Impact of irradiation defects annealing on long-term thermal creep of irradiated Zircaloy-4 cladding tube

Functions that can be allocated to cladding during interim storage depend on the evolution of cladding properties with time. The fuel rod cladding is strained by the end-of-life internal fuel rod pressure (40-60 bars NTP) and the potential release of fission gases and helium during dry storage, Within the temperature range that is expected during dry interim storage, long-term creep under over-pressure of the cladding might probably be a relevant strain mechanism, which could lead to breaching, Creep experiments were carried out on 4 cycles irradiated Cold Worked Stress Relieved (CWSR) Zircaloy-4 cladding under internal pressure within the temperature range of 470 520degreesC for up to 10 days, in order to estimate the eventual effect of a transient period at higher temperature on creep behavior during storage, Therefore, some of the tests consist of two periods: the first period at high temperature (470degreesC), followed by a second period at a lower temperature (320-400degreesC). A metallurgical characterization (TEM, optical microscopy) was carried out after the tests. A significant impact of the stress level is observed at the temperature of 470degreesC on creep strain. Tertiary creep is reached after a few days for 100 or 120 MPa. The effect of a first period at 470degreesC on the next creep behavior at 400degreesC for 150 MPa is confirmed. The probable induced annealing of irradiation defects contributes to increase the secondary creep rate at 400degreesC. Moreover, the creep kinetics of the tests conducted to rupture, show in all the cases a ductile rupture with ballooning instability, which might be partially the result of an annealing of irradiation defects.