The Metal-Enclosed Bus program was a new program, consistent with the
GALL Report, credited with managing the aging affects associated with loosening of bolted bus bar connections and reduced insulation and insulator resistance on bus ducts. Metal-enclosed buses are electrical buses installed on electrically insulated supports enclosed in a metal duct. The parameters monitored include loose connections, embrittlement, cracking, melting, swelling or discoloration of insulation, loss of material of bus enclosure assemblies, hardening of boots and
gaskets, and cracking of internal bus supports. The applicant included these non safety-related bus ducts because the ducts supported compliance with the station blackout regulations. The applicant scoped in the following non-segregated bus ducts (all conductors enclosed in a common metal enclosure) listed below:1,2,3 NBNA031,2,3 NBNA041,2,3 NBNA051,2,3 NBNA061,2,3 NBNA081,2,3 NBNA09Secondary of Engineered Safety Features Service Transformer E-NBN-X03 to NBN-A08 and NBN-A09Secondary of Engineered Safety Features Service TransformerE-NBN-X04 to NBN-A05 and NBN-A06Connection between Bus NBN-A04 and Vital Bus E-PBA-S03Connection between Bus NBN-A04 and Vital Bus E-PBB-S04Connection between Bus NBN-A03 and Vital Bus E-PBA-S03Connection between Bus NBN-A03 and Vital Bus E-PBB-S04The team reviewed
license renewal program basis documents,
aging management review documents, existing and new procedures, and preventive maintenance requirements. The team interviewed the
license renewal project personnel and the responsible plant and design engineers. The team walked down all the in-scope non-segregated bus ducts. The applicant had developed Procedure
82DP-OEE01, Electrical
Aging Management, Draft B, which included qualitative and quantitative acceptance criteria for the inspection of metal enclosed bus internal assemblies. The procedure established the following requirements to monitor for aging effects: (1) inspect a sample of the accessible bolted connections for evidence of overheating; (2) conduct contact resistance testing of a sample of the accessible splice plates to check for loose connections; (3) inspect each bus section for cracks, corrosion, foreign debris, excessive dust buildup, and evidence of water intrusion; (4) inspect the bus insulation for signs of embrittlement, cracking, melting, swelling, or discoloration; and (5) inspect the internal bus supports for structural integrity and signs of cracking. The procedure used visual inspection of bus enclosure assemblies to monitor for loss of material resulting from corrosion and hardening of boots and
gaskets. The applicant will allow no unacceptable indications of cracking and will use
thermography to determine if excessive heating occurred. The metal enclosed bus inspections will be completed before the period of extended operation and every 10 years as specified by the
GALL Report. The team determined that the applicant had an existing preventive maintenance program to inspect and clean bus ducts. The applicant inspected sections of the metal enclosed buses every other outage and performed
thermography on the bus at the transformer connections once every 6 months. At the time of the onsite inspection, from review of the corrective action database, the team determined that the applicant had not found any occurrences of corrosion, loss of material, hardening, foreign debris, excessive dust buildup, water intrusion, or overheating that changed the scope of this
aging management program. The team did review onsite operating experience that reinforced the need for these inspections. The previous operating experience related to Noryl insulation failures and ground faults on non-segregated bus ducts of components not within the scope of
license renewal that resulted in ground faults on the affected buses. Subsequent to the onsite inspection, on March 7, 2010, Unit 1 tripped when a ground fault occurred on Bus
1E-NAN-A03 connecting the non-class 1E 13.8 kV Bus
1E-NAN-S01 to Breaker
1E-NAN-S03B. Because of heavy rains, water intruded past the cover protecting the non segregated bus. The applicant preliminarily concluded the fault resulted from an improperly installed bus duct cover and a breakdown in theNoryl insulation because of aging mechanisms. Because additional information was needed to determine whether a recent age-related failure would impact the proposed
aging management program, this is an unresolved item: Unresolved Item 05000528, 05000529,
05000530/2010007-01, Impact of recent operating experience on Metal-Enclosed Bus program. For the Metal-Enclosed Bus program, although some questions remain related to the impact on this program of recent operating experience, the team concluded that the applicant had performed appropriate evaluations and considered pertinent industry experience and plant operating history to determine the effects of aging on the metal enclosed non-segregated bus ducts. The team concluded that, if implemented as described, the applicant provided guidance to appropriately identify and address aging effects during the period of extended operation
