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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RG-7.6 Rev 1
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Revision 1 March 1978 U.S. NUCLEAR REGULATORY COMMISSION

REGULATORY GUIDE

OFFICE OF STANDARDS DEVELOPMENT

REGULATORY GUIDE 7.6 DESIGN CRITERIA FOR THE STRUCTURAL ANALYSIS OF

SHIPPING CASK CONTAINMENT VESSELS

A. INTRODUCTION

those given in this guide on a case-by-case basis.

Sections 71.35 and 71.36 of 10 CFR Part 71,

"Packaging of Radioactive Material for Transport Section III of the ASME Boiler and Pressure Code t and Transportation of Radioactive Material Under contains requirements for the design of nuclear power Certain Conditions," require that packages used to plant components. Portions of the Code that use a transport radioactive materials meet the normal and "design-by-analysis" approach for Class 1 compo I ypothetical accident conditions of Appendices A and nents have been adapted in this guide to form accept B, respectively, to Part 71. This guide describes de able design criteria for shipping cask containment sign criteria acceptable to the NRC staff for use in the vessels. The design criteria for normal transport con structural analysis of the containment vessels of Type ditions, as defined in 10 CFR Part 71, are similar to B packages used to transport irradiated nuclear fuel. the criteria for Level A Service Limits (formerly Alternative design criteria may be used if judged ac called "normal conditions") of Section III, and the ceptable by the NRC staff in meeting the structural design criteria for accident conditions are similar to requirements of §§71.35 and 71.36 of 10 CFR Part those for Level D Service Limits (formerly called

71. "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

B. DISCUSSION

cable to fuel cask design.

At present, there are no design standards that can be directly used to evaluate the structural integrity of The criteria in this guide reflect the designs of re the containment vessels of shipping casks for ir cently licensed shipping casks. The containment ves radiated fuels. This guide presents containment ves sels having these designs were made of austenitic sel design criteria that can be used in conjunction stainless steel, which is ductile even at low temper with an analysis which considers the containment atures. Thus, this guide does not consider brittle frac vessel and other principal shells of the cask (e.g., ture. Likewise, creep is not discussed because the outer shell, neutron shield jacket shell) to be linearly temperatures of containment vessels for irradiated elastic. A basic assumption for the use of this guide fuel are characteristically below the creep range, is that the principle of superposition can be applied to even after the hypothetical thermal accident require determine the effect of combined loads on the con ment of 10 CFR Part 71. The nature of the design tainment vessel. However, use of this guide does-not cyclic thermal loads and pressure loads is such that preclude appropriate nonlinear treatment of other thermal ratchetting is not considered a realistic fail cask components (e.g., impact limiters and lead ure mode for cylindrical containment vessels. Con shielding). tainment vessel designs that are significantly differ Design criteria for nonlinear structural analyses are ent from current designs (in shape, material, etc.)

not presented in this guide because of the present lack may necessitate the consideration of the above failure of data sufficient to formulate substantial nonlinear modes.

criteria. The NRC staff will review criteria other than Copies may be obtained from the American Society of

• Lines indicate substantive changes from previous issue. Mechanical Engineers, United Engineering Center, 345 East

47th Street, New York, N.Y. 10017.

USNRC REGULATORY GUIDES Comments should be sent to the Secretary of the Commission, US. Nuclear Re u latory Commission. Washington, DC. 20555, Attention Docketing and Servie

9 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 able if they provide a basis for the fidigs requisite to the issuance or continuance 2. Research and Test Reactors 7. Transportation of a or by the Commiision.

fermnt r 3, Fuels and Materials Facilities 8. Occupational Health ncense 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 than load-controlled, and these stresses decrease as due to gross unrestrained yielding across a solid sec yielding occurs.

tion does not occur. Secondary stresses (i.e., stresses that are self-limiting) are not considered to cause The bending stress at a gross structural discon gross unrestrained yielding but are considered in tinuity, such as where a cylindrical shell joins a flat fatigue and shakedown analyses. head, is generally self-limiting and is considered to be a secondary stress. However, when the edge mo Regulatory position 3 ensures that fatigue failure ment at the shell and head junction is needed to pre does not occur, and regulatory position 4 ensures that vent excessive bending stresses in the head, the stress the structure will shake down to elastic behavior after at the junction is considered a primary stress. The a few cycles. Both of these positions address only the bending stress at a joint between the walls of a rec stress range of normal operation. Recent studies 2 tangular cross-section shell is considered a primary have shown that fatigue strength decreases beyond stress.

