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{{Adams
#REDIRECT [[05000458/LER-2016-005]]
| number = ML16126A229
| issue date = 04/25/2016
| title = LER 2016-005-00 for River Bend, Unit 1, Regarding Potential Loss of Safety Function of Onsite AC Sources and Operations Prohibited by Technical Specifications Due to Uncorrected Circuit Breaker Control Logic Design Causing Intermittent Fail
| author name = Maguire W F
| author affiliation = Entergy Operations, Inc
| addressee name =
| addressee affiliation = NRC/Document Control Desk, NRC/NRR
| docket = 05000458
| license number = NPF-047
| contact person =
| case reference number = RBG-47679
| document report number = LER 2016-005-00
| document type = Letter, Licensee Event Report (LER)
| page count = 6
}}
 
=Text=
{{#Wiki_filter:---(
Eintc*Oll' lne. Aivor Berni Station *** ..... *Entergy.
RBG-47679 April25.2016 U. S. Nuclear Regulatory Commission ATI"N: Document Control Desk Washington.
DC 20555
 
==Subject:==
RBF1*16-0051 Ucensee Event Report 50-45812016-00S-OO River Bend Statton -Unlt 1 Docket No. 50-458 Ucense No. NPF-47
 
==Dear Sir or Madam:==
5485 U.S. H!gh\':lay 61N St. Frcnci:;vill:J, LA 70775 Tel 225-391-4157 F. rn::gulro Vic'.l Pro:iid:mt tn accordance with 10 CFR enclosed is the subject Licensee Event Report. This document contains no commitments.
If you have any questions, please contact Mt'.. Joseph Clark at 225-381-4177.
-WFM/dhw Enclosure cc: U. S. Nuclear Regulatory Commission Region IV 1600 East Lamar Blvd. Arlington, TX 76011*4511 NRC Sr. Resident Inspector P. O. Box 1050 St. Francisville.
LA 70775 INPO (via ICES reporting) 
*' "°\ RBG'"47679 April 25, 2016 Page2of2 Central Records Clerk Public Utility Commission of Texas 1701 N. Congress Ave. -Austin, TX 78711 *3326 Department of Environmental Quality Office of Environmental Compliance RadlologiQSJ Emergency Planning and Response Section Ji Young Wiley P.O. Box 4312 eaten Rouge, LA 70821-4312 2
NRC FORM 366 U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY OMB: NO. 3150-0104 EXPIRES: 1013112018 (11-2015)
Estimated burden per response to comply with this mandatory collection request: 80 hours.
Reported lessons learned are incorporated into the licensing process and fed back to industry.
l .... Send comments regarding burden estimate to the FOIA, Privacy and Information Collecfions ff LICENSEE EVENT REPORT (LER) Branch (T-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by . .. internet e-mail to lnfocollects.Resource@nrc.gov, and to the Desk Officer, Office of Information and *1'11111.,_
I (See Page 2 for required number of Regulatory Affairs, NEOB-10202, (3150-0104), Offite of Management and Budget, Washington, DC 20503. If a means used to impose an information collecfion does not display a currently valid digits/characters for each block) OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collecfion.
