IR 05000309/1993021
| ML20059A045 | |
| Person / Time | |
|---|---|
| Site: | Maine Yankee |
| Issue date: | 10/19/1993 |
| From: | Ruland W, Richard Skokowski NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION I) |
| To: | |
| Shared Package | |
| ML20059A022 | List: |
| References | |
| 50-309-93-21, NUDOCS 9310260206 | |
| Download: ML20059A045 (11) | |
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" U~ S. NUCLEAR REGULATORY COMMISSION -
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'50-309/93-21 G
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50-309
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. Maine Yankee Atomic Power Comnany
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FACILITY.NAME:...
Maine Yankee Atomic Power Station
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INSPECTION AT:-
Wiscasset Maine m..
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INSPECTION DATES:
? August 23 - 27.1993
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' INSPECTOR:.
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Richard A. Skokowski, Reactor Engineer -
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Electrical Section, EB, DRS
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APPROVED BY::
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. William H.' Ruland[ Chief.
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Area Ins 9ected:- This was an announced inspection of Maine Yankee by regional personnel i
to review and determine the adequacy of the licensee's actions to resolve problems associated with the emergency diesel generator (EDG) load permissive relays and with the installed
~RMS-9 overcurrent trip devices.
.j Results:.' No violations or deviations were identified. However, the following issues were I
determined to be unresolved: 1) the corrective actions associated with the load permissive circuit wiring discrepancy; and 2) the significance of the as-found setting for the degraded
~ load permissive relay. The review in this inspection report only covers the licensee's j
- followup actions to the July 30,1993, spurious trips of the RMS-9 devices. A' review of the g
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July 30,- 1993, event and its impact on the operation of the plant is contained in Inspection Report No. 50-309/93-18.
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1;0 PURPOSE
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The purpose of this inspection was to review and determine the adequacy of the licensee's I?
. actions to resolve the following previously identifled issues at the Maine Yankee Atomic Power Station:
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the wiring configuration discrepancy found in the emergency diesel generator (EDG)
. load permissive circuits;
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the loose adjusting screw found within the cover of the "B" EDG load permissive relay; and 3)
the spurious trips of circuits breakers equipped with MicroVersaTrip RMS-9 overcurrent devices.
i 2.0
' EDG LOAD PERMISSIVE RELAYS 2.1 Background
' During preparations'to isolate the EDG load permissive relays (devices 59-DG-1 A and 59-
. DG-1B) for calibration, the licensee identified a potential wiring discrepancy in the load permissive circuit. To continue their effort to calibrate the load permissive relay for the "B" EDG (device 59-DG-1B) while they investigated the potential wiring configuration discrepancy, the licensee took the precautionary measures and performed a temporary modification installing a jumper wire in the load permissive circuit. This jumper allowed the licensee to remove 59-DG-1B while ensuring that the load permissive relay for the " A" EDG (device 59-DG-1 A) would have a complete current path if the suspected wiring discrepancy was verified to exist.
The purpose of the load permissive relays is to allow the EDG circuit breaker to close automatically and connect each generator to its respective safety bus only after the EDG establishes a minimum 90% of rated voltage. This prevents the EDG from powering emergency loads until the generator output voltage is adequate for performance of its intended safety function.
At 7:00 a.m., on August 15,.1993, prior to removing the 59-DG-1B device for calibration, the Central Maine Power (CMP) technician discovered a loose adjusting screw in the cover
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for the device.- The discovery was documented by the licensee in Unusual Occurrence
- Report (UOR)93-059, dated August 15, 1993. The licensee obtained a spare load F
permissive device from the warehouse, calibrated it, and used it to replace the previously L
- installed 59-DG-1B device. At this time, the installed jumper was removed and the p
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o associated temporary modification was closed. On August 17, 1993, the licensee verified that'a wiring configuration discrepancy in the load permissive circuit for the EDGs did exist,
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and reported the discrepancy to the NRC in accordance with 10 CFR 50.72(b)(2)(i), and
UOR 93-061 was written on August 18, 1993, to document this condition.
