Grain growth of NpO2 and UO2 nanocrystals

We report on the crystallite growth of nanometric NpO₂ and UO₂ powders. The AnO₂ nanoparticles (An = U and Np) were synthesized by hydrothermal decomposition of the corresponding actinide(iv) oxalates. NpO₂ powder was isothermally annealed between 950 °C and 1150 °C and UO₂ between 650 °C and 1000 °C. The crystallite growth was then followed by high-temperature X-ray diffraction (HT-XRD). The activation energies for the growth of crystallites of UO₂ and NpO₂ were determined to be 264(26) kJ mol^-1 and 442(32) kJ mol^-1, respectively, with a growth exponent n = 4. The value of the exponent n and the low activation energy suggest that the crystalline growth is rate-controlled by the mobility of the pores, which migrate by atomic diffusion along the pore surfaces. We could thus estimate the cation self-diffusion coefficient along the surface in UO₂, NpO₂ and PuO₂. While data for surface diffusion coefficients for NpO₂ and PuO₂ are lacking in the literature, the comparison with literature data for UO₂ supports further the hypothesis of a surface diffusion controlled growth mechanism.

Synthesis of Nanocrystalline PuO2 by Hydrothermal and Thermal Decomposition of Pu(IV) Oxalate: A Comparative Study

In recent years, the hydrothermal conversion of actinide (IV) oxalates into nanometric actinide dioxides (AnO₂) has begun to be investigated as an alternative to the widely implemented thermal decomposition method. We present here a comparison between the hydrothermal and the conventional thermal decomposition of Pu(IV) oxalate in terms of particle size, morphology and residual carbon content. A parametric study was carried out in order to define the temperature and time applied in the hydrothermal conversion of tetravalent Pu-oxalate into PuO₂ and to optimize the reaction conditions.

A multiscale in situ high temperature high resolution transmission electron microscopy study of ThO2 sintering

Two-grain model systems formed by ThO₂ nanospheres have been used to experimentally study for the first time the initial stage of sintering from room temperature to 1050 °C using high temperature high resolution transmission electron microscopy. In each grain, oriented attachment drove the reorganization and growth of the crystallites up to 300 °C to form a pseudo single crystal. Crystallite size kept growing up to 950 °C. At this temperature, a fast transformation probably corresponding to the elimination of stacking faults or dislocation walls led to the formation of single-crystals. The contact formed at room temperature between the two grains was stabilized during heat treatment by a slight reorientation of the crystallographic planes (T≈ 400 °C), leading the neck to be formed by numerous boundaries between the crystallites. At higher temperatures, the neck evolved and stabilized in the form of a plane of crystallographic orientation mismatch between the grains, which corresponds to the usual definition of the grain boundary. The growth of the neck by the addition of atomic columns was further observed in real time and quantified. At T = 950 °C, the evolution of the microscopic sintering parameter λ was obtained from HT-HRTEM images and indicated that the neck formation mostly proceeded through volume diffusion.

Supercritical hydrothermal synthesis of UO2+x: stoichiometry, crystal shape and size, and homogeneity observed using 23Na-NMR spectroscopy of (U, Na)O2+x

The hydrothermal synthesis of pure uranium dioxide under supercritical water (SCW) conditions was investigated using a starting material composed of a uranyl(vi) nitrate solution at 450 °C.

Homogeneous hydrolysis of thorium by thermal decomposition of urea

Thorium was precipitated homogeneously from a thorium nitrate solution by the thermal decomposition products of urea. The kinetics of the hydrolysis were studied at 90 and 100°C by pH measurement during the initial 5 h and the precipitation efficiencies of thorium and radium were measured over a 24 h period. Precipitation of the radium daughters was closely followed with the aim of co-precipitation of radium with thorium. The CO2 formed during urea decomposition dissolved in the solution, forming CO32− during the experiment upon reaching a sufficiently high pH level (>7). This allowed radium to co-precipitate partially, thus reducing the activity of the filtrate. After filtration or centrifugation, the precipitate is composed of nanocrystalline thorium dioxide (crystallite size ~10 nm), with weakly bound H2O and CO2.

Formation of a new type of uranium(iv) poly-oxo cluster {U38} based on a controlled release of water via esterification reaction

A new strategy for the synthesis of large poly-oxo clusters bearing 38 tetravalent uranium atoms {U38} has been developed by controlling the water release from the esterification reaction between a carboxylic acid and an alcohol. The molecular entity [U38O56Cl40(H2O)2(ipa)20]·(ipa) x (ipa = isopropanol) was crystallized from the solvothermal reaction of a mixture of UCl4 and benzoic acid in isopropanol at temperature ranging from 70 to 130 °C. Its crystal structure reveals the molecular assembly of the UO2 fluorite-like inner core {U14} with oxo groups bridging the uranium centers. The {U14} core is further surrounded by six tetrameric sub-units of {U4} to form the {U38} cluster. Its surface is decorated by either bridging- and terminal chloride anions or terminal isopropanol molecules. Another synthesis using the same reactant mixture at room temperature resulted in the crystallization of a discrete dinuclear complex [U2Cl4(bz)4(ipa)4]·(ipa)0.5 (bz = benzoate), in which each uranium center is coordinated by two chlorine atoms, four oxygen atoms from carboxylate groups and two additional oxygen atoms from isopropanol. The slow production of water released from the esterification of isopropanol allows the formation of the giant cluster with oxo bridges linking the uranium atoms at a temperature above 70 °C, whereas no such oxo groups are present in the dinuclear complex formed at room temperature. The kinetics of {U38} crystallization as well as the ester formation are analyzed and discussed. SAXS experiments indicate that the {U38} species are not dominant in the supernatant, but hexanuclear entities which are closely related to the [U6O8] type are formed.

Synthesis of size-controlled UO2 microspheres from the hydrothermal conversion of U(iv) aspartate

A wet chemistry route towards UO₂ spherical particles was designed through the hydrothermal conversion of uranium(iv) aspartate. A multi-parametric study led us to point out the conditions leading to monodisperse and size-controlled particles in the 400–2500 nm range. This simple protocol paves the way to applications in various scientific areas.

Novel approaches for the in situ study of the sintering of nuclear oxide fuel materials and their surrogates

Sintering is one of the key-points of the processing of ceramic materials. It is then of primary interest for the nuclear fuel cycle, in which it constitutes an important step in the fabrication of either UO2 or (U,Pu)O2 pellets used in current PWR reactors. The sintering of actinides oxides not only drives the final density and microstructure of the fuels, but also several characteristics that can impact significantly their behavior in the reactor. Dedicated tools are then needed to monitor the microstructure of such materials and forecast their evolution. In this frame, this paper presents the new potentialities offered by the use of environmental scanning electron microscope at high temperature (HT-ESEM) for the study of nuclear ceramics sintering. First, the results obtained from bulk pellets are detailed, either regarding original fundamental data at the grain level (such as grain boundaries and pores motion), or design of dedicated microstructures through the assessment of grain growth kinetics. Acquisition of sintering maps thanks to the combination of HT-ESEM observations and classical dilatometric measurements are also addressed. In a second part, observations undertaken at the 2-grain scale to monitor the first stage of sintering, dedicated to neck elaboration, are presented, and compared to the results currently provided by numerical models.