: 1. FACILITY NAME 2. DOCKET NUMBER 3. PAGE River Bend Station -Unit 1 05000 458 1 OF 4 4. TITLE Potential Loss of Safety Function of Onsite AC Sources and Operations Prohibited by Technical Specifications Due to Uncorrected Circuit Breaker Control Logic Design Causing Intermittent Failure to Close 5. EVENT DATE 6. LER NUMBER 7. REPORT DATE 8. OTHER FACILITIES INVOLVED I SEQUENTIAL I REV FACILITY NAME DOCKET NUMBER MONTH DAY YEAR YEAR NUMBER NO. MONTH DAY YEAR 05000 FACILITY NAME DOCKET NUMBER 02 24 2016 '2016 .. 005 -00 04 25 2016 05000 9. OPERATING MODE 11. THIS REPORT IS SUBMITTED PURSUANT TO THE REQUIREMENTS OF 10 CFR §: (Check all that apply) D 20.2201 (b) D 20.2203(a)(3)(i) 0 50.73(a)(2)(ii)(A) 0 50.73(a)(2)(viii)(A)'
4 D 20.2201 (d) D 20.2203(a)(3)(ii)
D 50.73(a)(2)(ii)(B) 0 50.73(a)(2)(viii)(B)
D 20.2203(a)(1>
D 20.2203(a)(4)
D so.73(a)(2)(iii) 0 50.73(a)(2)(ix)(A)
D 20.2203(a)(2)(i) 0 50.36(c)(1)(i)(A) 0 50.73(a)(2)(iv)(A) 0 50.73(a)(2)(x)
: 10. POWER LEVEL D 20.2203(a)(2)(ii) 0 50.36(c)(1)(ii)(A)
[{] 50.73(a)(2)(v)(A)
D 13.11(a)(4>
D 20.2203(a)(2)(m>
o so.3a(c)(2)
* 0 50.73(a)(2)(v)(B)
D 13.11(a)(s>
0 20.2203(a)(2)(iv)
D 0 50.73(a)(2)(v)(C)
D 1s.n(a)(1>
0 0 20.2203(a)(2)(v) 0 50.73(a)(2)(i)(A) 0 50.73(a)(2)(1i)(D)
D 1s.n(a)(2)(i) 0 20.2203(a)(2)(vi)
[{] 50.73(a)(2)(i)(B) 0 50.73(a)(2)(vii)
D 1s.n(a)(2)(ii)
D 50.73(a)(2)(i)(C)
D OTHER Specify in Abstract below or in NRG Form 366A 12. LICENSEE CONTACT FOR THIS LER LICENSEE CONTACT TELEPHONE NUMBER (Include Area Code) Joseph A. Clark, Manager -Regulatory Assurance (225) 381-4177 13, COMPLETE ONE LINE FOR EACH COMPONENT FAILURE DESCRIBED IN THIS REPORT CAUSE SYSTEM COMPONENT MANU-REPORTABLE f CAUSE SYSTEM COMPONENT MANU-REPORTABLE FACT UR ER TOEPIX FACTURER TOEPIX (see text) I 14. SUPPLEMENTAL REPORT EXPECTED 15. EXPECTED MONTH DAY YEAR D YES (If yes; complete 15. EXPECTED SUBMISSION DATE) [{] NO SUBMISSION DATE ABSTRACT (Limit to 1400 spaces, i.e., approximately 15 single-spaced typewritten lines) On February 24, 2016, with the plant in cold shutdown, the operations shift manager was made aware of a notification regarding a certain model ofMasterpact 480-volt circuit breakers that described a failure mode that could potentially prevent the automatic closure of the breakers.
Assessment of this information determined that the susceptible breakers included those powering the emergency ventilation fans in the Division 1 and 2 emergency diesel generator rooms and two auxiliary building unit coolers. This condition required that both diesel generators and both trains of shutdown cooling and to be declared inoperable.
This constituted a condition that could potentially prevent fulfillment of the safety function of onsite AC power sources and decay heat removal. The Division 2 residual heat removal loop was operating in shutdown cooling, satisfactorily maintaining reactor coolant temperature.
The cause of the event is that station personnel failed to recognize the breakers' vulnerability to this failure mode. This directly resulted in the failure to take corrective action prior to this industry notification.
The cause of the untimely corrective actions is that the breakers were incorrectly determined to be operable in 2014 when the condition was discovered.
All the affected breakers were modified to eliminate the failure mode prior to the subsequent plant 'startup.
NRG FORM 366 (11-2015) 
' Page 1of3 NRC FORM 366A (11-2015)
U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY OMB: NO. 3150-0104 EXPIRES: 10/31/2018 LICENSEE EVENT REPORT (LER) CONTINUATION SHEET Estimated burden per response to comply with this mandatory collection request: 80 hours. Reported lessons learned are incorporated into the licensing process and fed back to industry.
Send comments regarding burden estimate to the FOIA, Privacy and Information Collections Branch (T-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by internet e-mail to lnfocollects.Resollrce@nrc.gov, and to the Desk Officer, Office of Information and Regulatory Affairs, NEOB-10202, (3150-0104), Office of Management and Budget, Washington, DC 20503. If a means used to impose an information collection does not display a currently valid OMB control number, the NRG may not conduct or sponsor, and a person is not required to respond to, the information collection.
