Brunswick Unit 1 and 2 Electrical Bus Problem Causes Loss of Half of Emergency Cooling Systems AND a Plant SCRAM

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Brunswick events number 47687 and 47690

On 2/22/2012 at approximately 7 PM eastern time, the Brunswick site experienced a loss of balance of plant (BOP) Electrical Bus “Common C”. Several important loads that are fed by this bus include the Support Systems known as “Keep Fill Pumps” for:

  • Unit 1 ‘A’ loop of the Core Spray (CS) system
  • Unit 1 ‘B loop of the Residual Heat Removal (RHR) system
  • Unit 2 ‘A’ loop of the Residual Heat Removal
  • Unit 2 ‘B’ loop of the Core Spray system
Locations where RHR and Core Spray enter Reactor

The Brunswick FSAR describes the above functions of these systems:

6.3 EMERGENCY CORE COOLING SYSTEMS

6.3.1 DESIGN BASES AND SUMMARY DESCRIPTION

The Core Standby Cooling System (CSCS) or equivalently the Emergency Core Cooling System (ECCS), consists of the High-Pressure Coolant Injection (HPCI) System, Automatic Depressurization System (ADS), low-pressure coolant injection (LPCI) [an operating mode of the Residual Heat Removal (RHR) System], and Core Spray (CS) System.

The ECCS are provided to remove the residual and decay heat from the reactor core so that fuel cladding temperature is kept substantially below 2200°F and local oxidation of the cladding is kept substantially below 17 percent. The intent of the ECCS temperature and oxidation criterion is to prevent fuel cladding fragmentation. Under extreme conditions, highly oxidized Zircaloy is known to fracture on cooling. Based on experimental data, cladding fragmentation on cooldown is prevented if the maximum cladding temperature is limited to 2200°F and local oxidation is limited to 17 percent. These are therefore the design temperature and oxidation criteria for ECCS performance.

Keep Fill Systems (Automatic Fill Systems per Section 6.3.2.6 of the FSAR) are described:

Systems have been provided to protect against possible water-hammer damage during startup. The automatic fill systems for the RHR, CS, HPCI, and RCIC Systems are similar and operate as follows:

Demineralized water is supplied to pressure control valves at approximately 125 psig. The Core Spray and RHR Systems are equipped with two valves per system, one per each system loop. These valves are designed to maintain a positive pressure on the system pump discharge lines and are sized to overcome the maximum expected pump discharge check valve leakage. Pump discharge line pressure can be monitored by local pressure indicators, as well as by pressure switches and alarms in the control room. Should this pressure fall below an acceptable level, a control valve bypass is provided so that the system pump discharge lines can be filled with water and pressurized.

In short, the Automatic Fill Systems ensure that the Core Spray and the Residual Heat Removal Systems are ready to inject into the Core IF they are required.

The FSAR further states that in Section 8.3.1.1.6.4 for the 480 Volt Emergency Power Supply,

8.3.1.1.6.4 480 Volt Emergency Power

. . . . Bus Common C serves Unit 1 while unit substations 2E, 2F, E7, E8, and Common D serves Unit 2.

This may be an error in the FSAR because clearly the loss of Common Bus C affected both Units Keep Fill capability as shown above.

This condition required the Brunswick Units to commence a controlled shutdown due to the loss of the Automatic Fill Capability in accordance with Technical Specification 3.0.3. While performing the shutdown, the plant personnel installed “Temporary Power Supplies” to the Automatic Keep Fill Pumps in order to restore Full Power Operations. Unless specifically approved, Temporary Power Supplies have to undergo a strict Approval Process known as a Temporary Modification prior to their installation. It is not known whether this process was completed.

Further complications involved Unit 1’s subsequent Manual SCRAM also due to the loss of Common Bus C.

At 2319 hours, a manual Reactor Protection System (RPS) actuation was inserted on Unit 1 in anticipation of a loss of condenser vacuum. Shortly before the manual RPS actuation, Circulating Water Intake Pump (CWIP) 1B tripped due to high delta-pressure across the intake traveling screen. This caused the trip of the remaining pumps. Previously, at 1859 hours, balance of plant (BOP) bus Common C unexpectedly de-energized. This caused loss of power to the CWIP traveling screen motors which, in turn, lead to the high delta-pressure across the traveling screen(s).

Circulating Water Pumps ensure that there is Condenser Vacuum so that the Reactor Steam can be dumped to maintain Fueling Cooling.

 

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Conclusion:

While the Brunswick Units were not in great jeopardy due to this event, the cause of the loss of the Electrical Bus is unknown.

Further, there were unseen impacts to the loss of this Bus including a Reactor Trip (SCRAM).

This is an example of how one malfunction can be amplified.

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There were 17 documented Plant Inspection Findings at Brunswick 1 and Brunswick 2 in 2011

  1. Failure to Verify Bearing Oil Level Resulted in Residual Heat Removal Service Water Pump Failure – Inspection Report# : 2011005 (pdf)
  2. Failure to Identify and Correct a Degraded Flood Barrier for the Emergency Diesel Generator Fuel Oil Tank Rooms Inspection Report# : 2011012 (pdf)
  3. Inspection Report# : 2011014 (pdf)
  4. Inadequate Configuration Control Resulted in Rainwater Intrusion into the Unit 2 Reactor Building – Inspection Report# : 2011004 (pdf)
  5. Inadequate Corrective Actions for Control Building Air Conditioning Failures – Inspection Report# : 2010008 (pdf)
  6. Failure to Analyze MOV Operation with Transient Voltages – Inspection Report# : 2010008 (pdf)
  7. Failure to Promptly Identify and Correct Isolation Override Circuitry Testing Deficiencies – Inspection Report# : 2010008 (pdf)
  8. Inadequate Maintenance Results in Containment Isolation Valve Failure – Inspection Report# : 2011004 (pdf)
  9. Failure to follow procedures for analyzing radiological air samples for the presence of alpha emitters – Inspection Report# : 2011002 (pdf)
  10. Failure to Identify and Correct a Degraded Flood Barrier for the Emergency Diesel Generator Fuel Oil Tank Rooms – Inspection Report# : 2011012 (pdf)
  11. Inspection Report# : 2011014 (pdf)
  12. Inadequate Corrective Actions for Control Building Air Conditioning Failures – Inspection Report# : 2011004 (pdf)
  13. Failure To Adequately Evaluate And Correct A Condition Adverse To Quality Involving A Manufacturing Defect Of Barton Model 199 Dual Dampener Differential Pressure Units – Inspection Report# : 2011003 (pdf)
  14. Failure to Analyze MOV Operation with Transient Voltages – Inspection Report# : 2010008 (pdf)
  15. Failure to Promptly Identify and Correct Isolation Override Circuitry Testing Deficiencies – Inspection Report# : 2010008 (pdf)
  16. Failure to Perform Appropriate Corrective Actions for Deficiencies with Opening a Service Water Valve – Inspection Report# : 2010008 (pdf)
  17. Failure to follow procedures for analyzing radiological air samples for the presence of alpha emitters – Inspection Report# : 2011002 (pdf)

 

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