High temperature oxidation behaviour of alloy Zr-1Nb-1Sn-0.08Fe

The drive for improved efficiencies of nuclear steam supply system has led to higher fuel burnups and higher coolant temperatures. This has necessitated the development of improved cladding alloys which can work satisfactorily to extended duty limits and corrosion design limits. A number of studies have been reported on a potential alloy Zr-1Nb-1Sn-0.08Fe showing its superiority over presently used alloys, zircaloy-2 and zircaloy-4, as regards its corrosion resistance, creep resistance and irradiation growth for burnups up to 60 GWD/MTU. The present study concerns the high temperature oxidation behaviour of this alloy in steam at one atmosphere in the temperature range 500-700 deg C. These temperatures are far above operating temperatures but are envisaged during off-normal or loss-of-coolant accident conditions. Kinetics of the alloy Zr-1Nb-1Sn-0.08Fe have been determined and compared with those of Zircaloy-2. The two alloys have been assessed on the basis of microstructural examination before and after oxidation, morphology of oxide layer, weight gain during oxidation and structural characteristics of oxide film, as determined by XRD. It is seen that weight gains during oxidation for the new alloy are higher compared to zircaloy-2 at 500 deg C, while at 600 and 700 deg C, the weight gain difference is very little. XRD has indicated that films on both the alloys are monoclinic, though they differ texturally. Microstructure of the new alloy is characterised by coarse second phase precipitates while zircaloy-2 has finely dispersed precipitates. This possibly could be contributing to difference in the oxidation behaviour of the two alloys in the lower temperature region. (author)