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

Flashing is frequently encountered in nuclear power systems for example as leakage occurring on the steam generator (SG) heat transfer tubes. Pressurized primary coolant flows rapidly through the crack and flashes into vapor. The pressure relief rate and loss rate of coolant, which affects largely the safety of fission reactors, are determined by the flashing phase change process. Information about the flashing phenomenon is of significance for the leakage online monitoring system, which ensures the normal operation of steam generator (SG) and safety of the reactor when tube rupture accidents occur. In this research, steady-state and transient 3D flashing flow inside a short micro-crack channel in the heat transfer tube wall of SG have been studied using FLUENT. The cavitation model and evaporation-condensation model, in combination with both the mixture two-phase flow and the Eulerian two-fluid model, are adopted to simulate the flashing phenomenon. The real geometry and operating conditions of AP1000 nuclear system are adopted to reflect the reality leakage phenomenon in SG. Two types of micro-crack shape including axial crack and circumferential crack, which both can happen in the reality, are considered. The CFD results gained from five different models have been compared with experimental data, and good agreement is demonstrated.
The model comparison shows that the evaporation-condensation model behaves superior to the cavitation model in simulating the flashing phenomenon. Finally, the leakage rates are gained under different crack shapes, sub-cooling degrees and backpressures with the most accuracy scheme. In addition, two-phase choking flow phenomenon is simulated by changing backpressure of cracked tubes. The simulation results in this research could be good reference for leakage prediction of micro-crack in SG to improve the operation performance of SG and safety of the whole nuclear power system.