Effect of Tin and Niobium on Corrosion and Hydrogen Pickup of Quaternary Zirconium Alloys in Ultra-Long-Term Autoclave Exposures

Corrosion and hydrogen pickup of zirconium alloys can be life-limiting factors for fuel rods in light water reactors (LWRs). Extensive work has been performed and is still in progress to improve the specification of cladding materials to enhance the performance as well as to further increase the understanding of the involved mechanisms and to model the corrosion and hydrogen pickup kinetics. In this scope, knowledge regarding the behaviors of quaternary alloys (Zr-Sn-Fe-Cr) with different chemical composition is relevant. Two of the experimental alloys studied here also include niobium. These R&D quaternary alloys, including Alloy A, Alloy B, and Alloy C, were corrosion-tested for a long duration in an autoclave at EDF's R&D laboratories. Tests were performed in a water-chemistry representative for the primary water in pressurized water reactors (PWRs) at 320°C, 340°C, and 360°C. The weight gains were measured periodically, and some specimens were removed for hydrogen pickup measurements. The effect of tin and niobium on corrosion is discussed with respect to other metallurgical parameters. Cyclic features of the corrosion kinetics are studied, and comparisons between transition kinetics and oxide stratification are discussed. The effects of tin and niobium contents on hydrogen pickup are also studied. For all materials, oxide thickness and hydrogen content are linearly correlated irrespective of the duration of the experiment. Finally, the results from the ultra-long-term autoclave tests are assessed using experience gained previously from irradiation of the same materials in commercial PWRs as well as in the Halden test reactor.