ML20138A724
| ML20138A724 | |
| Person / Time | |
|---|---|
| Site: | Vermont Yankee File:NorthStar Vermont Yankee icon.png |
| Issue date: | 04/24/1997 |
| From: | Sojka R VERMONT YANKEE NUCLEAR POWER CORP. |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| BVY-97-54, NUDOCS 9704280206 | |
| Download: ML20138A724 (5) | |
Text
1 ERMONT YANKEE NUCLEAR POWER CORPORATION hx P.O. Box 157, Governor Hunt Road
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vernon, vermont 05354-0157 s
.j (802) 257-7711 April 24,1997 BVY 97-54 United States Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555 l
References:
(a) License No. DPR-28 (Docket No. 50-271)
(b) Letter, USNRC to VYNPC, NVY 97-22, dated February 25,1997 (c) Letter, VYNPC to USNRC, BVY 97-40, dated March 26,1997 1
(d) Letter, USNRC to VYNPC, NVY 97-50, dated April 17,1997 (e) Letter, VYNPC to USNRC, BVY 89-36, dated April 12,1989 (f) Letter, VYNPC to USNRC, BVY 90-38, dated March 30,1990 4
(g) Letter, VYNPC to USNRC, BVY 91-21, dated February 28,1991 (h) Letter, VYNPC to USNRC, BVY 91-69, dated July 17,1991 1
(i) Letter, VYNPC to USNRC, BVY 93-12, dated February 1,1993 (j) Letter, VYNPC to USNRC, BVY 93-94, dated September 2,1993 (k) Letter, VYNPC to USNRC, BVY 94-33, dated March 11,1994
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Subject:
Vermont Yankee Response to NRC Comments Regarding Offsite Power
System Design
In Reference (b) the NRC requested additionalinformation regarding the adequacy of the delayed offsite power circuit at Vermont Yankee Nuclear Power Station (VY.NPS) which Vermont Yankee provided in Reference (c). In Reference (d) the NRC responded with additional comments regarding Vermont Yankee's offsite power system design, in particular questioning whether the Vernon Hydroelectric Station may be credited as an AAC source to satisfy the requirements of 10CFR50.63 [ Station Blackout, (SBO)) if it is also considered a 1
source of offsite power to satisfy 10CFR50, Appendix A, Criterion 17 (GDC 17). The purpose of this letter is to address these additional NRC comments.
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The Vermont Yankee FSAR and Technical Specifications currently describe the Vernon Tie
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line as a delayed access source of offsite power. However, designating the 115 kV transmission system feeding the startup transformers as the immediate access source of
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offsite power, Backfeed from the 345kV switchyard through the main generator step-up and 1
unit auxiliary transformers as the delayed access source of offsite power, and the Vernon Tie line from the nearby Vernon Hydroelectric station as Vermont Yankee's approved SBO AAC source, meets the requirements of both GDC 17 and the SBO rule.
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5 United States Nuclear Regulatory Commission VERMONT YANKEE NUCLEAR POWER CORPORATION April 24,1997 Page 2 of 3 Vermont Yankee believes the Vernon Tie line can be used as a power source in a limited role to demonstrate GDC 17 compliance. The NRC's Standard Review Plan (SRP) states that the required demonstration for plants using a backfeed is that there is sufficient time to make the delayed access circuit available " assuming the availability of the grid itself but the unavailability of the immediate access circuit and the onsite power supplies." (SRP, p.8.2-6, emphasis adced). The corresponding NRC finding is that"[e]ach circuit can be made available to the onsite power system assumina loss of the onsite a-c standby power supplies and loss of the other offsite circuit to assure that fuel design limits and design conditions of the reactor coolant pressure boundary are not exceeded."(Id., p.8.2-9, emphasis added). Both of these statements appear to allow a licensee to credit power sources other than the immediate access and onsite standby sources (e.g. a SBO AAC source) to demonstrate that there is sufficient time to establish the Backfeed as the delayed access source in the event the immediate access source and onsite standby supplies are lost. This is in fact the limited role of the Vernon Tie. It provides an AC source of power while the Backfeed is being established.
When used in this capacity, the Vernon Tie line would not be normally connected to Vermont Yankee's offsite power systems. It is connectable to but not normally connected to Vermont Yankee's emergency Class 1E,4160V buses. Because the Vernon Tie line can be established within 10 minutes of a loss of the immediate access offsite power source and failure of the onsite emergency AC power, and is subsequently replaced with the Backfeed, and because both the Vernon Tie line and the Backfeed circuits have sufficient capacity to supply the necessary loads and prevent the fuel design limits and reactor coolant pressure boundary design conditions from being exceeded, GDC 17 compliance is demonstrated.
Therefore, based on the position in the SRP, designating the 115kV system as the immediate access source of offsite power, the Backfeed capability as the delayed access source of offsite power and the Vernon Tie line as the SBO AAC source meets the requirements of both GDC 17 and the SBO rule.
The reliability of the Vernon Hydroelectric station and its independence from Vermont Yankee's preferred offsite power systems have been extensively documented, principally in our SBO submittals [ References (e) through (k)). The Attachment provides a summary description of these capabilities which provide reasonable assurance and confidence that, in the event of a trip of the unit, failure of Vermont Yankee's immediately available oiisite power and failure of both emergency diesel generators to start, the station can be maintained in a safe condition until the Backfeed is be established.
