ML20138H308

Compliance W/Sbo Regulation

ML20138H308
Person / Time
Site:	Vermont Yankee File:NorthStar Vermont Yankee icon.png
Issue date:	04/28/1997
From:	VERMONT YANKEE NUCLEAR POWER CORP.
To:
Shared Package
ML20138H292	List: ML20138H286 ML20138H308 ML20138H335
References
BMO-97-18, NUDOCS 9705070137
Download: ML20138H308 (4)

Text

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Basis for Maintaining Operation BMO No: 97- 1% Compliance with Station Blackout Regulation 10CFR50.59 Safety Evaluation Required Yes__ No x Safety Eval No._

References (a) Event Report 97-(b) USNRC letter to VYNPC, NVY 97 22, dated February 25,1997 1 (c) VYNPC letter to USNRC, BVY 97-40, dated March 26,1997 (d) USNRC letter to VYNPC, NVY 97-50, dated April 17,1997 (c)VYNPC letter to USNRC, BVY 97-54, dated April 24,1997 (f) VY Technical Specifications, Section 3.10 l

(g) 10CFR50.63, Station Blackout

Background

Vermont Yankee has proposed to use the upgraded Vemon Tie to meet the requirements of the Station Blackout Rule. The Vernon Tie has been upgraded to meet the defmition of a Station Blackout Alternate AC source. The line was converted from a panially overhead line to one that is run underground from Vernon station to the Vermont Yankee switchgear. Modifications were unplemented to the 4kV switchgear to provide diverse control power sources for the Vernon tie line circuit breakers to nunimize common cause failures. The NRC had issued a Safety Evaluation accepting the Vernon Tie u a SBO <

AAC source; however, recent correspondence with the NRC staff contained in References c), d), e) and f) indicate that the Vernon tie may not be an acceptable Station Blackout Source.

The Vennont Yankee FSAR describes the Vernon Tie as both an offsite source of power and a source ofpower used to meet the Station Blackout Rule. The NRC staffhas indicated that it cannot be used for an AAC source ofpower and also credited by the VY licensing basis as an offsite power source [ Reference d)]. VY has determined that the Vernon Tie Line is considered an c,ffsite power source in our licensing basis. Therefore, VY is not in compliance with the requirements of the Station Blackout Rule because Vermont Yankee does not have a current SB0 coping study or an acceptable Alternate AC source of power.

A BMO is required tojustify operation if Vermont Yankee is not meeting the NRC requirements of the Station Blackout Rule.

I Deficiency and the equipment involved:

Reference g) 10CFR50.63. " Station Blackout" requires all plants to "be able to withstand for a specified duration and recover from a station blackout as dermed in { 50.2. The i specified station blackout duration shall be based on the following factors:

(i) The redundancy of the onsite emergency ac power sources; 9705070137 970429 PDR P ADOCK 05000271 PDR l

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(ii) The reliabi ity of the onsite emergency ac power sources; (iii) The expec ed frequency ofloss of offsite power; and (iv) The protaUe time needed to restore offsite power."

VY does not meet the requdements of the regulation because the Vernon Tie has been determined to be an unaccepttble SBO AAC source since it is also used for meeting offsite power requirements.

II Identification of the potential adcarse e'~ect on safety caused by the condition:

The NRC published the Station Blackout Rule because safety studies, such as the 1975

" Reactor Safety Study" (NUREG - 75/140) had identified that a Station Blackout could be a significant contributor to total risk of core damage. These studies together with concerns on the reliability of onsite and offsite emergency power systems, led to the designation of Station Blackout as an Unresolved Safety Issue, USI A-44.

Background information published in the Notice of Rulemaking j for the Station Blackout mle indicates that a core damage frequency of near 1 EWcould be readily achievable by @

implementation of the rule . NUREG 1560 (draft), " Individual Plant Examination Program: Perspectives on Reactor Safety and Plant Performance" evaluates the results of  ;

IPEs against NRC expectations. It confirms that the goal of the SBO rule was to reduce total risk of core damage from Station Blackout events to less than 1 E-5. 1 i

Non compliance with the requirements of the Station Blackout Rule could result in  !

unacceptable risk of core damage from station blackout events.

