ML20054K059
| ML20054K059 | |
| Person / Time | |
|---|---|
| Site: | Millstone  |
| Issue date: | 05/13/1982 |
| From: | Selan J LAWRENCE LIVERMORE NATIONAL LABORATORY |
| To: | NRC |
| Shared Package | |
| ML20054K056 | List: |
| References | |
| CON-FIN-A-0250, CON-FIN-A-250 UCID-19139, NUDOCS 8206300359 | |
| Download: ML20054K059 (11) | |
Text
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UcID.19139
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TECHNICAL EVALUATION REPORT ON THE PROPOSED DESIGN MODIFICATIONS AND TECHNICAL SPECIFICATION CHANGES ON GRID VOLTAGE DEGRADATION FOR THE MILLSTONE NUCLEAR POUER STATION, UNIT 1
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(Docket No. 50-245)
James C. Selan May 13, 1982
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This is an informal report iatended primarily for internal or limited external distribution.
-ate The opinions and concluuons stated we those of the author and may or may not be those
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of the Laboratory.
This work was supported by the United States Nuclear Regulatory Commission under a Memorandum of Understanding with the United States Depanment of Energy.
FIN A-0250 8206300359 820623 PDR ADOCK 05000245 p
ABSTRACT This report documents the technical evaluation of the proposed design modifications and Technical Specification changes for protection of Class lE equipment from grid voltage degradation for the Millstone Nuclear Power Station, Unit 1.
The review criteria are based on several IEEE standards and the Code of Federal Regulations.
The evaluation finds that
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inf ormation of the un3ervoltage the licensee has not provided sufficient protection system to allow a complete evaluation into the adequacy of protecting the Class 1E equipment from sustained voltage degradation.
FOREWORD This report is supplied as part of the Selected Electrical, Instrumentation, and Control Systems Issues Program being conducted for the U. S. Nuclear Regulatory Commission, Office of Nuclear Reactor Regulation, Division of Licensing, by Lawrence Livermore National Laboratory.
The U. S. Nuclear Regulatory Com=ission funded the work under the author 1=ation entitled " Electrical, Instrumentation and Control System Support," B&R 2019 04 031, FIN A-0250.
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TABLE OF CONTENTS Page Y
1.
INTRODUCTION.
1 2.
DESIGN BASIS CRITERIA 2
3.
EVALUATION 2
3.1 Existing Undervoltage Protection 2
3.2 Modifications' 3'
3.3 Discussions.
'3 3.3.1 NRC Staff Position 1:
Second Level of Udder-Voltage or Overvoltage Protection with a Time Delay 4
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3.3.2 NRC Staff Position 2:
Interaction of Onsite Power Sources with Load Shed Feature.
6 3.3.3 NRC Staff Position 3:
Onsite Power Source Testing 6
3.4 Technical Specifications 6
4.
CONCLUSION 7
REFERENCES 8
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TECFJ;ICAL EVALUATION REPORT ON THE PROPOSED DESIGN MODIFICATIONS AND TECIUi1 CAL SPECIFICATION CHANGES i
ON GRID VOLTAGE DECRADATION FOR THE MILLSTONE NUCLEAR POWER STATION, UNIT 1 (Docket No. 50-245)
I James C. Selan Lawrence Livermore National Laboratory
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1.
INTRODUCTION By letter dated June 3, 1977 [Ref. 1], the U. S. Nuclear Regulatory Co= mission (NRC) requested the Northern Utilities (NU), the licensee, to assess the susceptibility of the Class lE electrical equipment to sustained degraded voltage conditions at the offsite power sources and to the interaction between the offsite and onsite emergency power systems at the Millstone Nuclear Power Station, Unit 1.
In addition, the NRC requested that the licensee compare the current design of the emergency power systems at the plant facilities with the NRC staff positions as stated in the June 3, 1977 letter [Ref. 1), and that the licensee propose plant modifications, as necessary, to meet the NRC staff positions, or provide a detailed analysis which shows that the facility design
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has equivalent capabilities and protective features.
Further, the NRC required certain Technical Specifications be incorporated into the facility's operating license.
By letters dated August 1, 1977 [Ref. 2], August 10, 1979 [Ref.'3),
April 29, 1980 [Ref. 4), July 16, 1980 [Ref. 5),. August 20, 1980 [Ref. 6),
and April 21, 1982 [Ref. 7), the licensee submitted certain design modification details, additions to the Technical Specifications, and limiting conditions for operation (LCO's). The design modification details include a degraded voltage protection system for the Class lE equipment.
