Information Notice 1985-68, Diesel Generator Failure at Calvert Cliffs Nuclear Station Unit 1

Diesel Generator Failure at Calvert Cliffs Nuclear Station Unit 1

ML031180218
Person / Time
Site:	Beaver Valley, Millstone, Hatch, Monticello, Calvert Cliffs, Dresden, Davis Besse, Peach Bottom, Browns Ferry, Salem, Oconee, Mcguire, Nine Mile Point, Palisades, Palo Verde, Perry, Indian Point, Fermi, Kewaunee, Catawba, Harris, Wolf Creek, Saint Lucie, Point Beach, Oyster Creek, Watts Bar, Hope Creek, Grand Gulf, Cooper, Sequoyah, Byron, Pilgrim, Arkansas Nuclear, Three Mile Island, Braidwood, Susquehanna, Summer, Prairie Island, Columbia, Seabrook, Brunswick, Surry, Limerick, North Anna, Turkey Point, River Bend, Vermont Yankee, Crystal River, Haddam Neck, Ginna, Diablo Canyon, Callaway, Vogtle, Waterford, Duane Arnold, Farley, Robinson, Clinton, South Texas, San Onofre, Cook, Comanche Peak, Yankee Rowe, Maine Yankee, Quad Cities, Humboldt Bay, La Crosse, Big Rock Point, Rancho Seco, Zion, Midland, Bellefonte, Fort Calhoun, FitzPatrick, McGuire, LaSalle, 05000000, Zimmer, Fort Saint Vrain, Shoreham, Satsop, Trojan, Atlantic Nuclear Power Plant, Skagit, Marble Hill
Issue date:	08/14/1985
From:	Jordan E L NRC/IE
To:
References
IN-85-068, NUDOCS 8508120603
Download: ML031180218 (4)

SSINS No.: 6835IN 85-68UNITED STATESNUCLEAR REGULATORY COMMISSIONOFFICE OF INSPECTION AND ENFORCEMENTWASHINGTON, D.C. 20555August 14, 1985IE INFORMATION NOTICE NO. 85-68: DIESEL GENERATOR FAILURE AT CALVERTCLIFFS NUCLEAR STATION UNIT 1

Addressees

All nuclear power reactor facilities holding an operating license (OL) or aconstruction permit (CP).

Purpose

This information notice is provided to alert licensees of a potentially signif-icant safety problem involving cracked interpolar connecting bars that connectthe damper circuit of each rotor pole to the damper circuit of the adjacentrotor pole of the emergency diesel generator.It is suggested that recipients review this information for applicability totheir facilities and consider actions, if appropriate, to preclude similarproblems at their facilities. However, suggestions contained in this informa-tion notice do not constitute NRC requirements; therefore, no specific actionor written response is required.

Description of Circumstances

On May 14, 1985, Calvert Cliffs Unit 1 was shut down for refueling. An overspeedtest on the No. 11 emergency diesel generator (EDG) was initiated with thegenerator not connected to its essential bus. During the test, plant personnelwho were stationed in the proximity of the EDG* heard loud, metallic rappingsounds coming from the generator section of the EDG and immediately terminatedthe overspeed test. Examination of the generator revealed that insulation onthe stator windings had been rubbed and abraded to the point where one statorwinding had been exposed. Further examination determined the cause of thefailure to be a broken interpolar connecting bar on the rotor. The bar initiallyhad broken free on one end and damaged the stator windings. Because thegenerator was not connected to its essential bus, electrical field*Calvert Cliffs is a 2-unit PWR station with three EDGs that are designated asfollows: Unit 1 (EDG 11), Unit 2 (EDG 21), and shared (EDG 12).8508120603 IN 85-68August 14, 1985 excitation had not been applied and no electric arcing occurred when the statorwindings were damaged. The damaged generator was replaced with a spare genera-tor and a detailed investigation was undertaken involving the vendor, LouisAllis, and the licensee.A metallurgical analysis on the failed interpolar connecting bar determined thepredominant cause of failure to be high stresses resulting from periodiccentrifugal loading. To a lesser degree, bending of the connecting bar, duringinitial installation, and thermal expansion also were considered to be contrib-uting factors in the failure. The licensee reports that the analysis indicatedthe problem was of a design nature and not the result of a material defect.Radiographic testing on the damaged generator showed several cracks in theremaining interconnecting bars.The investigation into the generator design determined that the interpolarconnections between rotor poles are not necessary (a) if the EDGs are notoperated in parallel, which could cause power pulsations between units, and (b)if the plant is operating with a balanced three-phase electrical load. CalvertCliffs does not operate its EDGs in parallel with each other. Analysis showsthat the potential three-phase electrical load unbalance factor (i.e.,single-phase load) does not exceed 10% of the emergency three-phase load;therefore, it is not considered a concern. Consequently, the licensee initiateda program to remove the interpolar connecting bars from the three EDGs inservice at Calvert-Cl-iffs Units 1 and 2.On the basis of these findings, the licensee removed EDG No. 21 from Unit 2service on May 26, 1985 to determine if the interpolar connections on that EDGwere degraded in a similar manner. Test results indicated cracks were evident.On this basis, the EDG No. 12 (which applies emergency power to either Units 1or 2) was declared inoperable. Under this set of conditions, Calvert CliffsUnit 2, which was operating at 100% full power, started shutting down. Inorder to continue power operation of Unit 2, the replacement EDG from Unit 1was connected and aligned to Unit 2 to provide the necessary emergency powersource and the shutdown was terminated.The licensee's corrective action, to terminate the limiting condition foroperation, discussed above, involved the immediate removal of the interpolarconnections from EDGs 12 and 21. Postmodification qualification testing,conducted on both EDGs, proved to be satisfactory.The licensee's longer term corrective action plans are to remove the interpolarconnections from the replacement (spare) EDG.Discussion:Typically, a diesel generator that employs the continuous damper circuit design(i.e., using interpolar connecting bars) uses 16 connecting bars. These barsare installed so that each of the eight rotor poles on the generator has twoconnecting bars, one installed on the front and one on the back of each rotorpole. Noncontinuous damper circuit design does not employ interpolar connec-tions between the damper circuits on the rotor poles.

