A numerical method for thermal hydraulic phenomena in a hemispherical narrow gap flow passage was developed to evaluate a cooling capability with gap formation between the molten core and the reactor pressure vessel.The gap cooling mechanism was modeled as gas-liquid two-phase flow in the narrow gap with two-dimensional spherical coordinate system. The analytical model is based on a modified drift flux model for multi-dimensional two-phase flow analysis. Numerical results showed that liquid phase intrusion into the gap in the counter direction of gas phase upward flow kept down a rise of void fraction as gap cooling phenomena. Under the high heat flux condition,expansion of the high void fraction region due to the counter-current flow limitation was reproduced as a dryout phenomenon. Characteristics of gap cooling limitation predicted by the numerical analyses were verified by comparison with various experimental data and correlations of critical heat flux.

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