Regulatory Guide 7.8

Load Combinations for the Structural Analysis of Shipping Casks

ML13350A233
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Issue date:	05/31/1977
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RG-7.008
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U.S. NUCLEAR REGULATORY COMMISSION May 1977 REGULATORY GUIDE

OFFICE OF STANDARDS DEVELOPMENT

REGULATORY GUIDE 7.8 LOAD COMBINATIONS FOR THE STRUCTURAL ANALYSIS OF

SHIPPING CASKS

A. INTRODUCTION

Regulatory Position C.I.a of this guide mentions environmental initial conditions. The external ther- Appendix A, "Normal Conditions of Transport," mal environmental limits for which a shipping cask and Appendix B, "Hypothetical Accident Con- must be designed are stated in Appendix A of 10

ditions," of 10 CFR Part 71, "Packaging of Radioac- CFR Part 71 as being 130°F (54 0 C) in direct sunlight tive Material for Transport and Transportation of and -40°F (-40'C) in shade. These limits are applied Radioactive Material Under Certain Conditions." without an)y additional loading. For the other condi- describe normal conditions of transport and tions of Appendix A and for tht hypothetical acci- hypothetical accident conditions that produce ther- dent conditions, this guide:presents a range of am- mal and mechanical loads that serve as the structural bient temperatures. from -20°F (-29 0 C) to 1000F

design bases for the packaging of radioactive material (38'C) as a part of the initial conditions. In the con- for transport. tiguous United States.there Is a 99.7 percent probability that, any hourly temperature reading will However, initial conditions must be assumed fall within this range (Ref. l).The insolation data before analyses can be performed to evaluate the provided in'.the International Atomic Energy response of structural systems to prescribed loads. Agency's safet)"standards (Ref. 2) have been adopted This regulatory guide presents a set of initial condi- for this guide.because they have sufficient conser- tions that is considered acceptable by the NRC staff vatism When. compared with other solar radiation for use in the structural analyses of type B packages data (RCf. 3).

used to transport irradiated nuclear fuel in the con- tiguous United States. The values in this set supple-..: ac- .- Regulatory Position C. I.c mentions initial pressure ment the normal conditions and the hypothetical c.,,onditions. It should be noted that the pressure inside cident conditions of the regulations in forming-r.a.. ."the containment vessels and neutron shields of ir- more complete basis from which structural intigrity. radiated fuel shipping casks depends on several fac- may be assessed. tors. These factors include pre-pressurization of the vessels, the cask temperature distributions associated

B. DISCUSSION

with the ambient temperatures and the decay heat of the fuel rods, and any gas leakage from the nuclear To ensure safe structural behavior of shipping fuel rods.

casks used to transport irradiated nuclear fuel, specific load conditions must be established that will Regulatory Position C. .e states that the values for encompass the static, dynani-ic, and thermal loadings initial conditions given in this guide are maximums or that may be experiencl.d,by.:the.casks during trans- minimums. However, intermediate values could pos- port, This regulatory.guide presents initial conditions sibly create a more limiting case for some cask that can be used ididddition to parts of Appendices A designs. For example, a seal design might be more and B of 10 CER Par "ý7.1 to fully delineate thermal susceptible to leakage at a pressure less than the max- and mechanicalibad combinations for purposes of imum internal pressure, or a local structural response structural.'analysis'7.4tis intended that this guide be might be greater during an impact test if the weight of used in conjunction with Regulatory Guide 7.6, the contents was less than the maximum.

"Stress All0owiobles for the Design of Shipping Cask Containment Vessels," for the analytic structural Appendices A and B of 10 CFR Part 71 outline re- evaluation of the heavy (i.e., several tons in weight) quirements for packages used to transport type B

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quirements do not pertain to irradiated fuel shipping c. The internal pressure used in evaluating normal casks, however, because of the heaviness of the casks and hypothetical accident conditions should be con- or because the requirements are not structurally sistent with the other initial conditions that are being significant to cask design. Casks that are designed to considered.

transport one or. more commercia' fuel assemblies weigh many tons because of the large quantities of d. The release of all of the pressurized gases inside structural and shielding materials used. This mas- the fuel assemblies should be considered in determin- siveness causes a shipping cask to have a slow ther- ing, the maximum containment vessel pressure.

