05000499/LER-2010-005-02, Regarding Startup Feed Pump 24 Breaker Failure and Unit 2 Reactor Trip

Regarding Startup Feed Pump 24 Breaker Failure and Unit 2 Reactor Trip

ML11195A021
Person / Time
Site:	South Texas
Issue date:	07/07/2011
From:	Peter L South Texas
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
NOC-AE-11002693, STI: 32893768 LER 10-005-02
Download: ML11195A021 (7)

LER-2010-005, Regarding Startup Feed Pump 24 Breaker Failure and Unit 2 Reactor Trip

Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(iv)(A), System Actuation 10 CFR 50.73(a)(2) 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)(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)(B), Prohibited by Technical Specifications
4992010005R02 - NRC Website

text

ILMIpMN Nuclear Operating Company South Texas Project Electric Genmratin$ Station P.. Box 289 Wadsworth, Tcxas 77483 x *n NOC-AE-1002693 File No.: G25 10 CFR 50.73 STI: 32893768 U. S. Nuclear Regulatory Commission Attention: Document Control Desk One White Flint North 11555 Rockville Pike Rockville, MD 20852-2738 South Texas Project Unit 2 Docket No. STN 50-499 Revision 2 of Licensee Event Report 2010-005 Startup Feed Pump 24 Breaker Failure and Unit 2 Reactor Trip

References:

1. Letter dated January 3, 2011, from L. W. Peter, STPNOC, to NRC Document Control Desk, "Licensee Event Report 2010-005 Startup Feed Pump 24 Breaker Failure and Unit 2 Reactor Trip," (NOC-AE-10002630) (ML110070064)

2. Letter dated April 4, 2011, from L. W. Peter, STPNOC, to NRC Document Control Desk, "Revision 1 of Licensee Event Report 2010-005 Startup Feed Pump 24 Breaker Failure and Unit 2 Reactor Trip," (NOC-AE-1 0002659) (ML111010019)

Pursuant to 10 CFR 50.73, STP Nuclear Operating Company (STPNOC) submits the attached Unit 2 Licensee Event Report (LER) 2010-005 Revision 2 to address the Unit 2 Reactor trip that occurred on November 3, 2010.

This condition is considered reportable under 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) of this section.

This event did not have an adverse effect on the health and safety of the public.

The attached LER is the planned LER supplement described in Ref. 2 above including a correction to the reporting criteria where criterion 20.2203(a)(3)(i) was inadvertently checked. This LER supplement provides the results of the respective Root Cause Evaluation and revises the appropriate reporting criteria accordingly.

There are no commitments contained in this LER.

If there are any questions on this submittal, please contact either J. A. Loya at (361) 972-8005 or me at (361) 972-7158.

L. W. Peter Plant General Manager JAL Attachment: Revision 2 of Licensee Event Report 2010-005

NOC-AE-1 1002693 Page 2 of 2 cc:

(paper copy)

(electronic copy)

Regional Administrator, Region IV U. S. Nuclear Regulatory Commission 612 East Lamar Blvd, Suite 400 Arlington, Texas 76011-4125 Balwant K. Singal Senior Project Manager U.S. Nuclear Regulatory Commission One White Flint North (MS 8 B13) 11555 Rockville Pike Rockville, MD 20852 Senior Resident Inspector U. S. Nuclear Regulatory Commission P. 0. Box 289, Mail Code: MN1 16 Wadsworth, TX 77483 C. M. Canady City of Austin Electric Utility Department 721 Barton Springs Road Austin, TX 78704 A. H. Gutterman, Esquire Morgan, Lewis & Bockius LLP Balwant K. Singal U. S. Nuclear Regulatory Commission John Ragan Catherine Callaway Jim von Suskil NRG South Texas LP Ed Alarcon Kevin Polio Richard Pena City Public Service Peter Nemeth Crain Caton & James, P.C.

C. Mele City of Austin Richard A. Ratliff Texas Department of State Health Services Alice Rogers Texas Department of State Health Services

NRC FORM 366 U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY OMB: NO, 3150.0104 EXPIRES: 10/31/2013 (10-2010)

, the NRC may digits/characters for each block) not conduct or sponsor, and a person is not required to respond to the information collection.

