IR 05000369/1987022
| ML20238A159 | |
| Person / Time | |
|---|---|
| Site: | McGuire, Mcguire  |
| Issue date: | 08/28/1987 |
| From: | Peebles T NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION II) |
| To: | |
| Shared Package | |
| ML20238A137 | List: |
| References | |
| 50-369-87-22, 50-370-87-22, NUDOCS 8709090158 | |
| Download: ML20238A159 (27) | |
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- 8 Rfcg UNITED STATES
/
.'o NUCLEAR REGULATORY COMMISSION
[' I HEGION 11 n
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,j 101 h* 4RIETTA STREET, N.W.
e ATL ANTA. G EORGI A 30323
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9 '.**+
Report Nos.:
50-369/87-22 and 50-370/87-22 Licensee:
Duke Power Company 422 South Church Street Charlotte, NC 28242 Docket Nos.:
50-369 and 50-370 License Nos.:
NPF-9 and NPF-17 Facility Name: McGuire 1 and 2 Inspection Conducted:
July 7-10 and July 29-30, 1987 Inspectors:
D. S. Hood W. T. Orders A. 8. Ruff A. L. Toalston Contributing Inspectors:
K. R. Naidu P. K. Van Doorn Consultant:
G. J. Toman, Franklin Research Center Approved by:
o[d
_f/Ar/8')
. Peeb,lef, Team Ljader'
Date' Sfgned
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p vision WReactor P'rojects SUMMARY Scope: This special announced augmented inspection was conducted for the McGuire reactor trip breaker (RTB) failure of July 2,1987.
The inspection team was charged with investigation of the RTB failure in order to determine the root cause and what corrective actions are needed.
The areas inspected included:
sequence of events, description of failure, breaker evaluation and testing, human f actor aspects, safety significance, equipment history and regulatory compliance and conclusions.
Results:
In the areas inspected, no violations or deviations were identified.
8709090158 G70831 PDR ADOCK 05000369 G
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REPORT DETAILS 1.
Persons Contacted Licensee Employees
&*T. McConnell, Plant Manager
- B. Travis, Superintendent of Operations
- D. Rains, Superintendent of Maintenance
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- B. Hamilton, Superintendent of Technical Services
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- N. McCraw, Compliance Engineer
- M. Sample, Superintendent of Integrated Scheduling
&*N. Atherton, Ccmpliance
- G. Gilbert, Operations Engineer
- J. Snyder, Performance Engineer
- W. Reeside, Operations Engineer
- E. Estep, Project Engineer
- R. Blake, MNS/0PS
- R. C. Futrell, Nuclear Safety Assurance, Manager, G0
- C.
S. Geerken, MNS/MSRG
- A.
R. Sipe, MNS/MSRG
- A. Rose, CNS/ Transmission
- K. D. Leuschner, Transmission (GSS)/McGuire
- G. W. Hallman, Nuclear Maintenance, GO
- R. Wilkinson, Nuclear Rel. Assurance, GO
- G. A. Copp, Maintenance, Planning Engineer, MNS
&*J. E. Thomas, Design Engineering
- D. W. Murdock, Design Engineering
- R. L. Gill, NPD, NTS, Licensing
- W. H. Messer, Nuclear Maintenance, Mnt, Engineer, G0 t,-
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- L. S. Charles, D/E, Technical Specification, GO
- A. F. Batts, QA, McGuire
- M. D. McIntosh, NPD/ General Office
- T. Cline, MNS/IAE
&K. Wilkinson, Transmission-McGuire
&R. G. Bledsoe, Transmission Department Westinghouse Employees
- John M. Roth, McGuire Station Representative A. K. Deb, RCS Engineering G. C. Steinel, RCS Engineering B. J. Metro, Nuclear Safety Other licensee employees contacted included transmission circuit breaker technicians, operators, quality cortrol technicians, and office personnel.
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Resident Inspectors
- W. T. Orders
&*S. F. Guenther
- D. J. Nelson
- Attended exit interview July 10
& Attended' exit interview July 29 2.
Exit Interview The inspection scope and findings were summarized on July 10 and July 29, 1987, with those persons indicated in paragraph 1 above. The licensee did not identify as proprieta ry any of the information reviewed by the inspectors during the course of their inspection. No dissenting comments were received from the licensee.
3.
Unresolved Items No unresolved items were identified.
4.
Augmented Inspection Team (AIT) Activities l
The NRC was promptly informed of the Emergency Safety Feature (ESF)
actuation and reactor trip breaker (RTB) failure at 1:12 a.m. on July 3.
The licensee continued prompt corrective actions to assure the safety of the plant and in the process localized the problem to the specific RTB at 4:00 a.m.
A discussion between the Region, the Senior Resident Inspector and the licensee on July 3 and 6 concluded that a quarantine of the RTB was warranted. The licensee finished successful testing of the installed Unit 2 RTBs at 2:10 p.m. on July 3 and startup testing commenced at 2:20 p.m.
The AIT arrived at the site on July 7 at 9:00 a.m. and began a cooperative evaluation of the failed breaker with the licensee and the RTB vendor (Westinghouse Electric Corp.).
The AIT exited on July 10.
5.
Overview During the evening of July 2,1987, reactor control rod drop timing tests were being conducted on McGuire Unit 2 as part of startup testing following the refueling outage.
After several rod banks had been successfully tested, the B RTB failed to open during a manually initiated
trip from the main control panel. The coil of the shunt trip attachment (STA) had overheated, shorted, and opened; and the fuse to the STA
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circuity had opened. Three operators in the control room each stated that the RTB indicating lights (red and green) showed that the RTB had tripped.
l Subsequent observation at the RTB and the events recorder printout both
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indicated that it had not opened.
