Reactor scram of Unit 6

Nuclear Facility: Unit 6, Kozloduy NPP
Date: 30.10.2021 Time: 07:42
Event title: Reactor scram of Unit 6
Event description:
On 30.10.2021 Unit 6 operated at rated power. During the implementation of activities to reaching the minimum controllable reactor power level after annual outage and reloading, the automatic reactor scram has occurred. The reason for the scram was false signal for reducing “inlet-outlet” pressure difference of one of four main circulation pumps. Activities to identify the reason for the false signal have been performed. At 09:10 a leakage was found from a valve block of one of the three pressure sensors taking part in the reactor’s safety protection. As a result of this leakage the other two sensors were set on and as a result the reactor scram was triggered. At 02:10 after restoring the functionality of the valve block, the implementation of activities to reaching the minimum controllable reactor power level was continued. There has been no violation of the operational limits and conditions and respectively there have not been any changes in the radiological conditions.  
INES rating of the event: Below Scale/Level 0 No safety significance
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Decrease to 45% of the the rated power of Unit 5

 
Nuclear Facility: Unit 5, Kozloduy NPP
Date: 11.09.2021 Time: 04:55
Event title: Decrease to 45% of the the rated power of Unit 5
Event description:
On 11.09.2021 Unit 5 operated at rated power. At 04:55 one of the two turbine driven feedwater pumps (TDFWP-2) switched off due to mechanical malfunction of the regulating valve of the pump. The reactor power was reduced up to 45 % in accordance with the design protections algorithms. Activities have been undertaken to establish and eliminate the reasons for pump’s switch off, as a result of which the valve functionality was restored and at 08:25 TDFWP was switched on after the successful testing. Power increase was started in accordance with the requirements of the Technical Specifications for Safe Operation of Unit 5.   There has been no violation of the operational limits and conditions and respectively there have not been any changes in the radiological conditions.  
INES rating of the event: Below Scale/Level 0 No safety significance
   
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Decrease to 50% of the rated power of Unit 6

Nuclear Facility: Unit 6, Kozloduy NPP
Date: 25.05.2021 Time: 11:30
Event title: Decrease to 50% of the rated power of Unit 6
Event description:
On 25.05.2021 Unit 6 operated at rated power. When routinely switching the oil pump of the control system of turbine driven feedwater pump No.2 (TDFWP*-2), the control valves of the drive turbine opened at 100 %. Despite the actions of the operators, the water flow increased and when it reached 4500 m3/h, TCP-2 tripped on “high water flow” protection signal. The technological protections unloaded the unit up to 48.9% according to the design algorithms. Activities have been undertaken to establish the reasons for switching off the TCP-2, as a result of which elements of the pump control system have been replaced. After a successful inspection of the control system, at 20:30 TDFWP-2 was switched on and the power increase was started in accordance with the requirements of the Technical Specifications for Safe Operation of Unit 6. There has been no violation of the operational limits and conditions and respectively there have not been any changes in the radiological conditions. * TDFWP supplies water to the steam generators and is part of the conventional (non-nuclear) part of the plant.
INES rating of the event: Below Scale/Level 0 No safety significance
   

Reactor scram of Unit 5

Nuclear Facility: Unit 5, Kozloduy NPP
Date: 22.01.2021 Time: 01:16
Event title: Reactor scram of Unit 5
Event description:
On 22.01.2021 Unit 5 operated at rated power. During the implementation of activities to secure equipment for planned maintenance (maintenance of power supply switchgear of the train one of a safety system) loss of power supply to the sensors from that train has occurred. The loss of power supply to the sensors has led to activation of technological protections and as a result four main circulation pumps have switched off and automatic reactor scram has occurred. At 01:16 Unit 5 has been disconnected from the electrical grid. The technological protection algorithms have operated without any deviations. Activities to restore the power supply of the sensors have been performed. The unit equipment condition has been inspected. In the period from 03:32 to 03:38 the four main circulation pumps consecutively have been switched on and a process for reaching minimum controlled power level of the reactor has been initiated. There has been no violation of the operational limits and conditions and respectively there have not been any changes in the radiological conditions.  
INES rating of the event: Below Scale/Level 0 No safety significance