The balance between the amount of plutonium produced by reprocessing and recycled in the PWR leads EDF, the French Utility, in cooperation with CEA to investigate the feasibility of dry interim storage for the spent fuel assemblies, especially the MOX ones. The safety criteria require to prevent the cladding from any loss of integrity. The thermal creep of the cladding due to the internal fuel rod pressure is assumed to be the most relevant mechanism susceptible to lead to failure. Therefore, the simulation of the creep behavior for quite long term dry storage and under variable loading conditions (temperature and internal rod pressure) requires a creep law integrating the suitable parameters. Using a step-by-step approach based on available data base on as-received and irradiated CWSR Zircaloy 4 cladding, a creep law under biaxial loading has been developed, this paper presents the creep state equation.