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
Download: ML003739501 (7)

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

Section 71.71, "Normal Conditions of Trans port," and Section 71.73, "Hypothetical Accident Conditions," of 10 CFR Part 71, "Packaging and Transportation of Radioactive Material,"

describe normal conditions of transport and hypothetical acci dent conditions that produce thermal and mechanical loads that serve as the structural design bases for the packaging of radioactive material for transport.

Initial conditions must be assumed before analy ses can be performed to evaluate the response of structural systems to prescribed loads. This regulatory guide presents the initial conditions that are consid ered acceptable by the NRC staff for use in the struc tural analysis of Type B packages used to transport radioactive material in the contiguous United States.

Any information collection activities mentioned in this regulatory guide are contained as requirements in 10 CFR Part 7 1, which provides the regulatory ba sis for this guide. The information collection require ments in 10 CFR Part 71 have been cleared under OMB Clearance No. 3150-0008.

B. DISCUSSION

To ensure safe structural behavior of shipping casks used to transport radioactive material, specific load conditions must be established that will encom pass the static, dynamic, and thermal loadings that may be experienced by the casks during transport.

This regulatory guide presents initial conditions that USNRC REGULATORY GUIDES

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 staff In evaluating specific problems or postulated accidents, or to pro vide guidance to applicants. Regulatory Guides are not substitutes for regulations, and compliance with them Is not required. Methods and solutions different from those set out in the guides will be acceptable if they provide a basis for the findings requisite to the issuance or continu ance of a permit or license by the Commission.

This guide was issued after consideration of comments received from the public. Comments and suggestions for improvements in these guides are encouraged at all times, and guides will be revised, as ap propriate, to accommodate comments and to reflect new information or experience.

Written comments may be submitted to the Regulatory Publications Branch, OFIPS, ARM, U. S. Nuclear Regulatory Commission, Washing ton, DC 20555.

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

71.73 of Part 71 to fully delineate thermal and me chanical load combinations for purposes of structural analysis. This guide should be used in conjunction with Regulatory Guide 7.6, "Design Criteria for the Structural Analysis of Shipping Cask Containment Vessels," for the analytical structural evaluation of the heavy (i.e., weighing several tons) casks used to transport irradiated nuclear fuel.

Regulatory Position 1.1 of this guide addresses initial environmental conditions. The external ther mal environmental limits for which a shipping cask must be designed are stated in § 71.71 as being

100'F (38'C) in direct sunlight and -40*F (-40'C)

in shade. The regulations specify that these two nor mal conditions are to be applied separately from the other normal conditions. For the other conditions of

§ 71.71 and for the hypothetical accident conditions, this guide presents a range of ambient temperatures from -20*F (-29°C) to 100'F (38QC) as a part of the initial conditions. In the contiguous United States, there is a 99.7 percent probability that any hourly temperature reading will fall within this range.'

Regulatory Position 1.3 addresses initial pressure conditions. The pressure inside the containment vessels and neutron shields of irradiated fuel shipping casks depends on several factors. These factors include prepressurization of the vessels, the cask I M. B. Gens, The Transportation and Handling Environment, SD-DC-72-1386, Sandia Laboratories, Albuquerque, New Mexico, September 1972.

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temperature distributions associated with the ambient temperatures and the decay heat of the irradiated fuel, and any gas leakage from such fuel.

Regulatory Position 1.5 addresses the possibility that fabrication and installation stresses may result during cask construction. These stresses depend on many different processes and should be considered when evaluating the casks.

Regulatory Position 1.6 states that the values for initial conditions given in this guide are maximums or minimums. However, intermediate values could pos sibly create a more limiting case for some cask de signs. For example, a seal design might be more sus ceptible to leakage at a pressure less than the maxi mum internal pressure, or a local structural response might be greater during an impact test if the weight of the contents were less than the maximum.

Sections 71.71 and 71.73 of Part 71 outline re quirements for packages used to transport Type B

quantities of radioactive materials. However, some of these requirements do not pertain to irradiated fuel shipping casks because of the massiveness of the casks or because the requirements are not structurally significant to cask design. Casks that are designed to transport one or more commercial fuel assemblies weigh many tons because of the large quantities of structural and shielding materials used. This massive ness causes a shipping cask to have a slow thermal response to sudden external temperature changes such as those that might be produced by quenching after a thermal exposure. The NRC staff feels that the water immersion test of § 71.73 and the water spray test of § 71.71 are not significant in the struc tural design of large casks. Therefore, they are not

.discussed in this guide. (Note, however, that these conditions may be significant to criticality and other nonstructural aspects of cask design.)

