Characteristics of autoclave and in-reactor nodular corrosion of zircaloys

This paper reports on nodular corrosion characteristics of recrystallized Zircaloy-4 that were investigated in static autoclave tests at 500 degrees C and 10.3 MPa. The roles of annealing temperature, cooling rate after beta-treating at 1050 degrees C, cold work, and surface treatment in corrosion tests were correlated with the results of microstructural characterization by scanning and transmission electron microscopies. A good correlation was obtained between average size of intermetallic precipitates and weight gain, in contrast to nodule coverage and nodule number density. These results could be best explained by the hypothesis that nodules nucleate in local regions that are depleted of Fe and Cr alloying elements. Some observations were inconsistent with the premise that nodules nucleate on or near intermetallic precipitates. Nodular corrosion characteristics and microstructures of commercial Zircaloy-2 cladding of fuel and gadolinia rods, obtained from several boiling water reactors (BWRs) after burnup of 11 to 30 MWd/kgU, were also examined. Partial amorphization of intermetallic precipitates in BWR Zircaloy-2, and virtual dissolution, and in an extreme case spinodal-like fluctuations of dissolved alloying elements in PWR Zircaloy-4 cladding were observed