ML20235L162
| ML20235L162 | |
| Person / Time | |
|---|---|
| Site: | Mcguire |
| Issue date: | 09/30/1987 |
| From: | Carfagno S, Fishman H, Toman G CALSPAN CORP. |
| To: | Hood D NRC |
| Shared Package | |
| ML20235L165 | List: |
| References | |
| CON-NRC-05-86-168, CON-NRC-5-86-168 I-6177-5-1, NUDOCS 8710050374 | |
| Download: ML20235L162 (19) | |
Text
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ENCLOSURE e
0 Interim Report on McGuire 2B Reactor Trip Breaker Failure Evaluation and Recomended Corrective Actions from Westinghouse NRC Docket No. 50-370 FRC Project No. 6177-005 NRC Contract NRC-05-86-168, Task EL-305 FRC Report I-6177-5-1 FRC Group Leader:
NRC Group Leader:
G. J. Toman/H. M. Fishman D. Hood Prepared for U.S. Nuclear Regulatory Comission Washington, DC 20555 a
6 September 30, 1987 Prepared by:
Reviewe) by:
Approved by:
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FRANKLIN RESEARCH CENTER l
DIVISION OF ARVIN/CALSPAN 30th & RACE 5T898T1 MeLA00LpMLA,M M103 97/e503?ift XIF i
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'I-6177-5-1 i
.s' CONTENTS Section' Title hge 1
1-INTRODUCTION 2
2
SUMMARY
OF FAILURE EVALUATION AT NSID l
4 3
DISCUSSION OF FAILURE MODE.
4-DISCUSSION OF WESTINGHOUSE RECOMMENDATIONS FOR EVALUATION-10 OF WELD DEFICIENCIES-5 FRC COMMDJTS ON WESTINGHOUSE RECOMMENDATIONS 12 6
OTHER ISSUES AND CONCERNS 14 16 i
7 CONCLUSIONS f
17 a
RECOMMENDATIONS.
I 6
i 111 r
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- 1. ~ INTRODUCTION
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On July.2, 1987, the 2B reactor trip breaker'(RTB) of the'McGuire Unit 2 plant failed to open upon receipt of an electrical command signal..It had jammed mechanically. On July 3,1987, Duke power Company personnel. observed a
~
second jamming of the RTB during investigation of the problem; however, the f
1 jamming of the RTB was overcome before the.cause of the jamming was determined.
- )
On July 7 to 9, 1987, further attempts were made:to have the'jarned condition recur in the presence of the NRC Augmented Inspection Team. However,'the circuit breaker did not jam during multiple operations even with purposeful attempts to put the RTB-mechanism in an unfavorable position with regard to tripping. Thereupon, it was determined that the RTB should be shipped to the Nuclear Services Integration Division (NSID)'of Westinghouse for further 1
evaluation. During the evaluation at the McGuire plant, it was determined that the weld between the center-pole ~ lever and the pole shaft of the'RTB had completely separated.
It was believed that this weld failure was related to the. jamming of the RTB, but the exact relationship was not known.
This report describes the failure evaluation at NSID, discusses the recommendations made by Westinghouse on September 11,1987 to Westinghouse I
Owners' Group members with DS-type circuit breakers; critiques Westinghouse's presentation to the NRC on September 23, 1987; and provides reconnendations for further actions regarding DS-type circuit breakers, i
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I-6177-5-1 2.
SUMMARY
OF FAILURE EVALUATION AT NSID i
on August 24 through. September 1, 1987, an evaluation of the failure of the McGuire 2B RTB (No. 24Y98508, #4) took place at Westinghouse's NSID facility in Monroeville, PA.
The RTB was tested in the as-received condition starting with basic measurements of key components, which were compared to
]
those of an operable circuit breaker. After initial evaluation, the RTB was f
cycled 42 times with various means employed for tripping the RTB (undervoltage l
trip, shunt trip, and manual operation); no failure to trip occurred. Then, j
1 the RTB was partially disassembled, and the roller on the main drive link was j
lf found to'have been striking the right-hand (viewed from rear of RTB) side frame plate. The weld between the. pole shaft and center-pole lever was observed to be completely broken, which. allowed the roller end of the main drive link to be skewed to the right (viewed from rear) and the roller axis to be canted l
t counterclockwise approximately 3 to 5. degrees.
