A method to evaluate the effect of geometric structure and thermal hydraulic parameters on the passive circulation flow rates in the annular flow channel between the insulation structure and the lower head of the reactor pressure vessel and critical heat flux on the equator of lower head outer surface under core degradation accidents has been proposed in this work. Based on the one-dimensional steady-state mass, momentum and energy conservation equation, a numerical calculation model has been established. The drift flux model is adopted in the numerical calculation model since velocity difference between the liquid and vapor phase in the induced upward two-phase flow could be significant. The calculation results are compared with the available experiment data collected from the literature under the same condition for the validation of the numerical calculation model proposed in this work. The effects of the heating power, loss coefficient, flooding levels, gap clearance and inlet area on the passive circulation flow are studied in this study. Furthermore, combined with the CHF correlation proposed for the determination of the CHF on the downward heating surface, the variation of the CHF at the equator of lower head outer surface with those geometric structure and thermal hydraulic parameters are analyzed utilizing the numerical calculation model for the better understanding of the limitation of the in-vessel retention through external reactor vessel cooling.

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