05000338/FIN-2011011-02
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Finding | |
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| Title | Failure of 2H Emergency Diesel Generator Jacket Water Cooling Gasket Resulting in Inoperability during Dual Unit LOOP |
| Description | To adequately evaluate the performance of the EDGs in response to the seismically induced LOOP (including the 2H EDG coolant leak and any identified anomalies), the team performed the following activities: FnConducted walk-downs of the EDGs to evaluate the material condition FnConducted interviews with plant personnel (maintenance, engineering, and operations; root cause investigation team) to determine an accurate account of events related to the EDGs FnReviewed design and engineering documents to verify appropriateness of licensee actions in accordance with design and licensing basis FnObserved corrective maintenance and testing to assess the licensees actions to restore the EDGs In addition, the team reviewed corrective action CRs to evaluate the licensees response to identified deficiencies associated with the EDGs. The vendor manual was referenced to verify alignment with licensee maintenance procedures. Industry operating experience was referenced to identify any potential generic industry issues similar to what was observed at North Anna with respect to the EDGs performance. The team found some issues with EDG performance and identified two URIs that are described in this section. Following the seismic event on August 23, 2011, at 1:51 p.m., all four EDGs started and loaded their respective emergency buses due to a loss of offsite power on both units. About 45 minutes after the EDGs started, a coolant leak was observed on the 2H EDG. At 1:40 p.m., the 2H EDG was manually tripped and secured and the associated emergency bus de-energized. The 2H emergency bus was subsequently re-energized by the SBO diesel. Additionally, the 1J EDG was observed to have minor frequency oscillations. This issue is discussed in further detail in Section 6.0 of this report. Upon further investigation, it was determined that the 2H EDG coolant leak was caused by failure of a fiber gasket located between the exhaust belt and the jacket water cooling inlet jumper on the opposite control side (OCS) of the diesel engine. Initial discovery found the gasket soft and extruding from the flange edge. Due to the excessive coolant leak and in response to a High Jacket Coolant Temperature annunciator that came in during the event, the licensee inspected the cylinder liners, pistons, and rings for damage. No engine damage was found to have occurred. During restoration of the 2H EDG, a small exhaust leak was also identified during the post-maintenance test. The licensee subsequently replaced one exhaust gasket and the extension pipe. The small leak did not have an impact on the EDG to perform its safety function. In May 1999, EDG vendor Fairbanks-Morse issued a Marketing Information Letter, Vendor Technical Manual (VTM) Addenda 72, detailing a new, fiber gasket to replace the previous rubber gaskets for the cooling water bypass fittings. The licensee began installation of the new gaskets in 2001. One major difference was the new fiber gasket was 1/8 thick as opposed to 1/16 for the rubber gasket. The letter also provided recommendations for gasket installation. These recommendations included: FnAllowing a minimum dry time of 10 minutes following application of the gasket adhesive; FnEnsure the fitting surfaces for the exhaust belt and the water inlet flange have the appropriate finish; FnAssemble fitting to exhaust belt and torque nuts to 70 ft/lbs +/- 10 Maintenance procedure, 0-MCM-0701-27, Replacement of Emergency Diesel Generator Cylinder Liners, Revision 19, was used for replacement of the gaskets on 2H EDG in May 2010. The procedure did not include a dry time following application of the adhesive (RTV). Improper curing time for the adhesive could impact the proper alignment of the gasket; too short a time can allow the gasket to move out of place, too much time can harden the adhesive. Following overhaul of the 2H EDG in May 2010, which included replacement of the gaskets, the licensee performed a hydrostatic test to ensure proper restoration. During this test, water pressure was applied (at approx. 50 psi) to the engine block above the normal operating pressure (approx. 30 psi) to ensure no external leakage was occurring; however, coolant leakage was observed on all of the gaskets. It was determined at this time, as documented in Condition Report (CR) 383161 and Corrective Action (CA) 172549, that the RTV adhesive should be allowed to set for 30 V 60 minutes on the gaskets prior to installation for improved sealing. The 2H EDG gaskets were removed and re-installed and passed a subsequent hydrostatic post maintenance test. A subsequent revision to the procedure was approved and implemented in September 2010 to include the adhesive cure time. When the 2H EDG was taken out of service for corrective maintenance following discovery of the coolant leak on August 23, 2011, the licensee removed the OCS heat shields and stress bars, drained the remainder of the coolant, and removed the exhaust components as necessary to gain access to the jacket water inlet elbow. Initial inspection of the water by-pass inlet revealed the gasket protruding past the inlet fitting indicating that the gasket might not have been properly aligned when originally installed in May 2010, despite having been installed twice and satisfactory completion of the hydrostatic testing. Additional investigation by the licensee revealed that in addition to potential misalignment of the water bypass inlet gasket, the jacket water bypass inlet header adjustable screw and jam nut were potentially inappropriately installed. The adjustable screw and jam nut act as a cantilever on the engine block and bypass inlet fittings. Excessive tightening of the adjusting screw can place more compression on the top of the gasket and cause the gasket to extrude and leak on the bottom of the inlet pipe joint. There was no guidance in procedure 0-MCM-0701-27 for tightening the adjustment screw and jam nut; the procedure has since been revised to include detailed instructions. Following installation of the gaskets in May 2010, 0-MCM-0701-27 required the water bypass fitting bolts be torqued to 50 -55 ft-lb; however, this was in conflict with the vendor recommended 70 Ft.