]
To allow further evaluation of the operation of the RTB, the racking (levering) mechanism, charging motor, and the undervoltage trip attachment j
were removed from the RTB and the remaining portions of the mechanism were returned to operating condition. Then, the RTB was operated approximately 15 more times. Some operations were performed with shims between the roller and side of the drive link in an attempt to cancel the effects of wear. No failure to trip occurred. Spring tension was applied to the side of the main drive link to force it harder against the side frame plate. The circuit a
breaker still tripped. An attempt to force the roller higher onto the cam a
-l (above top dead center) also failed to prevent tripping.
The right side plate and the roller were then replaced to put the RTB in a less worn condition. The RTB was operated, and the new roller was observed to strike the side plate upon closing. At the 22nd cycle, the RTB failed to open. The closing cam was observed to have not completely rotated by 18 degrees (The closing springs were not completely discharged.). The trip shaft was completely free of the trip latch. The. edge of tho' trip latch had moved approximately 1/16 inch above the lower edge of the trip shaft,-indicating that th<a trip latch had operated. The roller of the main drive link was found to be wedged between the right side frame plate and the left-harfd* cam segment of the closing cam. The roller was not canted about its axis as far as it had.
2-6177-5-1
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i been in past closings (it was now riding 1 to 2 degrees counterclockwise).
The condition of the roller with respect to the cam edge was photographed via fiber optics to record the jammed condition.
After deliberation by the evaluation team, a lever was used to relieve the pinching of the roller by the cam and side plate and thus free the roller.
The failure was replicated twice more by manipulation of the closing cam and
'l the roller on the main drive link.
During the first replication, the con-straining link'was removed to verify that the failure was not partially or fully related to binding in the trip latch bearing (evaluation of the bearing in the original side plate had~ revealed damage to the trip latch bearing).
During the second replication of the' failure, the roller'was released from the bound condition by operating the manual charging lever. This verified that further rotation of the closing cam caused the roller to be pushed out of the jammed condition as had occurred during the failure to open in service at the 1
McGuire plant.
In an attempt to determine if the failure of the polo shaft weld was necessary to allow jamming to occur, two additional pole shafts were installed in the circuit breaker.
Both had center-pole levers that were out of alignment in a condition,similcr to the failed pole shaft lever. The two were selected from a batch of 18 pole shafts that were available at NSID where the failure evaluation was being performed. Multiple attempts were made to cause the RTB to fail to open none was successful. Although the roller could be forced to rest between the cam edge and the frame, the roller would not bind.
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'I-6177-5-1 1'
3.
DISCUSSZON OF FAILURE noog 4
Figure 1 provides a labeled view of the RTB mechanism. ' Item 2 is the
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close cam,. item 15'is the roller, item'3 is the constraining-link, and item 19 j
is the' side frame plate-(only. partially.shown). Figure 2 shows the position.
of'the RTB mechanism with the RTB closed. The rear of the RTB is at the>right
'in both figures. Figure 3 conceptually.shows the roller pinched between the side frame plate and the left cam segment as viewed from the rear of thelRTB.'
To actually observe the condition, the arc. chutes must be removed and inspec-J tion. mirrors or fiber optics must be.used to observe the' area where the pinching occurs.
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The roller becomes pinched during the closing action.. As the closing cam rotates, the edge of the roller-is caught (see Figure 4).- Continued rotation' j
of the cam causes the roller to straighten in e clockwise rotatiori about :its axis. This action causes the edge of the roller (marked "W" in Figure 4A) to attempt to separate the cam and the side plate. However, they are not free to move and therefore pinch the roller in place. When the trip latch is released, l
allowing the constraining link (item 3, Figure 1) to be free, the jaming of f
the roller prevents the roller from rolling down the cam face to allow the
-l circuit breaker to open. The jamming of the roller also prevents full dis-l charge of the closing springs, leaving the closing cam 18 degrees from fully l
rotated.
