Des systemes simplifies aux combustibles modeles etude in situ du frittage d'oxydes d'actinides et de lanthanides

International audienceresume JSM 201

Dynamic aspects of cerium dioxide sintering: HT-ESEM study of grain growth and pore elimination

International audienceSintering of CeO2 is studied in situ by high temperature scanning environmental microscopy (HT-ESEM) at T = 1400 °C. The morphological modifications of a single grains population are recorded for 6 h. Kinetic parameters are extracted from image series. The local grain growth determined from the single population studied in situ is compared to the general grain growth obtained by classical ex situ technique. Using HT-ESEM for sintering study is validated. The grain boundary velocities range between 0 and 5 μm h−1, with a mean value of about 1 μm h−1. The migration of the intragranular surface pores is described. Their velocities range between 0.4 and 1.2 μm h−1 and depend on pore diameters: the smaller the pore, the faster the pore velocity. The time required to fill a pore that arises at the sample surface is determined as a function of pore diameter. The time for pore elimination dependence with the pore diameters is also established

From thorium phosphate hydrogenphosphate hydrate to β\beta-thorium phosphate diphosphate: Structural evolution to a radwaste storage ceramic

$\beta$-Thorium phosphate diphosphate ($\beta$-TPD), considered as a very promising radwaste storage material, was obtained from thorium phosphate hydrogenphosphate hydrate (TPHPH) precursor through dehydration and hydrogen phosphate condensation. The structures of TPHPH, intermediate $\alpha$-thorium phosphate diphosphate ($\alpha$-TPD) and its hydrate ($\alpha$-TPDH) have been resolved ab initio by Rietveld analysis of their synchrotron diffraction patterns. All were found orthorhombic (space group Cmcm) and similarly composed of [ThPO$_4$]$_4^{4+}$ slabs alternating with disordered layers hosting either [HPO$_4$·H$_2$O]$_2^{4-}$ (TPHPH), [P$_2$O$_7$·2H$_2$O]$^{4-} ($\alpha$-TPDH), or [P$_2O_7$]$^{4-}$($\alpha$-TPD), unlike the 3D structure of$\beta$-TPD. The diphosphate groups of$\alpha$-TPD and$\alpha$-TPDH are strongly bent. The irreversible transition to the final$\beta$-TPD consists in a shearing of the slabs and a reduction of the interslabs cavities that explains the stability of this high-temperature form

Charge compensation mechanisms in Nd-doped UO2 samples for stoichiometric and hypo-stoichiometric conditions : Lack of miscibility gap

The evolution of the crystal lattice of samples made of UO2 doped with different concentrations of Nd in stoichiometric and hypo-stoichiometric conditions has been systematically studied by X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS). The substitution of a trivalent cation for the U4+ initial position is responsible for creating local structural disorder and changes in the oxidation states. In this scenario, the lattice parameter is affected and the concentration of U5+ and formation of oxygen vacancies as well, since these are the mechanisms necessary to maintain the charge neutrality. The systematic oxidation of U4+ as predominant charge compensation mechanism over the formation of vacancies can be reduced by performing a thermal treatment under reducing conditions. This paper presents an experimental characterization of the uranium oxidation state mixture and local structure using XAS techniques in samples with chemical formula (U1-yNdy)O2-x, with y = 0.04, 0.17 and 0.25 (Peer reviewe

Thermodynamics and Stability of Rhabdophanes, Hydrated Rare Earth Phosphates REPO4 · n H2O

Rare earth phosphates comprise a large family of compounds proposed as possible nuclear waste disposal forms. We report structural and thermodynamic properties of a series of rare earth rhabdophanes and monazites. The water content of the rhabdophanes, including both adsorbed and structural water, decreases linearly with increase in ionic radius of the rare earth. The energetics of the transformation of rhabdophane to monazite plus water and the enthalpy of formation of rhabdophane from the constituent oxides was determined by high temperature drop solution calorimetry. The former varies linearly with the ionic radius of the lanthanide, except for cerium. By combining the enthalpy of formation determined by high temperature drop solution calorimetry and the free energy of formation determined previously by solubility experiments, a complete set of thermodynamic data was derived for the rhabdophanes. They are thermodynamically metastable with respect to the corresponding monazites plus water at all temperatures under ambient pressure conditions. This conclusion strengthens the case for monazites being an excellent nuclear waste form

