ML20138H335

Tech Specs for Offsite Power

ML20138H335
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-97-03, BMO-97-03-R01, BMO-97-3, BMO-97-3-R1, NUDOCS 9705070141
Download: ML20138H335 (6)

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Basis forMaintaining Operation 1 i

BMO No: 97- 03 Rev i Technical Specification for Offsite Power 10CFR50.59 Safety Evaluation Required Yes No x Safety Eval No._,

l References (a) Event Report 97-0196 (b) Type A Commitment VYB97007, dated 2/10/97 (c) Supplement to NRC Confirmatory Action Letter to Maine Yankee, dated 1/30/97 (d) VY FS AR, Chapter 8 I (e) VY Technical Specifications, Section 3.10 (f) 10 CFR 50, Appendix A. General Design Criteria 17, Electric Power Systems (g) USNRC Regulatory Guide 1.93, Availability of Electric Power Systems i (h) Draft Improved Technical Specifications (ITS) for VY, Section 3.8 (i) USNRC letter to VYNPC, NVY 97-22, dated February 25,1997  ;

(j) Calculation VYC-1088, Vermont Yankee 4160/480 V Short Circuit / Voltage Regulation Study (k) Calculation VYC-1512, Station Blackout Voltage Drop and Short Circuit Study j (1) VYNPC letter to USNRC, BVY 97-40, dated March 26,1997 (m) USNRC letter to VYNPC, NVY 97-50, dated April 17,1997  ;

l Backstround Vermont Yankee initiated Reference a) to address potential Technical Specification improvement opportunities identified as a result of an operating experience review of l issues identitled in Reference c) with offsite power system Technical Specifications at j Maine Yankee (MY). The discussion at the ER screening meeting pointed out that current Tech Specs credit the backfeed through the main step up transfonner and the Vernon Tie 4 as a delayed access sources of offsite power and permit unrestricted plant operation if one 115 kV transmission line is available. The ER recommended that administrative limits be adopted which would limit plant operation if the Vernon Tie becomes unavailable for more than 3 days or if the 345 kV /115 kV autotransformer becomes unavailable for more I

than 3 days unless a BMO authorizes further operation. This administrative limit in essence, credits the path from the 345 kV transmission lines through the autotransformer as the immediate access source of offsite power and credits the Vernon Tie as the delayed access source of power. The 3 day operation limit is consistent with the guidance contained in References c) and g).

An investigation of the issues at MY was previously initiated by Reference b).

Investigation of the issues identified in Reference b)is scheduled to be completed by May 30th.

Vermont Yankee submitted Reference 1) to the NRC on March 26th. The NRC issued additional questions on adequacy of offsite power and compliance with the Station 9705070141 970429 PDR ADOCK 05000271 P PDR

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. 4 Blackout Rule on April 17th, in Reference m). Subsequent discussions and correspondence have led VY to conclude that the Vernon Tie is being used to meet i delayed access otTsite power source requirements. Because Vernon Tie is being used to l

meet offsite power requirements. VY has concluded it cannot be used to meet the requirements of the SBO mle. VY Technical Specifications address offsite power requirements. SBO nequirements are not addressed in Technical Specifications.

Smnmary of the Maine Yankee Issur I As a result of the Independem Safety Assessment (ISA) of MY, in Reference c) the NRC questioned the adequacy of the MY offsite power system design. The MY FSAR states f that two immediate access offsite power sources, consisting of two independent 115 kV transmission lines, are available. Analyses performed in 1995 by MY indicate that one of the two offsite lines may not be able to supply all the required loads under all grid operating conditions.

In November of 1996, with one of the i 15 kV lines out of senice, the remaining line tripped. MY operated for 10 hours1.157407e-4 days <br />0.00278 hours <br />1.653439e-5 weeks <br />3.805e-6 months <br /> without the two inunediate access offsite sources.

M Y's Technical Specifications permit operation for up to 7 days with no 115 kV lines available.

'MY's FSAR describes its electrical power system and offsite power system as complying with Criterion 39 of the AEC draft Ocneral Design Criteria.

