Hydrogen absorption by zirconium alloy cladding tube with surface oxide film

Hydrogen absorption kinetics of Zircaloy (2 and 4) and Zr-Nb (1% and 2.5%) cladding tubes were studied by heating in hydrogen gas after oxide film formation in steam, oxygen or air. Hydrogen absorption rate depended on the degree of pre-oxidation. In Zr-Nb, the absorption rate was also sensitive to the atmosphere used for pre-oxidation, whereas in Zircaloy the rate was relatively independent of the kind of oxidant. In all materials, pre-oxidation to the transition point was found to bring about high absorption rate in the subsequent hydriding step. After pre-oxidation to the post-transition region, hydrogen absorption rate by Zircaloy showed constant or slightly decreasing tendency with increasing oxidation level, whereas in Zr-Nb, particularly in Zr-2.5%Nb, the rate showed a clearly decreasing tendency depending on the pre-oxidation atmosphere. Different characteristics of Zircaloy and Zr-Nb can partly be explained in terms of different valencies of alloying elements which influence the lattice defect concentrations in the oxide films. (author)