ML20214X111

Safety Evaluation Supporting Util Analytical Methods Used to Evaluate Stresses of Critical Components for Vacuum Breaker Integrity Re Mark I Containment Program

ML20214X111
Person / Time
Site:	Quad Cities
Issue date:	11/26/1986
From:	Office of Nuclear Reactor Regulation
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Shared Package
ML20214H117	List: ML20214H115 ML20214X090 ML20214X111
References
NUDOCS 8612100546
Download: ML20214X111 (2)

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1. UNITED STATES 8[ o NUCLEAR REGULATORY COMMISSION WASHINGTON, D. C. 20555 h

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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATING TO MARK I CONTAINMENT PROGRAM - VACUUM BREAKER INTEGR:TY COMMONWEALTH EDISON COMPANY t

00AD CITIES NUCLEAR POWER STATION UNITS 1 AND 2 DOCKET NOS.: 50-254 AND 265

1.0 INTRODUCTION

In addition to the Staff's Safety Evaluation (SE) dated February 15, 1986 .

on the suppression chamber, torus attached piping, and pressure relieving lines, under the newly defined loadings, the Mark I containment program I reouired the assurance of the structural integrity of vacuum breakers during operation in all Mark I plants. This additional requirement was categorized

. as a separate effort, as the adequacy of other components was already discussed in the separate SE dated referenced above.

The staff's contractor, Franklin Research Center (FRC), has performed an evaluation of the structural integrity of vacuum breakers in the Quad Cities Nuclear Power Station Units 1 and 2 (Quad Cities) for the NRC staff.

Results of the review are reported in the attached document, TER-C5506-326,

" Structural Evaluation of the Vacuum Breakers (Mark I Containment Program),

Coninonwealth Edison Company, Quad Cities Station Units 1 and 2." FRC has concluded that the analytical methods used to evaluate stresses of critical components are adequate and that the actions taken by the licensee are adequate to restore the original design margin of safety for its vacuum breakers under the revised loadings in the Mark I containment.

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, The staff's and licensee's letters for this evaluation are discuued in the attached TER. The findings of the TER are given below. NRC staff has reviewed the attached document and concurs with the FRC findings.

2.0 DISCUSSION During steam condensation tests on BWR Mark I containments, the wetwell-l to-drywell vacuum breakers cycled repeatedly during the transient phase of steam blowdown. This load was not included in the original lead combinations used in the design of the vacuum breakers. Consequently, the repeated impact of the pallet on the valve seat and body created stresses that may impair its capability to remain functional.

A vacuum breaker is a check valve installed between the wetwell and the drywell. Its primary function is to prevent the formation of a negative pressure on the drywell containment during rapid condensation of steam in the drywell and in the final stages of a LOCA. The vacuum breaker maintains a wetwell pressure less than or equal to the drywell pressure by permitting air flow from the wetwell to the drywell when the wetwell is pressurized and the drywell is depressurized slowly.

8612100546 861126 PDR ADOCK 05000254 P PDR

3.0 EVALUATION In each of the Quad Cities Mark I Containments there are twelve 18" internal type Atwood - Morrill vacuum breakers. The vacuum breakers are mounted on flanges attached to the 20" nozzle at the intersection of each vent line and the ring header in the suppression chamber. There are two vacuum breakers on six of the eight vent line/ vent header junctions. Loadings on Mark I structures and vacuum breakers are based on the General Electric Company Report, NED0-21888, " Mark I Containment Program Load Definition Report," Revision 2, dated November 1981. For vacuum breakers, the loadings included are gravity, seismic, and hydrodynamic loads. The hydrodynamic forcing functions were developed by Continuum Dynamics, Inc. by using a dynamic model of a Mark I pressure suppression system and the full scale test facility data. The system model was capable of predicting pressure transients at specific locations in the vent system. Loading across the vacuum breaker disc caused by pressure differentials based on test data was thus quantified as a function of time. -

This issue was reviewed and approved by NRC on December 24, 1984. Loadings were combined according to the Final Safety Analysis Report commitments.

The licensee, to determine the structural integrity of the vacuum breakers, compared results from a finite element model and ANSYS program analyses with the design limits specified in the ASME Boiler and Pressure Vessel Code,Section III, Division 1, Subsection NC, 1977 Edition and addenda up to Summer 1977. It found that several components including the valve shaft and the weight lever will be overstressed under the newly defined design loadings. The licensee then replaced these critical components with construction of adequate high strength materials. The resulting predicted stresses for the modified vacuum breakers are within allowable limits and the minimum margin of safety was determined to be 40% by the licensee. This action to restore the required safety margins is judged by the staff to be adequate and acceptable.

4.0 CONCLUSION

The analytical methods used to evaluate stresses of critical components have been reviewed and judged adequate. The licensee has restored the safety margins of the Quad Cities vacuum breakers by replacing critical parts with adequate materials. The staff concludes that this corrective action is acceptable and sufficient, and that this issue is closed.

Principal Contributor: H. Shaw, T. Rotella, J. Donohew Dated: November 26, 1986

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