]
Upon removal of the original right side frame plate, the trip. latch a
bearing was found to have a broken edge. The concern that binding of the trip,
latch was partially causing the jamming was alleviated during the second jamming of the RTB when the constraining link was removed and the RTB remained jammed. Binding of the crank shaft on which the close cam is mounted could also have partially caused the binding. However, the crank shaft was found to be free and turning properly when the RTB mechanism was disassembled. Elimi--
l nation of trip latch and crank shaft binding confirmed that the sole cause of the failure was pinching of the roller between the edge of the close cam and i
the right side frame plate, The attempts to jam the circuit breaker with pole shafts with unbroken center-pole-lever welds showed that both lateral displacement of'the roller end of the main drive link and 3-to 5-degree canting of the axis of the __ _ _ _ _ - _ _ _ _ - _ _.
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Position of Mechanism with RTB Closed (Source: Instructions for Low-Voltage Power Circuit Breakers, Types DS and DSL; Westinghouse Electric Corp., Instruction Bulletin 33-790-IE) l J l
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Z 6177-5
- Left Side Frame Plate (When Viewed from Rear of RTB)
-Roller f'
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Roller Wedged Between Left Cam Segment and Right Side Frame
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(Conceptual Drawing, Not Fully to Scale)* *-
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A n*
Y I-6177-5-1, roller was necessary to allow jamming. The, pole shafts with unbroken welds allowed the lat'eral~ displacement and even allowed the roller to strike the side frame. However, they'did not allow. sufficient axial rotation of'the roller.
The roller could not straighten'out.(have the axis turn clockwise) to cause the displacement of the cam and side plate and thereby jam. However,.
f these were new pole' shafts and as such had not experienced wear that could.
lead to allow additional canting of the roller axis.
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.1 ik 1-61Ti-5-11
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4.
DISCUSSION OF WESTINGHOUSE RECOMMENDATIONS l
FOR EVALUATION OF WELD DEFICIENCIES Ir. addition.to the failure of the center-pole lever to pole-shaft weld on the McGuire 2B RTB, three similar weld failures are known to have occurre'd on DS-type circuit breakers. One occurred at a Duke hydroelectric power plant approximately 14 years ago and the two others occurred at nuclear plants within the last year.
In one case, the' circuit breaker. failed to close when both the center-pole lever and anti-bounce lever to pole-shaft welds. failed.
In response to an NRC request, Westinghouse developed inspection recommen-dations for Westinghouse. Owners Group members to use on DS-type breaker pole-shaft welds. These recommendations were issued September 11, 1987, and were provided for all Class 1E DS-416, DSL-416, DS-420, DS-206, and DSL-206 circuit i
breakers. The basic recommendation is to perform a weld inspection with the pole shaft in place in the circuit breaker and the top bracket removed from the RTB. Although removal of the top bracket improves the visibility of the weld on the center-pole lever, the weld is still located between the two side.
l frame plates that are 1 inch apart and the weld is approximately 3 inches below the top of the plates. Thus, assessment of the weld is difficult, especia11j because a closed crack may be difficult to observe visually under the best of conditions.
Short-term evaluations are recommended at the next surveillance. The criteria for short-term acceptance of the three pole-lever welds are as follows:
a a
1.
Completely separated welds:
remove the circuit breaker from service.
2.
Cracked weld:
remove the circuit breaker from main RTB service and use it only as a reactor trip bypass breaker until the weld condition is corrected.
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3.
Excluding the ends of the weld, which may show evidence of a ccid start, the weld should have at least 3/16-in fillet for 90' contin-l uously around the pole shaft. If the fillet is less than 3/16 in, then the weld must have at least a 1/8-in fillet for 120' contin-I uously around the pole shaft.
If these dimensions are not met,'then the RTB should only be used as bypass RTB until the weld condition is corrected.
