ML20246J542
| ML20246J542 | |
| Person / Time | |
|---|---|
| Site: | Arkansas Nuclear  |
| Issue date: | 07/07/1989 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20246J521 | List: |
| References | |
| IEB-79-27, NUDOCS 8907170394 | |
| Download: ML20246J542 (14) | |
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- pa ucugo UNITED STATES f ~ h[g 7,, .gE g NUCLEAR REGULATORY COMMISSION WASHINGTON, D. C. 20555
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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGLh ATION RELATED TO THE AUDIT OF THE RESOLUTION OF IE BULLETIN 79-27 CONCERNS ARKANSAS POWER AND LIGHT COMPANY j ARKANSAS NUCLEAR ONE, UNIT 1 DOCKET NO. 50-313
1.0 INTRODUCTION
On November 10, 1979, an event occurred at the Oconee Power Station, Unit 3, which is a Babcock & Wilcox (B&W)-designed nuclear power plant. The event started with a loss of power to a Non-class 1E 120-Vac, single-panel that supplied power to the integrated control system (ICS) and thephase power non-nuclearinstrumentation(NNI) system. This loss of power resulted in control system malfunctions and a significant loss of information to the control room operator. l The event at Oconee, Unit 3, occurred as the result of a Non-Class IE inverter feilure and the failure of its automatic bus transfer (ABT) switch to transfer the instrumentation and control loads from the failed inverter to a designated alternate regulated 120-Vac power source. The resulting loss of power to the NNI rendered control room indicators and recorders for the reactor coolant system (except for one wide-range resctor coolant system pressure recorder) and most of the secondary plant systems inoperable. Loss of power also caused the !
loss'of instrumentation associated with the systems used for decay heat removal and coolant addition to the reactor vessel and steam generators. In addition, upon the loss of power, all valves controlled by the ICS assumed their failure positicns.
On November 30, 1979, the NRC issued IE Bulletin 79-27, " Loss of Non-Class IE ,
InstrumentationandControlPowerSystemBusDuringOperation"(Reference 1). l IE Bulletin 79-27 required licensees to review the effects of loss of power to ea'ch Class IE and Non-Class 1E bus supplying power to plant instrumentation and controls and to determine the resulting effect on the capability to achieve a safe (cold)shutdownconditionusingplantoperatingproceduresfollowin0the power loss. The intent of IE Bulletin 79-27 was to ensure that the loss of power to any bus in the plant electric distribution system would not result in control system actions that would cause a plant upset / transient condition requiring operator action concurrent with the loss of control room information (indications, alarms, etc.) upon which these actions would be based.
On February 26, 1980, an event that involved a loss of NNI system power occurred at the B&W-designed Crystal River, Unit 3, nuclear plant. In this event, failed input signals provided ta the ICS from the NNI system caused reactor coolant system (RCS) overpressure +tation and the subsequent retcase of reactor coolant into the reactor building, This loss of power also resulted in the failure of most of the instruments needed by the operator to respond to the event, making operator action very difficult. On March 7, 1980, the NRC issued 8907170394 890707 PDR ADOCK 05000313 Q FDC
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IE Information Notice 80-10 (Reference 2), which expanded the scope of IE Bulletin 79-27 for B&W-designed reactors to include the implications of the Crystal River event. The NRC review of utility reponses to IE Bulletin 79-27 focused on whether there was reasonable assurance that the concerns of the .
bulletin had been properly addressed. This assurance was based on an affirmative I or clearly implied statement of conformance to all bulletin requirements and a positive indication that all required buses were reviewed.
. Following the issuance of IE Bulletin 79-27 and IE Information Notice 80-10, two events occurred at the B&W-designed Rancho Seco nuclear plant involving the 1 loss of ICS/NNI power and loss of control room information. These events, ]
occurring on March 19, 1984 and December 26, 1985 demonstrated that the concerns identified in IE Bulletin 79-27 continued to exist in B&W-designed plants.
Additional background information regarding licensee responses to IE Bulletin 79-27 and the NRC evaluation of these responses can be found in Section 7,
" Precursors to the December 26, 1985 Incident at Rancho Seco and Related NRC and SMUD Actions," of NUREG-1195 (Reference 3).