Brain metastasis and renal cell carcinoma : prognostic scores assessment in the era of targeted therapies

Aim: This study aimed at exploring several brain metastatic prognostic scores in patients with renal cell carcinoma. Patients and Methods: We retrospectively analyzed data of 93 metastatic renal cell carcinoma patients who were diagnosed with brain metastases between October 2005 and July 2016 who received targeted therapy. Potential prognostic factors (RTOG RPA, BS-BM, and a newly developed score CERENAL) were analyzed. Results: A total of 75 patients received targeted therapy. All scores showed prognostic value in progression-free survival after first-line treatment with CERENAL being the sole independent prognostic factor associated with improved duration of first-line treatment. Both RTOG RPA and CERENAL were potential prognosticators for overall survival, whereas only the CERENAL score was associated with prolonged disease-specific survival. Conclusion: Several prognostic scores can be useful to predict survival of patients with brain metastases from renal cancer, especially the newly developed CERENAL score

Outcomes in newly diagnosed elderly glioblastoma patients after concomitant temozolomide administration and hypofractionated radiotherapy

This study aimed to analyze the treatment and outcomes of older glioblastoma patients. Forty-four patients older than 70 years of age were referred to the Paul Strauss Center for chemotherapy and radiotherapy. The median age was 75.5 years old (range: 70-84), and the patients included 18 females and 26 males. The median Karnofsky index (KI) was 70%. The Charlson indices varied from 4 to 6. All of the patients underwent surgery. O6-methylguanine-DNA methyltransferase (MGMT) methylation status was determined in 25 patients. All of the patients received radiation therapy. Thirty-eight patients adhered to a hypofractionated radiation therapy schedule and six patients to a normofractionated schedule. Neoadjuvant, concomitant and adjuvant chemotherapy regimens were administered to 12, 35 and 20 patients, respectively. At the time of this analysis, 41 patients had died. The median time to relapse was 6.7 months. Twenty-nine patients relapsed, and 10 patients received chemotherapy upon relapse. The median overall survival (OS) was 7.2 months and the one- and two-year OS rates were 32% and 12%, respectively. In a multivariate analysis, only the Karnofsky index was a prognostic factor. Hypofractionated radiotherapy and chemotherapy with temozolomide are feasible and acceptably tolerated in older patients. However, relevant prognostic factors are needed to optimize treatment proposals

Elaboration de phosphate-diphosphate de thorium et d'uranium (beta-PDTU) et de matériaux composites beta-PDTU/monazite à partir de précurseurs cristallisés.Etudes du frittage et de la durabilité chimique

Le phosphate-diphosphate de thorium (beta-PDTU) est actuellement considéré comme une matrice céramique potentielle en vue de l'immobilisation des actinides en formation géologique profonde. Les études réalisées au cours de ce travail reposent sur la synthèse du phosphate-hydrogénophosphate de thorium hydraté (PHPTH) en tant que précurseur du beta-PDT. La structure cristalline du PHPTH a été élucidée puis le mécanisme conduisant à sa transformation en beta-PDT a été établi.Ce dernier met en jeu plusieurs composés intermédiaires dont le alpha-PDT, forme monoclinique jusqu'alors inconnue. Par ailleurs l'existence d'une solution solide entre le PHPTH et le PHPUH a été démontrée. Les conditions expérimentales de frittage conduisant à une densification optimale des compacts ont été déterminées. La densité relative est toujours comprise entre 95 et 100% de la valeur calculée et une nette amélioration de l'homogénéité a été constatée. Dans le butd'incorporer simultanément des actinides tri et tétravalents, le procédé basé sur la précipitation de précurseurs cristallisés puis sur leur frittage a également été appliqué à l'élaboration de matériaux composites beta-PDTU/monazite.NI

Elaboration de phosphate-diphosphate de thorium et d'uranium (b-PDTU) et de matériaux composites b-PDTU/Monazite à partir de précurseurs cristallisés (études du frittage et de la durabilité chimique)

