The decreasing of NC driving forces, the small temperature difference across U-Tubes of steam generators, and the occurrence of the CounterCurrent Flow Limiting Phenomenon (CCFL) at the entrance of U-tubes are at the origin of wide system oscillations of core inlet flowrate, e. g. Ref. [10]. The phenomenon has been investigated in Refs. [5] and [11], based on a natural circulation experiment performed in Lobi facility, Ref. [12]. Evidence of the phenomenon has been found also in other facilities. At mass inventories of the primary system around 70% of the nominal value, the efficiency of the condensation heat transfer across U-Tubes causes the release of almost all core thermal power in the ascending side of U-Tubes. Liquid level builds up and is prevented to drain down by the steam-liquid mixture velocity at the tube entrance, i. e. the CCFL condition occurs. Therefore, liquid level rises in the U-Tubes till reaching the top. During this period, typical duration of the order of 10 s, flowrate at core inlet is close to zero and core boil-off occurs. Once the liquid level reaches the upper bend of U-Tubes, the siphon effect occurs and causes the emptying of the ascending side of U-Tubes and the re-establishment of core inlet flowrate. A new cycle starts. The phenomenon is made more complex by the interaction of the several thousands of U-Tubes that constitute a SG tube bundle. Different groups of tubes may stay at a different stage of the oscillation at the same time, also causing flow reversal in the tube bundle. Suitable core cooling still can be achieved in these conditions.