In situ XRD analysis of the oxide layers formed by oxidation at 743 K on Zircaloy 4 and Zr–1NbO

Two alloys, having different oxidation behaviour (Zy4 and Zr–1NbO), have been investigated during oxidation at high temperature (743 K) and low oxygen pressure (10 kPa) by in situ X-ray diffraction (XRD). Tetragonal phase content and ‘pseudo-stresses’ on the monoclinic phase have been measured as a function of the oxide layer thickness. The tetragonal phase contents are similar for both alloys and decreased with the oxide layer thickness. Pseudo-stresses were much more compressive on Zr–1NbO alloy, with limited changes at the corrosion kinetics transition. On cooling, the tetragonal fractions do not change, while ‘pseudo-stresses’ decreased in different ways for the two alloys. With respect to stress analysis, no correlation was found between ‘pseudo-stresses’ and tetragonal phase content. In addition, due to the thermoelastic properties of the highly anisotropic phases of the zirconia, large internal thermal stresses are expected to develop during any temperature changes. The orders of magnitude of them are similar to the stresses induced by swelling during oxidation from Zr to ZrO₂.     

Determination of boron in Zr–Nb alloys by glow discharge quadrupole mass spectrometry

Direct determination of boron in Zr–2.5%Nb, Zr–1%Nb alloys and zirconium metals which are extensively used as structural materials in nuclear reactors has been carried out by glow discharge quadrupole mass spectrometer (GD-QMS). Relative sensitive factor (RSF) values for boron were determined using different solid standard reference materials (Zircaloy and steel). A comparison of the GD-QMS results obtained using these RSF values, with DC–Arc-AES (direct current arc atomic emission spectrometry)/certified values showed reasonably good agreement in all the Zr-based materials analysed for boron in the range of 0.1–7 mg kg⁻¹. Quantitation of boron in Zr matrix is possible even with a steel standard when certified for Zr and B. Internal precision (intra-sample precision) was found to be typically ±4% RSD (relative standard deviation) and the inter-sample precision was ±10% RSD for boron at 0.1 mg kg⁻¹ levels. The overall accuracy of the procedure was found to be ±8% at 0.5 mg kg⁻¹ levels of boron using Zircaloy and steel standards. Under optimised experimental conditions the detection limit for boron was found to be ±13 μg kg⁻¹.     

Chemical thermodynamic representation of AmO2−x

The AmO_2−x solid solution data set for the dependence of the oxygen potential on the composition, x, and temperature was retrieved from the literature and represented by a thermodynamic model. The data set was analysed by least-squares using equations derived from the classical thermodynamic theory for the solid solution of a solute in a solvent. Two representations of the AmO_2−x data were used, namely the Am_5/4O₂–AmO₂ and AmO_3/2–AmO₂ solid solution. No significant difference was found between the two, and the Am_5/4O₂–AmO₂ solution was preferred on the basis of the phase diagram. From the results the Gibbs energy of formation of Am_5/4O₂ has been derived.     

Phase transition temperature in the Zr-rich corner of Zr–Nb–Sn–Fe alloys

The influence of small composition changes on the phase transformation temperature of Zr–1Nb–1Sn–0.2(0.7)Fe alloys was studied in the present work, by electrical resistivity measurements and metallographic techniques. For the alloy with 0.2 at.% Fe we have determined T_↔+β=741°C and T_+β↔β=973°C, and for the 0.7 at.% Fe the transformation temperatures were T_↔+β=712°C and T_+β↔β=961°C. We have verified that the addition of Sn stabilized the β phase.     

Reactions between U–Zr alloys and Fe at 923 K

Interdiffusion experiments were carried out at 923 K with the diffusion couples consisting of U–23 at.% Zr/Fe and U–23 at.% Zr–1 at.% Ce/Fe. The reaction layer adjacent to the Fe was a single Zr-depleted UFe₂ phase. The phases in the reaction layers were estimated consistently with the calculated U–Zr–Fe ternary isotherm. The diffusion path obtained in this study was similar to that reported for the U–Pu–Zr/HT9-steel couple at 923 K, when those paths were expressed on the (U+Pu)–Zr–(Fe+Cr) composition triangle. The reaction layers grew in proportion to the square root of the annealing time. The addition of approximately 1 at.% of Ce to the U–23 at.% Zr alloy has little effect on the reaction between U–23 at.% Zr and Fe.     

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.     

