05000368/LER-2013-004, Regarding Fire and Explosion of the Unit Auxiliary Transformer Resulted in an Automatic Reactor Scram and Initiation of the Emergency Feedwater System
| ML14037A205 | |
| Person / Time | |
|---|---|
| Site: | Arkansas Nuclear  |
| Issue date: | 02/05/2014 |
| From: | Jeremy G. Browning Entergy Operations |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| 2CAN021401 LER 13-004-00 | |
| Download: ML14037A205 (7) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation 10 CFR 50.73(a)(2)(i)(A), Completion of TS Shutdown 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(i) 10 CFR 50.73(a)(2)(ii)(A), Seriously Degraded 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition 10 CFR 50.73(a)(2)(iii) 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)(vii), Common Cause Inoperability 10 CFR 50.73(a)(2)(viii)(A) 10 CFR 50.73(a)(2)(viii)(B) 10 CFR 50.73(a)(2)(ix)(A) 10 CFR 50.73(a)(2)(x) 10 CFR 50.73(a)(2)(iv)(B), System Actuation 10 CFR 50.73(b)(5) |
| 3682013004R00 - NRC Website | |
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2CAN021401 February 5, 2014 U. S. Nuclear Regulatory Commission Attn: Document Control Desk Washington, DC 20555-0001
Subject:
Licensee Event Report 50-368/2013-004-00 Automatic Reactor Trip and Emergency Feedwater Actuation Arkansas Nuclear One - Unit 2 Docket No. 50-368 License No. NPF-6
Dear Sir or Madam:
Pursuant to the reporting requirements of 10 CFR 50.73(a)(2)(iv)(A), attached is the subject Licensee Event Report concerning an automatic reactor trip on December 9, 2013, with a subsequent Emergency Feedwater actuation.
There are no new commitments contained in this submittal. Should you have any questions concerning this issue, please contact Stephenie Pyle, Manager, Regulatory Assurance at 479-858-4704.
Sincerely, Original Signed by Jeremy G. Browning JGB/car Attachment: Licensee Event Report 50-368/2013-004-00 10CFR 50.73 Entergy Operations, Inc.
1448 S.R. 333 Russellville, AR 72802 Tel 479-858-3110 Jeremy G. Browning Site Vice President Arkansas Nuclear One
2CAN021401 Page 2 of 2 cc:
Mr. Marc L. Dapas Regional Administrator U. S. Nuclear Regulatory Commission Region IV 1600 East Lamar Boulevard Arlington, TX 76011-4511 NRC Senior Resident Inspector Arkansas Nuclear One P.O. Box 310 London, AR 72847 Institute of Nuclear Power Operations 700 Galleria Parkway Atlanta, GA 30339-5957 LEREvents@inpo.org
NRC FORM 366 (10-2010)
U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY OMB: NO. 3150-0104 EXPIRES: 10/31/2013
, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.
LICENSEE EVENT REPORT (LER)
(See reverse for required number of digits/characters for each block)
- 1. FACILITY NAME Arkansas Nuclear One - Unit 2
- 2. DOCKET NUMBER 05000368
- 3. PAGE 1 OF 5
- 4. TITLE Fire and Explosion of the Unit Auxiliary Transformer resulted in an Automatic Reactor Scram and Initiation of the Emergency Feedwater System
- 5. EVENT DATE
- 6. LER NUMBER
- 7. REPORT DATE
- 8. OTHER FACILITIES INVOLVED MONTH DAY YEAR YEAR SEQUENTIAL NUMBER REV NO.
MONTH DAY YEAR FACILITY NAME Arkansas Nuclear One - Unit 1 DOCKET NUMBER 05000313 12 09 2013 2013
- - 004 -
00 02 05 2014 FACILITY NAME DOCKET NUMBER
- 9. OPERATING MODE 1
- 11. THIS REPORT IS SUBMITTED PURSUANT TO THE REQUIREMENTS OF 10 CFR §: (Check all that apply) 20.2201(b) 20.2201(d) 20.2203(a)(1) 20.2203(a)(2)(i) 20.2203(a)(2)(ii) 20.2203(a)(2)(iii) 20.2203(a)(2)(iv) 20.2203(a)(2)(v) 20.2203(a)(2)(vi) 20.2203(a)(3)(i) 20.2203(a)(3)(ii) 20.2203(a)(4) 50.36(c)(1)(i)(A) 50.36(c)(1)(ii)(A) 50.36(c)(2) 50.46(a)(3)(ii) 50.73(a)(2)(i)(A) 50.73(a)(2)(i)(B) 50.73(a)(2)(i)(C) 50.73(a)(2)(ii)(A) 50.73(a)(2)(ii)(B) 50.73(a)(2)(iii) 50.73(a)(2)(iv)(A) 50.73(a)(2)(v)(A) 50.73(a)(2)(v)(B) 50.73(a)(2)(v)(C) 50.73(a)(2)(v)(D) 50.73(a)(2)(vii) 50.73(a)(2)(viii)(A) 50.73(a)(2)(viii)(B) 50.73(a)(2)(ix)(A) 50.73(a)(2)(x) 73.71(a)(4) 73.71(a)(5)
OTHER Specify in Abstract below or in (Continued)
A reactor and main turbine trip normally fast-transfers power to in-house loads from 2X-02 to 2X-03. With 2X-03 unavailable due to the lockout of the switchyard auto transformer, the system automatically transferred one train of in-house loads to the redundant offsite power source provided by ANO-2 Startup 2 Transformer (X-04) [EA][XFMR] which is designed to be shared between ANO-1 and ANO-2. The remaining in-house electrical train initially de-energized followed by a start of 2K-4B, restoring power to the vital electrical equipment on the affected train. All safety systems remained available. Because the non-vital 6900V buses are not designed to remain energized in this configuration, power to reactor coolant pumps [AB][P] was not available, Operations established safe and stable plant operation using natural circulation cooling. The station Fire Brigade assisted by the local fire department responded to the 2X-02 explosion and contained the fire, which was extinguished approximately 1.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br /> following the initial event.
