Thermodynamic properties of nonstoichiometric urania-gadolinia solid solutions in the temperature range 700–1100° C

Partial molar thermodynamic quantities for urania-gadolinia solid solutions of compositions U_1−yGd_yO_2+x, with y values of 0.04 to 0.27, have been obtained using a solid electrolyte galvanic cell technique. The measurements were made for O/M ratios ranging from near stoichiometry to 2.20, and for temperatures ranging from 700 to 1100°C. The results for pure UO_2+x are in accordance with data reported earlier. The oxygen potentials for U_1−yGd_yO_2+x are higher than for pure UO_2+x and increase positively with increasing Gd content or excess oxygen. They can be represented as a function of the mean U valence, except at the stoichiometric composition. Both the partial molar entropy and enthalpy increase negatively with increasing Gd content or excess oxygen.     

Sputtering yield of Ti1−xBx films by 2 keV deuterium bombardment

We deposited titanium borides (Ti_1−xB_x; 0.40 < x < 0.77) by the co-sputter coating method and measured their sputtering yield by 2 keV deuterium ion bombardment as a function of their chemical composition at room temperature. The total sputtering yield is found to increase with increase of the boron content in Ti_1−xB_x. The total sputtering yield of stoichiometric TiB₂ is estimated to be 2.8 × 10⁻², about the same as those reported previously. Concerning the partial sputtering yield, that of the titanium does not depend on the chemical composition, but that of the boron increases with increase of the boron content. These experimental results could be explained by assuming that the partial sputtering yield is proportional to the spatial concentration of each atom in the Ti_1−xB_x matrix.     

Thermal stability and brazing characteristics of Zr0.7−xMxBe0.3 (M=Ti or Nb) ternary amorphous filler metals

The effects of Ti or Nb substitution on the thermal stability and brazing characteristics of Zr_0.7−xM_xBe_0.3 (M=Ti or Nb) ternary amorphous alloys were investigated in order to improve properties of Zr–Be binary amorphous alloy as a new filler metal for joining zirconium alloy. The Zr_0.7−xM_xBe_0.3 (M=Ti or Nb; 0x0.1) ternary amorphous alloys were produced by melt-spinning method. In the selected compositional range, the thermal stability of Zr_0.7−xTi_xBe_0.3 and Zr_0.7−xNb_xBe_0.3 amorphous alloys are improved by the substitution of titanium or niobium for zirconium. As the Ti and Nb content increases, the crystallization temperatures increase from 610°C to 717°C and 610°C to 678°C, respectively. These amorphous alloys were put into practical use in joining bearing pads on zircaloy cladding sheath. Using Zr–Ti–Be amorphous alloys as filler metals, smooth interface and spherical primary particles (proeutectic phase) appear in the brazed layer, which is the similar microstructure of using Zr_0.7Be_0.3 binary amorphous alloys. In the case of Zr–Nb–Be amorphous alloys, Ni-precipitated Zr phase that may cause some degradation in ductility and corrosion-resistance is formed at both sides of the brazed layer.     

Mass spectrometric investigation of the vaporization of li2tio3(s)

The vaporization of Li₂TiO₃(s) has been investigated by the mass spectrometric Knudsen effusion method. Partial pressures of Li(g), LiO(g), Li₂O(g), Li₃O(g) and O₂(g) over Li₂TiO₃(s) have been obtained in the temperature range 1180–1628 K. When the vaporization of Li₂TiO₃(s) proceeds, the content of Li₂O in the Li₂TiO₃(s) sample decreases. The phase of the sample is a disordered Li₂TiO₃ solid solution above 1486 K. The enthalpies of formation and the atomization energies for LiO(g) and Li₃O(g) have been evaluated from the partial pressures to be ΔH^o_f0(LiO, g) = 65.4 ± 17.4 kJ/mol, ΔH^o_f0(Li₃O, g) = − 207.5 ± 56.6 kJ/mol, D^o₀(LiO) = 340.5 ± 17.4 kJ/mol and D^o₀(Li₃O) = 931.6 ± 56.6 kJ/mol, respectively.     

