Etude de la reactivite du dioxyde d'uranium monocristallin vis-a-vis des processus de dissolution en milieu acide cinetique et reactivite aux interfaces.

International audienc

Transfert de radioéléments en zone non saturée (étude expérimentale et modélisation appliquée au site pilote de Tchernobyl)

Notre étude est dédiée à l'identification et à la quantification des mécanismes prépondérants impliqués dans la migration, en régime d'écoulement d'eau permanent, du césium et du strontium dans le sol non saturé du Site Pilote de Tchernobyl. Le transport de radioéléments dans la zone non saturée des sols résulte de la contribution de plusieurs mécanismes élémentaires couplés : la diffusion, l'advection, l'échange de matière entre fluides et phases solides. La réalisation d'expériences au laboratoire et l'utilisation d'un traceur de l'écoulement et de marqueurs des éléments réactifs, permettent d'isoler les contributions de chacun des phénomènes et de travailler dans des conditions physico-chimiques contrôlées. Avec un souci de représentativité des conditions de terrain, nous nous sommes orientés vers la réalisation d'expériences en colonne de sol non saturée en eau. Des expériences en réacteurs agités nous ont permis de caractériser les lois d'interactions radioéléments/sol.GRENOBLE1-BU Sciences (384212103) / SudocPARIS-BIUSJ-Sci.Terre recherche (751052114) / SudocSudocFranceF

Monazite, rhabdophane, xenotime & churchite: Vibrational spectroscopy of gadolinium phosphate polymorphs

International audienc

Reply to comment by Konings and Plyasunov on “First experimental determination of the solubility constant of coffinite” [Geochim. Cosmochim. Acta 181 (2016) 36–53]

International audienc

Evolution of silica walls of nanopores filled of water and ions

International audienc

Reactive transport of 85Sr in a chernobyl sand column: static and dynamic experiments and modeling

International audienceThe effects of nonlinear sorption and competition with major cations present in the soil solution on radioactive strontium transport in an eolian sand were examined. Three laboratory techniques were used to identify and quantify the chemical and hydrodynamic processes involved in strontium transport: batch experiments, stirred flow-through reactor experiments and saturated laboratory columns. The major goal was to compare the results obtained under static and dynamic conditions and to describe in a deterministic manner the predominant processes involved in radioactive strontium transport in such systems. Experiments under dynamic conditions, namely flow-through reactor and column experiments, were in very good agreement even though the solid/liquid ratio was very different. The experimental data obtained from the flow-through reactor study pointed to a nonlinear, instantaneous and reversible sorption process. Miscible displacement experiments were conducted to demonstrate the competition between stable and radioactive strontium and to quantify its effect on the 85Sr retardation factor. The results were modeled using the PHREEQC computer code. A suitable cation-exchange model was used to describe the solute/soil reaction. The model successfully described the results of the entire set of miscible displacement experiments using the same set of parameter values for the reaction calculations. The column study revealed that the stable Sr aqueous concentration was the most sensitive variable of the model, and that the initial state of the sand/solution system had also to be controlled to explain and describe the measured retardation factor of radioactive strontium. From these observations, propositions can be made to explain the discrepancies observed between some data obtained from static (batches) and dynamic (reactor and column) experiments. Desorbed antecedent species (stable Sr) are removed from the column or reactor in the flow system but continue to compete for sorption sites in the batch system. Batch experiments are simple and fast, and provide a very useful means of multiplying data. However, interpretation becomes difficult when different species compete for sorption sites in the soil/solution system. A combination of batches, flow-through reactor and column experiments, coupled with hydrogeochemical modeling, would seem to offer a very powerful tool for identifying and quantifying the predominant processes on a cubic decimeter scale (dm3) and for providing a range of radioactive strontium retardation factor as a function of the geochemistry of the soil/solution system

