CHF model development based on optimal experiments of reactor vessel external cooling

Abstract

IVR-ERVC is one of the severe accident mitigation facilities at the nuclear power plant, and it terminate the accident by keeping the molten corium inside the reactor pressure vessel. This IVR-ERVC has a great advantage in that it can minimize the leakage of radioactive materials to the environment. In this thesis, in order to evaluate the actual CHF value during the ERVC, the optimal experiment was conducted to evaluate the CHF, and a CHF prediction model was developed based on this. For optimal evaluation, an experimental specimen was made of carbon steel, and the experiment was performed under the severe accident environment. Particularly, it was also experimentally confirmed that an oxide layer may be formed on the outer wall of a pressure vessel under the normal operation conditions of a nuclear power plant, and that the oxide layer had effect on the CHF. The cross-section of the oxide layer was observed with focused ion beam equipment to analyze the structure and material of the oxide layer, and during the boiling, the oxide layer producing and losing process were presented based on this result. Finally, the effect of the oxide layer on the CHF was described. A CHF model based on the liquid film dry-out mechanism was developed. The CHF model was constructed based on physical phenomena and principles, and has a total of two correction constants. Optimized correction constants were presented depending on the heat transfer surface material, and this study presented values for carbon steel, stainless steel, and copper. Finally, based on the CHF results and the model, the thermal margin of ERVC in a LOCA condition was evaluated. Compared to previous studies, it was confirmed that a higher thermal margin was evaluated. In particular, the thermal margin was high under pressurized conditions, because the decay heat was constant while the CHF was increased by pressurization. Finally, this paper suggests that oxidized carbon steel should be used as a specimen when evaluating the CHF of ERVC.