05000387/LER-2002-007

LER-2002-007, 1 OF 3

Susquehanna Steam Electric Station - Unit 1
Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications
3872002007R00 - NRC Website

Susquehanna Steam Electric Station - Unit 1 05000387 2002 � - � 007 � - � 00 2 � OF 3

EVENT DESCRIPTION

On September 3, 2002 with Unit 1 in Mode 1 (Power Operation) at 100% power, Local Leak Rate Testing (LLRT) of the Unit 1 High Pressure Coolant Injection (HPCI; EIIS Code: BJ) exhaust check valve (155F049) could not be satisfactorily completed as a result of excessive leakage past the valve. The check valve (outboard Primary Containment Isolation Valve; EIIS Code: ISV), in conjunction with the inboard Unit 1 HPCI exhaust isolation valve (HV155F066), provides containment isolation between the suppression pool and the Unit 1 HPCI turbine exhaust. The excessive leakage was identified when the LLRT test volume could not be filled to obtain as-found hydrostatic test results in preparation for a planned inspection/corrective maintenance effort on the check valve. Inspection of the valve had been initiated when air was found to be leaking to the suppression chamber through 155F049. Because the inspection/maintenance effort was a pre-planned activity, appropriate Limiting Conditions for Operation and associated compensatory measures were already in place when the valve was discovered to be inoperable. Subsequent disassembly of HPCI check valve 155F049 revealed that the resilient valve seal seat was missing, the retaining ring that held the seal in place was separated from the disc and found to be loose in the valve, and that 12 screws used to hold the retaining ring and resilient seal in place were either damaged or missing.

Although it is probable that the retaining ring screws backed out over an extended period, a successfully performed LLRT in March 2002 suggests that the resilient seal was still in place at that time. Conservative interpretation of HPCI system operational history indicates that the earliest and most likely time the last of the retaining ring screws failed, thus allowing the resilient seal to be swept downstream, was during HPCI overspeed testing or startup testing that was performed during the spring 2002 outage. Accordingly, it has been concluded that the valve was inoperable since the beginning of the 13th Fuel Cycle (April 21, 2002) until the time the valve was repaired, reassembled and tested on September 4, 2002, a period of 135 days.

CAUSE OF EVENT

The HPCI exhaust line check valve (155F049) chatters at low steam flow. This chattering and associated vibration occurs after each turbine run and during overspeed testing but is not a factor during normal HPCI operating conditions. The stainless steel retaining ring had signs that indicated the screws used to hold it in place were tack welded. Failure of these tack welds, combined with valve back out. The remaining 2 screw shanks were found in place with their screw heads broken off. The apparent cause of the retaining ring displacement and subsequent loss of the resilient seal is attributed to tack weld failure most probably caused by vibration that occurs when the valve disc impacts the seat during low steam flow conditions. Others factors that may have contributed to the tack weld failures include initial weld quality, age, operational thermal expansion cycles and number of vibration/impact cycles. Dissimilar metal of the disc and the retaining ring screws could also have contributed to tack weld stress. The tack weld failures likely occurred over a several year period but remained undetected as successful routine operation of the system and strong LLRT performance had been considered sufficient condition monitoring to forego periodic internal inspections.

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• Susquehanna Steam Electric Station - Unit 1 05000387 _ � 2002 � - � 007 � - � 00 3 � OF 3

ANALYSIS/SAFETY SIGNIFICANCE

This event is reportable as a condition prohibited by Technical Specifications per 10CFR50.73(a)(2)(i)(B) for Unit 1 in that the Primary Containment Isolation Valve 155F049 was apparently inoperable for a period of 135 days. This period exceeds the action completion times allowed in Technical Specification 3.6.1.3.

However, the HPCI Exhaust Line Primary Containment penetration was still able to maintain the Primary Containment Isolation safety function because the inboard isolation gate valve HV155F066 remained operable to complete the isolation as necessary. The as-left LLRT performed on September 4, 2002 fully verified the integrity of this valve. In addition, boroscopic inspection of piping downstream of the check valve did not reveal the presence of any check valve parts that could potentially affect inboard containment isolation valve operation. It has been concluded that missing check valve parts were swept through the inboard isolation valve to the suppression pool during one of several HPCI turbine surveillance runs this year. The small amount of debris that is now in the suppression pool did not, and will not, interfere with HPCI exhaust line isolation or exhaust line sparger function. The debris, by virtue of its limited size and content, will not impact suppression pool functions or ECCS suction strainer capacity, or create adverse consequences to any plant equipment. At no time was the HPCI injection function adversely affected by the condition of the exhaust check valve. There were no actual adverse consequences to the health and safety of the public as a result of this event.

In accordance with guidance in NUREG-1022, Revision 2, the due date for this report is November 4, 2002.

CORRECTIVE ACTIONS

Corrective actions that have been completed:

• The Unit 1 HPCI exhaust check valve 155F049 was reworked and satisfactorily tested.

• The Unit 2 HPCI exhaust check valve 255F049 was tested and inspected. Although no problems were identified, the retaining ring, resilient seat and screws (tack welded) were replaced as a precautionary measure. The valve was successfully tested following this maintenance work effort.

• Equipment Reliability Preventive Maintenance tasks (ERPM) have been initiated to perform periodic internal inspections for both the Unit 1 and the Unit 2 HPCI exhaust check valves (155F049 and 255F049).

• Equipment Reliability Preventive Maintenance tasks (ERPM) have been initiated to perform periodic internal inspections for both the Unit 1 and Unit 2 Reactor Core Isolation Cooling (RCIC) exhaust check valves (149F040 and 249F040) which maintain a similar design to the failed HPCI check valve.

• As part of the determination of the extent of condition, both the Unit 1 and Unit 2 RCIC exhaust check valves were tested per the LLRT program and inspected for problems with the resilient seat, set screws and tack welds. No problems were found with those subcomponents. The Unit 2 valve failed the LLRT due to an unrelated problem that will be described in a separate LER.

Past Similar Events:

Failed Component:

Manufacturer:

Model Number:

None HPCI Exhaust Check Valve 155F049 Anchor Darling Valve Co.

W8121409 rev. D