Oxidation effect on pool boiling critical heat flux enhancement of Cr-coated surface for accident-tolerant fuel cladding application

Abstract

The oxidation of an accident-tolerant fuel (ATF) cladding alters the surface roughness, which, in turn, affects the capillary wicking characteristics and critical heat flux (CHF) during the boiling process. To evaluate the effect of oxidation on the boiling performance of ATF coating, this study fabricated Cr-coated surfaces by varying the substrate temperature, resulting in various sizes of deposited particulate nanostructures by using a direct-current magnetron sputtering technique. To consider the oxidation effect, sputtered test specimens were oxidized at 400 °C dry air for 20 days, forming a Cr2O3 oxide layer with a thickness of approximately 0.1 μm. Pool boiling CHF experiments were conducted in deionized water under atmospheric pressure. The resultant CHF on the fresh and oxidized Cr-coated surfaces was enhanced by up to 36% and 48%, respectively. A higher CHF enhancement was achieved with a lower substrate temperature during sputtering, which resulted in a higher nanoscale surface roughness. The spreading velocities on each specimen were measured through the characterization experiments of liquid droplet spreading, which can be interpreted as the capillary wicking momentum. The pool boiling CHF was predicted with spreading velocity and roughness parameters. The predicted CHF showed a good agreement with the measured CHF. This study suggests that the oxidation of Cr-coated ATF specimens has a positive effect on the enhanced CHF, which can be confirmed through the altered surface roughness and capillary wicking momentum.