05000388/LER-2002-006

LER-2002-006, 1 OF 3

Susquehanna Steam Electric Station - Unit 2
Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications
3882002006R00 - NRC Website

Susquehanna Steam Electric Station - Unit 2 05000388 2002 � - � 006 � - � 00 2 � OF 3

EVENT DESCRIPTION

On October 30, 2002 with Unit 2 in Mode 1 (Power Operation) at 100% power, Local Leak Rate Testing (LLRT) of the Unit 2 Reactor Core Isolation Cooling (RCIC; EIIS Code: BN) exhaust check valve (249F040) could not be satisfactorily completed as a result of excessive leakage past the valve. The check valve (outboard Primary Containment Isolation Valve; EIIS Code: 1SV), in conjunction with the inboard Unit 2 RCIC exhaust isolation valve (HV249F059), provides containment isolation between the suppression pool and the Unit 2 RCIC 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. By virtue of their design similarities, inspection of the valve had been initiated as part of the corrective action plan developed following failure of the Unit 1 High Pressure Coolant Injection (HPCI; EIIS Code BJ) exhaust check valve on September 3, 2002 (reference LER 50-387/2002-007-00). 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. Disassembly of RCIC check valve 249F040 revealed that the disk assembly was off its seat. When the minor force associated with the weight of a removal tool was applied to the valve disk, the disk returned to a fully seated position.

Additionally, there was no apparent degradation of the check valve's seating surfaces.

Investigation has concluded that the Unit 2 RCIC Turbine Exhaust Check Valve, 249F040, failed to fully seat following the last previous operation of the RCIC turbine on October 3, 2002. The RCIC system had been placed in the pressure control mode of operation at that time to support the Unit shutdown initiated in response to a fire at Startup Transformer No. 20 (EIIS Code: FK) (reference LER 50-387/2002-006-00).

Although actual reverse flow through the check valve may have provided enough force to dislodge the valve disk and effect isolation, the RCIC Check Valve 249F040 has conservatively been classified as inoperable and in non-compliance with Technical Specifications from Unit Startup on October 12, 2002 until the time the valve was removed from service on October 30, 2002, a period of 18 days.

CAUSE OF EVENT

The cause of the Unit 2 RCIC Turbine Exhaust Check Valve failure to fully close was mechanical interference between the guide rib and disk assembly. This interference was likely a combination of:

A small amount of dirt and/or corrosion products caught between one or more of the guide ribs and the valve disk assembly, and Contact between the unchamfered edge(s) of one or more of the guide rib notches made during the manufacturing process against the disk assembly.

Neither of the above stated conditions, alone, would likely have acted to hold the valve open. The valve only became susceptible to these conditions when the disk assembly was positioned, by virtue of operation at low steam flow conditions, in a lower vertical location during the October 3 Unit shutdown.

The failure mode of the Unit 2 RCIC Turbine Exhaust Check Valve was substantially different from that experienced on the Unit 1 HPCI Turbine Exhaust Check Valve as described in LER 50-387/2002-007- 00. The failure modes identified through these two events were not found to be present during inspections of all other similarly constructed valves (Unit 1&2 HPCl/RCIC exhaust check valves).

Susquehanna Steam Electric Station - Unit 2 05000388 2002 � - � 006 � - � 00 3 � OF 3

ANALYSIS/SAFETY SIGNIFICANCE

This event is reportable as a condition prohibited by Technical Specifications per 10 CFR 50.73(a)(2)(i)(B) for Unit 2 in that the Primary Containment Isolation Valve 249F040 was likely inoperable and in non- compliance with Technical Specifications for a period of 18 days. This period exceeds the action completion times allowed in Technical Specification 3.6.1.3. However, the RCIC Exhaust Line Primary Containment penetration was still able to maintain the Primary Containment Isolation safety function because the inboard isolation gate valve HV249F059 remained operable to complete the isolation as necessary. Two as-left LLRTs performed on November 1 and 3, 2002 fully verified the integrity of this valve. In addition, a review of system operation revealed that the RCIC turbine was at low speed and steam flow during the recent pressure control mode of operation. These conditions would not have allowed the valve to open to the same degree as that which would have been attained during full flow conditions. At the lower flow, the disk may have been placed vertically in the valve body at a location where the forces acting on the disk assembly were in dynamic equilibrium. At the higher steam flows associated with a quarterly surveillance run, the disk assembly is located higher in the valve and would more readily have overcome any mechanical resistance that may have existed. Based on these arguments, the valve would likely have reseated following the next Unit 2 RCIC full flow run. More importantly, the valve would likely have reseated to perform its isolation function if subjected to the forces present during actual reverse flow conditions. At no time was the RCIC 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 December 30, 2002.

CORRECTIVE ACTIONS

Corrective actions that have been completed:

• Per vendor recommendation, all unchamfered guide rib notches on 249F040 were chamfered to eliminate this potential failure contributor. In addition, the neck bore was cleaned to allow the disk assembly smooth insertion into the valve body.

• The valve was successfully leak rate tested following performance of the above stated corrective actions.

• The LLRT volume was filled following turbine operation under low-pressure steam. No leakage was observed at an upstream drain to 249F040 thus providing additional confidence that the valve would fully seat following operation under low flow conditions.

Past Similar Events:

Failed Component:

Manufacturer:

Model Number:

None RCIC Exhaust Check Valve 249F040 Anchor Darling Valve Co.

W8121410