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e The failed RTB was found.when a technician observed that the B RTB was still closed and there was smoke in the area.
Pressing the manual trip plate at the RTB did not open it.
The RTB was subsequently opened by manipulating the manual spring charging handle. The RTB was then removed from its cubicle.
m The licensee continued attempts to isolate the problem and to assure that the plant was in a known safe condition.
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The failed RTB was taken to the test room and placed on a bench. The inspection of the RTB showed that the STA coil was burned and had opened.
The RTB was then electrically closed, and successfully opened by de-energizing the undervoltage trip attachment (UVTA). The RTB was again
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electrically closed, but mechanically bound and failed to trip during a manual trip force test using a push force gauge.
Then continuing to
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duplicate the original scenario, the technician manipulated the manual spring charging handle and the breaker opened.
Die further test of the the RTB for opening was successful. The RTB was then quarantined.
A McGuire Unit 1 bypass RTB was used to replace the failed breaker. This RTB and the associated circuity was tested in place satisfactorily and no abnormal conditions were found. The other three breakers on Unit 2 were then tested in place satisfactorily.
The licensee determined that the
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problem was isolated to the one RTB and the other RTBs were operable. The rod drop tests were then resumed.
A methodical procedure for inspecting the breaker for the mechanical binding and erroneous position indication was developed by representatives from Duke Power Company, the RTB vendor and the AIT.
The mechanical inspection revealed a looseness of the center pole lever of the pole shaft
and in particular a broken weld between the pole shaf t and center pole lever.
This broken weld and a general loo ser,e s s contributed to an approximately 5 degrees of angular rotation and a skewing of tne the main drive link and its roller with respect to the closing cam and its mounting
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frame.
It was hypothesized that the skewing of the main drive link such that it could contact the mounting frame could result in jamming of the RTB in the closed position. The main drive link is the only substantial component that is oriented such that its jamming could hold the breaker closed; therefore, this was tentatively concluded to be the probable cause of the breaker's failure to open and the focus of further efforts of the investigation.
The electrical inspection of the internal breaker wiring was completed with no abnormalities found.
Subsequent attempts to duplicate the failure to trip were minimally successful.
All parties agreed that dismantling of the breaker and further testing should be conducted under laboratory conditions.
The breaker has been
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shipped to the vendor's laboratory where testing is to begin on August 24, 1987. There the causes of failure will be further investigated under the direction of the licensee and with participation by the NRC.
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6.
Description of Event On July 2,1987, McGuire Unit 2 was in mode 3 (Hot Standby) undeigoing control rod drop timing tests.
At 11:41 p.m.,
B RTB, one of two in series, failed to open when a planned, test induced, manual reactor trip was initiated in the control room.
Control room RTB position status
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lights erroneously indicated that both RTBs had opened. (Green Lights).
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At this point in time, the reactor control rods were inserted and there was no indication of a problem.
l At 11:43 p.m., both RTBs were given a manual demand to close from the control room in preparation for the continuance of rod timing tests.
RTB A closed and B remained closed. Control room indication was normal; both RTBs indicated closed.
(Red Lights).
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As the reactor operator (RO) attempted to withdraw control rod bank E, a
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demand counter (rod position) problem was encountered and the operator reinserted the bank of rods.
An electronics technician (IAE) was dispatched to the rod control logic cabinets, which are in the vicinity of the reactor trip breakers, to troubleshoot the problem.
The IAE technician informed the Shift Supervisor (SS) of smoke in the vicinity of the reactor tiip breakers. The SS then directed the R0 to initiate a manual reactor trip. At 11:46 p.m.,
the control room indication reflected both R1Bs open. At the RTB cubicle, smoke was found coming from the B breaker which was found to be closed.
The operator at the RTB informed the R0 that the B RTB was closed. The
R0s in the control room did not verify that the breaker open indication was still present.
Attempts to open the breaker locally by depressing the trip button on the
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breaker were unsuccessful. Manipulation of the manual spring charging ratchet handle on the front of the breaker resulted in the breaker opening at 11:58 p.m., some 12 minutes af ter the reactor trip had been initiated from the control room.
When the breaker opened, it resulted in a train B feedwater isolation
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since the operators in the control room were not expecting the reactor trip signal generated by the RTB opening and had not blocked the feedwater initiation signal as is normally the case during testing.
The feedwater isolation was of no major consequence and was quickly recovered.
During troubleshooting of the breaker on the morning of July 3,1987, it was determined that the breaker had mechanically bound.
Technicians at that time were successful in reproducing the binding which had led to the failure to trip.
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The binding had resulted in the electrical failure of the STA coil which I
remains energized subsequent to the initiation of a reactor trip signal until the breaker actually opens. The overheated coil was the source of the smoke.
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Ultimately, a unit 1 bypa'.ss breaker was placed in the Unit 2 B breaker cubicle, all four Unit 2 breakers (2 trip breakers, 2 bypass breakers)
l were functionally tested satisfactorily, and rod drop timing tests were resumed.
No further difficulties were encountered.
l Extensive testing performed on the B breaker during the week of July 7-10, 1987, failed to reproduce the binding which had been previously observed.
l 7.
Sequence of Events July 2, 1987 11:41 p.m.
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Manual reactor trip initiated in control room at completion of testing control rod bank D
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Reactor trip breaker A opens
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Reactor trip breaker B remains closed Status lights in control room indicate both A and B
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reactor trip breakers open (operator statements)
11:43 p.m.
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Reactor trip breakers A and B given close signal to begin testing of control rod bank E
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Reactor trip breaker A closes
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Reactor trip breaker B remains closed
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Status lights in control room indicate both breakers closed (operator statements)
11:44 p.m.
