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 NRC/IE
To:
References
IN-85-068, NUDOCS 8508120603
Download: ML031180218 (4)

SSINS No.: 6835 IN 85-68 UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF INSPECTION AND ENFORCEMENT

WASHINGTON, D.C. 20555 August 14, 1985 IE INFORMATION NOTICE NO. 85-68: DIESEL GENERATOR FAILURE AT CALVERT

CLIFFS NUCLEAR STATION UNIT 1

Addressees

All nuclear power reactor facilities holding an operating license (OL) or a

construction permit (CP).

Purpose

This information notice is provided to alert licensees of a potentially signif- icant safety problem involving cracked interpolar connecting bars that connect

the damper circuit of each rotor pole to the damper circuit of the adjacent

rotor pole of the emergency diesel generator.

It is suggested that recipients review this information for applicability to

their facilities and consider actions, if appropriate, to preclude similar

problems at their facilities. However, suggestions contained in this informa- tion notice do not constitute NRC requirements; therefore, no specific action

or written response is required.

Description of Circumstances

On May 14, 1985, Calvert Cliffs Unit 1 was shut down for refueling. An overspeed

test on the No. 11 emergency diesel generator (EDG) was initiated with the

generator not connected to its essential bus. During the test, plant personnel

who were stationed in the proximity of the EDG* heard loud, metallic rapping

sounds coming from the generator section of the EDG and immediately terminated

the overspeed test. Examination of the generator revealed that insulation on

the stator windings had been rubbed and abraded to the point where one stator

winding had been exposed. Further examination determined the cause of the

failure to be a broken interpolar connecting bar on the rotor. The bar initially

had broken free on one end and damaged the stator windings. Because the

generator was not connected to its essential bus, electrical field

• Calvert Cliffs is a 2-unit PWR station with three EDGs that are designated as

follows: Unit 1 (EDG 11), Unit 2 (EDG 21), and shared (EDG 12).

8508120603

IN 85-68 August 14, 1985 excitation had not been applied and no electric arcing

occurred when the stator

windings were damaged. The damaged generator was replaced

with a spare genera- tor and a detailed investigation was undertaken involving

the vendor, Louis

Allis, and the licensee.

A metallurgical analysis on the failed interpolar connecting

predominant cause of failure to be high stresses resulting bar determined the

from periodic

centrifugal loading. To a lesser degree, bending of

the connecting bar, during

initial installation, and thermal expansion also were

considered to be contrib- uting factors in the failure. The licensee reports that

the analysis indicated

the problem was of a design nature and not the result

of a material defect.

Radiographic testing on the damaged generator showed

several cracks in the

remaining interconnecting bars.

The investigation into the generator design determined

that the interpolar

connections between rotor poles are not necessary (a)

if the EDGs are not

operated in parallel, which could cause power pulsations

between units, and (b)

if the plant is operating with a balanced three-phase

electrical load. Calvert

Cliffs does not operate its EDGs in parallel with each

other. Analysis shows

that 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 initiated

a program to remove the interpolar connecting bars from

the three EDGs in

service at Calvert-Cl-iffs Units 1 and 2.

On the basis of these findings, the licensee removed

EDG No. 21 from Unit 2 service on May 26, 1985 to determine if the interpolar

connections on that EDG

were 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 1 or 2) was declared inoperable. Under this set of conditions, Calvert Cliffs

Unit 2, which was operating at 100% full power, started

shutting down. In

order to continue power operation of Unit 2, the replacement

EDG from Unit 1 was connected and aligned to Unit 2 to provide the necessary

emergency power

source and the shutdown was terminated.

The licensee's corrective action, to terminate the limiting

condition for

operation, discussed above, involved the immediate removal

of the interpolar

connections from EDGs 12 and 21. Postmodification qualification

conducted on both EDGs, proved to be satisfactory. testing, The licensee's longer term corrective action plans are

to remove the interpolar

connections 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 bars

are installed so that each of the eight rotor poles

on the generator has two

connecting bars, one installed on the front and one

on the back of each rotor

pole. Noncontinuous damper circuit design does not

employ interpolar connec- tions between the damper circuits on the rotor poles.

IN 85-68 August 14, 1985 The safety concern of this event is the common cause failure mechanism if

similar cracked connecting bars exist on all diesel generators at a nuclear

power plant (as was the case at Calvert Cliffs). Cracked connecting bars can

lead to a condition that adversely affects the operating voltages which are

necessary 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 on

May 21, 1985. A followup letter, dated May 29, 1985, from Louis Allis to the

NRC, briefly discussed the problem and identified other facilities that use

similar Louis Allis generator units. Colt Industries also forwarded a Louis

Allis report dated June 3, 1985, that identified the major cause for interpolar

connecting bars cracking and provided a basis for requesting removal of the

interconnecting bars from similarly designed generators in service at other

facilities. The June 3, 1985 report from Louis Allis to Colt Industries also

was 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 either

removed the interpolar connections or have established plans to have them

removed 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 other

than Louis Allis may have similar design features.

Additionally, licensees are reminded that if corrective actions are taken to

resolve observed degradation of interpolar connecting bars this action should

be reported to the NRC using the existing reporting requirements specified in

10 CFR 50.72 and 10 CFR 50.73.

No specific action or written response is required by this notice. If you have

any questions about this matter, please contact the Regional Administrator of

the appropriate Regional Office or this office.

A/

.Edward.L. ordan, Director

Division of Emergency Preparedness

andlEngineering Response

Office'of Inspection and Enforcement

Vincent D. Thomas, IE

(301) 492-4755 Attachment: List of Recently Issued IE Information Notices

\I-/

Attachment 1 IN 85-42, Rev. 1 August 14, 1985 LIST OF RECENTLY ISSUED

IE INFORMATION NOTICES

Information Date of

Notice No. Subject Issue Issued to

85-42 Loose Phosphor In Panasonic 8/12/85 Materials and fuel

Rev. 1 800 Series Badge Thermo- cycle licensees

luminescent Dosimeter (TLD)

Elements

85-67 Valve-Shaft-To-Actuator Key 8/8/85 All power reactor

May Fall Out Of Place When facilities holding

Mounted Below Horizontal Axis an OL or CP

85-66 Discrepancies Between 8/7/85 All power reactor

As-Built Construction facilities holding

Drawings And Equipment an OL or CP

Installations

85-65 Crack Growth In Steam 7/31/85 All PWR facilities

Generator Girth Welds holding an OL or CP

85-64 BBC Brown Boveri Low-Voltage 7/26/85 All power reactor

K-Line Circuit Breakers, With facilities holding

Deficient Overcurrent Trip an OL or CP

Devices Models OD-4 and 5

85-63 Potential for Common-Mode 7/25/85 All power reactor

Failure of Standby Gas Treat- facilities holding

ment System on Loss of Off- an OL or CP

Site Power

85-62 Backup Telephone Numbers to 7/23/85 All power reactor

the NRC Operations Center facilities holding

an OL and certain

fuel facilities

85-61 Misadministrations to Patients 7/22/85 All power reactor

Undergoing Thyroid Scans facilities holding

an OL and certain

fuel facilities

85-60 Defective Negative Pressure 7/17/85 All power reactor

Air-Purifying, Fuel Facepiece facilities holding

Respirators an OL or CP

OL = Operating License

CP = Construction Permit