ML19242C964
| ML19242C964 | |
| Person / Time | |
|---|---|
| Site: | Arkansas Nuclear  |
| Issue date: | 07/05/1979 |
| From: | Eisenhut D Office of Nuclear Reactor Regulation |
| To: | Harold Denton Office of Nuclear Reactor Regulation |
| References | |
| NUDOCS 7908140267 | |
| Download: ML19242C964 (9) | |
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JUL 5 1979 MEMORANDUM FOR:
H. Denton, Director, Office of Nuclear Reactor Regulation FROM:
D. Eisenhut, Acting Director, Division of Operating Reactors
SUBJECT:
EXPANSION OF SCOPE FOR D0R GENERIC ISSUE IMPLEMENTATION
" DEGRADED GRID VOLTAGE" The purpose of this memorandum is to obtain your approval of our proposal to expand the subject generic issue implementation to ensure that adequate voltage will be available at the Engineered Safety Feature (ESF) buses during all electrical transients.
This expansion is required ta meet a commitmer.t made in the NRC Abnormal Occurrence Event Report 759s-01 regarding systems at Arkansas Nuclear One (AN0).
degradation of engineering sa#
v This subject is on our list i < c0R backlog items and was highlighted by the recent ads.
We believe that this expansion is necessary to ensure that a nuclear station's electric power distribution is designed and operated to provide adequate voltage to safety buses during all modes of operation as intended.
We are proposing to request infomation from the licensees within 60 days in response to our proposed letter (enclosed) regarding " Adequacy of Station Electric Distribution Systems Voltages". We estimate that the staff review of the associated 44 actions will be completed ir approximately 6 months of receipt of the replies.
D. Eisenhut, Acting Dig ctor Division of Operating Reactors Office of Nuclear Reactor Regulation
Enclosure:
As stated cc w/ enclosure:
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R. Vollmer B. Grimes W. Gammill A. Schwencer D. Ziemann T. I.polito n
R. Reid G. Lainas S. Hanauer F. Rosa D. Tondi M. Chiramal
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UNITED STATES NUCLEAR REGULATORY COMMisslON ev 4
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,e Docket No.
Licensee:
Gentl emen:
RE: ADEQUACY OF STATION ELECTRIC DISTRIBUTION SYSTEMS VOLTAGES We are currently reviewing the licensee's submittals in response to the NRC generic letter of June 2,1977 regarding undervoltage protec-tion of the safety related electric equipment from loss of capability of redundant safety loads, their control circuitry, and associated
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electrical components required for performing safety functions as a result of sustained degraded voltage from the offsite electric grid system. This generic action was based on the Millstone Event which occurred on July 5,1976.
The recent event at the Arkansas Nuclear One (AND) station on September 16, 1978 brought into question the confomance of the e
station electric distribution system to GDC-17, in two separate regards.
Each of two units at the ANO station has a dedicated startup transformer powered through a single share) autotransformer (common source of offsite power) from the station switchyard. Operation of an autotransformer overturrent relay caused the loss of the two dedicated startup transformers. The station electrical distribution system thus automatically transferred the full auxiliary loads of both units to the backup startup transformer exceeding its rated capacity and degrading the voltage level at the safety buses.
Secondly, during our review of the einctrical system at the ANO station, the licensee's analysis indic.ted tY,t the "immediate access offsite power riT1:2 fit" '(tiedicated T artup transformer) lacked
" sufficient capacity and capability" to eccommodate the simultaneous starting demands of the emergency loads concurrent with the full house loads, in the event cf a loss of coolant accident (LCCA). The condition would result in all safety loads remaining on the dedi-cated startup transformer with unacceptably degraded voltage. A voltage degradation during the +!ectrical starting condition becomes a safety concern either if the (cgradation causes the starting condition to be prolonged so as to beccme a sustained undervoltage (iO 'l
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. or if the voltage degradation causes frequent spurious sheading of the ESF loads from the preferred power source, the offsite electric grid. This event was described in NRC's IE Information Notice No. 79-04.
Additional background information is provided in Enclosure 1.
The IE Information Notice No. 79-04 stated that NRC would follow with specific actions to be taken by licensees This letter identifies those actions.
Based oa the ANO event, the NRC has expancad its generic review of the adequacy of the electric power systems for all operat;ng nuclear power facilities.
Specifically, we must now confirm the acceptability ___
of the voltage can' itions on the station electric distribution systems d
with regard to both (1) potential overloading due to transfers of either safety or non-safety loads, and (2) potential starting transient problems in addition to the concerns expressed in our June 2,1977 correspondence with regard to degraded voltage conditions due to conditions originating on the grid.
