Volume 2, Issue 6 (June 2005)
Improved ZIRLOTM Cladding Performance through Chemistry and Process Modifications
Since the introduction of the ZIRLO™ alloy for use in commercial nuclear light water reactors (LWRs) in the early nineties, work has continued on identifying chemistry and process changes to improve further the in-reactor corrosion performance of the alloy. Relative to the reference ZIRLO chemistry of Zr-1%Nb-1%Sn-0.1%Fe, a series of Zr-xNb-ySn-zFe alloys was fabricated to strip material and autoclave tested. The goal was to identify Zr-Nb-Sn-Fe compositions that would result in lower uniform corrosion rates in pure water and steam environments, while retaining resistance to accelerated corrosion in abnormal chemistry conditions, such as elevated lithium levels. The results of the study identified a reduction in tin content as having the greatest impact on improving the uniform corrosion performance. However, a minimum tin content was required to avoid accelerated corrosion in water containing 70-ppm Li. In the processing area, an extensive study was conducted to evaluate the relationship between thermal-mechanical processing, alloy microstructure (e.g., second phase particles), and corrosion performance. Processing variables following β-phase heat treatments included cold work, annealing temperature, and annealing time. Long-term autoclave tests performed in 633°K water containing 70-ppm lithium showed the uniform corrosion rate of the alloy is essentially optimized in cold-worked material after only 1 h of anneal at temperatures higher than 823°K. The autoclave results also showed a lower corrosion rate with decreasing processing temperatures. A low tin version of the ZIRLO alloy, with approximately 0.75 % tin content, was fabricated into fuel cladding and inserted as fuel rods in a commercial pressurized water reactor. After two 18-month cycles at high fuel duty and average burnups greater than 52 GWD/MTU, the peak oxide thickness in the low tin ZIRLO was significantly lower than reference ZIRLO.