05000458/LER-2007-006, Regarding Unplanned Reactor Scram Due to Transformer Fault
| ML080080031 | |
| Person / Time | |
|---|---|
| Site: | River Bend  |
| Issue date: | 01/02/2008 |
| From: | Lorfing D Entergy Operations |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| G9.25.1.3, G9.5, RBF1-08-001, RBG-46776 LER 07-006-00 | |
| Download: ML080080031 (6) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability 10 CFR 50.73(a)(2)(ii)(A), Seriously Degraded 10 CFR 50.73(a)(2)(viii)(A) 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition 10 CFR 50.73(a)(2)(viii)(B) 10 CFR 50.73(a)(2)(ix)(A) 10 CFR 50.73(a)(2)(iv)(A), System Actuation 10 CFR 50.73(a)(2)(x) 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)(v), Loss of Safety Function 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 4582007006R00 - NRC Website | |
text
vwEn terg Entergy Operations, Inc.
River Bend Station 5485 U.S. Highway 61 N St. Francisville, LA 70775 Tel 225 381 4157 Fax 225 635 5068 dlorfin@entergy.com David N. Lorfing Manager-Licensing January 2, 2008 U. S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555
Subject:
Licensee Event Report 50-458 / 07-006-00 River Bend Station - Unit 1 Docket No. 50-458 License No. NPF-47 File Nos.
G9.5, G9.25.1.3 RBG-46776 RBF1-08-0001 Ladies and Gentlemen:
In accordance with 100CFR5O.73, enclosed is the subject Licensee Event Report.
This document contains n o commitments.
Sincerely, Manager - Licensing DNL/dhw Enclosure
'ff1~
Licensee Event Report 50-458 / 07-006-00 January 2, 2008 RBG-46776 RBF1 000 1 Page 2 of 2 cc:
U.* S. Nuclear Regulatory Commission Region IV 611 Ryan Plaza Drive, Suite 400 Arlington, TX 76011 NRC Sr. Resident Inspector P.O0. Box 1050 St. Francisville, LA 70775 INPO Records Center E-Mail Mr. Jim Calloway Public Utility Commission of Texas 1701 N. Congress Ave.
Austin, TX 78711-3326 Mr. Jeff Meyers Louisiana Department of Environmental Quality Office of Environmental Compliance P.O. Box 4312 Baton Rouge, LA 70821-4312
NRC FORM 366 U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY 0MB: NO. 3150-0104 EXPIRES: 08/31/2010 (9-20071Estimated
, the NRC may digits/characters for each block) not conduct or sponsor, and a person is not required to respond to, the information collectior.
- 3. PAGE River Bend Station - Unit 1 000481o
- 4. TITLE Unplanned Reactor Scram Due to Transformer Fault
- 5. EVENT DATE
- 6. LER NUMBER
- 7. REPORT DATE
- 8. OTHER FACILITIES INVOLVED SEOENTALREV MONH FACILITY NAME DOCKET NUMBER MONTH DAY YEAR YEAR NUMBER NO.
MNH DAY YEAR 05000 IFACILITY NAME DOCKET NUMBER 11 07
.2007 2007 - 006 -
00 01 02 2008 05000
- 9. OPERATING MODE
- 11. THIS REPORT IS SUBMITTED PURSUANT TO THE REQUIREMENTS OF 10 CFR§: (Check all that apply)
El 20.2201 (b)
[I 20.22D3(a)(3)(i)
El 5D.73(a)(2)(i)(C)
El 50.73(a)(2)(vii) 1El 20.2201 (d)
El 20.22D3(a)(3)(ii)
El 50.73(a)(2)(ii)(A)
El 50.73(a)(2)(viii)(A)
El 20.2203(a)(1)
El 20.2203(a)(4)
El 50.73(a)(2)(ii)(B)
El 50.73(a)(2)(viii)(B) 171 20.2203(a)(2)(i)
[1 50.36(c)(1 )(i)(A)
El 5D.73(a)(2)(iii)
El 50.73(a)(2)(ix)(A)
- 10. POWER LEVEL El 20.2203(a)(2)(ii)
El 50.36(c)(1)(ii)(A)
[0 50.73(a)(2)(iv)(A)
[] 50.73(a)(2)(x)
El 20.2203(a)(2)(iii)
El 50.36(c)(2)
El 50.73(a)(2)(v)(A)
El1 73.71 (a)(4)
El 20.2203(a)(2)(iv)
El 50.46(a)(3)(ii)
El 50.73(a)(2)(v)(B)
El 73.71 (a)(5) 75El 20.22D3(a)(2)(v)
El 5D.73(a)(2)(i)(A)
El 50.73(a)(2)(v)(C)
El OTHER El 20.2203(a)(2)(vi)
El1 50.73(a)(2)(i)(B)
El 50.73(a)(2)(v)(O)
Specify in Abstract below or in NRC Form 366A
- 12. LICENSEE CONTACT FOR THIS LER FACILITY NAME TELEPHONE NUMBER (include Area Code)
David N. Lorfing, Manager - Licensing 225-381-4157CAS YTMCMOET MANUI-REPORTABLE CASISSEMCMPNN MANU-REPORTABLE CASE SYTE CMPNET FACTURER TO EPIX CAS YTMCMOETFACTURER TO EPIX (see text)_______________
- 14. SUPPLEMENTAL REPORT EXPECTED 115. EXPECTED MONTH DAY YEAR SUBMISSION El YES (If yes, complete 15. EXPECTED SUBMISSION DATE) 0 No DATE ABSTRACT (Limit to 1400 spaces, i.e., approximately 15 single-spaced typewritten lines)
