Regulatory Guide 7.6

(Revision 1), Design Criteria for Structural Analysis of Shipping Cask Containment Vessels

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Issue date:	03/31/1978
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Revision 1 March 1978 U.S. NUCLEAR REGULATORY

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GUIDE 7.6 DESIGN CRITERIA FOR THE STRUCTURAL

ANALYSIS OF SHIPPING CASK CONTAINMENT

VESSELS

A. INTRODUCTION

Sections 71.35 and 71.36 of 10 CFR Part 71, "Packaging of Radioactive Material for Transport and Transportation of Radioactive Material Under Certain Conditions," require that packages used to transport radioactive materials meet the normal and I ypothetical accident conditions of Appendices A and B, respectively, to Part 71. This guide describes de sign criteria acceptable to the NRC staff for use in the structural analysis of the containment vessels of Type B packages used to transport irradiated nuclear fuel. Alternative design criteria may be used if judged ac ceptable by the NRC staff in meeting the structural requirements of §§71.35 and 71.36 of 10 CFR Part 71.

B. DISCUSSION

At present, there are no design standards that can be directly used to evaluate the structural integrity of the containment vessels of shipping casks for ir radiated fuels. This guide presents containment ves sel design criteria that can be used in conjunction with an analysis which considers the containment vessel and other principal shells of the cask (e.g., outer shell, neutron shield jacket shell) to be linearly elastic. A basic assumption for the use of this guide is that the principle of superposition can be applied to determine the effect of combined loads on the con tainment vessel. However, use of this guide does-not preclude appropriate nonlinear treatment of other cask components (e.g., impact limiters and lead shielding).

Design criteria for nonlinear structural analyses are not presented in this guide because of the present lack of data sufficient to formulate substantial nonlinear criteria.

The NRC staff will review criteria other than *Lines indicate substantive changes from previous issue.those given in this guide on a case-by-case basis. Section III of the ASME Boiler and Pressure Code t contains requirements for the design of nuclear power plant components.

Portions of the Code that use a "design-by-analysis" approach for Class 1 compo nents have been adapted in this guide to form accept able design criteria for shipping cask containment vessels. The design criteria for normal transport con ditions, as defined in 10 CFR Part 71, are similar to the criteria for Level A Service Limits (formerly called "normal conditions")

of Section III, and the design criteria for accident conditions are similar to those for Level D Service Limits (formerly called "faulted conditions").

However,Section III was de veloped for reactor components, not fuel casks, and many of the Code's requirements may not be appli cable to fuel cask design. The criteria in this guide reflect the designs of re cently licensed shipping casks. The containment ves sels having these designs were made of austenitic stainless steel, which is ductile even at low temper atures. Thus, this guide does not consider brittle frac ture. Likewise, creep is not discussed because the temperatures of containment vessels for irradiated fuel are characteristically below the creep range, even after the hypothetical thermal accident require ment of 10 CFR Part 71. The nature of the design cyclic thermal loads and pressure loads is such that thermal ratchetting is not considered a realistic fail ure mode for cylindrical containment vessels. Con tainment vessel designs that are significantly differ ent from current designs (in shape, material, etc.) may necessitate the consideration of the above failure modes. Copies may be obtained from the American Society of Mechanical Engineers, United Engineering Center, 345 East 47th Street, New York, N.Y. 1001

7. USNRC REGULATORY

GUIDES Comments should be sent to the Secretary of the Commission, US. Nuclear Re 9 u latory Commission.

Washington, DC. 20555, Attention Docketing and Servie Regulatory Guides are issued to describe and make available to the public methods Branchy acceptable to the NRC staff of implementing specific parts of the Commission's regulations, to delineate techniques used by the staff in evaluating specific problems The guides are issued in the following ten broad divisions or postulated accidents, or to provide guidance to applicants, Regulatory Guides are not subsirtutes for regulations, and compliance with them is not required 1. Power Reactors 6 Products Methods and solutions different from those set out in the guides will be accept 2. Research and Test Reactors 7. Transportation able if they provide a basis for the fidigs requisite to the issuance or continuance

3, Fuels and Materials Facilities

8. Occupational Health of a fermnt or r ncense by the Commiision.

4. Environmental and Siting 9 Antitrust Review 5. Materials and Plant Protection

10, General Comments and suggestions for improvements in these guides are encouraged at all Requests for single copies of issued guides which may be reproduced)

yr for place times, and guides will be revised, as appropriate, to accommodate comments and ment on an automatic distribution list for single copies of future guides in specilic to reflect new information or experience.

This guide was revised as a result of divisions should be made in writing to the U.S. Nuclear Regulatory Commission substantive comments received from the public and additional staff review. Washington, 0.C 20555, Attention Director.