1 In order to resolve the concerns raised in NUREG-1195, the B&W Owners Group .
submitted a description of the B&W program entitled " Safety and Performance teprovement Program (SPIP)" in their document BAW-1919 on May 15, 1986. The NRC staff reviewe1 BAW-1919 through Re.<ision 5 and presented its evaluation in NUREG-1231, dated November 1987, and in Supplement No. I to NUREG-1231, dated March 1988 (Reference 4).
Included in the SPIP program are specific tasks to be completed by each utility; however, the SPIP tasks do not include a review to determine whether the specific concerns of IE Bu11etir.79-27 have been properly addressed and resolved. The NRC staff believes that proper resolution of IE Bulletin ,
79-27 concerns, in conjunction with implementation of SPIP recommendations, should significantly reduce the frequency and severity of loss of power transients at B&W-designed plar,ts, including those transients resulting from loss of power to the ICS/NNI. As part .f the staff audit of the SPIP program, the Instrumentation and Control Systems Branch (ICSB) is conducting an audit of each B&W facility to verify the resolution of IE Bulletin 79-27, concerns. !
2.0 AUDIT METHODOLOGY The Arkansas Nuclear documentation One-1 (ANO-1)(audit review comprised consisted of a) examining of twedistribution plant electric parts: 1) a pre-audit system single line dia
, system descriptions, and reactor trip and shutdown Analysis Rep (ort procedures Reference (FSAR) 5), and grams (b) preparing a li n ofalong with other the equipment, instru- drawings ments, controls, and indications identified in the procedures and needed to bring the plant from an operating state with a reactor trip to a safe shut-down and cooldown condition; 2) on-site audit to determine if a safe shutdown can be achieved in the event of a postulated worst case bus failure using established operating procedures. The audit team met with the licensee's representatives (Reference 6) to determine the sources of power to each of the instruments and equipment in the list prepared during the pre-audit 1
documentation review. Three buses were selected for review by the audit team based upon the majority of the components identified on the list and supplied from these buses, and their downstream connections, which failed due to the l cascading' power loss. The failure of the three selected buses appeared to represent the worst case scenarios due to the consequential loss of a substantial number of instruments and equipment that could increase the complexity of the operator actions required to stabilize the plant and to achieve a safe shutdown following a reactor trip.
The applicable sections of the reactor trip, plant shutdown, and cooldown procedures were examined by the audit team and the utility representatives to determine how each' step would be performed while failing the selected buses one at a time. For those steps that were affected b licensee described how the step would be. performed (y the bus for example, by usingfailure, a the redendant instrument, switching to another power source, or performing manual actions to achieve a safe shutdown). The audit team also examined annunciator response procedures to determine if specific directions were provided to the operator for dealing with the loss of power to the plant distribution system.
3.0 EVALUATION The audit team evaluated the effects of the loss of power to each selected bus by analyzing the combined effects of the loss of power to the bus loads (instra-ments, controls, pumps, valves,etc.)andtheresultingeffectontheability to proceed to cold shutdown using approved procedures. The review included an evaluation of the indication and annunciation provided to alert the operator in the control room to the loss of bus power. Equipment and component losses that result from the failure of the selected buses were evaluated along with the cumulative effects of loss of power to loads due to cascading power losses, to determine the overall effect on the plant during operation.
The audit team selected three specific cases of bus failure and performed a detailed evaluation to determine operator capability for achieving a safe shutdown using the applicable procedures in each case. These three cases are described below.
