ML20058K610
| ML20058K610 | |
| Person / Time | |
|---|---|
| Site: | Millstone, Monticello, Prairie Island  |
| Issue date: | 08/13/1976 |
| From: | Ziemann D Office of Nuclear Reactor Regulation |
| To: | Mayer L NORTHERN STATES POWER CO. |
| References | |
| NUDOCS 9102120643 | |
| Download: ML20058K610 (5) | |
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DISTRIBUTION j
Docket (3) i NRCPDR(3) l Local PDR l
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AUG 131976 PWO Connor MGrotenhuis Northern States Power epany EAReeves j
ATTN: Mr. L. O. Maye n l'.anaaer RMdiggs (2) j Duclear Suppor: iervices DEisenhtt 414 Nicollet itall - 8th Floor TBAbernatny Pinneapolis, l'innesota 55401 JRBuchanan ACRS (16)
Gentlemen:
VStello Dross EVerdery DE: H0HTICELLO !!UELEAR GE'JERATING PLAT T PRAIDIE ISLAHD tJVCLEAP GENEPATING PLANT U:IT 1495.1 AHD 2 Provided herein as Enclosure 1 is a description of events which j
occurred at Millstone Unit ho. 2 during July 1975 relatinn to plant operation and equipment failures durine a degraded crid voltage condition.
I On July 27, 1976, all utilities with operating reactor facilities received telephone notification from tre t1RC of the events at the Millstone Unit No. 2 facility. At tFat time ner..bers of your staff were asked to investigate the vulnerability of your facility to sinilar degraded voltage conditions and to provide a response by f
i telephone within a bours, ts a result of our initial investigation and evaluation of the potential generic implicatians of the events at Millstene and our preliminary ciscussions with sever 31 licersocs, we consincr it necessary to require all operating reector licensees to conduct a thorough evaluation of the problem am' to sub,i t. femal reports.
Therefore, we request that you conduct an investigation of the issue as it affects your facility usina the Recuest for Infomation detailed in Enclosure 2 as a guide, and provice ti.c analyses and rasults within 30 days of your receipt of this letter.
The sinned original and 39 copies of your response vill be necessary.
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Torm ALC-318 (Rev. 9-3 3) AICM D240 j
9102120643 760813 CF ADOCK 05000263 CF-
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DO 'd N 2-Northern States Power Company l
This' request for generic information was approved by GA0 under a
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blanket clearance number B-160225 (R0072); this clearatice expires 3
I July 31, 1977.
i Sincerely, c
-e signed by}:t
. 1. W e nna m
d Dennis L. Ziemann, Chief j
Operating Peactors Branch #2 j
Division of Operating peactors i
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Enclosures:
1.
Description of Events Millstone Unit No. 2
]
2.
Request for Information t
cc w/ enclosures:
Gerald Charnoff, Escuire Mr. Kenneth Dzugan
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Shaw, Pi ttaan, Potts and Environnental P1anning Consultant Trowbridge Office of City Planner 1800 M Street, N. W.
Grace Building Washinaton, D. C.
20036 421 Wabasha Street St. Paul, Itinnesota 55102 j
Arthur Renout st, Escuire Vice President - Law Sandra S. Gardebring, Esquire Northern States Power Company Special Assistant Attorney General 414 Nicollet Mall Minnesota Pollution Control Agency 7
j flinneaoclis, Minnesota 55401 1935 W. County Road R2 Roseville, Minnesota 55113 f
iloward J. Vor.el, Escuire Legal Counsel Anthony Z. Poisman, Esouire i
j 2750 Dean Partway Roisaan, ressler and Cashdan l
l'innespclis, flinr,cnnta 55416 1712 H Street, N. W.
l Washington, D. C.
2003G j
1 Mr. Steve J. Gadler i
2120 Carter Avenue The Envirornental Conservation Library St. Paul, i:innesota 55108 l'.inneapolis Public Library l
300 nicollet Hall
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Daniel L. Ficker, Esquire Minnearolis, Pinnesota 5S401 l
tssistant City Attorne, Crininal Division j
63S City Hall 7
St. Paul,!!innesota 66102 f,$
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RPSnaider:ah MGrotenhuis EVerdery DLziemann
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ENCLOSURE NO. 1
' DESCRIPTION OF EVENTS MILLSTONE UNIT NO. 2 On July 20, 1976, Northeast Nuclear Energy Company (NNECO) reported 4
that, following a trip of Millstone Unit No. 2 on July 5,1976, several motors powered from 480 volt (v) inotor 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 kv to 333 kv. This voltage drop, l
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 blowd.
Subsequent testing by NNECO showed that the contactors required at least 410 y to function properly.
NNECO concluded that under similar low voltage conditions, the opera-bility of 480 v Engineered Safety Feature equipment could not be assured.
NNEC0's immediate 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 leads 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-This de-energized the emergency buses, caused voltage relay setting.
load shedding to occur, started the diesel generators and began sequencing l
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 emergency buses with no leads supplied.
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ENCLOSURE NO. 2 t
REQUEST FOR INFORMATION 1.
Evaluate the design of your facility's Class IE electrical distribution system to detemine if the operability of safety related equipment, including associated control circuitry or instrumentation, can be adversely affected by short tem or long term degradation in the grid l'
system voltage within the range where the offsite power is counted on to supply important equipment.
Your response should address all but not 4
be limited to the following i
a.
Describe the plant conditions under which the plant auxiliary systems (nfety related and non-safety related).will be supplied by offsite 1
power.
Include an estimate of the fraction of normal plant operating time in which this is the case.
b.
The voltage ? sed to describe the grid distribution system is usually a " nominal" value. Define the normal operating range of your grid system voltage and the corresponding voltage values at the safety j
rel ated -buses.
c.
The transformers utilized in power systems for providing the required voltage at the various system distribution levels are i
nomally provided with taps to allow voltage adjustment.
Provide i
the results of an analysis of your design to detemine if the voltage profiles at the safety rel'ated 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 j
generator, provide the voltage profiles at the safety buses for grid l
voltage at the normal maximum value, the nomal minimum value, and at
-i the degraded conditions (high or low voltage, current, etc.) which would !
require generator trip.
j 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.
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f.
Assuming operation on offsite power and degradation of the grid system j
voltage, provide the voltage values at the safety related buses j
corresponding to the maximum value of grid voltage and the degraded 9
grid voltage corresponding to the undervoltage trip setpoint.
l g.
Utilizing the safety related bus voltage values identified in (f),
f
, _i evaluate the capability of all safety related loads, including related control circuitry and instrumentation, to perform their safety functions.
Include a definition of the voltage range over which the safety related components, and non-safety components, can operate continuously in the performance of their design function.
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Describe the bus voltage monitoring and abnormal voltage alarms available in the control room.
2.
The functional safety requircaent of the undervoltage trip is to detect the loss of offsite (preferred) power systen voltage and initiate the necessary actions required to transfer safety related buses to the Describe the load shedding feature of your onsite power systen.
2 design (required prior to transfering to the onsite [ diesel generator]
systems) and the capability of the onsite systems to perform their function if the load shedding feature is maintained after the diesel 4
i Describe generators are connected to their respective safety buses.
the bases (if any) for retention or reinstatement of the load sheddin[
function after the diesel generators are connected to their respective l
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 provisions presently l
in effect for assuring that your facility is being operated within these limits.
4.
Provide a description of any proposed actions or r.odifications to your facility based or the results of the analyses performed in response to items 1-3 above.
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