The Characteristics of the Oxide Films Formed on Ni-base Alloy Cladding in Simulated Pressurized Water Reactor Primary Water Environment

Abstract

The microstructure and the oxide films formed on Ni-base alloy cladding in simulated pressurized water reactor (PWR) primary water environments at 325 o C were characterized. There was a steep drop of iron content and increase of Ni and Cr content in the Ni-base alloy cladding near the fusion line, and a relatively mild drop of iron content and the resultant increase of Ni and Cr contents at the location about 3.8 mm away from the fusion line. The surface oxide morphologies were observed by scanning electron microscopy and the cross-section morphologies and chemical composition of the oxide films were measured by transmission electron microscopy (TEM) and energy dispersive spectrometer. After 146 h immersion in PWR primary water, there were more oxide particles on the cladding alloy at the location closer to the cladding fusion line. After 1860 h immersion in PWR primary water, fine oxide particles on the specimen surface were covered with sparsely distributed large oxide particles. The oxide film after 1860h immersion exhibited a duplex structure, which was thicker at the location closer to the fusion zone, according to TEM results. The outer layer was rich in iron and the inner layer was rich in chromium. Both the inner and the outer oxide films were depleted in Ni, despite of the high Ni content in the alloy matrix.