Division of Document Control Regulatory positions

2 and 6 ensure that failure due to gross unrestrained yielding across a solid sec tion does not occur. Secondary stresses (i.e., stresses that are self-limiting)

are not considered to cause gross unrestrained yielding but are considered in fatigue and shakedown analyses.

Regulatory position 3 ensures that fatigue failure does not occur, and regulatory position 4 ensures that the structure will shake down to elastic behavior after a few cycles. Both of these positions address only the stress range of normal operation.

Recent studies 2 have shown that fatigue strength decreases beyond 10' cycles for certain materials.

Regulatory position 3.b addresses the possibility of fatigue strength re duction beyond 10' cycles. Regulatory position 5 states that buckling of the containment vessel should not occur. While it is rec ognized that local or gross buckling of the contain ment vessel could occur without failure (i.e., leak age), the stress and strain limits given in this guide are based on linear elastic analysis and are inappro priate for determining the integrity of a postbuckled vessel. If the analysis of a containment vessel indi cates the likelihood of structural instability, the de sign criteria of this guide should not be used. Regulatory position 7 places a limit on the extreme range of the total stresses due to the initial and fabri cation states (see definition

9 below) and the normal operating and accident states of the containment ves sel. The 10-cycle value of Sa (taken from the ASME design fatigue curves) is used. Because this value is in the extreme low-cycle range, this regulatory posi tion is actually a limit on strain rather than stress. Design criteria for bolted closures are not pre sented in this guide. Insufficient information exists, particularly for response to impact loading, to estab lish such criteria.

The following terms are presented with the defini tions used in this guide: 1. Stress intensity means twice the maximum shear stress and is equal to the largest algebraic difference between any two of the three principal stresses.

2. Primarv stress means a stress that is necessary to satisfy the laws of equilibrium of forces and mo ments due to applied loadings, pressure loadings, and body (inertial)

loadings.

Primary stresses are not self-limiting because local yielding and minor distor tions do not reduce the average stress across a solid section.

3. Secondary stress means a stress that is self limiting.

Thermal stresses are considered to be sec ondary stresses since they are strain-controlled rather 2 C. E'. Jaske and W. J. O'Donnell, 'Fatigue Design Criteria for Pressure Vessel Alloys,' ASME Paper 77-PVP-12.

than load-controlled, and these stresses decrease as yielding occurs. The bending stress at a gross structural discon tinuity, such as where a cylindrical shell joins a flat head, is generally self-limiting and is considered to be a secondary stress. However, when the edge mo ment at the shell and head junction is needed to pre vent excessive bending stresses in the head, the stress at the junction is considered a primary stress. The bending stress at a joint between the walls of a rec tangular cross-section shell is considered a primary stress. 4. Primary membrane stress means the average normal primary stresses across the thickness of a solid section. Primary bending stresses are the com ponents of the normal primary stresses that vary linearly across the thickness of a solid section.

5. Alternating stress intensity, Sait, means one half the maximum absolute value of S'2, Sý3, S;,, for all possible stress states i and j where 0-, 0" 2 , and (" 3 are principal stresses and S'12 = (o1i -G" 1 ,) -(0"'i Sý3 = (0r 2 i -92i) -(o`3 1 S'3 1= (0-3i -0-3 i) -(0'H 0-2 i)0-7, etc., follow the principal stresses as their direc tions rotate if the directions of the principal stresses at a point change during the cycle. 6. Stresses caused by stress concentrations means stress increases due to local geometric discontinuities (e.g., notches or local thermal "hot spots"). These stresses produce no noticeable distortions.

7. Type B quantity is defined in §71.4(q) of 10 CFR Part 71. Normal conditions of transport and hypothetical accident conditions are defined in Ap pendices A and B, respectively, to 10 CFR Part 71. 8. Containment vessel means the receptacle on which principal reliance is placed to retain the radioactive material during transport.

9. Fabrication means the assembly of the major components of the casks (i.e., the inner shell, shield ing, outer shell, heads, etc.) but not the construction of the individual components.

Thus, the phrase fab rication stresses includes the stresses caused by inter ference fits and the shrinkage of bonded lead shield ing during solidification but does not include the re sidual stresses due to plate formation, welding, et

c. The prefabrication

2tate is designated as the initial state and is treated as having zero stress. 10. Shakedown means the absence of a continuing cycle of plastic deformation.

A structure shakes down if, after a few cycles of load application, the deforma tion stabilizes and subsequent structural response is elastic.7.6-2 L

C. REGULATORY

POSITION The following design criteria are acceptable to the NRC staff for assessing the adequacy of designs for containment vessels of irradiated fuel shipping casks in meeting the structural requirements in §§7 1.35 and 71.36 of 10 CFR Part 71. References to the ASME Boiler and Pressure Vessel Code indicate the 1977 edition.

