05000370/LER-2010-002
| Docket Number | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(v)(B), Loss of Safety Function - Remove Residual Heat 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| LER closed by | |
| IR 05000369/2011004 (28 October 2011) | |
| 3702010002R01 - NRC Website | |
BACKGROUND:
The following information is provided to assist readers in understanding the event described in this LER. McGuire unique system and component identifiers are contained within parentheses. Applicable Energy Industry Identification [EIIS] system and component codes are enclosed within brackets.
The principal safety related function of the Nuclear Service Water System (RN) [BI] is the removal of decay heat from the reactor.
The RN System provides assured cooling water for various Auxiliary Building and Reactor Building heat exchangers during all phases of station operation.
Each unit has two redundant "essential headers" serving two trains of equipment necessary for safe station shutdown, and a "non-essential header" serving equipment not required for safe shutdown.
The RN system is further designed to tolerate a single failure following a Loss of Coolant Accident (LOCA) on one unit with a controlled shutdown on the alternate unit concurrent with a Loss-of-Offsite Power (LOOP) on each unit, or a seismic event causing loss of Lake Norman resulting in controlled shutdown on both units concurrent with a LOOP on both units.
Strainers [STR] are installed immediately upstream of the RN pumps. The related support function of the RN strainer is to ensure adequate suction pressure and flow rate are maintained during normal and accident conditions.
Each strainer can be backwashed either automatically or manually.
The strainers are of the automatic back flush type, and normally each will back flush with service water from RN pump discharge when the pressure drop across the strainer reaches a predetermined value. The strainer differential pressure instrumentation is safety related. Strainer back flush drive motors are powered by normal and emergency sources. A safety injection signal will fail closed the backwash return valve and a loss of instrument air will fail closed the backwash supply and return valve. In these cases, backwash will be controlled manually based on strainer DP. In addition, the manual control of the return valve prevents unnecessary loss of water from the system when aligned to the Standby Nuclear Service Water Pond (SNSWP).
EVENT DESCRIPTION:
On September 15, 2010, to perform required periodic testing, the 2A RN Train was declared inoperable and the suction was realigned to the SNSWP. The periodic test places the 2A RN Train in the emergency alignment to flush stagnant and seldom used piping associated with 2A RN Train supply and return headers.
While aligned to the SNSWP for performing a flush of the suction header piping, the 2A RN Strainer DP indicator failed low after flush flow rates were achieved for the 1A RN and 2A RN Trains. As flow is increased, it is expected that the DP indicator will show an increase in pressure differential. Also when significant clogging in the strainer occurs, the associated DP pressure indication is expected to reflect an increase in pressure differential. Failure of the DP transmitter to properly function could compromise the ability to recognize that a high strainer DP condition exists, which could fail the initiation of backwash, to eliminate strainer clogging, when it may be required during an event.
During the September 15, 2010 testing, the DP decreased as RN system flow was increased. This is an unexpected condition and action was taken to secure from the test. A work request was written to investigate the abnormal DP reading and the event was entered into the site's Problem Identification Process for evaluation and corrective actions. Subsequent action was taken to place the system into an alignment in which continuous backwash could be assured to the affected strainer and 2A RN was returned to operable.
Investigation discovered that a leaking fitting on the low pressure side of the DP loop caused the DP reading to reach a value below zero at high system flows. The instrument line was repaired.
The root cause failure analysis report determined the root causes to be the improper assembly of a tube fitting at the elbow above the low side manifold and an inadequate functional verification which failed to detect air in leakage across the fitting.
The leak manifested as a result of the maintenance activity completed July 24, 2010 which included a calibration of the 2A RN Pump strainer pressure instrumentation used for local indication, control of strainer backwash and alarms. The fitting was galled during a maintenance activity and provided a leak path during vacuum conditions. The galled condition was not discovered during the functional verification completed after the maintenance activity.
The functional verification performed after the maintenance activity verified no leakage at pressure but did not include verification at high flow conditions where DP instrumentation readings can be affected by air in- leakage.