-Mal response to sudden external temperature changes such as those that might be produced by quenching e. It is the intent of this guide to specify discrete in- after a thermal exposure. The NRC staff feels that the itial conditions that will serve as bounding cases for water immersion test of Appendix B and the water structural response. Maximum or minimum values of spray test of Appendix A are not significant in the initial conditions are given. However, if a larger structural design of large casks. Therefore, they are structural response is suspected for an initial condi- not discussed in this guide. (Note, however, that these tion that is not an extreme (e.g., an ambient conditions may be significant to criticality and other temperature between -19 0 F (-28 0 C) and 990 F

nonstructural aspects of cask design.) (370 C)), intermediate initial conditions should also be considered in the structural analysis.

Similarly, the corner drop and the compression tests of Appendix A are not discussed because they 2. Normal Conditions of Transport pertain only to lightweight packages. The penetration test of Appendix A is not considered by the NRC Each of the following normal conditions of trans- staff to have structural significance for large shipping port is to be applied separately to determine its effect casks (except for unprotected valves and rupture on the fuel cask.

disks) and will not be considered as a general re- quirement. a. Hot environment-The cask should be struc- turally evaluated for an ambient temperature of

C. REGULATORY POSITION

130°F (54°C) in still air and with maximum insola- tion (see Table I). If the cask has auxiliary cooling The load conditions given here are considered ac- systems for the containment or neutron shield fluids, ceptable to the NRC staff for use in the analytical these systems should be considered to be inoperable structural evaluation of shipping casks used to trans,- during the hot environment condition.

port type B quantities of irradiated nuclear fuels.

b. Cold environment-The cask should be I. General Initial Conditions To Be Used for Both evaluated for an ambient temperature of -401F

Normal and Hypothetical Accident Conditions (- 40°C) in still air and with no insolation. The case of maximum fuel heat load and maximum internal pres- a. All initial cask temperature distributions should sure should be considered in addition to the case of be considered to be at steady state. The normal and no internal heat load. The possibility and conse- hypothetical accident conditions should be con- quence of coolant freezing should also be considered.

sidered to have initial conditions of ambient temperature at -20°F (-29°C) with no insolation and c. Minimum externalpressure-Thecask should be of ambient temperature at 100 0 F (38 0 C) with the evaluated for an atmopheric pressure 0.5 times the maximum insolation data given in Table 1. Excep- standard atmospheric pressure.

tions to the above are made for the hot environment and cold environment normal conditions (which use d. Vibration and fatigue-The cask should be other steady state values) and for the thermal acci- evaluated for the shock and vibration environment dent condition (which considers the higher thermal normally incident to transport, This environment in- initial condition but not the lower one). cludes the quasi-steady vibratory motion produced by small excitations to the cask-vehicle system and b. The decay heat of the irradiated fuel should also intermittent shock loads produced by coupling, be considered as part of the initial conditions. switching, etc., in rail transport and by bumps, Generally, the maximum amount of decay heat potholes, etc., in truck transport. Repeated pres- should be considered in combination with the ther- surization loads and any other loads that may con- mal environmental conditions of Regulatory Position tribute to mechanical fatigue of the cask should be C.l.a. In addition, the free-drop and vibration parts considered.

of the normal conditions and the free-drop and punc- ture parts of the accident conditions should consider Factors that may contribute to thermal fatigue the case of no decay heat and the cask at -20OF should also be considered. These factors should in-

(-29'C). These initial thermal conditions are sum- clude the thermal transients encountered in the marized in Tahle 2. loading and unloading of irradiated nuclear fuel.

