Microstructural evolution of Y2O3 and MgAl2O4 ODS EUROFER steels during their elaboration by mechanical milling and hot isostatic pressing

Different ODS EUROFER steels reinforced with Y₂O₃ and MgAl₂O₄ were elaborated by mechanical milling and hot isostatic pressing. Good compromise between strength and ductility could be obtained but the impact properties remain low (especially for the Y₂O₃ ODS steel). The materials were structurally characterized at each step of the elaboration. During milling, the martensite laths of the steel are transformed into nano-metric ferritic grains and the Y₂O₃ oxides dissolve (but not the MgAl₂O₄ spinels). After the HIP, all the ODS steels remain ferritic with micrometric grains, surrounded by nano-metric grains for the Y₂O₃ ODS steels. The mechanisms in the Y₂O₃ ODS steels are complex: the Y₂O₃ oxides re-precipitate as nano-Y₂O₃ particles that impede a complete austenitization during the HIP. The quenchability of the ODS steels is modified by the milling process, the oxide nature and the oxide content. Eventually, the advantages and drawbacks of each oxide type are discussed.     

Magnetic studies on CoU2O6 and NiU2O6 by magnetic susceptibility, specific heat and neutron diffraction measurements

Magnetic properties of ternary uranium oxides CoU₂O₆ and NiU₂O₆ have been reported. X-ray and neutron diffraction measurements show that both compounds crystallize in the hexagonal Na₂SiF₆ structure (space group: P321), in which both cobalt (nickel) and uranium ions are in the distorted octahedral crystal field by six oxygen ions. Measurements of the magnetic susceptibility and specific heat reveal that CoU₂O₆ and NiU₂O₆ order antiferromagnetically at 32.5 and 35.3 K, respectively. Neutron diffraction measurements at 10 K show that the magnetic structure for CoU₂O₆ is a multi-sinusoidal structure with a propagation vector (0, 0, 1/6).     

Phase equilibrium of PuO2−x − Pu2O3 based on first-principles calculations and configurational entropy change

Combination of an oxygen vacancy formation energy calculated using first-principles approach and the configurational entropy change treated within the framework of statistical mechanics gives an expression of the Gibbs free energy at large deviation from stoichiometry of plutonium oxide PuO₂. An oxygen vacancy formation energy 4.20 eV derived from our previously first-principles calculation was used to evaluate the Gibbs free energy change due to oxygen vacancies in the crystal. The oxygen partial pressures then can be evaluated from the change of the free energy with two fitting parameters (a vacancy-vacancy interaction energy and vibration entropy change due to induced vacancies). Derived thermodynamic expression for the free energy based on the SGTE thermodynamic data for the stoichiometric PuO₂ and the Pu₂O₃ compounds was further incorporated into the CALPHAD modeling, then phase equilibrium between the stoichiometric Pu₂O₃ and non-stoichiometric PuO_2−x were reproduced.     

Cuboctahedral oxygen clusters in U3O7

When UO₂ is oxidised to U₃O₇, the positions in the crystal lattice of all the uranium atoms and of about 70% of the oxygen atoms are hardly affected. The remaining oxygen atoms occupy new sites which are located 310 pm along 〈1 1 0〉 vectors from the holes in the fluorite framework of UO₂. These results, which are based on the analysis of neutron diffraction powder data, are consistent with the concept that excess oxygen in U₃O₇ is accommodated in cuboctahedral anionic clusters.     

Interface strain study of thin Lu2O3/Si using HRBS

The interface of thin Lu₂O₃ on silicon has been studied using high-resolution RBS (HRBS) for samples annealed at different temperatures. Thin rare earth metal oxides are of interest as candidates for next generation transistor gate dielectrics, due to their high-k values allowing for equivalent oxide thickness (EOT) of less than 1 nm. Among them, Lu₂O₃ has been found to have the highest lattice energy and largest band gap, making it a good candidate for an alternative high-k gate dielectric. HRBS depth profiling results have shown the existence of a thin (∼2 nm) transitional silicate layer beneath the Lu₂O₃ films. The thicknesses of the Lu₂O₃ films were found to be ∼8 nm and the films were determined to be non-crystalline. Angular scans were performed across the [1 1 0] and [1 1 1] axis along planar channels, and clear shifts in the channeling minimum indicate the presence of Si lattice strain at the silicate/Si interface.     

