05000296/LER-2011-003
| Browns Ferry Nuclear Plant Unit 3 | |
| Event date: | 09-28-2011 |
|---|---|
| Report date: | 11-26-2012 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 2962011003R01 - NRC Website | |
I. PLANT CONDITION(S)
Browns Ferry Nuclear Plant (BFN) Unit 3 was at approximately 100 percent power when the event occurred.
II. DESCRIPTION OF EVENT
A. Event
automatically scrammed due to a main turbine generator load reject. Seven safety relief valves (S/RVs) [SB] cycled due to the reactor pressure transient. All systems responded as expected to the turbine trip. There were no Low Pressure Coolant Injection System (LPCI) [BO], Core Spray System (CS) [BG], High Pressure Coolant Injection System (HPCI) [BJ], or Reactor Core Isolation Cooling System (RCIC) [BN] reactor water level initiation set points reached. Primary containment isolation and initiation signals from groups 2, 3, 6, and 8 were received as expected. Reactor water level was automatically controlled by the Feedwater System [SJ].
B. Inoperable Structures, Components, or Systems that Contributed to the Event There were no inoperable structures, components, or systems that contributed to this event.
C. Dates and Approximate Times of Major Occurrences 1969 The BFN Unit 3 isolated-phase bus (IPB) C debris screen [SCN] was installed.
1972-1985 The BFN Unit 3 IPB C debris screen repaired by welding at various intersections.
September 28, 2011 at 0414 The BFN Unit 3 automatically CDT scrammed due to a main turbine generator load reject.
September 29, 2011 at 1751 Temporary alteration control form CDT (TACF) 3-11-007-262 was authorized to remove the BFN Unit 3 IPB A, B, and C debris screens.
September 30, 2011 at 0218 CDT The BFN Unit 3 entered Mode 1.
D. Other Systems or Secondary Functions Affected
There were no other systems or secondary functions affected.
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E. Method of Discovery
The BFN Unit 3 reactor automatically scrammed due to a main turbine generator load reject.
F. Operator Actions
Operations personnel entered emergency operating instruction 3-E0I-1, RPV Control, on reactor water level and abnormal operating instruction 3-A0I-100-1, Reactor Scram, for the reactor scram.
G. Safety System Responses
Seven S/RVs cycled due to the reactor pressure transient. Primary containment isolation and initiation signals for groups 2, 3, 6, and 8 were received. Reactor water level was automatically controlled by the Feedwater System.
III. CAUSE OF THE EVENT
A. Immediate Cause
The immediate cause of this event was a piece of the BFN Unit 3 IPB C debris screen found downstream in the bus duct [BDUC]. The debris screen caused a phase to ground fault on the BFN Unit 3 IPB C.
B. Root Cause
There were two root causes identified:
1. Electrical Preventive Instruction EPI-0-242-BUS001, Main Transformer, Unit Station Service Transformer and Associated Bus Work (Major Outage), did not contain adequate inspection criteria for a generation risk sensitive component (debris screen).
2. Weaknesses within the welding program allowed multiple repetitive welds on trip sensitive equipment.
C. Contributing Factors
A contributing factor was identified as failure to implement lessons learned from previous industry operating experience (OE).
IV. ANALYSIS OF THE EVENT
Description of System The IPB duct system is a 3 phase assembly in which each phase conductor is enclosed in a metal housing, and an air space is located between each phase to eliminate faults.
The IPB duct assembly is used as a cooling system to minimize heat flow as current travels from the main turbine generator [GEN] to the large power transformers [XFMR].
This current generated heat is minimized via a forced cooling system.
The cooling system is designed with two fans [FAN] and two motors [MO]. Each fan operates independently and produces approximately 48,000 standard cubic feet per minute (scfm) air flow. As a result, each individual fan is able to provide the necessary air flow to eliminate overheating the system. Each fan assembly has gravity and inlet dampers [DMP]. The gravity dampers are located directly above the fans and are provided to prevent backflow to the non-energized fan. The inlet dampers are used to control the capacity of the fan. The debris screens are located farther downstream of the inlet dampers. They are installed to prevent foreign material from entering the bus duct and causing a catastrophic failure.
Analysis of the Event
The Tennessee Valley Authority (WA) is submitting this report in accordance with 10 CFR 50.73(a)(2)(iv)(A), a condition that resulted in automatic actuation of the Reactor Protection System.
The failed Unit 3 IPB C debris screen and the intact Unit 3 IPB B debris screen were removed and sent to WA Central Laboratory for failure analysis. Laboratory analysis indicated multiple crack initiation sites at welded intersections on the BFN Unit 3 IPB C debris screen. The repair welding at various intersections allowed discontinuities to be introduced. Under repetitive cyclic loading, these discontinuities become sites for fatigue initiation due to residual tensile stress present as well as a high dislocation density.
Some of the fatigue initiation sites were not in weld metal of the intersections; however, the degree of residual stresses from the welding process makes the screen intersection corners highly likely to initiate cracks.
On a micro-scale, repetitive cyclic loading from flow induced vibration creates sharp peaks (extrusions) and troughs (intrusions) which result from dislocations moving under load. The end result is the formation of a crack tip which is then driven by cyclic stresses. This phenomenon was confirmed by laboratory analysis which showed fatigue striations. These striations on the fracture surface represent one stress cycle and were present at the welded intersections. Another contributor to the crack propagation is the residual stress left from the heating and cooling cycles of welding multiple intersections.
