05000259/LER-2011-001
| Browns Ferry Nuclear Plant (Bfn) Unit 1 | |
| Event date: | 04-27-2011 |
|---|---|
| Report date: | 06-27-2011 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iii) 10 CFR 50.73(a)(2)(iv)(B), System Actuation 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| Initial Reporting | |
| ENS 46793 | 10 CFR 50.72(b)(2)(iv)(B), RPS System Actuation, 10 CFR 50.72(a)(1)(i), Emergency Class Declaration, 10 CFR 50.72(b)(3)(xiii), Loss of Emergency Preparedness |
| 2592011001R00 - NRC Website | |
I. PLANT CONDITION(S)
At the time of the event, Browns Ferry Nuclear Plant (BFN) Units 1 and 2 were at approximately 75 percent power (Mode 1) and Unit 3 was at approximately 100 percent power (Mode 1).
II. DESCRIPTION OF EVENT
A. Event:
On April 27, 2011, following offsite power grid oscillations (due to severe weather including high winds and tornadoes) and subsequent Unit 1 and 2 power reductions from 100 percent to 75 percent to attempt to correct the condition, BFN experienced a complete loss of the 500-kV offsite power system. This resulted in automatic scrams of Units 1, 2, and 3.
All three units were in Mode 1 at the time of the event. All scram systems were actuated, all actuations were complete, and required systems started and functioned successfully with the exception of an indeterminate position indication for the Unit 3 B Inboard Main Steam Isolation Valve (MSIV)[SB]. All onsite safe shutdown equipment was available with the exception of the 3B Emergency Diesel Generator (EDG)[EK], which was inoperable and unavailable due to planned maintenance.
After the event, only one 161-kV line remained available for offsite power - all (seven) 500-kV lines and one (of two) 161-kV line were lost. All three units immediately entered Mode 3 (Hot Shutdown) with their respective 4-kV[EB] busses supplied by the onsite EDGs.
On April 27, 2011, at 1701 hours0.0197 days <br />0.473 hours <br />0.00281 weeks <br />6.472305e-4 months <br />, Central Daylight Time, a Notification of Unusual Event (NOUE) was declared due to the loss of normal and alternate supply voltage to all unit-specific 4-kV shutdown boards for greater than 15 minutes and at least two EDGs supplying power to unit-specific 4-kV shutdown boards. On May 2, 2011, at 2050 hours0.0237 days <br />0.569 hours <br />0.00339 weeks <br />7.80025e-4 months <br />, the NOUE was terminated following restoration of qualified offsite power sources.
The Tennessee Valley Authority (WA) is submitting this LER in accordance with 10 CFR 50.73(a)(2)(iii), any natural phenomenon or other external condition that posed an actual threat to the safety of the nuclear power plant or significantly hampered site personnel in the performance of duties necessary for the safe operation of the nuclear power plant. Additionally, TVA is reporting this event in accordance with 10 CFR 50.73(a)(2)(iv)(A), any event or condition that resulted in manual or automatic actuation of systems named in 10 CFR 50.73(a)(2)(iv)(B) - (1) Reactor Protection System including: reactor scram or reactor trip.
B. Inoperable Structures, Components, or Systems that Contributed to the Event:
None C. Dates and Approximate Times of Malor Occurrences:
April 27, 2011, at 1401 hours0.0162 days <br />0.389 hours <br />0.00232 weeks <br />5.330805e-4 months <br /> at 1539 hours0.0178 days <br />0.428 hours <br />0.00254 weeks <br />5.855895e-4 months <br /> at 1622 hours0.0188 days <br />0.451 hours <br />0.00268 weeks <br />6.17171e-4 months <br /> at 1636 hours0.0189 days <br />0.454 hours <br />0.00271 weeks <br />6.22498e-4 months <br /> at 1701 hours0.0197 days <br />0.473 hours <br />0.00281 weeks <br />6.472305e-4 months <br /> April 28, 2011, at 0243 hours0.00281 days <br />0.0675 hours <br />4.017857e-4 weeks <br />9.24615e-5 months <br /> April 28, 2011, at 0545 hours0.00631 days <br />0.151 hours <br />9.011243e-4 weeks <br />2.073725e-4 months <br /> April 28, 2011, at 1337 hours0.0155 days <br />0.371 hours <br />0.00221 weeks <br />5.087285e-4 months <br /> May 2, 2011, at 2010 hours0.0233 days <br />0.558 hours <br />0.00332 weeks <br />7.64805e-4 months <br /> May 2, 2011, at 2050 hours0.0237 days <br />0.569 hours <br />0.00339 weeks <br />7.80025e-4 months <br /> May 20, 2011 May 25, 2011 May 31, 2011 Operations personnel were notified that BFN was under a Tornado Warning. The actions of procedure 0-A01-107, Severe Weather, were addressed.
