Hydrogen terminal solid solubility determinations in Zr-2.5Nb pressure tube microstructure in an extended concentration range

In the present work the temperature of terminal solid solubility of hydrogen in the zirconium alloy Zr-2.5Nb was measured for material taken from a CANDU type pressure tube. The measurements were made in a wide concentration range (50-463 wppm) using two techniques, differential scanning calorimetry and differential dilatometry, with the additional purpose of checking the coincidence of the TSS points in both techniques. The work was complemented by microstructural studies pursuing the (β-Zr) → (α-Zr + β-Zrenr + β-Nb) transformation. This transformation is accelerated if the material remains at temperatures higher than 400 °C, which corresponds to a hydrogen solubility of about 180 wppm. Thus, microstructural studies were made by transmission electron microscopy and energy dispersive analysis, which allowed to relate changes in the solubility with these microstructural transformations.