3. PAGE South Texas Unit 2

4. TITLE Startup Feed Pump 24 Breaker Failure and Unit 2 Reactor Trip

5. EVENT DATE

6. LER NUMBER

7. REPORT DATE

8. OTHER FACILITIES INVOLVED MONTH DAY YEAR YEAR SEQUENTIAL REV MONTH DAY YEAR FACILITY NAME DOCKET NUMBER NUMBER NO.

___I N/A N/A 11 03 2010 2010 005 2

07 07 2011 N/A

'N/A

9. OPERATING MODE

11. THIS REPORT IS SUBMITTED PURSUANT TO THE REQUIREMENTS OF 10CFR§: (Check all that apply) 11 2012201(b) fl 20.2203(a)(3)(i)

[] 50.73(a)(2)(l)(C)

El 50.73(a)(2)(vii) rl 20,2201 (b)20.2201 20.2203(a)(3)(ii)

El 50.73(a)(2)(ii)(A)

Q 50.73(a)(2)(vili)(A) jJ20,22053(a.)(1) 202203(a)(4)

_J 573(a)(2)(1I)(B)

~

J50?(a)(2)(viii)(B)

10. POWER LEVEL El 20.2203(a)(2)(Q El 5.36(c)(1)(i)(A)

Q 50.73(a)(2)(i11)

[]

5073(a)(2)(ix)(A) 100%

20,2203[a)(2)(ii) 5&36(c)(1)(II(A) 51173(a)(2)(iv)(A) 511 3(a)(2)(x) 2l 2L.2203(a)(2)(1ii)

[]

5036(c)(2)

]

50.73(a)(2)(v)(A)

[]

73.71(a)(4)

[] 202203(a)(2)(Iv) r' 50.46(a(3)(Ii)

[J 50.73(a)(2)(v)(B)

[0 73.711(a)(5) rn 22203(a)(2)(v) 5073()(2)()(A)

El 573(a)(2)(v)(C)

E OTHER Specify in Abstract below El 20.2203(a)(2)(vi)

El 50.73(a)(2)(i)(B)

El 5%73(a)(2)(v)(D)

Or in NRC Form 366A

12. LICENSEE CONTACT FOR THIS LER FACILITY NAME TELEPHONE NUMBER (Include Area Code)

Joe Loya, Licensing Engineer 361-972-8005r YES (if yes, complete 15. EXPECTED SUBMISSION DATE)

ABSTRACT (Limit to 1400 spaces, i.e., approximately 15 single-spaced typewritten lines)

On November 3, 2010, the Startup Feed Pump (SUFP) 24 was being started to support a scheduled Partial Discharge Analysis as a preventive maintenance activity. At 1021 hours0.0118 days <br />0.284 hours <br />0.00169 weeks <br />3.884905e-4 months <br /> the SUFP was started but then after approximately 4 seconds the pump breaker tripped open. Computer data shows that approximately 8 seconds after the pump breaker tripped the voltage on Standby Bus 2H (which supplies power to the SUFP) spiked low to near zero volts and the Unit 2 reactor tripped on Reactor Coolant Pump (RCP) Undervoltage (the 2C RCP is also powered by Standby Bus 2H). Standby Diesel Generator 23 started and began supplying Engineered Safety Features (ESF) Bus E2C due to Loss of Offsite Power to the ESF bus.

Standby Bus 2F experienced a momentary voltage drop of approximately 1 second (Standby Buses 2F and 2H are both fed by the X winding of the Unit 2 Auxiliary Transformer) resulting in some A train loads being secured. Following the reactor trip, the plant was stabilized in MODE 3 at Normal Operating Pressure and Temperature.

At 1038 hours0.012 days <br />0.288 hours <br />0.00172 weeks <br />3.94959e-4 months <br /> an Unusual Event was declared for Unit 2 due to the breaker cubicle explosion associated with the SUFP breaker failure (ENS Event Number 46387). The breaker malfunction did not result in a fire. The Unusual Event was terminated at 1240 hours0.0144 days <br />0.344 hours <br />0.00205 weeks <br />4.7182e-4 months <br /> when the plant was stabilized in MODE 3.

