The Research On Reflood Models Of The Film Boiling Heat Transfer Based On RELAP5

The reflooding phase of loss of cooling accident(LOCA) refers to complex two-fluid and heat transfer, so it is always the importance of studying for international research institutions. The film boiling heat transfer is the major form of the post-CHF heat transfer of the reflooding phase, so whether the calculation of the heat transfer is reasonable or not will have effect on predicting the peak cladding temperature(PCT). Research shows that the current RELAP5 code underestimates the peak cladding temperature when simulates the reflooding phase and unreasonably predicts the quench time, which is related to the film boiling heat transfer model of reflooding. Thus it is necessary to study the film boiling heat transfer model of reflooding based on the RELAP5 code.In order to develop the film boiling heat transfer model of reflooding, first of all, it should find the shortage of the film boiling heat transfer model of RELAP5 code, and modifiy the wall-to-vapor convective heat transfer model and the wall-to-liquid heat transfer model respectively. When modifying the wall-to-vapor convective heat transfer model, we should consider the rod bundle configuration and the vapor flow state effects. Firstly, the wall-to-vapor convective heat transfer regime is divided in three sub-regimes based on the vapor flow state: the forced laminar flow regime, the forced turbulent flow regime and the forced laminar flow translate to turbulent flow regime. Then, the wall-to-vapor convective heat transfer model of sub-regimes is established based on the rod bundle configuration. When modifying the wall-to-liquid heat transfer model, we should consider the flow pattern effects. The film boiling heat transfer regime is divided in three sub-regimes based on the flow pattern: these are the inverted annular film boiling regime, the dispersed flow film boiling regime and the middle transition regime. With combining the characteristics of each sub-regime, the appropriate heat transfer models are applied. Finally, by joining the wall-to-vapor convective heat transfer model and wall-to-liquid heat transfer model together, the film boiling heat transfer models are integrally modified.By using the data of FLECHT SEASET, RBHT and FEBA experiments to verify the rationality of the new model. Through both single-effect validation and integral-effect validationand comparing the calculation results of the peak cladding temperature and the quench time given by the modified code and the corresponding results from the original code, it can be concluded that the modified reflooding model is reasonable.The model of AP1000 nuclear power plant based on RELAP5/MOD3.2 has been set up to analyze the Double Ended Cold Leg Guillotine Break(DECLGB) Large Break Loss-of-Coolant Accident(LB LOCA). The Peak Cladding Temperature during the accident is below the acceptance limit value of 1204℃ that meets the safety criterion. Results show that the Passive Core Cooling System(PXS) of AP1000 can provide enough cooling and ensure the residual heat remove.