ML18095A521

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Special Rept 90-4 Re Improper Operation of Westinghouse Type DB-50 Circuit Breakers.Caused by Deformed Contact Not Touching Breaker Cubicle Secondary Contact Stab.All Recommendations Incorporated Into Maint Procedure
ML18095A521
Person / Time
Site: Salem  PSEG icon.png
Issue date: 10/04/1990
From: Labruna S
Public Service Enterprise Group
To:
NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
90-4, NLR-N90190, NUDOCS 9010170049
Download: ML18095A521 (5)


Text

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PS~G Public Service Electric and Gas Company P.O. Box 236 Hancocks Bridge, New Jersey 08038 Nuclear Department 0 CT o 4 1990 NLR-N90190 United States Nuclear Regulatory Commission Document Control Desk Washington, D.C.

20555

Dear Sir:

SALEM GENERATING STATION LICENSE NOS.DPR-70 AND DPR-75 DOCKET NOS. 50-272 AND 50-311 UNIT NOS. 1 AND 2 SPECIAL REPORT 90-4 This Special Report addresses Westinghouse Type DB-50 Circuit Breakers.

Although not required by the Technical Specifications, Public Service Electric and Gas (PSE&G) is forwarding the attached information regarding improper operation of DB-50 circuit breaker secondary contacts because of the high visibility and sensitivity of the component involved (i.e., Reactor Trip Breakers)*.

The attached report provides detailed information about the circumstances, investigation, conclusions and corrective actions associated with this problem.

Sincerely, Attachment

...... ),,

.. J '~r 9o /t) I 7 OOCf- ~

rt£::i.~

I ir 95-2168 (40Mi 6-89.

Document Control Desk NLR-N90190 C

Mr. J. c. Stone Licensing Project Manager Mr. T. Johnson Senior Resident Inspector Mr. T. Martin, Administrator

.Region I Mr. Kent Tosch, Chief 2

New Jersey Department of Environmental Protection Division-of Environmental Quality Bureau of Nuclear Engineering CN 415 Trenton, NJ 08625 INPO Records Center Suite 1500 1100 Circle 75 Parkway Atlanta, Georgia 30339 Mr. s. B. Ungerer Manager - Joint Generation Projects Atlantic Electric

.PO*Box 1500 Pleasantville, NJ 08232 Mr. Jack Urban*

General Manager -

Fuels Supply Delmarva Power and Light Company 800 King Street, PO Box 231 Wilmington, DE 19899 Mr. J. T. Robb Director of Joint owner Affairs Philadelphia Electric 955 Chesterbrook Blvd.

51A-13.

Wayne, PA 19087-5691 OCT O 4 1990

NLR-N90190 Attachment EVALUATION OF WESTINGHOUSE DB-50 CIRCUIT. BREAKER SECONDARY MOVING CONTACTS Descriptio*n on June 12, 1990, PSE&G's Salem Maintenance Department was performing routine maintenance on Unit 2 Reactor T*rip Bypass Breaker B.

The testing section of the procedure requires verification of shunt trip operation by the local push-button.

The shunt trip function failed to trip the breaker.

Followup investigation determined that the shunt coil did not energize.

This coil is in series with the i7 moving*contact (P/N 204B536H01) installed in the breaker secondary moving contact assembly (P/N 18B1225.G09).

The contact was deformed to the extent that it was not touching the breaker cubicle second*ary contact stab.

The contact was replaced and the breaker retested satisfactorily.

PSE&G believes that a small retaining hook, at the front of the moving contact, may not prevent the contact finger from escaping the hook when the breaker is racked out.

If this happens, the lower portion of the contact may protrude too far below the bottom of the contact base.

Subsequent breaker rack-in is likely*

to deform the moving contact when the breaker cubicle secondary contact stab pushes against it.

PSE&G contacted Westinghouse Corporation on June 12, 1990 concerning this situation.

Several subsequent phone calls resulted in the following comments and recommendations by Westinghouse.

L If the retaining hook opening on the moving contact is not within 3/32 to 5/32 inches, the lower section of the moving contact finger may escape the retai~ing hook and protrude too far below the bottom of the contact base.

2.

Increase the hook opening (by bending the hook) until its*

tip contacts the inside of the molded base.

This will ensure the minimum 3/32 inch dimension and prevent the unhooking, if the rest of the contact is not bent or distorted.

After bending the hook, ensure the finger moves freely within the contact base (P/N 23A6388G05).

3.

This failure may be related to the breaker/cell alignment.

Recommend alignment inspection as part of normal inspection activities (section 9.6 of the Westinghouse Maintenance Program Manual MPM-WOGRTSDB50-0l).

4.

Recommend checking breaker/cell stationary secondary contacts for their location with respect to the upper main studs.

The center lines of.the secondary and main studs should be checked for the 6 5/8 inch +/- 1/16 inch dimension given in the outline diagram, provided in the original DB-50 instruction book (I.B. 33-850-30 or later).

Corrective Action We have incorporated all Westinghouse recommendations into our maintenance procedure entitled "Reactor Trip and Reactor Trip Bypass Air Circuit Breaker Inspection, Lubrication and Testing."

All Unit 1 and 2 trip and bypass breakers were inspected, with the following results:

  • Unit 1 Bypass Breaker A -

5 deformed contacts Unit 1 Bypass Breaker B -

1 deformed contact Unit 2 Bypass Breaker A 2 deformed contacts These deformed contacts were still functioning properly, but we elected to replace them.

PSE&G has inspected all contacts for proper retaining hook gap distance.

Any deficiencies were corrected by bending the retaining hook as recommended by Westinghouse.

Conclusions This problem could potentially result in the inability of the breaker to automatically trip.

These breakers employ redundant trip mechanisms (i.e., shunt and undervoltage).

Thus, both features must be rendered inoperable to prevent breaker tripping.

Both features on both breakers must be rendered inoperable to initiate an ATWS event.

PSE&G tests breaker trip functions prior to declaring ~ny breaker operable.

Since racking-in the breaker is hypothesized to cause contact deformation, subsequent testing.would detect trip function inoperability; as it did on June 12, 1990.

Although this problem could potentially result in breaker inoperability, administrative controls at Salem would result in detection prior to placing the breaker in-service.

A consensus was reached that 10CFR21 did not apply to this situation.

The following specifics support this conclusion:

The secondary moving contact does not contain an inherent defect.

Breaker misalignment, improper racking techniques etc. are required to cause contact deformation.

l

PSE&G conducted a PRA analysis to determjne if a "significant safety hazard" existed.

The Salem PRA does not model the reactor trip breakers.

We did identify the change in Core Damage Frequency (CDF) for the loss of an entire Reactor Protection System (RPS) logic train(s).

The initial CDF value was l.035E-4; loss of an entire logic train increases this value to 1.183E-4; simultaneous loss of both logic trains resulted in a value of 8.0141E-3.

We emphasize that the Salem PRA model can not adequately analyze the effect of a secondary contact or RTB failure.

The analysis assumes a total loss of RPS logic.

The calculated change in CDF is likely to be far more severe than if a more detailed model was available.

Although 10CFR21 does not apply, PSE&G and Westinghouse believe it is prudent to apprise the NRC and affected utilities of this observed problem.

Westinghouse agreed to issue a generic *letter to all DB-50 breaker users, describing the situation and providing the recommendations previously transmitted to Salem.

PSE&G is utilizing the Special Report format to convey this information to the NRC.

WPG:slc SORC Mtg.90-137 General Manager -

Salem Operations