Recent improvements in modelling of hydrogen isotopes diffusion in Zr-Nb pressure tubes

Evaluating the effect of hydrogen isotopes on DHC and fracture toughness in CANDU reactor pressure tubes requires determination of the concentration distribution along the pressure tube. The concentration distribution depends on the spatial distribution of the sources for hydrogen isotopes, as well as the redistribution of the isotopes by diffusion. Since CANDU reactors are heavy water reactors, the sources are expected to involve only deuterium, while protium is present from the initial manufacturing process for the pressure tube material. Typically, the approach to model and predict the concentration distribution considers the net effect of all isotopes in terms of an equivalent concentration of a single isotope. Measurements from surveillance pressure tubes can distinguish separate distributions of protium and deuterium, and some of the data indicate localized changes in the protium concentration from the initial as-manufactured values. A diffusion model is extended to include both the protium and deuterium isotopes, and solutions to applications are solved with a finite volume method. Results of the new model are presented and compared with the simplified equivalent concentration approach. (author)