05000529/LER-2013-001, Regarding Condition Prohibited by Technical Specifications Due to Emergency Diesel Generator Low Output Voltage
| ML13127A006 | |
| Person / Time | |
|---|---|
| Site: | Palo Verde  |
| Issue date: | 04/30/2013 |
| From: | Mims D APS, Arizona Public Service Co |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| 102-06693-DCM/FJO LER 13-001-00 | |
| Download: ML13127A006 (7) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 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)(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)(i)(A), Completion of TS Shutdown 10 CFR 50.73(a)(2)(v), Loss of Safety Function 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 5292013001R00 - NRC Website | |
text
10 CFR 50.73 DWIGHT C. MIMS Senior Vice President, Nuclear Oka; Regulatory & Oversight Palo Verde Nuclear Generating Station P.O. Box 52034 Phoenix, AZ 85072 Mail Station 7605 Tel 623 393 5403 102-06693-DCM/FJO April 30, 2013 ATTN: Document Control Desk U.S. Nuclear Regulatory Commission Washington, DC 20555-0001
Dear Sirs:
Subject:
Palo Verde Nuclear Generating Station (PVNGS) Unit 2 Docket No. STN 50-529 / License No. NPF 51 Licensee Event Report 2013-001-00 Enclosed please find Licensee Event Report (LER) 50-529/2013-001-00 that has been prepared and submitted pursuant to 10 CFR 50.73. This LER reports an event in which the Unit 2 train B emergency diesel generator did not reach required generator output voltage when started for surveillance testing. This resulted in a condition prohibited by Technical Specification Limited Condition for Operation 3.8.1, AC Sources - Operating.
In accordance with 10 CFR 50.4, copies of this LER are being forwarded to the Nuclear Regulatory Commission (NRC) Regional Office, NRC Region IV, and the Senior Resident Inspector. If you have questions regarding this submittal, please contact Mark McGhee, Operations Support Manager, Regulatory Affairs, at (623) 393-4972.
Arizona Public Service Company makes no commitments in this letter.
Sincerely, DCM/FJO/hsc Enclosure cc:
A. T. Howell, III NRC Region IV Regional Administrator L. K. Gibson NRC NRR Project Manager for PVNGS M. A. Brown NRC Senior Resident Inspector for PVNGS A member of the STARS (Strategic Teaming and Resource Sharing) Alliance Callaway-Comanche Peak.Diablo Canyon.Palo Verde-San OnofreSouth Texas-Wolf Creek
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 Palo Verde Nuclear Generating Station (PVNGS) Unit 2 05000529 1 OF 6
- 4. TITLE Condition Prohibited by Technical Specifications due to Emergency Diesel Generator Low Output Voltage
- 5. EVENT DATE
- 6. LER NUMBER__
- 7. REPORT DATE
- 8. OTHER FACILITIES INVOLVED MONTH DAY YEAR YEAR SEQUENTIAL REV MONTH DAY YEAR NUMBER NO.
FACILITY NAME DOCKET NUMBER 1
16 2013 2013 - 001 -
00 04 30 2013
- 9. OPERATING MODE
- 11. THIS REPORT IS SUBMITTED PURSUANT TO THE REQUIREMENTS OF 10 CFR§: (Check all that apply)
El 20.2201(b)
El 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)
[I 50.73(a)(2)(ii)(A)
[I 50.73(a)(2)(viii)(A)
El 20.2203(a)(1) 0l 20.2203(a)(4)
El 50.73(a)(2)(ii)(B)
E] 50.73(a)(2)(viii)(B)
[1 20.2203(a)(2)(i)
El 50.36(c)(1)(i)(A)
El 50.73(a)(2)(iii)
[I 50.73(a)(2)(ix)(A)
- 10. POWER LEVEL E] 20.2203(a)(2)(ii)
El 50.36(c)(1)(ii)(A)
-] 50.73(a)(2)(iv)(A)
[I 50.73(a)(2)(x)
El 20.2203(a)(2)(iii)
El 50.36(c)(2)
El 50.73(a)(2)(v)(A)
El 73.71(a)(4) 100 [1 20.2203(a)(2)(iv)
El 50.46(a)(3)(ii)
[I 50.73(a)(2)(v)(B)
C3 73.71(a)(5) 1E 20.2203(a)(2)(v)
El 50.73(a)(2)(i)(A)
[1 50.73(a)(2)(v)(C)
El OTHER El 20.2203(a)(2)(vi) 0 50.73(a)(2)(i)(B)
El 50.73(a)(2)(v)(D)
Specify in Abstract below or in Controls and instrumentation are provided in main and local control rooms for starting and stopping the EDGs, and for engine speed and voltage regulator adjustments. There are two modes of EDG operation, emergency mode and test mode. The test mode is used for non-emergency EDG operations and allows for automatic engine shutdown from lower priority engine protective features. Also, in the test mode, the generator voltage regulator and engine speed governor controls allow for parallel operations with other AC sources and the output frequency and voltage may be manually adjusted by operators in the main control room.