The additions to the Technical Specifications and LCO's are in regard to calibrations, surveillance require-ments, test requirements, and " action" statements associated with the existing undervoltage protection system.
The purpose of this report is to evaluate the licensee's proposed design modifications, Technical Specification changes, and proposed LCO's to determine that they meet the criteria established by the NRC for the protec-tion of Class lE equipment from grid voltage degradation.
2.
DESIGN BASIS CRITERIA The design basis criteria that were applied in determining the acceptability of the system modification to protect the Cl~ ass lE equipnent from degradation of grid voltages are as follows:
(1)
General Design Criterion 17 (GDC 17), "Electrig Pgwer Systems," of Appendix A, " General Desig"n' Criteria for Nuclear Power Plants," Code of Federal Regulations, Title 10, Part 50 (10 CFR 50) [Ref. 8].
(2)
IEEE Standard 279-1971, " Criteria for Protection Systems for Nuclear Power, Generating Stations" [Ref. 9].
(3) IEEE Standard 308-1974, " Class,lE Power Systems for Nuclear Power Generating Stations" [Ref. 10).
(4) NRC staff positions as stated in a letter dated June 3, 1977
[Ref. 1).
3.
EVALUATION '
3.1 EXISTING UNDERVOLTAGE PROTECTION The present undervoltage protection system design consists of the following:
(1) There are two undervoltage relays (induction disc type) connected to the Reserve Station Service Transformer (RSST) bushing potential devices to sense the 345 kV system.
The RSST is the preferred'off-site source.
These relays (level 1 of undervoltage protection) are used to sense a loss of offsite power condition.
The voltage setpoint for these relays.is 71%
345 kV (246 kV) with a time delay of 6 seconds at 50% of 345 kV.
The output of each relay supplies each of two identical and redundant " loss of normal power" (LNP) circuits.
The LNP circuits initiate the start of the onsite sources (diesel generator and gas turbine), isolation of the Class lE buses, and load shedding.
- Once the onsite sources reach operating speed, the output breakers are closed. A block of Class lE loads is energized i= mediately while the remaining loads are sequenced on by the load sequencer.
(2) There are also four solid state bistable voltage sensors installed the bushing potential devices on the RSST to sense d2 graded at voltage conditions (level 2 of undervoltage protection).
Two of
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e the bistable sensors monitor phases one and two and the,
remaining two sensors monitor phases two and three.
This sensing system makes up two protective channels, each requiring a signal from both devices (2-out-of-2 logic).
The setpoint for these devices is 336 kV (97% of 345 kV) with a ti=e delay of one second.
Actuatien of any one of the four sensors will alar = in che control room.
Ihe trip 7
signal from any one of the channels is interlocked with a SI signal.
Therefore, f or a degraded voltage condition concurrent with a SI, autenatic sconnection ffom'the RSST and transfer to the onsite source. -til occur.
Otherdise, operator action is required to resse+1 adequate voltage. The licensee's basis for not providing a
.satic disconnection under all degraded conditions is that the auto-tripping of the unit from the transmission grid could cause a " cascading effect". This'chscading' effect could cause other nuclear ~
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plants to subsequently trip, further degrading the system.
By means of corrective measures ~ the grid voltage could be increased to acceptable levels for continued plant operation.
The load shed feature is presently disabled once the onsite sources are supplying the Class lE buses.
Should the onsite sources trip, the load shed feature is not auto-reinstated.
The operator would then manually load shed and re-energize the buses.
3.2 MODIFICATIONS The licensee has proposed the following modifications to the existing undervoltage protection system:
(1) The present loss of offsite power (level 1) undervoltage protection scheme and the level 2 protection scheme will be redesigned to provide undervoltage protection directly at j
the Class lE buses as required by IEEE 279-1971.
The redesign will include new undervoltage protection setpoints and time I
delays to allow for short duration transients and Class lE equipment protection.
System level actuation will remain the same as the existing undervoltage protection schemes.
That is, automatic disconnection from the offsite source will only occur if a SI signal is present concurrent with level 2 actuation.
(2) Circuit design changes will be made, in addition to the above, to automatically reinstate the load shed feature following tripping of the onsite sources.
l 3.3 DISCUSSION 1
l This section presents a statement on the NRC staff position fro'm their June 3, 1977 letter [Ref. 1] followed by an evaluation of the licensee's design. I.
