05000455/LER-2008-001, Re Emergency Diesel Generators and Auxiliary Feedwater Pump Automatic Start Resulting from a Loss of Offsite Power Due to a Failed Insulator Causing a Differential Phase Overcurrent
| ML081480502 | |
| Person / Time | |
|---|---|
| Site: | Byron  |
| Issue date: | 05/27/2008 |
| From: | Hoots D Exelon Generation Co, Exelon Nuclear |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| 2.01.0700, BYRON 2008-0052 LER 08-001-00 | |
| Download: ML081480502 (5) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition 10 CFR 50.73(a)(2)(viii)(B) 10 CFR 50.73(a)(2)(iii) 10 CFR 50.73(a)(2)(ix)(A) 10 CFR 50.73(a)(2) 10 CFR 50.73(a)(2)(v)(A), Loss of Safety Function - Shutdown the Reactor 10 CFR 50.73(a)(2)(v)(B), Loss of Safety Function - Remove Residual Heat 10 CFR 50.73(a)(2)(i)(A), Completion of TS Shutdown 10 CFR 50.73(a)(2)(v), Loss of Safety Function 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 4552008001R00 - NRC Website | |
text
Nuclear May 27,2008 LTR:
BY RON 2008-0052 File:
2.01.0700 U. S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555-0001 Byron Station Unit 2 Facility Operating License No. NPF-66 NRC Docket No. STN 50-455
Subject:
Licensee Event Report (LER) 455-2008-001 -00, "Unit 2 Emergency Diesel Generators and Auxiliary Feedwater Pump Automatic Start Resulting from a Loss of Offsite Power Due to a Failed Insulator Causing a Differential Phase Overcurrent" Enclosed is an LER involving the March 25, 2008, Unit 2 event involving the Unit 2 Emergency Diesel Generators and Auxiliary Feedwater Pump automatic start resulting from a loss of offsite power due to a failed insulator. This event is reportable to the NRC accordance with10 CFR 50.73 (a)(2)(iv)(A).
There are no regulatory commitments contained in this report. Should you have any questions concerning this matter, please contact Mr. William Grundmann, Regulatory Assurance Manager, at (81 5) 406-2800.
Respectfully, David M. Hoots Site Vice President Byron Nuclear Generating Station Attachment LEU 455-2008-001 -00 cc:
Regional Administrator, Region Ill, NRC NRC Senior Resident Inspector-Byron Station
(9-2007)
, the NRC ma digitskharacters for each block) not conduct or sponsor, and a person is not required to respond to, th informatinn collectinn
- 13. PAGE Byron Station, Unit 2 I
05000455 I
1 of 4 hnit 2 Emeraencv Diesel Generators and Auxiliarv Feedwater Pump Automatic Start Resulting from a Loss of Offsite Power Due ib a ailed Insulator Causing a Differential Phase overcurrent
- 5. EVENT DATE
- 6. LER NUMBER
- 7. REPORT DATE
- 8. OTHER FACILITIES INVOLVED FACILITY NAME DOCKET NUMBER MONTH DAY YEAR YEAR MONTH DAY YEAR REV NUMBER NO.
NIA FACILITY NAME DOCKET NUMBER 03 25 2008 2008 001 00 03 27 2008
- 9. OPERATING MODE
- 11. THIS REPORT IS SUBMITTED PURSUANT TO THE REQUIREMENTS OF 10 CFR 5: (Check all that app&)
C] 20.2201(b)
II] 20.2203(a)(3)(i)
C] 50.73(a)(Z)(i)(C)
C] 50.73(a)(2)(vii) 1 C] 20.2201(d)
II] 20.2203(a)(3)(ii)
C] 50,73(a)(2)(ii)(A)
C] 50,73(a)(2)(viii)(A)
C] 20.2203(a)(l)
II] 20.2203(a)(4)
C] 50.73(a)(2)(ii)(B)
C] 50.73(a)(2)(viii)(B)
C] 20.2203(a)(2)(i)
C] 50.36(4(1 )(i)(A)
C] 50.73(a)(2)(iii)
C] 50.73(a)(2)(ix)(A)
- 10. POWER LEVEL C] 20.2203(a)(2)(ii)
C] 50.36(c)(l)(ii)(A)
IX) 50,73(a)(2)(iv)(A)
II] 50.73(a)(2)(~)
II] 20.2203(a)(2)(iii)
C] 50.36(~)(2)
II] 50.73(a)(2)(v)(A)
C] 73.71 (a)(4) 100
- 20.2203(a) (2)(iv)
- 50.46(a)(3)(ii) 50.73(a)(2)(v)(B)
- 73.71(a)(5) 20.2203(a)(2)(v)
- 50.73(a)(2)(i)(A)
- 50.73(a)(2)(v)(C)
II] OTHER C] 20.2203(a)(2)(vi)
C] 50.73(a)(2)(i)(B)
C] 50.73(a)(2)(v)(D)
Specify in Abstract below or in I
Unit 2 ESF buses having one of two qualified offsite circuits inoperable. This configuration is allowed for 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />.