Operation in the emergency mode is a more reliable configuration which limits automatic engine shutdowns to only those conditions which will result in damage to the generator or engine. Automatic, emergency mode actuation of the EDG is i'nitiated by the loss of power actuation, safety injection actuation system, or auxiliary feedwater actuation system. In emergency mode, the engine control system maintains engine speed and generator frequency at a preset value and generator output voltage is controlled to a fixed setpoint such that the required output voltage is maintained.
An AVR controls generator output voltage during all modes of operation. At startup, EDG field excitation is provided from battery supplied field flash power which is applied to the generator field winding to establish output voltage. Field flash power is removed when output voltage reaches greater than or equal to 60 percent of rated voltage and two voltage sensing relays, VR1 and VR2, are energized. With field flash removed, generator excitation is self-sustaining via generator output voltage and current.
The AVR functions by comparing the generator output voltage to an internal reference signal (setpoint) and then adjusting field excitation to maintain output voltage at the desired setpoint. In test mode, the AVR setpoint is a variable voltage provided by a motor operated potentiometer (MOP) which can be adjusted by the operator. In the emergency mode, the AVR setpoint is a fixed value provided by the instantaneous prepositioning (IPP) board.
For monthly surveillance testing, the EDG is typically started in the test mode. However, three times a year (January, May, and September) during monthly surveillance testing and during the 18 month (outage) surveillance testing, the EDG is tested in the emergency mode. The IPP board circuit functions are tested only during emergency mode testing.
When the EDG is tested in the emergency mode, the test procedures also place the EDG in test mode to facilitate paralleled operation with offsite power and for engine shutdown such that the MOP portion of the AVR is also tested.
The station TSs provide the voltage, frequency and start time requirements for the EDG output to assure AC powered safety related equipment will function as required when offsite power is not available.
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- 3.
INITIAL PLANT CONDITIONS
On January 16, 2013, Palo Verde Unit 2 was in Mode 1 (Power Operation), at 100 percent power and normal operating temperature and pressure. There were no structures, systems, or components inoperable that contributed to the event.
4. EVENT DESCRIPTION
On January 16, 2013, the 2B EDG was started in the emergency mode by operations personnel for scheduled monthly surveillance testing. The operator determined that the 2B EDG did not reach the required generator output voltage at startup. Operations personnel declared the 2B EDG inoperable and Unit 2 entered LCO 3.8.1, Condition B. Maintenance personnel performed troubleshooting and identified that the AVR malfunction was due to failure of an operational amplifier on the IPP board which provides the voltage setpoint for operation in the emergency mode. As an immediate corrective action, the faulty IPP board was replaced and the 2B EDG was successfully tested in the emergency mode on January 17, 2013. Operations personnel declared the 2B EDG operable on January 18, 2013, and Unit 2 exited LCO 3.8.1, Condition B, approximately 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br /> after entering the LCO.
The station initiated an investigation to identify and correct the cause of the IPP board failure in January, 2013. On March 1,2013, the investigation team completed the initial investigation. Although a root cause of failure was not determined, the investigation concluded that the IPP board operational amplifier failure was likely caused by malfunction of circuit components outside of the IPP board. As an interim corrective action, the circuit components which could potentially cause a failure of the operational amplifier on the IPP board were replaced in March 2013.