Standard molar Gibbs free energy of formation of PbO(s) over a wide temperature range from EMF measurements

The EMF of the following galvanic cells,

(render)

Kanthal,Re,Pb,PbOCSZO₂ (1 atm.),Pt

(render)

Kanthal,Re,Pb,PbOCSZO₂(1 atm.),RuO₂,Pt

were measured as a function of temperature. With O₂ (1 atm.), RuO₂ as the reference electrode, measurements were possible at low temperatures close to the melting point of Pb. Standard Gibbs energy of formation, Δ_fG⁰_mβ-PbO was calculated from the emf measurements made over a wide range of temperature (612–1111 K) and is given by the expression: Δ_fG⁰_mβ-PbO±0.10 kJ=−218.98+0.09963T. A third law treatment of the data yielded a value of −218.08 ± 0.07 kJ mol⁻¹ for the enthalpy of formation of PbO(s) at 298.15 K, Δ_fH⁰_mβ-PbO which is in excellent agreement with second law estimate of −218.07 ± 0.07 kJ mol⁻¹.     

Delayed neutron study using ENDF/B-VI basic nuclear data

The basic nuclear data of the latest releases of ENDF/B-VI were used in preliminary calculations with the CINDER'90 nuclide inventory code to simulate the activity of fission delayed-neutron precursors. Total delayed-neutron production was obtained at times during and following pulse (0.1-ms) and equilibrium (4-hr) fission histories for each of the sixty fission systems having fission-product yields in ENDF/B-VI. The equilibrium studies — at unit fission rate for constant fission periods sufficiently long that all precursors reached saturation inventories — yielded the value for each system. Delayed-neutron production rates at 54 decay times t, extending to 500 s following a fission pulse, were fit using the STEPIT code to the pulse function R(t) = ∑a_iλ_ie^−λ_it. Results following equilibrium irradiations were fit to the equilibrium function R(∞, t) = ∑a_iλ_ie^−λ_it. It was observed that functions from fits to pulse results did not well represent equilibrium results at long cooling times. Similarly, functions fit to the equilibrium results did not well represent pulse results at short cooling times.

A comprehensive series of CINDER'90 calculations was then made for irradiation times T of 0.1 ms, 1 s, 10 s, 100 s, 1000 s, and 4 hours; results were obtained at 60 decay times t extending to 800 s following irradiation. Comprehensive calculations were made using both the 1989 Pn data of England and Brady and the new Pn data of Pfeiffer, Kratz and Möller described elsewhere in this issue. The body of results for each system was included in fits to obtain the neutron production rate R(T, t) = ∑a_ie^−λ_it(1 − e^−λ_iT) for each system. Fits were made for the traditional sum of six exponentials, with all variables free to vary; additional fits were made for a sum of eight exponentials with decay constants set to values suggested by Piksaikin. The resulting pulse functions R(t), defined by the a_i and λ_i thus obtained, accurately represent calculated delayed-neutron production when integrated with any irradiation history.

The pulse functions thus produced and other published pulse functions fit to past measurements and calculations are compared numerically at several times after fission. Reactivity effects of all functions from measurements and calculations for each of the sixty systems are indicated by the asymptotic periods following positive 10¢– 50¢ reactivity steps simulated in point-reactor kinetics calculations using the AIREK-10 code.     

Ion-induced electron emission – monitoring the structure transitions in graphite

The temperature dependence of ion-induced electron emission yield γ under 30 keV Ar⁺ ion impacts at incidence angles θ = 0−80° under dynamically steady-state conditions has been measured for polygranular graphite POCO-AXF-5Q. The fluencies were 10¹⁸–10¹⁹ ion/cm², the temperatures varied from the room temperature (RT) to 400 °C. The RHEED has shown that same diffraction patterns correspond to a high degree of disorder at RT. At high temperature (HT), some patterns have been found similar to those for the initial graphite surfaces. The dependence γ(T) has been found to be non-monotonic and for normal and near normal ion incidence manifests a step-like increase typical for a radiation induced phase transition. At oblique and grazing incidence (θ > 30°), a broad peak was found at T_p = 100 °C. An analysis based on the theory of kinetic ion-induced electron emission connects the behavior of γ(θ,T) to the dependence of both secondary electron path length λ and primary ion ionizing path length R_e on lattice structure that drastically changes due to damage annealing.     

Plasma nitridation of thin SiO2 films: AES, ELS and IR study

Auger electron spectroscopy, low-energy electron loss spectroscopy and infrared spectroscopy are used to investigate the nitridation of thin (10–22 nm) thermal SiO₂ in RF soft NH₃ plasma. It is found that plasma action at a substrate temperature of 573 K can completely nitridate the thermal oxide to an oxynitride layer. The layers obtained are macroscopic mixtures of two phases SiO₂ and Si₃N₄, rather than amorphous polymers of Si, O and N.     

