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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...