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Reactor operator begins withdrawing control rod bank E for testing, step counter problem observed
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R0 reinserted control rods 11:44 +
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IAE technician dispatched to control rod logic cabinets to troubleshoot step counter problem.
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smells smoke... informs Shift Supervisor.
(SS)
11:46 p.m.
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Reactor operator initiates manual reactor trip as directed by SS Both breakers indicate open in Control Room (multiple
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operator statements)
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iAE technician and one reactor operator respond to
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area of e-ctor trip breakers, find B breaker still closed an. smoking
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Attempts to open breaker by depressing trip button on breaker futile Manipulation of manual spring charging ratchet handle 11:58
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results in breaker opening
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Train B feedwater isolation initiates due to breaker opening July 3, 1:12 a.m.
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NRC notified of ESF actuation (feedwater isolation)
and breaker failure July 3, 4:00 a.m
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Testing of B breaker in shop reveals STA coil overheated; mechanical binding reproduced
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July 3, 2:10 p.m.
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Unit 1 bypass breaker B placed in Unit 2 reactor trip breaker B cubicle.
Both reactor trip breakers and both bypass breakers tested satisfactorily July 3, 2:20 p.m.
Rod drop timing testing recommenced
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8.
Discussion of Equipment Failure
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The operator tripped the 2A and 2B RTBs from the manual control switches
at 11:46 p.m. on July 2.
By observation of the green light, the operator received indication that both RTB had opened.
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l The source of the smoke was traced to the 2B RTB, which was found to be l
closed, counter to the previously indicated status on the control board in
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the control room.
(0 pen indication was not verified at this time).
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Attempts to open the 2B RTB by pushing the manual trip button on the RTB
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were unsuccessful. Af ter approximately 12 minutes, the RTB opened when
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l the lever for manual charging of the closing spring was pulled approximately five times.
The source of the smoke was the STA coil that
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had shorted to ground and open circuited.
The STA coil is normally deenergized when the circuit breaker opens and, therefore, is energized only for approximately 100 milliseconds, from the time the trip signal is received until the circuit breaker opens.
However, during the period
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following the D bank rod drop tests until the E bank tests were started, the coil was energized (approximately 2 minutes); and again, after the tripping of the RTBs subsequent to detection of smoke, it was again energized until it burned open.
Two concerns arose from the failure of the 28 RTB to open. First, what was the cause of the jamming of the RTB and, second, what was the source of the erroneous open indication in the control room? Evaluation of alarm
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printer and events recorder data that receive status indications from the
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same set of 2B RTB auxiliary switches correctly indicated that the RTB did not open after the D rod drop tests or subsequent to the detection of smoke.
When the 2B RTB finally opened, the alarm printer and events recorded the position of the RTB correctly.
It should be noted that illumination of the control room green light for the 2B RTB failure was observed by three different operators.
During subsequent interviews, these operators were sure that they had indeed observed illumination of the green light.
9.
Discussion of Evaluation and Testing of the Unit 2 RTB Overview of Test Program On July 7 through 9, 1987, inspection and testing of the 28 RTB were performed. The breaker mechanism was not dismantled during these efferts.
The goal of this testing was to determine the cause of the 2B RTB failure l
to open and to determine if the erroneous open indication occurred due to improper operation of the auxiliary switches, or failure of or improper connections in the RTBs wiring harness. Between the start of the testing
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on July 7 and the time of the failure on July 2, a Duke Power circuit breaker technician had replicated the failure to open by slowly pushing the manual trip pin on the trip mechanism. Prior to this, the RTB tripped correctly by operation of the undervoltage trip attachment. The jamming of the RTB was agsin cleared by partially charging the closing springs by means of the manual charging lever. One further attempt at replicating the failure to open was unsuccessful.
Further tests were not attempted until the July 7 tests done in conjunction with the manufacturer and observed by the NRC.
The RTBs are Westinghouse model 0S-416 drawout circuit breakers.
These are stored-energy breakers in which the closing force is supplied by a set of springs that are charged by means of a motor and a ratchet system.
After release of the trip latch, the contacts are opened by means of another spring.
The tests and inspections that were to be performed on July 7 and 8 were:
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Determination of the electrical status of tha 'nternal control-
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circuits within the RTB including shorting between the circuit for the green light indication and other circuits in the RTB; and the condition of the STA coil.
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Evaluation of the condition of the circuit breaker operating mechanism, and the auxiliary switch and associated linkages without operating the circuit breaker.
3.
Evaluation of the circuit breaker after it was closed.
4.
Simulating a trip condition by pushing on the pin for the manual trip in an attempt to replicate the jamming condition. During the closing and opening (if any), the close and open indications from the auxiliary switch would be monitored by a digital recording oscilloscope to verify proper auxi lia ry switch operation.
Thereafter, further efforts would be taken as needed.
The results of the electrical tests with a digital ohmmeter were as follows:
No shorting or abnormal conditions were detected between the green light circuit and any other control circuit in the RTB.
The STA coil was open circuited and the positive leg was shorted to the RTB chassis with a 2.38 ohm resistance (a properly working coil is insulated from the chassis). The coil was observed to be charred.
Subsequent testinq on June 9 with a meggar and by applying 125 Vdc to I
the shunt coil reconfirmed that there were no short circuits between the green light circuit and the remainder of the control circuits.
The undervoltage trip attachment (UVTA) was confirmed as working properly.
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The visual inspection of t.he mechanism revealed that the weld between the pole shaft (Item 8 in Figure 1) and the center pole lever (Item 9) was completely cracked.
Figure 2 shows an overall view of the main mechanism.
