05000352/LER-2015-002
| Limerick Generating Station, Unit 1 | |
| Event date: | 08-27-2015 |
|---|---|
| Report date: | 10-22-2015 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 3522015002R00 - NRC Website | |
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Unit Conditions Prior to the Event Unit 1 was in Operational Condition (OPCON) 1 (Power Operation) at approximately 100% power. There were no other structures, systems or components out of service that contributed to this event.
Description of the Event
On Thursday, September 3, 2015, Limerick Unit 1 was operating at approximately 100% power preparing to conduct corrective maintenance on the OB Standby Gas Treatment System (SGTS) (EllS:BH) differential pressure (dP) controls (EllS:PDC). At 1657 hours0.0192 days <br />0.46 hours <br />0.00274 weeks <br />6.304885e-4 months <br />, OB SGTS was removed from service and declared inoperable to support repair of the degraded dP control instrument.
OB SGTS was restored to operable status on September 4, 2015, at 2050 hours0.0237 days <br />0.569 hours <br />0.00339 weeks <br />7.80025e-4 months <br />, after replacement of the degraded instrument (PDY-076-161B). SGTS dP control for Unit 2 was not affected. It was later identified by Engineering that the existing degraded dP control instrument rendered the OB SGTS subsystem inoperable. Also, firm evidence existed indicating that the condition was present since August 27, 2015, when the degraded instrument failed off-scale high. The degraded SGTS subsystem had been statused as degraded but operable.
TS 3.6.5.1.1 Reactor Enclosure Secondary Containment Integrity Surveillance Requirement (SR) 4.6.5.1.1.c.2 requires operating one SGTS subsystem for one hour maintaining greater than or equal to 0.25 inches of vacuum water gauge in the reactor enclosure at a flowrate not exceeding 2500 cfm. It was identified after the instrument was replaced that the degraded dP control on the OB SGTS subsystem could result in the flowrate exceeding the 2500 cfm TS limit during a reactor enclosure drawdown. Per TS 3/4.6.5 Bases, the 2500 cfm limit ensures that the reactor enclosure design leak tightness is maintained.
When one SGTS subsystem is inoperable TS 3.6.5.3 Standby Gas Treatment System, Action a, requires restoration of the inoperable SGTS subsystem within 7 days. OB SGTS was declared inoperable on September 3, 2015, at 1657 hours0.0192 days <br />0.46 hours <br />0.00274 weeks <br />6.304885e-4 months <br />, to support corrective maintenance. However, firm evidence of OB SGTS inoperability due to the degraded dP channel was identified to be present from August 27, 2015, at 0239 hours0.00277 days <br />0.0664 hours <br />3.95172e-4 weeks <br />9.09395e-5 months <br />, to September 4, 2015, at 2050 hours0.0237 days <br />0.569 hours <br />0.00339 weeks <br />7.80025e-4 months <br /> (approximately 8 days and 18 hours2.083333e-4 days <br />0.005 hours <br />2.97619e-5 weeks <br />6.849e-6 months <br />).
This event involved a condition prohibited by Technical Specifications. Therefore, this LER is being submitted pursuant to the requirements of 10CFR50.73(a)(2)(i)(B).
Analysis of the Event
There was no actual safety consequence associated with this event. The potential safety consequence of this event was minimal.
On September 4, 2015, Operations requested an Engineering review of the operability determination for the degraded OB SGTS dP control instrument. On October 1, 2015, Engineering completed a technical evaluation which identified that, with the degraded SGTS dP control instrument, OB SGTS flow following a reactor enclosure drawdown would exceed the 2500 cfm SGTS TS limit. Therefore, the degraded instrument rendered OB SGTS inoperable.
Per the UFSAR Section 6.5.1.1, SGTS is designed to exhaust sufficient filtered air from the reactor enclosure and/or refueling area to maintain a negative pressure of about 0.25 inch wg.
in the affected volumes during secondary containment isolation. The failed dP control instrument would have resulted in the OB SGTS subsystem operating at maximum flow during a secondary containment isolation. This would have caused the TS limit of 2500 cfm to be exceeded.
Cause of the Event
The cause of the OB SGTS subsystem inoperability was a degraded SGTS dP control instrument.
Corrective Actions Completed The degraded SGTS dP control instrument was replaced and the OB SGTS subsystem was restored to operable status.
Previous Similar Occurrences There were no previous similar occurrences in the last five years of reportable unplanned SGTS inoperability due to a condition prohibited by TS.
Component data:
System: BH Emergency/Standby Gas Treatment System Component: PDC Control, Differential, Pressure Component number: PDY-076-161B Manufacturer: M430 Siemens Energy & Automation.
Model number: 77-16