Corrosion behavior of Zr alloys with a high Nb content

The corrosion behavior of the Zr alloy with a high Nb content was evaluated in the water loop system containing 2.2 wppm Li and 650 wppm B. The characteristics of the precipitates were analyzed by transmission electron microscopy (TEM) and the oxide was characterized by an X-ray diffraction method using a synchrotron radiation source. On the basis of the results obtained by these measurements, the relationship among the oxidation behavior, the precipitate characteristics and the oxide properties was discussed. It was shown that the Cu addition was of benefit to the corrosion resistance of the Zr alloy with a high Nb content and the corrosion resistance of the Cu-containing alloy (Zr–1.5Nb–0.5Sn–0.2Fe–0.1Cu) was superior to that of the Cr-containing alloy (Zr–1.5Nb–0.5Sn–0.2Fe–0.1Cr). The fine β-Nb precipitates were found more frequently in the Cu-containing alloy than the Cr-containing alloy when heat-treated in the same condition. The fraction of the tetragonal zirconia in the region of the metal/oxide interface was higher in the Cu-containing alloy than the Cr-containing alloy, suggesting that the stabilization of the tetragonal phase in the oxide was promoted more when the smaller precipitates are incorporated into the oxide. It is concluded that the fine distribution of β-Nb is desirable for stabilizing the tetragonal phase in the oxide, thereby increasing the corrosion resistance of the Zr alloy with a high Nb content.     

Influence of the surface conditions on permeation in the deuterium–MANET system

The condition of the surfaces is of crucial importance for the deuterium permeation through materials. In this work a study of the surface constants for the adsorption (σk₁) and release (σk₂) of deuterium under different surface conditions on the martensitic steel DIN 1.4914 (MANET) has been carried out. The growth of an oxide surface layer (Cr₂O₃) of about 25–30 nm in a MANET sample, heat treated in an oxidizing environment, compared to the bare MANET that have a ‘natural' oxide of about 5 nm has provoked a reduction of both the permeation rate and the recombination coefficient (about 3 orders of magnitude). In addition, the permeation governing process has changed from diffusion-limited to surface-limited. The measurements of the permeation rate of deuterium were performed by a gas-phase permeation technique over the temperature range 574–746 K and for deuterium driving pressures in the range from 3 to 10⁵ Pa.     

Spectroscopic and microscopic study of the corrosion of iron–silicon steel by lead–bismuth eutectic (LBE) at elevated temperatures

The performance of iron–silica alloys with different silicon composition was evaluated after exposure to an isothermal bath of lead–bismuth eutectic (LBE). Four alloys were evaluated: pure iron, Fe–1.24%Si, Fe–2.55%Si and Fe–3.82%Si. The samples were exposed to LBE in a dynamic corrosion cell for periods from 700 to 1000 h at a temperature of 550 °C. After exposure, the thickness and composition of the oxide layer were examined using optical microscopy, scanning electron microscopy (SEM) and X-ray photoelectron spectrometry (XPS), including sputter depth profiling. Particular attention was paid to the role, spatial distribution, and chemical speciation of silicon. Low-binding-energy silicon (probably silicates or ) was found in the oxide; while elemental silicon (Si) was found in the metal as expected, and silica (SiO₂) was found at the bottom of the oxide layer, consistent with the formation of a layer between the oxide and the metal. Alloys with low concentrations of Si contained only silicate in the oxide. Alloys with higher concentrations of Si contained a layer of silica at the boundary between the oxide and the bulk metal. All of the alloys examined showed signs of oxide failure. This study has implications for the role of silicon in the stability of the oxide layer in the corrosion of steel by LBE.     

The crystal structure of oxides grown on Zr–20Nb alloy

Oxides that were grown on Zr–20Nb in water at 300°C for 3 d, or in air at 400°C for 2 h were characterized by analytical electron microscopy. In both oxides, a similar microstructure was observed and similar electron diffraction patterns and high resolution lattice images were obtained. Analyses of the results showed that the crystal structure of the oxides was identical to that of an incommensurate modulated Nb₂Zr_x−2O_2x+1 phase, with x ≈ 10.     

Thermodynamic studies on Cs4U5O17(s) and Cs2U2O7(s) by emf and calorimetric measurements

The oxygen potentials over the phase field: Cs₄U₅O₁₇(s)+Cs₂U₂O₇(s)+Cs₂U₄O₁₂(s) was determined by measuring the emf values between 1048 and 1206 K using a solid oxide electrolyte galvanic cell. The oxygen potential existing over the phase field for a given temperature can be represented by: Δμ(O₂) (kJ/mol) (±0.5)=−272.0+0.207T (K). The differential thermal analysis showed that Cs₄U₅O₁₇(s) is stable in air up to 1273 K. The molar Gibbs energy formation of Cs₄U₅O₁₇(s) was calculated from the above oxygen potentials and can be given by, Δ_fG⁰ (kJ/mol)±6=−7729+1.681T (K). The enthalpy measurements on Cs₄U₅O₁₇(s) and Cs₂U₂O₇(s) were carried out from 368.3 to 905 K and 430 to 852 K respectively, using a high temperature Calvet calorimeter. The enthalpy increments, (H⁰_T−H⁰₂₉₈), in J/mol for Cs₄U₅O₁₇(s) and Cs₂U₂O₇(s) can be represented by, H⁰_T−H⁰_298.15 (Cs₄U₅O₁₇) kJ/mol±0.9=−188.221+0.518T (K)+0.433×10⁻³T² (K)−2.052×10⁻⁵T³ (K) (368 to 905 K) and H⁰_T−H⁰_298.15 (Cs₂U₂O₇) kJ/mol±0.5=−164.210+0.390T (K)+0.104×10⁻⁴T² (K)+0.140×10⁵(1/T (K)) (411 to 860 K). The thermal properties of Cs₄U₅O₁₇(s) and Cs₂U₂O₇(s) were derived from the experimental values. The enthalpy of formation of (Cs₄U₅O₁₇, s) at 298.15 K was calculated by the second law method and is: Δ_fH⁰_298.15=−7645.0±4.2 kJ/mol.     