I YEAR SEQUENTIAL REV D NUMBER NO. u-1 °0 s 1-0 3. LER NUMBER 1. FACILITY NAME . I
!Riv" Bond Station -Unit 1 NARRATIVE REPORTED CONDITION On February 24, 2016, with the plant in cold shutdown, the operations shift manager was made aware of a notification regarding a certain model of Masterpact 480-volt s11fety-re(ated circuit breakers that described a failure mode that could potentially prevent the automatic closure of the breakers.
The initial information regarded the emergency ventilation fans in the Division 1 and 2 emergency diesel generator rooms. These fans are not in Technical Specifications, but they provide a support function to the emergency diesel generators
(**DG**).
This condition required that both diesel generators to be declared inoperable.
This constituted a condition that could potentially prevent fulfillment of the safety function of onsite AC power sources [ED]. Later on the same c:late, further evaluation of the notification identified four other circuit breakers affected by the same condition.
These breakers supply power to Division l*and 2 containment unit coolers and the Division 1and2.auxiliary building 141 ft. elevation general area unit coolers. The auxiliary building unit coolers are not in Technical Specifications, but they provide a support function to the electrical distribution system. The required action in Technical Specification is to declare both trains of the residual heat removal system (shutdown cooling mode) [BO] inoperable.
This inoperability constituted a condition that could have potentially prevented the fulfillment of the decay heat removal safety function.
Division 2 residual heat removal was operating in shutdown cooling, satisfactorily maintaining reactor coolant temperature.
The affected breakers can be manually operated to start/stop their associated equipment, if necessary for operation INVESTIGATION AND IMMEDIATE ACTIONS Masterpa'ct breakers were installed at RBS as replacements for obsolete General Electric AKR breakers.
Subsequently, at RBS and elsewhere, there were isolated failures ofthe breakers to close upon demand. Failure analyses had beer:i conducted by both the vendor and the utilities, but no such analyses determined that a potentially generic problem existed. * *
* In 2014, the breaker vendor issued a non-conformance report that concluded that the closure failures were from intermittent mechanical binding of the anti-pump mechanism.
The binding was the result of a "standing close signal," which is the condition where the relay that actuates the closing mechanism remains energized, either indefinitely or for a defined period of time following a closure actuation.
The control logic for any given breaker is an inherent design feature of the circuitry of the switchgear, not internal to the breaker itself. It was eventually determined by the vendor that this condition could cause any Masterpact breaker to intermittently fail to close. The vendor indicated that the failure mode could not be predicted, but was repeatable, 'and occurred on both new and old breakers.
When RBS evaluated the vendor's nonconformance report, a population of affected circuit breakers was identified, and the served systems were evaluated for adverse effects on their ability to perform their safety functions.
The operational history of the. breakers was examined, and it was determined that the observed increase in failure rate was not statistically significant.
The affected systems were determined to be operable.
The nonconformance report stated that the failure mode could be averted by pressing the PUSH TO OPEN button on the front of the breaker after remote opening. This information was used to develop an Operations Standing Order, and this action was performed after every breaker opening for any of the affected breakers during power operations or hot NRG FORM 366A (11-2015) 
\ NRC FORM 366A (11-2015)
: 1. FACILITY NAME River Bend Station -Unit 1 LICENSEE EVENT REPORT (LER) CONTINUATION SHEET 2. DOCKET NUMBER 05000-I .s. Page 2 of 3 U.S. NUCLEAR REGULATORY COMMISSION
: 3. LER NUMBER NUMBER NO I YEAR SEQUENTIAL REV u-1 oos 1-GJ shutaown until the moamcat1on to remove t 1e standing close signal was installed.
No ailures of these oreakers occurred when standing order was utilized.
Isolated failures occurred in breakers serving the control building air conditioning system in the refueling outage in 2015, and vendor recommendations for modifying the breaker control circuitry were implemented.
Those modifications were limited, however, to breakers with continuously energized closure relays. In the February 2016 notification, the vendor provided information regarding the identification of a potential defect, with a recommendation that Masterpact breakers designed for electrical closing operation be evaluated.
The vulnerable breakers identified in connection with the notification were not surfaced during previous extent-of-condition reviews due to being limited to the population with the. continuously energized closing relay. The new information provided in February 2016 revealed that even breakers with a time-delayed removal of power from the closing relay could be vulnerable while the relay was energized.
