Is hydrogen preferentially picked- up in IPHWR's pressure tubes over deuterium?

Hydrogen embrittlement is one of the major life limiting factors for zirconium-based Indian pressurized heavy water reactor (IPHWR) pressure tube materials. As part of the lifetime management of these pressure tube materials, slivering from the pressure tubes is performed during reactor shutdown condition. These samples were analyzed for hydrogen (H) and deuterium (D) content. During the analysis of these samples, it was observed that the H content is always higher than the D content, although the initial H content is maintained less than 5 ppmw and nuclear grade D₂O (D isotopic purity≥98.6%) is used as a coolant and also used to pressurize the scraping tool. According to the well-accepted mechanism for hydrogen pick-up, the nascent hydrogen/deuterium generated due to the reduction of H⁺/D⁺ at the metal and metal oxide interface is partially absorbed by the structural materials. Therefore, to understand the preferential pick-up of hydrogen isotope, Zr-2.5%Nb alloy specimens were electrolytically charged using a suitable electrolyte with a different amount of H & D. In the charged specimens, H & D contents were determined employing HVE-QMS technique. The hydrogen was preferentially absorbed by the Zr-2.5%Nb alloy between 5-9 times compared to deuterium for all mole fractions of H₂O & D₂O in the solution under study. To investigate the effect of adsorption on isotopic preferentiality, Zr-2.5%Nb alloy samples were exposed to different compositions of H₂ and D₂ gases at 633 K. Hydrogen has 2-3 time preferentiality over deuterium for adsorption and no correlation of the composition with the preferentiality was found. Therefore, in summary, the electrolyte proton/deuteron exchange reaction, their reduction (solvation energy difference), and adsorption may be responsible for the observed preferentiality.