Modeling primary creep for Zircaloy claddings during load reversals and drops in BISON

It is desired to accurately predict creep strains in zirconium claddings, subjected to cases such as sudden load drops or reversals, using the BISON fuel performance code. It has been experimentally proven that primary creep reinitializes at sudden load drops and reversals, which is different than normal operating conditions, where stresses increase gradually. A new visco-elastic primary creep model is added to BISON, which is capable of handling situations of load drops or reversals. The model is coupled with secondary creep models namely Hayes-Hoppe and Limback-Anderson, available in BISON to make predictions on cladding creep strains. The results are compared to Halden in-pile experiments IFA-585 and IFA-699. Based on the comparison, it is recommended to use the visco-elastic primary creep model coupled with Hayes-Hoppe secondary model for consistency and performance.