Oxidation behavior of Zry-4 in steam-air mixtures at high temperature

Negyesi, M.; Amaya, Masaki  

This study dealt with oxidation behavior of Zry-4 nuclear fuel cladding under a severe nuclear reactor accident scenario. Influence of the reaction extent on cladding mechanical properties was also treated. Zry-4 fuel cladding segments were exposed to steam atmosphere containing different amount of air in composition ranging from 0 to 100 vol.%. Exposure times covered both pre- and post-breakaway regimes. Metallographic observations along with microhardness testing were carried out to study the effect of air fraction on the material microstructure. Hydrogen pick-up was measured and residual mechanical properties were assessed employing Ring Compression Tests. The results show that the addition of air in steam can lead to more than 300% increase of weight gain in case of 50% or higher air fraction after the exposure of 60 min., namely, "late post-breakaway regime". Approximately three times thicker oxide scales were observed in such cases, indicating that most of the weight gain was due to the oxide layer increase. Nitride phase was preferentially observed close to the interface between oxide and metal layers. The -Zr(O) sublayer thickness decreases with the increasing fraction of air in steam. Microhardness, measured within the prior -phase region, slightly depends on the air fraction. Hydrogen uptake, on the other hand, depends strongly on the air fraction in steam. In the case of shorter exposure times, the effect of air addition in steam was suppressed. Eventually, specimen plastic strains drastically reduced in the late post-breakaway regime.