III Factors w hich compensate for the ndverse safety impact including but not limited to: redundant or backup equipment, safety margins, probability, inspections, historica!information, compensatory nctions, equipment test results, operating experience, current physical condition.

Background information published in the Notice ofRulemaking for the Station Blackout rule indicates that a core damage frequency of near 1 E-5 could be readily achievable by implementation of the rule . NUREG 1560 (draft), " Individual Plant Examination Program: Perspectives on Reactor Safety and Plant Performance" evaluates the results of IPEs against NRC expectations. It confirms that the goal of the SBO rule was to reduce total risk of core damage to less than 1 E-5.

Vermont Yankee's individual Plant Examination (IPE) determined that the total risk of core damage events from Station Blackout is in the order of 1 E-6. The IPE models VY's actual equipment configurations, power system, diesel generators and the Vernon Line.

The model is independent of whether Vernon Tie is designated "offsite power" or "a Station Blackout source" The IPE models ac power system interaction and interdependencies. Although not an IPEEE, the analysis address-s frequency ofloss of offsite power as an " external" event, including consideration of severe weather. It conservatively models losses of offsite power using generic industry methodology.

Because VY's plant specific IPE demonstrates that the risk from a station blackout is approximately an order of magnitude lower than the goal of the station blackout l

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regulation, operation of the plant isjustified although Vermont Yankee is not in literal compliance with the regulation. '

The Vernon Tie is considered a highly reliabic source ofpower that should be available in the event of trip of the unit, failure of the immediate access source of power and failure of ,

the diesel generators.

The Vernon Tie is physically and electrically independent of the other sources of power to the station emergency buses. These include the normal source ofpower, the main generator; the preferred source of offsite power, the connection from the transmission system to the startup transformers, and both emergency generators.

• It is physically and electrically independent of other circuits from the transmission system that supply Vermont Yankee. It shares no protective relaying or automatic signals with other circuits.

The cable from the VY switchyard to the Vernon switchyard is buried and protected i

from the weather. The new 13.2 kV / 4.16 kV Vernon Tie Line transformer, located  !

in the VY owner controlled area, was designed to be protected from severe weather.

The Vernon switchyard and the hydroelectric station building has been analyzed to be able to withstand severe weather. Therefore the connection would be available when other typical offsite power sources are expected to be unavailable.

In the event of an area wide blackout, the Vernon Hydrostation has the capability to restart within a short time to provide emergency power to Vermont Yankee through the tie line.

Although it is categorized as "offsite power" by the Vermont Yankee licensing basis, it provides unique features that set it apart from other offsite power sources. These features l

provide assurance that VY will not have an extended loss of all ac power and therefore, VY meets the intent of the SBO rule.

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l IV Recommendations and time estimate for correcting the condition: 4 Vennont Yankee is studying alternatives for coming into fuU compliance with the requirements of the SBO regulation. Alternatives being evaluated include plant modifications and completion of a SB0 Coping Analysis. Plans for achieving full  !

compliance will be established by May 25th 1997. Full compliance will be established  ;

prior to startup from the next refueling outage. l l 1 l V Conclusion, and supporting basis that there is reasonable assurance that the plant i

may continue to safely operate:

l The Station Blackout Rule was written by the NRC to assure that all plants would have reduced risk from Station Blackout Events. Vermont Yankee has performed a station blackout risk analysis as part ofits IPE. The IPE demonstrates that VY's risk from core i damage from a SBO event is an order ofmagnitude lower than the goal established by the rule, i

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l Unique design and configuration features of the Vernon Tie provide assurance that VY will not have an extended loss of all ac power and therefore VY meets the intent of the iB O rule. i Ve .;7clude that VY meets the intent of the regulation and that there is reasonable asarance that the plant may continue to safety operate untilissues associated with corraliance with the SBO rule are resolved.

Prepared b --4 db_ <//20/7 v g s

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