It was subsequently determined that air in-leakage from the galled condition affected the past operability of the 2A RN Train between July 24, 2010 through September 15, 2010. The duration of past operability exceeded the 72 hour8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> completion time allowed by Technical Specifications (TS) 3.7.7 and is reported as a condition prohibited by plant TS in accordance with 10 CFR 50.73(a)(2)(i)(B). In addition, scheduled maintenance or testing performed during these periods rendered Unit 2 "B" Train NSWS inoperable resulting in two trains being simultaneously inoperable for a total period of approximately 22 hours2.546296e-4 days <br />0.00611 hours <br />3.637566e-5 weeks <br />8.371e-6 months <br />; however, the "B" Train was always available during the inoperable time period. Two trains simultaneously inoperable are reported as an event or condition that could have prevented the fulfillment of a safety function in accordance with 10 CFR 50.73(a)(2)(v)(B).
The relevant sequence of events includes:
- 7/24/10: Preventive Maintenance (PM) performed on 2A RN Pump Strainer DP backwash control instruments. The PM includes calibration, instrument venting, and instrument line flushing.
- 9/15/10: The 2A RN Train was declared inoperable to perform periodic testing. An abnormal DP reading was identified during 2A RN Train Periodic Test (PT). DP indication decreased unexpectedly when RN 2A RN flow increased above 9000 GPM and decreased to zero when flow approached 10,000 GPM. The abnormal condition was entered into the site Problem Identification Process to investigate the unexpected DP indication.
- 9/16/10: Operations took action in accordance with station Test Acceptance Criteria (MCTC-1574-RN.V049-01) necessary to restore and maintain 2A RN Train operability when RN strainer back wash supply is incapable of automatically opening. The train was placed in an alignment that allowed backwash to be continuously supplied to the strainer.
- 9/17/10: Vacuum decay test showed excessive in-leakage at a fitting above the low side manifold at the elbow and tubing connection. The tubing and fitting were replaced. A vacuum test was rerun and passed successfully after replacement.
- 9/20/10: The station resumed periodic testing, which also served as the PMT. The "A" Train RN flush was successfully completed and RN strainer DP instrumentation did not unexpectedly decrease to zero at high flow rates. The test also identified an unexpected bias between indicated and expected flow which was documented in site's Problem Identification Process.
- On October 22, 2010, it was determined air in-leakage past a threaded fitting on the low pressure side of the DP loop affected the past operability of the 2A RN Train.
CAUSAL FACTORS:
The root cause analysis identified two root causes. The first root cause was the improper assembly of a tube fitting at the elbow above the low side manifold. The second root cause was an inadequate functional verification completed after PM. The functional verification was performed under normal system operating pressure and did not detect the air in-leakage. The PMT should have verified the function of the DP instrumentation at high flow conditions (vacuum operating conditions).
CORRECTIVE ACTIONS:
Immediate:
1. Testing was terminated and action was taken to restore 2A RN Train to operable status.
2. Placed maintenance procedures that could make the remaining trains vulnerable to the same problem on hold and took action to verify proper DP loop indication on the other loops when performing testing at a vacuum condition.
Subsequent:
1.Identified and corrected a fitting leak on the low side of instrument loop.
2.Performed PMT to demonstrate acceptable performance of the 2A [BI] (RN) strainer DP loop under all design basis conditions. Test was performed at high system flow rates using RN system flush procedure and test instruments were installed to validate assumptions on expected strainer DP.
Planned:
1. Develop PMT at vacuum conditions and revise station processes to ensure the revised PMT (or functional verification) is performed after restoring disconnected field process tubing or after any suspected field process tubing leaks.
SAFETY ANALYSIS:
Engineering analysis was used to make a quantitative assessment of the safety significance for the 2A [BI] (RN) DP indication failure. Using a conservative strainer clogging probability, a Conditional Core Damage Probability (CCDP) was calculated considering the duration of the LCO non-compliance and was determined to be less than 1E-06. This would be considered to be of no significance to the health and safety of the public.
ADDITIONAL INFORMATION:
A recurring event determination was completed during the root cause evaluation and determined this event was not a recurring event.
Nuclear Energy Institute (NEI) 99-02, "Regulatory Assessment Performance Indicator Guideline", defines a Safety System Function Failure identical to 10 CFR 50.73(a)(2)(v) criteria. Consequently, reporting an event as an event or condition that could have prevented the fulfillment of the safety function of structures, systems and components constitutes a Safety System Function Failure for the same basis used to deem the reporting criteria was met.� because the This event is being reported under 10 CFR 50.73(a)(2)(v)� 2B RN Train was declared inoperable for short durations to perform work activities within the period of time that the 2A RN Train was inoperable.