In this figure, the pole shaft is Item 21 and the center pole lever has been labeled "A".
This weld is critical to correct breaker operation in that the bulk closing force exerted by the closing cam is transmitted by this lever to the pole shaft which causes the main contacts to close.
Although the weld was broken, motive force was still being supplied to the pole shaft from the main drive link (Item 27 in Figure 2) by means of a parallel lever, the anti-bounce lever (which is not shown in the figures).
The anti-bounce lever is welded to the pole shaft and connected to the upper pin of the main drive link that connects to the center pole lever.
With the cracked weld on the center pole lever and the good weld on the anti-bounce lever, the center pole lever will skew to one side of the RTB and cause the bottom of the main drive link to cant towards the latch side of the RTB when the ci"cuit breaker is closed.
Evaluation of the close cam (Item 5 in Figure 2) detected flattened (mushroomed) spots on the laminations, especially in the area of the stop roller (Item 6 in Figure 2).
These flattened areas indicated that the roller on the main drive link was riding well off of center towards the trip latch side of the RTB.
Visual inspection of the auxiliary switches (see Figure 3) indicated no apparent problems.
There are three auxiliary switches stacked upon one another.
The linkages between the pole shaft and the auxiliary switches were found to be correctly connected and set up for proper operation. No loose or misaligned contacts were observed.
Following the visual inspection, the RTB closing springs were charged and the RTB was closed by energization of the electrical closing circuit.
Correct operation of the auxiliary switch was observed. The green light circuit opened; then, the shunt coil circuit closed.
With the circuit breaker closed, the center pole lever was observed to be canted at the bottom towards the charging motor side of the RTB, which, in turn, caused the bottom of the main drive link to be canted towards the trip shaft side of the RTB by approximately 1/4 in.
Subsequent observation showed that the roller on the main drive link was forcing the roller constraining link (Item 3, Figure 2) to the trip shaf t side of the RTB, which, in turn, was exerting a similar force on the trip latch (Item 4, Figure 2).
In an attempt to replicate jamming the manual trip pin was slowly pushed to see if the circuit breaker would jam.
It opened properly.
Correct auxiliary switch operation occurred with a 9 millisecond delay between shunt coil circuit opening and closing of the green light circuit. The closing of the green light and event recorder circuit were nearly simultaneous, f
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Fourteen more attempts were made to allow the circuit breaker to jam. It did not.
On July 9, purposeful, but nondamaging attempts of manipulating mechanism components were made to cause the circuit breaker to jam but none were successful.
Weld Inspections The inspector visually inspected welds between the levers and the pole shaft on the defective breaker and the two spare breakers. The welds were a single pass for approximately 180 of each lever (six per breaker).
Westinghouse standards and documentation were not available for this inspection, and in addition the surfaces were apparently plated after welding, inhibiting a proper visual inspection of the welds.
The welds were therefore inspected for obvicus defects and general appearance.
The weld of the center lever on the defective breaker was separated between the weld and the lever for approximately one-half the weld length.
At the end of the separation a crack had propagated across the weld bead thus allowing the lever and pole shaft to move independently to some degree.
No obvious defects were identified in the other welds of the defective breaker; however, portions of the other welds appeared to be of poor quality exhibiting a high probability of poor fusion or lack of fusion of the welds to the base material. This was evidenced by a lack of characteristic weld bead ripple, little evidence of base metal melting and notches at the edge of the weld beads.
The welds on the newer vintage spare breakers appeared to be of better quality. The welds were larger in bead size with only a few small areas where lack of fusion might be questioned and some porosity was noted at several weld starts and stops. A licensee welding inspector also visually inspecteo the welds.
His opinion paralleled that of the NRC inspector.
It is recommended that a failure analysis be conducted of the failed weld with NRC observation and that the quality control program for these welds be reviewed. Consideration should be given to inspection of these welds, especially on older vintage breakers, utilizing a qualified welding inspector.
Green Light Anomaly Investigation l
Three operators in the control room stated that the two RTB open green l
lights on the control panel had come on, indicating that both RTBs had
tripped, even though one had not.
The manual control switch is a three l
position handle-operated, spring-return type switch located on the vertical panel in the main control room. The handle's normal position is
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top dead center. It is turned to the left about 30 (counterclockwise) to trip the reactor trip breakers, and to the right about 30 to close the associated reactor trip breaker. When released it returns to the center position.
The two switches are on the same vertical plane about three l
feet apart. Each control switch will trip its reacter trip brecker (this l
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is a McGuire specific design) and the bypass breaker if it is racked-in to the operate position.
The operator is trained to operate both control switches simultaneously, one with each hand. Directly below each control switch are red and green indicating lights.
The operator is trained to interpt et the lights to indicate the actual position of the reactor trip breaker contacts. The red light indicates that the associated reactor trip breaker is closed, and the green light indicates it is open.
In reality, this is not always true and may raise some human factor concerns as discussed elsewhere in this report.
Both the red and green lights are energized from the same 125V dc source.
In series with the green light is an auxiliary "b" contact.
An 'a'
contact follows the breakers movement and a 'b'
contact acts opposite to the breaker.
(See Figure A).
The auxiliary contacts are mechanically operated by linkage from the breaker pole shaft. The moving mein contact blades are connected to this same shaft.
Thus, the auxiliary switch position reflects the actual position of the main contact b'ades.
Each auxiliary switch is a rotary switch with wiping type contar.ts.
A field measurement made as the main contact blades were slowly opened showed that a main contact blade opening of 5/8 of an inch was required before the auxiliary "b" contacts began to close.