3.1 Loss of Class 1E 480-Vac Motor Control Center (MCC) B61 MCC B61 has only one source of power (no provisions for an alternate power source) and many components and equipment powered from this MCC are not essential for the safe shutdewn. However, for those that are needed for safe shutdown, there were alternate or redundant components and equipment available or alternate methods were identified to achieve a safe shutdown (For example, the steam generator B main feedwater isolation valve cannot change position electrically when MCC B61 is failed; however, when procedurally required to terminate main feedwater, the operator will be able to trip the main feedwater pumps to accomplish this step of the procedure). The major loads on MCC B61 are 120-Vac Non-Class IE instrument and control Bus YO2, inverters Y22 and Y24, and battery charger D04. All of the loads on MCC B61 are listed in Attachment C of Procedure 1107.02, "ES Electical System Operation."(Reference 5),
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l Battery charger D04 supplies the 125-Vdc Bus D02 loads including inverters Y22 and Y24 and charges the associated battery bank. Should the loss of MCC B61 cause disabling of battery charger D04, the battery bank will resume supplying power to the 125-Vdc bu: 002. Alternate battery charger DOS can also be connected manually to Bus 002. Non-Class IE 120-Vac Bus YO2 is powered from 1100 B61 through a stepdown transformer, and supplies a variety of instruments and controls that are listed in Procedure 1107.01. The audit team reviewed Bus Y02 loads and determined that the loss of most of the instruments and controls powered by this bus has no effect on achieving a safe shutdown condition. For those instruments and controls whose loss could impact the shutdown procedures, either alternate power sources or completely redundant channels are provided, or alternate procedures are used (For example, the operating procedures require a manual local trip of the main feedwater pumps when the loss of Y02 causes loss of pump control power, or the proportional pressurizer heater controls lost with the loss of bus YO2 will not affect plant operation as the redundant pressurizer heater bank will still be available). The loss of power to bus YO2 is annunciated in the control room and the required operator responses are provided in procedure 1203.12A. In addition to various guidelines, this procedure directs the operator to manually connect bus YO2 through bus tie '
breakers to redundant bus Y01. This manual action is performed only when there is no fault on bus Y02.
Based on the above design features, plant operating prccedures, and operator training, the audit team concluded that following the loss of power to 480-Vac MCC B61, the operator has sufficient instrumentation, indication, and equipment available to achieve cold shutdown using approved procedures.
3.2 Loss of 125-Vdc Bus 001 The audit team chose to examine 125-Vdc bus 001 (Diagram 2) because it is the dc source of electrical power to instrument inverters Yll, Y13, and Y25. Inverters Y11 and 713 provide vital instrument power for the reactor protection system, while Y25 provides power to the plant computer, Bus DOI also supplies the engineered safety actuation system (ESAS) control bus RA1 and the station switchyard controls along with 125-Vdc distribution panel D11 to power control circuits such as the turbine trip circuitry, the generator lockout circuitry, the main feedwater pump turbine auxiliaries, and ac switchgear controls. Bus 001 is powered by battery charger D03 and has a backup supply from a 1350 ampere-hour,125-Vdc battery bank. A spare battery charger, DOS, can be manually ccnnected to bus D01 when battery charger D03 is inoperable.
Annunciator response procedure 1203.12A (Reference 5) deals specifically with the luss of bus 001 and provides procedures for its recovery. This procedure also lists the the affected dc buses and distribution panels, and the ac switchgear that lose control power as a consequence of the failure of bus D01.
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In the ANO-I design, the loss of o jower to an inverter causes trip of both the normal ac and de sources to that inverter. However, the bus loads including NNI and ICS, will not be affected by the loss of power to the inverter because the inverter supplied instrument buses have an alternate source of 120-Vac i' regulated power available within the inverter cabinet. An automatic power seekingbustransfer(ABT)switchselectsthisalternatesourceonthelossof de power to the inverter. Thu:;, the loss of inverters Y11 and Y13 will not affect their associated instrument buses. Similarly the loss of inverter Y25 will not affect computer power supply. Additionally, the loss of power to the computer bus RC1 has no affect on the ability of the operators to bring the >
plant to a cold shutdown condition Computer indication is not referenced in the shutdown procedures and the comp.ter has no control function. It is used for data logging and as a backup indicator to the primary instrument readouts.
The loss of ESAS control bus RAI will cause a reactor trip if the reactor is operatir.g at greater than 50 percent power. Loss of distribution panel D11 will not cause an automatic turbine trip or open the generator output breakers follewing the reactor trip. Thus, the generator will remain connected to the switchyard. Manual operator action is required to either re-energize D11 from redundant power source D02 per Section 9 of procedure !
1107.01 or to use alternate methods to trip the turbine and open the generator cutput breakers per E0P 1202.01 (Reference 5). The main feedwater pumps will continue operating as distribution panel 011 provides one of two power sources for the main feedwater pump controls. The turbine-driven energency feedwater (EFW) pump control power is independent from bus D01 and will be available to st' t and fced both steam generators when the manual trip of the main ]
feedwater pumps is procedurally required. The needed valves to establish '
flow are powered by the redundant dc distribution system.
