CHF experiments and prediction model under subcooling flow boiling condition in passive IVR-ERVC system

In-Vessel Retention (IVR) with External Reactor Vessel Cooling (ERVC) is proved to be the effective strategy for the protection of the Reactor Pressure Vessel (RPV) integrity during the severe accidents. In the prototype reactor, the lower head is cooled by natural circulation of the subcooled coolant in the In-Containment Refueling Water Storage Tank (IRWST). In such flow boiling process, the heat transfer coefficient (HTC) and CHF are affected by complex issues. The safety margin of the IVR strategy is the difference between the Critical Heat Flux (CHF) on external vessel and the imposed heat flux conducted by the molten pool. The subcooling flow boiling is beneficial for the removal of the heat on the vessel wall to improve the safety margin. In this paper, the flow boiling experiments of natural circulation in downward-facing surface were conducted to study the effect of subcooling. The CHF results with different subcooling degrees was obtained. The results show that as the temperature of water decreases from 100 degrees C to 75 degrees C, CHF is enhanced 24.5%-42.6% for different inclination angles. The CHF enhancement by subcooling effect obviously decreases with the increase of horizontal inclination angle of RPV lower head. A CHF prediction model considering the geometric effect, inclination, pressure and subcooling is derived. The prediction model was verified with the experimental data obtained by the rectangular channel experiments under the IVR conditions, which covered the following operational ranges: local pressure 1-1.8 bar, mass flux 100-1800kg/(m2 & sdot;s), subcooling 0-40K, gap size 30-156 mm and inclination 0-90 degrees. The developed model showed good prediction performance within +/- 15% error.