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

Abstract

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 -25 m 2-s-~ at room temperature and 6-10 -19 m L s -z 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. Introduction ExperimentalThe hydrogen uptake behavior of zircaloy is an important problem to evaluate the performance of fuel claddings of water-cooled reactors. It is well known that the hydrogen uptake behavior of zircaloy is strongly dependent on its alloy composition and heat treatments during fabrication.l In order to investigate the mechanism of influence of heat treatments, we have recently examined the influence of size distribution of intermetallic precipitates, such as Zr(Fe,Cr)2 and Zr2(Fe,Ni) in zircaloy-2 and Zr(Fe,Cr) 2 in zircaloy-4, on the hydrogen uptake behavior of zircaloy during oxidation in steam and found out that the specimen containing coarse precipitates took up larger amount of hydrogen than the specimen containing fine ones. 2,3 The influence of the precipitate size on the hydrogen transport in the oxide film, however, has not been fully clarified.In the present study, the diffusion behavior of hydrogen in the oxide films of two different types of zircaloy-4 specimens containing fine and coarse Zr(Fe,Cr)2 precipitates was examined at high temperature (673 K) and room temperature. In the former case, the specimens were oxidized in D20 steam, and the depth profile of deuterium was measured with secondary ion mass spectroscopy (SIMS). The secondary ion yield of deuterium was calibrated by measuring the radioactivity of tritium in the oxide film with a liquid scintillation counter. In the latter case, the diffusion coefficient of tritium in the oxide film was estimated from the leaching rate of tritium into the scintillation cocktail. The mechanism of the influence of precipitate size on the hydrogen transport in the oxide film is discussed.The alloy composition of zircaloy-4 used in the present study is shown in Table 1. Two disk-type specimens (8.5 mm in diameter, 3 mm in thickness) containing fine and coarse Zr(Fe,Cr) 2 precipitates were prepared with heat treatments. The detailed procedures of heat treatment were described in the previous paper. 3 The former specimen wilI be hereafter described as specimen A and the latter as specimen B. In the specimen A, the diameter of precipitates was less than 50 nm. 3 In the specimen B, the average and maximum diameter of precipitates were 0.5 and 1.7gm, respectively. 3 The specimens were mechanically polished with abrasive paper and finished by polishing with 0.3 gm A1203 powder.The specimens were oxidized in H20 steam at 673 K and 0.1 MPa, and a coherent oxide film whose thickness was 1...

Mechanism of corrosion of zirconium hydride and impact of precipitated hydrides on the Zircaloy-4 corrosion behaviour

“…Especially, it has been reported that the intermetallic precipitates such as Zr(Fe, Cr) 2 play a role in the hydrogen pick-up process [35]. These second phase particles in a metallic state close to the metal/oxide interface would be cathodic sites for hydrogen according to Ramasubramanian et al [36] and "windows" or "bridges" for hydrogen to cross the metal/oxide interface [37].…”

Interactions of hydrogen with zirconium alloying elements and oxygen vacancies in monoclinic zirconia

“…Consequently the type of hydrogen species diffusing through the oxide vary depending on the process under study [1][2][3][4][5][6][7][8][9].…”

Ab-initio calculations of hydrogen diffusion coefficient in monoclinic zirconia

“…Indeed Zircaloy-4 and M5 alloys contain mainly tin and niobium, respectively. Hydrogen diffusion coefficient in the oxide layer formed on M5 alloy is slightly higher than the one measured on Zircaloy-4 [13,14,16,17]. This difference is likely due to these alloying elements.…”

“…Anyway, research was initially focused on hydrogen diffusion through the oxide layer. In the past, the diffusion coefficient of hydrogen in the corrosion film was widely investigated either by 1) isotopic tracers, or 2) hydrogen implantation followed by Secondary Ion Mass Spectrometry (SIMS) or nuclear reaction analysis (NRA) [12] [13] [14] [15] [9] [16] [17] [18] [19] . However, due to the presence of two sub-layers in the oxide with different hydrogen transport properties [12], as well as the two types of hydrogen interaction within the oxide [9] or the polycrystalline structure of the oxide layer [19], apparent diffusion coefficient of hydrogen is difficult to evaluate accurately from measurements.…”