05000259/LER-2011-007, Regarding Multiple Containment System Isolations from Loss of RPS M-G Set 1B

Regarding Multiple Containment System Isolations from Loss of RPS M-G Set 1B

ML11285A048
Person / Time
Site:	Browns Ferry
Issue date:	10/05/2011
From:	Polson K Tennessee Valley Authority
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
LER 11-007-00
Download: ML11285A048 (9)

LER-2011-007, Regarding Multiple Containment System Isolations from Loss of RPS M-G Set 1B

Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(iv)(A), System Actuation 10 CFR 50.73(a)(2)(iv)(B)(2) 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)(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)(iii) 10 CFR 50.73(a)(2)(ix)(A) 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)(i)(A), Completion of TS Shutdown 10 CFR 50.73(a)(2)(v), Loss of Safety Function
2592011007R00 - NRC Website

text

n1VA Tennessee Valley Authority, Post Office Box 2000, Decatur, Alabama 35609-2000 October 5, 2011 10 CFR 50.73 ATTN: Document Control Desk U.S. Nuclear Regulatory Commission Washington, D.C. 20555-0001 Browns Ferry Nuclear Plant, Unit 1 Facility Operating License No. DPR-33 NRC Docket No. 50-259

Subject:

Licensee Event Report 50-25912011-007-00 On August 6, 2011, at 0522 hours0.00604 days <br />0.145 hours <br />8.630952e-4 weeks <br />1.98621e-4 months <br /> Central Daylight Time, Browns Ferry Nuclear Plant Unit 1 Reactor Protection System (RPS) Motor-Generator (M-G) set 1 B failed resulting in a partial loss of power to Primary Containment Isolation System (PCIS) groups and an invalid actuation of those PCIS groups. The combination of RPS M-G set 1 B loss and PCIS group 6 isolation resulted in the isolation of the Drywell Floor Drain Sump and the Drywell Continuous Atmospheric Monitor for both particulate and gaseous activity. Thus, both means of automatic monitoring of Reactor Coolant System leakage became inoperable.

The Tennessee Valley Authority is submitting this report under 10 CFR 50.73(a)(2)(iv)(A),

as any event or condition that resulted in manual or automatic actuation of any of the systems listed in paragraph 10 CFR 50.73(a)(2)(iv)(B)(2), general containment isolation signals affecting containment isolation valves in more than one system. This report also includes reporting under 10 CFR 50.73(a)(2)(i)(B), as any operation or condition which was prohibited by the plant's Technical Specifications.

U.S. Nuclear Regulatory Commission Page 2 October 5, 2011 There are no new regulatory commitments contained in this letter. Should you have any questions concerning this submittal, please contact J. E. Emens, Jr., Nuclear Site Licensing Manager, at (256) 729-2636.

Respectfully, K. J. Poison Vice President

Enclosure:

Licensee Event Report 259/2011-007 Multiple Containment System Isolations from Loss of RPS M-G Set 1B cc (w/ Enclosure):

NRC Regional Administrator - Region II NRC Senior Resident Inspector - Browns Ferry Nuclear Plant

Enclosure Browns Ferry Nuclear Plant Unit I Licensee Event Report 259/2011-007 Multiple Containment System Isolations from Loss of RPS M-G Set I B See Attached

NRC FORM 366 U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY OMB NO. 3150-0104 EXPIRES 10/31/2013 (10-2010)

, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.

3. PAGE Browns Ferry Nuclear Plant Unit 1 05000259 1 of 6

4. TITLE: Multiple Containment System Isolations from Loss of RPS M-G Set 1 B

5. EVENT DATE

6. LER NUMBER

7. REPORT DATE

8. OTHER FACILITIES INVOLVED SEQUENTIALý REV MOT A

ER FACILITY NAME DOCKET NUMBER MONTH DAY YEAR YEAR NUMBER REV MONTH DAY YEAR N/A 05000 FACILITY NAME DOCKET NUMBER 08 06 2011 2011 007 -

00 10 05 2011 N/A 05000

9. OPERATING MODE

11. THIS REPORT IS SUBMITTED PURSUANT TO THE REQUIREMENTS OF 10 CFR §: (Check all that apply)

El 20.2201(b)

E] 20.2203(a)(3)(i)

El 50.73(a)(2)(i)(C)

El 50.73(a)(2)(vii)

El 20.2201(d)

E] 20.2203(a)(3)(ii)

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

El 50.73(a)(2)(viii)(A)

E] 20.2203(a)(1)

El 20.2203(a)(4)

