On 09.09.00 Beloyarsk NPP Unit 3 was in normal operation at 99.8% nominal power (595 MWe). Safety systems were in hot standby.
At 12:03 a disturbance in power system operation occurred resulting in voltage and frequency oscillations in the range from 45 to 53 Hz (the normal frequency is 50 Hz).
At 12:11 as a result of progression of the above operational event a voltage decrease from 6 kV to 3 kV occurred in the unit auxiliary power buses. Rotation speed of the primary circuit's main coolant pumps (MCPs) dropped. Reactor fast-acting emergency protection actuated on low-low MCP rotation speed followed by turbine generators trip.
At 12:24 the tripping of the 110/220 kV Open Switchgear (OS) and deenergization of the 6 kV auxiliary buses occurred. According to the 'loss of power' design algorithm all the diesel generators (DGs) started thus feeding power to the appropriate 6 kV essential buses. The unit's essential loads were fed from the storage batteries through the reversible motor-generators (RMGs). The 'loss of power' algorithm enabled the actuation of safety systems' features and components that ensure Unit 3 cooldown.
At 12:31 voltage supply from the Urals power system to Beloyarsk NPP OS was resumed, power to the 6 kV and 0.4 kV normal buses was restored, and later the essential buses were returned to their normal configuration with the DGs being stopped. Voltage oscillations in the 110/220 kV buses of Beloyarsk NPP were observed during 10-15 minutes. All the change-overs in the auxiliary power system of Beloyarsk NPP were completed before 4 p.m.
At 05:10 a.m. on 12.09.00 Beloyarsk NPP Unit 3 was returned to service.
Breach of unit safe operation limits and conditions, interruptions in unit power supply did not occur. This event had no radiological consequences.
Real initiator (loss of off-site AC power, including consideration of voltage and frequency disturbances) - expected
According to the International Nuclear Event Scale (INES) this event was rated as Level 0 under defence-in-depth.
Event date: Sat, 09-09-2000