Thermal creep of Zircaloy-4 tubes containing corrosion-induced hydrogen

Abstract

Increased burn-up results in more corrosion as well as hydrogen pickup in zirconium alloys. The effect of absorbed hydrogen and surface oxide layer on creep properties was investigated for the Zircaloy-4 fuel cladding tube. Creep test was performed at 400 °C and 120 MPa for 10 days using ring-shaped tube specimens. Test samples were prepared by conducting a corrosion test in a steam atmosphere. Through the corrosion process, hydrogen was charged into zirconium matrix (16–756 wppm) and a surface oxide was formed (2.4–11.7 μm) at the same time. The absorbed hydrogen decreased the creep rates of the cold-worked stress-relieved Zircaloy-4. On the other hand, the effect of hydrogen was different depending on its phases, i.e. solid-solution and zirconium hydride, in the recrystallized Zircaloy-4. The hydrogen in solution almost doubled the creep rates. However, the hydride reduced the creep rates by one third as in the case of stress-relieved microstructures. The surface oxide was also observed to increase the creep resistance irrespective of the microstructures of Zircaloy-4.