05000388/LER-2005-004

LER-2005-004, Common Mode Failure of the Inboard 'A' and 'B' Loop RHR Shutdown Cooling Testable Check Valves due to Vibration-Induced Seat Damage

Susquehanna Steam Electric Station Unit 2
Event date:	03-02-2005
Report date:	07-18-2005
Reporting criterion:	10 CFR 50.73(a)(2)(vii), Common Cause Inoperability
3882005004R00 - NRC Website

PLANT CONDITIONS AT TIME OF EVENT

Unit 2, Mode 5, 0% power

EVENT DESCRIPTION

On March 2, 2005, the containment penetration for the 'A' RHR Shutdown Cooling line failed to pressurize during as-found Local Leak Rate testing (LLRT). On March 4, 2005, during performance of the as-found LLRT for the 'B' RHR Shutdown Cooling containment penetration line, the penetration failed to pressurize. An investigation concluded that the LLRT failures were due to leakage past the inboard HV251F050A and B RHR Shutdown Cooling testable check valves. Both valves were disassembled for examination and were found to have significant damage on the body and disk seats.

PPL conducted an investigation, including analysis performed by Structural Integrity and Kalsi Engineering, to determine the cause of the valve damage. The investigation was completed on May 31, 2005, and concluded that the damage was caused by fretting of the valve seats due to cyclic disk motion during plant operation.

This LER is being submitted in accordance with 10 CFR 50.73(a)(2)(vii) for any event where a single cause or condition resulted in at least one independent train or channel to become inoperable in multiple systems or two independent trains or channels to become inoperable in a single system.

CAUSE OF THE EVENT

The cause of the body seat and disk seat damage was due to relative motion between the disk and the seat, which resulted from a combination of the following circumstances:

• Leakage through the Unit 2 inboard RHR HV251F050A and B testable check valves and/or leakage past the F122 bypass valve enables the section of pipe between the check valves and the HV251F015A and B outboard isolation valves to pressurize, eventually to a point of equilibrium with pressure on the reactor side. Upon reaching the point of equilibrium, the only force keeping the swing check valve disk closed is gravity. The valve disk is then free to move relative to the seat as a result of structural vibration and/or hydraulic Recirculation System pump pressure pulsations.

• Extended operation at high core flows accelerates the damage mechanism. Since there is an incubation period associated with fretting failure, the damage was only observed after cumulative operation at high Recirculation System pump speeds associated with high core flow.

• Side-to-side disk clearance greater than vendor recommended specification on the HV251F050B valve aggravated the relative motion of the valve disk.

ANALYSIS / SAFETY SIGNIFICANCE

The Unit 1 and 2 inboard RHR testable check valves HV151(251)F050A and B are 24-inch Atwood-Morrill swing check valves with cast stainless steel bodies and disks, and stellite hard facing on the body and disk seats. The testable check valves are typically in standby service. The valves are stroked approximately eight times per operating cycle for surveillance testing and are opened with system flow for shutdown cooling for approximately 100 hours0.00116 days <br />0.0278 hours <br />1.653439e-4 weeks <br />3.805e-5 months <br /> per cycle.

X-13P(B) Containment Penetration To Reckc Loop 'A' M251FD15P(B) RHR Outboard Isolation Valve Testable Check Valve HI25IFO5ONIE9 Figure 1 HV2S1F122/01 Bypass Valve The LLRT is performed such that it tests the leakage through both the RHR HV151(251)F050A (or B) testable check valves and the HV151(251)F122A (or B) bypass valves (Figure 1). A review of previous Unit 2 LLRT results show that the Unit 2 RHR HV251F050A testable check valve successfully passed its LLRT or the 1000# leak test during the last two refueling outages. However, prior to this recent LLRT failure, the 'B' RHR testable check valve failed its LLRT during the last two refueling outages. One of the failures was attributed to the RHR 50B bypass valve F122B. No seat damage was noted, but due to radiation dose considerations, the 'B' RHR testable check valve disk was replaced and the valve rebuilt. The second failure was attributed to the 'B' RHR check valve. Although no seat damage was found, the valve seat was lightly lapped and the disk was replaced.

A review of the Unit 1 LLRT history identified that the RHR testable check valves (i.e., HV151F050A and B) were tested during the last three refueling outages and the results were acceptable. The valves have no prior record of seat damage. The Unit 1 RHR injection check valves are not expected to be subject to the same Susquehanna Steam Electric Station Unit 2 05000388 2005 - 04 - 00 damage mechanism observed in the Unit 2 valves because the Unit 2 recirculation pumps operate at higher pump speeds than the Unit 1 pumps to achieve the same core flow.

Actual Consequences The HV251F050A and B testable check valves are Primary Containment Isolation Valves (PCIVs) and have a function to open to allow Low Pressure Coolant Injection (LPCI) and Shutdown Cooling flow to the reactor vessel. Additionally, these valves are part of the reactor coolant pressure boundary (RCPB).

The disassembly and inspection of the Unit 2 RHR HV251F050A and B valves during the recent refueling outage revealed significant as-found damage to only the body and disk seats. Internal hardware such as the disk nut and pin, hinge arm, washers and bushings, were found in generally good condition. In the HV251F050B valve, one shaft bushing was cracked and there was evidence of wear at the shaft end cover.

However, this wear would not have adversely impacted the valve's operation. No internal parts were found to be missing and therefore damage due to loose parts was not a concern.

refueling outage and the valves would have opened to allow flow for LPCI, if required. There was no challenge to primary containment that would have required the valves to perform their primary containment isolation function. Additionally, there was no evidence of leakage noted at the pressure seal or shaft packing area, so the RCPB function was not compromised. Therefore, there were no actual safety consequences as a result of the damage found in the RHR HV251F050A and B testable check valves.

Potential Consequences In the event an accident had occurred which required the affected valves to perform their PCIV function, the affected penetrations could have been isolated by the outboard PCIVs (i.e., the HV251F015A and B valves).

These are normally closed valves and were operable for the primary containment function when they were opened (e.g. for shutdown cooling). The HV251F015A and B outboard isolation valves are not routinely stroked at power. Any potential consequence would have required an accident with primary containment isolation during the time one of the outboard PCIVs was open, and a failure of the outboard PCIV to close. In this unlikely case, one penetration would not have been completely isolated. The HV251F050A and B testable check valves would still have closed, but significant seat leakage would be expected. However, any leakage through this penetration would have been contained by the closed system of RHR piping beyond the outboard PCIVs. Therefore, any potential consequence of this condition would have been minimal.

CORRECTIVE ACTIONS

Completed Actions

• The Unit 2 RHR HV251F050A and B testable check valves were disassembled and repaired. Satisfactory as-left LLRT results were obtained.

• Vibration monitoring instrumentation was installed inside the Unit 2 drywell on selected points in the RHR and Reactor Recirculation systems to allow for vibration monitoring during the fuel cycle.

RHR HV251F050A and B inboard testable check valves.

Planned Actions

• Procedure TP-264-034, Reactor Recirculation/RHR Injection Loop Hydraulic Response evaluation will be performed allow stroking of the Unit 2 HV251F015A and B outboard isolation valves at power and monitoring of the vibration effects.

• Evaluate a possible modification solution to upgrade the Unit 1 RHR HV151F015A and B outboard isolation valves and/or the Unit 2 RHR HV251F050A and B testable check valves to solve the issue of swing check valve disk positive closure.

ADDITIONAL INFORMATION

Past Similar Events: None