2.2 Load Permissive Relay Wiring Configuration Discrepancy The wiring configuration discrepancy identified at Maine Yankee revealed that the two load permissive circuits shared a common wire. The load permissive circuit for each EDG
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consists of a potential transformer and the load permissive relay. The potential transformer steps down the voltage as seen at the output of the EDG (4160v) to a level that can be used by the load permissive relay _(120v). A predetermined voltage is required to be available at
' the load permissive relay to allow for the EDG output circuit breaker to close automatically as designed. A single break of the identified common wire would cause the voltage to be split between both EDGs' load permissive relays. Therefore, even if adequate voltage was available to close the EDG output circuit breakers, the load permissive relays may not have sensed the required voltage, and th's would not allow the EDG output circuit breakers to close automatically as desi;;ned.
Through discussions with the licensee and a review of the associated plant drawing and wiring diagrams, the inspector ascertained that this condition existed since the original construction of the plant. Furthermore, the wiring of the load permissive did meet the Maine Yankee original design criteria. The design criteria for Maine Yankee are those described in -
Federal Register, July 11, 1967. Of these design criteria, Criterion 39, Emergency Power for Engineered Safety Features, states that "As a minimum, the onsite power system and the offsite power system shall each, independently, provide this capacity assuming a failure of a single active component in each power system." However, the current Final Safety Analysis Report (FSAR), Section 8.3.7.5, " Cable Routing," states that " Components within control boards, panels, and relay racks are arranged to suit operational flexibility, and the wiring to any redundant component is physically separated." This statement was added to the FSAR by a December 1970 revision.
The licensee is planning to install an alternate return path connection for 59-DG-1 A to clim.inate the common wire between both load permissive relays. The exact locations of this alternate return path has not yet been determined by the licensee. However, Maine Yankee has committed to complete its installation ~ prior to plant startup. Furthermore, the licensee has committed to develop a plan to search for other possible single failure common wiring discrepancies prior to startup. This item is unresolved pending completion of this effort by the licensee and subsequent NRC review (50-309/93-21-01).
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2.3 Load Permissive Relay 59-DG-1B Loose Adjusting Screw The inspector reviewed Work Order 92-04956-00, Revision 1, dated August 15,1993. The purpose of this work order was to test and calibrate the 59-DG-1B relay. Additionally, this
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work order describes a loose part found in the cover of the device as an " adjusting screw
- (residual)." According the vendor manual for the General Electric undervoltage relay, type NGV15A and NGV15B, the residual screw is used to adjust the ratio of the pickup to dropout voltage of the relay. Since these relays are undervoltage relays, and they are being used to sense a voltage condition, this pickup to dropout voltage ratio is required to be set adequately to ensure proper operation on the load permissive' circuit. This work order also documented the as-found pickup voltage of the degraded 59-DG-1B device to be 102V on the secondary side of the potential transformer. Therefore, the relay would have allowed the L
EDG output circuit breaker to close automatically and connect the EDG to its associated safety-related bus with a voltage of 3570v, or 85.82% of rated voltage.
According to Technical Evaluation (TE) 431-92, " Design Basis for 59-DG-1 A and 59-DG-1B Relay Settings," Revision 0, dated January 6,1993, the load permissive relay must pickup at
_._90% of the rated 4160v, which is equal to 106.97 on the secondary side of the potential
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transformer. During this determination of the load permissive relay setting, the licensee
. documented communications with the EDG vendor indicated that the load permissive relay should pickup between 80% to 90% of EDG rated voltage. The licensee is in the process of determining the overall significance of the as-found value of 85.82% of the degraded 59-DG-
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1B device. This item is unresolved pending completion of this effort by the licensee and review by the NRC (50-309/93-21-02).
The relay vendor manual indicated that the relays should be checked every one to two years until a plant specific time interval can be established. The inspector examined the licensee calibration program for the load permissive relays and noted that, prior to 1990, the relays were not calibrated on a periodic basis. However, these relays were functionally tested on a refueling basis with the loss of ac test. The inspector also reviewed the 1990 calibration of the load permissive relays and found no indications of loose parts associated with either relay, and the as-found setting for both relays were found to be acceptable. Furthermore, a review of previously performed loss of ac tests revealed that the relays operated properly in the past.