Effects of water content on reactive transport of 85Sr in Chernobyl sand columns

International audienceIt is known that under unsaturated conditions, the transport of solutes can deviate from ideal advective–dispersive behaviour even for macroscopically homogeneous porous materials. Causes may include physical non-equilibrium, sorption kinetics, non-linear sorption, and the irregular distribution of sorption sites. We have performed laboratory experiments designed to identify the processes responsible for the non-ideality of radioactive Sr transport observed under unsaturated flow conditions in an Aeolian sandy deposit from the Chernobyl exclusion zone. Miscible displacement experiments were carried out at various water contents and corresponding flow rates in a laboratory model system. Results of our experiments have shown that breakthrough curves of a conservative tracer exhibit a higher degree of asymmetry when the water content decreases than at saturated water content and same Darcy velocity. It is possible that velocity variations caused by heterogeneities at the macroscopic scale are responsible for this situation. Another explanation is that molecular diffusion drives the solute mass transfer between mobile and immobile water regions, but the surface of contact between these water regions is small. At very low concentrations, representative of a radioactive Sr contamination of the pore water, sorption and physical disequilibrium dominate the radioactive Sr transport under unsaturated flow conditions. A sorption reaction is described by a cation exchange mechanism calibrated under fully saturated conditions. The sorption capacity, as well as the exchange coefficients are not affected by desaturation. The number of accessible exchange sites was calculated on the basis that the solid remained in contact with water and that the fraction of solid phase in contact with mobile water is numerically equal to the proportion of mobile water to total water content. That means that for this type of sandy soil, the nature of mineral phases is the same in advective and non-advective domains. So sorption reaction parameters can be estimated from more easily conducted saturated experiments, but hydrodynamic behaviour must be characterized by conservative tracer experiments under unsaturated flow conditions

From Th-Rhabdophane to Monazite-Cheralite Solid Solutions: Thermal Behavior of Nd 1–2 x Th x Ca x PO 4 · n H 2 O ( x = 0–0.15)

International audienc

Role de l'acide nitreux sur la dissolution des oxydes a base d'uranium (IV) Mecanisme et suivi operando de l'interface solide/solution

International audienceResume JSM 201

Synthèse, caractérisation et propriétés thermodynamiques de PuSiO4_4 – cas des systèmesPuSiO4_4 et CeSiO4_4

International audienceLe thorium et l’uranium au degré d’oxydation IV sont connus pour former naturellement dessilicates de formule brute AnSiO$_4$. Ces composés étant relativement abondants dansl’environnement, les études menées sur la formation de ces espèces supposent que celles-ci seformeraient en milieu réducteur et riche en ions silicate, par altération des oxydes d’actinidecorrespondant. De plus, des études réalisées à température ambiante pour des milieux faiblementbasiques et riches en carbonates ont permis d’observer la formation d’espèces colloïdales du typeoxy-hydroxy-silicate d’actinide pour le Th(IV), U(IV) et Np(IV). Ces observations amènent à sequestionner sur le devenir et la mobilité des actinides dans le cadre du stockage géologiqueprofond, en particulier dans le cas d’un stockage direct des déchets nucléaires. Dans le cas duplutonium, la formation de composés du type silicates de Pu(IV) a déjà été suspectée dans le casde précipités observés en milieu basique et pour l’altération de verres en phase vapeur. Parconséquent, l’étude de la formation du silicate de plutonium (IV) et la détermination des propriétésthermodynamiques associées à celui-ci pourraient enrichir significativement la compréhension ducomportement du plutonium dans l’environnement.Cependant, même si la synthèse de PuSiO$_4$ a déjà été reportée une fois dans la littérature,les conditions de formation de cette phase ne sont pas parfaitement connues. Afin de mieuxcomprendre et maîtriser cette synthèse, trois analogues chimiques (le cérium, le thorium etl’uranium) couramment utilisés pour simuler la chimie du plutonium, et formant des silicates demême structure cristallographique (structure de type zircon, ZrSiO$_4$, groupe d’espace I4$_1$/amd) quePuSiO$_4$ ont été étudiés et des conditions de synthèse ont été optimisées pour chacun de cessystèmes en vue d’une transposition à la synthèse de PuSiO$_4$