Regulatory Guide 7.8
| ML13350A233 | |
| Person / Time | |
|---|---|
| Issue date: | 05/31/1977 |
| From: | NRC/OSD |
| To: | |
| References | |
| RG-7.008 | |
| Download: ML13350A233 (5) | |
U.S. NUCLEAR REGULATORY COMMISSIONMay 1977REGULATORY GUIDEOFFICE OF STANDARDS DEVELOPMENTREGULATORY GUIDE 7.8LOAD COMBINATIONS FOR THE STRUCTURAL ANALYSIS OFSHIPPING 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 ofRadioactive Material Under Certain Conditions."describe normal conditions of transport andhypothetical accident conditions that produce ther-mal and mechanical loads that serve as the structuraldesign bases for the packaging of radioactive materialfor transport.However, initial conditions must be assumedbefore analyses can be performed to evaluate theresponse of structural systems to prescribed loads.This regulatory guide presents a set of initial condi-tions that is considered acceptable by the NRC stafffor use in the structural analyses of type B packagesused to transport irradiated nuclear fuel in the con-tiguous United States. The values in this set supple-..:ment the normal conditions and the hypothetical ac-cident conditions of the regulations in forming-r.a..more complete basis from which structural intigrity.may be assessed.
B. DISCUSSION
To ensure safe structural behavior of shippingcasks used to transport irradiated nuclear fuel,specific load conditions must be established that willencompass the static, dynani-ic, and thermal loadingsthat may be experiencl.d,by.:the.casks during trans-port, This regulatory.guide presents initial conditionsthat can be used ididddition to parts of Appendices Aand B of 10 CER Par "ý7.1 to fully delineate thermaland mechanicalibad combinations for purposes ofstructural.'analysis'7.4tis intended that this guide beused in conjunction with Regulatory Guide 7.6,"Stress All0owiobles for the Design of Shipping CaskContainment Vessels," for the analytic structuralevaluation of the heavy (i.e., several tons in weight)casks used to transport irradiated nuclear fuel.USNRC REGULATORY GUIDEStL' ait: tiueci ii tit cl-1 0,- h itl1110 P ,Ae wmlablv to the 1)U.btl mrithth$dsawM~l to , hi WINC %tallci itIneit li ~ all'ci .W~n rl i,'Corv ~ sorf.ev %h,0)r1s. to tfhl0 .lct' defltn s umit ty tht, .f th I i:j.tl i , gceýIw ic tobwmtceriWi twIl uittil aCCciil'nl%. 4 t, i t mlrivctdi quid.-ur t m .i i , it fleq, s lixu,, G jKles,Me ,lot u l fw,, timlln.licllfl. '.nlri fthii ' ivutim thetn is riot ,etaclmI 't- .M,'llhotis Indl Qilufirrtms ifihll ont trall. thos' set owi it, the qultu? m. Il tvi .lhe Cctl.dblbe I ti-V 1ri uviltv .) ts.oSl for the lrndiiii l.ui to the or COfli liuWarCCof a pmemil or licenw bv On Corrrnit n anrd Surpj t,! li iin% fant l OVei irit itI t i i. i rqii ides are eicouiaqd at e11f1M. , aril guilt tihl Ih' r..v.lt. a ,lcitrt)a wo l-ate. to .t: ioiii e.i1irieits t0 itect t ~,," l ii.tilot itO -iii ' t. t 'ne,. Ho,,,etm'vm .cornilmiit IIti thus gimlti',ilecrivei( imitt.... .i lw.' t t in tht .0ell. ,, its Will tIw icutllly I1 '.is4 itwRegulatory Position C.I.a of this guide mentionsenvironmental initial conditions. The external ther-mal environmental limits for which a shipping caskmust be designed are stated in Appendix A of 10CFR Part 71 as being 130°F (540C) in direct sunlightand -40°F (-40'C) in shade. These limits are appliedwithout an)y additional loading. For the other condi-tions of Appendix A and for tht hypothetical acci-dent conditions, this guide:presents a range of am-bient temperatures. from -20°F (-290C) to 1000F(38'C) as a part of the initial conditions. In the con-tiguous United States.there Is a 99.7 percentprobability that, any hourly temperature reading willfall within this range (Ref. l).The insolation dataprovided in'.the International Atomic EnergyAgency's safet)"standards (Ref. 2) have been adoptedfor this guide.because they have sufficient conser-vatism When. compared with other solar radiationdata (RCf. 3)..- Regulatory Position C. I.c mentions initial pressurec.,,onditions. It should be noted that the pressure inside."the containment vessels and neutron shields of ir-radiated fuel shipping casks depends on several fac-tors. These factors include pre-pressurization of thevessels, the cask temperature distributions associatedwith the ambient temperatures and the decay heat ofthe fuel rods, and any gas leakage from the nuclearfuel rods.Regulatory Position C. .e states that the values forinitial conditions given in this guide are maximums orminimums. However, intermediate values could pos-sibly create a more limiting case for some caskdesigns. For example, a seal design might be moresusceptible to leakage at a pressure less than the max-imum internal pressure, or