CAUSAL ANALYSIS The first direct cause of the event is that station personnel failed to recognize the breaker vulnerability to this mode. As a result, an additional population of breakers that had a time-delayed de-energization of the closing relay was identified in February 2016. These additional breakers included components that supported the emergency diesel generators and portions of the safety-related AC distribution system in the auxiliary building.
The second direct cause of the untimely corrective actions is that the breakers were incorrectly determined to be operable when the original condition with Masterpact breakers was discovered in 2014. Specifically, the procedure for development of operability determinations contained inadequate guidance on the use of reliability-based evaluations.
One contributing cause of the event was the insufficient detail in the procedure regarding distribution of OE information for screening.
This caused the component engineer for circuit breakers to be bypassed in the OE review cycle, likely preventing early detection of the condition.
Additionally, the OE notices did not contain detailed information regarding the failure mode. ' CORRECTIVE ACTIONS TO PREVENT RECURRENCE The following actions have been completed to prevent recurrence of the reported condition.
*All safety-related Masterpact circuit breakers with the control circuit configuration that applied a* s.tanding close signal have been modified to eliminate this condition.
*The fleet procedure for operability evaluations has been revised to address conditions involving reduced reliability of safety-related components.
PREVIOUS OCCURRENCE EVALUATION The reported condition resulted from a very specific design feature of a certain model of circuit breakers.
No previous event reports regarding this condition have been submitted by RBS in the last three years. SAFETY SIGNIFICANCE NRC FORM 366A (11-2015) 7 1 \ NRC FORM 366A (11-2015)
: 1. FACILITY NAME River Bend Station -Unit 1 LICENSEE EVENT REPORT (LER) CONTINUATION SHEET 2. DOCKET NUMBER 0500-0-I .,. Page 3 of 3 U.S. NUCLEAR REGULATORY COMMISSION
: 3. LER NUMBER D NUMBER NO. I . YEAR SEQUENTIAL REV u-1 °0 s 1-0 The plant was in cold shutdown when the vendor notification was received.
Technical Specification limiting condition for operations for the affected system were entered, and the required actions were completed.
Dedicated operators were assigned to implement the standing order to mitigate the failure mode. All safety-related Masterpact circuit breakers with the control circuit configuration that applied a standing close signal were modified to eliminate this condition prior to plant startup. A probabilistic risk analysis of this condition has concluded that this condition of very low risk significance. (NOTE: Energy Industry Identification System component function identifier and system name of each component or system referred to in the LER are annotated as (**XX**) and [XX], respectively.)
\' NRC FORM 366A (11*2015) 
---(
Eintc*Oll' lne. Aivor Berni Station *** ..... *Entergy.
RBG-47679 April25.2016 U. S. Nuclear Regulatory Commission ATI"N: Document Control Desk Washington.
DC 20555
 
==Subject:==
RBF1*16-0051 Ucensee Event Report 50-45812016-00S-OO River Bend Statton -Unlt 1 Docket No. 50-458 Ucense No. NPF-47
 
==Dear Sir or Madam:==
5485 U.S. H!gh\':lay 61N St. Frcnci:;vill:J, LA 70775 Tel 225-391-4157 F. rn::gulro Vic'.l Pro:iid:mt tn accordance with 10 CFR enclosed is the subject Licensee Event Report. This document contains no commitments.
If you have any questions, please contact Mt'.. Joseph Clark at 225-381-4177.
-WFM/dhw Enclosure cc: U. S. Nuclear Regulatory Commission Region IV 1600 East Lamar Blvd. Arlington, TX 76011*4511 NRC Sr. Resident Inspector P. O. Box 1050 St. Francisville.
LA 70775 INPO (via ICES reporting) 
*' "°\ RBG'"47679 April 25, 2016 Page2of2 Central Records Clerk Public Utility Commission of Texas 1701 N. Congress Ave. -Austin, TX 78711 *3326 Department of Environmental Quality Office of Environmental Compliance RadlologiQSJ Emergency Planning and Response Section Ji Young Wiley P.O. Box 4312 eaten Rouge, LA 70821-4312 2
NRC FORM 366 U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY OMB: NO. 3150-0104 EXPIRES: 1013112018 (11-2015)
Estimated burden per response to comply with this mandatory collection request: 80 hours.
