IR 05000334/1990004
| ML20006F387 | |
| Person / Time | |
|---|---|
| Site: | Beaver Valley |
| Issue date: | 02/13/1990 |
| From: | Young F NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION I) |
| To: | |
| Shared Package | |
| ML20006F382 | List: |
| References | |
| 50-334-90-04, 50-334-90-4, NUDOCS 9002270461 | |
| Download: ML20006F387 (8) | |
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U. S. NUCLEAR REGULATORY COMMISSION
REGION I
Docket No.
50-334 License Nos.
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Report No.:
~50-334/90-04
' Licensee:.
- Duquesne Light Company One Oxford Center 301 Grant Street
.Pittsburgh, Pennsylvania 15279 Facility Name: Beaver Valley Power Station, Unit 1 Location:
Shippingport, Pennsylvania-Dates:-
January 22 - 26, 1990-Inspector:
P. R. '..'ilson, Senior Resident Inspector.
- Approved:
hwa No w cQf/3l90 Francis You
'Date Reactor Proj% Actf@ Chief.
e s SeYtion No. 4B Division of: Reactor Projects Inspection Summary:
Inspection Report No. 50-334/90-04 for January 22 - January 26,-1990
' Area Inspected:
Special-inspection of Unit 1 operation between January 14 through January 19, 1990,- with the required No. 3 Uninterruptibic Power Supply
.being out of service.
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>Results:
One unresolved item was identified concerning the operation of the No. 3 120
'VAC Vital Instrument Bus without its associated uninterruptible power source
.for approximately 137 hours0.00159 days <br />0.0381 hours <br />2.265212e-4 weeks <br />5.21285e-5 months <br />. Another unresolved item was identified concerning
. technical' specification associated with defining operability of certain power sources. An-apparent weakness was identified concerning the willingness of
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the licensee to operate.in a degraded condition without restrictions despite
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recommendations to the contrary from the licensee's Independent Safety
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. Evaluation Group.
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DETAILS Within this report period, interviews and discussions were conducted with various licensee personnel, including senior control room operators, instrument and control engineers, licensing engineers, members of the licensee's Indepen-dent Safety Evaluation Group, and the licensee's management staff.
A.
Overview
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On January. 19, 1990, a Unit I controlled shutdown was performed due to an inoperable 120 VAC vital bus required by Technical Specifica-
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tions. Unit I had been operating at full power. since January 14, 1990, with the No. 3 Uninterruptible Power Supply (UPS) out of service due to a blown fuse. The No. 3 vital bus was being energized from an alternate source (480 VAC/120 VAC transformer connected to a
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safety relateu 480 VAC bus). The licensee had taken the position-that the 120 VAC vital bus could be considered operable when energized from the alternate source.
Following discussions with the Resident Inspector and HRC Region I staff, the licensee committed to shut down Unit 1 if the No. 3 UPS could not be immediately naturned to service. The No.s3 UPS required some minor adjustments and could not be immediately placed in service, therefore, a controlled shutdown was initiated.
B.
System Description and Requirements
.1.
System Description Unit 1 utilizes four independent vital instrument buses (VIB)
which provide power to the four channels of the Solid State
. Protection-System including its associated instrumentation trains and to various primary and secondary control systems.
Each VIB is energized from its associated UPS or an alternate source.
Each UPS is comprised of three major components; a rectifier, an inverter, and a solid state static transfer switch. During normal operation, the 120 VAC VIBs are supplied
.from the 480 VAC emergency unit substations 1-8 (Orange Train)
or 1-9 (Purple Train) through the rectifier and inverter.
If
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normal power supply is lost,-the VIB is supplied from the corresponding safety related 125 VDC station battery through the inverter, w
There are two VIBs in each Class 1E electric division or Train.
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No.1_VIB and No. 3 VIB are part of the Orange Train and No. 2 VIB and No. 4 VIB are part of the Purple Train.
If a UPS fails, an alternate source of power from the associated 480 VAC emergency unit substation through a 480 VAC to 120 VAC line voltage regulator is provided via the solid state static transfer switch. The static transfer switch will transfer the VIB loads on loss of power to the UPS to its associated alter-nate source within one cycle.
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Each solid state transfer switch has a three position manual transfer switch..In'the " Normal" position, if the UPS is lost, the solid state transfer switch automatically transfers the VIB to its associated alternate source.
