Degradation processes in thermal creep of Zr1%Nb alloy cladding at high temperatures

Short-term constant stress creep tests in tension were performed on pre-oxidized and pre-hydrided zirconium Zr1wt.%Nb cladding alloy in the form of a thin-walled cladding tube in the temperature range of 550-900°C and at the applied tensile stress ϭ from 5 to 60 MPa to provide further information of it thermal creep behavior in the α-Zr and (α+β) -Zr phase regions. The (α+β)-Zr region is considered the first stage of the LOCA accidental conditions. The microstructure evolution of the studied alloy was investigated using SEM and TEM microscopy in the as-received state and after creep tests. Based on the determined values of the stress exponent n of the minimum creep rate m, n = (∂ln m / ∂ln ϭ)T , the main creep deformation mechanism in the (α+β)-Zr region was proposed as dislocation glide, which is not altered by oxygen and hydrogen. Oxygen and dissolved hydrogen decrease the creep rate, with the effect being greatest at the highest oxygen and hydrogen concentrations and higher testing temperatures. The dominant creep strengthening effect provides oxidation hardening.