Based on the experience at ANO, the NRC is requiring all licensees to review the electric power systems at each of their nuclear power plants to determine analytically if, assuming all onsite sources of AC power are not available, the offsite power system and the onsite distribution system is of sufficient capacit7 and capability to automatically start as well as operate all required safety loads within their required voltage ratings in the event of (1) an antici-pated transient (such as unit trip) or (2) an accident (such as a LOCA) regardless of other actions the electric power system is < designed to automatically initiate and without the need for manual shedding of any electric loads.
Protection of safety loads from undervoltage conditions must be designed to provide the required protection without causing voltages in excess of maximum volicpe ratings of safety loads and without causing spurious separations of safety buses from offsite power.
NRC should be informed of any required sequential loading of any portion of the offsite power system or the onsite distribution system which is needed Aaassem that er +rovided to all 4efety - "
loads is wittrin required voltage limits for.th?se safety loads..
G Guidance on evaluating-t+e Terformance of eier:ric. power systems with regard to voltage drops is provided % Enclosure 2.
The adequacy of the ansite distribution of power from the offsite circuits shall be verified by test to assure that analysis results are valid.
Please provide: (1) a description of the eethod for performing this verification, and (2) the test results. If previous tests verify the results of the analysis, then test results should be submitted and additional tests need not be performed.
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In addition, you are requested to review the electric power syster.s of your nuclear station to determine if there ar
- any events or conditions which could result in the simultaneous or consequential loss of both required circuits to the offsite network to determine if any potential exists for violation of GDC-17 in this regard.
These reviews should be completed, and a copy of the analyses provided to NRC witnin 60 days of the date of this letter.
In the event that any violations or potential violations of GDC-17 or voltage requirements of safety loads are discovered remedial action should be taken immediately.
You should provide the Commission with Prompt Notification with Written Followup pursuant to the reporting r2quirements of your Technical Specifications.
If the above required reviews have been completed by you a's part of your response to our June 2,1977 request NRC should be inforced within 30 days of the date of this letter. Approved by GAO, B-180225 (R0072), clearance expires 7-31-80.
Approval was given under a blanket clearance specifically for identified generic problems.
. Chief Operating Reactors Branch #
Division of Dpergting Reactors.
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. ~ _ 2, ENCLOSURE 1 to Licensees Letter BACKGROUND INFORMATION ON ANO EVENT' The event that occurred at the Arkansas Nuclear One station on September 16, 1978, brought into qJestion the conformance of the station electric distribution system design to GDC-17 with regard to the capacity rd the capability of the onsite system.
Each of two units at the ANO station has a dedicated startup transfomer connected to a single shared autotransformer (common source of offsite power) from the station switchyard. The incident was initiated by Unit I reactor trip concurrent with trip of -the unit's starrbine-senerator.
The Unit 1 auxiliary loads were automatically transferred to Startup Transfomer 1.
The power being supplied to Startup Transformer ~3 (Unit 2 dedicated startup ? ransfomer), which was feeding Unit 2, and being t
supplied to Startup Transfomer 1 resulted in operation of an autotransformer overcurrent relay and consequent trippirg of the incoming circuit breaker of the autotransformer. The autotransfomer has the capacity to pmvide power for both units, but due to an error, the overturrent relay was still set for the operation, of Unit 1 only. Loss of input power to the two Startup transfomers automatically transferred the auxiliary loads for both units to the backup Startup Transfomer, ST 2.
However, this trans-fomer is designed as an alternate supply for one unit and is not designed to carry full auxiliary loads for both units. This overload caused a voltage degradation at the safety busas. The event to thi point demonstrated that the design of the orisite sower systera to the ANO station Units 1 and 2 did not fully meet GDC-17.
In the circurzstances experienced at ANO the failure of one of the two offsite electric power circuits resulted in failure of the other electri ;,ower circuit. GDC-17 requires, in part, that (1) electric power from the transmission network to the onsite distribution system shall be supplied b.Y two physically independent circuits (not necessarily on separ*.te rignts of way) designed and located so as to minimize to the extent practical tne likelihood of their simultaneous failure under operating and environmental conditions and (2) provision shall be included to minimize the probability of losing electric power from any of the remaining supplies as a result of, or coincident with, the loss of power generated by tha nuclear unit, or the lo_s of-power from the transe.ission network. The R:0 tid mo' fuTly meet these requirements.
Initially, the sequence of events on September '16,1978 did not indicate any problem with the electrical distribution systen of Unit 1.
