Real-time Computing Modeling Research Of A Small-break LOCA For PWR

SBLOCA is one of important subjects for PWR thermal-hydraulic safety analysis, and it is crucial to determine the mass discharge from the break for evaluating the accident level and designing the accident mitigating system. Different system safety analysis programs have their corresponding real-time simulation programs. At present, the choked flow models for real-time simulation are all derived under some specific assumptions, which can have a good agreement with experiment data within a certain range. But when simulating the blowdown process from the single-phase to the two-phase flow, the accuracy of the simulation results are defective which cannot reflect the practical blowdown process. Therefore, this thesis selected advanced mathematical models. A choked flow code for the Theatre program for the SimExec platform, which can accurately simulate the real-time mass flow in the small break loss of coolant accident, was developed based on their application conditions of mathematical models.Specific mathematical model can only apply to a specific situation. This thesis studied the blowdown process of SBLOCA. Based on the main features, the blowdown process was divided into different stages, which provided theoretical basis for the study of relevant mathematical models in different scenarios. Combining with the study of mathematical models which can be used in thermal hydraulic analysis at present, the more appropriate mathematical models combination for the real-time thermal-hydraulic simulation of SBLOCA was established, which made the foundation of the development of real-time program used for simulating the single-phase to the two-phase blowdown process.Through the study of low-quality blowdown process, the presence of metastable choking flow condition during the single-phase to two-phase transition was confrimed. Considering the complexity of the transition region, the thermal nonequilibrium effect in low-quality conditon was pointed out as the main characteric of the choking flow in the thesis.Henry-Fauske model can accurately simulate the choking flow process. For the two-phase thermodynamic equilibrium region with high quialiy, Moody model is better than the models based on Homogeneous Equilibrium assumptionsFinally, in order to evaluate the accuracy of developed SBLOCA thermal-hydraulic real-time simulation program, the real-time simulation of the imaginary SBLOCA of Qinshan Nuclear Power Plant was conducted by the developed program. In comparison with the results of the original Theatre program, the RELAP5 program and the experiment, the accuracy and applicability of mathematical models which used to develop this program was verified. The rationality of Henry-Fauske model adopted in low-quality region was also discussed in this thesis.