ML20202J421
| ML20202J421 | |
| Person / Time | |
|---|---|
| Site: | Vermont Yankee File:NorthStar Vermont Yankee icon.png |
| Issue date: | 03/31/1986 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20197F824 | List: |
| References | |
| NUDOCS 8604160034 | |
| Download: ML20202J421 (4) | |
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UNITED STATES 8
NUCLEAR REGULATORY COMMISSION n
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rj WASHINGTON, D. C. 20555 SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION VERMONT YANKEE NUCLEAR POWER CORPORATION VERMONT YANKEE NUCLEAR POWER STATION DOCKET NO. 50-271 DEGRADED GRID VOLTAGE PROTECTION FOR THE CLASS 1E SYSTEM INTRODUCTION AND
SUMMARY
The criteria and staff positions pertaining to degraded grid voltage.
protection were transmitted to Vermont Yankee Nuclear Power Corporation (VYNPC) by NRC Generic Letter dated June 3, 1977.
In response to this, by letters dated July 18, 1977, December 6, 1979, March 17, 1980, July 24, 1980, Fehruary 6,1981, June 8,1982, July 22,1982, May 15,1984 and June 21, 1984, the licensee proposed certain design modifications and 5
changes to the Technical Specifications. A detailed review and technical evaluation of these proposed modifications and changes to the Technical Specifications was performed by Lawrence Livennore Laboratory (LLL) in
" Degraded Grid Protection for Class 1E Power Systems Vermont Yankee Nuclear Power Station" dated August 25, 1982.
EVALUATION CRITERIA The criteria used by LLL in its technical evaluation of the proposed changes include GDC-17 (" Electric Power Systems") of Appendix A to 10 CFR Part 50, IEEE Standard 279-1971 (" Criteria for Protection Systems for Laclear Power Generating Stations"); IEEE Standard 308-1974 (" Class 1E Power Systems for Nuclear Power Generating Stations"); and staff positions defined in NRC Generic Letter to VYNPC dated June 3, 1977.
_ Existing Undervoltage Protection The existing undervoltage protection at Vennont Yankee consists of the following:
1.
Two inverse time loss of voltage relays on each 4160 volt Class IE bus arranged in a two-out-of-two logic. These relays have a setpoint of 46% of nominal voltage. Actuation of these relays will automatically disconnect the offsite power source, initiate load shedding, start the emergency diesel generators and sequence the Class 1E loads on the emergency diesel generators. The load shedding feature is automatically bypassed when the diesel generator. breaker is closed.
This feature is not automatically reinstated on a diesel generator trip.
8604160034 860331 DR ADOCK 0500 1
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. l 2.
Two voltage transducers are installed on each 4160 volt and 480 volt Class 1E bus. These sensors will actuate an alarm (in the control l
room) above the minimum voltage required for continuous operation of j
the Class IE equipment.
3.
Two undervoltage (degraded grid protection) relays on each 4160 volt Class 1E bus arranged in a two-out-of-two logic. These relays have a setpoint of 3700 40 volts (88.9 1% of nominal) with a time delay of 10 1 second. Operation of these relays is as follows:
a.
If the 4160 volt Class 1E bus should degrade to 88.9 1% of nominal under nonaccident conditions for greater than 10 1
second the undervoltage relays will' initiate an alarm in the l
control room. Upon receipt of this alarm the operator will contact the Rhode Island, Eastern Massachusetts, and Vermont Energy Control (REMVEC), the system dispatch center and request
. an assessment of the degraded voltage condition.
If imminent restoration of voltage is not possible, the operator will star.t I the onsite emergency diesel generator, separate the Class IE buses from the degraded offsite power source and place these buses on the onsite emergency diesel generators.
b.
If the 4160 volt Class 1E bus voltage should degrade below the above setpoints concurrent with an accident signal (SI), the disconnection of the degraded offsite power source, load i
shedding, diesel generator starting, and subsequent load sequencing will occur automatically.
Modifications The licensee is proposing a design change which will automatically reinstate the load shed features if the diesel generator output breaker should trip while it is supplying the safety loads.
The licensee's degraded grid undervoltage protection design will provide automatic separation of the Class 1E power system for offsite power if a degraded grid exists coincident with a safety injection signal (LOCA).
This approach provided protection to the Class 1E equipment needed to mitigate the consequences of an accident under degraded grid condition and is acceptable.
For a degraded grid condition without a LOCA an alarm will be actuated and operator action will be taken to restore the grid to an acceptable level.
In the event operator action is not successful in restoring grid voltage within an acceptable time period, the operator will manually start the onsite diesel generators and separate the Class 1E buses from the offsite power system. The Class 1E loads would then be sequenced onto the onsite emergency generators. This approach deviates from the staff position that requires automatic isolation of the offsite power system from such undervoltage after a time delay. Acceptability of this
. alternateapproachrequiresdemonstrationbythelicenseethathidequate safety systems will be available for safe shutdown of the reactor for these conditions and that appropriate plant operating procedures are developed and available to the operator for the required manual operator action.
In response to the above concerns, the licensee in a submittal dated June 8, 1982 provided a list of systems, that will not be exposed to or rendered inoperative by degraded grid voltage which would be available to place the plant in a safe shutdown under nonLOCA conditions. The Reactor Systems Branch (RSB) and Auxiliary Systems Branch (ASB) have reviewed the listing and concur with the licensee's approach that this equipment provides the capability to place the plant in a hot shutdown condition. This equipment additionally has the capability to maintain the plant in a hot shutdown condition for the time required to reset any overload protective devices, or replace fuses that may have actuated as a result of the degraded voltage.
On.the basis of the above and that protection devices, i.e., circuit breakers, fuses, relays, etc., are provided to prevent damage to the
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equipment required for long term plant safe shutdown and that alarms are proyided to alert the operator to this abnormal condition, we find the licensee's approach using operator action under degraded grid conditions without an accident acceptable. Acceptability of this approach is subject to completion of all proposed modifications and institution of adequate procedures to be taken by the operator during a degraded grid under nonaccident conditions.
CONCLUSIONS We have reviewed the licensee submittals and the LLL technical evaluation report and find that:
1.
The proposed degraded grid modifications will protect the Class IE equipment from sustained degraded voltage of the offsite power system during accident conditions and is acceptable.
2.
The licensee's proposal to use operator action instead of automatic disconnection of the Class 1E buses from a degraded offsite power source under non-accident conditions does not meet the staff position. To justify this alternate approach, the licensee has shown that redundant safety systems, which are not exposed to or rendered inoperative by degraded grid voltage, are available to place and maintain the plant in a safe shutdown condition. The Reactor Systems Branch and Auxiliary Systems Branch have reviewed the licensee.
proposal and concur that safety systems are available to effect a plant safe shutdown under nonaccident conditions. Based on the above, we find the licensee alternate approach acceptable.
-. 3.
The load shedding feature will be bypassed when an onsite emergency diesel generator is supplying power to the Class 1E buses. This feature will be reinstated automatically if the onsite emergency diesep. generator breaker should trip.
4.
The licensee is required to provide the following:
(a) Plant operating procedures to cover operator actions under degraded grid and nonaccident conditions.
(b) Technical Specifications to cover the setpoints and tolerances, limiting conditions fcr operation and surveillance testing for the undervoltage protective relaying system.
We therefore, find the Vermont Yankee Nuclear Power Station design acceptable.
Principal Contributors:
J. Emmani and R. Dudley
Attachment:
Technical Evaluation Report Dated: March 31, 1986 1
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