ML23251A022
| ML23251A022 | |
| Person / Time | |
|---|---|
| Issue date: | 08/31/2023 |
| From: | NRC/OCIO |
| To: | - No Known Affiliation |
| Shared Package | |
| ML23251A034 | List: |
| References | |
| FOIA-2023-000163 | |
| Download: ML23251A022 (1) | |
Text
ACCELERATED DIS UTION DEMONST TION SYSTEM REGULATORY INFORMATION DISTRIBUTION SYSTEM (RIDS)
ACCESSION NBR: 9110280380 DOC. DATE: 91/10/23 NOTARIZED: NO DOCKET FACIL:50-410 Nine Mile Point Nuclear Station, Unit 2, Niagara Moha 05000410 AUTH. NAME AUTHOR AFFILIATION SYLVIA,B.R. Niagara Mohawk Power Corp.
RECIP.NAME RECIPIENT AFFILIATION TAYLOR,J.M. Ofc of the Executive Director for Operations
SUBJECT:
Discusses 910813 uninterruptible power supply failures,per 911018 discussions w/commissioners.Suggests that NRC info notice & IIT rept (NUREG-1455) be reve.sed to accurately reflect design of uninterruptible power supply units. D DISTRIBUTION CODE: AOOID COPIES RECEIVED:LTR 2 ENCL 0 SIZE:
TITLE: OR Submittal: General Distribution NOTES:
RECIPIENT COPIES RECIPIENT COPIES ID CODE/NAME LTTR ENCL ID CODE/NAME LTTR ENCL D PD1-1 LA PD1-1 PD BRINKMAN,D. D INTERNAL: ACRS NRR/DET/ECMB 7D NRR/DET/ESGB NRR/DOEA/OTSBll NRR/DST 8E2 NRR/DST/SELB 7E NRR/DST/SICB8H7 NRR/DST/SRXB 8E NUDOCS-ABSTRACT OC/LF OGC/HDS1 E " ZGh RES/DSIR/EIB 03.'SIC EXTERNAL: NRC PDR D
D D
NOTE TO ALL "RIDS" RECIPIENTS:
PLEASE HELP US TO REDUCE WASTE! CONTACT THE DOCUMENT CONTROL DESK, I'ISTS ROOM Pl-37 (EXT. 20079) TO ELIMINATEYOUR NAME FROM DISTRIBUTION FOR DOCUMENTS YOU DON'T NEED!
TOTAL NUMBER OF COPIES REQUIRED: LTTR 24 ENCL ~ '0
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hl Y NIAGA A A kl IVNWAWK NIAGARAMOHAWKPOWER CORPORATION/301 PLAINFIELDROAD, SYRACUSE. NEW YORK 13212/TELEPHONE (31 5) 428-7494 B. Ralph Sylvia Executive Vice President October 23, 1991 Nuclear NMP2L 1323 Mr. James M. Taylor Executive Director for Operations U. S. Nuclear Regulatory Commission Vashington, D. C. 20555 Nine Mile Point Unit 2 Docket No. 50-410 NPF-69
Dear Mr. Taylor:
SUBJECT:
NINE MILE POINT UNIT 2 UNINTERRUPTIBLE POVER SUPPLY FAILURES As a result of the discussion that took place with the Commissioners on October 18, 1991, I feel it necessary for the NRC Staff and Niagara Mohawk Power Corporation to reach a common understanding of the design basis, operation, and failure mechanisms of the non-safety related uninterruptible power supplies (UPS) which failed at Nine Mile Point Unit 2 on August 13, 1991. It is important that other nuclear power licensees understand the event and appreciate the valuable lessons to be learned. For example, Niagara Mohawk believes that the conclusions stated in Information Notice 91-64 and NUREG-1455 that functional control logic power supply batteries or improved preventative maintenance, including battery pack replacement, would have prevented the loss of the UPS units need clarification or correction. The post-event testing performed, combined with other significant evidence, leads us to conclude that the uD" size batteries were not intended to supply logic power while the UPS units are in service. The following information supports our conclusions:
- 1) The vendor manual states that the purpose of these batteries is to power logic lights and aid in troubleshooting when the unit is shutdown.
