05000271/LER-2004-003, Regarding Automatic Reactor Scram Due to a Main Generator Trip as a Result of an Iso-Phase Bus Duct Two-Phase Electrical Fault
| ML042440686 | |
| Person / Time | |
|---|---|
| Site: | Vermont Yankee File:NorthStar Vermont Yankee icon.png |
| Issue date: | 08/16/2004 |
| From: | Bronson K Entergy Nuclear Northeast |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| BVY 04-080 LER 04-003-00 | |
| Download: ML042440686 (5) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition 10 CFR 50.73(a)(2)(ix)(A) 10 CFR 50.73(a)(2)(x) 10 CFR 50.73(a)(2)(iv)(A), System Actuation 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 |
| 2712004003R00 - NRC Website | |
text
-t
- - Entergy Entergy Nuclear Northeast Entergy Nuclear Operations, Inc.
Vermont Yankee 185 Old Ferry Rd.
RO. Box 500 Brattleboro, Vr 05302 Tel 802-257-5271 August 16, 2004 BVY 04-080 U.S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555
Subject:
Vermont Yankee Nuclear Power Station License No. DPR-28 (Docket No. 50-271)
Reportable Occurrence No. LER 2004-003-00 As defined by 10CFR50.73, we are reporting the attached Reportable Occurrence LER 2004-003-00. No Regulatory Commitments have been generated as a result of this event.
Sincerely, Entergy Nuclear Operations, Inc.
Vermont Yankee Kevin Bronson General Manager cc:
USNRC Region I Administrator USNRC Resident Inspector - VYNPS USNRC Project Manager - VYNPS Vermont Department of Public Service
NRC.kORM 366 U.S NUCLEAR REGULATORY APPROVED BY OMB NO. 3150-0104 EXPIRES 7-31-2004 l (7-2001)
COMMISSION Esfirnated burden per response bo comply with is rnandatory information cofectron request 50 hours5.787037e-4 days <br />0.0139 hours <br />8.267196e-5 weeks <br />1.9025e-5 months <br />. Reported lessons learned are Incorporated into the kensing process and fed back to industry. Send comrnrents regarding burden esrniate to the Records Managerment Branch (T-6 E6), U.S. Nudear Regulatory Cofrrwission, Washington, DC 20555-LICENSEE EVENT REPORT (LER) 0001,or by internet e-rnal to bis @nrc.gov, and to the Desk Ofcer, Office of Information and Regulatory Affairs, NEOB-(See reverse lor required number of 10202 (3150104) Ofce of Managernent and Budget. Washington, DC 20503.1/ a rneans used to irnpose infrmnation d(~See iraev erse for re ahre number f(
collecti on does not display a currently vatid OMB control riunner. the NRC may not conduct or sponso, arnd a person is digits/characters for each block) rnot reruired to respond to, the infornation collection.
- 4. TITLE Automatic Reactor Scram due to a Main Generator Trip as a result of an Iso-Phase Bus Duct Two-Phase Electrical Fault
- 5. EVENT DATE
- 6. LER NUMBER
- 7. REPORT DATE
- 8. OTHER FACILITIES INVOLVED FACILITY NAME DOCKET NUMBER MO DAY YEAR YEAR SEQUENTIAL REV MO DAY YEAR N
05000-NUMBER NO
____N/A J_______
Z II FACILITY NAME DOCKET NUMBER 06 18 2004 2004 003 00 08 16 2004 NIA N
05000-
- 9. OPERATING
- 11. THIS REPORT IS SUBMITTED PURSUANT TO THE REQUIREMENTS OF 10 CFR §: (Check all that apply)
MODE N
O l 20.2201(b) 20.2203(a)(3)(ii)
F1 50.73(a)(2)(ii)(B)
__50.73(a)(2)(ix)(A) 10 POWER 20.2201 (d)
LI 20.2203(a)(4)
El 50.73(a)(2)(x)
LE100 20.2203(a)(1) n 50.36(C)(1)(i)(A)
__ 50.73(a)(2)(iv)(A) r73.71 (a)_(4) 50.36_____
9c____1____
i 50.73(a)(2)(iv)(A) 73.71 (a)(5)
[71. - ;20.2203(a)(2)(i) 50.36(c)(1)(ii)(A)
[71 20.2203(a)(2)(ii)
[
50.36(c)(2)
[
50.73(a)(2)(v)(B)
OTHER
~ LI 20.203()(2)iii)~
- - Specify in Abstract below or In 20.2203(a)(2)(iii) 50.46(a)(3)(ii) a 50.73(a)(2)(v)(C)
Abstract
On 06/18/04 at 0640, with the plant at full power, a turbine load reject scram occurred due to a two phase electrical fault to ground on the 22 kV iso-phase bus. All safety systems responded as designed and the reactor was shutdown without incident. Offsite power sources and station emergency power sources were available throughout the event. Arcing and heat generated during the fault damaged an area around the iso-phase bus ducts and Main Transformer low voltage bushings. The electrical faults disrupted an oil line flange between the Main Transformer oil conservator (expansion tank) and the UC" phase low voltage bushing box, and the leaking oil ignited. Fire suppression systems activated automatically. An Unusual Event was declared at 0650 for a fire lasting greater than 10 minutes. The VY fire brigade and local community fire departments extinguished the oil fire at 0717. At 1245, the Unusual Event was terminated. The electrical grounds that initiated the event were caused by loose material in the "B" iso-phase bus duct as a result of the failure of a flexible connector. The grounds raised the voltage on the 'A" iso-phase bus contributing to the failure of the "A" phase surge arrester. The root causes of the event were determined to be inadequate preventative maintenance on portions of the iso-phase bus and failure to monitor age related degradation on the surge arresters. There was no release of radioactivity or personnel injury during this event.