Reported lessons learned are incorporated into the licensing process and fed back to industry.
l .... Send comments regarding burden estimate to the FOIA, Privacy and Information Collecfions ff LICENSEE EVENT REPORT (LER) Branch (T-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by . .. internet e-mail to lnfocollects.Resource@nrc.gov, and to the Desk Officer, Office of Information and *1'11111.,_
I (See Page 2 for required number of Regulatory Affairs, NEOB-10202, (3150-0104), Offite of Management and Budget, Washington, DC 20503. If a means used to impose an information collecfion does not display a currently valid digits/characters for each block) OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collecfion.
: 1. FACILITY NAME 2. DOCKET NUMBER 3. PAGE River Bend Station -Unit 1 05000 458 1 OF 4 4. TITLE Potential Loss of Safety Function of Onsite AC Sources and Operations Prohibited by Technical Specifications Due to Uncorrected Circuit Breaker Control Logic Design Causing Intermittent Failure to Close 5. EVENT DATE 6. LER NUMBER 7. REPORT DATE 8. OTHER FACILITIES INVOLVED I SEQUENTIAL I REV FACILITY NAME DOCKET NUMBER MONTH DAY YEAR YEAR NUMBER NO. MONTH DAY YEAR 05000 FACILITY NAME DOCKET NUMBER 02 24 2016 '2016 .. 005 -00 04 25 2016 05000 9. OPERATING MODE 11. THIS REPORT IS SUBMITTED PURSUANT TO THE REQUIREMENTS OF 10 CFR §: (Check all that apply) D 20.2201 (b) D 20.2203(a)(3)(i) 0 50.73(a)(2)(ii)(A) 0 50.73(a)(2)(viii)(A)'
4 D 20.2201 (d) D 20.2203(a)(3)(ii)
D 50.73(a)(2)(ii)(B) 0 50.73(a)(2)(viii)(B)
D 20.2203(a)(1>
D 20.2203(a)(4)
D so.73(a)(2)(iii) 0 50.73(a)(2)(ix)(A)
D 20.2203(a)(2)(i) 0 50.36(c)(1)(i)(A) 0 50.73(a)(2)(iv)(A) 0 50.73(a)(2)(x)
: 10. POWER LEVEL D 20.2203(a)(2)(ii) 0 50.36(c)(1)(ii)(A)
[{] 50.73(a)(2)(v)(A)
D 13.11(a)(4>
D 20.2203(a)(2)(m>
o so.3a(c)(2)
* 0 50.73(a)(2)(v)(B)
D 13.11(a)(s>
0 20.2203(a)(2)(iv)
D 0 50.73(a)(2)(v)(C)
D 1s.n(a)(1>
0 0 20.2203(a)(2)(v) 0 50.73(a)(2)(i)(A) 0 50.73(a)(2)(1i)(D)
D 1s.n(a)(2)(i) 0 20.2203(a)(2)(vi)
[{] 50.73(a)(2)(i)(B) 0 50.73(a)(2)(vii)
D 1s.n(a)(2)(ii)
D 50.73(a)(2)(i)(C)
D OTHER Specify in Abstract below or in NRG Form 366A 12. LICENSEE CONTACT FOR THIS LER LICENSEE CONTACT TELEPHONE NUMBER (Include Area Code) Joseph A. Clark, Manager -Regulatory Assurance (225) 381-4177 13, COMPLETE ONE LINE FOR EACH COMPONENT FAILURE DESCRIBED IN THIS REPORT CAUSE SYSTEM COMPONENT MANU-REPORTABLE f CAUSE SYSTEM COMPONENT MANU-REPORTABLE FACT UR ER TOEPIX FACTURER TOEPIX (see text) I 14. SUPPLEMENTAL REPORT EXPECTED 15. EXPECTED MONTH DAY YEAR D YES (If yes; complete 15. EXPECTED SUBMISSION DATE) [{] NO SUBMISSION DATE ABSTRACT (Limit to 1400 spaces, i.e., approximately 15 single-spaced typewritten lines) On February 24, 2016, with the plant in cold shutdown, the operations shift manager was made aware of a notification regarding a certain model ofMasterpact 480-volt circuit breakers that described a failure mode that could potentially prevent the automatic closure of the breakers.
