05000388/LER-2005-001

LER-2005-001, DDegradation of Primary Coolant Pressure Boundary due to Recirculation Pump Discharge Valve Bonnet Vent Connection Weld Flaw

Docket Number
Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(ii)(A), Seriously Degraded
Initial Reporting
ENS 41506	10 CFR 50.72(b)(3)(ii)(A), Seriously Degraded
3882005001R00 - NRC Website

PLANT CONDITIONS AT TIME OF EVENT

Unit 1, Mode 1, 100% Unit 2, Mode 4, 0%

EVENT DESCRIPTION

On March 20, 2005, during the performance of the Unit 2 ASME Class 1 Boundary Leakage Test, a socket weld at the connection to the bonnet vent piping for the "B" Reactor Recirculation Pump Discharge Valve HV243F031B (EIIS: AD) was found leaking approximately 10 drops per minute. Subsequent investigation revealed a crack in the toe of the weld at the valve's bonnet vent socket weld. The connection was an abandoned piece of 34-inch piping that contained a 16-inch long pipe stub that had been cut and capped in 1997 due to a previous vibration-induced weld failure.

This condition constituted pressure boundary leakage as defined in Technical Requirement 3.4.2 and TRO Action 3.4.2.0 was entered. On March 21, 2005, an ENS Notification (#41506) was made to the NRC. Based on guidance provided in NUREG-1022, Rev. 2, this material defect in the primary coolant boundary is unacceptable under ASME Code requirements and is reportable under 10 CFR 50.73(a)(2)(ii)(A).

CAUSE OF THE EVENT

The cause of the weld failure was due to inadequate implementation of GE SIL 512 recommendations. GE SIL 512 identified preventative measures for socket welds associated with cantilevered branch lines attached to recirculation gate valves. The GE SIL recommended plugging the branch lines or minimizing the relative motion of the branch line by strapping the line to the recirculation pipe. In 1997, PPL cut the 3/4-inch line and capped it leaving a 16-inch long pipe stub. However, the GE SIL recommended removing the line at the point of highest fatigue and plugging it.

ANALYSIS / SAFETY SIGNIFICANCE

Actual Consequences There were no safety consequences or compromises to public health and safety as a result of the failed weld.

With the reactor shutdown in Mode 4, the available systems were fully capable of compensating for the leakage.

Based on the location of the flaw, the leak path was also capable of being isolated.

Potential Consequences In Mode 4, a complete weld failure would have resulted in an isolation of the leak path and a controlled vessel depressurization by operators. Auxiliary systems, such as control rod drive and low pressure emergency core cooling systems, were also available to maintain vessel inventory, if required.

During Mode 1 operation, the weld flaw would have resulted in increased drywell unidentified leakage and an eventual failure of the 16-inch long pipe stub. This loss of reactor vessel inventory is bounded by the PPL analysis as described in Chapter 15 of the FSAR. Small pipe break LOCA events, including breaks on this particular line, have been evaluated as part of PPL's PRA model. These events contribute less than 2% of the risk of core damage frequency (CDF) and are evaluated as an insignificant contributor ( frequency. This particular line makes up approximately 10% of the total small liquid LOCA event frequency; therefore, the CDF for this particular line break would contribute less than 0.2% of the overall CDF.

CORRECTIVE ACTIONS

Completed Actions

The cracked weld was repaired by removing the pipe stub and welding a plug directly into the bonnet connection.

containment was performed to determine if similar small pipe connections existed. No similar connections were found.

Planned Actions

A new review of Unit 1 and Unit 2 recirculation lines identified by GE SIL 512 will be performed. All recirculation gate valve vent, drain, and stem leak-off lines, previously abandoned will be plugged at the welded joint. Other vibration-prone small bore piping will also be evaluated for susceptibility to weld cracking and corrective actions taken as necessary to prevent future weld failures.

ADDITIONAL INFORMATION

Past Similar Events: Docket No. 50-388, LER 93-009 and LER 97-006