Influence of Steam Pressure on the High Temperature Oxidation and Post-Cooling Mechanical Properties of Zircaloy-4 and M5 Cladding (LOCA Conditions)

摘要

Studies that deal with small and intermediate break loss of coolant accident (LOCA) conditions, for which steam pressure remains relatively high in the reactor primary system, are scarce, even though it has been sometimes observed that steam pressure could have a significant effect on the high temperature (HT) oxidation kinetics of zirconium alloys. Thus, in order to study the effect of steam pressure on the oxidation of low-tin Zircaloy-4 and M5 alloys, cladding samples were oxidized at temperatures between 750 and 1200°C in flowing steam at pressures ranging from 1 to 80 bars. It is shown that the oxidation kinetics of as-fabricated Zircaloy-4 is enhanced under high steam pressure within the 750 to 1000°C oxidation temperature range. The effect is quite low at such temperatures for as-fabricated M5, and at 1100 and 1200°C for both alloys. In order to examine the influence of burn up effects, oxidation tests were performed on Zircaloy-4 and M5 samples pre-hydrided at approximately their respective end-of-life hydrogen expected contents for high bum up claddings and on autoclaved specimens pre-oxidized (pre-existing oxide layer thicknesses from 3 up to 8 μm) at 360°C with typical pressurized water reactor (PWR) primary water chemistry. For both materials, it is shown that pre-hydriding (by gaseous charging) does not significantly modify the oxidation kinetics at 850 and 1000°C (temperatures at which the steam pressure effect is maximal for the fresh materials) under steam pressures of 1 and 80 bars. For the same HT oxidation conditions, the weight gains are lower for the pre-oxidized samples and the differences between sensitivities of Zircaloy-4 and M5 to a pressurized steam environment are significantly reduced after pre-oxidation. No significant hydrogen uptake was observed in the case of M5, whatever the investigated oxidation conditions. Metallurgical observations revealed that the enhanced oxidation kinetics of Zircaloy-4 under high pressure is associated with a significant but limited hydrogen uptake and a thickening of the oxide layers formed at HT. However, it is observed that the diffusion of the oxygen (coming from the cladding oxidation process) through the metallic substrate is not modified. The results show that, for Zircaloy-4 alloy, the steam pressure effect is associated with changes in the oxide microstructure (crystallites morphology, porosity, and cracks). These changes may be, for example, induced by a destabilization of the tetragonal phase of zirconia, so that oxygen and hydrogen transport through the oxide is easier. Ring compression tests performed at room temperature and at 135°C showed that the effect of steam pressure on the post-cooling mechanical properties of fresh, pre-hydrided, or pre-oxidized Zircaloy-4 and M5 samples after transient oxidation at HT is, for the conditions investigated, generally limited, although significant in a few cases for Zircaloy-4. Finally, it was determined that the impact of steam pressure observed for low-tin Zircaloy-4 is very low or negligible on the equivalent cladding reacted (ECR) and the hydrogen pickup of the cladding during typical in-reactor transients and has hence no significant consequence on the verification of LOCA cladding safety criteria.