Regulatory Guide 7.8

(Task Ms 527-4 and Ms 804-4), Load Combinations for Structural Analysis of Shipping Casks for Radioactive Material

ML003739501
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Issue date:	03/31/1989
From:	Office of Nuclear Regulatory Research
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References
MS 527, MS 804-4, Reg Guide 7.8, Rev 1
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U.S. NUCLEAR REGULATORY COMMISSION Revision 1 March 1989 REGULATORY GUIDE

OFFICE OF NUCLEAR REGULATORY RESEARCH

REGULATORY GUIDE 7.8 (Tasks MS 527-4 and MS 804-4)

LOAD COMBINATIONS FOR THE STRUCTURAL ANALYSIS OF

SHIPPING CASKS FOR RADIOACTIVE MATERIAL

A. INTRODUCTION

can be used in addition to parts of §§ 71.71 and

71.73 of Part 71 to fully delineate thermal and me Section 71.71, "Normal Conditions of Trans chanical load combinations for purposes of structural port," and Section 71.73, "Hypothetical Accident analysis. This guide should be used in conjunction Conditions," of 10 CFR Part 71, "Packaging and with Regulatory Guide 7.6, "Design Criteria for the Transportation of Radioactive Material," describe Structural Analysis of Shipping Cask Containment normal conditions of transport and hypothetical acci Vessels," for the analytical structural evaluation of dent conditions that produce thermal and mechanical the heavy (i.e., weighing several tons) casks used to loads that serve as the structural design bases for the transport irradiated nuclear fuel.

packaging of radioactive material for transport.

Regulatory Position 1.1 of this guide addresses Initial conditions must be assumed before analy initial environmental conditions. The external ther ses can be performed to evaluate the response of mal environmental limits for which a shipping cask structural systems to prescribed loads. This regulatory must be designed are stated in § 71.71 as being guide presents the initial conditions that are consid 100'F (38'C) in direct sunlight and -40*F (-40'C)

ered acceptable by the NRC staff for use in the struc in shade. The regulations specify that these two nor tural analysis of Type B packages used to transport mal conditions are to be applied separately from the radioactive material in the contiguous United States. other normal conditions. For the other conditions of Any information collection activities mentioned § 71.71 and for the hypothetical accident conditions, in this regulatory guide are contained as requirements this guide presents a range of ambient temperatures in 10 CFR Part 7 1, which provides the regulatory ba from -20*F (-29°C) to 100'F (38QC) as a part of sis for this guide. The information collection require the initial conditions. In the contiguous United States, ments in 10 CFR Part 71 have been cleared under there is a 99.7 percent probability that any hourly OMB Clearance No. 3150-0008. temperature reading will fall within this range.'

Regulatory Position 1.3 addresses initial pressure

B. DISCUSSION

conditions. The pressure inside the containment vessels and neutron shields of irradiated fuel shipping To ensure safe structural behavior of shipping casks depends on several factors. These factors casks used to transport radioactive material, specific include prepressurization of the vessels, the cask load conditions must be established that will encom pass the static, dynamic, and thermal loadings that IM. B. Gens, The Transportation and Handling Environment, may be experienced by the casks during transport. SD-DC-72-1386, Sandia Laboratories, Albuquerque, New Mexico, This regulatory guide presents initial conditions that September 1972.

USNRC REGULATORY GUIDES The guides are Issued in the following ten broad divisions:

Regulatory Guides are issued to describe and make available to the pub lic methods acceptable to the NRC staff of Implementing specific parts of the Commission s regulations, to delineate techniques used by the 1. Power Reactors 6. Products staff In evaluating specific problems or postulated accidents, or to pro 2. Research and Test Reactors 7. Transportation vide guidance to applicants. Regulatory Guides are not substitutes for 3. Fuels and Materials Facilities 8. Occupational Health regulations, and compliance with them Is not required. Methods and 4. Environmental and Siting 9. Antitrust and Financial Review solutions different from those set out in the guides will be acceptable if 5. Materials and Plant Protection 10. General they provide a basis for the findings requisite to the issuance or continu ance of a permit or license by the Commission.

Copies of issued guides may be purchased from the Government Printing This guide was issued after consideration of comments received from Office at the current GPO price. Information on current GPO prices may the public. Comments and suggestions for improvements in these be obtained by contacting the Superintendent of Documents, U.S.

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propriate, to accommodate comments and to reflect new information or 20013-7082, telephone (202)275-2060 or (202)275-2171.

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1. GENERAL INITIAL CONDITIONS TO

temperature distributions associated with the ambient BE USED FOR BOTH NORMAL AND

temperatures and the decay heat of the irradiated HYPOTHETICAL ACCIDENT CONDITIONS

fuel, and any gas leakage from such fuel.

