Hot-leg loop-seal clearance behavior and effects on two-phase natural circulation experiments during SB-LOCA at VVER-440 PWR model

During a Small Break Loss of Coolant Accident (SB-LOCA) in a PWR Nuclear Power Plant (NPP) type different forms of Natural Circulation (NC) govern the transient process in the primary circuit. Perhaps the most important of them is the loop seal behavior and its effect on the core cooling. It is known that the above effect is more significant in a Russian type PWR NPP (VVER-440) where both the hot and cold legs are designed and built with U-tube sections producing double loop seals in the system. In this paper the results of the investigation on the hotleg loop-seal effects are presented. The experimental investigation has been carried out in the PMK-NVH facility which is a VVER-440 primary model scaled 1:2070. The results obtained by stepwise draining of coolant inventory from the primary circuit are compared with calculation. The calculations were preformed by applying the well-known SMABRE code for the problem. A satisfactory agreement was found between measured and calculated data, and it can be concluded that the loop seal clearances play an important role and it should be investigated in a much wider range of parameters