The 'Boil-Over in Milk-Pot' symptom at opened VVER-440 reactor vessel

In shutdown conditions two of the six loops are in operation to remove the core fission product decay heat at reduced primary coolant level - down to 1.4 m above the elevation of hot leg axis - in single-phase natural circulation heat transfer mode in VVER-440/V-213 type NPPs. The present paper gives an overview on results of two plant calculations in shutdown conditions performed by RELAP5/MOD3.3 code version where the inadvertent isolation of the operating SGs by closing of main loop isolation valves in both loops is the initiating event. The presentation comprises the 'Boil-Over in Milk-Pot' symptom at opened VVER-440 reactor vessel before refuelling but after removing the vessel head when about 30 t of primary coolant get lost during half a minute by positive feedback of decreasing hydrostatic pressure. (author)