NRC FORMM 366 (7-2001)
l (If more space is required, use additional copies of NRC Form 366A)
DESCRIPTION
On 06/18/04 at 0640, with the plant operating at full power, a two-phase electrical fault-to-ground occurred on the 22kV System (EIIS=IPBU, BDUC). The 'B" phase faulted to ground in the low voltage bushing box on top of the Main Transformer (EIIS=XFMR), and the "A" phase faulted to ground in the surge arrester cubicle of the Generator Potential Transformer (PT) Cabinet through the "A" phase surge arrester (EIIS=LAR).
Within less than one cycle (11 milliseconds) of the initial electrical fault, the Main Generator protective relaying sensed the condition and isolated the generator from the grid within the following 5 cycles (80 milliseconds). A generator load rejection reactor scram then occurred. Approximately 400 milliseconds following the initial electrical faults to ground from 'A" and "B" phases, arcing and ionization in the "B" phase low voltage bushing box carried over to the "C" phase low voltage bushing box on top of the Main Transformer. The electrical faults disrupted a flange in the oil piping between the Main Transformer oil conservator (expansion tank) and the "C" phase low voltage bushing box. The arcing or heat from the fault ignited the oil, resulting in a fire. Fire suppression systems activated automatically as expected.
The plant response following the scram was as expected, with the exception that both Recirculation pumps tripped and other AC voltage effects were observed as a result of the voltage transient associated with the high fault current. All safety systems functioned as designed and the reactor was shutdown without incident.
There was no release of radioactivity and no personnel injuries.
The VY fire brigade was dispatched at 0641. An Unusual Event was declared at 0650 due to "Any unplanned on-site or in-plant fire not extinguished within 10 minutes". The VY fire brigade initiated fire hose spray from a nearby hydrant and quenched the fire. Local fire departments began arriving at 0705. The fire was completely extinguished at approximately 0717and re-flash watches were established. Offsite power sources and station emergency power sources were available at all times throughout the event.
The States of Vermont, New Hampshire and Massachusetts were provided with initial notification of the event at 0721. The NRC Operations Center was notified of the event at 0748, recorded as NRC Event 40827. In addition to the declaration of the emergency classification, a 4-Hour NRC Non-Emergency Notification was completed due to an RPS actuation with the reactor critical, pursuant to 10 CFR 50.72(b)(2)(iv)(B). At 1245, the Unusual Event was terminated.
The isophase bus flexible connector that failed (expansion joints) was part of the original bus supplied and designed by H.K. Porter, Drawing Numbers G-1 91144 & G-191146. All flexible connectors were replaced with an upgraded design supplied by Delta-Unibus. The surge suppressors were GE Alugard Station Arrestors, Model Number 9L1 1 LAB, installed as original plant equipment. All of the surge suppressors were replaced.
CAUSES
The electrical grounds that initiated the event were caused by loose material in the "B" iso-phase bus duct as a result of the failure of a flexible connector (EIIS=FCON) that allows the iso-phase bus to thermally expand and contract. The grounds raised the voltage on the "A" iso-phase bus, contributing to the failure of the "A" phase surge arrester. The root causes of the event were determined to be inadequate preventative maintenance for cleaning and inspections during outages and failure to monitor age related degradation.
Although the iso-phase bus is subjected to preventative maintenance cleaning and Doble Testing each refueling outage, the cleaning and inspection is limited to the stand-off insulators. Additional inspections to evaluate the condition of the bus (including its flexible connectors) would have detected the degraded flexible connectors or the presence of loose/foreign material with the potential to ground the bus. The need for (If more space is required, use additional copies of NRC Form 366A) (17)
Prior to Plant Start Up:
- 1. The phase A, B, and C 22 kV surge arresters and capacitors were replaced prior to energizing the 22kV bus.
- 2. The phase A, B, and C 22 kV flexible connectors were replaced with an upgraded design supplied by Delta-Unibus prior to energizing the 22kV bus.
- 3. A cleanliness inspection was performed and documented as part of Iso-Phase Bus Duct Modification.
- 4. Maintenance department personnel inspected the cooler and leads fans for foreign material. Following operation of the fans, an additional inspection of the fans and coolers was performed.
- 5. Operator Alarm response sheets were revised to enhance operator actions in the event of future ground faults.
- 6. A preventative maintenance schedule was established for increased sampling of transformer oil for the main, auxiliary, and two startup transformers for four weeks after start-up.
- 7. The isophase bus duct system was monitored after assembly with the fans running to ensure that vibration levels are acceptable.
- 8. VY discussed this event and associated issues with the Entergy Fleet and industry experts as necessary to gather information pertinent to the root cause investigation and equipment recovery.
Long Term:
- 1. Include the 22kV surge arresters and capacitors in the preventative maintenance program and define periodic testing requirements.
- 2. Revise the 22kV isophase bus preventative maintenance program and periodic inspection requirements as necessary to improve performance and to prevent recurrence of this event.
- 3. Complete the testing of selected components involved in the event to validate the initial conclusions of the root cause investigation team, and revise the root cause analysis report if needed.
ADDITIONAL INFORMATION
No similar events with a related cause have occurred at Vermont Yankee.