Etude de la dissolution de britholites et de solutions solides monazite / brabantite dopées avec des actinides

In the field of the radwaste storage in underground repository, several matrices were considered as promising ceramics for the specific immobilization of actinides. Two of them, britholites and monazite / brabantite solid solution, have been considered during this work. In order to examine the dissolution mechanisms occurring at the solid liquid interface, several leaching experiments have been conducted on (LnIIIPO4 ), brabantite (CaIIAnIV(PO4)2 : An = Th, U) and britholites (Ca9Nd0.5An0.5 IV(PO4)4.5(SiO4)1.5F2 : An = Th, U). Some steady experiments, performed in undersaturation conditions for various pH and temperature conditions allowed to evaluate the long term behaviour of such matrices through their chemical durability. On the contrary, the thermodynamic equilibria were examined through the leaching experiments performed near the saturation conditions. By the way, various secondary phases, precipitated onto the surface of altered samples have been identified and characterized. Among them, the (Nd, Ca, Th) – rhabdophane, novally prepared in over- saturation experiments for a thorium weight loading lower than 11% appeared to be metastable. Indeed, it turns into TPHPH (Th2(PO4)2HPO4 · H2O) and Nd – rhabdophane (NdPO4 · ½ H2O) when increasing leaching time.Dans le contexte d'un stockage de déchets nucléaires en formation géologique profonde, plusieurs matrices de confinement spécifiques des actinides ont été retenues. Deux de ces matrices, les britholites et les solutions solides de monazite / brabantite ont été étudiées au cours de ce travail. L'une des propriétés requises pour assurer leur comportement à long terme résidant dans une forte résistance à l'altération, des expériences de dissolution ont été menées sur des échantillons de monazites (LnIIIPO4 ), les brabantites (CaIIAnIV(PO4)2 : An = Th, U), les britholites (Ca9Nd0,5An0,5 IV(PO4)4,5(SiO4)1,5F2 : An = Th, U. Les expériences menées sous contrôle cinétique de la dissolution ont permis de déterminer les vitesses de dissolution normalisées pour chacun de ces solides dans plusieurs milieux et à diverses températures. L'approche des conditions saturantes, conduisant à l'établissement d'équilibres thermodynamiques, a été menée en utilisant deux approches (qualifiées de sous- et sursaturation). A travers ces études, les diverses phases secondaires précipitant à la surface des échantillons altérés ont été caractérisées et notamment d'une rhabdophane de formulation Nd1-2xCaxThxPO4 , ½ H2O, inédite à ce jour. Cette phase, obtenue en système clos pour des pourcentages massiques en thorium inférieurs à 11%, apparaît métastable. En effet, une prolongation des tests de lixiviation s'accompagne de sa transformation en PHPTH, d'une part et en NdPO4 , ½ H2O d'autre part. Toutes ces phases néoformées sont associées à des produits de solubilité très faibles et peuvent rapidement jouer un rôle protecteur par rapport au matériau initial dans le cadre d'un stockage à long terme, en ralentissant le passage des radionucléides au sein de la solution

Actinide solubility-controlling phases during the dissolution of phosphate ceramics

International audiencePhosphate ceramics (britholites, monazite/brabantite solid solutions, thorium phosphate diphosphate, i.e. β-TPD, and associated β-TPD/monazite composites) are often considered as potential candidates to immobilize tri- and tetravalent actinides. In order to study the properties of such materials on the retention of actinides during aqueous alteration, phosphate-based neoformed phases were prepared using under- and over-saturation processes then extensively characterized (involving grazing XRD, EPMA, μ-Raman, IR or SEM). In over-saturation conditions, lanthanides (used as surrogates of trivalent actinides) are quickly precipitated as three hydrated forms (monazite, rhabdophane or xenotime) depending on the temperature, the heating time and the ionic radius of the element. Moreover, as already described for thorium, tetravalent actinides (Th, U, Np, Pu) are more often immobilized as phosphate hydrogenphosphate compounds. However, samples of (Ln,Ca,Th)-rhabdophane can also precipitate in the presence of large concentrations of calcium. Such neoformed phases were also precipitated at the surface of leached phosphate-based ceramics during under-saturation experiments. The associated thermodynamic solubility constants at infinite dilution were estimated. Due to their rapid precipitation and their very low solubility constants, these actinide phosphate solubility-controlling phases appear of significant interest in the field of the evaluation of the long-term performance of actinide-doped phosphate ceramics

Sintering of bˆeta\^beta-Thorium-Uranium(IV) Phosphate-Diphosphate Solid Solutions from Low-Temperature Precursors