Once various intersections began failing by fatigue, the neighboring intersections were loaded incrementally more until they began failing. The end result is that the final intersections were overloaded and the screen segment broke away from the rest of the screen. Once free in the Unit 3 IPB C duct, airflow pushed the screen segment downstream until it contacted the Unit 3 IPB C and the Unit 3 IPB C duct at the same time. The screen segment caused a momentary ground that tripped the main turbine generator neutral overvoltage relay due to the screen being electrically conductive. This was confirmed by Power System Operations Oscillography that demonstrated the neutral bus was gaining voltage from C phase at the time of the automatic scram. The automatic scram occurred as a result of the tripping of the main turbine generator neutral overvoltage relay.
Extent of Condition During the BFN Unit 3 forced outage, BFN Unit 3 IPB A, B, and C debris screens were removed by TACF 3-11-007-262. The BFN Unit 3 debris screens were replaced with non-welded screens during BFN Unit 3 refueling outage 15.
The extent of condition includes the BFN Units 1 and 2 IPB A, B, and C debris screens.
The extent of condition for these six screens will be addressed by verifying installation of non-welded debris screens and, if applicable, replacing the existing welded screens with a non-welded debris screen.
There are also screens above the Unit Service Station Transformer (USST) for BFN Units 1, 2, and 3 buses A and B. These six screens are also part of the extent of condition. The extent of condition for these screens will be addressed by inspecting the screens (and replacing, if necessary) during the next applicable refueling outage for each unit followed by a determination of the inspection preventive maintenance (PM) frequency based on the results of the initial inspections.
Extent of Cause The extent of cause was identified to be programmatic weaknesses have the potential to impact other plant processes, equipment, or human performance.
V. ASSESSMENT OF SAFETY CONSEQUENCES
When the BFN Unit 3 scram occurred, all control rods fully inserted. All systems responded as expected. The plant was supplied from offsite power and was in normal shutdown configuration. There were no LPCI, CS, HPCI, or RCIC reactor water level initiation set points reached. Reactor water level was automatically controlled by the Feedwater System. The main steam isolation valves were open with decay heat being removed via steam to the main condenser [SG] using the bypass valves.
Therefore, WA concluded that there was no significant reduction to the health and safety of the public for this event.
VI. CORRECTIVE ACTIONS - The corrective actions are being managed by TVA's corrective action program.
A. Immediate Corrective Actions
The immediate corrective action was to remove the BFN Unit 3 IPB A, B, and C debris screens.
B. Corrective Actions
1. Revised Electrical Preventive Instruction EPI-0-242-BUS001 to include an inspection of the debris screens with the first inspection taking place one outage after implementation of the new screens. Any welded intersections, cracks, or deformation should be the basis for screen replacement.
2. Initiated and created a PM activity to perform a debris screen inspection once every four years.
1 1 3. Implement Operating Experience Extractor at BFN to improve review methods and implementation of OE.
4. Perform an inspection of BFN Units 1, 2, and 3 USST debris screens. Any welded intersections, cracks, or deformation should be the basis for screen replacement.
5. Initiate and create/revise a PM activity to perform a USST screen inspection using Electric Power Research Institute's Nuclear Maintenance Applications Center: Isolated Phase Bus Maintenance Guide and inspection data as a guide to set PM frequency.
6. Performed welding surveillances of at least 10 existing bus duct or sheet metal welds. Note any excessive number of weld repairs and document the results of the surveillances.
C. Corrective Actions to Prevent Recurrence
1. Verified installation of a non-welded debris screen on BFN Unit 3 IPB A, B, and C ducts.
2. Verify installation of a non-welded debris screen on BFN Units 1 and 2 IPB A, B, and C ducts.
3. Revise Electrical Preventive Instruction EPI-0-242-BUS001 to remove the requirement that allows the lead performer or system engineer to waive evaluation of USST bus housing, to incorporate additional generation risk significant components by systems engineering into the inspection criteria for EPI-0-242-BUS001, and to include an inspection of the debris screens.
4. Generate a list of other IPB duct generation risk significant components (e.g., debris screens) and other specific inspection attributes to be incorporated into EPI-0-242-BUS001.
5. Engineering will perform an assessment and gap analysis to determine why the IPB duct debris screens were not identified as a single point vulnerability (SPV) (generation risk). Upon completion of the gap analysis, apply the lessons learned to procedure NPG-SPP-09.18, Integrated Equipment Reliability Program, and identify additional SPVs.
6. Engineering will review all SPVs identified to date and determine if the inclusion of mitigating strategies should be added to relevant inspection procedures.
7. Revise procedure MMDP-10, Controlling Welding, Brazing, and Soldering Processes, to include controls that limit the number of attempts to repair the same weld area on plant equipment.
VII. ADDITIONAL INFORMATION
A. Failed Components
The failed component was the BFN Unit 3 IPB C debris screen.
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B. Previous Similar Events
In February 1997, a section of BFN Unit 1 bus duct cooling fan exhaust ductwork broke out of the aluminum exhaust duct. The section was sent to TVA Central Laboratory for analysis. The analysis concluded that fatigue cracking was initiated from the weld root on the inside of a corner on the duct. The failure mechanism was identified to be low cycle fatigue.
C. Additional Information
The corrective action document for this report is Problem Evaluation Report 440359.
D. Safety System Functional Failure Consideration
This event was not a safety system functional failure in accordance with NEI 99-02.
E. Scram With Complications Consideration
This event was not a complicated scram in accordance with NEI 99-02.
VIII. COMMITMENTS
There were no commitments.