ML19309B811
| ML19309B811 | |
| Person / Time | |
|---|---|
| Site: | Prairie Island  |
| Issue date: | 03/07/1980 |
| From: | Gilberts D NORTHERN STATES POWER CO. |
| To: | James Keppler NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION III) |
| References | |
| IEB-79-27, NUDOCS 8004070210 | |
| Download: ML19309B811 (2) | |
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NORTHERN -5TATES POWER COMPANY M I N N E A PO L.l S. M S N N E S OTA S 5 401 March 7, 1980 Mr. J. G. Keppler, Director Region III Office of Inspection and Enforcement U. S. Nuclear Regulatory Commission 799 Roosevelt Road Glen Ellyn, IL 60137
Dear Mr. Keppler:
PRAIRIE ISLAND NUCLEAR GENERATING PLANT Docket Nos. 50-282 & 50-306 In response to IE Bulletin 79-27, an investigation of Prairie Island's instrumenc buses was conducted with the following conclusions:
(Delay of our response to IE Bulletin 79-27 was approved by Mr. D. C. Boyd of your office by telephone on February 28, 1980.)
1.
Instrument and control power is supplied by six inverters for each unit.
Four of the inverters are Class 1E safety related and supply each of four primary system instrument and control channels. One non-Class lE inverter supplies backup power to the Rod Position Indicator system.
The other non-Class lE inverter supplies computer, fire detection and plant loud speakers and is provided with automatic transfer to its alternate AC source. Trans-fer to alternate AC for all other inverters is manual. Normal, alternate and DC sources for both Class lE and non-Class lE inverters are the safety grade station batteries and 480 volt buses.
See FSAR Figures 8.3-3 and 8.3-4.
2.
Class lE instrument channels are alarmed and annunciated in the control room upon loss of power. In addition, normally lighted status lights for respective channels would go out.
This includes one status light each specifically for channel power supply. Rod Position Indicator power supply failure is also alarmed and annunciated. The remaining inverter's loss is not alarmed or annunciated, but none of its loads are vital to plant operation.
3.
There are at least two independent indications for each vital function pro-vided in the control room. The loss of a single inverter, therefore, does not remove any indicator necessary for achievement of cold shutdown in a safe, orderly manner. In addition, process functions necessary to achieve a normal shutdown are provided with sufficient redundancy so that the loss of one does
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.Mr. J. G. Keppler Page 2 March 7,'1980
' not-prevent achievement of cold shutdown, or loss of automatic control does
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not prevent manual control from the control room. As an example of the.for-mer, feedwater flow control for each loop is powered from a separate inverter.
As an example of the latter, automatic pressurizer pressure control is powered from the channel I inverter and manual control is powered from the channel II inverter.
4.
Abnormal operating procedures are in effect covering the alarms / indications received upon loss of an inverter and methods for restoring power to its bus. Due to the inherent safety of redundancy and backup designed into the instrument and control systems, it is not necessary to develop an emergency procedure for loss of an inverter. To do so would unnecessarily dilute the effectiveness of our existing emergency procedures.
- 5..The condition described in Bulletin 79-27, loss of indication and control necessary for achievement of cold shutdown due to inverter failure, does not exist at Prairie Island. Inverter failures have occurred here, resulting
~ in reactor trips without -loss of essential shutdown controls.
A design change has been initiated to replace the instrument inverters with in-verters capable of automatic transfer to alternate AC.
These uninterruptible supplies satisfy the concerns expressed in IE Circular 79-02 and will signifi-cantly reduce the probability of inadvertent trips or ESF actuation due to inverter failure.
Yours truly, D. E. Gilberts Vice President Power Production ec:
Mr. G. Charnoff NRC Office of Inspection and Enforcement Division of Reactor Operations Inspection DEG:nk