Modeling of creep behavior of Zircaloy-4 by considering metallurgical effect

The creep model for Zircaloy-4 has been improved by including the metallurgical effect of Zircaloy cladding. Based on the experimental results in which the creep strain rate is highest for stress relief annealed cladding (SRA) and lowest for recrystallized cladding (RXA), the annealing factor was introduced and derived by iterative calculations until the best predictions for all the rods were obtained. The creep model has been incorporated into the KAERIs fuel performance code COSMOS and then verified with 4 cladding creep data, of which 3 cases exhibit creepdown and the other one creepout. The model predicts well the creep behavior of UO2 fuel rods in PWRs. The prediction does not show any discernable difference between the high- and low-Sn claddings. With satisfactory agreement between predicted and measured values, the comparison indicates no difference in creep behavior between MOX and UO2 fuel rods as expected. Although there are controversies over the experimental method and the analysis procedures for clad creepout, COSMOS generally well predicts the creep behavior, even in the case of creepout, if the creepout factor is applied.