In-the Bypass position, both the UPS and the alternate source are connected to the VIB.
In the " Bypass Isolate" position, the VIB is connected directly to the alternate source.
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The 480 VAC Emergency Unit Substations are designed such that if
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offsite power is lost, the substation would be reenergized by the associated emergency diesel generator-(i.e., the unit substation is not stripped.from the 4160 VAC emergency bus on'a loss of offsite power). Assuming no failures, a VIB aligned to the alternate source would be reenergized within ten seconds-following a loss of offsite power.
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2.
Requirements Beaver. Valley Unit 1 Technical Specification'(TS) limiting condition for operations (LCO) 3.8.2.1, "Onsite Power Distribu-tion-Systems," requires that the 120 VAC VIBs be operable and energized from sources of power other than,the emergency diesel j
generators...This LCO is applicable in Modes 1 through 4.
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'The action statement associated with this LC0 requires that an inoperable VIB be restored to an operable status within eight hours or be' in at least Hot Standby (Mode -3) within the next six hours and.in Cold Shutdown (Mode 5) within the following 30 hours3.472222e-4 days <br />0.00833 hours <br />4.960317e-5 weeks <br />1.1415e-5 months <br />.
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The Unit 1 TS definition 1.6 for operability.contains a state-ment?that the normal and emergency electric power sources.
. required for that system, subsystem, train, component or device-U must also be capable for performing their related safety n
function (s).
Unit 1 TS LC0 3.0.5 allows for components not to be declared
.i inoperable solely because its emergency or normal power source is inoperable, provided the corresponding. normal or emergency source is operable.
. The Unit 1 Final Safety Analysis Report (FSAR) Sections 8.5.3 J
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and 8.5.4 describes the power sources for the.VIBs.
Section-h.
8.5.3 states:
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Vital buses normally receive power from the inverters o
through a rectifier AC supply and the battery and charger
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are not required under this condition.
The battery will i
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supply the vital bus inverters on loss of AC power.
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?i Section 8.5.4 of the FSAR states:
During normal operation the 120 VAC vital buses-are
supplied from the 480 VAC emergency, bus inverter with the unit substations 1-8 or 1-9 through a rectifier and a vital battery floating.
In the event inverter fails, an alter-
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nate_ source of power from the 480 VAC emergency unit sub-a station through a 120/240 VAC distribution transformer is provided as a bypass.
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C.
Detailed Description:
At approximately 3:20 a.m. on January 14, 1990, the main power fuse
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for the No. 3 UPS inverter blew. The loads on the No. 3 VIB auto-matica11y transferred to the associated alternate power source via the static transfer switch.
Control room operators responded in-
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accordance with procedure and placed the manual transfer switch in
" Bypass Isolated." A clearance was subsequently placed on the manual switch and a Maintenance Work Request (MWR) was prepared. The MWR
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was classified as priority "3" work.
Site Administrative Procedure'
30, "The Maintenance Work Reque t," describes a priority "3" work as
" work to be performed on technical specification and balance of plant j
equipment or components that show signs of degraded performance, but
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do not warrant an immediate fix. Equipment operability and station
' power operation are not compromised."
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On January 9,1990, discussions with the' Resident Inspector and Regional Staff were held concerning the operability of the No. 3 VIB
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without its associated UPS.
During the discussions, the licensee committed to shut.down Unit I-if:the No. 3 UPS could.not be immediately returned to service. The No. 3 inverter. Main Power Fuse was replaced I
and the UPS energized. Technicians observing the UPS observed that L
the UPS did not energize as expected and determined that a calibra-
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L tion check was-required.
Therefore, at approximately 4:22 p.m., a
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Unit I controlled shutdown was commenced.
Following some minor
adjustments, the No. 3 UPS was returned to service at approximately 8:13 p.m. that evening.
The cause of the blown fuse could not I
determined. The licensee continued the shutdown to repair a leaking charging header relief valve on the Chemical-and Volume Control System.
A Standing Night Order was prepared directing that TS 3.8.2.1 should
be-applied whenever a UPS should fail. A similar order was imple-mented at Unit 2.
Further discussions were held between the licensee and NRC on
o January 25, 1990. During the discussion, the licensee described their interpretation of TS 3.8.2.1.