In Reference c), the NRC instmeted MY to modify its Tech Specs to require both 115 kV transmission lines to be operable during power operation to provide reasonable assurance that MY will be operated within its licensing basis. MY attempted to credit its backfeed through the generator step up transformer as an additional source of delayed offsite power. The NRC staff responded that it searched the MY docket and the staff had never credited the delayed access source of power at MY in meeting licensing requirements. i Vermont Yankee Offsite Power System Discussion l

l Reference b) requested Design Engineering to perform a formal review ofissues identified l in Reference c) for shnilarities with the Vermont Yankee design, procedures and technical specifications. 1 The Vermont Yankee FSAR Section 8.3 describes the offsite power system as consisting  ;

of three 345 kV transmission lines, a 345 kV /115 kV autotransformer connecting the 345 l kV transmission system to the 115 kV switchyard and a 115 kV transmission line from Keene. The supply from the 345 kV yard through the autotransformer and the startup  ;

transromiers is described as the immediate access circuit. The Keene Line is described as an " alternate immediate access circuit" whose " availability is dependent on its preloading, !

which must be limited by system dispatchers prior to it being declared an immediate access source." Delayed access circuits are described as the backfeed from the 345 kV switchyard through the main step up transformer and the Vernon Tie.

Previous versions of the FSAR have described the Keene line as having " sufficient capacity to carry the engineered safety features." The description of the backfeed through the main step up transformer was added to Section 8.3 in Revision 12 of the FSAR. It

was previously described in Appendix F of the FSAR which describes conformance to General Design Criteria, and the Technical Specifications.

A review of the original AEC Safety Evaluation of Vermont Yankee, indicates that the 3

design of the offsite power system was determined to be acceptable based on the i

immediate access source from the transmission system through the stanup transformer, and the delayed access source from Vernon.

Analyses of Adeauacy of Offsite Power The original design of Vermont Yankee consisted of two 115 kV transmission lines, two 345 kV transmission lines and a single 115 kV / 4.16 kV startup transformer. The original FSAR stated that both 115 kV lines were acceptable sources of offsite power. Shortly after commercial operation, the 115 kV line to Coolidge was converted to 345 kV. In 1974 the original startup transformer failed and was replaced by existing startup transformers T3A and T3B. Studies performed during the design of the replacement ,

transformers examined the ability of the transformers to support plant startup and l shutdown Cases examined the ability of the plant to startup when only the Keene hne i I

was in service. The studies identified that the Keene line had limited capacity and may not be able to support the plant under all conditions. The FSAR was modified at that time to state that the line had " sufficient capacity to carry the engineered safety features", l l

in August of 1979, the ?,'RC issued a letter to all licensees requesting the licensees to 1 submit analyses on the < %.sacy of electrical distribution system voltages. The analyses had to document that offsite power system could supply the automatically connected load in the event of a unit trip and start of accident loads, Vermont Yankee's analyses for the immediate access source through the startup transformer, using the NRC guidance contained in the letter, used minimum system voltage but assumed that the i autotransformer and all lines were in service. VY received a subsequent request from the  !

NRC regarding a backfeed through the main step up transformer. VY responded that it  ;

had such backfeed capability and an analysis of the adequacy of the backfeed was submitted. The NRC SER documents the adequacy of the inunediate access source l

through the startup transformers and the backfeed through the main step up transformer.

Reference j) documents, in a fonnal calculation, the current analyses on adequacy of offsite power system voltages for VY.

In 1989, VY identified issues associated with loss of the autotransformer. ' Additional l studies were performed on the adequacy of the Keene line. The studies reconfirmed that the Keene line had limited capacity and plant procedures were revised to acknowledge the limitation. Procedure OT 3155 , Loss of Autotransformer was revised to incorporate guidance and administrative limits to address situations when the autotransformer was out ofservice. ,

As a result of recent discussions and correspondence with the NRC, VY concluded that the Vernon Tie line is being used to meet delayed access offsite power requirements.

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., t 1 Deliciency and the equipment involved:

i Technical Specifications Section 3.10.A.4 requires at least one offsite transmission ime and one delayed access source consisting of either the backfeed through the main step up transformer or the Vernon Tie to be in service during power operation.

As discussed above, j the Keene line has not been demonstrated to be an acceptable immediate access l source of power.

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• The backfeed through the main step up transformer was not credited as a  !

i delayed access source in the original AEC review of VY. It takes up to 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> to establish the backfeed. An analysis does not exist to demonstrate that j the backfeed can meet the timing requirements of a delayed c.ccess source to be  !

available in " sufficient time following a loss of all onsite attemating current power supplies and the other offsite electric power circuit, to assure that .

specified acceptable fuel design limits and design conditions of the reactor coolant pressure boundary are not exceeded."  :

Technical Specification Section 3.10.A.4 could permit reactor operation with offsite power supplies degraded to below an acceptable limit because it could permit operation with the Keene line as the only source of power to the startup transformers and it could permit operation with the main step up transformer backfeed as the only delayed access ,.

source. As discussed previourly the main step up transformer backfeed was not credited l in the originallicensing of the plant as a delayed access source and may not be available in  !

sutlicient time to meet the timing requirements of a delayed access source. -

VY currently uses the Vernon Tie as a delayed access source. Therefore the VY l Technical Specifications should require that the Vernon Tie be operable to meet delayed i access offsite power source requirements.

