ML20210R140
| ML20210R140 | |
| Person / Time | |
|---|---|
| Site: | Limerick  |
| Issue date: | 07/08/1986 |
| From: | Kemper J PECO ENERGY CO., (FORMERLY PHILADELPHIA ELECTRIC |
| To: | Murley T NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION I) |
| References | |
| REF-PT21-86-334-000 179, PT21-86-334, PT21-86-334-000, NUDOCS 8610070131 | |
| Download: ML20210R140 (5) | |
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2863060390 PHILADELPHIA ELECTRIC COMPANY 2301 MARKET STREET P.O. BOX 8699 PHILADELPHI A. PA.19101 JOHN S. KEMPER
..........-P R E SI D E N T VIC E July 8, 1986
......u..c" Dr. Thomas E. Murley, Administrator Region I U.S. Nuclear Regulatory Commission 631 Park Avenue King of Prussia, PA 19406
SUBJECT:
Significant Deficiency Report No. 179 Reactor Protection System Power Supply Circuit Breakers Limerick Generating Station, Unit 2 NRC Construction Permit No. CPPR-107 FILE:
QUAL 2-10-2 SDR No. 179 REF:
a) Telecon, P. K. Pavlides, PECo to R. Gallo, USNRC dated 6/13/86 b) 10CFR Part 21 Report, S. L. Daltroff, PECo to T. E. Murley, USNRC dated 6/16/86
Dear Dr. Murley:
In compliance with 10CFR50.55(e), we are submitting our final Significant Deficiency Report concernin'g the subject Reactor Protection System Power Supply Circuit Breakers.
A 10CFR Part 21 Report (reference b) on this subject was previously submitted for Limerick Unit 1 and Peach Bottom Units 2 and 3.
We trust that this satisfactorily resolves this item.
If further information is required, please do not.hes'itate to contact us.
Sincerely,
/
CBT/la la626861130 Attachment copy to: Director of Inspection and Enforcement U.S. Nuclear Regulatory Commission Washington, DC 20555 E. M. Kelley, LGS Senior Resident Site Inspector St91 8610070131 860708 if)
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j 2863060390 Significant Deficiency Report - SDR No. 179 Reactor Protection System Power Supply Circuit Breakers Limerick Generating Station, Unit 2 NRC Construction Permit No. CPPR-107 INTRODUCTION:
This deficiency involves Westinghouse Type LBB Circuit Breakers, Catalog Number LBB 22250, 2-pole, 400 amp, magnetic trip only, with 125 VDC shunt trip attachment, used in the Reactor Protection System (RPS) breaker panels.
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The power supply circuits served by the RPS circuit breakers are provided with relay protection against undervoltage, overvoltage and underfrequency.
Any one of these abnormal conditions in the power supply will cause the shunt trip coils to actuate causing the breakers to open.
Limerick utilizes two of these breakers in series in each of the two RPS power supplies.
These breakers and their associated protective relays are designed to prevent abnormal conditions in the power supply from damaging or making the RPS equipment inoperable.
Upon loss of power or upon opening of these circuit breakers, the s
RPS will deenergize and initiate a scram signal. If one or two of the series breakers in one of the two RPS power supplies opens,' a half--
scram signal is generated.
If one or two breakers in both of the RPS power supplies opens, a full reactor scram will be initiated.
In compliance with the requirements of 10CFR50.55(e), this Final Report is issued to describe the deficiencies, analyze their safety implications, and describe the action taken to correct them.
This significant deficiency was reported via telecon to R. Gallo, USNRC Region I by P. K. Pavlides on June 13, 1986.
DESCRIPTION OF DEFICIENCY:
4 On one occasion at Limerick Unit 1, the shunt trip coils of the RPS power supply breakers were found to be damaged after apparent normal operation of the breakers.
An investigation into this matter i
has detcrmined that the failure was, in all likelihood, caused by the combination of manual misoperation of the breaker in the presence of a minor manufacturing defect.
A After a successful trip of the breaker, the shunt trip coil can be damaged if attempts are made to reclose the breaker with a trip signal present.
This damage will occur provided that the breaker also has a manufacturing defect known as " insufficient spin-over." The spin-over process is performed to remove excess metal from the bottom of a rivet
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on the breaker handle post.
The presence of the extra =ctal can cause binding between the breaker handle post and breaker operating mechanism. Attempts to close a non-defective breaker with a trip signal still present apparently does not cause damage to the shunt trip coils.
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_2-2863060390 The following sequence of event? and conditions provides a mechanism for common mode failure of these shunt trip coils.
Following operation of the circuit breaker (either after testing or required actuation from abnormal power supply conditions), plant personnel attempt to reset the breaker with the trip signal from the relays still present. This is contrary to *.or al practice.
The handle is pushed to reset and then to the closed position.
Excess metal on the bottom of a rivet on the breaker handle post causes binding between the handle and the breaker operating mechanism.
This binding does not prevent the main contacts from opening.
The binding prevents the 'a' contact of the auxiliary cutoff switch in series with the shunt trip coil frw. opening and current passes through the shunt trip coil (which is designed to be a momentarily energized coil) until it fails.
The failed shunt trip coil is not detected and the breaker is subsequently successfully reset.
The breaker is now back iri service without the capability to trip for undervoltage, overvoltage or under frequency protection of the RPS power supply.
ANALYSIS OF SAFETY IMPLICATIONS:
Failure of both shunt trip coils of one RPS power supply would prevent tripping of that RPS power supply for undervoltage, overvoltage or underfrequency protection.
Operation of the RPS system with these degraded conditions has the potential to prevent a successful scram.
However, this could only occur if all of the N.owing conditions occurred.
Both shunt coils on the breakers c-w US supply had failed open.
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A degraded power supply condition exists on that RPS Bus for a period exceeding four seconds.
The RPS equipment being supplied by the degraded power supply failed to operate properly with the abnormal condition.
A valid RPS trip signal was present.
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. 2863060390 CORRECTIVE ACTIONS:
Four breakers of this type are utilized on Limerick Unit 2 and have been received but not yet installed. The following steps will be implemented prior to fuel load to preclude failures of this type:
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Operation aids (labels) will be affixed to the appropriate breaker panels instructing the operators to assure that trip signals are not present when attempting to close the breakers.
Utilizing a test procedure provided by Westinghouse, each breaker will be tested to assure that the spin-over binding defect is not present.
A circuit interlock will be added to each of the RPS breaker shunt trip circuits.
This interlock consists of wiring the second unused main contact of each breaker in series with its shunt trip coil and 'a' contact.
The additional interlock contact will open the shunt trip coil circuit whenever the breaker is opened or tripped, assuring that the shunt trip coil is not energized beyond its momentary rating.
The neon indicating lamps which are used to determine if a shunt trip signal is present will be replaced with multi-light emitting diode lamps.
These lamps have better visibility and, therefore, will make it easier to determine if the trip signal is present.
Westinghouse has advised us that they are revising the breaker manufacturing drawings to change the amount of spin-over on the handle post.
They are also testing these breakers with the operating handle in the closed position to ensure that binding, which could cause shunt trip coil failures in this adverse operating condition, does not exist.
CONCLUSIONS:
The corrective action described above will eliminate the factors responsible for the shunt trip coil failures.
No further action is planned.
CBT/la la62686m845
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