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**UNITED STATESNUCLEAR REGULATORY COMMISSIONOFFICE OF NUCLEAR REACTOR REGULATIONWASHINGTON, D.C. 20555-0001**

September 18, 1998

**Addressees**

All holders of operating licenses for nuclear power reactors.

**Purpose**

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice (IN) to alert addressees to several recentevents in which inadequate or poorly controlled, non-safety-related maintenance activities resulted in unnecessarychallenges to safety systems. It is expected that recipients will review the information for applicability to their facilities andconsider action, as appropriate to avoid similar problems. However, suggestions contained in this information notice are notNRC requirements; therefore, no specific action or written response is required.

**Description of Circumstances**

BRAIDWOOD UNIT 2

On January 18, 1996, with Braidwood Unit 2 operating at full power, offsite power was lost. Inadequately secured roofingrepair materials located on the roof of the adjacent service building were blown off the roof during a rain storm, causing aphase-to-ground arc on one of two station auxiliary transformers (SATs). As designed, the second SAT tripped when thefirst SAT failed. Both emergency diesel generators (EDGs) automatically started and all safe-shutdown loads automaticallysequenced on to the engineered safety feature (ESF) buses. All automatic bus transfers occurred as designed, maintainingpower to non-ESF buses from the unit auxiliary transformers (UATs). The reactor remained stable at full power throughoutthe event. To restore offsite power to Unit 2, the licensee cross-tied both Unit 2 ESF buses to the respective ESF buses ofUnit 1, which also remained at full power throughout the event.

Inspection of the faulted SAT revealed damage to (1) the Phase B and C bushings and (2) a gas detector relay on top of thetransformer where heavy arcing occurred. The licensee determined that the initiating event was a Phase C arc-to-ground,followed by a Phase B to Phase C arc. A 345-kilovolt switchyard breaker, which connected the SAT to the switchyard ringbus, was also damaged. Approximately one square foot of the Phase B bushing was blown out.

BYRON UNIT 1

On May 23, 1996, with the Unit 1 reactor shut down in excess of a month for a refueling outage, offsite power was lostwhen one of the two Unit 1 SATs isolated because of a Phase A to Phase B current differential relay operation. The secondSAT tripped as designed when the first SAT failed. Both EDGs automatically started and all safe-shutdown loadsautomatically sequenced on to the ESF buses. Because the reactor was shut down, power to non-ESF buses was lost andtwo service water pumps and a station air compressor common to both Byron units tripped. Without the service waterpumps, Unit 2, which was operating at full power, had no cooling water to many loads, including the main generatorauxiliaries, station air compressors, condensate pumps, and condensate booster pumps. Subsequently, Unit 2 was manuallytripped.

The licensee determined that the SAT that initially tripped underwent a phase-to-ground fault on a Phase B insulator. Thefailed insulator was located outdoors, about halfway between the SAT and the auxiliary building. After initiating, the arccontinued down the bus bar, increasing in width until it involved the other two phases. Air and water present in the bus barduct was heated by the arc and resulted in pressurization of the duct.

Chronic water leakage through inadequately caulked insulator mounting holes and through an improperly compressedgasket caused degradation of the insulator metal inserts and insulator material between these inserts. Eventually, the busflashed to ground through the degraded insulator and initiated the transient.

PERRY

On June 5, 1997, Perry was operating at full power when an automatic reactor scram occurred following a three-phase faultin one of two high-voltage secondary winding termination compartments of the UAT. The fault resulted in the actuation ofthe auxiliary transformer phase differential relay, a main generator lockout, a main turbine trip, and fast closure of theturbine control valves.

At the time of the transient, safety-related electrical loads were being supplied by the in-service startup transformer. Thenon-safety-related electrical loads supplied by the transformer automatically transferred to the startup transformer exceptfor the bus that was faulted. Operators stabilized the plant without major complication.

The licensee determined that the fault had developed as a result of an accumulation of moisture and dust that had enteredthe termination compartment housing where a 2-inch-long piece of gasket material was missing from the compartmenthousing-to-transformer sealing surface. The missing piece of gasket material may have been lost during work performedwhen replacing the auxiliary transformer following failure of the transformer on May 30, 1996. The cause of that failurewas attributed to inadequate original design or poor assembly practices or both.

POINT BEACH UNIT I

During a severe storm on January 8, 1998, a ground in the low-voltage-side bus duct caused the failure of the Unit I SAT.This failure and a failure of the 13.8-kilovolt automatic bus transfer to the Unit 2 SAT resulted in the loss of normal powerto the Unit 1 safeguards buses. The EDGs subsequently started and restored power to the buses. Unit 1 remained at 98percent power throughout the transient. Unit 2 was in cold shutdown.

The licensee attributed the failure of the transformer to the buildup of condensation internal to the buses and long-terminsulation degradation in the bus duct on the low-voltage side of the transformer. Before the transformer failed, the circuitbreaker for a heater designed to keep the bus duct dry would not remain closed. Consequently, in August 1996, the licenseeleft the circuit breaker open and wrote a work order for its repair. However, work schedulers did not recognize that the busduct heater was inoperable with the circuit breaker open, and the work order was erroneously classified as "minormaintenance" and given low priority. At the time the transformer failed, the work order was in the "backlog" ofmaintenance items. In addition to the inoperable heater, the licensee determined that the lack of a routine inspectionprogram for the bus ducts may have contributed to the event. Inspection of the bus ducts after the transformer failedrevealed insulation breakdown that would likely have been identified through a periodic inspection program.

**Discussion**

These events illustrate the potentially significant consequences of inadequate or poorly performed maintenance activities onnon-safety-related plant components. Unnecessary challenges to plant safety systems or control room operators or bothcould result from inadequate control or inattention to detail during non-safety-related maintenance activities. The potentialimpact on safety systems of such activities as the re-roofing of nuclear power plant support buildings, the installation ofgaskets, or the caulking of outdoor components, could easily be overlooked. In addition, several of the events discussedhere highlight the need for licensees to have better control over the maintenance activities of contractor and non-stationutility employees, and the need for thorough periodic walkdowns or preventive maintenance on auxiliary transformers andthe associated outdoor bus ducts.

This information notice requires no specific action or written response. However, recipients are reminded that they arerequired to consider industry-wide operating experience (including NRC INs) where practical, when setting goals andperforming periodic evaluations under Section 50.65, "Requirements for Monitoring the Effectiveness of Maintenance atNuclear Power Plants," of Part 50 of Title 10 of the Code of Federal Regulations. If you have any questions about theinformation in this notice, please contact one of the technical contacts listed below or the appropriate Office of NuclearReactor Regulation (NRR) project manager.

Attachment: List of Recently Issued NRC Information Notices

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