-lbs. as outlined in VTM Addenda 72. According to the vendor, the 50 ft-lb torque specification was applicable to the previous rubber gasket and was specified to reduce the thickness of the gasket from 1/16 (.062 ) to .040-.050 . The new gasket was thicker at 1/8 and the 70 Ft.-lbs. was the specified torque. There are two bolts per fitting and are torqued to ensure appropriate compression was applied between the bypass fitting, the gasket, and the exhaust belt. This discrepancy in torque values was identified by the licensee and documented in CR 347658 in September 2009. After discussion with the EDG vendor, the licensee determined that the lower torque value was acceptable given no leakage up to that time had been observed during hydrostatic testing or operation of the diesel; however, the vendor maintained a recommendation of 70 Ft.-lbs. if leakage was observed. In response to the 2H EDG coolant leak on August 23, 2011, the licensee conducted follow-up discussions with the vendor to determine if 50 Ft.-lbs. was acceptable. The vendor restated the recommendation of 70 Ft.-lbs. and performance of a hydrostatic test at 50 psi. The team questioned whether the lower 50 Ft.-lbs. torque value being applied to the new thicker gasket provided the appropriate compression for sealing. A lack of compression can allow the gasket to absorb water and soften, which can lead to gasket extrusion from the flange edge. The licensee was going to perform a technical evaluation to demonstrate adequate compression was available to the gasket. The procedure has since been revised to include the recommended 70 Ft.-lbs. torque specification. Additionally, in September 2009, the licensee documented in CR 347783 that the EDG water bypass fittings had the incorrect surface finish and were not in accordance with the VTM Addenda 72 recommendation of ensuring the exhaust belt had a 125 micro-inch finish and the inlet flange had a 250 micro-inch finish. Though the CR was written to resolve the discrepancy before the next EDG outage (1J), the procedure was not revised until August 2011, following the 2H EDG coolant leak. The team concluded the licensee failed to properly incorporate or evaluate vendor recommendations regarding installation of the cooling water gaskets. At the time of the teams review, the licensee planned to continue evaluating whether the seismic event accelerated the failure of the gasket. Though the licensee eventually inspected all four EDGs following the discovery of the leak on 2H EDG, the team questioned why the licensee initially determined the leak to be an isolated event without having known the cause. The TS requires a common cause evaluation if one EDG is determined to be inoperable. If the cause cannot be confirmed not to exist on the remaining EDGs, the EDGs should be tested to provide reasonable assurance and the corrective action program should continue to evaluate the common cause possibility for the other EDGs. In the case of the 2H EDG leak, the apparent cause was known to be the gasket failure as documented in CR 439091 on August 24, 2011. At the time, the other EDGs were running at full load to support plant shutdown; however, it was not known if the gaskets were installed properly on these EDGs. The CR recognized that previous related issues existed (i.e., multiple coolant leaks across multiple EDGs); however, the licensee still determined the leak was an isolated event. The team observed that this conclusion was based on lack of visible evidence or result (i.e., coolant leakage), but not on a determination of the actual cause. The licensee did submit work orders to inspect the gaskets on the remaining EDGs, but the initial assessment of this being an isolated event did not appear in accordance with proper corrective action program common cause evaluations. The failure of the jacket water cooling gasket caused a leak on the 2H EDG and consequently, inoperability of the 2H EDG during a dual unit LOOP following a seismic event on August 23, 2011. Additional review by the NRC will be needed to determine whether the lack of adequate procedural guidance for EDG cooling water gasket installation represents a performance deficiency. An unresolved item will be opened pending completion of this review. The issue will be identified as unresolved item (URI) 05000338, 339/2011011-02: Failure of 2H Emergency Diesel Generator Jacket Water Cooling Gasket Resulting in Inoperability During Dual Unit LOOP |
| Site: | North Anna  |
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| Report | IR 05000338/2011011 Section 4OA5 |
| Date counted | Dec 31, 2011 (2011Q4) |
| Type: | URI: |
| cornerstone | Mitigating Systems |
| Identified by: | NRC identified |
| Inspection Procedure: | IP 93800 |
| Inspectors (proximate) | G Kolcum L Suggs M Chakravorty M Franke R Jackson S Park S Walker V Mccree Y Lie Leag Kolcum G Mccoy L Lake M Yoo R Clagg |
| INPO aspect | |
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Finding - North Anna - IR 05000338/2011011 | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Finding List (North Anna) @ 2011Q4
Self-Identified List (North Anna)
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