Oxide film characteristics under PWR primary coolant conditions

Abstract

In pressurized water reactors (PWRs), the deposition of corrosion products (crud) on the fuel cladding surface causes Crud-Induced Power Shift (CIPS), which shifts the neutron flux distribution. Since corrosion products found on the cladding are rich in Ni, the Ni-based alloy steam generator (SG) tubing is the primary concern for corrosion product inventory that has led to the development of CIPS. This study was carried out to study the effects of SG alloy composition and heat treatment, boron concentration and zinc addition on oxide film formation under PWR primary coolant conditions. The corrosion tests were performed on specimens of Alloy 600, Alloy 690, Alloy 800, 304 stainless steel and Zirc-4 in an autoclave (Titanium autoclave and stainless steel autoclave), simulating PWR primary coolant conditions. After exposure for several days, the oxides on the samples were characterized with several techniques. The results revealed that nickel-based alloys and SS304 were covered with Fe-rich crystallites overlaying an amorphous Cr-rich layer, while Zircaloy-4 was covered with a ZrO₂ layer. For the same heat treatment, the higher-Cr alloys apparently produced more protective oxide (finer and more compact crystallites) than the lower-Cr alloys. The heat treatment effects were observed on all alloys, but were overshadowed by effects of alloy composition. The compactness of the oxide films was strongly boron dependent; the compact oxides were formed in the coolant containing boron. In addition, the oxides on Ni-based alloys and SS304 seem to be more protective by the addition of 20 ppb Zn in the coolant.