Investigation of zirconium hydrides and hydrogen solubility limit in Zr-2.5 Nb alloy by small angle neutron scattering

Abstract

In zirconium and its alloys, hydride phases form as needle-shaped ([gamma]-phase) or plate-shaped ([delta]-phase) precipitates when the terminal solid solubility (TSS) of hydrogen in the metal is exceeded. The hydrogen TSS in zirconium is almost negligible at room temperature, but at 620 K the solubility limit increases to about 1 at%. Zr-2.5 Nb alloy pressure tubes are the structural components in CANDU-PHW reactors and, if hydrides are present, the materials may be embrittled. A precise knowledge of the hydrogen TSS is required for assessing the fitness-for-service of Zr-2.5 Nb pressure tubes. However, in TSS measurements a hysteresis behavior is observed where the temperatures attributed to hydride dissolution and precipitation are different. This hysteresis occurs even in the absence of applied stress and may contribute to a variation in TSS values. To evaluate the feasibility of SANS for future experiments that will study zirconium hydride precipitation and dissolution, Zr-2.5 Nb samples containing several distributions of hydrides were tested at both room temperature and up to 600 K.