ML19317F763
| ML19317F763 | |
| Person / Time | |
|---|---|
| Site: | Millstone, Davis Besse  |
| Issue date: | 10/01/1976 |
| From: | Stolz J Office of Nuclear Reactor Regulation |
| To: | Roe L TOLEDO EDISON CO. |
| References | |
| NUDOCS 8001280693 | |
| Download: ML19317F763 (5) | |
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Docket File R. H. Vollmer NRC PDR M. L. Ernst Local PDR W..P. Gammill 7"
..N LWR 1 File ELD R. C. DeYoung IE (3)
F. J. Williams ACRS (16)
J. Stolz J. Miller e
- fn Docket !:o. St-046 e
R. Heineman
[) @@p Toledo Edison Company h*,Kn ht, SS Od ATT:1: Mr. Lowell E. Roe R. Tedesco Pn M ' Q
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Vice President, Facilities V. A. fioore t
I O{jI h_k f Development H. Denton Edison Plaza 300 !!adison Avenue Toledo, Ohio 43662 bcc:
J. R. Buchanan,flSIC T. B. Abernathy, liC Gentlarxn:
EQUIP. EliT FAILURES CURI!iG A DEGRADED GRID YOLTAGE CO.?iDITImi AT liILLST3;iE, UNIT 2 Provided nerein as Enclosure 1 is a description of events which occurred at :lillstone Unit :!o. 2 during July 1976 relating to plant operation and equipment failures during a degraded grid voltage cnndition.
Da July 27, 1976, all utilities with operacing reactor facilities received telepnone notification from the :lRC of the events at the liillstone Unit No. 2 facility. At that time nenbers of your staff were asked to investigate the vulneracility of your facility to sinilar degraded voltage conditions and to provide a response by telaphone within 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />.
As a result of our Sitial investigation and evalt.ation of the potential generic imolications of tne events at Millstone and our prelininary discussions with several licensees, we consider it necessary to re:;uire all plants presently in review for an operating license to conduct a thorough evaluation of the probica and to sut: nit fornal reports. Therefora, we request that you conduct an investi-gation of the issue as it affects ytur facility using tne Request for Information detailed in Enclosure 2 as a guide, and provide the analyses and results within 30 days of receipt of this letter or explain rety you cannot meet ta;; ?rhedule and provide the schedule ycu will meet.
The signed original and 39 copies of your response will be necessary.
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Fone AIC.318 (Rev. 9 53) AECM 0240 W v. s. eovannesswv entestene orrects so74.ose. nee soo12eo 673 s
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Toledo Edison Company 2-This request for generic information was approved by GAO under a blahkut clearance number B-130225 (R0072); this clearanco expires July 31, 1977.
Sincerely,
.Ori;;inal s;;; ed by John F.Stolz 2
John F. Stolz, Chief Light Water Reactors Branch flo.1 Division of Project."anagenent
Enclosures:
1.
Description of Events
- lillstone Unit :10. 2 2.
Request for Infomation cc: Mr. Donald 11. Hauser, Esa.
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cleveland, Ohio 44101 U
Gerald Charnoff, Esq.
Snaw, Pittr.an, Potts and Trowtridge 31017th Street, i;. 'i.
Washington, Q. C.
20036 Laslie Henry, Esq.
Fuller, Seney, Henry and flodge 303 fiadison Avenue Toledo, Ohio 43604 l
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n ENCLOSURE NO. 1
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- "DESCRIPTI0tr OF' EVENTS
' MILLSTONE UNIT NO. 2 On July 2b,.1976, Northeast Nuclear Energy CompaIiy (NNECO) reported that, following a trip of Millstone Unit No. 2 on July 5,1976, several motors powered from 480 volt (v) motor control centers failed to start as required. The failure of the 480 y motors to start was traced to blown control power fuses on the individual motor controllers. These controllers receive control power through 480 v/120 y transformers within the controller.
NNECO's investigation disclosed that, as a result of the plant trip, the grid voltage dropped from352 ky to 333 kv. This voltage drop, in conjunction with additional voltage drops associated with the transformers involved, reduced the control power and voltage within individual 480 y controllers to a voltage which was insufficient to actuate the main line controller contactors. As a result, when the motors were signalled to start, the' control power fuses were blown.