Assessment of this information determined that the susceptible breakers included those powering the emergency ventilation fans in the Division 1 and 2 emergency diesel generator rooms and two auxiliary building unit coolers. This condition required that both diesel generators and both trains of shutdown cooling and to be declared inoperable.
This constituted a condition that could potentially prevent fulfillment of the safety function of onsite AC power sources and decay heat removal. The Division 2 residual heat removal loop was operating in shutdown cooling, satisfactorily maintaining reactor coolant temperature.
The cause of the event is that station personnel failed to recognize the breakers' vulnerability to this failure mode. This directly resulted in the failure to take corrective action prior to this industry notification.
The cause of the untimely corrective actions is that the breakers were incorrectly determined to be operable in 2014 when the condition was discovered.
All the affected breakers were modified to eliminate the failure mode prior to the subsequent plant 'startup.
NRG FORM 366 (11-2015) 
' Page 1of3 NRC FORM 366A (11-2015)
U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY OMB: NO. 3150-0104 EXPIRES: 10/31/2018 LICENSEE EVENT REPORT (LER) CONTINUATION SHEET Estimated burden per response to comply with this mandatory collection request: 80 hours. Reported lessons learned are incorporated into the licensing process and fed back to industry.
Send comments regarding burden estimate to the FOIA, Privacy and Information Collections Branch (T-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by internet e-mail to lnfocollects.Resollrce@nrc.gov, and to the Desk Officer, Office of Information and Regulatory Affairs, NEOB-10202, (3150-0104), Office of Management and Budget, Washington, DC 20503. If a means used to impose an information collection does not display a currently valid OMB control number, the NRG may not conduct or sponsor, and a person is not required to respond to, the information collection.
I YEAR SEQUENTIAL REV D NUMBER NO. u-1 °0 s 1-0 3. LER NUMBER 1. FACILITY NAME . I
!Riv" Bond Station -Unit 1 NARRATIVE REPORTED CONDITION On February 24, 2016, with the plant in cold shutdown, the operations shift manager was made aware of a notification regarding a certain model of Masterpact 480-volt s11fety-re(ated circuit breakers that described a failure mode that could potentially prevent the automatic closure of the breakers.
The initial information regarded the emergency ventilation fans in the Division 1 and 2 emergency diesel generator rooms. These fans are not in Technical Specifications, but they provide a support function to the emergency diesel generators
(**DG**).
This condition required that both diesel generators to be declared inoperable.
This constituted a condition that could potentially prevent fulfillment of the safety function of onsite AC power sources [ED]. Later on the same c:late, further evaluation of the notification identified four other circuit breakers affected by the same condition.
These breakers supply power to Division l*and 2 containment unit coolers and the Division 1and2.auxiliary building 141 ft. elevation general area unit coolers. The auxiliary building unit coolers are not in Technical Specifications, but they provide a support function to the electrical distribution system. The required action in Technical Specification is to declare both trains of the residual heat removal system (shutdown cooling mode) [BO] inoperable.
This inoperability constituted a condition that could have potentially prevented the fulfillment of the decay heat removal safety function.
Division 2 residual heat removal was operating in shutdown cooling, satisfactorily maintaining reactor coolant temperature.
The affected breakers can be manually operated to start/stop their associated equipment, if necessary for operation INVESTIGATION AND IMMEDIATE ACTIONS Masterpa'ct breakers were installed at RBS as replacements for obsolete General Electric AKR breakers.
Subsequently, at RBS and elsewhere, there were isolated failures ofthe breakers to close upon demand. Failure analyses had beer:i conducted by both the vendor and the utilities, but no such analyses determined that a potentially generic problem existed. * *
* In 2014, the breaker vendor issued a non-conformance report that concluded that the closure failures were from intermittent mechanical binding of the anti-pump mechanism.
The binding was the result of a "standing close signal," which is the condition where the relay that actuates the closing mechanism remains energized, either indefinitely or for a defined period of time following a closure actuation.
The control logic for any given breaker is an inherent design feature of the circuitry of the switchgear, not internal to the breaker itself. It was eventually determined by the vendor that this condition could cause any Masterpact breaker to intermittently fail to close. The vendor indicated that the failure mode could not be predicted, but was repeatable, 'and occurred on both new and old breakers.
When RBS evaluated the vendor's nonconformance report, a population of affected circuit breakers was identified, and the served systems were evaluated for adverse effects on their ability to perform their safety functions.
