Nodular corrosion characteristics of recrystallized Zircaloy-4 were investigated in static autoclave tests at 500°C and 10.3 MPa. The roles of annealing temperature, cooling rate after beta-treating at 1050°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. Occurrence of nodular oxidation of Zircaloy-2 in BWRs could best be correlated to average size of intermetallic precipitates before irradiation and to fuel cladding operating temperature. For an intermetallic size range of 250 to 700 nm, nodular oxides were observed at 288°C, but only thick uniform oxide was observed at 307°C. For extended-burnup operation, increased precipitation of hydrides, suboxides, and small oxide particles, apparently formed via an irradiation-induced process, is expected near the metal/oxide boundary and results in more pronounced microcracking along the metal/oxide boundary. In addition to its association with metal/oxide boundary temperature, which increases for increasing oxide thickness, boundary microcracking may be associated with accelerated corrosion of extended-burnup fuel of >45 MWd/kgU (rod-average burnup).
Author Information
Jeong, YH
Korea Atomic Energy Research Institute, Daeduk-Danji, Choong-Nam, Republic of Korea
Rheem, KS
Korea Atomic Energy Research Institute, Daeduk-Danji, Choong-Nam, Republic of Korea
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