Critical heat flux and heat transfer behaviors on aged RPV outer wall surface in flow boiling under IVR conditions

Critical Heat Flux (CHF) plays an important role in the nuclear power plant safety strategy both during normal operation and under accident conditions. During normal operation, the outside wall is exposed to high temperature environment. The surface is oxidized by air, which is so-called "aged surface". To specify the characteristics of aged surface and the effect on CHF, the specimen aging experiments and flow boiling experiments under External Reactor Vessel Cooling (ERVC) were conducted. The results of specimen aging experiments show that surface characteristics get steady after 500 h aging and the oxide layer is the Fe-Fe2O3-Fe3O4 mixture. After aging, the surface gets rougher and has better hydrophily. In the flow boiling experiments, characteristics of heat transfer and CHF are studied. Heat transfer is enhanced by extended surface area and increased nucleation sites at lower angle. The influence of capillary suction is not obvious and the enhancement of CHF is limited. As the inclination angle increases, the departure of bubbles is more frequent and capillary suction is dominant to assist liquid supply into the liquid sublayer and delay the boiling process. At higher angle, CHF is more obviously enhanced but heat transfer gets worse. Overall, CHF of RPV outer wall is increased after surface aging, and it is beneficial to improving the safety margin of IVR mitigation measures.