides that can migrate from radioactive wastes buried within consolidated argillaceous rocks, I-129 poses a particular problem due to its high mobility in geomedia and its very long half-life of 1.6 10(7) years. Previous studies carried out on such rocks showed the existence of a low iodide retardation regarding the other anion species. The purpose of the present study was two-fold. It was hoped (1) to confirm this retardation by means of radial diffusion experiments and (2) to estimate the influence of the initial iodide concentrations towards the iodide uptake. The results obtained from in-diffusion stage show that (1) no iodide uptake occurs at high initial concentrations and (2) an uptake at low initial concentrations cannot be clearly evidenced. Finally. the most singular behaviour of iodide was highlighted for all the initial iodide concentrations, from the out-diffusion stage. during which some amount of iodide, available for out-diffusion, were not involved in the diffusion process. Although the origin of such phenomenon remains unclear, a kinetic effect for the iodide uptake was proposed, assuming that the period of 55days (corresponding to the in-diffusion stage duration) would be necessary for inducing the iodide uptake. This kinetic effect, already shown elsewhere by (Descostes. M., Blin, V., Bazer-Bachi, F., Meier, P., Grenut, B., Radwan. J., Schlegel, M.L., Buschaert, S.. Coelho, D, Tevissen, E. Diffusion of anionic species in Callovo-Oxfordian argillites and Oxfordian limestones. In press for Appl. Geochem.) on similar materials, needs to be deepened

Impact of γ\gamma-radiation on carbon steel anaerobic corrosion and H2_2 production.

International audienceThis article reports investigations on the anoxic corrosion process impacting a carbon steel/ultrapure deaerated water system when it is exposed to gamma radiation (50 Gy h$^{-1}$ ), and on the associated molecular hydrogen gas generation. On the one hand, a dynamic semi-open experimental setup allowing to distinguish between the H$_2$ production related to water radiolysis and the one related to anoxic corrosion has been adopted to estimate the corrosion rate associated to three successive temporal phases: pre-irradiation, irradiation and post-irradiation phases. On the other hand, post mortem analyses performed on both liquid and solid samples permitted to (i) quantify the chemical changes occurring in solution due to the formation of water radiolysis products and (ii) observe and characterize the thin corrosion layer composed of magnetite (Fe$_3$O$_4$) at the surface of the metallic coupons. We show that gamma radiation induces a slight diminution of the carbon steel corrosion rate. However, by comparing the mean corrosion rate values obtained for the three studied phases (i.e., before, during and after irradiation), the chemical changes caused by radiolytic species seem to inhibit the formation of a thick magnetite protective layer. Additionally, the corrosion rate observed in post-irradiation phase does not decrease over time, which suggests that irradiation could modify the properties of the corrosion product layer by making it less protective. A structural modification of this oxide film might be caused by the oxidizing water radiolysis products. Such mechanisms may favour continuous active corrosion processes rather than passivation of the metallic surface and thus lead to a reduced lifetime of the component

Anoxic corrosion of carbon steel in high pH cementitious media and high temperature conditions: New insights on the formation of (Fe,Al)Si-hydrogarnet corrosion product

International audienceThe corrosion of carbon steel in alkaline anaerobic conditions and high temperature (80◦C) was studied throughmock-up tests representative of imperfect contact between steel and cementitious material. The evolution of thewhole system, including cement mineralogy, reservoir solution chemistry, corrosion product and rate, was fol-lowed over one year. SEM-EDX and XRD analyses indicated that the corrosion product layer was mostlyconstituted of (Fe,Al)Si-Hydrogarnet ((Fe,Al)Si-HG) and that its densification progressively inhibited corrosion.Thermodynamic and reactive transport modelling helped to better constrain the chemical and temperatureconditions of (Fe,Al)Si-HG formation

Diffusion of HTO, Cl-36(-) and I-125(-) in Upper Toarcian argillite samples from Tournemire: Effects of initial iodide concentration and ionic strength

