ML16154A356
| ML16154A356 | |
| Person / Time | |
|---|---|
| Site: | Oconee  |
| Issue date: | 10/29/1993 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML16154A357 | List: |
| References | |
| IEB-79-27, NUDOCS 9311080237 | |
| Download: ML16154A356 (5) | |
Text
UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 20555-0001 SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION IE BULLETIN 79-27, SUPPLEMENTAL RESPONSE DUKE POWER COMPANY OCONEE NUCLEAR STATION, UNITS 1. 2. AND 3 DOCKET NOS. 50-269, 50-270, AND 50-287
1.0 INTRODUCTION
On November 10, 1979, an event occurred at the Oconee Nuclear Station, Unit 3, that resulted in loss of power to a non-class-1E 120 volt bus that supplied power to the Integrated Control System (ICS) and the Non-Nuclear Instrumentation (NNI). This loss of power resulted in control system malfunctions and significant loss of information to the control room operator.
The event was discussed in IE Information Notice 79-29 followed by the IE Bulletin 79-27. The Bulletin requested licensees to take certain actions to ensure the adequacy of plant procedures for accomplishing cold shutdown upon loss of power to any Class 1E or non-Class 1E bus supplying power for instruments and controls. The actions included a review of these buses, the loads served, the bus failure alarm and indications provided in the control room, the effects of bus failure on the ability to achieve cold shutdown, and the emergency procedures available for use in the event of such a bus failure.
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 or transient condition requiring operator action concurrent with the loss of control room information (indications, alarms, etc.) upon which these actions would be based. In response to the Bulletin, DPC submitted their analysis results, and our SER dated June 26, 1982 (Reference 1), found their response acceptable. In addition to this evaluation, the Oconee Nuclear Station design and procedures were reviewed under the B&W Owners Groups (BWOG) Safety Performance Improvement Program (SPIP) in a 1989 general audit of all B&W power plants to verify resolution of IE Bulletin 79-27 concerns. In this audit, three instrumentation and control power buses (120 Vac, non-1E computer power panel; 125 Vdc, 1E bus; 120 Vac, 1E vital instrument power panel) were postulated as failed, one at a time, and its effect on the control room operator's ability to achieve hot and subsequent cold shutdown using procedures was assessed.
Our audit report dated December 7, 1989 (Reference 2), found the Oconee Station design to have adequately resolved the IE Bulletin 79-27 concerns.
However, in October 1992, licensee event report (LER) (Reference 3), DPC reported that the Oconee switchyard (offsite power source to Oconee) and the Keowee Hydro Station Unit (standby power source to Oconee) were not included in their 1980 review. DPC committed in the LER to review both switchyard and Keowee Hydro Station instrumentation and control power buses for the IE 9311060237 931029 PDR ADOCK 05000269 g)
-2 Bulletin 79-27 concerns, and by letters dated January 28, 1993 (Reference 4),
and July 14, 1993 (Reference 5), DPC submitted their review results as a supplemental response to the Bulletin's concerns.
2.0 EVALUATION Oconee Nuclear Station startup and preferred power source is a.230 kV switchyard through the station startup transformer. If the 230 kV switchyard is isolated by the grid protective relaying, the startup transformer supply is automatically switched from the switchyard to the Keowee Hydro Units through an overhead circuit. Unavailability of the overhead circuit causes the Keowee Units to supply emergency power to the station distribution system through a pre-aligned underground feeder and a separate power transformer. The Oconee Nuclear Station is licensed with two Keowee Hydro Units as the standby source of emergency power to the station Class lE distribution system.
The 230 kV switchyard instrumentation and control system power supply is comprised of six 125 Vdc, Class 1E power panels.
Four of these panels energize the power circuit breakers' trip coils in two divisions, while the remaining two panels energize two channels of grid protection relaying.
"Power circuit breakers trip coil trouble statalarm" and "channel 1 or channel 2 dc supply failure statalarm" indicate a loss of power to their respective group of 125 Vdc power panels. DPC stated that loss of any one of these panels will not affect the ability of the switchyard instrumentation and control circuits to isolate the 230 kV switchyard from the grid and provide emergency power source to Oconee Nuclear Station.
The Keowee Hydro Station instrumentation and control system is supplied by four 125 Vdc, class 1E distribution panels and two 120 Vac, Non-Class 1E distribution panels located at the Keowee Station. In addition to these sources, the Keowee emergency startup channels A and B logic circuits and Keowee supervisory instrumentation is supplied by seven 125 Vdc power sources from the Oconee vital instrumentation and control power system. These power sources are divided between the two Keowee Hydro Units to supply their individual instrumentation and controls and likewise the two groups of power sources are separated from each other on the basis of their unit association.
