A model for analysis of the effect of final annealing on the in- and out-of-reactor creep behavior of Zircaloy cladding

The creep behavior of Zircaloy cladding materials depends on materials texture, degree of recrystallization, and chemical composition. This study is devoted mainly to the analysis of the effect of the final annealing on the creep characteristics. For this purpose, data from a series of thermal creep tests are presented and evaluated. The out-of-reactor tests are performed under internal pressurization, and the test matrix covers seven conditions with temperatures from 330 to 400 C and hoop stresses between 80 and 160 MPa. Three lots of Zircaloy-2 claddings and one lot of Zircaloy-4 are considered. The difference between the three Zircaloy-2 lots is in their final annealing conditions. The claddings are either stress relief annealed (SRA), recrystallization annealed (RXA), or partially recrystallization annealed (PRXA). The materials used when fabricating the Zircaloy-2 claddings are from the same ingot, and the chemical compositions of the three types of claddings are almost identical. The Zircaloy-4 cladding included in the test is SRA, and the tin content in this material is similar to that in the Zircaloy-2 materials. The creep data are analyzed by separating the primary and the secondary creep. In this analysis, the Matsuo creep model, which accounts for both primary and secondary creep, is modified, calibrated, and verified using the new thermal creep data. Based on in-reactor data, the thermal creep model is extended to cover also the creep behavior under irradiation. The claddings considered in the in-reactor test were of both SRA and RXA types, and the experiments were made under external pressure. It is observed that for moderate hoop stresses the measured hoop creep rates, both out-of-reactor and in-reactor, decrease with increasing final annealing temperature. However, for higher stresses, the steady state creep rate has a minimum for partially recrystallized claddings