Phasengleichgewichte in den systemen UO2-UO2,67-ThO2 und UO2 + x-NPO2

Solid-state chemical investigations have established that in the compositional range UO₂-UO_2.67-ThO₃ of the U-Th-O ternary system, the following single-phase domains exist: U₃O₈, which does not dissolve any ThO₂ in the solid state; an ordered M₄O₉ phase on the section between U₄O₉ and U₂Th₂O₉, below ≈ 1150 °C; and a phase with fluorite structure which occupies a large part of the system and which at 1250 °C is bounded by the compositions UO₂-UO_2.25 (U_0.43, ThO_0.57)O_0.25-ThO₃. The maximum O/M ratio of the “fluorite” phase is O:(U + Th) = 2.25. The highest oxidation valency of uranium is 5.30; this value falls as more thorium oxide is incorporated in the (U.Th)O_{2 + x} “fluorite” phase.     

Studies on phase diagram of Pb-Cr-O system

Equilibrium phase fields of the Pb-Cr-O ternary system were identified by long-term equilibrations and characterization of phases by XRD. Existence of the following phase fields was identified: (1) PbCrO₄-Pb₂CrO₅-Cr₂O₃, (2) Pb₂CrO₅-Pb₅CrO₈-Cr₂O₃ (3) Pb₅CrO₈-PbO-Cr₂O₃, (4) Pb-PbO-Cr₂O₃ and (5) Pb-Cr-Cr₂O₃. Based on these results, a partial phase diagram of Pb-Cr-O system has been reported.     

Damage evolution in Au-implanted Ho2Ti2O7 titanate pyrochlore

Damage evolution at room temperature in Ho₂Ti₂O₇ single crystals is studied under 1 MeV Au²⁺ ion irradiation by Rutherford backscattering spectroscopy along the 〈0 0 1〉 direction. For a better determination of ion-induced disorder profile, an iterative procedure and a Monte Carlo code (McChasy) were used to analyze ion channeling spectra. A disorder accumulation model, with contributions from the amorphous fraction and the crystalline disorder, is fit to the Ho damage accumulation data. The damage evolution behavior indicates that the relative disorder on the Ho sublattice follows a nonlinear dependence on dose and that defect-stimulated amorphization is the primary amorphization mechanism. Similar irradiation behavior previously was observed in Sm₂Ti₂O₇. A slower damage accumulation rate for Ho₂Ti₂O₇, as compared with damage evolution in Sm₂Ti₂O₇, is mainly attributed to a lower effective cross section for defect-stimulated amorphization.     

Laser-joined Al2O3 and ZrO2 ceramics for high-temperature applications

A laser process is presented that has been specially developed for joining oxide ceramics such as zirconium oxide (ZrO₂) and aluminium oxide (Al₂O₃). It details, by way of example, the design of the laser process applied for to producing both Al₂O₃-Al₂O₃ and ZrO₂-ZrO₂ joints using siliceous glasses as fillers.The heat source used was a continuous wave diode laser with a wavelength range of 808-1010 nm. Glasses of the SiO₂-Al₂O₃-B₂O₃-MeO system were developed as high-temperature resistant brazing fillers whose expansion coefficients, in particular, were optimally adapted to those of the ceramics to be joined. Specially designed measuring devices help to determine both the temperature-dependent emission coefficients and the synchronously determined proportions of reflection and transmission.The glass-ceramic joints produced are free from gas inclusions and macroscopic defects and exhibit a homogenous structure. The average strength values achieved were 158 MPa for the Al₂O₃ system and 190 MPa for the ZrO₂ system, respectively.     

Phase diagram investigations of K-Sr-U-O system

In continuation of our previous work on phase diagram investigations of alkali metal-alkaline earth metal-uranium oxides, the sub-solidus phase relations in K-Sr-U-O quaternary system were determined at 850 °C in air. A pseudo-ternary phase diagram of UO₃-SrO-K₂O was drawn using reported ternary compounds in K₂O-UO₃ and SrO-UO₃ systems, a single quaternary compound, K₈Sr₂U₆O₂₄ and various phase mixtures. These compounds and mixtures were synthesized by solid route, by heating K₂CO₃, SrCO₃ and UO₃ in different molar proportions at 850 °C. The phase fields of K₂O-SrO-UO₃ pseudo-ternary phase diagram were established by X-ray powder diffraction analysis. Gibbs energies of formation of binary and ternary uranates were estimated. The Gibbs energies of formations of A₈Sr₂U₆O₂₄ (A = Na/K/Rb/Cs) type compounds for different alkali metals were compared to find a reason for the stability of the compound for sodium, potassium and rubidium and its instability for cesium.     