There were no personnel injuries, no offsite radiological releases, and no damage to safety-related equipment.

This LER is the planned LER supplement described in Revision 1 including a correction to the reporting criteria where criterion 20.2203(a)(3)(i) was inadvertently checked. This LER supplement provides the results of the respective Root Cause Evaluation and revises the appropriate reporting criteria accordingly.

NRC FORM 366 (10-2010)

SUMMARY OF THE EVENT On November 3, 2010, the Startup Feed Pump (SUFP) 24 was being started to support a scheduled Partial Discharge Analysis as a preventive maintenance activity. Following the pre-job brief, a Licensed Operator Training (LOT) Trainee made a plant announcement and then placed the hand switch to Start at 1021 hours0.0118 days <br />0.284 hours <br />0.00169 weeks <br />3.884905e-4 months <br />. The SUFP started but then after approximately 4 seconds the pump breaker tripped open. Computer data shows that approximately 8 seconds after the pump breaker tripped the voltage on Standby Bus 2H (which supplies power to the SUFP) spiked low to near zero volts and the Unit 2 reactor tripped on Reactor Coolant Pump (RCP)

Undervoltage (the 2C RCP is also powered by Standby Bus 2H). Standby Diesel Generator (SDG) 23 started and began supplying Engineered Safety Features (ESF) Bus E2C due to Loss of Offsite Power (LOOP) to the ESF bus. Standby Bus 2F experienced a momentary voltage drop of approximately 1 second duration (Standby Buses 2F and 2H are both fed by the X winding of the Unit 2 Auxiliary Transformer) resulting in some A train loads being secured, however the low voltage condition cleared on Bus 2F prior to the point at which associated time delay relays would have started Standby Diesel Generator 21 (ESF Train A). Following the reactor trip, the plant was stabilized in MODE 3 at Normal Operating Pressure and Temperature.

At 1038 hours0.012 days <br />0.288 hours <br />0.00172 weeks <br />3.94959e-4 months <br /> an Unusual Event was declared for Unit 2 due to the breaker cubicle explosion associated with the SUFP breaker failure (ENS Event Number 46387). The breaker malfunction did not result in a fire. The Unusual Event was terminated at 1240 hours0.0144 days <br />0.344 hours <br />0.00205 weeks <br />4.7182e-4 months <br /> when the plant was stabilized in MODE 3.

An Operator in the Turbine Generator Building (TGB) reported substantial damage had occurred to the SUFP Breaker. The front door and access panels for Cubicle 1A in 13.8 KV Standby Bus 2H had been blown open or deformed.

FORM 366 (10-2010)LICENSEE EVENT REPORT (LER)

U.S. NUCLEAR REGULATORY COMMISSION CONTINUATION SHEET

1. FACILITY NAME

2. DOCKET

6. LER NUMBER

3. PAGE YEAR SEQUENTIAL REV. NO South Texas Unit 2 05000499 NUMBER 3OF5 2010 005 02 All prO[eCUve relay s red flags were actuated on Lhe burr "IToior ureaKer cubicle except Tor me lower unit of the 46 current balance relay. Most of the relay flags were probably caused by shock/vibration from the breaker explosion except for the 86 Lockout Relay, which requires rotary motion to actuate. It is unclear whether the 86 Lockout Relay was actuated by shock/vibration causing one of the associated protective relays (50/51, 51 G, 87, 46) to momentarily close an output contact resulting in the electrical actuation of the 86 relay or whether the 86 relay was actuated when one or more of the protective relays was actuated by a valid signal.

The three arc chute assemblies (one per phase) showed signs of damage and were dark with black soot. The three blow-out coil return straps showed substantial damage. These straps are approximately 1 inch wide by 1/8 inch thick. When a breaker trips and the contacts open to interrupt the current, the resulting arcs from the arcing contacts transfer to the blow-out coil assembly, which includes the blow-out coil return straps. The coils produce a magnetic field which helps to push the arcs into the arc chutes where the arcs are dissipated and cooled. The three return straps had each been melted through which indicates multiple arcs had existed across the breaker contacts during the event.

E.

METHOD OF DISCOVERY

The breaker failure, reactor trip, and automatic actuation of the systems listed below were self-revealing.

