Fukushima accidents showed the importance of safety systems using passive concept against severe accidents to reinforce a safety of nuclear power plant. The core catcher has been developed to stabilize the molten corium while avoiding molten core concrete interaction in the containment during the severe accident. Phenomena in the core catcher system are characterized by multi-dimensional flow behaviors with weak driving force by two-phase flow. Therefore, a multi-dimensional analysis is required for investigating this natural circulation.
Objectives of this paper is a validation of the CUPID code for the analysis of two-phase natural circulation in the core catcher. The CUPID code which has an advantage in transient two-phase flow seems to be a promising tool for the analysis of core catcher system. The CUPID code adopts a two-fluid, three-field model for two-phase flow. The physical modeling and the numerical solution method of the CUPID code put an emphasis on versatile and robust simulations of complicated two-phase flows.
For the validation of the CUPID, CE-PECS(Cooling Experiment-Passive Ex-vessel corium retaining and Cooling System) experiment which was carried out for assesing the cooling performance of the core catcher was selected. Before the validation, some improved models for bubble departure diameter, active nucleation site density and bubble size was implemeted into the CUPID code for the evaluation of the flow behaviors in an inclined channel with downward faced heating surface which is main phenomena in the core catcher. These three models in the CUPID code were improved. And the model evaluation test showed that bubble size is a dominant factor in the two-phase natural circulation.
For the quantitative evaluation of Yun`s bubble size model which is adopted in the present study, flow boiling test performed by Korea Maritime University was calculated. The result showed that Yun`s model predicted more resonable bubble size than Yoneda model which is default model in the CUPID code.
Finally, CE-PECS experiments were calculated by the default and the improved model in the CUPID code. Comparing with the experimental data, the loop flow rate was well predicted by the CUPID code when Yun`s bubble size model is adopted.