Structure and electrophysical properties of liquid Pb83Mg17 and Pb83Li17 eutectics

Short range order structure of Pb₈₃Mg₁₇ and Pb₈₃Li₁₇ liquid alloys has been studied by means of X-ray diffraction method. The structure factors and pair correlation functions are analyzed. Experimental structure data were used to calculate the partial structure characteristics by means of Reverse Monte Carlo method. It is shown also that Li₄Pb significantly affects the structure of Pb₈₃Li₁₇ eutectic melt. For Pb₈₃Mg₁₇ eutectic melt the electrical resistivity and thermo-e.m.f. were measured in the temperature range of 550-1300 K. Their analysis confirms the diffraction data concluding the heterocoordinated atomic distribution Pb and Mg atoms.     

Pu2O3 and the plutonium hydriding process

The role of cubic Pu₂O₃ in the corrosion of PuO₂-coated Pu by H₂ was investigated. Experiments were conducted to demonstrate that nucleation of hydriding is promoted by formation of Pu₂O₃ sites in the oxide layer. The nucleation mechanism based on diffusion of hydrogen through the PuO₂ layer was evaluated and an alternative mechanism based on formation of catalytic Pu₂O₃ sites via the Pu-PuO₂ reaction is proposed. The possibility of active participation of other impurities and inclusions in the dioxide is also discussed.     

Swift heavy ion irradiation-induced microstructure modification of two delta-phase oxides: Sc4Zr3O12 and Lu4Zr3O12

Swift gold ions (185 MeV) were used to systematically investigate the radiation damage response of delta phase compounds Sc₄Zr₃O₁₂ and Lu₄Zr₃O₁₂ in the electronic energy loss regime. Ion irradiation-induced microstructural modifications were examined using X-ray diffraction (XRD) and transmission electron microscopy (TEM). XRD investigations indicate a phase transformation from ordered rhombohedral to disordered fluorite (O-D) in both compounds, with the Sc compound transforming at a higher ion fluence compared with the Lu compound. This result is consistent with our previous study on Sc₄Zr₃O₁₂ and Lu₄Zr₃O₁₂ under displacive radiation environment in which the nuclear energy loss is dominant. High resolution TEM revealed that individual ion tracks maintain crystalline structure, while the core region experiences an O-D phase transformation. TEM observations also suggest that for the doses in which the tracks overlap, the O-D phase transformation occurs across the entire ion range.     

Study on the phase behavior of CayU1−yO2+x solid solution

The lattice parameter and O/M ratio of the solid solution, Ca_yU_1?yO_2+x, prepared in a stream of helium were investigated. The single phase region of the cubic fluorite type solid solution extended from $\text{0.03 ? y ? 0.33}$. The lattice parameter was expressed as a linear equation of $\text{x}$ and $\text{y}$ in the range $\text{x ? 0}$ to be $\text{a = 5.4704 ? 0.102x ? 0.310y}$ (Å).The oxidation states of uranium in the solid solution were found to be U⁴⁺ and U⁵⁺ from a simple ionic model assuming rigid sphere ions. The partial molar enthalpies were estimated using calculated partial molar entropy. The partial molar enthalpy showed a maximum near $\text{x = 0.01}$.     

Phase transitions of U4O9

The phase transitions which occur above room temperature in U₄O₉ are reviewed. In the low-temperature transition, the transition temperature varies with O/U ratio from 66 to 97°C, and the variation with O/U ratio in the high-temperature transition is between 530 and 620°C. It was concluded that both transitions are of the order-disorder type involving configurational change of U(IV) and U(V) ions in the U₄O₉ lattice and with oxygens on normal lattice sites entering into interstitial sites through thermal promotion at the transition temperature. These interstitial oxygens intensify the superlattice reflections and also result in lattice contraction. Calorimetry at the low-temperature transition revealed that the entropy change associated with the transition can be divided into two terms: one is due to the order-disorder rearrangement of U (IV) and U(V) ions, the other is due to the displacement of oxygen ions. The phase diagram of U₄O₉ is presented and is based on a combination of electrical conductivity and X-ray data.     