II. EVENT-DRIVEN INFORMATION A.

SAFETY SYSTEMS THAT RESPONDED All required safety systems responded as expected including the following actuations:

1. Reactor Coolant Pump Undervoltage Reactor Trip

2. Reactor Protection System P-1 6, Turbine Trip

3. Feedwater Isolation Actuation

4. CRE HVAC Emergency Recirculation (C Train LOOP)

5. Reactor Containment Fan Coolers (C Train LOOP)

6. Auxiliary Feedwater Actuation (All AFW pumps actuated)

7. Primary Pressure Control (Pressurizer Spray and Heaters actuated as required)

8. Secondary Pressure Control Actuation (Steam Dumps Actuated)

B.

DURATION OF SAFETY SYSTEM INOPERABILITY

N/A FORM 366 (10-2010)LICENSEE EVENT REPORT (LER)

U.S. NUCLEAR REGULATORY COMMISSION CONTINUATION SHEET

1. FACILITY NAME

2. DOCKET

6. LER NUMBER

3. PAGE YEAR SEQUENTIAL REV. NO South Texas Unit 2 05000499 NUMBER 4OF5 2010 005 02 C.

SAFETY CONSEQUENCES AND IMPLICATIONS OF THE EVENT I

v.

There was no impact to radiological safety, safety of the public, or safety of station personnel during this event.

The Incremental Conditional Core Damage Probability (ICCDP) for the Reactor Trip in Unit 2 on November 3, 2010 is 2.82E-07.

The resulting Incremental Conditional Large Early Release Probability (LERP) given a turbine trip is 7.21 E-09.

Ill. CAUSE OF THE EVENT The root cause of the breaker failure in 13.8 KV Standby Switchgear 2H / Cubicle 1 A (SUFP 24) and the resulting Unit 2 reactor trip is that the arc failed to extinguish when the breaker opened due to moisture induced contamination shorting out the arc runner spacer between the first and second blow out coils in the upper arc chute assembly.

This caused the arc current to bypass the second coil rather than running through the coil so a magnetic field was not created in the second coil that would have driven the arc further into the arc chute.

This caused damage to the arc chutes and caused plasma (i.e., ionized air) in the cubicle. Eventually the plasma caused a conduction path in the cubicle for the three phases to ground.

The grounding of the three phases caused the voltage on the associated buses to drop to zero and resulted in an explosion (pressure wave) that caused physical damage to the cubicle. The voltage drop caused the Unit 2 reactor to trip on Reactor Coolant Pump (RCP) undervoltage.

IV. CORRECTIVE ACTIONS

1. Revise Preventive Maintenance (PMs) for the 13.8 KV and 4160 V breakers to:

(1) Include AC Hi-Pot testing of the whole breaker including arc chutes; (2) Include resistance measurements of the whole arc path through the arc runners and blow out coils [Digital Low Resistance Ohm (DLRO) or ductor testing];

(3) Specify that velocity testing of the breaker opening is preferred over the currently performed speed testing.

(4) Perform "as found" velocity or speed testing at the beginning of the PMs rather than just performing the tests at the end of the PMs.

2. Revise procedures OPMP05-NA-0001, 0PMP05-NA-0005, and OPMP05-NA-0009 to include the following:

(1) Address and match the changes made to the 13.8 KV and 4160 V PMs (as discussed above);

(2) Ensure that the DLRO/ductor testing includes the whole arc path through the arc runners and blow out coils [currently just checks resistance of breaker contacts];

FORM 366 (10-2010)LICENSEE EVENT REPORT (LER)

U.S. NUCLEAR REGULATORY COMMISSION CONTINUATION SHEET

1. FACILITY NAME

2. DOCKET

6. LER NUMBER

3. PAGE YEAR SEQUENTIAL REV. NO South Texas Unit 2 05000499 NUMBER 5OF5 2010 005 02 (3) Address possible misassembly of arc chutes as discussed in the root cause report.

3. Evaluate the adequacy of the cubicle heaters in the 2H/1A cubicles (SUFPs 14 and 24.

V. PREVIOUS SIMILAR EVENTS

There have been no similar events within the last three years.

VI. ADDITIONAL INFORMATION

None.

FORM 366 (10-2010)