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• e. Free drop-The cask should be evaluated for a length as to cause maximum damage to the cask. The one-foot free drop onto a flat unyielding surface. The cask should contain the maximum weight of contents cask should contain the maximum weight of contents and should hit the bar in a position that is expected to and should strike the impact surface in a position that inflict maximum damage.

is expected to inflict maximum damage. *c. Thennal-The cask should be evaluated for a

3. Hypothetical Accident Conditions thermal condition in which the whole cask is exposed to a radiation environment of 1.475OF (800 0 C) with The following hypothetical accident conditions are an emissivity coefficient of 0.9 for 30 minutes. The to be applied sequentially in the order indicated to surface absorption coefficient of the cask should be determine the maximum cumulative effect. considered to be 0.8. The structural response of the cask should be considered up to the time when the a. Free drop-The cask should be evaluated for a temperature distributions reach steady state. The free drop through a distance of 30 feet (9 meters) possibility and consequence of the loss of fluid from onto a flat unyielding horizontal surface. It should the neutron shield tank should be considered for strike the surface in a position that is expected to in- casks that use this design feature.

flict maximum damage and should contain the max- imum weight of contents. Table 2 summarizes the loading combinations given above.

In determining which position causes maximum damage, the staff currently requests evaluations of

D. IMPLEMENTATION

drop orientations where the top and bottom ends, the top and bottom corners, and the sides are the cask The purpose of this section is to provide informa- impact areas. The center of gravity is usually con- tion to applicants and licensees regarding the NRC

sidered to be directly above these impact areas. staffs plans for using this regulatory guide.

However, evaluations of oblique drop orientations are requested when appropriate. This regulatory guide will be used by the staff after January 1, 1978, in assessing the structural adequacy b. Puncture-The cask should be evaluated for a of designs for irradiated fuel shipping casks with free drop of 40 inches (I meter) onto a stationary and respect to the requirements in 10 CFR Part 71, vertical mild steel bar of 6 inches (15 cm) diameter §§71.35 and 71.36. Alternative methods that satisfy with its top edge rounded to a radius of not more the requirements in the Commission's regulations than 0.25 inch (6mm). The bar should be of such a will also be considered acceptable.

REFERENCES

I. M.B. Gens, The Transportationand Handling En- Series No. 6, Regulations for the Safe Transport of vironment, SC-DC-72-1386, Sandia Laboratories, Radioactive Materials, 1973.

Albuquerque, New Mexico, Sept. 1972.

3. Department of the Army, Research, Development.

Test, and Evaluation of Materialfor Extreme Climatic

2. International Atomic Energy Agency, Safety Conditions, AR 70-38, May 1969.

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TABLE 1 MAXIMUM INSOLATION DATA

Form and location Insolation for 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> of surface per day Flat surfaces transported horizontally:

Base None Other surfaces 800gcal/cm 2 (2,950 Btu/ft 2 )

Flat surfaces not transported horizontally:

Each surface 200 gcal/cm 2 (737 Btu/ft 2 )*

Curved surfaces 400 gcal/cm 2 (1,475 Btu/ft' 2 )*

',Alternati'ely, a sine function may be used. adopting an absorptioncoefficientand neglecting the effects of possible reflection from neighboring objects.

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TABLE 2 SUMMARY OF LOAD COMBINATIONS FOR

NORMAL AND HYPOTHETICAL ACCIDENT CONDITIONS OF TRANSPORT

Applicable initial condition Ambient Insolation Normal or accident temperature Decay heat U

condition a. 0

I-

U

Y I 4 1

LL.

a

0

r'j C0 C0

Normal conditions -I _ i _ i Hot environment -

130°F ambient temp. X x x Cold environment - x x x

-40*F ambient temp. x x x Minimum external pressure - x x x x

0.5 atm. x x x x Vibration & shockt - x x x x Normally incident to x x x x the mode of transport x x x x Freedrop - x x x x x I foot drop x x x x x x x x x x Accident conditions x x X x X

Free drop - 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

__

__ __ __ __ __ _ _ _ K _ x _ _ x x x Thermaitt -

Fire accident

'See Table I.

"*See Regulatory Position C.I.c and C.I.d.

tSce Regulatory Position C.2.d for "Vibration and fatigue."

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

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