 INES 3 |
UNIT SCRAM CAUSED BY GRID DISTURBANCES DUE TO TORNADO |
The unit was operating at 370 MWe power. As a result of tornado the 330 kV, 154 kV and 110 kV transmission lines in the "Kolenergo" power system were damaged. At 02:27 and 02:32 TG-1 and TG-2 were tripped by system emergency protection as a result of strong frequency oscillations in the system. At 02:32 AZ1 protection (scram) actuated on core pressure difference increase above setpoint because of RCPs increased flowrate as a result of grid frequency rise up to 52.5 Hz. The normal DGs 1,2 and the standby diesel (SD) started for coast down as designed. Since the auxiliary power buses were under voltage, DGs 1,2 after 1.5 mins. of operation tripped according to design algorithm. Personnel transferred DGs 1,2 and SD to manual control. Unit auxiliary power was supplied from service transformers. Main steam header pressure maintaining and residual heat removal were performed using BRU-K turbine bypass check valves (venting steam to condensers) and BRU-SN steam dump valves (controlled bleed to station service collector). SG levels were nominal. Preparations were started to bring the reactor to the minimum controlled power level. At 03:33 AZ1 signal was formed again. DGs 1,2 and SD failed to start due to their unavailability for the startup (they were on manual control). Significant voltage oscillatios at 6 kV auxiliary buses subsequently followed. By 04:00 the primary cause of AZ1 signal was eliminated. DGs 1,2 and SD were brought to automatic control.
At 04:11 at voltage decrease AZ1 signal was formed on RPS feeders deenergization. DGs 1,2 and SD started. DGs 1,2 failed to get connected to essential buses since the latter were under voltage and after 1.5 mins. of operating tripped. Since the AZ signal for DGs start persisted, DGs 1,2 periodic restarts followed which led to exhausting the air inventory in the DG startup air bottles. Core cooling was maintained through natural circulation. The SD remained in service at coast down. At 04:26 SD was tripped by the lo - lo oil pressure protection. Subsequent attempts to supply power to auxiliary buses from the DGs were unsuccessful due to absence of air in the startup bottles.
At 06:15 as a result of system accident progression unit total loss of power occurred. DGs 1,2 and SD failed to start due to insufficient air pressure in the DG startup air bottles. The vital buses were connected via the reversible motor generators (RMG) to the batteries. Core cooldown was performed via SGs in the natural circulation mode with steam dumping to BRU-SN lay-out. At 06:45 unit power supply from the off-site source was restored. Throughout the transient core status was being monitored with the help of design-provided (normal) instrumentation.
Basis for rating:
1. No off-site impacts:
2. No on-site impacts.
3. This event is rated under the "Degradation of defence in-depth" criterion.
Real initiators:
Loss of off-site AC power sources including voltage and frequency fluctuations (initiator frequency - high/expected);
Reactor scram (initiator frequency - high/expected)
Safety function availability: adequate.
According to section III-2.4.2 of the INES User's Manual and Table II (C1) level 2/3 is assigned.
Violation of unit safe operation limits and conditions occurred: efforts to bring the reactor to the minimum controlled power level were initiated with unavailable DGs 1,2 and SD. DGs 1, 2 and SD unavailability to perform safety function at unit total loss of power.
Taking into account additional factors (safety culture deficiencies) level 3 has been chosen.
Event date: Tue, 02-02-1993
