Research of flow instability in OTSG under low load conditions

Integrated reactors generally use once-through steam generators (OTSGs). There are a lot of parallel channels in OTSG interior, and heat transfer tube secondary side sees severe boiling and phase transition phenomenon. At low load conditions, the feed water flow is small. The single-phase region in OTSG heat transfer tube secondary side is relatively short, and the effect of throttling element is not obvious at low flow rate conditions. Thus a slight flow disturbance is likely to cause various types of two-phase flow instability in OTSG secondary side. In this paper, a comprehensive study of two-phase flow instability of OTSG was performed using RELAP5 code. Flow instability boundaries of OTSG are found by gradually reducing the load of secondary loop, and the influence of system parameters on instability boundary is studied. Reducing the primary coolant average temperature, increasing the steam pressure, reducing the primary coolant flow rate, and using OTSG grouping run strategy can improve the stability of OTSG.