The operational history of the. breakers was examined, and it was determined that the observed increase in failure rate was not statistically significant.
The affected systems were determined to be operable.
The nonconformance report stated that the failure mode could be averted by pressing the PUSH TO OPEN button on the front of the breaker after remote opening. This information was used to develop an Operations Standing Order, and this action was performed after every breaker opening for any of the affected breakers during power operations or hot NRG FORM 366A (11-2015) 
\ NRC FORM 366A (11-2015)
: 1. FACILITY NAME River Bend Station -Unit 1 LICENSEE EVENT REPORT (LER) CONTINUATION SHEET 2. DOCKET NUMBER 05000-I .s. Page 2 of 3 U.S. NUCLEAR REGULATORY COMMISSION
: 3. LER NUMBER NUMBER NO I YEAR SEQUENTIAL REV u-1 oos 1-GJ shutaown until the moamcat1on to remove t 1e standing close signal was installed.
No ailures of these oreakers occurred when standing order was utilized.
Isolated failures occurred in breakers serving the control building air conditioning system in the refueling outage in 2015, and vendor recommendations for modifying the breaker control circuitry were implemented.
Those modifications were limited, however, to breakers with continuously energized closure relays. In the February 2016 notification, the vendor provided information regarding the identification of a potential defect, with a recommendation that Masterpact breakers designed for electrical closing operation be evaluated.
The vulnerable breakers identified in connection with the notification were not surfaced during previous extent-of-condition reviews due to being limited to the population with the. continuously energized closing relay. The new information provided in February 2016 revealed that even breakers with a time-delayed removal of power from the closing relay could be vulnerable while the relay was energized.
CAUSAL ANALYSIS The first direct cause of the event is that station personnel failed to recognize the breaker vulnerability to this mode. As a result, an additional population of breakers that had a time-delayed de-energization of the closing relay was identified in February 2016. These additional breakers included components that supported the emergency diesel generators and portions of the safety-related AC distribution system in the auxiliary building.
The second direct cause of the untimely corrective actions is that the breakers were incorrectly determined to be operable when the original condition with Masterpact breakers was discovered in 2014. Specifically, the procedure for development of operability determinations contained inadequate guidance on the use of reliability-based evaluations.
One contributing cause of the event was the insufficient detail in the procedure regarding distribution of OE information for screening.
This caused the component engineer for circuit breakers to be bypassed in the OE review cycle, likely preventing early detection of the condition.
Additionally, the OE notices did not contain detailed information regarding the failure mode. ' CORRECTIVE ACTIONS TO PREVENT RECURRENCE The following actions have been completed to prevent recurrence of the reported condition.
*All safety-related Masterpact circuit breakers with the control circuit configuration that applied a* s.tanding close signal have been modified to eliminate this condition.
*The fleet procedure for operability evaluations has been revised to address conditions involving reduced reliability of safety-related components.
PREVIOUS OCCURRENCE EVALUATION The reported condition resulted from a very specific design feature of a certain model of circuit breakers.
No previous event reports regarding this condition have been submitted by RBS in the last three years. SAFETY SIGNIFICANCE NRC FORM 366A (11-2015) 7 1 \ NRC FORM 366A (11-2015)
: 1. FACILITY NAME River Bend Station -Unit 1 LICENSEE EVENT REPORT (LER) CONTINUATION SHEET 2. DOCKET NUMBER 0500-0-I .,. Page 3 of 3 U.S. NUCLEAR REGULATORY COMMISSION
: 3. LER NUMBER D NUMBER NO. I . YEAR SEQUENTIAL REV u-1 °0 s 1-0 The plant was in cold shutdown when the vendor notification was received.
Technical Specification limiting condition for operations for the affected system were entered, and the required actions were completed.
Dedicated operators were assigned to implement the standing order to mitigate the failure mode. All safety-related Masterpact circuit breakers with the control circuit configuration that applied a standing close signal were modified to eliminate this condition prior to plant startup. A probabilistic risk analysis of this condition has concluded that this condition of very low risk significance. (NOTE: Energy Industry Identification System component function identifier and system name of each component or system referred to in the LER are annotated as (**XX**) and [XX], respectively.)
\' NRC FORM 366A (11*2015)}}

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