Similarly, the corner drop and the compression tests of § 71.71 are not discussed because they per tain only to lightweight packages. The penetration test of § 71.71 is not considered by the NRC staff to have structural significance for large shipping casks (except for unprotected valves and rupture disks)

and will not be considered as a general requirement.

C. REGULATORY POSITION

The load conditions given here are considered acceptable to the NRC staff for use in the analytical structural evaluation of shipping casks used to trans port Type B

quantities of radioactive materials.

Table 1 lists the load combinations that should be used. Analyses should combine the initial conditions with both the normal conditions and the hypothetical accident conditions.

1. GENERAL

INITIAL

CONDITIONS

TO

BE USED

FOR

BOTH

NORMAL

AND

HYPOTHETICAL ACCIDENT CONDITIONS

1.1. All initial cask temperature distributions 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 perature at -20'F (-29 °C) with no insolation and of ambient temperature at 100'F (38'C) with maxi mum insolation. Insolation should be in accordance with paragraph 71.7 1 (c) (1). Exceptions to the above are made for the cold environment normal condition (which uses -40'F) and for the thermal accident condition (which considers the higher thermal initial condition but not the lower one).

1.2. The decay heat of the radioactive material should be considered as part of the initial conditions.

Generally, the maximum amount of decay heat should be considered for the hot environment and no decay heat should be considered for the cold envi ronment. These conditions should include the insola tion considerations of Regulatory Position 1. 1. In ad dition, the free-drop and vibration parts of the nor mal conditions and the free-drop and puncture parts of the accident conditions should consider the cases of maximum decay heat with an ambient temperature of 100'F (38'C) and of no decay heat with an ambi ent temperature of -20'F (-29QC).

1. 3. The internal pressure used in evaluating nor mal and hypothetical accident conditions should be consistent with the other initial conditions that are being considered. Minimum internal pressure should be taken as atmospheric or, for designs where inter nal pressures are less than atmospheric, the appropri ate negative value.

1.4. For commercial nuclear power plant fuels, the release of all the pressurized gases inside the irra diated fuel should be considered in determining the maximum resultant containment vessel pressure.

1.5. Fabrication 2 and installation stresses used in evaluating transportation loadings should be consis tent with the joining, forming, fitting, and aligning processes employed during the construction of casks.

Unless subsequent steps are taken to eliminate these stresses, they should be considered in determining the maximum resultant vessel stress.

2Fabrication means the assembly of the major components of the casks (i.e., the inner shell, shielding, outer shell, heads, etc.)

but not the construction of the individual components. Thus, the phrase fabrication stresses includes the stresses caused by in terference fits and the shrinkage of bonded lead shielding dur ing solidification but does not include the residual stresses due to plate formation, welding, etc.

7.8-2 L

Table 1 Summary of Load Combinations for Normal and Hypothetical Accident Conditions of Transport Applicable Initial Condition Ambient Internal Fabrication Temperature Insolation Decay Heat Pressure2 Stresses3

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 shock4:

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.

2See Regulatory Positions 1.3 and 1.4.

3See Regulatory Position 1.5.

4See Regulatory Position 2.5.

5Evaluations 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 crete initial conditions that will produce bounding cases of structural response. Maximum or minimum values of initial conditions are given. However, if a larger structural response is suspected for an initial condition that is not an extreme (e.g., an ambient temperature between -20'F (-29°C) and 100°F

(38°C)), intermediate initial conditions or other combinations of initial conditions should also be con sidered in the structural analysis.

2.

NORMAL CONDITIONS OF TRANSPORT

Each of the following normal conditions of trans port is to be applied separately to determine its effect on the fuel cask. These normal conditions are also to be combined with all the initial conditions as shown in Table 1.

2.1. Hot environment - The cask should be structurally evaluated for an ambient temperature of

100'F (38'C) in still air and with maximum insola tion (see Regulatory Position 1.1). If the cask has mechanically operated auxiliary cooling systems, these systems should be considered to be inoperable during the hot environment condition.

