The code development and thermo-hydrodynamic analysis of the reflood during severe accident in PWR

The reflooding, i.e. injecting water to the uncovered degraded core, is the most important accident management measure to terminate a severe accident and to stop the core from being melt in LWR. The reflooding of LWR core during severe accident may lead to the core cooling and cessation or to the temperature escalation and further development of the accident. That will depend on several important parameters, characterizing the core state and the way of the reflooding. Appropriate understanding of the complex core reflooding phenomena is necessary for the prediction of the system evolution. Based on the reflood flow and heat transfer characteristics, a thermo-hydraulic analysis model for the reflood phase was developed and presented in this paper. Based on this model, a code module was developed for the thermo-hydraulic analysis of reflood. The calculation results for the different conditions are compared with the FLECHT data and the QUENCH data, respectively. The results showed that the calculation results and the experiment data are in substantial agreement. Then the influences of the system pressure, the subcooling of coolant and the wall temperature on reflood characteristics were studied, too. The quench velocity falls when the wall temperature went up. The quench velocity increases when the system pressure increases. The quench velocity drops with the increasing of water temperature. The paper also provides a theoretical basis for safety analysis of fuel element cladding during reflood phase.