Browns Ferry Nuclear Plant, Unit 3 LER: Automatic Reactor Scram Due to an Actuation of a Main Transformer Differential Relay

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by Bob Meyer

Inadequate procedures, inadequate management oversight challange Safety and Operators.

On May 29, 2012, at 0331 Central Daylight Time, the Browns Ferry Nuclear Plant (BFN), Unit 3, reactor automatically scrammed due to fast closure of turbine control valves, initiated by a load reject signal on the Main Generator. The cause of the load reject signal was actuation of newly installed main transformer differential relay 387T, which caused the scram. All systems responded as expected to the load reject signal. Main steam isolation valves remained open and reactor pressure was controlled by the main turbine bypass valves. No Emergency Core Cooling System or Reactor Core Isolation Cooling System reactor water level initiation set points were reached. Primary Containment Isolation System isolations from Groups 2, 3, 6, and 8 were received, and reactor water level was controlled by the Feedwater System.

Three root causes were identified: 1) inadequate procedure, instructions, and testing methodology/equipment used for current transformer (CT) bench testing; 2) inadequate acceptance review by Tennessee Valley Authority (TVA) Engineering of a vendor prepared design change; and 3) inadequate management, oversight, and accountability by the BFN Maintenance organization for work performed by the Protective Relay Group. 

The corrective actions to prevent recurrence include: 1) revise the CT bench test procedure; 2) revise the human performance procedure to incorporate technical conscience principles, focus technical task risk factors, mitigating strategies, and decision making; and 3) using the Nuclear Operating Model, utilize the TVA's strategic performance management process to ensure management alignment in the ownership and accountability for leadership expectations at BFN.

Event

On May 29, 2012, at 0331 Central Daylight Time (CDT), the BFN, Unit 3, reactor automatically scrammed due to fast closure of turbine control valves [VM that was initiated by a load reject signal on the Main Generator [TB]. The cause of the load reject signal was the actuation of the newly installed main transformer differential relay [RLY] 387T which caused the scram.

All systems responded as expected to the load reject signal. Main steam isolation valves (MSIVs) remained open and reactor pressure was controlled on the main turbine bypass valves. No Emergency Core Cooling System (ECCS) [BJ][BO][BM][SB] or Reactor Core Isolation Cooling (RCIC) System [BN] reactor water level initiation set points were reached. Primary Containment Isolation System (PCIS) [BD] isolations from Groups 2, 3, 6, and 8 were received, and reactor water level was controlled by the Feedwater System [SJ].

CAUSE OF THE EVENT

A. Immediate Cause

The immediate cause of this event was the vendor (Asea Brown Boveri) manufactured a current transformer (CT) [XCT] with a reversed polarity and the Tennessee Valley Authority (TVA) installed this CT in the plant.

B. Root Cause

There were three root causes identified:

1. The Energy Delivery (ED) group procedure, TOM-FTM-6-INXF-002 (Testing Instrument Transformers), instructions and testing methodology/equipment were inadequate and did not meet the requirements of a Nuclear Power Group (NPG) procedure per NPG-SPP-01.2, Administration of Site Technical Procedures, such as inclusion of the human performance tools proven to reduce errors for testing high risk trip-sensitive CT components.

2. The TVA's acceptance review by Engineering of the design change prepared by an outside vendor was inadequate for modification to high risk trip-sensitive components.

3. Management, oversight, and accountability by the BFN Maintenance organization for work performed by the Protective Relay Group (PRG) were inadequate.

C. Contributing Factor

The written instructions for the on-line commissioning test of high risk trip-sensitive components were inadequate.

CORRECTIVE ACTIONS - The corrective actions are being managed by TVA's corrective action program.

A. Immediate Corrective Actions

1. The CT polarity input to the main transformer differential relay 387T was corrected by swapping leads.

2. The polarity was tested satisfactorily on all remaining newly installed CTs.

Corrective Actions

1. Revised the vendor manual for the newly installed main transformer differential relay to provide adequate guidance on when and how to perform an on-line commissioning test.

2. Revise procedure NPG-SPP-09.3, Plant Modifications and Engineering Change Control, and procedure NPG-SPP-06.9.3, Post-Modification Testing, to require monitoring of all trip risk components that are capable of being monitored during return to operations.

3. Review the transformer and relay connections as well as the relay logic on the CSSTs at BFN to determine if a CT with reverse polarity would be self-revealing or if a field test will be required to ensure polarity is correct. For those CTs where reverse polarity is not considered self-revealing, enter the condition into the Corrective Action Program for resolution.

4. Revise the ED group field test manual procedures used by PRG at TVA nuclear facilities and issue as approved NPG procedures. The revised procedures will include required testing equipment and methodology, critical steps, second party verification, specific acceptance criteria, and NPG standards and expectations for human error techniques and risk sensitive activities.

5. Retested the new BFN, Unit 1, Main Generator CTs prior to their installation during BFN, Unit 1, refueling outage 9.

6. Revise procedure NEDP-5, Design Document Reviews, to establish definition, objective criteria, and requirements for each type of review (e.g., owner's acceptance review, technical review), and the situations when each type of review is appropriate.

C. Corrective Actions to Prevent Recurrence

1. Revise the CT bench test procedure TOM-FTM-6-INXF-002 and issue as an approved NPG procedure per NPG-SPP-01.2, Administration of Site Technical Procedures.

2. Revise procedure NPG-SPP-18.2.2, Human Performance, to incorporate technical conscience principles, focus technical task risk factors, mitigating strategies, and decision making.

3. Using the Nuclear Operating Model, utilize TVA's strategic performance management process to ensure management alignment in the ownership and accountability for leadership expectations at BFN.

Previous Similar Events

A search of BFN, Units 1, 2, and 3, LERs for approximately the past five years did not identify any similar events.

A search was performed on the BFN corrective action program. Similar concerns regarding management oversight, change management, and inadequate procedures were identified in problem evaluation reports (PERs) 139781, 151772, 162391, and 177395.

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