2.2. Cold environment - The cask should be evaluated for an ambient temperature of -40'F

(-40'C) in still air and with no insolation. The case of no internal heat load and minimum internal pres sure should be considered. The possibility and conse quence of coolant freezing should also be considered.

2.3. Increased external pressure should be evaluated for an external

20 psia (140 kilopascal).

2.4. Minimum external pressure should be evaluated for an external

3.5 psia (24.5 kilopascal).

- The cask pressure of

- The cask pressure of

2.5. Vibration and fatigue -

The cask should be evaluated for the shock and vibration environment normally incident to transport. This environment in cludes the vibratory motion produced by small excita tions to the cask-vehicle system and also intermittent shock loads produced by coupling, switching, etc., in rail transport and by bumps, potholes, etc., in truck transport. Repeated pressurization loads and any other loads that may contribute to mechanical fatigue of the cask should be considered.

Factors that may contribute to thermal fatigue should also be considered. These factors should in clude the thermal transients encountered in the load ing and unloading of irradiated nuclear fuel from the cask and any restraint against thermal expansion that may be provided by the tiedown system.

7.8-4 Although there are a number of different load combinations that could occur during normal trans port and for which separate fatigue analyses may be performed, this evaluation should be based on the most unfavorable initial conditions for the specific design consistent with a credible spectrum represent ing the life-cycle for normal shock and vibration envi ronments, Table 1 identifies the cases that the staff believes are the most unfavorable.

2.6. Free drop - The cask (assuming a weight of 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 expected to inflict maximum damage. Impacts with maximum and minimum weights of contents should be considered.

3.

HYPOTHETICAL ACCIDENT CONDITIONS

The following hypothetical accident conditions are to be applied sequentially to the same cask in the order indicated (dropped, then punctured, then ex posed to fire) to determine the maximum cumulative effect. These hypothetical accident conditions are also to be combined with the initial conditions as shown in Table 1.

3.1. Free drop - The cask should be evaluated for a free drop through a distance of 30 feet (9 m)

onto a flat unyielding horizontal surface. It should strike the surface in a position that is expected to inflict maximum damage. Impacts with the maximum and minimum weights of contents should be considered.

In determining which position causes maximum damage, applicants should consider impact orienta tions in which the cask strikes the impact surface on its top end, top corner, side, bottom end, and bottom corner and the center of gravity of the cask is directly over the point of impact. If the design of the cask is such that an intermediate oblique orientation could be more damaging, the applicant should also evaluate the impact of the cask in those orientations. These latter evaluations should include impacting on appur tenances that are part of the cask design such as those used for handling, tiedown, or for other func tions during transport.

3.2. Puncture - The cask should be evaluated for a free drop of 40 inches (1 m) onto a stationary and vertical mild steel bar of 6 inches (0.15 m) di ameter with its top edge rounded to a radius of not more than 0.25 inch (6 mm). The bar should be of such a length as to cause maximum damage to the cask; however, it should not be less than 8 inches

(0.2 m) long. The cask should hit the bar in a posi tion that is expected to inflict maximum damage, and impacts with maximum and minimum weights of con tents should be considered.

3.3. Thermal - The cask should be evaluated for a thermal condition in which the whole cask is

exposed to a radiation environment of 1,4750 F

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

minutes. The surface absorption coefficient of the cask should be considered to be 0.8. The structural response of the cask should be considered up to the time when the temperature distributions reach steady state. The possibility and consequence of the loss of fluid from the neutron shield tank should be considered for casks that use this design feature.

Table I summarizes the loading combinations given above.

D. IMPLEMENTATION

The purpose of this section is to provide informa tion to applicants and licensees regarding the NRC

staff's plans for using this regulatory guide.

Except in those cases in which an applicant or licensee proposes an acceptable alternative method for complying with specified portions of the Commis 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 age designs to be designated as Type B packages for all applications and requests.

7.8-5

REGULATORY ANALYSIS

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

7.8-6

UNITED STATES

NUCLEAR REGULATORY COMMISSION

WASHINGTON, D.C. 20555 FIRST CLASS MAIL

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USNRC

PERMIT No G-67 OFFICIAL BUSINESS

PENALTY FOR PRIVATE USE, $300