Numerical simulation of micro-crack leakage on steam generator heat transfer tube

Highlights: • Several kinds of flashing model are compared for flashing in micro-crack channel. • A superior guideline for flashing model chosen is developed for micro-crack. • Leakage prediction accuracy is improved for micro-crack compared with LBB code. • Critical two phase flow phenomenon is obtained for micro-crack channel. • A comprehensive CFD results is obtained for leaking coolant migration in SG. The pressure relief rate and loss rate of coolant, which affect largely safety of fission reactor systems, are determined by the flashing process inside cracked passages. Information about flashing phenomenon is of great significance for the leakage online monitoring system of SG (Steam Generator), which ensures the normal operation of SG and safety of the reactor system during early stage of tube rupture accidents. In this research, flashing flow inside a thin micro-crack in heat transfer tube of SG has been studied using ANSYS-FLUENT. The classic cavitation model and evaporation–condensation model, in combination with both the mixture two-phase flow model and Eulerian two-fluid model, are adopted to simulate flashing phenomenon. The real geometry and operation conditions of AP1000 nuclear system are adopted to reflect the reality leakage process in SG. Two types of micro-cracks including axial crack and circumferential crack, which both could happen in the reality operations, are considered. The CFD (Computational Fluid Dynamic) results gained from this research have been compared with test data, and good agreement is demonstrated. The results show that the evaporation–condensation model under Eulerian two-fluid model framework behaves superior to the classic cavitation models in simulating the flashing phenomenon after evaluating accuracy of leakage rate and rationality of thermal hydraulic parameters distribution. In addition, two-phase choking flow phenomenon is obtained by decreasing backpressure of cracked tubes. The simulation in this research reveals the flashing process in short and micro-crack and the results could be good reference for leakage prediction of SG under LBB (Leak Before Break) operations to improve operation performance of SG and safety of the whole nuclear power system.