Summary Report for Conduct of Kozloduy NPP Stress Tests

“Kozloduy NPP” PLC
SUMMARY REPORT
for Conduct of Kozloduy NPP
Stress Tests
· Multiple sources of water can be used for the steam generators makeup depending on
the source of power;
· All available sources of power supply can be used.
At failure to all power sources (primary, backup, emergency DGs, additional DGs, and
mobile DG) before the start of core damage the operator has not less than 16 hours.
The worst case of loss of the ultimate heat sink is disconnecting the power units from the
spray pools. In this case under the above conditions, the shortest time to the core damage is over 28
hours.
Reactors are most vulnerable in cold depressurized primary circuit as:
· The ultimate heat sink is atmospheric air, the connection to it is ensured only through
the spray pools;
· There are no systems powered from mobile DG to provide heat removal from the
primary circuit or supplement it.
The minimum time to fuel damage is up to 7 hours and 30 minutes at failure to bothe the
emergency and the additional DGs.
At loss of the ultimate heat sink, except in the event of disconnection of the units from the
spray pools, time to core damage is extended with the time for the depletion of water in the
emergency inventory of the cold channel (197 hours or 8.2 days). Minimum time in cold state and
with reactor depressurized amounts to 19 hours and 45 minutes.
The heat from the SFP of Units 5 and 6 is only removed to the spray pools. No systems
powered from additional or mobile DG ensuring heat removal or filling of the pools. The loss of
off-site power supply and the emergency DGs is equivalent to disconnection of the units from the
spray pools. Minimum time reserves at the beginning of fuel uncovering are:
· Over 5 days in storage of spent fuel only;
· Less than 17 hours when moving all the fuel from the reactor to the SFP.
The critical element for the robustness of Units 5 and 6 and their pools are spray pools and
their connections with the diesel generator stations, as:
· Spray pools are the only ultimate heat sink of the core with the cold and
depressurized primary circuit and fuel in the SFP;
· Through them heat is removed from the emergency diesel generators and from the
safety systems pump motors.
Therefore, the priority measures to improve the robustness of the units to extreme external
events are:
· Technical solutions to prevent loss of water from spray pools;
· Ensuring alternative means of supplementing spray pools from independent sources;
160/202