El 50.73(a)(2)(ii)(B)

El 50.73(a)(2)(viii)(B)

E] 20.2203(a)(2)(i)

E] 50.36(c)(1)(i)(A)

E] 50.73(a)(2)(iii)

El 50.73(a)(2)(ix)(A)

10. POWER LEVEL

[] 20.2203(a)(2)(ii)

El 50.36(c)(1)(ii)(A)

[D 50.73(a)(2)(iv)(A)

El 50.73(a)(2)(x)

[1 20.2203(a)(2)(iii)

El 50.36(c)(2)

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

El 73.71(a)(4)

[I 20.2203(a)(2)(iv)

El 50.46(a)(3)(ii)

El 50.73(a)(2)(v)(B)

El 73.71(a)(5) 099

[] 20.2203(a)(2)(v)

El 50.73(a)(2)(i)(A)

El 50.73(a)(2)(v)(C)

[E OTHER E] 20.2203(a)(2)(vi)

ED 50.73(a)(2)(i)(B)

El 50.73(a)(2)(v)(D)

Specify in Abstrac below or in NRC Fonrm 366A

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

Eric Bates, Licensing Engineer 256-614-7180CAUSE SYSTEM COMPONENT MANU-REPORTABLE

CAUSE

SYSTEM COMPONENT MANU-REPORTABLE FACTURER TO EPIX FACTURER TO EPIX E

JC RG G080 Y

14. SUPPLEMENTAL REPORT EXPECTED

15. EXPECTED SUBMISSION MONTH DAY YEAR El YES (Ifyes, complete 15. EXPECTED SUBMISSION DATE)

[

NO DATE N/A N/A N/A ABSTRACT (Limit to 1400 spaces, i.e., approximately 15 single-spaced typewritten lines)

On August 6, 2011, at 0522 hours0.00604 days <br />0.145 hours <br />8.630952e-4 weeks <br />1.98621e-4 months <br /> Central Daylight Time, Browns Ferry Nuclear Plant Unit 1 Reactor Protection System (RPS) Motor-Generator (M-G) set 1B failed resulting in a partial loss of power to Primary Containment Isolation System (PCIS) groups and an invalid actuation of those PCIS groups. PCIS groups 1 and 2 received partial isolation signals with no subsequent system isolations, as designed. PCIS groups 3, 6, and 8 received partial isolation signals with resulting system isolations, as designed. The combination of RPS M-G set 1 B loss and PCIS group 6 isolation resulted in the isolation of the Drywell Floor Drain Sump and the Drywell Continuous Atmospheric Monitor for both particulate and gaseous activity. Thus, both means of automatic monitoring of Reactor Coolant System leakage became inoperable. Unit 1 entered Technical Specification (TS) Limiting Condition for Operation (LCO) 3.4.5 Condition D, all required leakage detection systems inoperable, and immediately entered TS LCO 3.0.3 as required.

Trouble-shooting activities did not identify a specific condition that caused the voltage drop on the RPS M-G set 1 B. However, there were two potential causes: (1) a loose connector or cable assembly at a terminal on the RPS M-G set control panel due to the machine design and operational characteristics (i.e., vibration induced), and (2) discontinuity within the voltage adjustment potentiometer.

NRC FORM 366 (10-2010)

PLANT CONDITION(S)

Browns Ferry Nuclear Plant (BFN) Unit 1 was at approximately 99 percent power when the event occurred.

II.

DESCRIPTION OF EVENT

A. Event On August 6, 2011, at 0522 hours0.00604 days <br />0.145 hours <br />8.630952e-4 weeks <br />1.98621e-4 months <br /> Central Daylight Time (CDT), BFN Unit 1 Reactor Protection System (RPS) [JC] Motor-Generator (M-G) [MG] set 1 B failed resulting in a partial loss of power to Primary Containment Isolation System (PCIS) [BD] groups and an invalid actuation of those PCIS groups. PCIS groups 1 and 2 received partial isolation signals with no subsequent system isolations, as designed. PCIS groups 3, 6, and 8 received partial isolation signals with resulting system isolations, as designed. The combination of loss of RPS M-G set 1 B and PCIS group 6 isolation resulted in the isolation of the Drywell Floor Drain Sump Monitoring System and the Drywell Continuous Atmospheric Monitoring System for both particulate and gaseous activity. Thus, both means of automatic monitoring of Reactor Coolant System (RCS) [IJ] leakage became inoperable. Unit 1 entered Technical Specification (TS) Limiting Condition for Operation (LCO) 3.4.5 Condition D, all required leakage detection systems inoperable, and immediately entered TS LCO 3.0.3 as required. TS LCO 3.0.3 requires Unit 1 to be in MODE 2 within 10 hours1.157407e-4 days <br />0.00278 hours <br />1.653439e-5 weeks <br />3.805e-6 months <br />, MODE 3 within 13 hours1.50463e-4 days <br />0.00361 hours <br />2.149471e-5 weeks <br />4.9465e-6 months <br />, and MODE 4 within 37 hours4.282407e-4 days <br />0.0103 hours <br />6.117725e-5 weeks <br />1.40785e-5 months <br />.

