Renaud Podor

Versatile Monazite: Resolving geological records and solving challenges in materials science. Monazite as a promising long-term radioactive waste matrix: Benefits of high-structural flexibility and chemical durability

Abstract Monazite (Ln3+PO4) and related solid solutions are a well-known source of rare earth elements on earth. They may also accommodate large amounts of thorium and uranium without sustaining damage to the structure by self-irradiation. Such observations led to monazite-type structures being proposed as a potential host matrix for sequestering long-lived radionuclides produced during the nuclear fuel cycle and/or plutonium and americium from dismantled nuclear weapons. Monazite has two main advantages as a matrix for the containment of radioactive waste (or “radwaste”). The first is a highly flexible structure that permits accommodation of high concentrations of actinides. The incorporation of trivalent elements may be achieved by direct synthesis of An3+PO4 (An3+ = plutonium, Pu to einsteinium, Es), while tetravalent cation incorporation requires coupled substitutions, either on the anionic site (leading to monazite-huttonite solid solution) or on the cationic site (monazite-cheralite solid solution). Various methods developed for the preparation of such compounds are summarized here, as well as the experimental conditions required for the production of sintered pellets, with a particular focus on plutonium-bearing compositions. The second highly favorable property of monazite is its high chemical durability. Several experimental procedures developed to determine normalized leaching rates are reviewed, as well as results obtained from natural and synthetic monazite. Potential phases formed during dissolution were considered because they also partially control the concentration of actinides in the media. A preliminary list for such phases of interest, as well as corresponding thermodynamic data, is presented.

Stability and Structural Evolution of CeIV1–xLnIIIxO2–x/2 Solid Solutions: A Coupled μ-Raman/XRD Approach

Several CeO(2)-based mixed oxides with general composition Ce(1-x)Ln(x)O(2-x/2) (for 0 ≤ x ≤ 1 and Ln = La, Nd, Sm, Eu, Gd, Dy, Er, or Yb) were prepared using an initial oxalic precipitation leading to a homogeneous distribution of cations in the oxides. After characterization of the Ce/Nd oxalate precursors and then thermal conversion to oxides at T = 1000 °C, investigation of the crystalline structure of these oxides was carried out by XRD and μ-Raman spectroscopy. Typical fluorite Fm ̅3m structure was obtained for relatively low Ln(III) contents, while a cubic Ia ̅3̅ superstructure was evidenced above x ≈ 0.4. Moreover, since Nd(2)O(3) does not crystallize with the Ia ̅3̅-type structure, two-phase systems composed with additional hexagonal Nd(2)O(3) were obtained for x(Nd) ≥ 0.73 in the Ce(1-x)Nd(x)O(2-x/2) series. The effect of heat treatment temperature on these limits was explored through μ-Raman spectroscopy, which allowed determining the presence of small amounts of the different crystal structures observed. In addition, the variation of the Ce(1-x)Ln(x)O(2-x/2) unit cell parameter was found to follow a quadratic relation as a result of the combination between increasing cationic radius, modifications of cation coordination, and decreasing O-O repulsion caused by oxygen vacancies.

New silicides for new niobium protective coatings

Efforts to improve at high temperature the oxidation resistance of pure niobium or commercial niobium alloys have led to the development of a pack cementation process for the co-deposition of Si, Ti, Cr and Fe. Owing to the knowledge of the quaternary Nb(Ti)-T-Cr-Siphase diagrams (T5Fe or Co or Ni) and of the crystallographic features of phases present in the silicide coatings, new protective coatings have been applied on pure niobium and Cb752 alloy. The results of the crystallographic study of three new silicides isostructural with Nb Fe CrSi , in which Nb is substituted by Ti and Fe by Co or Ni are reported. The oxidation performances of two 33 6 new coatings mainly consisting of such a silicide are also outlined.