The ed light is in series with another contact on the same auxiliary switch as the green light. A field measurement made as the main contact blades were slowly opened showed an opening of 1/16 of an inch before the
"a" auxiliary switch contact began to open. Thus, the "a" contact began to open before the "b" contact began to close. Oscilloscope measurements during the testing confirmed this.
The STA coil is in series with the red light and the "a" auxiliary contact (See Figure A).
The red light has a dual function of indicating the continuity of the shunt trip circuit as well as the breaker status. While the RTB is closed, a small current flows through the red light and the shunt trip coil.
When the manual control switch is rotated to the trip position, a contact (CS-Trip, top of Figure A) closes to bypass the red light and energize the STA coil. Release of the manual control switch to the center position opens this contact.
Another contact in the control switch (CS-Trip, bottom of Figure A)
interrupts current to the UVTA and also to the (S) relay.
When the control switch is released, the design is such that the UVTA circuit remains deenergized, and thus the (S) relay remains deenergized until the RTB is closed. When the (S) relay is deenergized, it closes contact (S)
(Top of Figure A).
Closure of this contact energizes the STA and also bypasses the red light.
If the breaker fails to open, the STA remains energized.
The STA is not designed to carry full current for a long period of time and will burn out if the breaker's auxiliary contact does not open to interrupt the current.
In this case, the RTB did not open, the STA coil burned out and shorted to the frame, and the fuse blew.
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i The initiatio.n of the event recorder is b.v a 'b' contact on a different auxiliary switch than the auxiliary switch which' controls the red and green lights. However, this switch is mechanically ganged to the other, and both operate from the same pole shaft. - Visual inspection revealed no abnormality of the ganging mechanism.
Field measurements, confirmed by the oscillograph plots, showed that the contact initiations to' the green light and to the event recorder were simultaneous. Thus, the green light indication at the control panel should have been consistent with the event recorder printout.
The RTB fa11ed to open, as evidenced by the events recorder, the alarm typer, and the STA coil burnup. Yet, three different operators state that the green light was on even though the RTB was still closed. 'This would suggest that something was wrong with the wiring to the green light.
However, the green light was operating properly during every other trip prior to the event. It was also tested to be operating properly after the failed RTB was replaced by another RTB, without any wiring changes.
No measurements or inspections at the failed RTB explained the anomaly.
Comprehensive measurements and checking of the circuitry external to the RTB could not be safely mad: with the reactor in operation.
However, drawings are being checked and an extensive checkout of the actual wiring is being pursued by Duke Power Company personnel as conditions permit.
Discussion of Results of Test'ng While the jamming of the circuit breaker could not be replicated during the testing of July 7 to 9,1987, the observation of the failed weld and the associated misalignment of the main drive link, coupled with the wear marks on the close com and frame, strongly suggested that this combination was responsible for the jamming of the circuit breaker.
The testing of the electrical control circuits and the auxiliary switch showed no abnormal condition that would result in erroneous operation af the green open. position indication in the control room.
The shorting of the STA coil to ground should not have been the cause of the erroneous indication because the operators stated that, upon the call for opening of the RTBs following the 0 rod tests, there was an immediate green light. At that time, the STA coil probably had not yet failed and the B RTB had not pt operly opened.
The electrical control ' circuits within the RTB were found to be working correctly.
Therefore, the incorrect control status appears to be caused by a problem in the circuits between the reactor trip breaker cubicle and the control room indicating light and control switches.
Duke Power is evaluating these circuits. However, only limited testing can be performed
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with the unit at power.
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9.
Human Factor Aspects The principle human factor concern disclosed by the event was the failure by the operator to discern that one of the RTBs had failed to open.
If the green indicating light had indeed come on even though the RTB did not open, this would indicate a wiring deficiency, and not a human factor deficiency.
This is still under investigation.
If the green light did not come on, as designed, and the operator failed to notice that it had not, or thought it had come on when indeed it had not, this could indicate a human factor deficiency.
If an operator is performing a series of consecutive tests every few minutes, with each test requiring a series of steps and checks, and the checks are successful time after time as they were in this case, it can be postulated that there would be a possibility of missing one of the checks, particularly if the checks 5:ere not individually recorded and that the check was merely cor. firming what the previous checks had already shown.
In accordance with procedures and training, in tha event of a reactor trip, the operator first observes rod insertion on the rod position indication display, next, that the reactor flux has decreased, and next that the manual reactor trip annunciator is on and that the green breaker indication lights are lit. Further, there are two green lights to observe (one for each RTB) about 3 feet apart, one to each side of where the operator was standing to manually trip the RTBs. The manual reactor trip annunciator is initated by the maneal reactor trip switch.
Another possible human factor concern is that the circuit design causes the red light to go out before the RTB physically opens, and even if the RTB fails to open. First the manual trip switch contacts bypass (shorts)
the red light, and then the S relay contacts bypass the red light. This falsely indicates to the operator that the RTB has opened, when indeed it may not have as in this case.
Consideration should be given to redesigning the circuitry so that the red light truly indicates the closed position of the RTB.
A dif ferent color light could be added to monitor the continuity of the shunt trip circuitry.
Safety _ Significance and Radiological Consequences The reactor trip breaker and its associated switcngear provides the safety related function of interruption of power to the control rod drive mechanisms (CROM) in response to an auzomatic reactor trip signal or manual command.
Interruption of the power causes the magnetic latches of the CRDMs to be denergized and the full-length control rods to fall by gravity into the core, decreasing reactivity to shut down the reactor.
Two main reactor trip breakers, each receiving separate control power and an independent reactor protection system trip signal, are connected in
_ _ _ _ _ _ _ _ _ _
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13 i
series. Thus, failure of a single breaker to open does not ' prevent a reactor. trip; failure of both RTBs to open in response to an anticipated i
operational occurrence could esult in an anticipated. transient without scram (ATWS).