As allowed by TS 3.8.1, at 8:04 pm, on March 25, 2008, the two Unit 2 ESF buses were cross-tied with their respective Unit 1 ESF buses and the Unit 2 DGs were secured. An investigation team was immediately convened to ascertain the cause/s and to effect repairs. As a result of the troubleshooting efforts, the likely cause was narrowed down to the SAT 242-2 protection circuitry. At 11:09 pm, on March 27, 2008, SAT 242-1 was reenergized and connected to both Unit 2 ESF buses. SAT 242-2 remained out of service pending further investigation. The NOUE was terminated and TS 3.8.1 Condition A exited.
I This event is reportable to the NRC in accordance with 10 CFR 50.73 (a)(2)(iv)(A), as an event or condition that resulted in the valid actuation of the Auxiliary Feedwater and/or Emergency AC electrical power systems.
I
C. Cause of the Event
The root cause was identified as the failure of a ceramic insulator on the B-Phase of the 4kV non-segregated bus associated with SAT 242-2. This failure was attributed to internal corona discharges that over time led to a breakdown of the insulating material.
I Due to the configuration of the current transformers associated with the differential relays, a B-phase fault is also sensed by the C-phase differential relay. Post event calibration of the B and C phase relays indicated the C-phase was set more sensitively the B-phase relay and was expected to actuate first.
A contributing cause was identified as water vapor in the bus duct accelerating the degradation process of the insulators.
I
D. Safety Analysis
There were no actual safety consequences to this event. The 2A and 26 DGs and the Motor Driven AF Pump started as designed. Unit 2 remained stable and at full power. The AC electrical power sources are designed with suEficient redundancy to ensure the availability of necessary power to ESF Systems.
The Unit 2 ESF buses could have been powered by their respective DGs or the Unit 1's ESF buses, which were capable of being powered by either their offsite sources or their respective DGs.
I Unit 2 ESF buses were energized by the DGs for approximately 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> 29 minutes and then crosstied to Unit 1 ESF buses for approximately 53 hours6.134259e-4 days <br />0.0147 hours <br />8.763227e-5 weeks <br />2.01665e-5 months <br />. A probability risk assessment of these Unit 2 configurations and durations concluded they were of low safety significance.
I
E. Corrective Actions
I SAT 242-2 bus duct insulators have received a high potential test to identify other degraded insulators.
No other degraded insulators were found with this failure mechanism. PRINTED ON RECYCLED PAPER
LICENSEE EVENT REPORT (LER)
CONTINUATION SHEET yron Station, Unit 2 The periodic inspection procedures for both non-segregated and isolated-phase bus ducts will be revised to require the performance of a high potential test of the insulators.
Water vapor intrusion prevention activities were completed on the non-segregated bus ducts associated with SAT 242-2. The preventive maintenance procedure will be revised to provide enhanced instructions on preventing water vapor intrusion into the bus duct.
F. Previous Occurrences
LER 454-96-007, "Loss of Offsite Power Due to a Failure of an Insulator on Phase B of the Unit 1 System Auxiliary Transformer From Water Intrusion."
G. Component Failure Data
Insulator, bus, a-30, 4.1 6 kv., non-segregated, non-insulated phase, metal, enclosed.
Part #: 10501 005 Manufacturer: Delta-Unibus. PRINTED ON RECYCLED PAPER