A Study on Aperiodic Instability for a Once-Through Steam Generator with a Tube-in-tube Modular Feedwater Line

A once-through steam generator (OTSG) with a modular feedwater line (MFL) is located inside a reactor vessel. A sufficient heating of the feedwater in the MFL can cause an aperiodic instability especially at low flow and low pressure conditions. Aperiodic instability is an instability related to the change of a flow direction in individual steam generating U-shaped channels operating at given pressure difference. The nature of an aperiodic instability is close to a Ledinegg instability related to the presence of multiple flows at the full hydraulic curve of a U-shaped channel. In this paper, the conditions for a reverse flow at the OTSG with a tube-in-tube MFL are studied. From the results of the studies, it is revealed that the change of a flow direction in the MFL is due to the boiling of the feedwater in the downcomer branch of the U-shaped pipe and that multiple flows start in the area of the extreams corresponding to the minimum pressure difference of the hydraulic curves.