I. The values for material properties, design stress intensities (Sm), and design fatigue curves for Class 1 components given in Subsection NA of Section III of the ASME Boiler and Pressure Vessel Code should be used for the materials that meet the ASME specifi cations. For other materials, the method discussed in Article III -2000 of Subsection NA should be used to derive design stress intensity values. ASTM material properties should be used, if available, to derive de sign stress intensity values. The values of material properties that should be used in the structural analy sis are those values that correspond to the appropriate temperatures at loading.

2. Under normal conditions, the value of the stress intensity resulting from the primary membrane stress should be less than the design stress intensity, Si, and the stress intensity resulting from the sum of the primary membrane stresses and the primary bending stresses should be less than 1.5Si. 3. The fatigue analysis for stresses under normal conditions should be performed as follows: a. Sa1t is determined (as defined in the Discus sion). The total stress state at each point in the nor mal operating cycle should be considered so that a maximum range may be determined.

b. The design fatigue curves in Appendix I of Section III of the ASME Boiler and Pressure Vessel Code should be used for cyclic loading less than or equal to 106 cycles. Cornsideration should be given to further reduction in fatigue strength when loading ex ceeds 10' cycles. c. SaIt should be multiplied by the ratio of the modulus of elasticity given on the design fatigue curve to the modulus of elasticity used in the analysis to obtain a value of stress to be used with the design fatigue curves. The corresponding number of cycles taken from the appropriate design fatigue curve is the allowable life if only one type of operational cycle is considered.

If two or more types of stress cycles are considered to produce significant stresses, the rules for cumulative damage given in Article NB-3222.4 of Section III of the ASME Boiler and Pressure Ves sel Code should be applied.

d. Appropriate stress concentration factors for structural discontinuities should be used. A value of 4 should be used in regions where this factor is un known.4. The stress intensity, Sn, associated with the range of primary plus secondary stresses under nor mal conditions should be less than 3 Sm. The calcula tion of this stress intensity is similar to the calcula tion of 2 Salt; however, the effects of local stress con centrations that are considered in the fatigue calcula tions are not included in this stress range. The 3Sm limit given above may be exceeded if the following conditions are met (these conditions can generally be met only in cases where the thermal bending stresses are a substantial portion of the total stress): a. The range of stresses under normal condi tions, excluding stresses due to stress concentrations and thermal bending stresses, yields a stress inten sity, Sn, that is less than 3Sm. b. The value Sa used for entering the design fatigue curve is multiplied by the factor Kg, where: K. = 1.0, for Sn--3Sm =1.0+n(m -)(-m- ), for 3Sm<Sn<3mSn

-, for Sn > 3mSm n Sn is as described in regulatory position 4.a. The values of the material parameters m and n are given for the various classes of materials in the fol lowing table: Low-Alloy Steel Martensitic Stainless Steel Carbon Steel Austenitic Stainless Steel Nickel -Chromium-Iron m 2.0 2.0 3.0 1.7 1.7 n 0.2 0.2 0.2 0.3 0.3 Tmax 'F °C 700 371 700 371 700 371 800 427 800 427 c. The temperatures do not exceed those listed in the above table for the various classes of materials.

d. The ratio of the minimum specified yield strength of the material to the minimum specified ul timate strength is less than 0.8. 5. Buckling of the containment vessel should not occur under normal or accident conditions.

Suitable factors, should be used to account for eccentricities in the design geometry and loading. An elastic-plastic buckling analysis may be used to show that structural instability will not occur; however, the vessel should also meet the specifications for linear elastic analysis given in this guide. 6. Under accident conditions, the value of the stress intensity resulting from the primary membrane stresses should be less'than the lesser value of 2.4Sm and 0.7S, (ultimate strength);

and the stress intensity resulting from the sum of the primary membrane stresses and the primary bending stresses should be less than the lesser value of 3.6Sm and Su.7.6-3

7. The extreme total stress intensity range between the initial state, the fabrication state (see definition

9 in the Discussion), the normal operating conditions, and the accident conditions should be less than twice the adjusted value (adjusted to account for modulus of elasticity at the highest temperature)

of Sa at 10 UNITED STATES NUCLEAR REGULATORY

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WASHINGTON, D. C. 20555 OFFICIAL BUSINESS PENALTY FOR PRIVATE USE, $300 cycles given by the appropriate design fatigue curves. Appropriate stress concentration factors for struc tural discontinuities should be used. A value of 4 should be used in regions where this factor is unknown.

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