Thermal-hydraulic stability investigations of once through helically coiled steam generators for SMR applications

Abstract

Once-through steam generators with helically-coiled tubes (OTSG-HC) are susceptible to density wave instability phenomena. In order to investigate the stability performance of the CAREM-25 OTSG-HC, a new stability model is presented. Such a model being linear and strictly non-diffusive is able to represent arbitrary heat flux profiles, which is essential to study realistic cases. The OTSG-HC tubes are modeled by three different regions representing the liquid zone, the boiling zone, and the superheated vapor zone. Moreover, the coolant from the primary side is coupled with a model representing the phenomena present in CAREM-25. A convenient parameter space for stability representation is found and used to characterize the CAREM-25 OTSG-HC stability performance.In this work, special attention is paid to the calculation of the friction pressure losses in the coolant flowing through the helically coiled tubes. For this reason, a best estimate correlation is implemented and used in the model together with a sophisticated axial power profile model which allows representing realistic boundary conditions. As a result, it is confirmed the crucial importance of correctly representing the axial power profile transferred in the helically coiled tubes. In addition, it is found the commonly used uniform axial power profile leads to non-conservative stability predictions particularly at low power conditions. The latter is explained by the underestimation of the boiling and superheating lengths, which is proved to have a destabilizing effect.Stability maps are presented in terms of the transferred power and the primary coolant operational pressure, i.e. Qsg-Ppri plane. It is observed that when modeling realistic power profiles, the less stable operational condition of the OTSG-HC is found at very low powers, for all analyzed range of primary pressures. In addition, a weak stability dependence of the OTSG-HC with the pressure of the primary side is observed. Finally, the system is stabilized in the region of interest and its stability performance quantified.