Kiyoko Takeda

Thermal properties of zirconium hydride

Abstract Zirconium hydride specimens with hydrogen contents of 1.45–1.70 H/Zr were fabricated directly from zirconium metal in a modified UHV Sieverts apparatus. All the zirconium hydride fabricated in the present study were found from X-ray diffraction analysis to be CaF2-type δ-ZrH2−x. At temperatures of 350–700 K, the heat capacity of the zirconium hydride was measured using an enthalpy method by means of a differential scanning calorimeter (DSC), and the experimental results were consistent with the values estimated from the sound velocities and the thermal expansion coefficient. In the temperature range of 300–700 K, the thermal diffusivity of the zirconium hydride was examined by a laser flash method. The thermal diffusivity decreased with increasing temperature and was not markedly influenced by the hydrogen content. The thermal conductivity of the zirconium hydride was calculated from thermal diffusivity and heat capacity and found to have slightly lower thermal conductivity than pure zirconium metal.

Characteristics of zirconium hydride and deuteride

The electrical and thermal properties of zirconium hydride and deuteride have been measured. The lattice parameter of δZrD2−x was smaller than that of δZrH2−x, and the deuteride had higher elastic moduli than the hydride. The electrical and thermal conductivities of δZrH2−x were slightly different from pure Zr metal. The electronic structure of the zirconium hydride was found from XPS measurements to differ from those in the pure Zr metal, and there was a peak due to the Zr–H bond at 6.4 eV below Fermi energy in the XPS spectra. The density of states was estimated by a molecular orbital calculation, and the agreement between the XPS valence band measurements and the calculations was quite satisfactory. Some of the mechanical and thermal properties of zirconium hydride and deuteride were interpreted in terms of the results of the molecular orbital calculation.

Analysis of the fracture behavior of a hydrided cladding tube at elevated temperatures by fracture mechanics

An analysis, based on fracture mechanics at elevated temperatures, has been carried out for several types of hydrided Zircaloy cladding tubes to elucidate the fracture behavior of high burn up light water reactor fuel cladding during reactivity initiated accidents. Fracture mechanics parameters such as stress intensity factor, J-integral and plastic yield load were estimated by a finite element method analysis, and the material properties of α-phase of zirconium required for the analysis were obtained by tensile tests at elevated temperatures. The failure assessment diagram (FAD) was constructed using the fracture mechanics parameters to estimate the failure stress of the cladding. It was found from the FAD that the predicted failure stress of the cladding qualitatively agreed with the experimental results obtained by burst tests at elevated temperatures for the hydrided Zircaloy cladding tube at Japan Atomic Energy Research Institute.

Analysis of the electronic structure of zirconium hydride

Abstract The electronic structure of zirconium hydride was analyzed by a discrete-variational (DV)-Xα molecular orbital method. The density of states of zirconium hydride estimated by DV-Xα calculation agreed with X-ray photoemission spectroscopy spectra of zirconium hydride. The net charges of Zr atom and H atom were found to be small and independent of the hydrogen content. While the Zr–Zr bond order decreased with the hydrogen content, the Zr–H bond order was hardly affected by the hydrogen content. The mechanical properties such as elastic modulus were qualitatively discussed on the basis of the results for the molecular orbital calculation.

Tensile test of hydrided Zircaloy

In order to examine the influence of precipitated zirconium hydride on the failure behavior and fracture strength of light water reactor (LWR) cladding tubes, tensile tests were performed at room temperature for non-hydrided and hydrided Zircaloy sheet-type specimens with gauge section of 10.0×5.0 mm and thicknesses of 0.5, 1.0, 1.5, 2.0, 2.5, and 3.0 mm. For specimens with thickness more than 2.5 mm, the ultimate tensile strength of the specimens appeared to be independent of thickness, which implied that plane strain condition was attained. For the specimen with 2.5 mm thickness, the ultimate tensile strength increased slightly with increasing average hydrogen concentration. Through microscopic observation of the hydrided specimen surface by scanning electron microscopy (SEM), it was found that matrix/hydride debonding was not generated but that micro-cracks perpendicular to the axial direction were produced at the hydride layer.

Photoelectrochemical study of hydrogen in zirconium oxide

Abstract The chemical states of hydrogen in a zirconium oxide film were studied by photoelectrochemical (PEC) measurements and electrochemical impedance spectroscopy (EIS) measurements. The oxide films were prepared by the oxidation of a zirconium sheet (purity 99.7%) in air at 673 K for 18 h. In order to obtain a hydrogen-implanted oxide film, the oxide film was implanted with 100 keV H + ions to a dose of 10 17 cm −2 at room temperature. From the PEC measurements, the band gap energy for the hydrogen-free oxide film was determined to be about 4.8 eV and it was reduced to 3.5 eV by hydrogen implantation. The distribution of the carrier density in the band gap was obtained by EIS measurements using Mott–Schottky analysis. Hydrogen implanted into the oxide film caused the impurity level in the original band gap. This result was consistent with that of PEC measurements.

Electronic states of hydrogen in zirconium oxide

Abstract First-principles molecular orbital calculations for the electronic states of hydrogen in ZrO 2 have been carried out using the discrete variational-Xα cluster method. Both the band gap energy and the valence band structure of ZrO 2 are in good agreement with experimental results. By the introduction of a hydrogen atom, a new impurity level appears below the conduction band and the band gap is greatly reduced. Calculated partial density of states of the H 1s component in the band gap reproduces the experimental results obtained by means of Mott–Schottky analysis. The dominant chemical bond in ZrO 2 is found to be the strong ionic bond between Zr and O atoms, and the bonding character is hardly modified by the hydrogen content.

Effect of Metallographic Factor on Hydrogen Pick-up Properties of Zircaloy-2

Zirconium alloy sheets were prepared with varying Fe, Cr and Ni systematically. The corrosion and hydrogen pickup property were estimated in steam at 673 K, in water at 633 K and in super critical water at 673 K. The effect of the SPP and the oxide film on the hydrogen pick-up was studied from the hydrogen pick-up route using D2O and the microstructure of the oxide film and secondary phase particle (SPP) in the oxide film. The hydrogen pick-up ratio decreased with increase of Fe and decrease of Ni. It was affected by Fe/Ni ratio of the matrix. The hydrogen pickup was not related to SPP but was related to the oxide film when the oxide film was relatively thick. The tetragonal ZrO2 is considered to act as a barrier for hydrogen pick-up.

Characterization of Al rich oxide layers on austenitic stainless steels by fluorescence spectroscopy

Abstract An Al rich oxide passivation technique has been developed to improve the corrosion resistance of the stainless steel to ozone added ultra-pure water. Fluorescence spectroscopy was applied to the study on the selective oxidation of aluminum containing austenitic stainless steel in low oxygen atmosphere at 1,353K. It was found that in the thermal oxidation under low oxygen pressure, the minor alloying constitution of Al resulted in the formation of thin oxide layers. The frequency shifts of fluorescence spectra show that the compressive stresses exist in the oxide layers as a result of the difference of the thermal expansion coefficients between substrate steels and α-Al2O3, and depends on the thickness of the oxide layers. It is confirmed that pure α-Al2O3 protective layers grown on the stainless steels, which remain stable and attached to the stainless steels in ozone added ultra-pure water. These act as a diffusion barrier and protect the stainless steels from the metal dissolution.