The System Mean Void Fraction (SMVF) model combined with the void propagation equation is developed to predict the transient thermal hydraulic behavior during core uncovery with or without ECCS flow. The developed model is based on an integral formulation approach, and solved for power and pressure transients without involving the quasi-steady assumption. Unlike the previous System Mean Void Fraction model with the assumption that the void fraction is time invariant, the present model does not have the assumption of time invariance and the time variation of void fraction in this model is derived by solving the void propagation equation. In order to obtain the mean void fraction it is assumed that the quality has a linear profile in the axial direction. For powr transient cases the results predicted by the present model are compared to the EPRI``s rod bundle experiment with a uniform power profile and non-uniform powr profile. Also, for pressure transient cases, the results are compared to Marviken data and GE experimental data. The comparisons show that the present model can describe fast transient better than any other previous models. The heat transfer model is implemented in the present model to calculate the local cladding temperatures under core uncovery. In comparison with the 336 rod bundle experiment performed by EPRI, it is noticed that there are discrepances between the predicted results and the data around the just above the interface where there are large cooling effects on cladding by a considerable amount of droplets.