Degradation of Zirconium Alloy Pressure Tube in Qinshan CANDU-6 Heavy Water Reactor and Its Mitigation

Pressure tubes are critical components in heavy water reactors,and its ageing and degradation directly has effect on the service life of the reactor.The material of the pressure tube of Qinshan Phase Ⅲ CANDU-6 reactors are zirconium alloy（Zr-2.5Nb）,and the dimensions and mechanical properties of pressure tubes change under long-time service conditions of high temperature,high pressure,and high radiation.Also there is risk that delayed hydride cracking（DHC） may happen on pressure tubes due to the pickup of deuterium by zirconium alloy.To improve the radial creep rates of pressure tubes,reduce the residual stress and risk of DHC,and follow up the degradation extent of pressure tubes to provide important information and evidences for life assessment,Qinshan Phase Ⅲ implemented the improvements of manufacture and installation process,control of service chemical conditions and periodic inspections,and will implement preventative maintenance.