Regulatory Guide 7.8

Load Combinations for the Structural Analysis of Shipping Casks

ML13064A084
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Issue date:	05/31/1977
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RG-7.008
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U.rS. NUCLEAR REGULATORY

COMMISSION

May 1977 REG ULATOR Y G U I DIE OFFICE OF STANDARDS

DEVELOPMENT

REGULATORY

GUIDE 7.8 LOAD COMBINATIONS

FOR THE STRUCTURAL

ANALYSIS OF SHIPPING CASKS

A. INTRODUCTION

Appendix A, "Normal Conditions of Transport," and Appendix B, "Hypothetical Accident Con-ditions," of 10 CFR Part 71, "Packaging of Radioac-tive Material for Transport and Transportation of Radioactive Material Under Certain Conditions," describe normal conditions of transport and hypothetical accident conditions that produce ther-mal and mechanical loads that serve as the structural design bases for the packaging of radioactive material for transport.

Regulatory Position C.l.a of this guide mentions environmental initial conditions.

The external ther-mal environmental limits for which a shipping cask must be designed are stated in Appendix A of 10 CFR Part 71 as being 130'F (54 0 C) in direct sunlight and -40'F (-40'C) in shade. ThescJimits are applied without any additional loadingn#or the other condi-tions of Appendix A and focqiheypothetical acci-dent conditions, this guid preseintýta range of am-bient temperatures frna °F (-F'°C) to 100°F (38°C) as a part of t,ýjniti ditions. In the con-tiguous UnitedAat rcas a 99.7 percent prohabilitv thiat4,v " rnirl ~mnerntntrp rpgelin uill However, initial conditions must be assumed fall, within i qe -Af. 1).The insolation data before analyses can be performed to evaluate the provided 07' the national Atomic Energy response of structural systems to prescribed loads. Agency's salty5und~ards (Ref. 2) have been adopted This regulatory guide presents a set of initial condi- for tiog- adt b use they have sufficient conser-tions that is considered acceptable by the NRC staff valismr'W-'Nbmpared with other solar radiation for use in the structural analyses of type B packages ReR eV used to transport irradiated nuclear fuel in the con-tiguous United States. The values in this set suppleiCc n snapru Position C. L c mentions initial pressure ment the normal conditions and the hypothetical ac- -d tions. It should be noted that the pressure inside cident conditions of the regulations in c s els an noteu more complete basis from which structural :,,gt _ 41er daecontainmentue vessels and neutron shields of ir-masegrt, rdiated fuel shipping casks depends on several fac-may be assessed.

BV 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 iof pping fuel rods.casks used to transport irradiated nuclear fuel, specific load conditions must be established that will Regulatory Position C.l.e states that the values for encompass the static, dye , and thermal loadings initial conditions given in this guide are maximums or that may be

b asks during trans- minimums.

However, intermediate values could pos-port. This regulato 1.guIdresei ts initial conditions sibly create a more limiting case for some cask that can be used on.parts of Appendices A designs. For example, a seal design might be more and B of 10 C L Part.I to fully delineate thermal susceptible to leakage at a pressure less than the max-and mechanial

7'ad cebinations for purposes of imum internal pressure, or a local structural response struc is. in tended that this guide be might be greater during an impact test if the weight of used in c~ijinction with Regulatory Guide 7.6, the contents was less than the maximum."Stress Allo .es 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 casks used to transport irradiated nuclear fuel. quantities of radioactive materials.

Some of these re-USNRC REGULATORY

GUIDES Comments should be sent to the Secretary of the Commission, US. Nuclear Regu-Regulatory Guide% are issued to describe and make available to the public methods latory Commission, Washington, D.C 20555, Attention:

Docketing and Service acceptable to the NRC staff of impiementing specific parts of the Commission's Branch.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 substituths for regulations, and compliance with them is not required.

1. Power Reactors 6 Products Metnhods and solutions dilferent 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 findings requisite to the issuance or continuance

3. Fuels and Materials Facilities

8. Occupational Health of a permit or license by the Commission.

4 Environmental and Siting 9. Antitrust Review S. Materials and Piant Protection

10. General Comments and suggestions for improvements in these guides are encouraged at all times, and guides wil be revised, as appropriate to accommodate comments and Req uests for single copies of issued guides (which may be reproduced)

or for place-to reflect new information or experience.

However, comments on this guide,if ment on an automatic distribution list fOT single copies of future guides in specific received within about two months after its iuarice. will be particularly useful int divisions should be made in writing to the US. Nuclear Regulatory Commission evaluating the need for an early revision.

Washington, D.C 20555, Attention Director.

Division of Document Control.

quirements do not pertain to irradiated fuel shipping casks, however, because of the heaviness 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 mas-siveness causes a shipping cask to have a slow ther-mal 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 Appendix B and the water spray test of Appendix A are not significant in the structural 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 Appendix A are not discussed because they pertain only to lightweight packages.

The penetration test of Appendix A 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 re-quire ment.C. REGULATORY

POSITION The load conditions given here are considered ac-ceptable to the NRC staff for use in the analytical structural evaluation of shipping casks used to tran's, port type B quantities of irradiated nuclear fuels.1. General Initial Conditions To Be Used for Both Normal and Hypothetical Accident Conditions a. All initial cask temperature distributions should be considered to be at steady state. The normal and hypothetical accident conditions should be con-sidered to have initial conditions of ambient temperature at -20'F (-29'C) with no insolation and of ambient temperature at 100°F (38°C) with the maximum insolation data given in Table 1. Excep-tions to the above are made for the hot environment and cold environment normal conditions (which use other steady state values) and for the thermal acci-dent condition (which considers the higher thermal initial condition but not the lower one).b. The decay heat of the irradiated fuel should be considered as part of the initial conditions.