ATWS: mitigation actuation circuitry required' by
,
'10 CFR 50.62 is scheduled for installation during the 1988 refueling i
outages of each McGuire unit.
"
The specific event had small safety significance and no radiological j
consequences because the reactor was in hot standby i with all rods inserte and one of the RTBs operated properly. The event would have had more saiety significance if a significant common cause failure mechanism for this vintage of RTB is discovered ' during the continuation of the
! analysis and investigation.
No common cause' failure has been discovered.
to-date.
An erroneous green light indicating that the breaker is' open when it is
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closed can reduce the probability of detection or delay timely corrective action oy the reactor operator. This specific green light anomaly, and the fail are of the' operator to detect that the RTB had failed to trip had no safety significance in this instance because the problem was discovered through.t he smoking coil. If the smoke had not been detected,~the opening of the cc il, or the absence of voltage across the coil due to the blown fuse, should have been detected by the absence of the red indicating light.
T1us, the event discloses that the actual position of the RTBs should tv checked prior to resetting them and positive indication of
- breaker position is highly desirable.
11.
Equipment History and Review of Licensee and NRC's Requirements The four RTBs (main and bypass) for each McGuire unit are Westinghouse type DS-416 circuit breakers of pre-1984 vintage.
The breakers were delivered to the site in 1973 and are some of the earliest of this vintage breaker. The licensee estimates that McGuire RTB B had experienced about 3000 cycles as of July 2,1987. The actual number of cycles could not be determined because counters were not initially included on the breakers.
The' counters are not required to remain functional and the counter for RTB B was not operating.
Many of the accrued cycles are associated with periodic surveillance / maintenance of the breakers which is performed on each McGuire unit monthly and at 6 month intervals in accordance with Unit 2 License Condition 2.C(12)c, Table 1.
Each breaker accumulates 250-300 cycles per year due to testing and maintenance of the breaker and associated equipment. Although no overall limitation on the number of cycles is specified in the Westinghouse Maintenance Program Manual for the DS-416 breakers, replacement recommendations are included for certain components.
A recommended service life of about 2500 operations is specified for both the UVTA and STA. Certain parts such as breaker arcing and main contacts are specified for replacement based upon specified dimensional check.
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The inspectors reviewed the following document sections and procedures:
a.
Technical Specifications Section 3/4.3.1 b.
License Amendment No. 2 for McGuire Unit 2, Reactor Trip Breaker Section c.
Maintenance and Periodic Test Procedures:
(1) MP/0/A/2001/06, Westinghouse 05-416 Air Circuit Inspection and Maintenance (2) PT/2/A/4600/56, Manual Reactor Trip Function Test (3) PT/0/A/4601/07A, Response Time Testing of Reactor Trip Breakers RTA and/or BYB
.
(4) PT/0/A/4601/078, Response lime Testing of Reactor Trip Breakers
'
RTB and/or BYA (5) PT/0/A/4601/088, Solid State Protection System (SSPS) Train "B" Periodic Test With NC System Pressure Greater Than 1955 psig (6) PT/0/A/4601/09, Solid State Protection System (SSPS) Train "B" Periodic Test With NC System Pressure Less Than 1955 psig.
The procedures impicment the Technical Specification and licensing amendment (a. and b, above). These documents require functional tests of the UVTA the STA manual reactor trip from the control room, the response time testing of the UV/ breaker on signal from the reactor protection system and the periodic surveillance / maintenance of the reactor trip (RT)
and bypass (BY) breakers.
The procedure appeared to be complete, accurate, and when applicable, the vendor's requirements (Westinghouse Program Manual MPM-WOGRTSDS416-01) were incorporated.
A review of recent work requests for breaker, S/N 24Y9850B4, that failed to open was made. The following tabulation shows the various locations of this breaker and the tests and maintenar.ce that were performed over the past 18 months.
Breaker Location Procedure From c. Above Date
- 2BYA
12/85 2BYA
01/86 2BYA
01/86 2BYA
06/86 2BYA
06/86 2BYA
12/86 2BYA
12/86
//2RTB
12/86 2RTB
12/86 2RTB 2&5 01/87 2RTB
02/87 2RTH
03/87 2RTS
04/87 2RTB
05/87 2RTB 2&6 06/87 Breaker failed July 2
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" Unit 2 r.eactor trip bypass breaker Train A
- Unit 2 reactor trip breaker Train B During these recent inspections, no major anomalies were recorded for the RTB that failed. These inspections are performed for both the Catawba and McGuire stations by the Duke Power Company Transmission Department personnel.
These craftsmen are exceptionally i
l knowledgeable of the breaker's construction and working mechanisms.
The inspections include 'an examination for cracked or broken welds.
None were recorded and in a discussion with these craf tsmen, they stated that. if a broken weld had exis"d during the last inspection period, they would have seen it and
- would have been noted and corrected.
However, welds with partial cracks would have been difficult to notice.
The time response testing of the breaker over this time period showed that the breaker was functioning as desiyaed and no adverse trends were evident.
The opening times were random between 60 and 90 milliseconds which is well within the acceptance criterion of 150 milliseconds.
In crder to assure that no major malfunctions had occurred, the
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inspectors reviewed the overall maintenance records on the failed RTB and determined.the following.
Breaker Serial No. SN24Y9850 B-4 Type DS416 Initially installed as Bypass breaker A in Unit 2 Designated CR 9 Initial Inspection July 26, 1979.
A.
March 26, 1983 As a result of NRC Bulletin 83-02, Duke inspected and determined that the clearance between the operating arm and the moving plunger of the Undervoltage Trip Attachment (UVTA) was. 037 inches. The trip shaf t was removed for lubrication and it was observed that one end of the shaft had been ground on a grinding machine.