1.1. All initial cask temperature distributions Regulatory Position 1.5 addresses the possibility that fabrication and installation stresses may result during cask construction. These stresses depend on should be considered to be at a steady state. The nor mal and hypothetical accident conditions should be considered to have initial conditions of ambient tem L

many different processes and should be considered when evaluating the casks. perature at -20'F (-29 °C) with no insolation and of ambient temperature at 100'F (38'C) with maxi Regulatory Position 1.6 states that the values for mum insolation. Insolation should be in accordance initial conditions given in this guide are maximums or with paragraph 71.7 1 (c) (1). Exceptions to the above minimums. However, intermediate values could pos are made for the cold environment normal condition sibly create a more limiting case for some cask de (which uses -40'F) and for the thermal accident signs. For example, a seal design might be more sus condition (which considers the higher thermal initial ceptible to leakage at a pressure less than the maxi condition but not the lower one).

mum internal pressure, or a local structural response

1.2. The decay heat of the radioactive material might be greater during an impact test if the weight of should be considered as part of the initial conditions.

the contents were less than the maximum.

Generally, the maximum amount of decay heat Sections 71.71 and 71.73 of Part 71 outline re should be considered for the hot environment and no quirements for packages used to transport Type B decay heat should be considered for the cold envi quantities of radioactive materials. However, some of ronment. These conditions should include the insola these requirements do not pertain to irradiated fuel tion considerations of Regulatory Position 1. 1. In ad shipping casks because of the massiveness of the dition, the free-drop and vibration parts of the nor casks or because the requirements are not structurally mal conditions and the free-drop and puncture parts significant to cask design. Casks that are designed to of the accident conditions should consider the cases transport one or more commercial fuel assemblies of maximum decay heat with an ambient temperature weigh many tons because of the large quantities of of 100'F (38'C) and of no decay heat with an ambi structural and shielding materials used. This massive ent temperature of -20'F (-29QC).

ness causes a shipping cask to have a slow thermal

1. 3. The internal pressure used in evaluating nor response to sudden external temperature changes mal and hypothetical accident conditions should be such as those that might be produced by quenching consistent with the other initial conditions that are after a thermal exposure. The NRC staff feels that being considered. Minimum internal pressure should the water immersion test of § 71.73 and the water be taken as atmospheric or, for designs where inter spray test of § 71.71 are not significant in the struc nal pressures are less than atmospheric, the appropri tural design of large casks. Therefore, they are not ate negative value.

.discussed in this guide. (Note, however, that these conditions may be significant to criticality and other 1.4. For commercial nuclear power plant fuels, nonstructural aspects of cask design.) the release of all the pressurized gases inside the irra diated fuel should be considered in determining the Similarly, the corner drop and the compression maximum resultant containment vessel pressure.

tests of § 71.71 are not discussed because they per tain only to lightweight packages. The penetration 1.5. Fabrication 2 and installation stresses used in test of § 71.71 is not considered by the NRC staff to evaluating transportation loadings should be consis have structural significance for large shipping casks tent with the joining, forming, fitting, and aligning (except for unprotected valves and rupture disks) processes employed during the construction of casks.

and will not be considered as a general requirement. Unless subsequent steps are taken to eliminate these stresses, they should be considered in determining

C. REGULATORY POSITION

the maximum resultant vessel stress.

The load conditions given here are considered acceptable to the NRC staff for use in the analytical 2 Fabricationmeans the assembly of the major components of structural evaluation of shipping casks used to trans the casks (i.e., the inner shell, shielding, outer shell, heads, etc.)

port Type B quantities of radioactive materials. but not the construction of the individual components. Thus, Table 1 lists the load combinations that should be the phrase fabricationstresses includes the stresses caused by in used. Analyses should combine the initial conditions terference fits and the shrinkage of bonded lead shielding dur ing solidification but does not include the residual stresses due with both the normal conditions and the hypothetical to plate formation, welding, etc.

accident conditions.

7.8-2

Table 1 Summary of Load Combinations for Normal and Hypothetical Accident Conditions of Transport Applicable Initial Condition Ambient Internal Fabrication Temperature Insolation Decay Heat Pressure 2 Stresses 3

100°F -20'F Max.1 Zero Max. Zero Max. Min.

NORMAL CONDITIONS

(Analyze Separately)

Hot environment:

100°F ambient temp. X X X X

Cold environment:

-40'F ambient temp.

X X X X

Increased external pressure: 20 psia X X X X X

Minimum external pressure: 3.5 psia X X X X X

Vibration and shock 4 : X X X X X

normally incident to the mode of transport X X X X X

Free drop: X X X X X

1-foot drop X X X X X

ACCIDENT CONDITIONS

(Apply sequentially)

Free drop: X X X X X

30-foot drop X X X X X

Puncture: X X X X X

drop onto bar X X X X X

Therma l:

fire accident X X X X X

'See Regulatory Position 1.1.

2 See Regulatory Positions 1.3 and 1.4.

3

4 See Regulatory Position 1.5.

5 See Regulatory Position 2.5.

Evaluations should be made 30 minutes after start of fire and at postfire steady-state conditions.

