Yuji Hatano

Influence of size distribution of Zr(Fe, Cr)2 precipitates on hydrogen transport through oxide film of Zircaloy-4

Abstract The hydrogen uptake behavior of two types of Zircaloy-4 specimens containing fine or coarse Zr(Fe,Cr)2 precipitates was studied at 623–723 K in the pre-transition period of oxidation in steam. The amount of hydrogen uptake of the specimen containing fine precipitates was smaller than that of the specimen containing coarse ones, and the former was independent of the oxidation temperature while the latter increased with decreasing temperature. These results were successfully explained with the model that the Zr(Fe,Cr)2 precipitates remaining unoxidized act as the short-circuiting route in the hydrogen transport through the oxide film. The amount of hydrogen uptake of the present Zircaloy-4 specimens containing coarse precipitates was larger than that of the Zircaloy-2 specimens containing comparable sizes of precipitates. This was attributed to the fact that the concentration of precipitates remaining unoxidized in the oxide film of Zircaloy-4 was higher than that of Zircaloy-2.

Tracer diffusion coefficient of tritium in vanadium and trapping effect due to zirconium impurity

The tracer diffusion coefficient of tritium in the α-phase of vanadium is measured in a temperature region from 323 to 523 K by making use of the glow discharge implantation method recently developed by the present authors. The obtained results are in good agreement with the data previously reported by Qi et al. (J. Phys. F: Metal Phys. 13 (1983) 2053). The trapping effect of zirconium on the diffusion of hydrogen isotopes is examined by using specimens containing a small amount of zirconium impurity. The existence of at least two kinds of trapping sources is deduced by analyzing the concentration dependence of tracer diffusion coefficient of tritium: one is due to zirconium and the other is due to a certain complex of zirconium with interstitial impurities such as oxygen and nitrogen. Trapping energies of 0.15 and 0.36 eV are assigned to the former and the latter, respectively.

Observation of the spatial distribution of hydrogen in Zircaloy-2 oxidized in H2O steam at 723 K by a technique of tritium microautoradiography

The spatial distribution of hydrogen in Zircaloy-2, which was oxidized in H2O steam at 723 K and quenched down to room temperature, was examined by a technique of tritium autoradiography. Tritium was introduced into the specimen through the oxide layer by the cathodic charging method using tritium water at room temperature. The radiograph of the cross-section was observed by a scanning electron microscope. Hydrogen was densely distributed on the grain boundary and in the grain of the inner base alloy, but it was scarcely distributed in the oxide layer. Hydrogen was locally distributed, however, in the region corresponding to the intermetallic precipitates embedded in the oxide layer. The mechanism of penetration of tritium through the oxide layer during the cathodic charging was discussed.

Hydrogen isotope electromigration and impurity effect in vanadium

Abstract The effective charge of protium and tritium in the α-phase of vanadium has been measured by means of a stationary state electromigration experiment in a temperature region from 323 to 523 K. The obtained results have shown a definite mass dependence such as Z * (T)> Z * (H) in a lower temperature region and a tendency that the effective charge of H and T approaches a same value in a higher temperature region. By comparing this result with the data for protium and deuterium previously reported by other investigators, we have concluded a mass dependence according to the sequence Z * (T)> Z * (D)> Z * (H) in a lower temperature region. The origins of this mass dependence has been discussed by decomposing the effective charge into the direct and wind charges. The trapping effect due to a small zirconium impurity in the investigated vanadium specimen on the electromigration of hydrogen isotopes has been also examined and the trapping model has been discussed.

Observation of spatial distribution of tritium in zirconium alloy with microautoradiography

Abstract Spatial distribution of tritium in the oxide layer formed on zirconium alloy was examined by tritium microautoradiography. The zirconium alloy (Zircaloy-2) was oxidized in tritiated steam at 673 K. The autoradiographs of the surface and cross-section of the formed oxide films were observed with a scanning electron microscope (SEM). They indicated that the concentration of tritium in the zirconium oxide phase was appreciably high at the oxide surface and decreased sharply with depth. Inside the oxide film, tritium was concentrated in the intermetallic precipitates such as Zr(Fe,Cr) 2 and Zr 2 (Fe,Ni).

Transport mechanism of hydrogen through oxide film formed on zircaloy-4

The diffusion behavior of hydrogen in the oxide films of zircaloy-4 specimens containing different size of Zr(Fe,Cr)2 precipitates was examined. In the case of the specimen containing fine precipitates, hydrogen diffused uniformly through the zirconium oxide phase. The diffusion coefficient was 2·10−21 m2·s−1 at room temperature and 6·10−19 m2·s−1 at 673 K. The transport rate of hydrogen in the oxide film of the specimen containing coarse precipitates was significantly higher than that of the specimen containing fine ones at both room temperature and 673 K.

Measurement of Diffusion Coefficient of Tritium in Alpha-Phase of Zirconium Based on Glow Discharge Implantation Method

A tritium implantation method based on a glow discharge of tritium gas was applied to the measurement of diffusion coefficient of tritium in alpha-phase of zirconium. The diffusion experiment was carried out in a temperature region from 673 K to 873K, and the diffusion coefficient of tritium was determined as follows, D/(cm 2 /s) = 3.14x10 -3 exp? The present data were in good agreement with our previous data which were determined with an usual implantation method based on a nuclear reaction 6 Li(n,a)T.

Characterization of surface barrier effect upon tritium permeation through stainless steel with auger electron spectroscopy

The surface barrier effect on tritium permeation through SUS-316 stainless steel was characterized with Auger electron spectroscopy for the surfaces which had been confirmed to have different barriers from our previous study. The surface which was prepared by heat treatment at 1273K for 1 hr in vacuum(10 -4 Pa) was not contaminated with oxygen and carbon but covered uniformly with a large amount of sulfur. The surface exposed to air at room temperature after the vacuum annealing was covered with duplex oxide layers: the top layer consisted of iron oxide and the inner layer consisted of chromium, iron and nickel oxides. The iron oxide in the top layer was easily reduced with hydrogen gas at elevated temperatures but inner oxide layer was not completely reduced under the present conditions. These results were correlated to the surface barrier effect on tritium permeation based on our previous experimental results concerning the dissolution rate of gaseous tritium into stainless steel.

Observation of hydrogen distribution in oxidized zircaloy-2 with tritium microautoradiography

ABSTRACT The hydrogen distribution in the oxidized Zircaloy-2 was related to its microsturcuture by tritium microautoradiography based on a cathodic tritium charging method. It was found out that hydrogen atoms were concentrated in the intermetallic precipitates such as Zr(Fe,Cr)2 and Zr2(Fe,Ni) existing in the oxide film, and on the grain boundary of zirconium matrix. It was also found out that hydrogen atoms were scarcely present in the thin metallic region adjacent to the oxide layer, the thickness of which was about 10–15 µm.