B.
April 16-18 1983 x
The trip shaf t was replaced and was coated with Poxylube and molybdenium disulphide and alchohol mixture.
5. ' quently, the
,.
breaker was operated for 25 cycles and the UVTA oc. ability was successfully verified.
7-26-83 Regular preventive maintenance (RPM) was performed and the breaker was tested on the test bench.
No abnormalities were documented.
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1-7-84 RPM was performed, followed by routine bench tests.
No abnormalities were documented.
7-18-84 New trip latches were installed and the operability of the breaker was verified.
1-7-85 RPM was performed followed by routine bench tests. No abnormalities were documented.
7-10-85 RPM was performed followed by routine bench tests. No abnormalities were documented.
1-2-86 RPM was performed and counters were installed on the breakers.
The reading was 99749.
6-24-86 RPM was performed and the breaker was operated 95 times.
12-18-86 RPM was performed. Triplatch overlap adjustments were made and locktite was applied on the threads.
7-2-87 The breaker did not open on signal from the control room subsequently, on July 8,1987, it was determined that mechanical binding occurred due to a weld separation.
The weld connecting the pole shaf t and the center pole shaft separated.
Conclusion The inspectors found the testing, inspections, and maintenance l
of the breakers to be satisfactory, and considers that NRC and the licensee's requirements were met.
12.
Evaluation of Failure to' Trip _of McGuire Reactor Trip Breaker No. 2AY98498 During Periodic Testing of UVTA on July _21 1987 Inspection conducted on July 29 and 30, 1987 Location: McGuire and Catawba Nuclear Plants Personnel Involved: T. Peebles, NRC Region II 0. Hood, NRC K. Naidu, NRC J. Thomas, Duke Power R. Bledson, Duke Power G. Toman, Franklin Research Center
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_
_
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Background on July 28, 1987, during periodic maintenance and inspection of the McGuire Unit 1 RTBs, RTB No. 24Y98498 failed to trip during post-travel margin testing of the associated UVTA. The RTB failed to trip because the UVTA did not cause.the trip shaft to rotate far enough to release the trip
{
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latch. The RTB was left in the "fa' led to trip"' condition until arrival of members of the.NRC AIT that had previously evaluated the RTB failure'of July 2, 1987 at McGuire.
Discussion of Findings Subsequent to the Salem ATWS events in February -1983, which were due to problems with the UVTAs of Westinghouse D8-50 RTB, proble.as were recognized on the UVTAs of the Westinghouse DS-416 RTB at McGuire.
The evaluation of these problems led to development of acceptance criteria for the UVTA and its interface with the RTB trip shaft.
No criteria were necessary for the STA.
These acceptance criteria for the six month inspections are:
1.
The UVTA must have a static output force of at least 3 lb. (A new UVTA will produce approximately 6 lb.),
2.
The UVTA must operate (trip the circuit br'eaker) when the. applied voltage is reduced to no more than 28.8 V and no less than 14.4 V.
3.
The force required to trip the breaker via the trip shaft shall be no more than 2 lb (normal values are generally 1 to 1 1/4 lb).
4.
In the reset and energized state, with the RTB closed, a gap of at least 0.030 in must exist between the trip lever of the UVTA and the associated pin on the trip shaft.
5.
The UVTA must be able to trip its RTB with a 0.070 inch shim in place that limits full travel of the UVTA trip lever. This is a go/no go test that proves that margin exists in the' travel of the UVTA trip lever such that a UVTA is assured of causing the trip shaft to rotate far enough to release the RTB trip latch.
6.
The trip shaft adjustment must be set by turning its screw clockwise until the trip latch releases (trips the RTB) and then it is backed off four complete turns.
The trip shaft adjustment determines the distance that the trip shaft will rotate beyond the trip point when the trip shaft resets. If this setting is backed off too far, a larger rotation of the trip shaf t will be required before the breaker trips.
If the trip shaf t adjustment is too little, the trip latch could fail to hold and the circuit breaker could trip spuriously due to vibration or at the time of closing (trip free).
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The failure o.f the RTB on July 28, 1987, at McGuire is related to the post travel margin test of Item 5 above. With the 0.070 in shim.n place, the UVTA failed to trip the RTB when the UVTA was deenergized. The visual inspection of the VVTA and RTB by the AIT verified Duke Power's observation that a gap existed between the trip lever of the UVTA and the trip shaft pin;, indicating that the UVTA trip lever had struck the trip shaft pin, ;and has caused the trip shaft to rotate.
However, the
' combination o f the travel and force of the UVTA trip lever was insufficient 'to cause adequate rotation of the trip shaft to trip the RTB.
~
Duke Power personnel proceeded to evaluate the actual margin of UVTA travel under static actuation (slowly allowing the UVTA to operate rather l
than allowing normal rapid operation as would occur when the UVTA was deenergized). 'The static trip point occurred with a shim of 0.019 inches, but did not occur with a shim of 0.031 inches. Subsequent to these tests, the RTB was tripped successfully three times with the 0.070 inches shim in place when the UVTA was deenergized.
These results indicated that while margin = existedt the UVTA was at the borderline of the acceptance criterion. <The force required at the trip shaft to trip the RT8 was verified to be 1 1/4 lb.
Duke Power personnel replaced the UVTA and demonstrated that the new UVTA met the acceptance criteria with much improved tolerance margins.
13.
Catawba RT1]nspection on July 30, 1987 During the visit to McGuire on July 29, 1987, Duke Power personnel stated that cracks had been found in a Catawba RTB center pole lever to pole shaft weld.
