Numerical study on thermal hydraulic phenomena in a hemispherical gap under core meltdown accident

Numerical analytical method for thermal hydraulic phenomena in a hemispherical narrow gap flow passage was developed to evaluate heat removal capability with gap formation between the reactor pressure vessel (RPV) and the molten core debris. The problem of gap cooling mechanism was modeled as vapor-liquid two-phase flow in the gap with two-dimensional spherical coordinates system. The analytical model is based on a modified drift flux model for multi-dimensional two-phase flow analyses. Numerical results showed that liquid-phase intrusion into the gap in the counter direction of gas-phase upward flow keeps down a rise of void fraction as gap cooling effect. The applied heat flux, which leads to expansion of high void fraction region with the state of partial counter-current flow limitation (CCFL), was regarded as predictive value for partial critical heat flux (CHF). The predicted partial CHF in the analyses were compared with other measurements and correlations obtained from some CHF experimental studies.