International audienceDiffusion parameters for HTO, 36Cl−, and 125I− were determined on Upper Toarcian argillite samples from the Tournemire Underground Research Laboratory (Aveyron, France) using the through diffusion technique. The direction of diffusion was parallel to the bedding plane. The purpose of the present study was 3-fold; it was intended (i) to confirm the I− interaction with Upper Toarcian argillite and to verify the effects of initial I− concentration on this affinity, as previously observed by means of radial diffusion experiments, (ii) to highlight any discrepancy between Cl− and I− diffusivity, and (iii) to investigate the effect of an increase of the ionic strength of the solution on the anionic tracers' diffusive behaviour. The results show that the effective diffusion coefficient (De) and diffusion accessible porosity (εa) values obtained with an ionic strength (I.S.) synthetic pore water of 0.01 eq L−1 are: De = 2.35-2.50 × 10−11 m2 s−1 and εa = 12.0-15.0% for HTO, and De = 14.5-15.5 × 10−13 m2 s−1 and εa = 2.5-2.9% for 36Cl−. Because of anionic exclusion effects, anions diffuse slower and exhibit smaller diffusion accessible porosities than HTO, taken as a water tracer. The associated effective diffusion coefficient (De) and rock capacity factor (α) obtained for 125I are: De = 7.00-8.60 × 10−13 m2 s−1 and α = 4.3-7.2%. Such values make it possible to calculate low 125I distribution ratios (0.0057 < RD < 0.0192 mL g−1) which confirm the trend indicating that the 125I rock capacity factor increases with the decrease of the initial I− concentration. Additional through-diffusion experiments were carried out with a higher ionic strength synthetic pore water (I.S. = 0.11 eq L−1). No evolution of HTO diffusion parameters was observed. The anionic tracers' effective diffusion coefficient increased by a factor of two but no clear evolution of their accessible porosity was observed. Such a paradox could be related to the particularly small mean pore size of the Upper Toarcian argillite of Tournemire. The most significant finding of this study is the large discrepancy (factor of two) between the values of the effective diffusion coefficient for 125I and 36Cl. Whatever the ionic strength of the synthetic solution used, 125I exhibited De values two times lower than those of 36Cl. A detailed explanation for this difference cannot be given at present even if a hypothesis based on ion-pairing or on steric-exclusion cannot be excluded. This makes questionable the assumption usually made for quantifying 125I sorption and postulating that 36Cl and 125I would diffuse in the same porosity. In other terms, at Tournemire, 125I sorption could be more pronounced than previously indicated

Anoxic corrosion of carbon steel in high pH cementitious media and high temperature conditions: New insights on the formation of (Fe,Al)Si-hydrogarnet corrosion product

International audienceThe corrosion of carbon steel in alkaline anaerobic conditions and high temperature (80◦C) was studied throughmock-up tests representative of imperfect contact between steel and cementitious material. The evolution of thewhole system, including cement mineralogy, reservoir solution chemistry, corrosion product and rate, was fol-lowed over one year. SEM-EDX and XRD analyses indicated that the corrosion product layer was mostlyconstituted of (Fe,Al)Si-Hydrogarnet ((Fe,Al)Si-HG) and that its densification progressively inhibited corrosion.Thermodynamic and reactive transport modelling helped to better constrain the chemical and temperatureconditions of (Fe,Al)Si-HG formation

Effet des bactéries sur la corrosion d’un acier en contexte nucléaire : caractérisation par une approche multi-techniques (microscopie optique,

Une expérience innovante de mise en corrosion a été réalisée dans ce travail : des coupons d’acier au carbone ont été corrodés pendant neuf mois à 25 °C au contact d’argilite du Toarcien provenant de Tournemire, d’une solution simulant l’eau porale et en présence ou non d’un cocktail de bactéries représentatives de la biodiversité en milieu argileux profond. Les expériences de corrosion ont été menées en conditions aérées, étape intervenant après la fermeture des dispositifs de stockage des déchets nucléaires et précédant la transition oxique-anoxique. Une approche multi-techniques a été choisie pour caractériser les produits de corrosion et ceux d’origine biologique : microscopie optique, spectroscopie μRaman, microscopie électronique à balayage à effet de champ (MEB-FEG) couplée à l’analyse EDS. L’ensemble des techniques a permis de montrer que le système aéré évolue vers un système désaéré lorsque les bactéries sont ajoutées à l’expérience de corrosion

Effet des bactéries sur la corrosion d’un acier en contexte nucléaire : caractérisation par une approche multi-techniques (microscopie optique, μ\muRaman et MEB-EDS)

International audienceEffet des bactéries sur la corrosion d'un acier en contexte nucléaire : caractérisation par une approche multi-techniques (microscopie optique, $\mu$Raman et MEB-EDS) Résumé – Une expérience innovante de mise en corrosion a été réalisée dans ce travail : des coupons d'acier au carbone ont été corrodés pendant neuf mois à 25° C au contact d'argilite du Toarcien provenant de Tournemire, d'une solution simulant l'eau porale et en présence ou non d'un cocktail de bactéries représentatives de la biodiversité en milieu argileux profond. Les expériences de corrosion ont été menées en conditions aérées, étape intervenant après la fermeture des dispositifs de stockage des déchets nucléaires et précédant la transition oxique-anoxique. Une approche multi-techniques a été choisie pour caractériser les produits de corrosion et ceux d'origine biologique : microscopie optique, spectroscopie $\mu$Raman, microscopie electronique à balayage à effet de champ (MEB-FEG) couplée à l'analyse EDS. L'ensemble des techniques a permis de montrer que le système aéré a évolué vers un système désaéré lorsque les bactéries sont ajoutées à l'expérience de corrosion