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- l I-6177-5-1 1
l The long-term actions, to be performed at the next refueling outage 1
according to the Westinghouse letter, are to inspect the remainder of the l
welds on the pole' shaft' excluding the stop lever selds, to. replace the pole j
-shaft if necessary, and to check the alignment of the breaker mechanism.
In the alignment check, the roller on the main-drive link is to be verified as a
riding on the two outside cam segments and, while the breaker is in the closed j
position, Ja not in contact with either side-frame plate.
1 The Westinghouse letter states that the short-and long-term inspection-criteria for the DS-416 apply to the-DSL-416 and DS-420.
The timing of the inspections of the DS-206 and DSL-206 circuit breakers, which Westinghouse clains are less stressed, is left'to the utilities, but is not to exceed the next refueling outage.
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I-5177-5-1
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5.
FRC COMMENTS ON WESTINGHOUSE RECOMMENDATIONS l
The recomrnendations contained in the Westinghouse letter of September 11, 1987 have many shortcomings.
It is clear from the letter that a circuit breaker with a completely severed weld should be removed from service.
How-ever, according to Westinghouse, welds with cracks that are not completely broken or are shorter than 90* (3/16-in fillet) or 120' (1/8-in fillet) may be l
used as a bypass RTB.
No criteria are given for just how degraded these welds must be to be considered inadequate for bypass service, and no reasoning is provided for stating that an RTB with a degraded weld is acceptable for bypass service.
Is 10' of weld acceptable? Is 90' of cracking acceptable? At present, the criteria merely state that if the weld is not completely severed, the circuit breaker is acceptable for a bypass RTB. This is not a logical assumption.
In addition, Westinghouse claimed that the originally specified 180' weld had a conservatively calculated safety factor of 3.5.
This value was never substantiated either in the letter or in the subsequent presentation on September 23, 1987.
Complete inspection criteria for the weld are not given in the letter.
Neither enough information nor a specific standard required for use in the inspection is given. During a meeting between Westinghouse and the NRC held on September 23, 1987 in NRC offices in Bethesda, MD, Westinghouse personnel stated that a lack of fusion would be grounds for rejection of a weld and would require corrective action. However, the Westinghouse letter does not have any such requirement. During the meeting, the selection of 90' of con-tinuous weld was described by Westinghouse as being acceptable for permanent use. The original design required at least 180' of weld with a 3/16-in fillet. The 180* weld design had been proven to be adequate by a 4000-cycle qualification test. The new 90*/120' weld acceptance criteria are based on static analyses and deration factors that do not take dynamic loads and fatigue fully into account and do not account for stress concentrations at the edges of cold starts in the welds. The visual inspection criteria given in the Westinghouse letter allow cold starts even though there is no way of telling if good fusion has occurred after the cold start. No qualification testing has been performed with welds that are 90*/120' long that have cold starts (i.e., no breaker is known to have been cycled 4000 or more' times with such a weld). _ - _ _ _ _ _ _ _ _ - _
I-6177-5 l
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'The calculations of working and failure torquefpresentedlat the September; 23, 1987 meeting indicated that the working torque for the 180' 3/16-in weld
' is 1230 in-lb.
The formulae used are valid for a total.of 180* are length of '
f symmetrically placed welds, not for the_one-sided weld configuration specified by Westinghouse. No explicit formulae are presented!in the AISC* handbook for partial are fillets, but the calculation methodology that should be used is similar to that-described in the section for Eccentric Loads on. Weld Groups.
- The load capacity of partial are fillets is significant1y'less than the are length ratio times the capacity for an all-around circular weld.
Other loading effects, such as bending in the weld due to misalignment of the. roller, were neglected in the Westinghouse analysis..
Experimental validation data were presented to-indicate that, under l
dynamic conditions and without electrical-loed, the torque on the weld is less' l
than the allowable. The experiment involved putting strain gauge rosettes, wired to measure shear, on the shaft on each side of the. lever and closing and opening the RTB. The calibration and the data reduction techniques described 1
were incorrect and the torque data for the weld were meaningless,~but not-necessarily unconservative..This was pointed out to Westinghouse at the l
l meeting.
