High temperature oxidation of zirconium alloys cladding material in sfp-loca conditions examined with micro raman imaging and 18o tracer techniques

International audienceSince the Fukushima Daiichi accident, an increased attention is continuously paid to the vulnerability of the Spent Fuel Pools (SFPs). In SFPs, degradation of the fuel rods induced by cladding oxidation in air-steam mixtures after dewatering is a major safety concern.In this study, the high temperature oxidation of Zircaloy-4 and M5 cladding materials in oxygen, air and air - steam atmospheres was investigated. Because spent fuels are concerned, the presence of the corrosion oxide scale formed in reactor during in-service operation was considered. Oxidation tests covering the 700 – 950°C range were performed in a thermobalance, and mass spectroscopy measurements allowed to monitor in real time the oxidizing atmosphere. Raman imaging was used to examine the specimens after the oxidation tests, either directly at the oxide surface, or on metallographic preparations for cross-section examinations. Taking benefit of the ability of Raman spectroscopy to quantitatively extract with a fair accuracy the 18O distribution in a zirconia scale [1-4], two kinds of high temperature oxidation experiments using the 18O isotope as a tracer were performed: (i) Two-stage diffusion experiments, to extract high temperature oxygen transport data in thermally grown zirconia scales; (ii) Oxidation experiments in mixed 18O2 - H216O - N2 atmospheres, as an attempt to understand the respective contributions of oxygen and steam to the oxidation mechanism.Two stage oxidation tests were first performed with specimens pre-oxidized at low temperature, simulating in-service corrosion. Results gave clear evidence for different characteristic distributions of 18O in the scales: (i) Away from defects, the protective effect of zirconia scales formed at low temperature regarding subsequent high temperature oxidation in nitrogen-containing atmospheres was clearly demonstrated. Apparent oxygen diffusivities were derived using the radial 18O concentration profiles extracted from the Raman 18O distribution maps (Fig. 1). Nitrogen appears to have no or limited influence on the oxygen diffusion, but was observed to reach the metal/oxide interface much faster than oxygen. (ii) Where radial cracks have formed during the pre-oxidation process, the corroding medium penetrates locally in the scale and spreads though the circumferential crack network interspersing the scale. Here the protective influence is clearly lowered. For the second kind of experiments, mixed 1802 – H216O – N2 atmospheres were considered, to tentatively discriminate between the respective contributions of steam and oxygen on the oxidation mechanism. Here, bare Zircaloy-4 and M5 samples were oxidized. Oxidation durations have been adjusted to simultaneously show pre-transition as well as post-transition regions. Results show strong differences in the oxidation mechanisms between the pre- and post-transition regimes. Specifically, our current measurements strongly suggest that, in the pre-transition regime, bare Zy-4 samples preferentially react with steam (Fig. 2). As an attempt to explain this finding, further high temperature two-stage tests have been performed and will be discussed