The licensee's position was that a VIB need not be declared inoperable upon the failure of the
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associated UPS as long as the VIB was energized by its alternate source.
The licensee contended that the normal power source to the
- UPS and the alternate power source to the VIB ultimately come from the same source, the 480 VAC emergency unit substation and therefore
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the normal power supply to the VIB was not lost.
In addition, the emergency diesel generator would provide a backup source of power if a loss of offsite_ power occurred.
The_ licensee further contended the Technical Specification 3.0.5 permitted that the VIB not be declared inoperable if the emergency power supply (station battery) to the bus was not cperable as long as the normal power source was operable.
During the above discussions, the licensee committed to formally request,-within two weeks, a Technical Specification Amendment which.
would address the operability of UPSs.
Specifically, the revision would require that each VIB be energized vice its associated UPS.
It would restrict continued operation to 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> if a UPS was inoperable.
This approach is consistent with Revision 4 of Standard Westinghouse Technical Specifications.
D.
Inspector Findinns 1.
The inspecter reviewed the_ maintenance and operational histories of both Unit I and Unit 2 UPSs.
Inverters in Unit-l' had a history of recurring failures, especially the No. 3 inverter.
The Unit 2 UPSs have operated satisfactorily since the unit began commercial operation.
For approximately 14 months, between the fifth and sixth refueling outages, the No. 3 VIB was ene,rgized from its associated alternate source due to recurring inverter problems.
During this time, other Unit 1 VIBs were also energized from their alternate sources for short periods of time (no longer than two days).
No more than two VIBs were energized from alternate sources at the same time.
Prior to 1988, a number of plant transients and reactor trips had occurred-at Unit 1 due to' inverter failures. The original
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design of the power supplies to the Unit 1 VIBs did.not include a static transfer switch. When an inverter failed, operators had to manually reenergize the VIB via its alternate source. To improve the reliability of the Unit i VIBs, several modifica-tions were made. During the sixth refueling outage, static transfer switches were installed. Also, internal air recircu-lation fans were installed in the inverter cabinets to improve cooling. The majority of the components ir, the No. 3 inverter were also replaced.
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From the end of the sixth refueling outage (February,1988),
until the No. 3 inverter main power fuse blew on January 14, 1990, the Unit 1 UPSs operated satisfactorily during plant operation.
2.
The inspector reviewed maintenance and operation procedures for the Unit I and Unit 2 VIBs.
No restrictions or limits were found on the number or the duration that VIBs could be energized by the alternate sources. Therefore, it was possible to have all VIBs energized from the alternate sources simultaneously.
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Since the VIBs of each train (Orange Train - VIBs l' and 3; j
PurpleLTrain VIBs 2 and 4) share the same alternate power source; a single failure would result in the loss of power to both VIBs associated with one train.
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3.
The effects of operating with VIBs on the alternate source were
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reviewed and are discussed below:
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a.
A loss of offsite power would deenergize all VIBs aligned to the alternate source (s) for approximately 10 seconds until the emergency diesel generators reenergized the Units emergency buses. The effects of the momentary loss of power to the VIBs were not analyzed by the licensee.
b.
The loss of power to one VIB, depending on which'VIB was lost, could result in a plant transient which, in turn could lead to an inadvertent reactor trip, assuming no operator action.
If a Solid State Protection System (SSPS)
bistable energized from another unaffected VIB was tripped for maintenance or testing an inadvertent reactor trip could result,
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c.
The loss of power of both VIBs in one train would result in a reactor trip and an inadvertent safety injection.
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d.
Loss of power to a VIB, whether momentary or for longer duration, would affect operator response, especially if power was lost to more than one VIB. The loss of so' e or all vital. instrumentation increases the probability for operator errors and delays in' response to a plant trar ent or accident.
e.
The' alternate power source regulating transformer was designed to limit voltage variations to approximately that'
of the UPS.
The loads on the VIBs were not sensitive to small variations in frequency.
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4.
The inspector reviewed-the licensee annunciator response procedures concerning loss of power to a VIB.
The procedures were detailed ~and comprehensive prov_iding adequate instructions to prevent an inadvertent reactor trip. Operators interviewed were knowledgeable of the actions to be taken'on the loss of
Vital bus.
Simulator training for a loss of VIB has also been
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given to control room operators.
i 5.