7.8-3

1,6. It is the intent of this guide to specify dis Although there are a number of different load crete initial conditions that will produce bounding combinations that could occur during normal trans cases of structural response. Maximum or minimum port and for which separate fatigue analyses may be values of initial conditions are given. However, if a performed, this evaluation should be based on the larger structural response is suspected for an initial most unfavorable initial conditions for the specific condition that is not an extreme (e.g., an ambient design consistent with a credible spectrum represent temperature between -20'F (-29°C) and 100°F ing the life-cycle for normal shock and vibration envi

(38°C)), intermediate initial conditions or other ronments, Table 1 identifies the cases that the staff combinations of initial conditions should also be con believes are the most unfavorable.

sidered in the structural analysis. 2.6. Free drop - The cask (assuming a weight of

2. NORMAL CONDITIONS OF TRANSPORT over 30,000 pounds (13,600 kg)) should be evalu ated for a 1-foot free drop onto a flat unyielding sur face; it should strike the surface in a position that is Each of the following normal conditions of trans expected to inflict maximum damage. Impacts with port is to be applied separately to determine its effect maximum and minimum weights of contents should on the fuel cask. These normal conditions are also to be considered.

be combined with all the initial conditions as shown 3. HYPOTHETICAL ACCIDENT CONDITIONS

in Table 1.

The following hypothetical accident conditions

2.1. Hot environment - The cask should be are to be applied sequentially to the same cask in the structurally evaluated for an ambient temperature of order indicated (dropped, then punctured, then ex

100'F (38'C) in still air and with maximum insola posed to fire) to determine the maximum cumulative tion (see Regulatory Position 1.1). If the cask has effect. These hypothetical accident conditions are mechanically operated auxiliary cooling systems, also to be combined with the initial conditions as these systems should be considered to be inoperable shown in Table 1.

during the hot environment condition.

3.1. Free drop - The cask should be evaluated

2.2. Cold environment - The cask should be for a free drop through a distance of 30 feet (9 m)

evaluated for an ambient temperature of -40'F onto a flat unyielding horizontal surfac

e. It should

(-40'C) in still air and with no insolation. The case strike the surface in a position that is expected to of no internal heat load and minimum internal pres inflict maximum damage. Impacts with the maximum sure should be considered. The possibility and conse and minimum weights of contents should be quence of coolant freezing should also be considered. considered.

In determining which position causes maximum damage, applicants should consider impact orienta

2.3. Increased external pressure - The cask tions in which the cask strikes the impact surface on should be evaluated for an external pressure of its top end, top corner, side, bottom end, and bottom

20 psia (140 kilopascal).

corner and the center of gravity of the cask is directly

2.4. Minimum external pressure - The cask over the point of impact. If the design of the cask is should be evaluated for an external pressure of such that an intermediate oblique orientation could

3.5 psia (24.5 kilopascal). be more damaging, the applicant should also evaluate the impact of the cask in those orientations. These latter evaluations should include impacting on appur

2.5. Vibration and fatigue - The cask should tenances that are part of the cask design such as be evaluated for the shock and vibration environment those used for handling, tiedown, or for other func normally incident to transport. This environment in tions during transport.

cludes the vibratory motion produced by small excita 3.2. Puncture - The cask should be evaluated tions to the cask-vehicle system and also intermittent for a free drop of 40 inches (1 m) onto a stationary shock loads produced by coupling, switching, etc., in and vertical mild steel bar of 6 inches (0.15 m) di rail transport and by bumps, potholes, etc., in truck ameter with its top edge rounded to a radius of not transport. Repeated pressurization loads and any more than 0.25 inch (6 mm). The bar should be of other loads that may contribute to mechanical fatigue such a length as to cause maximum damage to the of the cask should be considered. cask; however, it should not be less than 8 inches

(0.2 m) long. The cask should hit the bar in a posi Factors that may contribute to thermal fatigue tion that is expected to inflict maximum damage, and should also be considered. These factors should in impacts with maximum and minimum weights of con clude the thermal transients encountered in the load tents should be considered.

ing and unloading of irradiated nuclear fuel from the cask and any restraint against thermal expansion that 3.3. Thermal - The cask should be evaluated may be provided by the tiedown system. for a thermal condition in which the whole cask is

7.8-4

exposed to a radiation environment of 1,475 0 F

D. IMPLEMENTATION

(800-C) with an emissivity coefficient of 0.9 for 30

minutes. The surface absorption coefficient of the The purpose of this section is to provide informa cask should be considered to be 0.8. The structural tion to applicants and licensees regarding the NRC

response of the cask should be considered up to the staff's plans for using this regulatory guide.

time when the temperature distributions reach steady Except in those cases in which an applicant or state. The possibility and consequence of the loss of licensee proposes an acceptable alternative method fluid from the neutron shield tank should be for complying with specified portions of the Commis considered for casks that use this design feature. sion's regulations, the NRC staff will use the methods described in this guide in evaluating applications for new package designs and requests for existing pack Table I summarizes the loading combinations age designs to be designated as Type B packages for given above. all applications and requests.

7.8-5

REGULATORY ANALYSIS

A draft regulatory analysis was published with the the final guide has not been prepared. A copy of the second proposed Revision 1 to Regulatory Guide 7.8 draft regulatory analysis is available for inspection (Task MS 804-4) when the draft guide was published and copying for a fee at the Commission's Public for public comment in September 1988. No changes Document Room at 2120 L Street NW., Washington, were necessary, so a separate regulatory analysis for DC, under Task MS 804-4.

7.8-6

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