It may be possible for Westinghouse to reinterpret the data and derive meaningful dynamic weld torque information.
1 To demonstrate that a 1/8-in fillet could be used for 120', Westinghouse performed a static load test. The weld configuration was a ground-down 3/16-in weld. Such a modified weld may have fewer surface' irregularities than a fillet with an 1/8-in bead, and may be significantly stronger than the' weld s
a it is intended to simulate.
In addition to problems with weld acceptance criteria, the 4000-cycle I
qualification limit is being approached by some RTBs. The estimated number of cycles on the McGuire 2B RTB that failed is between 2500 and 3500 cycles.
Westinghouse states that some test circuit, breakers have been cycled to at-least 10,000 cycles; however, it must be assumed that these circuit breakers had 180* welds unless the pole shafts are available and can be inspected to determine weld condition and length.
- Manual of Steel Construction, Eight Edition, American Institutt pf Steel Construction, Inc. (AISC) 1 1 /
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I-6177-5-1 r
6.
OTHER ISSUES AND CONCERNS The observed damage to tSe latch bearing is'the result of the torsional forces on the constraining link and trip latch caused by the unwanted lateral f
displacement of the roller end of the main drive link. The lateral displace-ment could be caused by the breaking of the center-pole-lever weld or by the It is center-pole lever not being fully perpendicular to the pole shaft.
possible that another type of failure could occur due to the misalignment of main drive link and roller if the damage to the trip latch bearing progresses and binds the trip latch pivot pin.
During inspection of the RTB components, damage was noted on the surfaces j
to the closing cam. The cam in the McGuire-RTB is composed of four steel The two outer segments that are sandwiched together and held by three rivets.
segments are heat-treated steel; the two inner segments are non-hardened steel. The surface of the segments is supposed to be of uniform shape except in the area of the stop roller that is fixed in a hollow in the two center segments. However, on the McGuire RTB, the two outer segments are slightly larger than the inner segments, providing the edge for the roller to catch upon.
The cam had also been mushroomed by the drive link roller in a number of Of key concern was mushrooming in the area of the stop roller (item 1 areas.
in Figure 1).
The stop roller holds the mechanism in readiness for release of the closing latch. The extreme mushrooming impeded rotation of the stop roller. It is possible that continued mushrooming could totally prevent stop-a roller operation, which could prevent closure of the circuit breaker upon demand. While not of concern for an RTB, a failure to close condition would be of concern for DS-type circuit breakers used in safety applications requiring energization of the connected loads.
Eighteen new pole shafts were evaluated for use in the RTB to determine the importance of a broken weld to a jamming condition. The welds on the center-pole lever are supposed to extend a minimum of 180 degrees around the surface of the shaft. Because of the geometry of the adjacent anti-bounce lever, the center-pole-lever weld must be made in two segments: one approxi-mately 120 degrees and the other approximately 60 degrees. It was noted that _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ - _ _ -
I-6177-5-1
.'l approximately half of the pole shafts only had the 120-degree segment of the-weld.
It was also noted that some of these welds appeared to lack fusion to the base metal of the lever and/or shaft. -It should be noted that the weld' that failed on the McGuire RTB also was only 120 degrees long and appeared to have a~1ack of fusion for more than two-thirds of its length. These conditions were probably key to its failure.
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I I-6177-5-1 7.
CONCLUSIONS The McGuire 2B RTB failed to open due to pinching of the main roller between the raised edge of the closing cam and the right-hand side plate.
Both lateral displacement of the roller end of the main drive link and axial rotation of roller are necessary to allow jaming.
Some pole shafts without broken welds will' allow the lateral displacement and even allow the roller to strike the side plate. However, a new pole shaft without broken welds would not allow sufficient axial rotation for jamming to occur. It may be possible that 3000 or more cycles could cause wear that would allow the necessary axial rotation.
The raised cam edge is also necessary to allow jaming. In addition, the distance between the inner surface of the cam edge and the side frame plate must be close to the width of the roller.