IN 85-68August 14, 1985 The safety concern of this event is the common cause failure mechanism ifsimilar cracked connecting bars exist on all diesel generators at a nuclearpower plant (as was the case at Calvert Cliffs). Cracked connecting bars canlead to a condition that adversely affects the operating voltages which arenecessary to operate essential equipment during accident conditions.Because of the generic implications of the Calvert Cliffs event, the vendor,Louis Allis, issued a 10 CFR 21 report on the potential problem to the NRC onMay 21, 1985. A followup letter, dated May 29, 1985, from Louis Allis to theNRC, briefly discussed the problem and identified other facilities that usesimilar Louis Allis generator units. Colt Industries also forwarded a LouisAllis report dated June 3, 1985, that identified the major cause for interpolarconnecting bars cracking and provided a basis for requesting removal of theinterconnecting bars from similarly designed generators in service at otherfacilities. The June 3, 1985 report from Louis Allis to Colt Industries alsowas sent to end-users of the Louis Allis generator units.As a result of the early notifications discussed above, the licensees of TMI/1,Vermont Yankee, and Peach Bottom Units 2 and 3 all report that they have eitherremoved the interpolar connections or have established plans to have themremoved from the affected generator units in service at their facilities.Although no similar interpolar connecting bar failures have been reported,related to other generator units, generators supplied by manufacturers otherthan Louis Allis may have similar design features.Additionally, licensees are reminded that if corrective actions are taken toresolve observed degradation of interpolar connecting bars this action shouldbe reported to the NRC using the existing reporting requirements specified in10 CFR 50.72 and 10 CFR 50.73.No specific action or written response is required by this notice. If you haveany questions about this matter, please contact the Regional Administrator ofthe appropriate Regional Office or this office.A/.Edward.L. ordan, DirectorDivision of Emergency PreparednessandlEngineering ResponseOffice'of Inspection and Enforcement

Technical Contact:

Vincent D. Thomas, IE(301) 492-4755Attachment: List of Recently Issued IE Information Notices

\I-/Attachment 1IN 85-42, Rev. 1August 14, 1985LIST OF RECENTLY ISSUEDIE INFORMATION NOTICESInformation Date ofNotice No. Subject Issue Issued to85-42Rev. 185-6785-66Loose Phosphor In Panasonic 8/12/85800 Series Badge Thermo-luminescent Dosimeter (TLD)Elements85-6585-64Valve-Shaft-To-Actuator KeyMay Fall Out Of Place WhenMounted Below Horizontal AxisDiscrepancies BetweenAs-Built ConstructionDrawings And EquipmentInstallationsCrack Growth In SteamGenerator Girth WeldsBBC Brown Boveri Low-VoltageK-Line Circuit Breakers, WithDeficient Overcurrent TripDevices Models OD-4 and 5Potential for Common-ModeFailure of Standby Gas Treat-ment System on Loss of Off-Site PowerBackup Telephone Numbers tothe NRC Operations Center8/8/858/7/857/31/857/26/857/25/857/23/85Materials and fuelcycle licenseesAll power reactorfacilities holdingan OL or CPAll power reactorfacilities holdingan OL or CPAll PWR facilitiesholding an OL or CPAll power reactorfacilities holdingan OL or CPAll power reactorfacilities holdingan OL or CPAll power reactorfacilities holdingan OL and certainfuel facilitiesAll power reactorfacilities holdingan OL and certainfuel facilitiesAll power reactorfacilities holdingan OL or CP85-6385-6285-61Misadministrations to Patients 7/22/85Undergoing Thyroid ScansDefective Negative Pressure 7/17/85Air-Purifying, Fuel FacepieceRespirators85-60OL = Operating LicenseCP = Construction Permit