While this configuration exceeds the SBO rule safety goals by an order of magnitude, Vermont Yankee is currently evaluating plant and procedural modifications to improve the Backfeed capability to meet GDC 17 performance criteria with no reliance on the Vernon Tie line.
Following completion of our evaluation of possible improvements, the FSAR and Technical Specifications will be revised, as necessary, to clearly describe our offsite power capability.
Vermont Yankee will provide the results of our evaluation to the NRC within 60 days.
Unite'd States Nuclear Regulatory Commission VERMONT YANKEE NUCLEAR POWER CORPORATION April 24,1997 Page 3 of 3 Should you have questions or require additionalinformation, please contact this office.
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Sincerely, VER NT YANKE, NUCLEAR POWER CORPORATION h',
t Robert E. Sojka Licensing Manager 2
Attachment - Description of the Vernon Tie Line c:
USNRC Region I Administrator USNRC Project Manager - VYNPS USNRC Resident inspector-VYNPS I
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E MEAR NER CO@ORAMN Unite'd States Nuclear Regulatory Commission April 24,1997 j
Attachment Page 1 of 2 i
Summary Description of the Capabilities of the Vernon Tie Line 1
Reliability of the Vernon Hydroelectric Station. the 69kV Transmission System and Vernon Tie The Vernen Hydroelectric station currently has eight generators: two are rated 4.2 MVA and six are rated 2.5 MVA. Current maximum station output is limited to 23.4 MVA. Vernon Hydroelectric station generation is connected to two 13.8 kV generation buses within the station indoor structure. The two buses are connected by a normally open tie breaker. Each bus supplies a 13.8 kV to 69 kV generator step up transformer by underground cable. Each step-up transformer, located outdoors in the switchyard, is sized to carry the full station output.
The 69 kV switchyard is connected in a dual bus configuration with one step up transformer 4
supplying each bus. Each 69 kV transmission line is normally lined up to one 69 kV bus, but it may be transferred to the opposite bus.
The 69kV transmission system feeding the Vernon switchyard consists of five separate 1
overhead lines. Two lines (A1 and B2) run southwest into Massachusetts, two lines (C3 and D4) run southwest tc Massachusetts, and one line (G33) runs north to Bellows Falls, Vermont.
These lines do not directly interface with the Vermont Yankee 345 kV or 115 kV switchyard or j
any of the 345 kV or 115 kV transmission lines that supply offsite power to Vermont Yankee.
1 The Vernon Tie line is supplied from the secondary of a 69kV /13.2 kV transformer located in the Vernon Hydroeiectric Station switchyard. The transformer is supplied from the switchyard 69kV bus #1 which is connected to the 69kV transmission system and the hydroelectric station output. The Vernon Tie line is an underground transmission line from the 13.2 kV portion of the switchyard to the 4160V Class 1E buses at Vermont Yankee. The tie is configured so that it can be connected to either Class 1E bus, but not both simultaneously. Since the majority of the Tie line is underground it is not prone to the same weather-induced failure as an overhead line.
In summary, the configuration of the station and switchyard at Vernon provides for a very reliable design. Power from the station can be connected to the 69 kV bus with one switchgear out of service or one step up transformer out of service. Power from the transmission lines can be connected to either 69 kV bus. The 69kV switchyard can thus be powered from either the hydroelectric station or the 69kV transmission system or both. Also, the Vernon Tie line is an underground transmission line directly from the switchyard to the emergency buses at Vermont Yankee. Since it is buried it is not prone to weather-induced failure.
Independence of Vernon Hydroelectric Station and 69kV Transmission System from Vermont Yankee's Transmission Systems The Vernon station 69 kV transmission system is not directly electrically connected to either of Vermont Yankee's transmission systems, thus providing electricalindependence and minimizing the potential for common cause failure due to electrical faults, switching problems or other grid related losses of power. Vernon Station switchyard is physically separated from Vermont Yankee's switchyard (by about a mile) and the transmission lines emanating from the i
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United States Nuclear Regulatory Commission ERMONNAWEE NUCLEAR POWER CORDORABON April 24,1997 Attachment Page 2 of 2 station are routed on separate rights of way than those serving Vermont Yankee's 345 kV and 115 kV switchyards. The majority of the lines emanating from Vernon Station are routed in completely different directions from the lines supplying offsite power to VY's switchyards.
Capability and Capacity of the Vernon Tie Line As documented in our response to the SBO rule [ Reference (e)] the Vernon Tie line is available within ten minutes in the event of a loss of immediate offsite power and the failure of the onsite emergency power sources and has sufficient capacity and capability to operate systems necessary to bring and maintain the plant in a safe shutdown condition. Vermont Yankee's Safe Shutdown Capability Analysis supports in excess of ten minutes to establish the Vernon Tie line. Establishing the Vernon Tie is controlled by plant procedure, is part of routine operator qualification training and is practiced on the training simulator. Additionally, the Vernon Tie line is load tested during each refueling outage per plant procedure.
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