Self-compensation in creep of pressurized cladding tubes: From self-limitation in dry storage to self-acceleration in burst tests

Abstract

Self-compensation in cladding creep under internal pressure is analyzed as a separate effect. The internal volume increase due to creep and the consequent decrease in internal pressure is compensated, fully or partly, by the direct effect of changing cladding dimensions on cladding stress. First, a simplified basic model is set up that reveals the difference between hollow cladding sample on the one side and real fuel rod filled with pellets or a cladding sample with mandrel on the other. Both isotropic and anisotropic cladding is considered, and three application examples are shown: pre-pressurized hollow cladding, pre-pressurized filled cladding, and pressurized cladding held at constant internal pressure. Second, a more elaborate model is developed that accounts additionally for cladding dimensions and outer pressure. Closed analytical solution of the model is presented and applied to various examples. Finally, the phenomenon of creep self-limitation in dry storage is assessed quantitatively and compared with full fledged calculation of the GRS-671 fuel rod dry storage benchmark. It can be concluded that the effect of self-limitation is practically negligible. Additionally, an upper bound of self-limitation in dry storage was estimated to 4% hoop stress decrease at 1% creep strain.