Nucleate boiling heat transfer and CHF enhancement with porous surface coatings on the RPV outer wall

The In-Vessel Retention (IVR) strategy has been widely applied in the existing and newly design Light Water Reactor (LWR). To give a sufficient safety margin for the further design of the large-scale reactors, the enhancement of the Critical Heat Flux (CHF) should be further studied. Porous coating technology is known as an effective CHF enhancement method. In this paper, the REPEC-III facility is modified to adapt the CHF enhancement conditions with higher heating power and wall temperature. The REPEC-III facility has 1:1 height ratio with the prototypic External Reactor Vessel Cooling (ERVC) environment. The flow channel is designed as a curved rectangular channel and the area ratio is 1:100 to the prototypic ERVC flow channel. The applicability of the porous coatings under the IVR-ERVC conditions are evaluated in this study. The cold spray technology is applied to prepare the porous coatings and prevent surface damage. The porous layer is composed of the dense basal layer and porous layer. The comparisons of the boiling phenomena are analyzed. On the porous surface, the larger amplitude and lower frequency of the pressure difference oscillation are observed and mean the more vapor slugs and the longer vapor period. During the experiments, the temperatures of the heating block with the porous surface are overall higher than the temperatures with the fresh surface. The heat transfer capability is worsened by the thermal resistance and improved liquid replenishment, which leads to a higher wall superheat under the same heat flux. The CHF is enhanced by the porous coatings and the enhancement effect is related to the orientations. The maximum percentage of the CHF enhancement is 43% at 87 degrees. The intense turbulent induced by the more vapor slugs and the capillary wicking in the porous layer are beneficial to the liquid replenishment and enhance the CHF. Through the full-height experimental analysis, the porous coating technology is an effective method to improve the safety margin of the IVR strategy. The application of the porous coatings on the Reactor Pressure Vessel (RPV) outer wall are still needed to be further studied.