On November 7, 2007,. at 3:06 a~m. CST, an unplanned manual reactor scram was initiated following the loss of the operating condensate pumps. The plant was operating at 75 percent power at the time. The main condensate pumps tripped when power was lost on the 'A" non-safety 13.8kv bus, in addition to numerous 480v load centers supplied by that bus. The loss of that 13.8kv bus occurred when a rodent inside a transformer enclosure supplied by that bus caused arcing across phases and to ground. The slow action of the circuit breaker feeding the faulted transformer caused the main feeder breaker to the "A" 13.8kv bus to trip. Following the scram, reactor water level decreased below the actuation setpoint of the high pressure core spray (HPCS) system and the reactor core isolation cooling (ROIC) system, both of which started automatically as designed.
Due to the loss of electrical power, the normal service water system was being supplied by only one 50 percent capacity pump, resulting in an abnormally low header pressure. The standby service water system was being placed into service to raise header pressure when the "C" standby service water pump started automatically due to the low pressure condition (this response was as designed, considering the configuration of the system at the time). The Level 2 signal also caused actuation of the primary containment isolation valves in numerous systems, which were confirmed to have responded as designed. This event is being reported in accordance with 1 0CFR50.73(a)(2)(iv)(A) as an unplanned manual actuation of the reactor protection system, as well as the automatic actuations of the HPCS, RCIC, standby service water, and primary Containment isolation systems.
NRC FORM 366 (9-2007)
PRINTED ON RECYCLED PAPERU.S. NUCLEAR REGULATORY COMMISSION (9-2007)
LICENSEE EVENT REPORT (LER)
- 1. FACILITY NAME
- 2. DOCKET
- 6. LER NUMBER
- 3. PAGE IYEAR SEQUENTIAL IREVISION I
NUMBER NUMBER River. Bend Station -Unit 1 105000-458 2007 006 00 I2 OF 4
REPORTED CONDITION On November 7, 2007, at 3:06 a.m. CST, an unplanned manual reactor scram was initiated following the loss of the operating condensate (SD) pumps. The plant was operating at 75 percent power at the time. The main condensate pumps tripped when power was lost on the "A" non-safety 13.8kv bus (EA), in addition to numerous 480v load centers supplied by that bus. Following the scram, reactor water level decreased below the actuation setpoint of the high pressure core spray (HPCS)
(BG) system and the reactor core isolation cooling (ROIC) (BN) system, both of which started automatically as designed. Due to the loss of electrical power, the normal service water system (KG) was being supplied by only one 50 percen't capacity pump, resulting in an abnormally low header pressure. The standby service water system (BS) was being placed into service to raise header pressure when the "C" standby service water pump started automatically due to the low pressure condition (this response was as designed, considering the configuration'of the system at the time). The Level 2 signal also caused actuation of the primary containment isolation valves in numerous systems, which were confirmed to have responded as designed. This event is being reported in accordance with 1 OCFR5O.73(a)(2)(iv)(A) as an unplanned manual actuation of the reactor protection system, as well as the automatic actuations of the HPCS, ROIC, standby service water, and primary containment isolation (JE) systems.
The actuation of the HPCS system is reportable in accordance with River Bend Technical Specification 5.6.9.2, "ECOS System Actuations." In the operating history of the plant, this event was the 14th injection cycle for the HPCS system. An engineering evaluation determined that the nozzle usage factor remained well below the TS administrative limit of 0.70.
BACKGROUND The 13.8kv electrical system comprises "A", "B", and "C" busses, which can be supplied from offsite power or from the main generator. In addition to powering the reactor feedwater pumps and condensate pumps, the "A" and "B" 13.8kv busses supply power to numerous 4160kv and 480v load centers throughout the plant via step-down transformers. The 13.8kv circuit breakers are General Electric Magneblast units. A manually-operated load break switch is installed on the high side (1 3.8kv supply) of each 480v (EC) step-down transformer for use during maintenance..Each 480v transformer and switch is enclosed in a housing connected to the associated switchgear.
