Out-of-reactor study on external-pressure creep of zircaloy-2 fuel cladding tubes

An out-of-reactor experiment was conducted for the purpose of studying the external- pressure creep behavior of zircaloy-2 fuel cladding tubes. The hour-glass shape of UO2 pellets acquired by thermal distortion was simulated by flanges machined out at both ends of stainless steel pellets 21 mm long. The mock-up specimens thus formed were pressurized externally in a furnace, and the changes brought upon the tube diameter were measured at intervals. The external-pressure creep deformation was observed to proceed in three steps- diametral decrease, elliptical deformation and final collapse. In the case of hollow tubes tested devoid of the pellets, elliptical deformation was observed, which accelerated with time until abrupt final collapse. Elliptical deformation was not observed on the pellet-filled tubes. Empirical equations were derived from the experimental results as functions of time, of hoop stress and of temperature, to express the external-pressure creep strain behavior of the stress-relieved tube, pellet-filled and thus internally supported at intervals of 20 mm. No difference in the external-pressure creep deformation was observed between stress-relieved and recrystallized tubes under the condition of 350°C and 72.5 kg/cm2. The pellet-filled specimens showed larger deformation than the hollow tubes in the process of diameter reduction. A certain length of unsupported distance in the range of 0–20 mm appeared to maximize the external-pressure creep deformation at the pellet center, under conditions similar to that of the present experiment.