ML19263B065

Responds to NRC Request of 781106 for Addl Info Re Containment Shell Buckling Criteria & Application of Criteria to Static Buckling Load

ML19263B065
Person / Time
Site:	Atlantic Nuclear Power Plant
Issue date:	12/21/1978
From:	Haga P OFFSHORE POWER SYSTEMS (SUBS. OF WESTINGHOUSE ELECTRI
To:	Baer R Office of Nuclear Reactor Regulation
References
NUDOCS 7901040069
Download: ML19263B065 (6)

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N s FNP-MNE-879 Offshore Power Systems  :

December 21, 1978 Mr. Robert L. Baer, Chief Light Water Reactors Branch No. 2 Division of Project Management U.S. Nuclear Regulatory Comission 7920 Norfolk Avenue Bethesda, Maryland 20852

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Re: Docket No. STN 50-437, Containment Shell Buckling Criteria and Application

Dear Mr. Baer:

The attached information is submitted in response to your request for additional information dated November 6, 1978. The subject was discussed at a meeting in Jackson-ville on November 16th and 17th.

We plan to incorporate the attached information in a revision to the reference report as soon as we have your concurrence that the responses are satisfactory.

Very,truly yours,

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P.B.Haga,ChefEngineer Mechanical and Nuclear Engineering

/lel Attachment CC: V. W. Campbell A. R. Collier 790104coM g\p t>

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USNRC STRUCTURAL ENGINEERING BRANCH .

REQUEST FOR INFORMATION Question 1 The stress analysis assumes that cutout reinforcement as prescribed by the ASME Pressure Vessel Code is sufficient to alleviate stress concentrations. How certain is this assumption? What studies have been made to substantiate this claim?

Answer The area replacement requirements of NE-3332 of the ASME Code replace about two times the cutaut material and dis-tribute the replacement in the nozzle as well as in the insert plate around the cutout. This method has been used successfully in the past for practically all ASME pressure vessel designs and has been verified by analyses and tests (Ref.1).

For loadings which cumulatively result in stresses ex-ceeding 10% of the primary stresses induced by the design pressure, NE-3131 requires the penetration reinforcement ,

be designed by the stress intensity approach of NE-3200, Design by Analysis.

Question 2 The buckling analysis replaces the two-dimensional stress distribution by uniform stress distributions. These cor-respond to combined axial, circumferential and shear stress.

The stress components at every point in the shell are com-pared to the critical uniform stress values. While the effectiveness of opening reinforcement can also be questioned, more questionable is the procedure of replacing the variable geometry shell by a shell having uniform properties. What is the justification for this method? Why isn't the stress analysis model also used for buckling analysis?

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Answer Based on a literature search (References 44 through 50 and Section 5.2.8 of Ref. 2) well-reinforced small penetrations have insignificant effects on the shell buckling. capacity. For well-reinforced large penetrations, the adequacy of replacing the variable geometry shell by a shell having uniform pro-perties in the buckling analysis will be verified by an independent analysis taking into account the variable geometries and loadings.

Question 3 Capacity reduction factors have been defined on the basis of Koiter's asymptotic imperfection sensitivity studies and assumed deformation amplitudes. In the present study, the deformation amplitudes are taken as the maximum out-of-round-ness values permissible under the ASME Pressure Vessel Code, the shell thickness. With such a large " imperfection" the method is conservative. The choice of amplitude is rather arbitrary, however, and may be too severe. Lesser amplitudes may yield conservative results since it is not at all certain that the ASME tolerances control all of the imperfections which reduce buckling loads. Why isn't aerospace industry experience in the form of empirical buckling criteria, NASA SP-8007, 8032, for example, used?

Answer Koiter's asymptotic imperfection sensitivity analysis is a unified theory which is consistent with the ASME design approach based on the controlled tolerance requirements of NE-4220. The empirical formulae of NASA SP-8007, 8032 are established from lower bound buckling values of test data without specific tolerance requirements. The use of NASA SP-8007, 8032 for containment vessels constitutes a significant departure from the ASME design practice. However, based on our preliminary evaluation, the use of SP-8007, 8032 in con-junction with a safety factor of 2.0 for the basic compressive

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al owable stresses of NE-3222.1 will not substantially alter the containment vessel design. Therefore, at the final design stage, the containment shell will be designed in accordance with the critical buckling stresses recommended in SP-8007, 8032 with the following clarification and amplification:

('a) For the particular cylinder geometry under consideration, a linear interaction equation is used for the relationship between the critical buckling stresses for axial compression and hydrostatic compression and also for the relationship between the hydrostatic compression and lateral compression.

Similarly, a linear interaction equation is also used for the relationships between the critical torsional stress and the critical axial compression, hydrostatic compression or lateral compression. This results in a critical buckling surface consisting of two planes.

(b) SP-8007 recommends that a knockdown factor of 0.75 be applied to the values calculated from Equation 37 (which is essentially an equation for calculating classical buckling values for axial compression) for cylinders with closely spaced, moderately large stiffeners. Since it is unclear whether the stiffeners com-monly used in the containment vessel construction belong to this category, the following conserrative knockdown factors to be applied to the classical buckling values for axial com-pression will be used for the design of cylindrical shells.

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(1) For As/tet = 0.1, use a knockdown factor of 0.5. The Symbols As, 10, t are found in Fig. 3.4-1 of Ref. 2 . The 0.5 factor is a lower bound value established from test data sumarized in Reference 57 of Ref. 2 .

(2) For As/tet < 0.1, use a linear equati, lween 0.5 and the knockdown factor for the unstiffened cylinder.

(3) For As/tet > 0.1, use 0.5. A value between 0.5 & 0.75 may be used for heavier stiffening.

(c) For loadings and geometries not specifically covered by SP-8032, the following knockdown factors from classical buckling sur-faces will be used for doubly curved ring segements, caps and shell panels under combined loadings.

(1) The knockdown factor for the longitudinal compression and the circumferential compression is 0.25. The same 0.25 factor is also used for all stress combinations between the longitudinal compression & the circumferential com-pression.

(2) The torsional knockdown factor is 0.5. The interaction of critical buckling stresses between torsion & longitudinal or circumferential compression is linear.

(d) For primary loadings concurrent with the Operating Basis Earthquake (ASME Service Level 8, which is the same as Service Level A), a minimum overall safety factor of 2 is required

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against the critical buckling values establfshed from the above procedures. For Level C Service Limits, allowable loads are 120% of those specified for Level A Service Limits.

Question 4 Dynamic reduction factors are in question since the literature indicates that for axial load, the dynamic buckling load is at least 70.9% of the static buckling load of the imperfect structura. Why, then, is the capacity reduction factor equal to unity when the dynamic stress is greater than 1.42 (1/.707) of the static stress?

Answer Dynamic reduction factors are applied to the static buckling load of the imperfect structure. The procedure is to apply the imperfection knockdown factors to the classical buckling load based on the equivalent static loading and then apply the dynamic reduction factors to arrive at the critical bucki-ing load. If the dynamic load factor is greater than 1.42, the dynamic reduction factor only is taken as 1.0.

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REFERENCES (I) Harvey, J. F. , Theory and Design of Modern Pressure Vessels, 2nd Edition, Van Nostrand Reinhold Co., New York,1974, Sections 6.6 and 6.7.

(2) OPS Report No. 7270-RP-16A51, " Buckling Criteria & Application,"

Draf t Rev. B , May 1, 1978.

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