10' cycles for certain material

s. Regulatory position

3.b addresses the possibility of fatigue strength re 4. Primary membrane stress means the average duction beyond 10' cycles. normal primary stresses across the thickness of a solid section. Primary bending stresses are the com Regulatory position 5 states that buckling of the ponents of the normal primary stresses that vary containment vessel should not occur. While it is rec linearly across the thickness of a solid section.

ognized that local or gross buckling of the contain ment vessel could occur without failure (i.e., leak 5. Alternating stress intensity, Sait, means one age), the stress and strain limits given in this guide half the maximum absolute value of S'2, Sý3, S;,, for are based on linear elastic analysis and are inappro all possible stress states i and j where 0-, 0"2 , and ("3 priate for determining the integrity of a postbuckled are principal stresses and vessel. If the analysis of a containment vessel indi cates the likelihood of structural instability, the de S'12 = (o1i - G"1,) - (0"'i 0-2 i)

sign criteria of this guide should not be used. Sý3 = (0r 2 i - 92i) - (o` 3 1 S'31 = (0-3i - 0-3 i) - (0'H

Regulatory position 7 places a limit on the extreme range of the total stresses due to the initial and fabri

0-7, etc., follow the principal stresses as their direc cation states (see definition 9 below) and the normal tions rotate if the directions of the principal stresses operating and accident states of the containment ves at a point change during the cycle.

sel. The 10-cycle value of Sa (taken from the ASME

design fatigue curves) is used. Because this value is 6. Stresses caused by stress concentrations means in the extreme low-cycle range, this regulatory posi stress increases due to local geometric discontinuities tion is actually a limit on strain rather than stress. (e.g., notches or local thermal "hot spots"). These stresses produce no noticeable distortions.

Design criteria for bolted closures are not pre sented in this guide. Insufficient information exists, 7. Type B quantity is defined in §71.4(q) of 10

particularly for response to impact loading, to estab CFR Part 71. Normal conditions of transport and lish such criteria. hypothetical accident conditions are defined in Ap pendices A and B, respectively, to 10 CFR Part 71.

The following terms are presented with the defini tions used in this guide: 8. Containment vessel means the receptacle on which principal reliance is placed to retain the

1. Stress intensity means twice the maximum shear radioactive material during transport.

stress and is equal to the largest algebraic difference between any two of the three principal stresses. 9. Fabrication means the assembly of the major components of the casks (i.e., the inner shell, shield

2. Primarv stress means a stress that is necessary ing, outer shell, heads, etc.) but not the construction to satisfy the laws of equilibrium of forces and mo of the individual components. Thus, the phrase fab ments due to applied loadings, pressure loadings, and ricationstresses includes the stresses caused by inter body (inertial) loadings. Primary stresses are not ference fits and the shrinkage of bonded lead shield self-limiting because local yielding and minor distor ing during solidification but does not include the re tions do not reduce the average stress across a solid sidual stresses due to plate formation, welding, etc.

section.

The prefabrication 2tate is designated as the initial

3. Secondary stress means a stress that is self state and is treated as having zero stress.

limiting. Thermal stresses are considered to be sec

10. Shakedown means the absence of a continuing ondary stresses since they are strain-controlled rather cycle of plastic deformation. A structure shakes down if, after a few cycles of load application, the deforma

2 C. E'. Jaske and W. J. O'Donnell, 'Fatigue Design Criteria for tion stabilizes and subsequent structural response is Pressure Vessel Alloys,' ASME Paper 77-PVP-12. elastic.

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7.6-2

C. REGULATORY POSITION

4. The stress intensity, Sn, associated with the range of primary plus secondary stresses under nor The following design criteria are acceptable to the mal conditions should be less than 3 Sm. The calcula NRC staff for assessing the adequacy of designs for tion of this stress intensity is similar to the calcula containment vessels of irradiated fuel shipping casks tion of 2 Salt; however, the effects of local stress con in meeting the structural requirements in §§7 1.35 and centrations that are considered in the fatigue calcula

71.36 of 10 CFR Part 71. References to the ASME tions are not included in this stress range.