II Identification of the potential adverse effect on safety caused by the condition:

1 The licensing basis requires Vermont Yankee to have both an offsite power system and an onsite power system. The safety function of each system (assuming the other system is unavailable) is to have sufficient capacity to supply the required loads.

Weaknesses have been identified in the existing Technical Specifications because they would allow unrestricted operation with the autotransformer out of service, or the Vernon Tie out of service.

With the autotransformer out of service, the offsite source of power through the startup transformers may not be capable of supplying the required loads under all conditions.

With the Vernon Tie out of service, Technical Specifications would permit operation with the backfeed inoperable. The backfeed would not be established in sufficient time to meet delayed access source requirements.

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. 6, HI Factors which compensate for the adverse safety impact including but not limited to: redundant or backup equipment, safety margins, probability, inspections, historical information, compensaton actions, equipment test results, operating experience, current physical condition.

Currently the autotransformer and the Vernon Tie are available. Reference j), Calculation VYC-1088, Vermont Yankee 4160/480 V Short Circuit / Voltage Reeulation Study supports the adequacy of the autotransformer as a source of offsite power. Reference k),

Calculation VYC-1512, Station Blackout Voltane Droo and Short Circuit Study supports the adequacy of the Vemon Tie as a delayed access source of offsite power.

Administrative controls have existed in Procedures OP 2140,"345 kV Electrical System";

OP 2141. "115 kV Switchyard"; OP 2143, "4 kV Electrical System and OT 3155, " Loss of Autotransformer" to address loss of the autotransformer or the Vernon Tie. These administrative controls have been made more conservative by changes in these procedures on February 28th,1997. The procedures now only permit operation for 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> when cither the Vernon Tie or autotransformer is inoperable. If the sources are not restored

, within 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> or a BMO prepared to support further operation, the plant must be in cold i shutdown within the next 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />.

On April 18,1997 an Event Report was generated that raised questions about the appropriateness of using the Vernon Tieline to satisfy both Station Blackout and offsite  ;

power requirements. Subsequent discussions resulted in a determination that the backfeed and the Vernon tie delayed sources are both assumed in our current licensing basis. i TV Recommendations and time estimate for correcting the condition:

Until such time as this issue is resolved, an additional adminsitrative limit, which is more restrictive than our current Technical Specifications, is established to require the Vernon Tie to be operable prior to making the reactor critical. The existing administrative limit which permits operation for 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> without the Vernon Tie should continue. The modified administrative limits should remain in place until a permanent TS change is made or resolution ofissues associated with immediate access and delayed access sources of offsite power are resolved.

V Conclusion, and sq porting basis that there is reasonable assurance that the plant i may continue. to safety operate:

References a) and b) initiated efforts to address potential weaknesses in Vermont Yankee's Technical Specifications. Reference i) also requires VY to provide information to the NRC staffen adequacy of existing Technical Specifications and delayed access sources of power.

At this time the inunediate access source of offsite power from the 345 kV transmission system through the autotransformer and a delayed access source of power from Vernon and the backfeed are operable. References j) and k) document the adequacy of these

• • - ' sources. As long as these sources continue to be opmble, ne safety issues' exist and plant op: ration may continue.

Reference a) initiated procedure changes which impose 3 day administrative LCOs if either the Vernon Tie or the feed through the autotransformer becomes inoperable. These

limits have been changed to add an additional administrative limit requiring the Vemon Tie to be operable prior to making the reactor critical because of reliance on the Vernon Tie as a delayed access source. The administrative limits incorporated into the existing procedures are consistent with the LCOs incorporated in Reference g) and the Standard Technical Specifications. These limits provide appropdate interim operating restrictions l which address the issues raised in Reference a). Operation within these limits proside reasonable assurance that the plant may continue to operate safely, h_-l A -

Prepared by:&/ 7 74 jad/

,f Independent Review by:[ 77 -

/V//h PORC: NrnOf4d7 97039/ 4/22/97 Q\

R Approved: - / PI97 Ti p p T

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