On August 12, 1989, the licensee's Independent Safety
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Evaluation Group (ISEG) issued a letter (NDISEG:0304 Rev 1)
addressed to General Manager of Nuclear Operations concerning the operation of the VIBs energized from sources other than the UPSs. The letter stated the operation with the No. 1 or No. 2 UPS inoperable was an unanalyzed condition due to additional
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in the actuation of safety related system following a loss of offsite power. The letter therefore recommended that when the No. 1 UPS or No. 2 UPS were not in service, the action statement of TS 3.8.2.1 should be invoked. The letter also recommended that operation without the No. 3 or No. 4 UPS should be limited to seven days.
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The inspector could find no evidence that the recommendations made in the above letter were implemented..No reason could be given as to why the recommendations were not evaluated and acted j
upon.
The inspector reviewed the licensee's process for the handling
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of ISEG recommendations. Site managers indicated that ISEG-
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recommendations, when received, are prioritized and acted upon l
depending on the significance of the recommendation.
The ISEG I
is responsible for tracking the status of the recommendations i
made by the group, however, the initial followup normally does l
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not occur until six to nine months af ter the letter was issued.
6.
Site management's response to the failure of the No. 3 UPS was reviewed.
The repair of the No. 3 UPS was given a routine
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priority by the on duty Nuclear Shift-Supervisor. This was
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consistent with the licensee's interpretation of TS 3.8.2.1.
Senior operations management was not aware that the No ~3 UPS-I was out of service. During the recovery from the last Unit 1
refueling outage (7R), senior operations management had placed a
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I hold'on the plant heat up from Mode 5 to Mode 4 until a UPS was
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satisfactorily tested. The Site Instrumentation-and Controls Manager directed that the repair of the UPS be given the highest precedence of all priority "3" MWRs, however a qualified replacement fuse was not readily available.
E.
Conclusions
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1.
The licensee's interpretation of VIB normal and cmergency power i
supplies was not consistent with the system operation descrip-
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tion in the-FSAR.
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The normal power supply is through the inverter and,
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The battery is the emergency power supply.
I With'the No. 3 inverter out of service, neither the normal nor
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the emergency power source was available.
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Also, the licensee's interpretation to Technical Specification 3.8.2.1 requirements was not consistent with the design of-the
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VIBs.
VIBs are designed to be uninterruptible. Using the l-
" bypass isolate" mode defeats this inherent design feature.
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Operation.of a VIB with its associated UPS out of service was a degraded condition which increased the probability for inadvertent reactor trips and safety injections.
The effects of a momentary loss of power to the VIBs resulting from_a loss of offsite power were not analyzed when VIBs are energized from the alternate sources.
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3..
The failure to enter the action statement for TS 3.8.2.1 following the failure of the No. 3 UPS constituted an apparent violation. This item is Unresolved pending further review by NRC(50-334/90-04-01).
4.
Technical Specification 3.8.2.1 apparently was inadeq'uate in that it did not specifically address the operability of uninterruptible power supplies.
This item.is Unresolved pending the submittal requesting an amendment to TS 3.8.2.1 that restricts the use of the alternate power source to no more than 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> after which time associated VIB would be declared inoperable (50-334/90-04-04).
5.
The licensee modifications to improve the reliability of
the Unit 1 VIBs were notable and have reduced the number of plant transients and inadvertent reactor trips.
6.
.There is an apparent weakness in site management's safety
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perspective as evident by the lack of limits or ' restriction on the number of VIBs or the time duration that a VIB(s) could q
be energized by the alternate source (s). The failure to evaluate and act upon the recommendations of-'the ISEG and
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the' assignment of a routine priority to the repair of the No. 3 inverter were other examples of this apparent weakness.
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' F.
Exit Meeting
The inspector met with-licensee representatives at'the. conclusion of the inspection on January 30, 1990.
The inspector summarized the scope of the inspection, the. findings, and the conclusions-as
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described in the inspection report.
The following members of the licensee's' staff attended the exit meeting:
T. Noonan, General Manager, Nuclear Operations
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K. Grada, Manager, Nuclear Safety (t
.M. Pavlich, Acting Manager, Quality Assure.nce
)'n R. Hecht, Director, Instrumentation and Controls J. Forney, I&C Engineer D. Murko, I&C Engineer L
S. Sovick, Licensing Engineer
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