Impact of Irradiation Defects Annealing on Long-Term Thermal Creep of Irradiated ircaloy-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 470–520°C 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 (470°C), followed by a second period at a lower temperature (320–400°C). 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 470°C on creep strain. Tertiary creep is reached after a few days for 100 or 120 MPa. The effect of a first period at 470°C on the next creep behavior at 400°C for 150 MPa is confirmed. The probable induced annealing of irradiation defects contributes to increase the secondary creep rate at 400°C. 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.

The microscopic characterization confirms the hypothesis of a partial annealing of the irradiation defects after a period of 10 days at 470°C, which leads to a microstructure intermediate between irradiated and as-received CWSR condition, while a temperature of 520°C leads to a microstructure that looks like recrystallized Zircaloy-4 condition.

After the creep test, hydrides morphology, distribution, and orientation appear rather different from usual post-irradiation hydrides characteristics. The hydrides are distributed uniformly throughout the thickness of the tube. A cooling under mechanical loading influences the hydrides precipitation, particularly by leading to a radial “reorientation.” A stress level of 80 MPa during cooling is sufficient to lead to radial hydrides formation as hydrogen precipites in the cladding.