Corrosion resistance of ceramics SiC and Si3N4 in flowing lead-bismuth eutectic

A corrosion test was conducted to investigate the corrosion characteristics of SiC and Si₃N₄ in a flowing lead-bismuth eutectic (Pb-Bi) with a fluid temperature of 550 °C, a velocity of 1 m/s, and an oxygen concentration of 1 × 10⁻⁶ wt% for an exposure time of 2000 h. The weight losses of the SiC and Si₃N₄ specimens were lower than those of corrosion-resistant steels tested under the same condition. The specimens showed excellent resistance against an element dissolution and an oxidation in the Pb-Bi flow. The surface was slightly damaged due to some stresses of the Pb-Bi flow and/or those generated by adhered Pb-Bi in the test procedure.     

Kinetics of precipitation of U4O9 from hyperstoichiometric UO2+x

For safe and reliable operation of fission reactors in space, the phase diagrams and reaction kinetics of systems used as nuclear fuels, such as U-O, U-N, U-C, are required. Diffraction allows identification of phases and their weight fractions as a function of temperature in situ, with a time resolution of the order of minutes. In this paper, we will provide results from a neutron diffraction experiment studying the U-O system. Using the neutron diffractometer HIPPO, the decomposition of UO_2+x into UO₂ and U₄O₉ as a function of temperature was investigated in situ. From the diffraction data, the participating phases could be identified as UO_2+x, UO₂ and U₄O_8.94 and no stoichiometric U₄O₉ was found. Results of the experiment were used to improve existing thermodynamic models. The presented techniques (i.e., neutron diffraction and thermodynamic modeling) are also applicable to the other systems mentioned above.     

Hydrothermal synthesis of Ni2FeBO5 in near-supercritical PWR coolant and possible effects of neutron-induced B fission in fuel crud

The hydrothermal synthesis of a nickel-iron oxyborate, Ni₂FeBO₅, known as bonaccordite, was investigated at pressures and temperatures that might occur at the surface of high-power fuel rods in PWR cores and in supercritical water reactors, especially during localized departures from nucleate boiling and dry-outs. The tests were performed using aqueous mixtures of nickel and iron oxides with boric acid or boron oxide, and as a function of lithium hydroxide addition, temperature and time of heating. At subcritical temperatures nickel ferrite NiFe₂O₄ was always the primary reaction product. High yield of Ni₂FeBO₅ synthesis started near critical water temperature and was strongly promoted by additions of LiOH up to Li/Fe and Li/B molar ratios in a range 0.1-1. The synthesis of bonaccordite was also promoted by other alkalis such as NaOH and KOH. The bonaccordite particles were likely formed by dissolution and re-crystallization by means of an intermediate nickel ferrite phase. It is postulated that the formation of Ni₂FeBO₅ in deposits of borated nickel and iron oxides on PWR fuel cladding can be accelerated by lithium produced in thermal neutron capture ¹⁰B(n,α)⁷Li reactions. The process may also be aided in the reactor core by kinetic energy of α-particles and ⁷Li ions dissipated in the crud layer.     

HfO2 and Ta2O5 as grain growth inhibitors in Eu2O3

Because of the high neutron capture cross section for five consecutive europium isotopes, Eu₂O₃ is of interest as a control material for nuclear reactors. A tendency toward excessive grain growth degrades its mechanical properties. Small amounts of HfO₂ and Ta₂O₅ were added to the Eu₂O₃ in attempts to suppress this grain growth. Three at % substitution of Hf for     

Mössbauer study and structural characterization of UO2–Gd2O3 sintered compounds

Samples of UO₂and up to 10 wt% of Gd₂O₃ were prepared by solid-state reaction under a reducing atmosphere, in a thermal path comprising ramps and dwell times in the temperature range of 900–1750 °C. The sintered material was analyzed by X-ray diffraction and ¹⁵⁵Gd Mössbauer spectroscopy. The results showed that for samples annealed up to 900 °C, the gadolinium sesquioxide remained unreacted. However, when the temperature was increased to 1300 °C, a solid-state reaction took place forming mixed oxides. For the more severe sintering condition, at 1750 °C, gadolinia left urania partially unreacted producing a material consisting of two compositions, UO₂ (with no dissolved gadolinium) and (U, Gd)O₂. The proposed heating cycle provided pellets free from Gd₂O₃ phase and may be used by the nuclear fuel industry as a suitable sintering process.