The inspector observed the calibration of the 59-DG-1A device by the CMP technician on August 26,1993. There were no indications of loose parts in this relay and the as-found setting was acceptable. Even though the 59-DG-1A device was found acceptable, Maine
~ Yankee has committed to evaluate the significance of the loose adjusting screw found in the 59-DG-1B device, and determine the potential for the same problem occurring in similar relays, prior to plant startu gg f,,
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2.4 Conclusion -
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The inspector reviewed Closeout Plan (COP) 93-17, "DG Voltage Sensing Relays Closcout Plan,". Revision 1, dated August 23,1993, and found it thorough. This COP was for both
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' the load permissive relay wiring configuration discrepancy and the loose parts. concern, and it n
addresses a number of pertinent questions including:
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determining the significance of the as found relay settings for the degraded 59-DG-1B
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performing the root cause evaluation of the loose adjusting screw. associated with the
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59-DG-1B device; I
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performing a 10 CFR Part 21 evaluation of the loose adjusting screw associated with
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. installing the alternate' connection for 59-DG-1 A device to eliminate the common s
. wiring discrepancy; and,
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.-assessing the need to evaluate other circuits for similar wiring configuration.
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. The inspector noted that the licensee's initial actions and plans to resolve both issues related
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' to the load permissive relays were good. With the exception of the two unresolved items c
identified above, the in.spector had no further questions regarding these issues.
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3.0 MICROVERSA TRIP RMS-9 OVERCURRENT DEVICES
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3.1. 1 Background
. On July 30,1993, at 10:22 a.m., while the plant. as shutdown for the beginning of the.
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. refueling outage, 480.V bus 8 feeder circuit breaker to service water pump P-29D tripped
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- open concurrent with a 480V ground alarm. Technicians investigated the P-29D motor and associated circuit breaker and found no indications of a ground. Nevertheless, at 12:00 noon f
'.th' licensee replaced the P-29D circuit breaker with a spare circuit breaker. At 1:45 p.m.,
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' P-29D and the motor control center (MCC) 8B circuit breakers tripped open, concurrent'with
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la ground on the.480V bus. Again an investigation by the technicians found no indications of y
. grounds'or other reasons for the circuit breakers to trip open. ' On July 31,1993, the
- technicians identified a ground on the bus 8 circuit for the boric acid mix tank (BAMT)
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heaters. : The' BAMT heater is a parallel circuit to the P-29D and the MCC.-8A circuit, each'
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having their own protective devices.. However, this ground in the BAMT. circuit did not
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. explain the spurious trips of the other circuit breakers on bus 8. Therefore, the licensee Y
l wrote UOR 93-047,',"P-29D Trip Due to Ground," dated July 31, 1993, and Safety Issue
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. Concern (SIC)93-004, " Spurious Trips of GE Type AK Circuit Breakers with RMS-9 trip H
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- devices,". dated August 3,1993, to address these events, f
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The event on July 30,1993, was the first spurious trip of safety-related circuit breakers
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- equipped with the General Electric (GE) MicroVersaTrip RMS-9 trip device. However,
' Maine Yankee has experienced spurious trips of MCC circuit breakers equipped with RMS-9 tr p ev ces otb h previously and subsequent to the July 30,1993 event. The evaluation id i contained in SIC 93-004 is based upon all information available, including events at the Browns Ferry Nuclear Plant (BFNP), and describes that the potential existed that the GE RMS-9 trip devices were more senative to transient or short duration current pulses, which
could cause the circuit breakers to trip spuriously. BFNP issued a 10 CFR Part 21 report regarding the GE RMS-9 trip devices equipped with an instantaneous trip function on November 25,1992. The licensee reviewed this information and determined that the defect in the RMS-9 instantaneous trip identified by BFNP was particular to BFNP's distribution -
setup, and was not a concern at Maine Yankee. The licensee's review of the BFNP 10 CFR Part 21 report is documented in the internal Plant Engineering Department memorandum 93-
- 13, dated April 7,1993.
