As part of a joint U.S.-Korean International Nuclear Engineering Research Initiative investigating methods to enhance external reactor vessel cooling (ERVC) under severe accident conditions, it was proposed that surface coating be used to facilitate the process of downward-facing boiling. Toward this end, metallic micro-porous layer coatings were selected and different compositions of the coating material were evaluated to obtain a mixture with desirable qualities. Durability and adhesion tests were also done to study the performance of each of the resulting coatings. Quenching of the candidate coatings were then conducted under downward facing boiling conditions using hemispherical test vessels to obtain the local boiling curves. Compared to the case of plain vessel surface under identical experimental conditions, considerable enhancement was observed in nucleate boiling heat transfer and the critical heat flux (CHF) for the case with surface coatings. Optical and SEM (Scanning Electron Microscope) photos showed that the candidate coating possessed the desired porous microstructure with interconnecting channels and pores, leading to appreciable increases in the boiling heat transfer and the local CHF limits.
An experimental study was performed to investigate the effect of surface coating on the critical heat flux for downward facing boiling on the outer surface of a hemispherical vessel. Steady-state boiling experiments were conducted in the SBLB (subscale boundary layer boiling) facility using test vessels with metallic microporous coatings to obtain the local boiling curves and the local CHF limits. Similar heat transfer performance was observed for microporous aluminum and microporous copper coatings. When compared to the corresponding data without coatings, the boiling curves for the coated vessels were found to shift upward and to the right. This meant that the CHF limit and minimum film boiling temperatures were located at higher wall superheats. In particular, the microporous coatings were found to enhance the local CHF values appreciably at all angular locations explored in the experiments. Results of the present study showed that the microporous aluminum coating was very durable. Even after many cycles of steady state boiling, the vessel coating remained rather intact, with no apparent changes in color or structure. Although similar heat transfer performance was observed for microporous copper coatings, the latter were found to be much less durable and tended to degrade after several cycles of boiling.
As part of joint U.S.–Korean International Nuclear Engineering Research Initiative (INERI) investigating methods to enhance external cooling of advanced reactor vessel under severe accident conditions, a scaling analysis has been performed to study the phenomena of external cooling of an advanced reactor vessel under severe accident conditions. Five key transfer processes have been considered and the characteristic time for each of these processes has been determined and compared with the residence time for external reactor vessel cooling (ERVC) in the flow channel. To complement the scaling analysis, an ERVC upward co-current two-phase flow model has been developed to predict the total mass flow rate induced in the annular channel by the process of downward facing boiling on the vessel outer surface. The model takes into account the wall heat flux level, the geometry of the vessel/insulation system, the local variation of the cross-sectional flow area, and the pressure drops through various segments of the channel. Based on the results of the ERVC flow calculations and the scaling analysis, criteria for experimental simulation have been established to assure that the ERVC phenomena simulated in laboratory-scale experiments would have the same effects as those anticipated for the full-scale reactor system.
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