Generally, the maximum amount of decay heat should be considered in combination with the ther-mal environmental conditions of Regulatory Position C.l.a. In addition, the free-drop and vibration parts of the normal conditions and the free-drop and punc-ture parts of the accident conditions should consider the case of no decay heat and the cask at -20°F (-29°C). These initial thermal conditions are sum-marized in Table 2.c. The internal pressure used in evaluating normal and hypothetical accident conditions should be con-sistent with the other initial conditions that are being considered.

d. The release of all of the pressurized gases inside the fuel assemblies should be considered in determin-ing the maximum containment vessel pressure.e. It is the intent of this guide to specify discrete in-itial conditions that will serve as bounding cases for structural response.

Maximum or minimum values of initial conditions are given. However, if a larger structural response is suspected for an initial condi-tion that is not an extreme (e.g., an ambient temperature between -19'F (-28°C) and 99 0 F (37°C)), intermediate initial conditions should also be considered 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.a. Hot environment-The cask should be struc-turally evaluated for an ambient temperature of 130'F (54°C) in still air and with maximum insola-tion (see Table 1). If the cask has auxiliary cooling systems for the containment or neutron shield fluids, these systems should be considered to be inoperable during the hot environment condition.

b. 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 maximum fuel heat load and maximum internal pres-sure should be considered in addition to the case of no internal heat load. The possibility and conse-quence of coolant freezing should also be considered.

c. Minimum external pressure-The cask should be evaluated for an atmopheric pressure 0.5 times the standard atmospheric pressure.d. Vibration and fatigue-The cask should be evaluated for the shock and vibration environment normally incident to transport.

This environment in-cludes the quasi-steady vibratory motion produced by small excitations 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 pres-surization loads and any other loads that may con-tribute 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 loading and unloading of irradiated nuclear fuel.7.8-2 e. Free drop-The cask should be evaluated for a one-foot free drop onto a flat unyielding surface. The cask should contain the maximum weight of contents and should strike the impact surface in a position that is expected to inflict maximum damage.3. Hypothetical Accident Conditions The following hypothetical accident conditions are to be applied sequentially in the order indicated to determine the maximum cumulative effect.a. Free drop-The cask should be evaluated for a free drop through a distance of 30 feet (9 meters)onto a flat unyielding horizontal surface. It should strike the surface in a position that is expected to in-flict maximum damage and should contain the max-imum weight of contents.In determining which position causes maximum damage, the staff currently requests evaluations of drop orientations where the top and bottom ends, the top and bottom corners, and the sides are the cask impact areas. The center of gravity is usually con-sidered to be directly above these impact areas.However, evaluations of oblique drop orientations are requested when appropriate.

b. Puncture-The cask should be evaluated for a free drop of 40 inches (1 meter) onto a stationary and vertical mild steel bar of 6 inches (15 cm) diameter with its top edge rounded to a radius of not more than 0.25 inch (6mm). The bar should be of such a length as to cause maximum damage to the cask. The cask should contain the maximum weight of contents and should hit the bar in a position that is expected to inflict maximum damage.c. Thermal-The cask should be evaluated for a thermal condition in which the whole cask is exposed to a radiation environment of 1,475°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 2 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.This regulatory guide will be used by the staff after January 1, 1978, in assessing the structural adequacy of designs for irradiated fuel shipping casks with respect to the requirements in 10 CFR Part 71,§§71.35 and 71.36. Alternative methods that satisfy the requirements in the Commission's regulations will also be considered acceptable.

Series No. 6, Regulations for the Safe Transport of Radioactive Materials, 1973.3. Department of the Army, Research, Development, Test, and Evaluation of Material for Extreme Climatic Conditions, AR 70-38, May 196

9. REFERENCES

1. M.B. Gens, The Transportation and Handling En-vironment, SC-DC-72-1386, Sandia Laboratories, Albuquerque, New Mexico, Sept. 1972.2. International Atomic Energy Agency, Safety 7.8-3 TABLE "I MAXIMUM INSOLATION

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

Base Other surfaces Flat surfaces not transported horizontally:

Each surface Curved surfaces None 800 gcal/cm 2 (2,950 Btu/ft 2)200 gcal/cm 2 (737 Btu/ft 2)*400gcal/cm

2 (1,475 Btu/ff 2)**Alternatively, a sine function may be used, adopting an absorptioncoefficientand neglecting the effects of possible reflection from neighboring objects.7.8-4 TABLE 2 SUMMARY OF LOAD COMBINATIONS

FOR NORMAL AND HYPOTHETICAL

ACCIDENT.

CONDITIONS

OF TRANSPORT Applicable initial condition Ambient Ambient Insolation Decay heat =Normal or accident temperature n condition 0 I I-I Normal conditions 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 at

m. x x x Vibration

& shockt -x x x x Normally incident to x x x x the mode of transport x x x x Free drop -x x x x x 1 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 x 30 foot drop x x x x x Puncture -x x x x x Drop onto bar x x x x x X X x x x Thermaltt -x x Fire accident*See Table 1.**See Regulatory Position C.l.c and C.l.d.tSee 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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