Boiler and Pressure Vessel Code indicate the 1977 edition. The 3Sm limit given above may be exceeded if the following conditions are met (these conditions can I. The values for material properties, design stress generally be met only in cases where the thermal intensities (Sm), and design fatigue curves for Class 1 bending stresses are a substantial portion of the total components given in Subsection NA of Section III stress):

of the ASME Boiler and Pressure Vessel Code should a. The range of stresses under normal condi be used for the materials that meet the ASME specifi tions, excluding stresses due to stress concentrations cations. For other materials, the method discussed in and thermal bending stresses, yields a stress inten Article III -2000 of Subsection NA should be used to sity, Sn, that is less than 3Sm.

derive design stress intensity values. ASTM material properties should be used, if available, to derive de b. The value Sa used for entering the design sign stress intensity values. The values of material fatigue curve is multiplied by the factor Kg, where:

properties that should be used in the structural analy K. = 1.0, for Sn--3Sm sis are those values that correspond to the appropriate

=1.0+n(m -)(-m- ), for 3Sm<Sn<3mSn temperatures at loading.

- , for Sn > 3mSm

2. Under normal conditions, the value of the stress n intensity resulting from the primary membrane stress should be less than the design stress intensity, Si, Sn is as described in regulatory position 4.a.

and the stress intensity resulting from the sum of the The values of the material parameters m and n are primary membrane stresses and the primary bending given for the various classes of materials in the fol stresses should be less than 1.5Si. lowing table:

Tmax

3. The fatigue analysis for stresses under normal m n 'F °C

conditions should be performed as follows: Low-Alloy Steel 2.0 0.2 700 371 a. Sa1t is determined (as defined in the Discus Martensitic Stainless Steel 2.0 0.2 700 371 sion). The total stress state at each point in the nor Carbon Steel 3.0 0.2 700 371 mal operating cycle should be considered so that a Austenitic Stainless Steel 1.7 0.3 800 427 maximum range may be determined. Nickel -Chromium-Iron 1.7 0.3 800 427 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 c. The temperatures do not exceed those listed equal to 106 cycles. Cornsideration should be given to in the above table for the various classes of materials.

further reduction in fatigue strength when loading ex d. The ratio of the minimum specified yield ceeds 10' cycles. strength of the material to the minimum specified ul timate strength is less than 0.8.

c. SaIt should be multiplied by the ratio of the modulus of elasticity given on the design fatigue 5. Buckling of the containment vessel should not curve to the modulus of elasticity used in the analysis occur under normal or accident conditions. Suitable to obtain a value of stress to be used with the design factors, should be used to account for eccentricities in fatigue curves. The corresponding number of cycles the design geometry and loading. An elastic-plastic taken from the appropriate design fatigue curve is the buckling analysis may be used to show that structural allowable life if only one type of operational cycle is instability will not occur; however, the vessel should considered. If two or more types of stress cycles are also meet the specifications for linear elastic analysis considered to produce significant stresses, the rules given in this guide.

for cumulative damage given in Article NB-3222.4 of Section III of the ASME Boiler and Pressure Ves 6. Under accident conditions, the value of the sel Code should be applied. stress intensity resulting from the primary membrane stresses should be less'than the lesser value of 2 .4Sm d. Appropriate stress concentration factors for and 0.7S, (ultimate strength); and the stress intensity structural discontinuities should be used. A value of 4 resulting from the sum of the primary membrane should be used in regions where this factor is un stresses and the primary bending stresses should be known. less than the lesser value of 3 .6Sm and Su.

7.6-3

7. The extreme total stress intensity range between cycles given by the appropriate design fatigue curves.

the initial state, the fabrication state (see definition 9 in the Discussion), the normal operating conditions, Appropriate stress concentration factors for struc and the accident conditions should be less than twice tural discontinuities should be used. A value of 4 the adjusted value (adjusted to account for modulus of elasticity at the highest temperature) of Sa at 10

should be used in regions where this factor is unknown.

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