Enhancement of Boiling Heat Transfer for Safety of Nuclear Systems using Surface Modification Techniques

Abstract

Since the nuclear system is of multiple scale and environments at each system are diverse, research directions for enhanced safety can be determined based on an application scale with different requirements. In order to improve the safety of nuclear system, we introduced two novel surface modification techniques, which are sputtering and layer-by-layer assembly technologies. Based on the system scale, each technique was utilized to fabricate different functional layers for the in-core and system scale applications. Extensive pool boiling critical heat flux (CHF) experiments were conducted to confirm the effects of the functional layer on the thermal safety margin of the fabricated heaters. For the component or in-core scale application, the FeCrAl layer on the metal substrate by DC sputtering was fabricated, and the effects of the FeCrAl-layer on the thermal safety margin were explored. We found that the substrate temperature and sputtering time affected the surface characteristics of the FeCrAl-layered heaters. A substantial increase in surface roughness was realized in the substrate temperature range from 150 to 600 oC during the DC sputtering process. Enhancement in CHF was achieved for the first time with all types of FeCrAl-layered heaters. The highest CHF enhancement (~42%) was observed for the layer of 1hr sputtering at a substrate temperature of 150 oC. The enhanced CHF was attributed to an increase in the roughness factor at the micrometer scale. This, in turn, could have a desirable effect on shortening the rewetting interval with the FeCrAl-layered heaters. For the system scale application, the LbL-assembled PEI/MWNCT multilayer was developed on the metal substrate, and the effects of the LbL assembly process on the thermal safety margin were discussed. It was confirmed that the fabricated layers exhibited the porous structures and hydrophilic nature as the bilayers increased after the LbL deposition. CHF was significantly improved up to ~94% for the sample with 40 bilayers. The developed porous layer with the increased wettability contributed to the substantial improvements. As similar to the cases of FeCrAl-layered heaters, reduced rewetting times were observed for the LbL-assembled heaters.