However,
suosequent analysis by the licensee indicated that in the cient cf a LOCA at Unit 1 during which time Startup Transformer No.1 would be required to provide power to both the non-safety auxiliary electrical t W ')
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2-loads and start the safety loads a voltage di-gradation would result.
The safety loads might not transfer to the Unit 1 diesel-generators but could remain on the startup transformer with unacceptably degraded voltage.
Although there is margin in the thermal capability of equipment such a situation could result in thermal damage in the safety equipment and/or blown fuses in control circuits fc these safety loads. Either event could result in disabling these ic_ds during a LOCA. 6DC-17 requires, in part, that electric power supplies for nuclear power plants provide sufficient capacity and capabili+' to assure that certain limits are not exceeded in the event of an*..cipated operational occurrences and that the core is cooled and containment integrity and other vital functions are naintained in the event of postulated failures.
The ANO design was not capable of providing the electric power of " sufficient capacity and capability."
l ENCLOSURE 2 to Licensees Letter GUIDELINES FOR VOLTAGE DROP CALCULATIONS 1.
Separate analyses should be perfonned assuming the power source to safety buses is (a) the unit auxiliary transformer; (b) the startuo transformer; and (c) other available connections to the offsite network one by one assuming the need for electric power is initiated by (1) an anticipated transient (e.g., unit trip) or (2) an accident, whichever presents the largest load demand situation.
2.
For multi-unit stations a separate analysis should be performed for each unit assuming (1) an accident in the unit being anlayzed and simultaneous shutdown of all Ather 4mits at that station; or (2) an anticipated transient in the unit being analyzed (e.g.,
unit trip) and simultaneous shutdown of all other units at that station, whichever presents the largest load detand situation.
3.
All actions the electric power system is desicned to automatically initiate should be assumed to occur as designed (e.g., accomatic bulk or sequential iuading or automatic transfers of bulk loads from one transformer to another).
Included shou d be consideration of starting of large non-safety loads (e.g., concensate pumps).
4.
Manual load shedding snould not be assumed.
5.
For each event analyzed, the maximum load necessitated by the event and the mode of operation of _the plant at the time of the event should be assumed in addition to all loads ca.usedly expected automatic actions and manual actions pennitted by administrative procedures.
6.
The voltage at the terminals of each safety load sho.ld be calculated based on the above listed considerations and assumptions and based on t'e assumptian that the grid voltage is at the " minimum expected value".
The " minimum expected value" should be selected based on the least of the following:
a.
The minimum steady-state voltage experienced at the connection
...to the offsite circcit.
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circuit due to contingency plans which ma:. nsult in reduced.
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c.
The trinimum predicted. grid voltace from gri: sta:ility acabsis.
(e.g., load flow studies).
In the report to NRC on this matter the licensee s7c ld state planned actions, including any proposed " Limiting Conditicqs for Operation" for Technical Specifications, in response to egerie cin; vo'tage at the connection to the offsite circuit which is leis tsar. the
" minimum expected value."
A, copy of the plant procedure in this regard should be provided.
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The voltage analysis should include documentation for each condition analyzed, of the voltage at the input and output of each transformer and at each intermediate bus between the connection to the offsite circuit and the terminals of each safety load.
8.
The analysis should document the voltage setpoint and any inherent or adjustable (with nominal setting) time delay for relays wnich (1) initiate or execute automatic transfer of loads from one source to another; (2) initiate or execute automatic load shedding; or (3) initiate or execute automatic load sequencing.
9.
The calcula.ed voltages at the terminals of each safety load should be compared with the required voltage range for normai operation and starting of that load.
Any identified inadequacies of calculated voltage require immediate remedial action and notification of NRC.
10.
For each case evaluated the calculated voltages on each safety bus should be compared with the voltage-time settings for the under-voltage relays on these safety buses.
Any identified inadequacies in undervoltage relay settings require inmediate remedial action and notification of NRC.
11.
To provide assurance that actions taken tn assure adequate voltage ~
levels for safety loads do not Tesult in excessive yoltage, assuming the maximum expected value of voltage at the connection to the offsite circuit, a determination should be made of the maximum voltage expected at the terminals of each safety load and its starting circuit.
If this voltage exceeds tne maximum voltage rating of any item of safety equipment immediate remediai action is required and NRC shall be notified.
12.
Voltage *.me settings for undervoltage relays shall be selected so as to avoid scurious separation of safety buses from offsite power during c] ant startup, normal operation and shutdown Aue to
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13.
Analysis tiocumentation should incitroe e statement -of the assurations for each case malyzed.
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