Solubility of hydrogen in Zircaloy-4: irradiation induced increase and thermal recovery

The solid solubility of hydrogen isotopes in neutron irradiated Zircaloy-4 core components removed after 10.3 years in service is studied by differential scanning calorimetry (DSC) in two cooling channels of the Atucha I Heavy Water Power Reactor, Argentina. For irradiated Zircaloy-4 samples containing 150–380 ppm-Heq the temperature for hydride dissolution, TSSdirrad, measured during the first DSC run was approximately similar to the reactor operating temperature (260–300 °C) and 50–150 °C lower than the TSSd for unirradiated material. This difference decreases in subsequent DSC runs and a stage of stable TSSdirrad intermediate values is reached. After further isothermal annealings the TSSdirrad values approach asymptotically TSSd. These results may be interpreted by assuming that a fraction of the total hydrogen isotope concentration, which is similar to the concentration that should be present in solid solution in unirradiated Zircaloy-4 at the operating temperature, is in solid solution in the irradiated Zircaloy-4 and the rest is trapped and slowly released from complex irradiation induced defects.