Subsequent testing by NNECO showed that the contactors required at least 410 y to function properly.
NNECO concluded that under s'imilar low voltage conditions, the opera-bility of 480 v Engineered Safety Feature equipment could~not be assured.
NNECO's insnediate corrective action was to raise the setpoint of the Engineered Safeguards Actuation System (ESAS) " loss of power" under-voltage relays to assure that the plant would be separated from the grid and emergency power system (dual) operation would be initiated before the control voltage fell below that required for contactor operation.
A trip of the undervoltage relays causes the emergency buses to be de-energized and a load shed signal to strip the emergency buses, the diesel generators to start and power the emergency buses, and required safety related loads to sequence start on the buses.
On July 21, 1976, NNECO reported that the earlier corrective action taken was no longer considered appropriate because during starting of a circulating water pump, the voltage dropped below the new ESAS under-voltage relay setting. This de-energized the e:nergency buses, caused load shedding to occur, started the diesel generators and began sequencing loads onto the emergency buses in accordance with the design.
- However, during sequencing of the loads onto the buses, the voltage again dropped below the undervoltage relay setting which caused the load shed signal to strip the buses. The result was energized eniergency buses with no' loads supplied.
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ENCLOSURE NO. 2 i -
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REQUEST FOR INFORMATION 1.
Evaluate the design of your facility's Class IE electrical distribution
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system to detennine if the operability of safety related equipment, including associated control circuitry or instrumentation, can be adversely affected by wet tenn or long tenn degradation in the grid system voltage within the range where the offsite power is counted on to supply important equipment. Your response should 6ddress all but not be limited to the following:
Describe the plant conditions under which the plant auxiliary systems a.
(safety related and non-safety related) will be supplied by offsite power.
Include an estimate of the fraction of nomal plant operating time in which this is the case.
b.
The voltage used to describe the grid distribution system is usually a " nominal" value. Define the nomal operating range of your grid '
system voltage and the corresponding voltage values.at the safety related buses.
g c.
The transfomers utilized in power systems for providing the required voltage at the various system distribution levels are nonnally provided with taps to allow voltage adjustment. Provide the results of an analysis of your design to determine if the voltage profiles at the safety related buses are satisfactory for the full load and no load conditions on the system and the range of grid voltage.
i d.
Assuming the facility auxiliary loads are being carried by the station generator, provide the voltage profiles at the safety buses for grid voltage at the nomal maximum value, the nomal minimum value, and at i
the degraded conditions (high or low voltage, current, etc.) which would l
require generator trip.
e.
Identify the sensor location and provide the trip setpoint for your
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facility's Loss of Offsite. Power (undervoltage trip) instrumentation.
Include the basis for your trip setpoint selection.
f.
Assuming operation on offsite power and degradation of the grid system voltage, provide the voltage values at the safety related buses corresponding to the maximum value of grid voltage and the degraded grid voltage corresponding to the undervoltage trip setpoint.
Utilizing the safety rei ted bus voltag'e values identified in (f),
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evaluate the capability of all safety related lo:ds, including related control circuitry and instrumentation,- to perfom their. safety functions.
Include a definition of the voltage range over which the safety related components, and non-safety conponents, can operate continuously in the perfomance of their design function.
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h.
Describe the bus voltage monitoring and abnormal voltage alarms.
available in the control room.
2.
The functional safety requirement of the undervoltage trip is to detect the loss of offsite (preferred) power system voltage and initiate the necessary actions required to transfer safety related buses to the onsite power system. Describe the load shedding feature of your design (required prior to transfering to the onsite [ diesel generator]
systens) and the capability of the onsite systems to perform their function if the load shedding ' feature is maintained after the diesel generators are connected to their respective safety buses. Describe the bases (if any) for retention or reinstatement of the load shedding function after the diesel generators are cannected to their respective buses.
3.
Define the ' facility operating limits (real and reactive power, voltage, frequency and other) established by the grid stability analyses cited in the FSAR.
Describe the operating procedures or other provision.' presently in effect for assuring that your facility is being operated wit.1n these limits.
4.
Provide a description of any proposed actions or modificat'ons to your facility based on the results of the analyses performed in response to items 1-3 above. _
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