Quenching behavior of high temperature Cr-coated Zr-alloy cladding during bottom reflood condition

Abstract

Chromium (Cr) coated cladding is the most promising candidate for near-term accident-tolerant fuel (ATF) for current light water reactors (LWRs) because of its excellent oxidation resistance at high temperatures. For deployment of the Cr-coated cladding as ATF, the performance should be evaluated under various conditions, including the loss-of-coolant accident (LOCA). In this research, a single-rod reflood facility was constructed to investigate the quenching behavior and performance of Cr-coated cladding under bottom reflooding at high temperature conditions up to 1200 °C. Quench tests were performed using an uncoated, cold spray Cr-coated, and PVD Cr-coated Zr-alloy at various initial temperature ranges from 600 °C to 1200 °C with 20 K of water subcooling and 4.5 cm/s of reflood velocity. The quench temperatures were obtained at four different axial locations based on the maximum curvature method developed in this study. The bottom reflood quench test results indicate that the quench temperature increases with increasing initial cladding temperature and decreasing axial distance from the bottom. The quench temperature was not notably different for uncoated Zr-alloy and Cr-coated Zr-alloy, regardless of the coating deposition method. However, above 1000 °C, the Cr-coated Zr-alloy showed a consistent and predictable quench temperature in contrast with the uncoated Zr-alloy for which the quench temperature data were severely scattered. The underestimation by the existing quench temperature model was observed with the current test using typical cladding thickness (0.57 mm). The thin cladding thickness was suggested as the reason for the underestimation. The modified Kim and Lee's correlation is proposed for better prediction of the thin-walled Zr-alloy cladding. The Cr-coating appears to mitigate the degradation of the mechanical integrity of the underlying Zr-alloy tube under severe oxidation and repeated thermal cycling, by virtue of superior oxidation resistance. It indicates that the coated cladding provides better integrity and performance in power transient conditions such as load-following operation. Post-test characterization showed the Cr-coating oxide layer thickness to be an order of magnitude lower than that of the uncoated cladding. The change in wettability was minimal in all test cases despite the trend of increasing roughness with the increasing initial temperature.