a local structural responsemight be greater during an impact test if the weight ofthe contents was less than the maximum.Appendices A and B of 10 CFR Part 71 outline re-quirements for packages used to transport type Bquantities of radioactive materials. Some of these re-AII -11-ts' %bo1ullf % l lH ie. n 4 I.t.'t .!tfq tt { .,;"11 " &'itw llt u ........... ,U S .t#,4 , l' l -*' vi3. I ueli tol Mitet .,I, F.it'ilit=.e II Oiicc.it~u~llul~ IW.:.lrth2. tA.ttet dc -il L ,%i m Pii. tT ... tO. ( ,mutt....v..Jit $ lit h iii I ., 1"t , i l e it ... U .. N &l li. i.iii ci Cif J- m tnI V~ij-tiiri t.uti .C. ?055S,. Api t ii D--11-11i1ucicc m. ~im'i i.imii quirements do not pertain to irradiated fuel shippingcasks, however, because of the heaviness of the casksor because the requirements are not structurallysignificant to cask design. Casks that are designed totransport one or. more commercia' fuel assembliesweigh many tons because of the large quantities ofstructural and shielding materials used. This mas-siveness causes a shipping cask to have a slow ther--Mal response to sudden external temperature changessuch as those that might be produced by quenchingafter a thermal exposure. The NRC staff feels that thewater immersion test of Appendix B and the waterspray test of Appendix A are not significant in thestructural design of large casks. Therefore, they arenot discussed in this guide. (Note, however, that theseconditions may be significant to criticality and othernonstructural aspects of cask design.)Similarly, the corner drop and the compressiontests of Appendix A are not discussed because theypertain only to lightweight packages. The penetrationtest of Appendix A is not considered by the NRCstaff to have structural significance for large shippingcasks (except for unprotected valves and rupturedisks) and will not be considered as a general re-quirement.
C. REGULATORY POSITION
The load conditions given here are considered ac-ceptable to the NRC staff for use in the analyticalstructural evaluation of shipping casks used to trans,-port type B quantities of irradiated nuclear fuels.I. General Initial Conditions To Be Used for BothNormal and Hypothetical Accident Conditionsa. All initial cask temperature distributions shouldbe considered to be at steady state. The normal andhypothetical accident conditions should be con-sidered to have initial conditions of ambienttemperature at -20°F (-29°C) with no insolation andof ambient temperature at 1000F (380C) with themaximum insolation data given in Table 1. Excep-tions to the above are made for the hot environmentand cold environment normal conditions (which useother steady state values) and for the thermal acci-dent condition (which considers the higher thermalinitial condition but not the lower one).b. The decay heat of the irradiated fuel shouldbe considered as part of the initial conditions.Generally, the maximum amount of decay heatshould be considered in combination with the ther-mal environmental conditions of Regulatory PositionC.l.a. In addition, the free-drop and vibration partsof the normal conditions and the free-drop and punc-ture parts of the accident conditions should considerthe case of no decay heat and the cask at -20OF(-29'C). These initial thermal conditions are sum-marized in Tahle 2.c. The internal pressure used in evaluating normaland hypothetical accident conditions should be con-sistent with the other initial conditions that are beingconsidered.d. The release of all of the pressurized gases insidethe 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 forstructural response. Maximum or minimum values ofinitial conditions are given. However, if a largerstructural response is suspected for an initial condi-tion that is not an extreme (e.g., an ambienttemperature between -190F (-280C) and 990F(370C)), intermediate initial conditions should alsobe considered in the structural analysis.2. Normal Conditions of TransportEach of the following normal conditions of trans-port is to be applied separately to determine its effecton the fuel cask.a. Hot environment-The cask should be struc-turally evaluated for an ambient temperature of130°F (54°C) in still air and with maximum insola-tion (see Table I). If the cask has auxiliary coolingsystems for the containment or neutron shield fluids,these systems should be considered to be inoperableduring the hot environment condition.b. Cold environment-The cask should beevaluated for an ambient temperature of -401F(- 40°C) in still air and with no insolation. The case ofmaximum fuel heat load and maximum internal pres-sure should be considered in addition to the case ofno internal heat load. The possibility and conse-quence of coolant freezing should also be considered.c. Minimum external pressure-The cask should beevaluated for an atmopheric pressure 0.5 times thestandard atmospheric pressure.d. Vibration and fatigue-The cask should beevaluated for the shock and vibration environmentnormally incident to transport, This environment in-cludes the quasi-steady vibratory motion producedby small excitations to the cask-vehicle system andalso 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 beconsidered.Factors that may contribute to thermal fatigueshould also be considered. These factors should in-clude the thermal transients encountered in theloading and unloading of irradiated nuclear fuel.7.8-2