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Bus D01 is a backup source of power to switchyard controls that normally ,
receive power f rom an independent switchyard battery and battery charger. Loss of power to the switchyard controls due to the loss of bus D01 will not occur !
with the normal distribution system alignment. Should bus D01 be supplying this power when bus D01 is lost, manual operation of the switchyard by the operator is still possible.
I Based on the above design features, plant operating procedures, and operator training, the audit team concluded that following the loss of 125-Vdc bus D01, the operator has sufficient instrumentation, indication, and equipment available to achieve cold shutdown using approved procedures.
3.3 Loss _of 24-Vdc NNI Power The audit team chose to examine 24-Vdc NNI-X and .424-Vdc NNI-Y buses (Diagram 3). Power indicators have been provided in the control room in response to IE Bulletin 79-27 for both NNI-X and NNI-Y buses.
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i Each of the two NNI-X buses (+24-Vdt bus and Vdc bus) receive auctioneered power from their respective set of two power supply modules. Each of the two modules in a set are supplied by independent and redundant sources of Class IE 120-Vac power. One power source is a division 1 inverter output bus while .
the other power source is a division 2 regulated instrument bus.' The inverter '
has a primary ac power source, a backup de power source, and an alternate ac power source. These design features ensure that a single failure of these ;
power sour:es will not degrade the 24-Vdc NNI-X instrument power buses. !
ThstwoNNI-Ybuses(+24-Vdcbusand-24-Vdcbus)areeachsuppliedbyasingle power supply module (non-auctioneered supply) being fed by a single source of 120-Vac power. However, the design includes an automatic bus transfer (ABT) switch to automatically transfer the 120-Vac input to both modules from inverter supplied power (preferred division I power source) to a regulated power source (fromredundantdivision2). The audit team identified that reactor controls are not supplied by the NNI-Y 24-Vdc buses and thus, a failure of the NNI-Y 24-Vdc buses will not affect reactor controls.
The audit team also examined the licensee's calculation number 87E-0084-01, "Known Safe State Evaluation for ANO-1 ICS/NNI" (Reference 5). This document evaluates partial and total loss of ICS/NNI power, and is divided into the following sections:
- 1. LossofICSpower(total)
- 2. Loss of ICS power (ac only) 3.
- 4. Loss ofNNI-X Loss of NNI-Ypower power(all(dc,)ac, or total) :
- 5. LossofNNI-Xpower(ac) '
- 6. Loss of NNI-X power (dt) i
- 7. Loss of NNI and ICS power (all)
- 8. Loss of NNI-X and -Y power (all) l In ilese sections, the licensee assumed (1) that there are no component failures beyond the equipment lost due to the failed bus; (2) that the power distribution system is in a worst-case alignment, namely, buses were assumed I
to be loaded so that they become a cascading failure; (3) that safety s
! were operable; (4) that the plant was operving at full power; and (5) ystems that no l operator action occured for ten minutes. This last assumption goes beyond the assumptions of the audit team in that the audit team assumed operator action.
Thus, the licensee's evaluation is more conservative than the audit reported here.
In each of the above analyzed events, the known safe state (hot shutdown) was attained. The licensee stated that cold shutdown is possible once hot shutdown is achieved because the decay heat removal system is available.
Based on the above design features, plant operating procedures, and operator training, the audit team concluded that following a loss of power to 24Vdc NNI-X or NNI-Y buses, the operator has sufficient instrumentation, indication, and equipment available to achieve cold shutdown using approved procedures.
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1' i 3.4 Automatic Bus Transfer Switches The continued power availability on certain buses, such as the vital instrument buses (Diagram 2) and the NNI-X and NNI-Y 120-Vac instrument buses (Diagram 3), ,
relies on the operation of ABT switches. These buses are normally supplied with j inverter-derived power with automatic bus transfer (ABT) switch operation to l connect to a regulated power supply on a low inverter output voltage (Section 3.2).
Although the licensee had cleaning and housekeeping procedures for these switches, they had no requirement or procedure to periodically test the opability of the switches to perform their design function. Since the ABTs are installed to transfer the power from one source to another and are assumed to operate, that capability should be demonstrated periodically. It is the staff's position that the licensee should institute a surveillance program that periodically tests the operability of these devices to demonstrate their reliability. This position is consistent with the B&W Owners Group SPIP recommendations, which include preventive maintenance and periodic testing of ABT switches used for maintaining ICS power supplies (TR-183-ICS).
