Total loss of on- and off-site power to the station - a possibility for accident management at Kozloduy NPP units 5 and 6

The event 'Total loss of electrical power supply' is defined as a total and ultimate loss of normal and reserve AC power supply and total failure of the emergency power supply system. As a result, the possibilities for heat removal from the primary to the secondary circuit are lost, leading to a severe accident. The possibility for increasing of the grace time before running into an irreversible core state by utilizing the water reserves in the feed water system and in the hydro-accumulators are analysed, based on proposed variants of an accident management procedure involving secondary bleed and passive feed of the steam generators. The present study shows that for the Kozloduy units 5 and 6 WWER-1000 plants properly timed (in the range of 2 to 3 hours after loss of power) secondary bleed will provide grace times on the order of 10.5 - 12 hours for restoration of electrical power supply. The utilization of the water reserves and the calculated grace time is the higher, the later the secondary bleed is initiated. The initiation time is limited by the discharge time of the batteries. Up to that grace time of 11 - 13 hours, neither the safety variables of the core have exceeded any DBA acceptance criteria nor has the integrity of the containment been jeopardized. (authors)