- e. Free drop-The cask should be evaluated for aone-foot free drop onto a flat unyielding surface. Thecask should contain the maximum weight of contentsand should strike the impact surface in a position thatis expected to inflict maximum damage.3. Hypothetical Accident ConditionsThe following hypothetical accident conditions areto be applied sequentially in the order indicated todetermine the maximum cumulative effect.a. Free drop-The cask should be evaluated for afree drop through a distance of 30 feet (9 meters)onto a flat unyielding horizontal surface. It shouldstrike 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 maximumdamage, the staff currently requests evaluations ofdrop orientations where the top and bottom ends, thetop and bottom corners, and the sides are the caskimpact areas. The center of gravity is usually con-sidered to be directly above these impact areas.However, evaluations of oblique drop orientationsare requested when appropriate.b. Puncture-The cask should be evaluated for afree drop of 40 inches (I meter) onto a stationary andvertical mild steel bar of 6 inches (15 cm) diameterwith its top edge rounded to a radius of not morethan 0.25 inch (6mm). The bar should be of such alength as to cause maximum damage to the cask. Thecask should contain the maximum weight of contentsand should hit the bar in a position that is expected toinflict maximum damage.* c. Thennal-The cask should be evaluated for athermal condition in which the whole cask is exposedto a radiation environment of 1.475OF (8000C) withan emissivity coefficient of 0.9 for 30 minutes. Thesurface absorption coefficient of the cask should beconsidered to be 0.8. The structural response of thecask should be considered up to the time when thetemperature distributions reach steady state. Thepossibility and consequence of the loss of fluid fromthe neutron shield tank should be considered forcasks that use this design feature.Table 2 summarizes the loading combinationsgiven above.
D. IMPLEMENTATION
The purpose of this section is to provide informa-tion to applicants and licensees regarding the NRCstaffs plans for using this regulatory guide.This regulatory guide will be used by the staff afterJanuary 1, 1978, in assessing the structural adequacyof designs for irradiated fuel shipping casks withrespect to the requirements in 10 CFR Part 71,§§71.35 and 71.36. Alternative methods that satisfythe requirements in the Commission's regulationswill also be considered acceptable.REFERENCESI. 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, SafetySeries No. 6, Regulations for the Safe Transport ofRadioactive Materials, 1973.3. Department of the Army, Research, Development.Test, and Evaluation of Material for Extreme ClimaticConditions, AR 70-38, May 1969.7.8-3 TABLE 1MAXIMUM INSOLATION DATAForm and location Insolation for 12 hour1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />sof surface per dayFlat surfaces transportedhorizontally:Base NoneOther surfaces 800gcal/cm2 (2,950 Btu/ft2)Flat surfaces not transportedhorizontally:Each surface 200 gcal/cm2 (737 Btu/ft2)*Curved surfaces 400 gcal/cm2 (1,475 Btu/ft'2)*',Alternati'ely, a sine function may be used. adopting an absorptioncoefficientand neglecting the effectsof possible reflection from neighboring objects.7.8-4 TABLE 2SUMMARY OF LOAD COMBINATIONS FORNORMAL AND HYPOTHETICAL ACCIDENT CONDITIONS OF TRANSPORTApplicableinitial conditionNormal or accidentconditionAmbienttemperatureInsolationDecay heatUa.I-U0Y I 4 1LL.a0r'j*C0C0Normal conditions -I _ i _ iHot environment -130°F ambient temp.XxxCold environment -x x x-40*F ambient temp. x x xMinimum external pressure -x x x x0.5 atm. x x x xVibration & shockt -x x x xNormally incident to x x x xthe mode of transport x x x xFreedrop -x x x x xI foot drop x x x x xx x x x xAccident conditionsx x X x XFree drop -x x x x30 foot drop x x x x xPuncture -x x x x xDrop onto bar "_ x x x x x__ __ __ __ __ __ _ _ _ K _ x _ _ x x xThermaitt -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.7.8-5