There are two batteries, one for each Keowee unit with their individual trouble alarm in the Keowee control room. Loss of any one battery would not affect the Keownee units' ability to provide emergency power to the Oconee Units. Loss of power to any 125 Vdc or 120 Vac instrumentation and control power supply bus is indicated in the Keowee control room on five "statalarm" panels. These alarm panels are powered by 120 Vac, Non-Class 1E buses. For loss of power to either alarm panel, an Oconee "statalarm" actuates to alert Oconee Control Room of the loss of Keowee "statalarm" panel.
Additionally, a dedicated ring-down telephone between Oconee Control Room and the Keowee Control Room has been installed to provide voice communication between the two stations.
In order to make the power supply to the Keowee "statalarm" panels more reliable, the licensee has replaced the "statalarm" panels power source from the power panel board to an uninterruptible power supply (battery backed
-3 inverter).
Loss of the inverter power supply would result in loss of power to the Keowee "statalarm" as well as to its computer and will be alarmed in the Keowee Control Room. However, this loss of power would not prevent either Keowee Unit from performing its safety function for the Oconee Nuclear Station.
The licensee's assessment of the loads fed from the Keowee instrumentation and control power panels concluded that these panels are not required for Oconee to reach a cold shutdown. Similarly, loss of any one ac or dc instrumentation and control power panel at Keowee will not affect the ability of its hydro units to supply emergency power to Oconee Nuclear Station. Loss of power supply to the Keowee emergency start up logic circuits or supervisory instrumentation is indicated by "statalarm" in the Oconee Control Room.
Additionally, the licensee's review of the associated loads on the Keowee Units auxiliary transformers and switchgear has not identified any single bus failure that would prevent both hydro units from being available to provide emergency power to the Oconee Station. A test has been performed to verify that the Keowee units can emergency start and continue to run for at least one hour without power from any of its 120 Vac control power sources. The licensee further stated that there are no specific emergency operating procedures to address a single failure of any one bus in the Keowee or Oconed Switchyard ac or dc distribution systems. 'However, for a loss of offsite power event, the Oconee emergency operating procedures provide directions, including primary alarms and alternate indication and controls, to restore emergency power (Keowee Source) to the Oconee Station.
IE Bulletin 79-27 also required the licensees to re-review IE Circular No. 79-02, "Failure of 120 volt Vital AC Power Supplies (Inverters)" dated January 11, 1979, and submit any proposed design modifications or administrative controls resulting from the re-review. DPC provided the following itemized response as applicable to the Keowee Inverter:
- 1.
The Keowee Inverter circuitry does not include time delay devices to delay transfer switching due to transient loading conditions. For synchronization purposes, the Keowee Static Switch does include a one second delay in the retransfer to normal power sequence. Transient loading of the Keowee Inverter has not been a historical problem.
- 2.
The Keowee Inverter System does not include a dedicated rectified ac input to the inverter component. It is possible to have excessive dc voltages if the battery bus is eliminated and the charger alone is supplying the inverter; however, it is prohibited by operating procedures to use the battery charger as a "battery eliminator." It is recognized that our present charger system is not sufficiently regulated without the battery bus connected.
- 3.
An alternate ac source is provided with the Keowee Inverter. The transfer switch will not transfer load unless an alternate source is available. To date there has not been any experience with the Keowee Inverter inadvertently switching due to transient loading conditions.
Sufficient load capacity remains on the Keowee Inverter System and transient loading conditions are not a problem. No adjustments are deemed necessary at this time.
- 4.
Normal post maintenance and post modification administrative controls are in place to ensure operability following maintenance or testing.
Neither the Keowee Inverter nor any component fed from the inverter is considered to be safety related.
In a September 16, 1993, telecon with Phil North and Joe Whitm&n of DPC, the licensee clarified that the subject inverter only feeds the Keowee Computers and the "statalarm" panel which do not perform any safety function. The licensee stated that failure of the Keowee Inverter or the loss of any of its associated loads will not prevent Keowee from being available to Oconee as a source of emergency power.
3.0 CONCLUSION
Based on our review of the DPC's submittal we conclude that the licensee's response provided reasonable assurance that the specifically identified concerns of IE Bulletin 79-27 have been adequately addressed for the Oconee Switchyard and Keowee Hydro Units (standby power source for Oconee) instrumentation and control power supplies. The licensee's analysis has provided sufficient assurance that loss of any one of the several power sources to instrumentation and control system of the Oconee Switchyard or Keowee Hydro Units will not affect the safety function of the Keowee Hydro Units and Oconee operator's capability to achieve safe shutdown using the operating procedures.
Principal Contributor:
Iqbal Ahmed Date:
October 29, 1993
-5
4.0 REFERENCES
(1) Memorandum from R.J. Mattson (DST/NRR) to Darrel G. Eisenhut (DL/NRR),
dated June 26, 1981.
(2) Letter from NRC (L. Wiens) to Duke Power Company (H.B. Tucker), dated December 7, 1989.
(3) LER No. 92-04-00, dated November 1992.
(4) Duke Power letter (J.W. Hampton) to NRC (document control desk), dated January 28, 1993.
(5) Duke Power letter (J.W. Hampton) to NRC (document control desk), dated July 14, 1993.