Beloyarsk NPP Unit 3 scram due to events in the operation of the Urals power system

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.
Rating Justification:
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.

Location: 
BELOYARSKY-3(BN-600)
Event date: 
Sat, 2000-09-09
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