Comparison of the High Burn-Up Corrosion on M5 and Low Tin Zircaloy-4

The corrosion and hydriding behavior of M5 and low tin Zircaloy-4 in PWR have been compared at high burn-ups (>70 GWd/t). Whereas low tin Zircaloy-4 reached about 90 μm in oxide thickness and is subjected to significant precipitation of hydride rims at the metal/oxide interface, the oxide layer on M5 remains below 20 μm, and a hydrogen content remains below 100 ppm. Owing to the modification of the oxide structure and hydrogen localization observed, it is proposed that, concerning the high burn-up corrosion acceleration of Zircaloy-4, the tin content is the parameter governing the onset of the high burn-up corrosion acceleration, which is further accelerated by hydride precipitation at the metal/oxide interface and dissolution of precipitates. On M5, since no significant evolution of the oxide structure was observed, and none of the parameters identified as contributors to acceleration are present, no acceleration of corrosion kinetics is expected at higher burn-ups.