|Clinton Power Station, Unit 1|
|Reporting criterion:||10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications|
10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition
|4612017002R01 - NRC Website|
|Person / Time|
|From:||Stoner T R|
Exelon Generation Co
Document Control Desk, Office of Nuclear Reactor Regulation
|Download: ML17192A259 (6)|
Infocollects Resource@nrc gov, and to the Desk Officer, Office of Information and Regulatory Affairs, used to impose an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection 3. LER NUMBER 1. FACILITY NAME Clinton Power Station 2017 002
PLANT AND SYSTEM IDENTIFICATION
General Electric—Boiling Water Reactor, 3473 Megawatts Thermal Rated Core Power Energy Industry Identification System (EIIS) codes are identified in the text as [XX]
Failure of the Division 1 Diesel Generator Ventilation Fan Load Sequence Relay Circuit During Concurrent Maintenance of RHR Division 2 Results in an Unanalyzed Condition A. Plant Operating Conditions before the Event Unit: 1 Mode: 1 Event Date: March 9, 2017 Event Time: 0319 Mode Name: Power Operation Reactor Power: 99 percent
B. DESCRIPTION OF EVENT
On March 7 at 2258, an area operator detected a "clicking sound" coming from safety related Unit Substation 1A (1AP11E). The sound was determined to be emanating from relay 427X2- 41A (X2 relay) which was cycling every ten (10) seconds. The model of this component is an Agastat Time Delay Relay (TDR) that provides the signal to reset the Load Shed and Resequencing circuit for Div. 1 Emergency Diesel Generator (EDG) Room Vent fan 1VDO1CA.
Div. 1 EDG was declared inoperable on March 9 when it was determined that 1VDO1CA was unable to respond to a demand signal.
An event investigation determined that relay 427X3-41A (X3 relay) in the undervoltage circuitry for 1AP11E was not operating as expected, causing the cycling of the X2 relay. The effect of this condition was that (1) the associated Fan 1VDO1CA would be unable to start automatically or manually when required, and (2) the ability of the Div. 1 fan to properly sequence following restoration from a bus undervoltage condition was impacted. During an assessment of relay replacement history, it was established that the X3 relay which was originally an ITE Gould J13 relay had been replaced with a General Electric CR120B relay in 2008. The replacement X3 relay combined with the installed X2 relay satisfactorily passed post maintenance testing in 2008 (including undervoltage functional testing) and also passed integrated surveillance testing in 2009. A "like-for-like" replacement of the X2 relay was performed in 2011 and passed integrated surveillance testing in 2011 and 2013. The investigation determined, however, that the combined operating characteristics of the X3 and X2 relays was adversely impacted by a used to impose an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information Regulatory Commission, Washington, DC 20555-0001, or by e-mail to Infocollects Resource@nrc gov, and to the Desk Officer, Office of Information and Regulatory Affairs, httpl/www.nrc.qov/readinci-rm/doc-collections/nureqs/staff/srl022/r3/) collection second "like-for-like" replacement of the X2 relay in May 2016. Through an error in work order planning, no testing was specified for this "like-for-like" replacement. However, the impact on the operating characteristics of the electrical circuit for this X2 replacement would not have been detected by the procedurally required post maintenance testing. Due to the successful integrated testing since 2008, variations in the dropout voltage and associated impacts of the relays used in this circuit had not been previously recognized.
The combination of replacement of the X3 (ITE Gould J13) relays with CR120B relays in 2008 and the perceived "like-for-like" replacement of the Agastat X2 relay in May 2016 established a relay coordination condition where the required relay sequence of operation was adversely affected due to variations in the manufacturer's electrical characteristics of the relays supplied for the electrical circuit. The cause of the event was attributed to not understanding the design basis of the circuit subject to relay coordination and the impact of the change in specific components as part of the 2008 design change.
Following the replacement of the CR120B relay with the original ITE Gould J13 relay design and successful undervoltage testing, the Div. 1 EDG was restored to OPERABLE status.
C. CAUSE OF EVENT
The cause of the event was attributed to not understanding the design basis of the circuit subject to relay coordination and the impact of the change in specific components as part of a 2008 design change. Since the critical characteristics of the EDG ventilation fan logic were not clearly documented or known, it was assumed that differences in relay dropout voltages were not material to the design function.
D. SAFETY ANALYSIS
There were no actual safety consequences associated with the event described in this report since offsite power remained available during the event and no other events occurred necessitating ECCS actuation. Plant systems necessary to conduct a safe and orderly shutdown remained available during this event. The Division 2 EDG, although inoperable intermittently to perform required surveillance testing would have been able to be restored expeditiously in the event of an accident in conjunction with a loss of offsite power.
Infocollects Resource@nrc gov, and to the Desk Officer, Office of Information and Regulatory Affairs, used to impose an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection 3. LER NUMBER 1. FACILITY NAME Clinton Power Station 2017 002 - 01 The event is reportable in accordance with 10 CFR 50.73(a)(2)(i)(B) as a condition which was prohibited by the plant's Technical Specifications. This event is also reportable in accordance with 10 CFR 50.73(a)(2)(ii)(B) as a condition that resulted in the plant being in an unanalyzed condition that significantly degraded plant safety. Additionally, a review of plant logs identified occurrences when the Div. 2 EDG was made inoperable to support surveillance testing during this time period. As a result, this event is also reportable as a loss of safety function per 10 CFR 50.73 (a)(2)(v)(D) which is considered a safety system functional failure.
E. CORRECTIVE ACTIONS
The corrective actions taken include replacing the existing CR120B relay with an original ITE Gould J13 relay design and successfully performing the undervoltage test. A circuit modification is in progress that will eliminate the vulnerability to the lower dropout voltage settings expected from use of the CR120B relays. A procurement engineering standard will be developed to address complex relay logic schemes and what critical characteristics should be selected for evaluation when performing a safety basis, a commercial grade dedication, and/or item equivalency evaluation for complex circuits involving relay coordination.
F. PREVIOUS SIMILAR OCCURENCES
No previous occurrences involving similar circuits and relay interactions have been identified.
G. COMPONENT FAILURE DATA
No CPS component failures associated with this event were identified.