Investigation of in-reactor creep and irradiation growth of zirconium-2.5 wt% niobium channel tubes

We summarize the diametral creep results obtained in the MR reactor of the Kurchatov Institute of Atomic Energy on zirconium-2.5 wt% niobium pressure tubes of the type used in RBMK-1000 power reactors. The experiments that lasted up to 30 000 h cover a temperature range of 270 to 350°C, neutron fluxes between 0.6 and 4.0 ×10¹³ n/cm² · s (E > 1 MeV) and stresses of up to 16 kgf/mm². Diametral strains of up to 4.8% have been measured. In-reactor creep results have been analyzed in terms of thermal and irradiation creep components assuming them to be additive. The thermal creep rate is given by a relationship of the type ε_th = A₁ exp [(A₂ + A _3σt) T] and the irradiation component by ε_rad = A_4σtø(T − A₅), where T = temperature, σ_t = hoop stress, ø = neutron flux and a₁ to A₅ are constants. Irradiation growth experiments carried out at 280° C on specimens machined from pressure tubes showed a non-linear dependence of growth strain on neutron fluence up to neutron fluences of 5 × 10²⁰ n/cm². The significance of these results to the elongation of RBMK reactor pressure tubes is discussed.     

Electrical conductivity and non-stoichiometry in the (U,Gd)O2±x system

The isothermal electrical conductivity and oxygen potential of the (U,Gd)O_2±x solid solution were measured in various oxygen partial pressure regions at 1200 °C and 1300 °C. The electrical conductivity gradually decreased with decreasing oxygen partial pressure even in the hypo-stoichiometric region. These findings were in contrast to the implication of a hypo-stoichiometry where the electrical conductivity is increased through the formation of oxygen vacancies. The (U_1−yGd_y)O_2−y/2 was defined as a new stoichiometric composition to determine the relationship between the deviation of the oxygen composition from stoichiometry and oxygen partial pressure. The dependence of the new oxygen deviation, z in (U_1−yGd_y)O_2−y/2+z, on the oxygen partial pressure corresponds to the dependence of the electrical conductivity, and thus a consistent defect structure model can be deduced from both the dependence curves. It suggests that the defect type is oxygen interstitial even below the oxygen composition of 2.     

Contribution to the study of the U---Cs---Mo---I---O system

The reaction in the U---Cs---Mo---I---O system at 1073 K have been studied as a function of oxygen potential. The chemical constitution and the morphology of the phases formed were examined by electron-probe microanalyzer and X-ray diffractometer. The existence of a two-phase field UO_2+x + Cs₂UO₄ was confirmed in the U---Cs---O system. The threshold oxygen potential was determined for the decomposition of CsI by UO_2+x to form Cs₂U₄O₁₂ and gaseous iodine. The effect of molybdenum on the UO_2+x−CsI reaction was also investigated. Predominance of Cs₂MoO₄ over Cs₂UO₄ was verified in a certain range of oxygen potentials. Some assessments and interpertations of the experimental results were made with the aid of thermodynamic calculations.     

Use of reference samples for more accurate RBS analyses

While one of the primary assets of RBS analysis is that it is quantitative without use of reference samples, for certain types of analyses the precision of the method can be improved by measuring RBS spectra of unknowns relative to the RBS spectra of a similar known sample. The advantage of such an approach is that one can reduce (or eliminate) the uncertainties that arise from error in the detector solid angle, beam current integration efficiency, scattering cross-section, and stopping powers. We have used this approach extensively to determine the composition (x) of homogeneous thin films of TaN_x using as reference samples films of pure Ta. Our approach is to measure R=(Tacount)_unknown/(Tacount)_standard and use RUMP to determine the function x(R). Once the function x(R) has been determined, this approach makes it easy to analyze many samples quickly. Other analyses for which this approach has proved useful are determination of the composition (x) of WN_x, SiO_xH_y and SiN_xH_y, using W, SiO₂ and amorphous Si as reference samples, respectively.     

High temperature high dose C ion implantation in epitaxial SiGe

Si_{1 − x}Ge_x epitaxial layers fully strained (x = 0.27) and relaxed (x = 0.55) have been implanted with C ions at 500°C. Implantation energy and doses were selected to obtain the C peak in the central region of the SiGe layer, with a concentration similar to the Ge content. The implanted layers have been analyzed by Raman scattering, X-ray diffraction, transmission electron microscopy and secondary ion mass spectroscopy. The data obtained show the direct synthesis of β-SiC precipitates aligned in relation to the SiGe lattice after the implantation, as well as a Ge enrichment and stress relaxation of the SiGe lattice. For the relaxed layer a significant Ge redistribution from the implanted region is observed.     

