Nano/microscale roghness control of accident-tolerant Cr- and CrAl-coated surfaces to enhance critical heat flux

Abstract

This study aimed to improve the thermal safety of accident-tolerant fuel (ATF) cladding by enhancing the pool boiling critical heat flux (CHF) through control of the nano/microscale roughness of ATF-functional Cr- and CrAl-coated surfaces. To diversify the surface structures at the nano/microscale, we ground the surface to achieve the typical roughness range of a nuclear fuel cladding and then deposited the ATF candidate materials of Cr and CrAl on the ground surface. 17 test surfaces were fabricated by grouping three types of surface structures: microstructure, nanostructure, and nano/microstructure. The structural feature was parametrically categorized based on the arithmetic roughness height Ra at microscale and the surface area ratio rn at nanoscale. While Ra was observed to influence both the nucleate boiling efficiency and CHF, rn was not capable of enhancing them both exclusively. Nonetheless, a synergistic effect of Ra and rn on the CHF was observed. The CHF values of microstructure, nanostructure, and nano/microstructure were enhanced by 19%, 9%, and 79%, respectively. A capillary wicking experiment showed that an increase in surface roughness leads to a decrease in the dry area; therefore, a potential increase in the liquid area fraction of the boiling surface contributes to additional evaporation.