3.3.1 NRC Staff Position 1:
Second level of Undervoltage or Overvoltage Protection with a Time Delay This position is to be met by the licensee meeting certain criteria.
Each criterion has been evaluated against the licensee's proposal and is addressed below.
I' (1)
"The selection of voltage and time setpoints shall be determined fro = an analysis of the voltage requirements of the safety-related loads at all onsite system dist.ri-bution levels."
The licensee has not sub=itted details on the selection of the new voltage and time delay setpoints as the system redesign is no,t,yet com_pleted.
(2)
"The voltage protection'shall include coin,cidence logic to preclude spurious trips of the offsite power sources."
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Since new design details have not been submitted, determin-ation for providing the required coincident logic cannot be made.
f (3)
"The time delay selected shall be based on the following conditions."
(a)
"The allowable time delay, including margin, shall not exceed the maximum time delay that is assumed in the FSAR accident analysis."
Since new design details have not been submitted, determination of the adequacy of the time delay selected cannot be made.
(b)
"The time delay shall minimize the effect of short-"
l duration disturbances from reducing the availability c.
of the offsite power sources."
The licensee states that the time delay associated with the redesign will be such that it will allow for short duration transients caused from grid voltage j
fluctuations or motors starting.
l (c) "The allowable time duration of a degraded voltage I
condition at all distribution system levels shall I
not result in failure of safety systems or components."
Since the system redesign is not complete, the licensee has not submitted an analysis to show that the setpo,ints selected will provide equipment protection at all Class lE power system distribution levels..
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"The undervoltage monitors shall automatically initiate the disconnectio.n of offsite power sources whenever the voltage setpoint and time delay li=its have been exceeded."
The level 2 of undervoltage protection system redesign will only allow for auto =atic disconnection from the degraded offsite sources whenever an accident condition occurs con-currently.
Without an accident condition present, actuation of the level 2 undervoltage protection scheme will initiate an alarm which signals the operator of d" degraded voltage condition.
Plant procedures will then direct the operator for corrective actions.
The licensee has not provided the following information to support this method of providing undervoltage protection for the ' Class iE'e'quipment from sustained voltage degradation:
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(a) Time versus voltage degradation limits for continued plant operation.
(b) Minimum voltage limit at which disconnection would occur if voltage restoration is not completed.
(c) Analysis of the Class lE equipment operating capabilities versus the time and voltage limits established.
(d) List of the nor= ally running Class lE equipment which could be lost as a result of the time / voltage' limits established for continued plant operation.
(e) Basis for the time limits for continued plant operation.
(f) Details of the corrective actions being taken to restore edequate voltage.
The licensee has submitted a list of reactor systems [Ref. 7]
still available to safely shutdown the plant should the normally running Class 1E equipment be lost as a result of continued plant operation under sustained voltage degradation conditions.
These systems are_ presently being evaluated by the Reactor Sys.tems Branch of the NRC.
(5)
"The voltage monitors shall be designed to satisfy the requirements of IEEE Standard 279-1971."
The licensee states that the proposed design codifications will co= ply with the requirements of IEEE 279-1971.
(6)
"The Technical Specifications shall include li=iting condi-tions for operation, surveillance requirements, trip setpoints with einimus and =axi=us li=its, and allowable values for the second-level voltage protection monitors."
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t Limiting conditions for operation, surveillance requirements and the trip setpoints for the existing undervoltage protection schemes are included in the licensee's proposed Technical Specifications.
i 3.3.2 NRC Staff Position 2:
Interaction of Onsite Power Sources with Load Shed Feature The second position requires the system be designed,to, prevent automatic load shedding of the emergency buses once tMe onsite sources are supplying power to all sequenced loads.
If an adequate basis can be provided for retaining the load-shed feature, the licensee must assign maximum and minimum values to the setpoint of the load-shed feature.
These setpoints must be documented in the Technical Specifications.
The load-shedding feature must also be reinstated if the onsite source supply breakers are tripped.
The licensee is bypassing the load,shed feature once the onsite i
sources are supplying the Class lE buses.
The licensee also states that design modifications vill be incorporated into the new undervoltage protection system redesign to auto =atically reinstate the' load-shed feature following the tripping of the onsite sources.
Design details for the modification should be s ubmitted.
3.3.3 NRC Staff Position 3:
Onsite Power Source Testing I
l The third position requires that certain test requirements be included in the Technical Specifications.