Also, TS LCO 3.3.2.1 Condition A, one rod block monitor channel inoperable, was entered due to the loss of the Rod Block Monitor B.

B. Inoperable Structures, Components, or Systems that Contributed to the Event RPS M-G set 1A was being supplied by its alternate power source, a regulating transformer, for scheduled maintenance. The alternate power source was not available to RPS M-G set 1 B. The alternate power source is interlocked so that both the RPS M-G set 1A and the RPS M-G set 1B cannot be simultaneously fed from the alternate power source.

C. Dates and Approximate Times of Major Occurrences

August 6, 2011 at 0522 CDT Power lost from Unit 1 RPS M-G set 1 B.

August 6, 2011 at 0524 CDT Unit 1 entered TS LCO 3.4.5 Condition D, TS LCO 3.0.3, and TS LCO 3.3.2.1.

August 6, 2011 at 0617 CDT Unit 1 began reducing power from approximately 99 percent power due to the TS LCO 3.0.3 entry.

August 6, 2011 at 1204 CDT Trouble-shooting was completed and power was restored to Unit 1 RPS M-G set 1B and to the RCS leakage monitors; exited TS LCO 3.3.2.1.

August 6, 2011 at 1208 CDT Unit 1 exited TS LCO 3.0.3 and TS LCO 3.4.5 Condition D.

August 6, 2011 at 1548 CDT Unit 1 began to increase power from approximately 91 percent power after the exit of TS LCO 3.0.3.

D. Other Systems or Secondary Functions Affected

RCS Leakage Detection Instrumentation was made inoperable.

E. Method of Discovery

The condition was discovered by an assistant unit operator.

F. Operator Actions

Operations personnel entered TS LCO 3.0.3.

G. Safety System Responses PCIS groups 1 and 2 received partial isolation signals with no subsequent system isolations. PCIS groups 3, 6, and 8 received partial isolation signals with resulting system isolations.

III.

CAUSE OF THE EVENT

A. Immediate Cause BFN Unit 1 lost power from the RPS M-G set 1 B. The RPS M-G set 1B voltage dropped from 120 volts-alternating current (VAC) to approximately 22 VAC.

B. Root Cause Trouble-shooting activities did not identify a specific condition that caused the voltage drop on the RPS M-G set 1 B. However, there were two potential causes:

(1) a loose connector or cable assembly at a terminal on the RPS M-G set control panel due to the machine design and operational characteristics (i.e., vibration induced), and (2) discontinuity within the voltage adjustment potentiometer.

C. Contributing Factors The Preventive Maintenance (PM) for the RPS M-G set 1 B to check the terminal connections is on a three year frequency. This three year PM was last performed on January 11, 2008. In the PM tracking system, the PM is currently due on November 14, 2011, but the PM should have been due around January 11,2011.

Consequently, the PM was not scheduled within the 25 percent grace period. This condition, which was considered a possible contributing factor, was identified in Problem Evaluation Report (PER) 438509.

IV. ANALYSIS OF THE EVENT

The Tennessee Valley Authority (TVA) is submitting this report under 10 CFR 50.73(a)(2)(iv)(A), as any event or condition that resulted in manual or automatic actuation of any of the systems listed in paragraph 10 CFR 50.73(a)(2)(iv)(B)(2), general containment isolation signals affecting containment isolation valves in more than one system.

Furthermore, TS LCO 3.4.5.D was entered because both means of automatic monitoring of RCS leakage became inoperable, requiring TS LCO 3.0.3 to be entered immediately. Therefore, this report also includes reporting under 10 CFR 50.73(a)(2)(i)(B), as any operation or condition which was prohibited by the plant's Technical Specifications.

Trouble-shooting for this event involved the voltage adjustment potentiometer, the voltage regulator [RG], and the wiring between the voltage adjustment potentiometer and the voltage regulator.

The voltage adjustment potentiometer was wiped (operated over a full range to clean internals) while monitoring the resistive output. Even though the results indicated smooth changes and no discontinuities, discontinuities could not be ruled out as a potential cause.

