Hydride induced embrittlement of zirconium alloy pressure tubes

Zirconium alloy tubes act as miniature pressure vessels in Pressurized Heavy Water Reactors (PHWR) and are subjected to aqueous corrosion resulting in hydrogen pick up. Hydrogen exceeding solid solubility precipitates out as brittle hydride phase and may cause embrittlement of the host matrix. Two forms of embrittlement have been recognized for dilute zirconium alloy pressure tubes - gross and localized. Gross embrittlement is caused due to uniformly distributed precipitate, whereas localized embrittlement is caused by hydrogen migration leading to damage accumulation within a small region over a period of time before catastrophic failure of the component may occur. The parameters like fracture toughness, threshold stress for reorientation of hydrides, threshold hydrogen concentration for blister formation, delayed hydride cracking (DHC) velocity, threshold stress intensity factor for DHC initiation are used as fitness criteria for service assessment of the pressure tubes. In this paper, some of the results generated using Indian pressure tube material will be discussed. (author)