Review of the recent operational history for the 2B EDG found that the last emergency mode test (prior to the January 16, 2013 failure) was a simulated loss of power start during periodic 18 month surveillance testing on October 26, 2012, which occurred during a refueling outage. During that outage, Unit 2 entered Mode 4, when LCO 3.8.1 becomes applicable to require that both EDGs are operable, on November 3, 2012. The unit returned to Mode 5 for unrelated activities and returned to Mode 4 again on November 4, 2012.
Based on the current results of the investigation, the condition prohibited by LCO 3.8.1 existed from November 3, 2012, through January 18, 2013.
The 2B EDG was started in the test mode during the monthly surveillance tests in November and December of 2012. Those test mode starts would not have provided an opportunity to detect a problem with the IPP board components because the IPP board output is not used as the AVR setpoint in test mode operations.
I Reviews of operational history determined that the Unit 2 train A EDG was not removed from service during the period from November 3, 2012, through January 18, 2013.
The event investigation is still in progress, and the results will be reported in a supplement to this report. To date no firm evidence that the condition existed prior to November 3, 2012, has been developed.
- 5.
ASSESSMENT OF SAFETY CONSEQUENCES
This event did not result in a challenge to the fission product barriers or result in the release of radioactive materials; and the event did not adversely affect the safe operation of the plant or health and safety of the public. This event did not result in a potential transient more severe than those analyzed in the Updated Final Safety Evaluation Report.
The probabilistic risk significance associated with this condition, based upon a review of the PVNGS probabilistic risk assessment model is small. In this condition, the 2B EDG was considered to be unavailable. Since the time of failure is indeterminate, established methods dictate that exposure time be taken as one half the time since the last successful test/operation of the EDG where the subject failure would have been detected. For this event, the exposure time was determined to be the 42 days between October 26, 2012, and January 18, 2013. The incremental conditional core damage probability due to this event over the exposure period of 42 days is 7E-7. This increase in risk is characterized as small per NRC Regulatory Guide 1.174, An Approach for Using Probabilistic Risk Assessment in Risk-Informed Decisions on Plant-Specific Changes to the Licensing Basis.
Based upon current information, this condition would not have prevented the fulfillment of a safety function as defined by 10 CFR 50.73 (a)(2)(v). However, the investigation to determine the cause of this event is still in progress at the time of this report. A supplement to this report will be provided following completion of the investigation.
6. CAUSE OF THE EVENT
The direct cause was failure of an operational amplifier on the IPP board used to provide the voltage setpoint to the 2B EDG AVR during operation in the emergency mode.
A preliminary conclusion of the investigation identified age related degradation of EDG electrical circuit components as the probable cause of the 2B EDG AVR malfunction.
However, the investigation to determine the root cause of this event is still in progress at the time of this report. A supplement to this report will be provided following completion of the investigation.
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- 7.
CORRECTIVE ACTIONS
As an immediate corrective action, the IPP board was replaced and post-maintenance testing was successfully completed on January 17, 2013.
The following interim corrective actions have been performed:
" All monthly testing of PVNGS EDGs now include emergency mode operation to ensure monitoring of the IPP board performance.
" 2B EDG electrical circuit components, which could potentially have caused the IPP board malfunction, were replaced on March 18, 2013.
" Selected electrical components that were removed from the 2B EDG have been sent to an offsite laboratory for further analysis in support of the cause evaluation.
" During monthly 2B EDG testing activities, additional instrumentation is used to monitor the AVR circuit performance.
Upon completion of the investigation, a supplement will be provided to this report.
- 8.
PREVIOUS SIMILAR EVENTS
In the past three years, PVNGS has not reported any similar events.
A malfunction of the 2B EDG AVR occurred during post-maintenance testing following a planned EDG maintenance outage on February 22, 2011. Troubleshooting determined that the AVR malfunction resulted from failure of an operational amplifier on the IPP board that was in use at that time. The IPP board was replaced. The investigation of the 2011 AVR malfunction did not identify firm evidence that the condition existed prior to the maintenance outage and no event report was required. Because both the 2011 and the 2013 AVR malfunctions involved failure of an operational amplifier with the same function on an IPP board, the on-going 2013 investigation is evaluating correlation between these two events.
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