At the time of the event, ANO-1 was operating at approximately 100% power with in-house loads powered from the ANO-1 Unit Auxiliary Transformer (X-02) [EA][XFMR].
C. Event Cause
Based on the physical evidence available, the initial fault is suspected to have occurred at the C phase 6900V flexible link on the 2X-02 6900V non-segregated bus, which propagated to the associated C phase bus. Damage from the explosion led to phase-to-phase and phase-to-ground faults on the 6900V and 4160V buses. Based on observations of the 2X-02 A and B phase flexible links at this location, there was evidence of corona exposure on the tapping around the bolted connections, moisture and corrosion on the copper flexible links, and no vendor recommended putty on the bolt heads. Without the putty, partial discharge (corona) occurred which degraded the tape insulation. The flexible links and insulation have been installed in this configuration since at least 1979. In addition, the duct design air gap in the flex link area had a marginal air gap as compared to applicable electrical codes that combined with the lack of putty lowered margin for fault protection.
2X-02 is protected by various protective relays including high speed phase differential relays to actuate the main generator lockout relays for isolation of the transformer and the associated fault. Upon fault detection, these relays are designed to initiate prompt actuation of the main generator lockout relays that open the main generator output breakers, exciter field breaker, and associated 4160V and 6900V bus breakers. Although the relays did actuate during this event as evidenced by the instantaneous element target flags, subsequent inspections identified the output contact for the 2X-02 differential relays were not terminated.
Failure of the relays to clear the fault allowed 2X-02 to source the fault for approximately 4 to 5 seconds prior to its failure, which exceeds the typical maximum through-fault current rating of 2 seconds for this class of transformer.
(Continued)
A Root Cause Evaluation (RCE) for the initial bus fault determined that the initial construction of the bus did not utilize the required putty around the bolted connections of the failed flexible link as required by the vendor technical manual. The absence of this putty is suspected to have led to a breakdown in the taped insulation caused by partial discharge or corona of the air space around the bolted flex connections. A contributing cause identified an inadequate design associated with the minimum air gap clearances between the flex links and the duct that in combination with a breakdown in the insulation of the bolted connection and allowed a phase-to-ground fault to occur. A second contributing cause was insufficient periodic maintenance necessary to identify insulation degradation over time at the location of the flexible link. The RCE for the fire and catastrophic failure of 2X-02 was determined to be a latent (1995) human performance error associated with the output contact for the 2X-02 differential relays which was not terminated, thus rendering the relays incapable of actuating the main generator lockout relays. This error is suspected to have occurred during implementation of a 1995 modification.
D. Corrective Actions
Corrective actions include replacement of the unit auxiliary transformer and its associated 4160V and 6900V buses with redesigned buses and ducts, revisions to the non-segregated bus Periodic Maintenance program for inspections, protective relay testing and reviews for controls for lifted leads.
E. Safety Significance Determination Systems and components required to shutdown the reactor, maintain safe shutdown conditions, remove residual heat, and control the release of radioactive material were available and performed as required. There were no actual consequences related to the subject event with regards to Nuclear Safety.
F. Basis for Reportability This event is reported pursuant to the 10 CFR 50.73(a)(2)(iv)(A): "Any event or condition that resulted in manual or automatic actuation of any of the systems listed in paragraph (a)(2)(iv)(B).
Applicable systems in 10 CFR 50.73(a)(2)(iv)(B):
Reactor protection system (RPS) including: reactor scram or reactor trip and PWR auxiliary or emergency feedwater system.(10-2010)
LICENSEE EVENT REPORT (LER)
CONTINUATION SHEET U.S. NUCLEAR REGULATORY COMMISSION
- 1. FACILITY NAME
- 2. DOCKET
- 6. LER NUMBER
- 3. PAGE Arkansas Nuclear One - Unit 2 05000368 YEAR SEQUENTIAL NUMBER REV.
NO.
5 OF 5 2013
- - 004 -
00
G. Additional Information
10 CFR 50.73(b)(5) states that this report shall contain reference to any previous similar events at the same plant that are known to the licensee. NUREG-1022, Revision 3 reporting guidance states that the term "previous occurrences" should include previous events or conditions that involved the same underlying concern or reason as this event, such as the same root cause, failure, or sequence of events.
A review of the ANO corrective action program and Licensee Event Reports for the previous three years revealed no relevant similar events.
Energy Industry Identification System (EIIS) codes and component codes are identified in the text of this report as [XX].