Other failure modes may also be developing in the DS-416 circuit breakers, j
If the roller hits the right-hand side frame plate (viewed from back of circuit breaker), the linkage exerts a lateral force on the constraining link, and thereby the trip latch, resulting in trip latch bearing damage. Ultimately, such bearing damage may jam the trip latch and prevent the circuit breaker from opening.
(On September 23, 1987, Westinghouse personnel discounted this theory, but presented no formal evaluation of the problem.)
In addition, a failure to close condition could occur due to the mush-roceing of the cam if it causes the binding of the stop roller. Such binding would prevent operation of the close release mechanism.
With respect to weld acceptance criteria, a complete standard is needed for evaluating the welds on the pole shaft. The standard must either be an existing industry standard or one specifically prepared for the pole shaft welds. The inspectors need to know exactly how to judge the welds and what is or is not acceptable. It is doubtful that an adequate inspection of the weld can be made with a pole shaft still in the circuit breaker, especially of the center-pole-lever weld. The restricted space and viewing angle do not allow a proper inspection. Use of dye-penetrant inspection may give better results with the pole shaft still in the circuit breaker.
The reduction of acceptable weld length to 90'/120' has not been proven acceptable by either qualification testing or sound engineering analysis. *
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I 8.
RECOMMENDATIONS The inspection of pole-shaft welds is highly desirable. However,.at.
o present, insufficient'information has'been'provided to the users of the circuit breakers to perform' uniform inspections.- TheLinspection' standard must be.specified or a complete' inspection guide provided.
o 'There is no established basis for permanent usage of 90' and-120*
welds. A qualification program should be performed for breakers lwith,
pole shafts with 90'and 120' welds.
If welds;with cold starts ~are to be allowed for unrestricted use, then the' qualification specimens must contain welds of 90' and 120' having cold starts.
For interim, usage of the 90*/120* welds, Westinghouse should compute.
o allowable torques based on. proper formulae with' conservative. stress concentration factors'.. In addition, it is suggested that the dynamic strain gauge' measurements be reperformed or reevaluated,, establishing the dynamic torque history during opening and closing of an RTB for comparison to calculated values.
o Since some circuit breakers are approaching the qualification limit-of 4000 cycles, refurbishment criteria must be established or.the qualification limit must be extended by ratest'or verification that cycling test data exist. The qualification limit for 180* and 90*/120' welds must be established.
l With respect to restriction of the stop roller by the mushrooming'of o
the' edges of the close cam segments, inspection criteria should be' added-to maintenance procedures for all Class 1E DS circuit breakers. Remedial actions should also be specified.
6 f #
E d
Mr. H. B. Tucker
. Duke Power Company McGuire N'uclear Station
'CC:
Mr. A.V. Carr, Esq.
Dr. John M. Barry Duke Power Company Department'of Environmental Health P. O.' Box 33189 Mecklenburg County 422 South Church Street 1200 Blythe' Boulevard Charlotte, North Carolina 28242 Charlotte, North Carolina 28203 County Manager of Mecklenburg County.
Mr. Dayne H. Brown, Chief
' 720 East Fourth Street Radiation Protection Branch Charlotte, North Carolina' 28202 Division of Facility Services Department of Human Resources.
701 Barbour Drive Mr. Robert Gill Raleigh, North Carolina 27603-2008 Duke Power Company Nuclear Production Department P. O. Box 33189 Charlotte, North Carolina 28242 J. Michael McGarry, III, Esq.
Bishop, Libennan, Cook, Purcell and Reynolds-1200 Seventeenth Street, N.W.
l Washington, D. C.
20036 l
Senior Resident Inspector c/o U.S. Nuclear Regulatory Comission l
Route 4, Box 529 Hunterville, North Carolina 28078 Regional Administrator, Region II U.S. Nuclear Regulatory Commission, 101 Marietta Street, N.W., Suite 2900 Atlanta, Georgia 30323 a
l L. L. Williams l
l Area Manager, Mid-South Area ESSD Projects Westinghouse Electric Corporation MNC West Tower - Bay 239 P. O. Box 355 Pittsburgh, Pennsylvania 15230 l
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