Formation of uranium mononitride by the reaction of uranium dioxide with carbon in ammonia and a mixture of hydrogen and nitrogen: II. Reaction rates

Kinetics of the carbothermic synthesis of UN from UO₂ in an NH₃ stream and a mixed 75% H₂ + 25% N₂ stream were studied in the temperature range of 1400–1600°C by X-ray analysis and weight change measurement of the sample. The weight change was divided into two parts; i.e. weight loss due to carbothermic reduction of UO₂ and weight loss due to removal of carbon by hydrogen. The former followed the first-order rate equation −1n(1 − ₀) = k₀t, and the latter the rate equation of phase boundary reaction 1 − (1 − _c)^1/3 = k_ct. The apparent activation energy of the former was in the range of 320–380 kJ/mol. The value of the latter in an NH₃ stream was 175–185 kJ/mol, which was smaller than that in a mixed 75% H₂ + 25% N₂stream (285 kJ/mol). In this method, the rate of the removal of carbon by hydrogen determines that of the formation of high purity UN.     

Density measurement of amorphous SixGe1 − x alloys

The atomic density of amorphous Si_xGe_{1 − x} alloys (x = 1, 0.85, 0.67, 0.50, 0.20 and 0) has been measured. Mono-crystalline Si_xGe_1−x layers Were implanted with 1.50–2.75 MeV Si²⁺ and Ge²⁺ ions to produce the amorphous material. Using surface profilometry and RBS/channeling, it was found that amorphous alloys are less dense than the crystalline alloys, and that Vegard's law underestimates the a-Si_xGe_1−x density.     

The behaviour of single atoms of molybdenum in urania

Computer simulation techniques are used to investigate the behaviour of single atoms of Mo in UO_2±x. In UO_2−x, Mo is calculated to be present as neutral atoms in bound Schottky trio sites. In UO_2+x, most of the Mo is calculated to be in isolated uranium vacancy sites with Mo ionisation increasing with the O/U ratio. The behaviour near stoichiometric composition is more complex and is found to be very sensitive to changes in O/U ratio. An approximation to the free energy change associated with Mo incorporation in urania is plotted as a function of O/U ratio and Mo concentration. Although this plot is found to be in agreement with the observed insoluble character of Mo in urania, at high O/U ratios and very low Mo concentrations, Mo in solution may be preferred over Mo in the gaseous state.     

A comparative EPR study of ion implantation induced damage in Si, Si1−xGex (x ≠ 0) and SiC

Electron Paramagnetic Resonance (EPR) measurements have been made to investigate the build up of damage in silicon in relaxed crystalline Si_1−xGe_x (x = 0.04, 0.13, 0.24, 0.36) and in 6H-SiC as a result of increasing the ion dose from low levels (10¹² cm⁻²) up to values (10¹⁵ cm⁻²) sufficient to produce an amorphous layer. Si, Si_1−xGe_x (x ≠ 0) and SiC were implanted at room temperature with 1.5 MeV Si, 2 MeV Si and 0.2 MeV Ge ions respectively. A comparison is made between the ways in which the type and population of paramagnetic defects depend on ion dose for each material.     

Role of FP iodine in irradiation behaviors of nuclear fuel pins

During the operation of a reactor, the fuel ($\text{(U,Pu)O}{}_{\mathrm{2-x}}$ or $\text{UO}{}_{\mathrm{2\; +\; x}}$) reacts with its cladding (stainless steel or zircaloy). The role of iodine, a fission product, in this reaction has been examined. Out-of-pile experiments have been done to study the chemical vapor transport of components of stainless steel or pure iron due to iodine released by the electron bombardment of CsI vapor, with or without the excess cesium vapor, using X-ray microprobe analysis. Though addition of excess cesium vapor diminishes the transport rate of stainless steel components, there is still a possibility that cladding components are transported due to iodine in a reactor. Thermodynamic marker tests have been done to check the iodine pressure in the electron-irradiated CsI atmosphere. A short review of the previous relevant studies is also given.     