Based on the information provided by the BFNP 10 CFR Part 21 Report, and the spurious
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i trips'of circuit breakers containing the RMS-9 devices at Maine Yankee, the licensee issued a separate 10 CFR Part 21 Report on August 20,1993. The difference between BFNP's Part 21 report and Maine Yankee's Part 21 report is that BFNP indicated that the problem-only existed with'RMS-9 devices with the instantaneous trip function. However, Maine Yankee observed problems with RMS-9 devices without the instantaneous trip units. The-only two cases at Maine Yankee where RMS-9 devices with the instantaneous trip function experienced spurious trips were the service water pump trips, which occurred on July 30,1993.
Since the RMS-9 appears to be susceptible to transients or short duration current pulses, it should be noted that BFNP and Maine Yankee are both designed with an ungrounded 480V distribution-system. An ungrounded system has no intentional connection between the system conductors and ground. The advantage of an ungrounded system is that a circuit is not automatically tripped at the occurrence of the first ground fault. Assuming that the fault remains a single line-to-ground fault, end no other fault occurs, the circuit may be permitted to continue in operation. If a sin;;13 Ene-to-ground fault was to occur on'a grounded system, the protective device nearest the fault should trip, isolating the remaining system from the
- fault. ;However, even with ungrounded systems there always exists a capacitive coupling between the systems conductors and ground. This capacitive coupling allows for the possibility of transients to occur from line-to-ground during the development of a line-to-ground fault or even the switching of circuits containing line-to-ground faults. Furthermore, the same ' condition will result from the repeated restriking of the arc in an arching fault from
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line-to-ground. These excessive transients would not occur on a grounded system, because, at the first occutrence of a line-to-ground fault, the device protecting the circuit containing the fault would activate, isolating it from the remainder of the distribution system.
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3.2 Safety Significance
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- Failures of nonsafety-related components powered from a safety-related bus could be the common mode cause for the loss of safety-related equipment powered by the same bus.
Furthermore, assuming a single failure of one train, concurrent with this common mode failure associated with the RMS-9 on the other train, could place the electric plant in a
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condition where it would be unable to perform its design function. In 1989, Maine Yankee g
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installed the RMS-9 trip devices in the following safety-related circuit breakers:
P-29A' Service Water Pump P-29B Service Water Pump P-29C Service Water Pump P-29D Service Water Pump MCC-7A Feeder MCC-7B Feeder.
MCC-8A Feeder j
MCC-8B Feeder In addition, there are approximately 65 nonsafety-related breakers that use the RMS-9 devices.
The licensee addressed the operability concerns associated with the RMS-9 devices in the 10
- CFR 50.59 determination, " Operation of RMS-9 devices with as-found design condition,"
dated August 13, 1993. In addition, the licensee has provided the operators with guidance r
. when ' responding.to a spurious RMS-9 breaker trip. This guidance was provided in the Night Orders, dated August 13,.1993. The inspector reviewed both of these documents and found them acceptable for short-term operation of the plant prior to the implementation of permanent corrective actions.
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The review in this inspection report only covers the licensee's followup actions to the
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July 30,1993, spurious trips of the RMS-9 devices. A review of the July 30,1993, event and its impact on the operations of the plant is contained in the Resident Inspector's Report Number 50-309/93-18, 3.3 -
Corrective Actions To address th'e issues associated with the RMS-9 trip devices, the licensee developed COP.-93-016, " Spurious Trips'of G.E. Type AK Circuit Breakers Equipped with the RMS-9
' Trip Device," written on August 5,1993. This COP addresses a number of pertinent
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determining if 10 CFR Part 21 applies; -
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determining if the installed RMS-9 devices were set properly; E
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determining if similar spurious circuit breaker trips have occurred; 4)
reviewing industry information for previous experience of this event at other plants;
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hiring a GE Power System Analyst to examine the problem; 6)
performing tests of the RMS-9 devices to aid in determining the cause of the spurious
trips; and 7)
replacing RMS-9s, for all Class IE circuits and four other circuits that were
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determined important to the safe operation of the plant, with EC trip devices.
