IMPROVEMENT IN OXIDATION RESISTANCE OF ZIRCALOY-4 BY SURFACE ALLOYING WITH A THIN-LAYER OF PALLADIUM

Economic considerations and environmental pressures have forced reactor operators world-wide to consider higher burnup, to the extent that standard Zircaloy-4 fuel cladding is now approaching its design limitations. One of the main factors limiting the safe in-reactor lifetime of PWR fuel is the waterside corrosion of the fuel cladding. In this investigation the oxidation resistance of Zircaloy-4, surface alloyed with a thin layer of palladium by annealing in vacuum at 950-degrees-C, was studied. Short term oxidation in air shows a retardation of the transition to linear oxidation kinetics. It is argued that this retardation is caused by the incorporation of intermetallic particles into the oxide layer which increases the ductility of the oxide, thereby retarding the onset of cracking from the outside surface of the oxide layer.