RADIOCHPure and single phase thorium-uranium(IV) phosphate-diphosphate -TUPD (orthorhombic form) dense pellets were prepared by two wet chemical routes from low-temperature precursors. The first method was based on the "direct evaporation" of a mixture containing tetravalent actinides and phosphoric acid while the second one involved the initial precipitation of the thorium-uranium(IV) phosphate-hydrogenphosphate hydrate (TUPHPH). Both methods led to single phase sintered samples following a two-step procedure using a uniaxial pressing at room temperature and then a heat treatment at high temperature. The dilatometric study and the determination of the linear shrinkage of the pellets showed that the best densification of the samples prepared via the "precipitation" method was obtained after heating the raw pellet at 1250 C for 5 h, for initial uranium amount lower than 44.5 wt %(U). For samples prepared via "direct evaporation", the heat treatment must be prolonged for at least 15 h at this temperature. Moreover, the complete characterization of the samples (EPMA, and so forth) confirmed that the homogeneity of the samples was significantly improved when using TUPHPH as a precursor, probably due to the better reactivity of the initial powder (higher specific surface area, smaller grain size, and so forth). Several leaching tests were performed in various acidic media and in natural waters on -TUPD sintered samples. All the corresponding normalized dissolution rates remained low (10-6 to 10-5 g/(m2 day)) even in aggressive media which confirms the high chemical durability of -TUPD, thus enhancing the potential use of this ceramic in the field of the efficient immobilization of tetravalent actinides

Kinetics of dissolution of thorium and uranium doped britholite ceramics

In the field of immobilization of actinides in phosphate-based ceramics, several thorium and uranium doped britholite samples were submitted to leaching tests. The normalized dissolution rates determined for several pH values, temperatures and acidic media from the calcium release range from 4.7 × 10−2 g m−2 d−1 to 21.6 g m−2 d−1. Their comparison with that determined for phosphorus, thorium and uranium revealed that the dissolution is clearly incongruent for all the conditions examined. Whatever the leaching solution considered, calcium and phosphorus elements were always released with higher RL values than the other elements (Nd, Th, U). Simultaneously, thorium was found to quickly precipitate as alteration product, leading to diffusion phenomena for uranium. For all the media considered, the uranium release is higher than that of thorium, probably due to its oxidation from tetravalent oxidation state to uranyl. Moreover, the evaluation of the partial order related to proton concentration and the apparent energy of activation suggest that the reaction of dissolution is probably controlled by surface chemical reactions occurring at the solid/liquid interface. Finally, comparative leaching tests performed in sulphuric acid solutions revealed a significant influence of such media on the chemical durability of the leached pellets, leading to higher normalized dissolution rates for all the elements considered. On the basis of the results of chemical speciation, this difference was mainly explained in the light of higher complexion constants by sulfate ions compared to nitrate, chloride and phosphate

Behavior of thorium–uranium (IV) phosphate–diphosphate sintered samples during leaching tests. Part II. Saturation processes

Sintered pellets of thorium–uranium (IV) phosphate–diphosphate solid solutions (β-Th4−xUx(PO4)4P2O7, β-TUPD) were altered in several acidic media. All the results reported in the first part of this paper confirmed the good chemical durability of the samples. The evolution of the normalized weight loss showed that, in several media, thorium quickly precipitates in a neoformed phosphate-based phase while uranium (IV) is released in the leachate due to its oxidation into the uranyl form. The characterization of neoformed phases was carried out through several techniques involving grazing XRD, infrared and μ-Raman spectroscopies, EPMA, SEM and TEM. SEM micrographies showed that the dissolution mainly occurs at the grain boundaries, leading to the break away of the grains: only the first 15 μm are altered for 2 months in 10−1 M HNO3. From EPMA and BET measurements, neither the chemical composition nor the specific surface area are significantly modified. Near equilibrium, two neoformed phases were observed and identified by grazing XRD and/or μ-Raman spectroscopy at the surface of the leached pellets: one is found to be amorphous and progressively turns into the crystallized thorium phosphate–hydrogenphosphate hydrate (TPHPH). From the results obtained, a chemical scheme of the dissolution of β-TUPD sintered samples is proposed. The behavior of the actinides in the gelatinous phase appears mainly driven by their oxidation state: thorium remains in the tetrapositive state and is quickly and quantitatively precipitated while uranium (IV) is oxidized into uranyl then released in the leachate. The Th-precipitation as TPHPH first appears scattered then covers the entire surface of the pellet, inducing a delay of the actinides release in the leachate. Both phases act as protective layers and should induce the significant delay of the release of actinides (Th, U) to the biosphere