Influence of Crystallization State and Microstructure on the Chemical Durability of Cerium–Neodymium Mixed Oxides

To underline the potential links between the crystallization state and the microstructure of powdered cerium-neodymium oxides and their chemical durability, several Ce(IV)(1-x)Nd(III)(x)O(2-x/2) mixed dioxides were prepared in various operating conditions from oxalate precursors and then leached. The powdered samples were first examined through several physicochemical properties (crystallization state and associated crystallite size, reactive surface area, porosity...). The dependence of the normalized dissolution rates on various parameters (including temperature, nitric acid concentration, crystallization state) was examined for pure CeO(2) and Ce(1-x)Nd(x)O(2-x/2) solid solutions (with x = 0.09 and 0.16). For CeO(2), either the partial order related to the proton activity (n = 0.63) or the activation energy (E(A) = 37 kJ·mol(-1)) suggested that the dissolution was mainly driven by surface reactions occurring at the solid-liquid interface. The chemical durability of the cerium-neodymium oxides was also strongly affected by chemical composition. The initial normalized dissolution rates were also found to slightly depend on the crystallization state of the powders, suggesting the role played by the crystal defects in the dissolution mechanisms. On the contrary, the crystallite size had no important effect on the chemical durability. Finally, the normalized dissolution rates measured near the establishment of saturation conditions were less affected, which may be due to the formation of a gelatinous protective layer at the solid/liquid interface.

Experimental study of Th-bearing LaPO 4 (780 degrees C, 200 MPa); implications for monazite and actinide orthophosphate stability

Abstract A complete solid solution has been hydrothermally synthesized between the two end- members LaPO4 and (Ca05Th0.5)PO4 at 780 ℃ and 200 MPa, indicating that there is no limitation in temperature and pressure conditions corresponding to those of granitic magmas for Th insertion in natural monazites. The composition limits of the (A3+1-2xB2+xC4+x)PO4 compounds crystallized in the monazite structure-type are determined by both raverage = (1 - 2x) [9]rA³+ + x [9]rB²+ + x [9]rC⁴+ and rratio = (1 - x)[9]rA³+ + [9]rB²+/(1-x)[9]rA³+ + x [9]rC⁴+ parameters (where [9]rA is the ionic radius of the A element in ninefold coordination). The upper and lower values of these parameters are 1.216 Å ≥ raverage ≥ 1.107 Å and 1.238 ≥ rratio ≥ 1. The incorporation of large amounts of trans-uranium elements in the monazite structure is deduced from this model. The limitations and geochronological inferences of this model are discussed.

Environmental SEM monitoring of Ce1−xLnxO2−x/2 mixed-oxide microstructural evolution during dissolution

The microstructural evolution of several Ce1−xLnxO2−x/2 (Ln = Nd, Er, x = 0.28; 0.59 and 0.76) sintered pellets was studied during their dissolution in 4 M HNO3 at 90 °C or 60 °C. Environmental Scanning Electron Microscopy (ESEM) experiments were developed to follow the changes in the sample microstructure and their consequences on the dissolution evolution by monitoring a constant zone of the material throughout the alteration process. The LnIII content was the parameter affecting most of the dissolution kinetics of such solid solutions. From a microstructural point of view, grain boundaries dissolve preferentially during the first dissolution step due to their relative weakness compared to the well-crystallized bulk material. Crystal defects, namely screw and edge dislocations, constitute the second kind of preferential dissolution zone and induce the formation and growth of corrosion pits, mainly of pyramidal shape on the whole surface. Both microstructural parameters are responsible for significant and non-linear increase of the reactive surface developed at the solid/liquid interface and thus important modifications of the topology. On sintered samples and even more clearly on the powdered Ce0.72Nd0.28O1.86 sample, the formation of a gelatinous layer resulting from processes occurring in the back-end of the dissolution process (e.g. local saturation of the leaching solution), was evidenced through ESEM observations. This gel is also presumably responsible for the progressive decrease of the normalized dissolution rate. From these observations, it appears that the study of material dissolution kinetics can clearly benefit from the monitoring of the microstructure evolution and particularly that of the reactive surface area during dissolution.

Development of an Integrated Thermocouple for the Accurate Sample Temperature Measurement During High Temperature Environmental Scanning Electron Microscopy (HT-ESEM) Experiments.