The formation of tritium permeation barriers by CVD

The effectiveness as permeation barriers of the following CVD coatings have been investigated: TiC (1 to 2 μm in thickness); a bi-layer of TiN on TiC (3 μm total thickness) and CVD A1₂O₃ on a TiN/TiC bi-layer. The substrate materials were TZM (a Mo alloy) and 316L stainless steel in the form of discs of diameter 48 mm and thickness 0.1 or 1 mm. Permeation measurements were performed in the temperature range 515–742 K using deuterium at pressures in the range 1–50 kPa. CVD layers were shown to form reasonably effective permeation barriers. At a temperature of 673 K TiC is around 6000 times less permeable to deuterium than 316L stainless steel.     

The kinetics and mechanism of the uranium-water vapour reaction — an evaluation of some published work

The published results of Grimes and Morris on the rate of the uranium-water vapour reaction which were obtained using interferometry have been recalculated using the best values derived from the literature for the complex refractive indices of uranium and uranium dioxide (3.1–3.91 for uranium and 2.2-0.51 for uranium dioxide). The kinetics have been described by Haycock's model and the linear rate constant is given by K₁ = 1.3 × 10⁴P^1/2_H2O exp( − 9.0 kcal/RT )mg U/cm² h, where P_H2O is the water vapour pressure in torr or K₁ = 3.48 × 10⁸r^1/2 exp( −14.1 kcal/RT)mg U/cm² h, where r is the fractional relative humidity, R is the gas constant and T is the absolute temperature.

A mechanism is described which accounts for the observed dependence of the rate of uranium-water vapour reaction on the square root of the water vapour pressure.     

Resonant coherent excitation of 2s electron of Li-like Fe ions to the n = 3 states

We observed resonant coherent excitation of the 2s electron to the n = 3 states of 83.5 MeV/u Li-like Fe²³⁺ ions planar-channeling in the plane of a Si crystal. A survival fraction of the Li-like ions was measured as a function of the angle between the incident beam and the [0 0 1] axis. Clear resonance dips corresponding to the transitions of a 2s electron to all the n = 3 states were observed. The transition of each resonance dip was identified by comparing with spectroscopic data. The resonance dips at the transition energies corresponding to the optically forbidden 2s_1/2–3s_1/2, 2s_1/2–3d_3/2 and 2s_1/2–3d_5/2 transitions were observed as well as the resonance dips at transition energies corresponding to the optically allowed 2s_1/2–3p_1/2 and 2s_1/2–3p_3/2 transitions.     

Corrosion of steels with surface treatment and Al-alloying by GESA exposed in lead–bismuth

Corrosion tests were performed for T91, E911 and ODS (oxide dispersion strengthened) with surface treatment and Al-alloying by pulsed electron beam (GESA—GepulsteElektronenStrahlAnlage) in flowing lead bismuth eutectic (LBE) with an oxygen content of 10⁻⁶ wt% at 550 °C for 2000 h. The result was that the surface treatment by GESA led to a faster growing multiphase oxide layer which was very homogenous in thickness. After exposure of specimens to LBE, the average oxide layer at the surface was 14–15 μm thick for ODS, 19–20 μm for E911 and 8–22 μm for T91. No dissolution attack occurred. On the surface of the Al-alloyed specimens, thin protective alumina layers were observed at the places where FeAl was formed by the GESA process, otherwise multiphase oxide layers or corrosion attack were observed.     

Melting behaviour of oxide systems for heterogeneous transmutation of actinides. II. The system MgO–Al2O3–PuO2

Isothermal sections of the phase diagram of the system MgO–Al₂O₃–PuO₂ at various temperatures were calculated using sublattice models. The results show that below 2133 K no liquid occurs in the system. Above 2133 K liquid starts to form at the Al₂O₃–PuO₂ side. The phase diagram of the pseudo-binary system PuO₂–MgAl₂O₄ was also obtained from an isopleth T–x calculation.