Le Phosphate-Diphosphate de Thorium (b-PDT) est actuellement considéré comme une matrice céramique potentielle en vue de l'immobilisation des actinides en formation géologique profonde. Les études réalisées au cours de ce travail reposent sur la synthèse du Phosphate-Hydrogénophoshate de Thorium Hydraté (PHPTH) en tant que précurseur du b-PDT. La structure cristalline du PHPTH a été élucidée puis le mécanisme conduisant à sa transformation en b-PDT a été établi. Ce dernier met en jeu plusieurs composés intermédiaires dont le a-PDT, forme monoclinique jusqu'alors inconnue. Par ailleurs, l'existence d'une solution solide entre le PHPTH et le PHPUH a été démontrée. Les conditions expérimentales de frittage conduisant à une densification optimale des compacts ont été déterminées. La densité relative est toujours comprise entre 95 et 100 % de la valeur calculée et une nette amélioration de l'homogénéité a été constatée. Dans le but d'incorporer simultanément des actinides tri- et tétravalents, le procédé basé sur la précipitation de précurseurs cristallisés puis sur leur frittage a également été appliqué à l'élaboration de matériaux composites b-PDTU/Monazite. Enfin, la durabilité chimique des frittés de b-PDTU a été évaluée. Les taux de lixiviation normalisés déterminés démontrent une bonne résistance des solides à l'altération. La vitesse de dissolution présente une faible dépendance vis-à-vis de la température, du pH et de plusieurs ions présents en solution. Dans tous les cas, le thorium précipite rapidement sous forme de phase néoformée à saturation du lixiviat. Celle-ci a été identifiée au PHPTH et une proposition relative au mécanisme de précipitation a été formulée.Thorium Phosphate-Diphosphate (b-TPD) is actually considered as potential host matrix for the immobilization of radionuclides, and especially actinides, in the field of an underground repository. The studies reported in this work are based on the precipitation of the Thorium Phosphate-HydrogenPhosphate Hydrate (TPHPH) as a precursor of b-TPD. The crystal structure of TPHPH was solved then the reactions involved during its transformation into b-TPD were established. It allows to put in evidence a new monoclinic variety of TPD, called a-TPD, acting as intermediate of reaction. Moreover, the existence of a complete solid solution between TPHPH and UPHPH was demonstrated.The experimental conditions of sintering leading to an optimal densification of the pellets were determined. The relative density of the samples was always between 95 and 100 % of the calculated value while a significant improvement of the homogeneity of the samples was noted. By this way, the process based on the precipitation of low-temperature crystallized precursors followed by their heat treatment at high temperature was applied to the preparation of b-TUPD/Monazite based composites in the aim to incorporate simultaneously tri- and tetravalent actinides. The chemical durability of b-TUPD sintered samples was evaluated. The normalized leaching rates determined in several experimental conditions revealed the good resistance of the solids to aqueous alteration. Moreover, the normalized dissolution rates exhibited a low dependance to temperature, pH as well as to several ions present in the leachate. For all the samples, thorium was quickly precipitated as a neoformed phosphate phase identified to TPHPH.STRASBOURG-Bib.Central Recherche (674822133) / SudocORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF

Impact of impurities on the fabrication and performances of yttrium-doped thoria electrolyte ceramics

International audienceThe effect of several impuritites on the life-cycle of yttria-doped thoria electrolyte was evaluated in this study. All the oxide exhibited the single-phase fluorite-type structure expected for such materials, while the variation of the unit cell parameter attested for the incorporation of the impurities in the lattice. Doping with 0.2, 0.5 or 1 wt.% of Al, Si or Zr only slightly affected the morphology of the powders, as well as the microstructure of the pellets obtained after sintering, even if a deleterious effect, leading to the embrittlement of the samples, was observed for high silicon contents. The presence of silicon and zirconium in the ceramic further modified its electrical properties. The grain conductivity was always found to decrease. Impedance spectroscopy also revealed a strong modification of the signal assigned to grain boundaries, which evidenced that impurities partially migrated during the sintering step. Finally, corrosion tests undertaken in liquid sodium at 500°C showed that the pellets were fragilized by the presence of silicon or high zirconium contents, possibly through the formation of Na2MO3 ternary oxides at the grain boundaries. From a general point of view, silicon seems to be the most harmful impurity that should be especially checked during the fabrication processes