Nano/microscale roughness 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 R-a at microscale and the surface area ratio r(n) at nanoscale. While R-a was observed to influence both the nucleate boiling efficiency and CHF, r(n) was not capable of enhancing them both exclusively. Nonetheless, a synergistic effect of R-a and r(n) 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.