ML20129E861
| ML20129E861 | |
| Person / Time | |
|---|---|
| Site: | Vermont Yankee File:NorthStar Vermont Yankee icon.png |
| Issue date: | 07/10/1985 |
| From: | Capstick R VERMONT YANKEE NUCLEAR POWER CORP. |
| To: | Vassallo D Office of Nuclear Reactor Regulation |
| References | |
| FVY-85-64, NUDOCS 8507170088 | |
| Download: ML20129E861 (13) | |
Text
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VERMONT YANKEE NUCLEAR POWER CORPORATION RD 5, Box 169, Ferry Road, Brattleboro, VT 05301
,,,,, 70 W
ENGINEERING OFFICE N
Y, 4 July 10, 1985 1671 WORCESTER ROAD FVY 85/64 FRAMINGHAM MASSACHUSETTS 01701 TELEPHONE 6t ?-872-8100 United States Nuclear Regulatory Commission Washington, DC 20555 ATTEN: Office of Nuclear Reactor Regulation Mr. Domenic B. Vassallo, Chief Operating Reactors Branch Division of Licensing
References:
(a) License Number DPR-28 (Docket Number 50-271)
(b) Letter, VYNPC to USNRC, FVY 85-36, dated April 22, 1985 (c) Letter, VYNPC to USNRC, FVY 85-43, dated May 14, 1985
Subject:
Structural and Mechanical Aspects of Vermont Yankee Recirculation Piping Analysis
Dear Sir:
The following information is provided in response to your staff's recent request for additional information to assist in the review of Vermont Yankee's request to utilize ASME Code Case N-411 for analysis of the replacement recirculation system. The following discussion and enclosed Figures are provided in response to the request for a description of the coupled building model and spectra comparison.
Figure 1 is a cross-section view of the reactor building complex at Vermont Yankee. The reactor building complex consists of the reactor building, drywell, reactor pedestal, reactor shield wall, reactor pressure vessel and associated equipment.
The four primary structures are coupled at the 235.48f t. elevation and the 309.96f t. elevation. The recirculation system is supported off the reactor pressure vessel and reactor pedestal. The highest elevation in the recirculation system is approximately elevation 280ft.
The reactor building is the most massive of the structures, possessing over 96 percent of the total mass of the complex.
In the original design of Vermont Yankee, the designer made a simplifying assumption that the reactor building would be the driving force for seismic excitation of the recirculation system. A reactor building amplified response spectrum from elevation 280f t. at 0.5 percent damping was used as input to the recirculation system seismic analysis. A copy of the input spectrum for the original Design Basis Earthquake (comparable to what is currently termed the OBE) is attached as Figure 2.
The Maximum Hypothetical Earthquake was exactly twice the DBE.
As discussed in Reference (c), the original scismic analysis consisted of one horizontal earthquake coupled with a 0.05g (or 0.10g for the MHE) vertical earthquake. The model used for developing the original ARS is shown as Figure 3.
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U.S.N.R.C. July 2, 1985 D.B. Vassallo The NRC currently allows more realistic damping values to be used for seismic analyses. Although we wanted to take advantage of the more realistic equipment damping we felt it would be inappropriate to do so if we did not assess the effect, if any, of the original simplifying assumption relative to the use of the reactor building as the input to the recirculation system seismic analysis. We concluded that the assumption would be accurate if there was no secondary amplification by the reactor vessel and pedestal, and could be non-conservative if there was secondary amplification. We discussed the issue with several organizations experienced in the analysis of BWR structures. The consensus was that 1) the assumption could be non-conservative, 2) the degree of non-conservation was probably small, 3) the actual magnitude would be plant specific, and 4) the extreme conservatism resulting from using one-half percent damping would more than compensate for any non-conservatism.
Thus, we concluded that in order to maintain the intended margins of safety we should develop a more accurate structural model to obtain amplified response spectra at the reactor pressure vessel, which is the true input to the recirculation cystem. A copy of the model used is attached as Figure 4.
A comparison of amplified response spectra at the same elevation from the reactor building and the reactor vessel indicate that the response at the reactor vessel is higher than the reactor building. See Figures 5 and 6.
Figure 7 is an ARS developed using ASME Code Case N-411.
It was developed for the jet pump inlet nozzles, and is at the same elevation as Figure 5.
Figures 2,5,6, and 7 are all based upon a Taf t earthquake at five (5) percent structural damping, to allow a consistent basis for comparison.
Figure 8 is equivalent to Figure 7, except it was developed using an NRC Reg Guide 1.60 ground response.
(As discussed in Reference (c), R.G. 1.60 was used for the analysis of the replacement recirculation system.) Figure 9 shows that the R.G. 1.60 input is conservative with respect to the Taf t l
earthquake at all frequencies greater than 3Hz. Since the primary structural response at Vermont Yankee is greater than 4Hz the new input bounds the L
FSAR requirements.
In conclusion, we feel that from an engineering standpoint, the non-conservatism l
resulting from neglecting the secondary amplification of the reactor pressure vessel is inconsequential for the original design, given the very conservative j
assumption of one-half percent equipment damping. However, we have chosen l
to adopt the more conservative input in order to allay any concerns about reduction of margin when utilizing "PVRC" damping.
Furthermore, in response to the recent request for clarification of the 7
I statement contained in our May 14, 1984 letter (Reference (c)), please be advised that the acceptance criteria to ensure, "...that adequate clearance exists to allow for all piping displacements" includes a review to ensure that mounted equipment can withstand any increased motion due to the increased flexibility of the system.
l l
c-U.S.N.R.C. July 2, 1985 D.B. Vassallo We trust this information is adequate to allow you to perform your review.
Should you have any additional questions, please contact Mr. J.R. Hoffman at the Framingham Office.
Very truly yours, Vermont Yankee Nuclear Power Corporation
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