Referring to single-phase natural circulation in steam generators with inverted U-tubes, various working conditions can be postulated in response to perturbations in the operating parameters. Clearly, such behavior is related to potential flow instabilities in parallel channels connected to a common manifold. A physical and analytical interpretation is given of the mechanism governing the onset and eventual divergence of flow surges observed in the LOBI-MOD2 steam generators during the evolution of thermal-hydraulic transients relevant to decay heat removal in pressurized water reactors. Secondary-side heat sink degradation is identified as the primary mechanism leading to the observed oscillations. A generalized analytical steady-state model is used to identify potential unstable operating conditions in the steam generators; a numerical method is then used to describe the transient condition.