INVESTIGATION AND IMMEDIATE ACTIONS The investigation of this event found that a fault occurred on the bus connections of the step-down transformer supplying a 480v load center in the auxiliary boiler building. This fault appears to have occurred when a rodent crawled between bus bars on the high side (1 3.8kv input) of the transformer, resulting in arcing to ground and then between phases. Significant damage was caused to the load break switch and other subcomponents in the enclosure.
When the fault occurred, the 13.8kv breaker supplying the step-down transformer should -have tripped to isolate the fault. That breaker was slow to operate, allowing fault current of sufficient magnitude and duration to trip the main feeder breaker supplying the "A" 13.8kv bus. Although itU.S. NUCLEAR REGULATORY COMMISSION (9-2007)
LICENSEE EVENT REPORT (LER)
- 1. FACILITY NAME
- 2. DOCKET
- 6. LER NUMBER
- 3. PAGE YEAR SEQUENTIAL I REVISION I
NUMBER NUMBER River Bend Station -Unit 1 105000-458 I2007 006 00 I3 OF 4
could not be conclusively determined, the slow operation of the 13.8kv breaker to the transformer was likely due to hardened, dirty grease in the breaker mechanism.
When the "A" 13.8kv bus feeder breaker tripped, both the-running condensate pumps tripped, resulting in a low suction header pressure to the reactor feedwater (SJ)-pumps. (The running feedwater pumps were powered by the "B' 13.8kv bus, and were not directly affected by the fault.)
This protective feature caused an automatic trip of the feedwater pumps after a designed time delay.
When the reactor operator recognized the condition, he manually initiated a reactor scram before the low water level scram signal could automatically actuate.
Reactor water level decreased to the Level 2 setpoint where the HPCS and ROIC systems automatically actuated. As reactor water level then started to increase, operators took manual control of those systems in accordance with procedural guidance, and stabilized water level. The operators manually closed the inboard main steam isolation valves to control the reactor cooldown rate. Due to the loss of electrical power, it was not possible to maintain vacuum in the main condenser, which necessitated the closure of the outboard MSIVs. The operators then manually cycled reactor safety-relief valves (SRVs) to maintain reactor pressure within a prescribed band.
Power was restored to the "A" 13.8kv bus at 12:45 p.m. that day. The residual heat removal "A" loop was placed in the shutdown cooling mode of operation at 4:26 p.m. Mode 4 (cold shutdown) was entered at 7:42 p.m.
The damaged transformer was isolated to preserve evidence for the investigation. The power supply leads to that transformer were disconnected from the 13.8kv bus.
CORRECTIVE ACTIONS TO PREVENT RECURRENCE A detailed examination of the damaged equipment was performed to determine the source of the fault. Company and vendor specialists assisted the site staff in retrieving and analyzing recorded data. This information was used to reconstruct the detailed sequence of events and to investigate the response of the affected circuit breakers.
Numerous corrective actions are being taken in response to this event.
- Rodent traps have been placed in vulnerable areas.
- A contract for a rodent abatement program for buildings inside the protected area will be instituted.
- Preventative maintenance (PM) procedures for switchgear and transformer enclosures will be revised to provide guidance on inspecting the equipment for signs of rodent intrusion.
- The existing plan for scheduled overhauls of Magneblast circuit breakers will be accelerated for completion in the upcoming refueling outage beginning in January 2008 (except for one breaker which is not in service).
- PM frequencies will be increased for switchgear located in buildings outside the power block to ensure that breaker grease remains fresh.U.S. NUCLEAR REGULATORY COMMISSION (9-2007)
LICENSEE EVENT REPORT (LER)
- 1. FACILITY NAME
- 2. DOCKET
- 6. LER NUMBER
- 3. PAGE YEAR SEQUENTIAL I REVISION I NUMBER NUMBER River Bend Station -Unit 1 105000-458 I2007 006 00 I4 OF 4
PREVIOUS OCCURRENCE EVALUATION No reactor scram with a similar initiating condition has occurred at River Bend Station in the last ten years.
SAFETY SIGNIFICANCE
At the time of this event, there were no safety-related systems out of service. The reactor operator responded to the event by manually actuating a reactor scram before reactor water level reached the automatic trip setpoint. Operators promptly established control of reactor pressure and water level, and proceeded with a
plant cooldown until the shutdown cooling system was put into service.
No reactor SRVs were required to operate in the safety mode. All affected safety-related systems operated as designed. This event was of minimal safety significance to the health and safety of the public.