4.0 CONCLUSION
S The audit team was reasonably assured that a loss of power to any of the three buses considered would not result in a loss of the control room indication necessary to bring the plant to a safe shutdown using app oved operating procedures. The audit results provided sufficient r,vidence that the operators, using the existing procedures, can achieve a safe (cold) shutdown condition at ANO-1 following the loss of power to any single Class 1E or Non-Ckss IE bus that supplies power to plant ~ instrument and control circuits. It is thet efore concluded that IE Bulletin 79-27 concerns have been adequately resol fed for AH0-1.
-The audit team believes that to ensure reliable operation of the devices that perform automatic switching of bus power sources from one power source to another, the devices should be periodically tested for their safety function.
A. periodic test program for these devices is recommended to be developed by the licertsee.
i Dated: July 7, 1989 Principle Contributor: I. Ahmed l
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5.0 REFERENCES
- 1. NRC IE Bulletin No. 79-27, " Loss of Non-Class 1-E Instrumentation and-Control Power System Bus During Operation," November 30, 1979.
- 2. NRC Information Notice No.'80-10, " Partial Loss of Non-Nuclear Instrument System Power Supply During Operation," March 7, 1980.
3.. NUREG-1195, " Loss of Integrated Control System Power and Overcooling Transient at Rancho Seco on December 26, 1985," February,1986.
4 NUREG-1231, " Safety Evaluation' Report Related to Babcock & Wilcox Owners Group Plant Reassessment Program," November 1987 and Supplement 1 to the NUREG, March 1988..
- 5. Schematics, drawings, and procedures listed in Appendix A of this report.
- 6. Licensee personnel contacted during the audit and listed in. Appendix B of this report.
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APPENDIX'A DOCUMENTS EXAMINED, The following documentation was examined as part of this audit.
FSAR Chapter 8 Rev. 11 Electrical' Systems Section 3.2.6 Rev. 11 Alternate Shutdown Systems 'and Components- 4 Figure 8-1 Rev. 11 Station Single Line' Diagram
, : Manuals Sections 1, 3, 4, 6'and 7 " Inverter Installation and Operation," TMS f
250.001D/0.0090(controlled-document).
-DRAWINGS.
Drawing Number Revision Title M-230 Sheet 1 62 P&ID Reactor Coolant System
'6600-MIR-189 -14 NNI Power Distribution Schematic Sheet 1 STM-1-32 fig. 32.44 480-Vac Distribution Fig. 32.47 125-Vdc Distribution Fig. 32.54 Inverters Functional Diagram
-Fig. 32.58 NNI-X Power Fig. 32.59 NNI-Y Power Fig. 32.60 ICS Power 1
PROCEDURES Procedure Procedure Number Revision Procedure Title 1102.10 29 Plant Shutdown and Cooldown 1107.01 3.1 Electrical System Operations 1107.02 1 ES Electrical System Operation 1202.01 11 Emergency Operating Procedure
.1203.12A 24 Annunciator K01 Corrective Action 1203.12B 19 Annunciator K02 Corrective Action
,1203.12L 19 Annunciator K14 Corrective Action 1307.002- 1 Vital ac' inverter Test and Inspection
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APPENDIX A'(con't)
DOCUMENTS EXAMINED y DESIGN REVIEW REPORT-H . Project calculation No. 87E-0084-01, "Known Safe State Evaluation for ANO ICS/NNI," July 15, 1985.
NUREG Section' 6.1.7, "ICS/NNI Reliability," WUREG 1231, Supplement 1, December 7
-1988,'
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I c APPENDIX B LICENSEE PERSONNEL CONTACTED R. Barnes . AP&L Design Engr.
.K. Camite- APal OPSS Training D. Howard- AP&L. Licensing.
P.-Lomax AP&L Licensing S. Quennez ANO General Manager LG. Sullins AP&L Design Engr.
R. Thorton- AP&L Licensing NRC PERSONNEL PARTICIPATING
-I. Ahmed 'SICB/NRR W. Johnson' NRC/ SRI R. McCormick INEL LA. C. Udy. INEL
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