These tests are to "... demonstrate the full functional operability and independence of the onsite power sources at least once per 18 months during shutdown." The tests are to simulate loss of offsite power in conjunction with a safety-injection actuation signal and to simulate interruption and subsequent reconnection of onsite power sources.
These tests will verify the proper operation of the load-shed system, the load-shed bypass circuitry, and that there is no adverse inter-action between the onsite and offsite power sources.
Existing Technical Specifications include tests to demonstrate the full functional operability and independence of the onsite power sources by simulating loss of offsite power in conjunction with a safety injection signal.
Since design modifications will include the auto-reinstatement of the load shed: feature following onsite' source tripping, a test must be included in the Technical Specifications to demonstrate the operability that the bypassing of the load shed feature will automatically be reinstated following onsite source tripping with subsequent load sequencing.
3.4 TECHNICAL SPECIFICATION The licensee submitted Technical Specifications on the existing undervoltage protection system.
The Technical Specifications include:
(1) Trip setpoints without tolerances for the level 1 (loss of voltage) and level 2 (degraded voltage) undervoltage protection schemes. l
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(2) Coincident logic (1-out-of,-2 for level 1 and 2-out-of-2 per each of the two channels for level 2).
(3) Surveillance requirements for channel calibration during refueling shutdown and a functional test during refueling shutdown. No channel check requir*7ents were included.
(4) Action statements reguarding li=iting conditions for operation when the nu=ber of trip syste=s required is reduced.
4.
CONCLUSIONS
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Sased on the infor=ation provided by_ Northeast U.tilities, it has been determined that the existing undervoltage protection design does not meet the requirements of NRC Staff Position 1.
The licensee's proposed syste=, redesign for providing sustained undervoltage protection for the Class 1E equipment cannot be fully evaluated with respect to the require-ments of NRC Staff Position 1 until the following inf ormation is provided:
(1) Details of the operation and design of the new undervoltage protection schemes.
This must include voltage setpoints and time delays with tolerances and the coincident logic circuitry.
(2) Verification that the system design meets the requirements of IEEE 279-1971.
(3) Time and voltage limits defined for continued plant operation should a degraded voltage condition exist without a concurrent accident condition and the basis for these limits.
(4) Hinimum voltage limit for tripping should voltage restoration
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fail.
(5) Analysis to verify the setpoints selected will provide equipment protection at all Class lE power system distribution levels and will minimize the effect of short duration transients. Analysis should also compare Class lE equipment operating capability with the voltage / time limits established for continued plant operation.
(6) Define which Class lE equipment could be lost as a result of con-tinued plant operation under sustained voltage degradation (with respect to the established voltage / time limits for operation).
(7) Details of the actions being taken to restore voltage when. the operator receives an alarm.
(8) Proposed Technical Specifications to include the setpoints with tolerances, surveillance requirements, testing and calibration requirements, and limiting conditions for operation.
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The licensee is proposing to modify the load shed circuitry to comply with NRC Staff Position 2.
Details of the circuitry modification is required to verify the compliance with this position.
Since the system redesign is not complete, a test has not been proposed in the Technical Specifications to denonstrate the operability of the auto-rein-of the load shed feature following onsite source tripping, thus all the statement requirements of NRC Staff Position 3 have not been met.
REFERENCES
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1.
NRC letter (A. Schwencer) to Northeast Nuc_ lear Energy. Company (NNECO), dated June 2,1977.
2.
NNECO letter (D. C. Switzer) to the NRC (G. Lear), dated August 1, 1977.
3.
NU letter (W. G. Counsil) to the NRC (D. L. Ziemann), dated August 10, 1979.
4 NU letter (W. G. Counsil) to the NRC (D. M. Crutchfield), dated April 29, 1980.
5.
NU letter (W. G. Counsil) to the NRC (D. M. Crutchfield), dated July 16,1980.
6.
NU letter (W. G. Counsil) to the NRC (D. M. Crutchfield), dated August 20, 1980.
- 7. 'NU letter (W. G. Counsil) to the NRC (D. M. Crutchfield and R. A. Clark),
dated April 21, 1982.
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8.
Code of Federal Regulations, Title 10, Part 50 (10 CFR 50), General Design Criterion 17 (GDC 17), " Electric Power Systems" of Appendix A
" General Design Criteria for Nuclear Power Plants."
9.
IEEE Standard 279-1971, " Criteria for Protection Syste=s for Nuclear Power Generating Stations."
10.
IEEE Standard 308-1974, " Criteria for Class lE Power Systems for Nuclear Power Generating Stations."
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