'l In addition to'the COP, the inspector reviewed Technical Evaluation (TE) 267-93, " Alternate Replacement of RMS-9 Devices in. General Electric AK-2A '25 Breakers," dated August 17, 1993. The purpose of this TE was to replace RMS-9 devices with the original installed EC overcurrent. devices for the 8 safety-related breakers mentioned earlier. The EC overcurrent devices were replaced by the RMS-9 devices by Engineering Design Change j-Request (EDCR)88-504 because, "These devices (EC) require frequent maintenance'and refurbishment and are difficult to calibrate." Maine Yankee intends to obtain new or refurbished EC devices from GE. Also, Maine Yankee intends M incorporate GE's recommended maintenance requirements for the EC devices and train maintenance personnel on the devices.. As-found testing will be performed to trend the operation of the individual EC devices. The as-found testing and trending will help to improve long-term reliability of
'the EC devices. The coordination of the EC devices will not be as precise as the RMS-9 devices, but the required coordination will be achieved. The inspector found both the COP and the TE adequate with respect to addressing the potential common mode failure associated-with the RMS-9 devices.
l 3.4 Conclusion The licensee's corrective actions to address the issues associated with the RN S-9 devices were found to be acceptable. Additionally, the licensee has committed to enst,re safety-related loads previously containing RMS-9 devices are operable with respect to the potential common mode failure prior to returning the unit to power operation. The inspector had no ( further questions regarding these issues.
4.01 ' UNRESOLVED ITEMS 1 Unresolved items are matters about which additional information is necessary to determine whether they are acceptable, a deviation or a violation. Two unresolved items are discussed in detail under Section 2.0.
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' 5.0 EXIT MEETING The inspector met with the licensee's personnel denoted in Attachment 1 of this report at the conclusion of the inspection period on August 27,1993. At that time, the scope of the
. inspection and the inspection results were summarized. In addition, during the exit meeting, the licensee confirmed their commitments to ensure that the following issues are addressed-prior to plant startup: 1) the elimination of the wiring configuration discrepancy in the EDGs' load permissive circuit; 2) the development of a plan to search for other possible single failure common wiring discrepancies; 3) the evaluation of the significance of the loose parts associated with the "B" EDG load permissive relay, and determine the potential for the same problem. occurring in similar relays; and 4) the implementation of corrective actions to ensure lthat the safety-related circuits previously containing RMS-9 devices are operable with
. respect to the potential common mode failure. The licensee acknowledged the inspection findings as detailed in this report and had no additional comments regarding the inspection results.
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i ATTACHMENT 1 PERSONS CONTACTED
" Maine Yankee Atomic Power Company W. Barry, Senior Engineer R. Bickford, Manager, Maintenance
R.' Blackmore, Plant Manager
J. Connell, Section Head, Nuclear Safety
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T. Gifford, Assistant Manager, Corporate Engineering
- R. Grant, Section Head, Electrical and I&C Engineering
J. Hebert, Manager, Licensing & Engineering Support ir
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R. Jordan, Senior Licensing Engineer g
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S. IxClerc, Section Head, Quality Assurance
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C. Lloyd, Supervisor, Quality Control
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' E. Pazdziorko, Electrical Engineer.
C. Shaw, Manager, Plant Engineering
G. Whittier, Vice President, Licensing and Engineering
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Yankee Atomic Electric Comoany J. Bonner, Lead Electrical Engineer
S. Urbanowski, Principal Electrical Engineer State of Maine-P. Dostie, State Nuclear Safety Inspector
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- U.S. Nuclear Regulatory Commission
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J. Beall, Acting Section Chief, Electrical Section C. Marschall, Senior Resident Inspector, Maine Yankee
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W. Olsen, Resident Inspector, Maine Yankee
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denotes attendance at exit meeting
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