The first 500-kV line was lost. Others followed with the last (seventh) line lost at 1636 hours0.0189 days <br />0.454 hours <br />0.00271 weeks <br />6.22498e-4 months <br />.
The first 161-kV line was lost. The other 161-kV line did not trip and provided the only sustained source of offsite power to the station during the event and recovery from it.
Units 1, 2, and 3 automatically scrammed due to loss of all 500-kV offsite power sources. Units 1, 2, and 3 entered Mode 3 (Hot Shutdown).
BFN declared a Notification of Unusual Event (NOUE) in accordance with EPIP-1, Emergency Classification Procedure, Emergency Action Level 5.1-U - Loss of normal and alternate supply voltage to all unit-specific 4-kV shutdown boards for greater than 15 minutes and at least two EDGs supplying power to unit-specific 4-kV shutdown boards.
Unit 3 entered Mode 4 (Cold Shutdown).
Unit 2 entered Mode 4 (Cold Shutdown).
Unit 1 entered Mode 4 (Cold Shutdown).
All shutdown boards are powered from qualified 161-kV offsite power sources, and all EDGs are shutdown and in standby readiness.
The NOUE was terminated.
Unit 1 returned to service with main generator breaker closure.
Unit 2 returned to service with main generator breaker closure.
Unit 3 returned to service with main generator breaker closure.
D. Other Systems or Secondary Functions Affected:
None
E. Method of Discovery:
The event was self-revealing. Severe weather in the Tennessee Valley Service Area caused grid instability and loss of all 500-kV offsite power sources that resulted in automatic scrams of all three units.
F. Operator Actions:
Following the automatic scrams, Operations personnel used the applicable post-scram procedures. Unit-specific emergency operating procedures were also used as the scrams were complicated by the loss of normal power to balance of plant systems.
G. Safety System Responses:
All scram systems were actuated, all actuations were complete, and required systems started and functioned successfully with the exception of an indeterminate position indication for the Unit 3 B Inboard MSIV. All onsite safe shutdown equipment was available with the exception of the 3B EDG, which was inoperable and unavailable due to planned maintenance.
After the event, only one 161-kV line remained available for offsite power - all (seven) 500-kV lines and one (of two) 161-kV lines were lost due to extensive damage to the area grid. All three units immediately entered Mode 3 (Hot Shutdown) with the respective shutdown 4-kV busses supplied by onsite EDGs.
III. CAUSE OF THE EVENT
A. Immediate Cause:
The immediate cause for this condition was loss of all 500-kV offsite power sources that resulted in a scram of all three units from automatic turbine trips when power load unbalance signals were detected.
B. Root Cause:
The TVA Transmission/Distribution system was subjected to severe wind speeds and wind induced forces which resulted in multiple failures of transmission towers and conductor elements. This resulted in the eventual loss of all 500-kV lines servicing the station and automatic shutdowns of all three units.
IV. ANALYSIS OF THE EVENT
On April 27, 2011, at approximately 1636 hours0.0189 days <br />0.454 hours <br />0.00271 weeks <br />6.22498e-4 months <br />, all three BFN units automatically scrammed with a first out signal of Power Load Unbalance. Problem Evaluation Report (PER) 364318 was initiated for this event. Required plant systems and components responded as expected except for an indeterminate Main Control Room position indication for the Unit 3 B Inboard MSIV.
Evaluation of Plant Systems / Components Based on review of plant system records (including scram reports), operating logs, and transmission/distribution system data, BFN plant systems responded as designed to facilitate shutdown of all three units.
Safety systems actuations following the initial scrams include:
- Reactor Protection System (RPS)[JC] for Units 1, 2, and 3 due to offsite power losses,
- Primary Containment Isolation System (PCIS)[JE][JM] Groups 2, 3, 6, and 8 isolations for Units 1 and 3 due to loss of A and B RPS power and for Unit 2 due to low reactor water level,
- PCIS Group 1 (e.g., MSIVs) isolations for Units 1 and 3 due to loss of A and B RPS power. The Unit 2 MSIVs did not close because the remaining 161-kV line continued to supply power to the Unit 2 RPS,
- High Pressure Coolant Injection (HPCI)[BJ] for Unit 1 only. Unit 1 HPCI auto-initiated on low reactor water level (less than minus 45 inches). Unit 1 Reactor Core Isolation Cooling (RCIC) [BN] had been manually initiated earlier in the event for level control and was already running when the low reactor water level signal was received.