The voltage regulator passed all failure analysis testing. The analysis was performed by PSI Repair Services, Inc. PSI Repair Services, Inc. disassembled the voltage regulator to clean and inspect it for damaged and missing components. The analysis identified the following: "a pinched wire with no physical damage and three electrolytic capacitors that should be changed due to their age; however, all components tested within specification." The Electric Power Research Institute (EPRI) guidance indicates that the expected shelf life for electrolytic capacitors is 14 years. This voltage regulator was received in 2004 and installed in 2005. The EPRI guidance also states the expected in-service life of this voltage regulator is 9.5 years at 48 degrees Celsius.

This type of voltage regulator should be able to achieve its 10 year replacement frequency in the controlled environment where it was located.

Vibration measurements are taken on all RPS M-G sets on a quarterly basis. Since the control panel is mounted directly to the M-G set, any vibrations would be transferred directly to the control panel; however, vibration data is not taken on the control panel.

Continuous vibrations could cause wiring terminals to loosen. The trouble-shooting section of the vendor manual lists "connectors or cable assemblies loose" as a cause for when the generator fails to build up rated voltage. A vibration-induced loose wire at a terminal was a potential cause of the output voltage drop. No specific loose connection was detected, but terminal connections were tightened during the trouble-shooting activities.

Extent of Condition The extent of condition is the RPS M-G Sets A and B for Units 1, 2, and 3. The RPS M-G sets are similar in size, continuously in service, and have the control panel bolted to the rotating equipment.

V.

ASSESSMENT OF SAFETY CONSEQUENCES

The RPS provides timely protection against the onset and consequences of conditions that threaten the integrity of the fuel barrier and the nuclear system process barrier.

The system is designed such that no single failure can prevent a reactor scram. The RPS is designed to automatically shutdown the reactor based on parameters that deviate from normal.

The failure of the RPS M-G set 1 B occurred when Unit 1 was at approximately 99 percent power. The failure did not reduce defense-in-depth because the RPS fails to a safe condition. The failure caused a half scram to the reactor and rendered all required leakage detection systems inoperable. TS LCO 3.0.3 was initiated within the required one hour. TS LCO 3.0.3 requires Unit 1 to be in MODE 2 within 10 hours1.157407e-4 days <br />0.00278 hours <br />1.653439e-5 weeks <br />3.805e-6 months <br />, MODE 3 within 13 hours1.50463e-4 days <br />0.00361 hours <br />2.149471e-5 weeks <br />4.9465e-6 months <br />, and MODE 4 within 37 hours4.282407e-4 days <br />0.0103 hours <br />6.117725e-5 weeks <br />1.40785e-5 months <br />. TS LCO 3.0.3 was exited within 10 hours1.157407e-4 days <br />0.00278 hours <br />1.653439e-5 weeks <br />3.805e-6 months <br /> which allowed Unit 1 to remain in MODE 1.

TS LCO 3.3.2.1 was exited within the required 24 hour2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> completion time.

Therefore, TVA concluded that there was no significant reduction to the health and safety of the public for this event.

VI. CORRECTIVE ACTIONS - The corrective actions are being managed by TVA's corrective action program.

A. Immediate Corrective Actions

The voltage regulator was replaced, the voltage adjustment potentiometer was wiped (operated over a full range to clean internals), and all terminals were tightened.

B. Corrective Actions

1.

Verify current three year RPS M-G set PMs are scheduled in next component outage window.

2.

PM frequency for all RPS M-G sets will be increased from 3 years to 2 years.

3.

Appropriate scheduling of the RPS M-G set PMs within grace will be addressed by PER 438509.

VII. ADDITIONAL INFORMATION

A. Failed Components The failed component was the Unit 1 RPS M-G set 1 B voltage regulator. The manufacturer of this component is General Electric and the model number is 3300A03B0044.

B. Previous LERs or Similar Events There was a previous similar event that occurred in 2004 on Unit 3. The voltage dropped on the RPS M-G set 3B. This occurrence was documented in PER 69358 and PER 70538. The cause for this event was determined to be premature degradation of the voltage regulator gain potentiometer due to a lack of periodic maintenance.

C. Additional Information

The corrective action documents for this report are PER 412934 and PER 438509.

D. Safety System Functional Failure Consideration This event was not a safety system function failure according to NEI 99-02.

E. Scram With Complications Consideration This event was not a complicated scram according to NEI 99-02.

VIII. COMMITMENTS

There were no commitments.