2 MeV Si ion implantation damage in relaxed Si1−xGex

The damage produced by implanting, at room temperature, 3 μm thick relaxed Si_1−xGe_x layers with 2 MeV Si⁺ ions has been measured as a function of Ge content (x = 0.04, 0.13, 0.24 or 0.36) and Si dose in the dose range 10¹⁰–10¹⁵ cm⁻². The accumulation of damage with increasing dose has been studied as a function of Ge content by Rutherford Backscattering Spectrometry, Optical Reflectivity Depth Profiling and Transmission Electron Microscopy and an increased damage efficiency in Si_1−xGe_x with increasing x is observed. The characteristics of implantation-induced defects have been investigated by Electron Paramagnetic Resonance. The results are discussed in the context of a model of the damage process in SiGe.     

Li4SiO4 pebbles reduction in He + 0.1% H2 purge gas and effects on tritium release properties

Lithium orthosilicate reduction was examined by Temperature Programmed Reaction (TPR) and Temperature Programmed Desorption (TPD) methods performed in He (or Ar) + H₂ purge gas flowing through pebble bed specimens. The parameters governing the kinetics and the steady-state of the reduction process to Li₄SiO_4−x were determined at 800°C. The level x of the O-vacancy concentration at steady-state (of the order of 1.5×10⁻³ mole fraction) was found to be compatible with the impurities content in the specimens. Pebble pre-annealing treatments were found to affect the microstructure and the reduction mechanism. Post-irradiation tritium release by TPD tests were performed on both stoichiometric and reduced pebbles with similar results. Tritium release properties of this breeder system seem to be independent from the material reduction state (x).     

Rapid thermal annealing of arsenic implanted relaxed Si1−xGex

The electrical activity and redistribution during rapid thermal annealing (RTA) of high concentrations of As implanted into epitaxially grown, relaxed Si_1−xGe_x for x≤0.5 have been studied as a function of composition x and RTA parameters. At a given RTA temperature the maximum carrier concentration decreases and the redistribution increases with increasing x. Maximum carrier concentrations and junction depths as a function of composition and RTA parameters are given.     

Influence of dynamic strain aging on the ductile tearing of C–Mn steels: modelling by a local approach method

C–Mn steels and associated welds can be susceptible to dynamic strain aging (DSA). In this case, fracture toughness passes through a minimum when the temperature increases from 20 to 300 °C. Since Charpy V-Notch data are not affected by the DSA phenomenon, the method for predicting the evolution of J_0.2 and dJ/da in the temperature domain where DSA occurs is still an open question. The purpose of the present study is the assessment of this decrease in fracture toughness using a local approach method. The results of J–R tearing resistance tests, characterized by crack initiation resistance J_0.2 and tearing modulus dJ/da relative to a base metal (A48 French standard steel) and manual metal arc deposited metals are presented and discussed. The local approach determination of J_0.2 was performed using the Rice and Tracey model. Comparisons between experimental results and the results of local approach modelling indicate that both J_0.2 and dJ/da decreases with DSA can be correctly predicted.     

Thermochemical study of vaporization of LiTiO by a mass spectrometric Knudsen effusion method

The vaporization of Li₄TiO₄ has been studied by a mass spectrometric Knudsen effusion method in the temperature range 1082–1582 K. Identified vapors are Li(g), LiO(g), Li₂O(g) and Li₃O(g). When the vaporization proceeds, the content of Li₂O in the Li₄TiO₄ sample decreases and the condensed phase of the sample changes to β-Li₄TiO₄ plus l-Li₂TiO₃ below 1323 K, to β-Li₄TiO₄ plus h-Li₂TiO₃ in the range 1323–1473 K and to h-Li₂TiO₃ plus liquid above 1473 K. On the basis of the partial pressure data, the enthalpies of formation for β-Li₄TiO₄ from elements and from constituent oxides have been determined to be ΔH_f,298^°(β-Li₄TiO₄,s) = −2247.8 ± 14.3 kJ mol⁻¹ and Δ_fox,298^°(β-Li₄TiO₄, s) = −107.3 ± 14.3 kJ mol⁻¹, respectively.     

Determination des proprietes elastiques dynamiques du systeme UO2-SiO2 dans la gamme de temperatures 0–300°C

The complex elastic modulus E*(=E₁ + iE₂) of UO₂-SiO₂ with 64 mol% UO₂ in the range 0–300°C was measured by means of the resonant constrained bar technique. The modulus was found to be constant in the range investigated. Using the elastic moduli of UO₂ and SiO₂, the modulus of the sample were estimated theoretically and found to be close to the measured values; the theoretical model used can thus justifiably be adopted for other volume fractions.