THEORETICAL AND NUMERICAL STUDY OF HYDROGEN DIFFUSION IN Zr-Nb ALLOYS

Abstract

Within the framework of the delayed hydride cracking phenomena reported for the pressure tubes of CANDU-type nuclear reactors, we study the influence of the Zr-β (∼ 20%Nb - ∼ 80%Zr) continuous phase on the hydrogen diffusion coefficient, DH, through the Zr-α / Zr-β biphasic alloy. We propose an improved phenomenological model for DH with respect to those found in the literature. Furthermore, we study the influence of the Nb content on DH for the cubic phase Zr-β, employing the transition state theory furnished with ab-initio parameters provided by the SIESTA code. In particular, 9 ordered alloys are considered with different Nb content and effective activation energies are computed by Arrhenius fits for each alloy. We find that activation energies vary in a non-monotonic way as Nb content increases, reaching a maximum value at about Zr-50%Nb. Finally, we observe that the predicted and measured larger diffusivity along the tube axis vs. the radial direction, is consistent with the material texture. Moreover, we conclude that the loss of continuity of the Zr-β sheets present in the tube microstructure, is consistent with the decrease of DH in time at a given temperature.