We have developed two integrated thermocouple (TC) crucible systems that allow precise measurement of sample temperature when using a furnace associated with an environmental scanning electron microscope (ESEM). Sample temperatures measured with these systems are precise (±5°C) and reliable. The TC crucible systems allow working with solids and liquids (silicate melts or ionic liquids), independent of the gas composition and pressure. These sample holder designs will allow end users to perform experiments at high temperature in the ESEM chamber with high precision control of the sample temperature.

Preparation and characterisation of uranium oxides with spherical shapes and hierarchical structures

An easy way of preparation, based on the precipitation of U4+ or UO22+ cations by urea in the presence of PEG at T = 90–120 °C, was set up to prepare shape-controlled spherical uranium oxides. The parametric study of heating time and temperature allowed us to tailor the size of the mesocrystals obtained, which varied from 50 to 250 nm. For U(IV)-based samples, advanced characterization by the combination of SEM and HR-TEM observations and PXRD and SAXS measurements revealed a hierarchical organization of the powder with three different levels. The first one corresponded to small crystallites of about 3 nm, which grouped into spherical agglomerates of 15–20 nm and then reaggregated to produce the bigger spheres observed (up to 200 nm in diameter). On the other hand, U(VI)-bearing spherical aggregates were found to be more likely in a metastable form, evolving towards the precipitation of crystalline metaschoepite. In the last step, the samples prepared at low temperature were fired between 700 and 1000 °C under various atmospheres in order to tailor the final O/M ratio in the resulting oxides. In spite of the important chemical modifications associated, the precursors were generally found to present pseudomorphic conversion towards the final high temperature oxides and still exhibited a spherical form, provided the conditions of calcination were properly selected.

Phase equilibria in the Nb-Fe-Cr-Si system

Abstract Efforts to improve the CVD process used to elaborate protective coatings against oxidation for niobium alloys have needed a good knowledge of the quaternary Nb–Fe–Cr–Si phase diagram. Many phase equilibria have been established at 1473 K and a new quaternary, Nb 6.6 Fe 1.4 Cr 4 Si 8 , phase has been identified and studied using single X-ray diffraction data. The crystal structure is of a new type, derived from the Cr 11 Ge 8 -type structure (space group Pnma, Z =4, a =13.649(2) A, b =4.953(1) A, c =16.386(3) A, R = R w =5.7%). Higher temperatures stabilize the ternary Nb 2 Cr 3 Si 3 silicide in the Mn 5 Si 3 -type structure (space group P6 3 /mcm, Z =2, a =7.190(2) A, c =4.860(1) A, R =3.2%, R w =2.7%). The two structures are characterized by presence of Si octahedra sharing faces and edges.

Mechanism of RuO2 Crystallization in Borosilicate Glass: An Original in Situ ESEM Approach

Ruthenium, a fission product arising from the reprocessing of spent uranium oxide (UOX) fuel, crystallizes in the form of acicular RuO(2) particles in high-level waste containment glass matrices. These particles are responsible for significant modifications in the physicochemical behavior of the glass in the liquid state, and their formation mechanisms are a subject of investigation. The chemical reactions responsible for the crystallization of RuO(2) particles with acicular or polyhedral shape in simplified radioactive waste containment glass are described. In situ high-temperature environmental scanning electron microscopy (ESEM) is used to follow changes in morphology and composition of the ruthenium compounds formed by reactions at high temperature between a simplified RuO(2)-NaNO(3) precursor and a sodium borosilicate glass (SiO(2)-B(2)O(3)-Na(2)O). The key parameter in the formation of acicular or polyhedral RuO(2) crystals is the chemistry of the ruthenium compound under oxidized conditions (Ru(IV), Ru(V)). The precipitation of needle-shaped RuO(2) crystals in the melt might be associated with the formation of an intermediate Ru compound (Na(3)Ru(V)O(4)) before dissolution in the melt, allowing Ru concentration gradients. The formation of polyhedral crystals is the result of the direct incorporation of RuO(2) crystals in the melt followed by an Ostwald ripening mechanism.