Stoce this is the weld that had failed on the RTB that did not trip at McGuire on July 3,1987, the leader of the AIT chose to visit Catawb'a'and observe the RTB. During periodic testing of the Catawba 1 tbs subsequent to the July 3, 1987 McGuire failure, Duke personnel had observed that the weld of the center pole lever to pole shaft on one of the RTBs appeared to be poorly made. However, the upper frame of the RTB obscured complete observation of the weld. The Duke personnel removed the upper frame and performed dye penetrant inspection of the weld and
.' verified that the previously obscured portion of the pole shaf t weld was indeed cracked. It should be noted that removal of the upper frame is not parta f normal inspection procedures.
The crack was not completely o
through and the weld was still holding the shaft to the lever. Duke Power will replace the pole shaft and the cracked weld will be examined by an outside laboratory to determine if the crack was propagating during operation.
Conclusions
- 1; The UVTA problem at McGuire of July 28s 1987 is unrelated to the July 2, 1987 RTB failure.
During the Julv 2 RTB failure, the RTB trip latch had actually released and the main operating mechanism jammed.
In the July 28, 1987 UVTA event, the UVTA did not cause sufficient travel of the trip shaft to allow the trip latch to releas __
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19'
2.
The observation of the crack in the Catawba.RTB is interesting in that it shows that detection of such cracks may be difficult since the frame of 'the RTB partiall; obr, cures the welds.
A one-time partial disassembly-of the RTB may be necessary to verify that no
'
manufacturing flaws exist in the welds.
Subsequently, evaluations may'be' limited to the readily observable portions of the welds during periodic inspections.
3.
. 0f the above criteria, only the ' firet two are. applicable to the initial RTB failure to trip on July 2 and both of these criteria appear to have been met at the time of the event. During inspection of the 2B RTB subsequent to the event, the trip force required to trip the CB was measured twice and found to be 1 1/4 lb..Therefore, the 2-lb limit was met.
During Duke Power's initial evaluation of the event, when the technician actually succeeded in having the RTB jam a second time, the latch of the RTB was observed to have been released, indicating that the trip shaft adjustment was not responsible for the problem.
Failure to meet the established tolerance criteria for the trip shaft and UVTA was not a cause of the malfunction of the RTB but is to be rechecked before the RTB is dismantled.
14.
Root Cause of the Event The root cause of the RTB f ailure will be determined by the ongoing inspections and evaluations described below.
15.
Findings and Conclusions The broken weld between the pole shaf t and' center pole lever resulted in unsynimetrical forces at several points of the RTB mechanism including the interfaces between the closing cam and the main drive link roller. These unsymmetrical forces occurred during both the closing and tripping of the RTB causing uneven wearing of the parts and loosening at the various pivot
~
points. Manufacturing tolerances for this pre-1984 vintage DS-416 breaker, and normal wear resulting from the 3000 cycles of operation (estimated by the licensee) undoubtedly also contributed to the twisting and lateral play observed in the mechanism.
It is possible that this permitted the roller to shif t sufficiently off center of the closing cam to cause it to bind against the frame, or for the trip latch to bind against other parts.
Since the inspection team was not able to observe such a binding, it was not possible to determine the location or cause of binding.
The fact that the bindirg phenomenon could not be repeated during the bench testing suggests that the original point at which the binding occurred had been worn to a smoother or more rounded surface by the subsequent tripping / closing of the breaker. The decrease in the measured force required to trip the breaker after the breaker was closed and tripped several times may not be significant, since this force would appear to be more closely associated with the trip shaf t than with the parts subject to binding due to the unsymmetrical forces.
_ _ - _ _ _ _ _ - - _ -
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.The - erroneous breaker open indication cannot be presently explained.
Physical inspection confirmed the correct operation of the RTB auxiliary.
switches.
Internal RTB wiring checks for grounds and circuit continuity disclosed no problems with the RTB wiring other than the open STA coil.
Correct operation of the indicating lights prior to and subsequent to the event indicates that any problem in the wiring external to the RTB'must be
. unusual.
It js possible that additional wiring checks, which Duke Power Company is performing as conditions permit, will di sclose J such a situation.
If no circuitry problems are found to exist, it would seem reasonable. to conclude - that the operators were probably remembering and referring to'the green light indication on the immediate previous normal trip operation of RTB rather.than at the time of the failed operation.
Recommendations and Long Term Actions
,
a.
, Consideration should be given to redesigning the breaker indication circuits so that an indicating light clearly indicates the position
'
f.the TB movabic blades.
If this design had been, a light wou'ld have.itayed on and the operator would have been alerted that the breaker had not opened even if the green light did incorrectly indicate that it had opened. An alternative is to check the RTB opened by observation at the breaker cubicle prior to re-closing the breaker.
'
l b.
NRC Information Notice No. 87-35 issued July 30, 1987, stated that enhanced visual inspection of the RTB is appropriate.
This visual inspection should consist of inspection for cracked welds or abnormal wear such that the main drive link roller could contact the support frame at the close cam or bind against the cam.
c.
A one time. inspection of the pole shaf t welds to assure their, l.
adequacy is appropriate.
The Catawba pole shaf t is being inspected l
for the NRC by an independent consultdnt.
Recommendations for l
inspection. criteria are forth coming from the inspection.
l
^
l-d.
The licensee submitted Licensee Event Report (LER) 370/87-09 dated August 3, 1987, describing the failure of the RTB to open.
The results and recommendations of the Duke - Westinghouse - NRC i
inspection and testing of the breaker will be submitted in a l
supplement to that LER.
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_ _ _ _ _ - _ - _ _ _ _ _ _ _ _ _ _ _ _
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