ML20136G361

Safety Evaluation Accepting Seismic Design Criteria Utilized for Evaluation of Modified Recirculation Sys

ML20136G361
Person / Time
Site:	Vermont Yankee
Issue date:	08/12/1985
From:	Office of Nuclear Reactor Regulation
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ML20136G359	List: ML20136G356 ML20136G361
References
NUDOCS 8508190381
Download: ML20136G361 (2)

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UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON D. C. 20555

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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION VERMONT YANKEE NUCLEAR POWER CORPORATION -

VERMONT YANKEE NUCLEAR POWER STATION PIPING REPLACEMENT ANALYSES - STRUCTURAL ENGINEERING DOCKET NO. 50-271 Vermont Yankee Nuclear Power Corporation (licensee) has requested approval v to utilize ASME Code case N-411 for analysis of the replacement -

recirculation system. The licensee has utilized current NRC criteria in the analysis of the proposed replacement recirculation system.

The original plant design basis utilized the N69W component of the 1952 Taft earthquake nomalized to 0.07 for the operating basis earthquake (OBE) and 0.14g for the safe shutdown earthquake (SSE). Five percent structural damping was assumed for concrete structures for both the OBE and the SSE.

One-half (1/2%) percent damping was assumed for piping for both the OBE and the SSE. Also, the horizontal seismic input for the original recirculation system was based on non-peak broadened reactor building amplified response spectra. The vertical input was based on two-thirds of the ground zero period acceleration.

The repl,acement system seismic input is based on USNRC Regulatory Guide 1.60, normalized to 0.079 for the OBE and 0.149 for the SSE. As stated above, structural damping as specified in USNRC Regulatory Guide 1,61 was utilized. The Regulatory Guide 1.60 ground spectra were detennined to be conservative with respect to the original seismic design basis. For the replacement system, a coupled structural model containing the reactor building, drywell, reactor pedestal, reactor vessel and reactor internals was developed. The amplified response spectra were developed utilizing USNRC Regulatory Guides 1.60, 1.61, 1.92, and 1.122. For the analysis of piping systems, the damping requirements of ASME Code Case N-411 are being used. Fifteen percent peak broadening and full consideration of horizontal and vertical structural amplification are employed.

In addition the seismic load combination for the original system was based upon a two-directional earthquake. The replacement system analysis utilized ;: three directional earthquake. The cutoff frequency for the original design was 20 Hz. For the replacement system, the cutoff frequency is 33 Hz. The licensee has provided, at the request of the staff, a comparison of the old and new seismic models, ground response spectrum and mass point response acceleration spectrum, along with an explanation of th! evaluation procedures.

The infomation provided by the licensee identified that the recirculation system is supported off the reactor pressure vessel and reactor pedestal with the highest elevation being approximately 280 feet, that the amplified 8500190381 050812 PDR ADOCK 05000271 P PDR

2 response of the reactor vessel is the valid seismic input for the recirculation system, and that the developed response utilizing turrent staff criteria results is more conservative than the original FSAF criteria for all frequencies greater than 3 Hz. The primary structural rssponses at Vermont Yankee is greater than 4 Hz. and therefore bounds the original FSAR requirements. In addition, the licensee has stated that adequate clearance is provided to allow for all piping displacements and that all related mounted equipment has been evaluated for any increased motion resulting from the increased flexibility of the system.

The staff concludes, based on the information provided by the licensee, that the seismic design criteria utilized for the evaluation of the modified recirculation system are acceptable and provide added conservatism from the original design.

• Principal Contributor: F. Rinaldi Dated: August 12, 1985

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