The Unit 3 B Inboard MSIV indicated indeterminate (PER 361532).
Other significant, post-event failures or issues were identified as follows:
1. Failure of the diesel-driven Fire Pump [KP](PER 361542), 2. Failure of the Nuclear Security diesel-driven generator [IA](PER 364675), 3. Significant loss of the Alert Notification System (PER 364674), 4. Loss of power to the plant Chemistry Lab (to counting equipment) [LQ](PER 362839), and 5. Potential damage to all turbine generators [TB](PER 362890).
As shown, PERs were initiated for each of these items.
Evaluation of Personnel Performance Personnel performance following the event and subsequent scrams was reviewed and evaluated. One issue was identified that had elements related to personnel performance.
PER 335574 was initiated as result of a subsequent valid Unit 1 scram on low water level following the initial scram reported in this LER. The specific issues related to this scram will be reviewed and addressed in PER 335574 and a separate LER. Otherwise, no additional personnel performance issues were identified.
V. ASSESSMENT OF SAFETY CONSEQUENCES
The event discussed in this root cause did impact nuclear safety and resulted in a reduction in the defense-in-depth. The loss of all 500-kV offsite power sources (only one 161-kV line remained) reduced margins to nuclear safety by causing the automatic scrams of all three units and actuation of required safety systems. Although there was a reduction in defense-in-depth, safety systems and automatic protective functions all performed as designed. Individual component failures are addressed by separate PERs. Collectively, the identified conditions posed no significant risk or substantial degradation to the station's ability to respond to a design basis event. These conditions are under analysis by separate PERs, and actions to mitigate will be documented in those PERs.
The NOUE was reported in accordance with the Emergency Plan (Reference Event Notification 46793).
There was no change in radiological risk during or following the event. Therefore, safety margin was maintained and this event was of very low safety significance.
VI. CORRECTIVE ACTIONS
A. Immediate and Corrective Actions:
Qualified 161-kV offsite power sources were provided on May 2, 2011. The EDGs were shutdown and each unit entered a non-refueling outage awaiting restoration of sufficient 500-kV lines to support unit restart and power generation.
Corrective actions to improve severe weather preparedness and performance were developed. These include the following provisions:
1. Standardized source for use of predictive weather information, 2. Work activity risk management process that integrates the application of predictive weather products ( i.e., 1, 3, and 5-day outlooks) based on weather thresholds relative to risks, 3. Action thresholds and protocols relative to risk due to weather for on-line and outage maintenance/modifications activities, 4. Site and WA Fleet long-term strategy for EDG operation during recovery from prolonged offsite power losses, 5. Procedures that ensure anticipated disruptions in communications services are minimized and alternate communication methods are available for prolonged offsite power losses, 6. Guidelines and protocol for exiting emergency situations relative to 10 CFR 26 nuclear fatigue rule compliance, 7. Emergency procedures that include a review of 10 CFR 26 nuclear fatigue rule compliance procedures prior to termination, 8. Severe weather procedures that improve specific criteria for entry and exit conditions, 9. A backup diesel generator for maintaining fire operations during prolonged offsite power losses, 10. Procedures for installation and activation of temporary diesel generators for selected plant components, machinery, equipment, and 11. Operational readiness activity for all non safety-related diesel generators including those leased/rented to BFN.
B. Corrective Actions to Prevent Recurrence:
No practicable actions exist to prevent offsite power losses for the type of extensive, region-wide weather that caused this event.
VII. ADDITIONAL INFORMATION
A. Failed Components:
None B. Previous LERS or Similar Events:
None
C. Additional Information:
The corrective action document for this report is PER 364318.
D. Safety System Functional Failure Consideration:
There were no safety system functional failures.
E. Scram With Complications Consideration:
Summaries for each unit's response to the event are as follows:
For Units 1 and 3, offsite power losses resulted in a loss of RPS power, which led to MSIV closure and subsequent loss of feedwater flow and main condenser vacuum.
Decay heat was rejected to the primary suppression chamber (torus) via manual operation of Main Steam Relief Valves [SB] and HPCI in pressure control mode.
For Unit 2, there was no loss of normal heat removal capability.
Based on NEI 99-02 Revision 2 guidance, following the scram/shutdown transient, operator actions were beyond that of a normal scram. Therefore, the event scrams of Units 1 and 3 are considered as unplanned scrams with complications.
VIII. COMMITMENTS
None