05000461/LER-2016-007

LER-2016-007, Main Steam Line Flexible Hose Intergranular Stress Corrosion Cracking Identified During Refueling Outage

Clinton Power Station, Unit 1
Event date:	05-17-2016
Report date:	07-15-2016
Reporting criterion:	10 CFR 50.73(a)(2)(ii)(A), Seriously Degraded
4612016007R00 - NRC Website

LER 16-007-00 for Clinton Power Station, Unit 1, Regarding Main Steam Line Flexible Hose lntergranular Stress Corrosion Cracking Identified During Refueling Outage

ML16201A232
Person / Time
Site:	Clinton
Issue date:	07/15/2016
From:	Stoner T R Exelon Generation Co
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
U-604297 LER 16-007-00
Download: ML16201A232 (5)

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2016 - 007 00

PLANT AND SYSTEM IDENTIFICATION

General Electric—Boiling Water Reactor, 3473 Megawatts Thermal Rated Core Power Energy Industry Identification System (ENS) codes are identified in the text as [XX]

EVENT IDENTIFICATION

Main Steam Line Flexible Hose Intergranular Stress Corrosion Cracking Identified During Refueling Outage A. Plant Operating Conditions before the Event Unit: 1 Event Date: May 17, 2016 Mode: 4 Mode Name: Cold Shutdown

B. DESCRIPTION OF EVENT

Event Time: 0945 Reactor Power: 0 percent On May 17, 2016 at 0945 while the plant was in Mode 4 (Cold Shutdown) during refueling outage C1R16, it was discovered that water was leaking from two separate flexible hoses connecting the main steam line (MSL) to flow instrumentation. Steam flow during power operations is measured in each MSL using instrument taps off the inside and outside of the respective piping elbow. Pressure sensed in each of the lines is used to derive the steam flow.

Flexible hose 1621-D372C - located at the inner elbow on MSL 'B' had water leaking slowly in a thin, steady stream. The leak originated from the collar on the end of the hose closest to MSL 'B'. No mechanical damage was noted on the flexible hose or attached insulation. The vacuum port protective jacket was in place. Flexible hose 1621-D372E - located at the inner elbow on MSL 'C' had water dripping out slowly, less than 5 dpm. The leak was coming from the area of the vacuum port near the top of the hose, going down the side, and dripping off the bottom. A failure analysis of the flexible hose failures identified the failure mechanism as lntergranular Stress Corrosion Cracking (IGSCC).

Both leaking flexible hoses 1621-D372C and 1621-D372E were replaced during the refueling outage and their respective high side flexible connections were also replaced. No additional leaks were found during an inspection of other flexible hoses connected to MSLs and the reactor recirculation system.

An examination of monitored drywell points prior to plant shutdown for C1R16 showed no change in temperature, pressure or airborne radiation levels.

C. CAUSE OF EVENT

IGSCC resulted in the failed flexible hose discovered during the C1R16 walkdown. The root cause evaluation for this event determined that the corrective actions to prevent recurrence of the condition identified June 18, 2007 (LER 2007-003) failed to eliminate or significantly reduce below threshold any of the three factors required for IGSCC to exist (susceptible material, tensile stresses, and aggressive environment).

D. SAFETY ANALYSIS

There were no safety consequences associated with this condition. This event is reportable under the provisions of 10 CFR 50.73(a)(2)(ii)(A) for the condition of the nuclear power plant including its principal safety barriers being seriously degraded. A plant shutdown was not required since the plant was in Mode 4 during refueling outage C1R16. The plant subsequently transitioned to Mode 5. Discovery of the reportable condition was the result of a planned walkdown to inspect the condition of MSL flexible hoses. Systems necessary to maintain the plant per Technical Specification requirements during the performance of refueling outage activities in Mode 5 were available to perform their safety function.

This event report does not identify any safety system functional failures.

E. CORRECTIVE ACTIONS

The leaking flexible main steam line hoses and the remaining flexible hoses on the MSLs B and C were replaced during C1R16. The remaining inner elbow flexible hoses on MSLs A and D have been scheduled for replacement during the next refueling outage C1R17. A design modification is planned to eliminate or significantly reduce at least one of the three factors required for IGSCC (susceptible material, tensile stress, or corrosive environment) to below the threshold where IGSSC can be initiated.

F. PREVIOUS SIMILAR OCCURENCES

On June 18, 2007, Operations performed a plant shutdown at 1241 hours0.0144 days <br />0.345 hours <br />0.00205 weeks <br />4.722005e-4 months <br /> to assess indications of a drywell steam leak and repair the leak. On June 19 at 0635, Maintenance personnel entered the drywell and found pressure boundary leakage on a one-inch diameter ASME Section III Class II stainless steel braided flexible hose assembly on the 'C' Main Steam Line flow elbow low-pressure instrumentation tap. Operators entered the actions of Technical Specification 3.4.5, which required a plant shutdown due to reactor coolant pressure boundary leakage. The cause of this event was IGSCC. Flexible hose assemblies installed in IGSCC susceptible locations were replaced.

Susceptible flexible hose assemblies that were not currently in service were cut out and the lines were capped. Preventive maintenance was established to periodically replace susceptible flexible hose assemblies installed in IGSCC susceptible locations.

a COMPONENT FAILURE DATA Manufacturer Nomenclature Metal Bellows Corp. 1-inch ASME III Class II, Flexible Braided High Pressure Stainless Steel Hose Senior Flexonics, Inc. 1-inch ASME III Class II, Flexible Braided High Pressure Stainless Steel Hose