05000263/LER-2014-004
| Monticello Nuclear Generating Plant | |
| Event date: | 2-10-2014 |
|---|---|
| Report date: | 4-11-2014 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| Initial Reporting | |
| ENS 49814 | 10 CFR 50.72(b)(3)(v)(A), Loss of Safety Function - Shutdown the Reactor, 10 CFR 50.72(b)(3)(v)(B), Loss of Safety Function - Remove Residual Heat, 10 CFR 50.72(b)(3)(ii)(B), Unanalyzed Condition, 10 CFR 50.72(b)(3)(v)(C), Loss of Safety Function - Release of Radioactive Material |
| 2632014004R00 - NRC Website | |
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EVENT DESCRIPTION
On February 10, 2014, Monticello Nuclear Generating Plant was in Mode 1 at approximately 88% power.
Plant personnel identified through an extent of condition review that the Emergency Diesel Generators (EDGs) [DG] would not energize permanently connected loads until 10.2 or 10.34 seconds for Division I and Division II, respectively, which exceeded the limitation of Technical Specification (TS) Surveillance Requirement (SR) 3.8.1.12. The surveillance requires that on a simulated or actual loss of off-site power signal, in conjunction with an actual or simulated Emergency Core Cooling System (ECCS) initiation signal, the EDGs auto-start from a standby condition and energize permanently connected loads in seconds. The loss of voltage logic for each division included a 10-second (nominal) time delay relay that must actuate to initiate a load transfer to the respective EDGs whenever the 1AR Transformer [XFMR] secondary voltage is above the secondary voltage relay dropout value. The 1AR Transformer secondary voltage relay is not part of the TS Loss of Power Instrumentation.
As a result, SR 3.8.1.12 was declared not met and both EDGs were declared inoperable at 1650. At 1803, operations isolated the 1AR Transformer which bypassed the time delay relay and declared the EDGs operable.
A review of past surveillances indicates that SR 3.8.1.12 would not have been satisfied since implementation of improved TS in 2006.
EVENT ANALYSIS
This event is reportable in accordance with 10 CFR 50.73(a)(2)(i)(B), Operation or Condition Prohibited by Technical Specifications, 10 CFR 50.73(a)(2)(v)(A-D), Event or Condition That Could Have Prevented Fulfillment of a Safety Function, and 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability of Independent Trains or Channels.
An engineering analysis was performed which determined that the increased EDG output circuit breaker automatic closure time delay would not have exceeded the performance credited in the Updated Final Safety Analysis Report ECCS performance, high energy line break (HELB), or alternate source term (AST) analyses. The condition did not impact manual circuit breaker operation, operation of the EDG itself, or the load carrying capability of the machine. As such, the identified condition does not constitute a Safety System Functional Failure as defined by NEI 99-02 Revision 7.
Additionally, since the condition was within the bounds of existing plant analyses the condition does not constitute an unanalyzed condition as reported in Event Notification 49814.
SAFETY SIGNIFICANCE
Subsequent evaluation determined that although the EDGs would not meet SR 3.8.1.12 there was no loss of safety function. The principal basis for this conclusion was that for the HELB, AST, and ECCS performance analyses assumed a 15 second delay for the EDGs to connect to the essential buses.
The condition identified would not have delayed the EDGs connection to the essential buses beyond 11 seconds, and even at the extreme limit of the testing acceptance criteria, the expected EDG performance remained bounded by the existing analyses.
CAUSE
The apparent cause of this event is that the description of the EDG SR was inadvertently changed during transition from custom TS 4.9.6.3 to improved SR 3.8.1.12 due to improper verification and validation practices of the Improved TS preparation team in 2004-2006.
CORRECTIVE ACTION
The set-points for applicable relays have been revised. The time delay set-point change provides a setting to support EDG performance meeting the 10 second acceptance criterion of SR 3.8.1.12. Additionally, the surveillance procedure will be revised as necessary to implement correct testing for SR 3.8.1.12.
PREVIOUS SIMILAR EVENTS
In 2012, an issue was identified during the NRC's component design basis inspection where the TS did not reflect the total time for the degraded voltage scheme to transfer to the EDGs. The cause of the event was determined to be a legacy issue and no apparent cause could be determined. Improved TS implementation was cited as a possible contributing factor. Actions were implemented to identify cascading logic associated with TS circuits that resulted in time delays not reflected in TS, the issue discussed in this License Event Report was discovered as a result of those actions.