- 2) Exide Corporation representatives made statements at the September 4, 1991 meeting among NMPC, NRC, and Exide officials at NRR headquarters, that the "D" size batteries only power the logic circuit when there is neither maintenance nor normal "ACu power available.
- 3) Testing conducted using the original design with the "D" size batteries still installed and effectively dead proved that complete loss of the maintenance supply would operate the K-5 relay to transfer the control power to the inverter output without tripping the unit. This was also stated by the Exide design engineer. In addition, testing proved that with dead batteries if an inverter trip occurs, the UPS unit will transfer to maintenance supply without loss of load. Consequently, it is concluded that the "D" size batteries were not needed for transfer, and were never intended to serve a power supply role for UPS operation.
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Mr. James M. Taylor Page 2
- 4) When the logi'c control circuit is powered from the normal "AC" supply, the associated "DC" logic voltage is 20 VDC. The nine "D" cells are two volts each for a total voltage of 18 VDC. The logic circuit trip setpoint is approximately 17 VDC, or very near the 18 VDC maximum voltage of the "D" size batteries. This is, in our judgement, because the batteries are not intended to power the logic circuit for UPS operation.
- 5) The testing that was done to investigate the trip of the UPS was a test designed to prove logic power susceptibility to transients on the maintenance supply. It proved that the logic could trip due to a voltage transient. It did not quantify how susceptible the circuit is to transients.
As part of the testing new batteries were installed into the UPS and the test re-performed. While the UPS did not trip with new batteries, "DC" voltage spikes in the range of 2 to 3 volts were seen on the oscilloscope traces. This indicates that the control logic is still susceptible to trips when connected to the maintenance supply even with fresh, fully-charged batteries.
- 6) The voltage output of the "D" size batteries cannot be measured or checked with the UPS in service; it can only be checked with the "AC" supply (20 VDC) disconnected and removed. There is no way to disconnect the normal supply to measure the "D" size battery voltage with the unit in service. This configuration further suggests that the batteries are not critical to the UPS in an operating condition.
- 7) The location of the batteries in the cabinet is such that these batteries were not intended to be replaced with the UPS in service.
Replacement requires a unit outage with loss of the load. Changing the batteries with any power being supplied to the loads would not only endanger the safety of personnel due to close proximity of 575 and 208 VAC, but would also risk loss of critical loads if these cables are touched. If the batteries are not required for reliability of the unit, which at this time they are not, then it is better to continue to supply the critical loads than to risk failure of same in order to replace a component that is not intended for reliable operation of the UPS.
I wish to emphasize Niagara Mohawk fully appreciates the need to maintain non-safety as well as safety related systems. As a matter of fact, the maintenance program on our UPS's goes beyond the manufacturer's recommendations. Several months prior to the event, the system engineer had initiated an action to upgrade the maintenance program of all UPS systems, with no differentiation made between safety related and non-safety related.
All UPS maintenance programs will be equally extensive, incorporating industry experience and INPO recommendations.
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Mr. James M. Taylor
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" 'Page 3 In addition, we are concerned that emphasis placed on the role of the "D" size batteries in the NRC's Information Notice and the IIT report (NUREG-1455) may cause other licensees with similar UPS units to focus on the wrong root cause and to possibly expose their personnel to unnecessary safety hazards.
In conclusion, Niagara Mohawk suggests that the Information Notice and NUREG be revised to accurately reflect the design of the UPS units and the true root cause which we have determined to be improper design. Please let me know you wish to meet with us, or require additional information regarding this if matter.
Very truly yours, Exec. Vice President-Nuclear DKG/mls 000049AC XC ~ Regional Administrator, Region I Commissioners Mr. W. L. Schmidt, Senior Resident Inspector Mr. R. A. Capra, Project Director, NRR Mr. J. E. Menning, Project Manager, NRR Mr. D. R. Haverkamp, Chief, Reactor Projects, Section 1B Mr. J. Groth, INPO Records Management
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