NUREG/CR-4829

UCID-20733

Vol. 1

Shipping Container Response to

Severe Highway and Railway

Accident Conditions

Main Report

Prepared by L. E. Fischer, C. K. Chou, M. A. Gerhard, C. Y. Kimura,

R. W. Martin, R. W. Mensing, M. E. Mount, M. C. Witte

Lawrence Livermore National Laboratory

Prepared for

U.S. Nuclear

Commission

Regulatory

Reprinted October 1988
NOTICE

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NUREG/CR-4829

UCID-20733

Vol. 1

RT

Shipping Container Response to

Severe Highway and Railway

Accident Conditions

Main Report

Manuscript Completed: April 1986

Date Published: February 1987

Prepared by

L. E. Fischer, C. K. Chou, M. A. Gerhard, C. Y. Kimura,

R. W. Martin, R. W. Mensing, M. E. Mount, M. C. Witte

Lawrence Uvermore National Laboratory

7000 East Avenue

Livermore, CA 94550

Prepared for

Division of Reactor System Safety

Office of Nuclear Regulatory Research

U.S. Nuclear Regulatory Commission

Washington, DC 20555

NRC FIN A0397
0
ABSTRACT

This report describes a study performed by the Lawrence Livermore

National Laboratory to evaluate the level of safety provided under severe

accident conditions during the shipment of spent fuel from nuclear power

reactors. The evaluation is performed using data from real accident histories

and using representative truck and rail cask models that likely meet 10 CFR 71

regulations. The responses of the representative casks are calculated for

structural and thermal loads generated by severe highway and railway accident

conditions. The cask responses are compared with those responses calculated

for the 10 CFR 71 hypothetical accident conditions. By comparing the

responses it is determined that most highway and railway accident conditions

fall within the 10 CFR 71 hypothetical accident conditions. For those

accidents that have higher responses, the probabilities and potential

radiation exposures of the accidents are compared with those identified by the

assessments made in the "Final Environmental Statement on the Transportation

of Radioactive Material by Air and other Modes," NUREG-0170. Based on this

comparison, it is concluded that the radiological risks from spent fuel under

severe highway and railway accident conditions as derived in this study are

less than risks previously estimated in the NUREG-0170 document.

iii

TABLE OF CONTENTS

Page

1. INTRODUCTION ................. .......... .................. ..... ......... 1-1

1.1 Background ................................................. . . . . 1-

1.2 Regulations and Past Assessments .................................... 1-4

1.2.1 Title 10, Code of Federal Regulations, Part 71 ................ 1-4

1.2.2 Transportation of Radioactive Material -

Environmental Statement (NUREG-0170) .......................... 1-7

1.3 Objective and Approach ............................................ 1-9

2. ACCIDENT-RATES, ACCIDENT SCENARIOS, AND LOADING PARAMETER

DISTRIBUTIONS ....................... .... ..... **..* ... * .... ... 2-1

2.1 Introduction ................................ .. ...................... 2-1

2.2 Highway Accident Rates .............................................. 2-3

2.3 Railway Accident Rates ............................................... 2-3

2.4 Accident Loading Data Requirements .................................. 2-4

2.5 Highway Accident Loading Parameters ................................ 2-10

2.5.1 Mechanical Loading Parameters ................................ 2-10

2.5.1.1 Accident Scenarios and Object Hardness .............. 2-10

2.5.1.1.1 Collision Accident Hardness Data ......... 2-11

2.5.1.1.2 Non-Collision Accident Hardness Data ..... 2-14

2.5.1.2 Impact Velocity ........... ........................... 2-17

2.5.1.2.1 Cask Velocity ............................. 2-17

2.5.1.2.2 Impact Angle .............................. 2-21

2.5.1.3 Cask Orientation .................................... 2-23

2.5.2 Thermal Loading Parameters ................................... 2-24

2.5.2.1 Accident Scenarios and Fire Frequency .............. 2-24

2.5.2.2 Fire Duration ........... .. ...... ... 2-26

2.5.2.3 Flame Temperature ...... ............................. 2-26

2.5.2.4 Fire Location ...... o........ ................. o. 2-27

2.6 Railway Accident Loading Parameters ................................. 2-27

2.6.1 Mechanical Loading Parameters ........... o ..................... 2-27

V
TABLE OF CONTENTS (continued)

2.6.1.1

2.6.1.2

2.6.1.3

2.6.2 Thermal

2.6.2.1

2.6.2.2

2.6.2.3

2.6.2.4

Accident Scenarios and Object Hardness

Impact Velocity..o.......... .......

2.6.1.2.1 Cask Velocity . .. . .. ............ . ... .. .

2.6.1.2.2 Impact Angle..... ..........

Cask Orientation . .. .. ... ..... o ........ o. .........

Loading Parameters ....... ... ............ .....

Accident Scenarios and Fire Frequency ...........

Fire Duration .. .. .... ..... ... ...........

Flame Temperature ....... .. ........... .......

F ire Location .......... .... .............

3. SELECTION OF REPRESENTATIVE SPENT FUEL CASKS FOR EVALUATION .......

3.1 Introduction .............. ..........................

3.2 Cask Functions and Design Features .........................

3.3 Cask Design Features Important to Safety ......................

3.3.1 Containment ......................................

3 .3 .2 Ra d iat ion Sh ield in g .............. . ............ ... . .. ...

3.3.3 Subcrlticality Assurance ...............................

3.4 Selection of Cask Shielding Material .............................

3.5 Definition of Representative Cask Designs ........... ...........

3.5.1 Shielding Features ...... .. . .. .. .. . ...................

3.5.2 Containment Features . .. ............. . . ... . .............

3.5.3 Subcritical ity Assurance Features . .... .. . .............

3.5.4 Damage-Mitigating Features ..................... ....

3.5.5 Representative Cask Design Description............

3.6 Margins of Safety .... . ........................ . ........ .. . .....

4. REPRESENTATIVE CASK RESPONSE STATES, LEVELS, AND REGIONS ............

4.1 Introduction ........................................

4.2 Response States and Levels for Mechanical Loads .... ........ ....

Page

2-28

2-30

2-31

2-34

2-34

2-34

2-35

2-35

2-35

2-37

3-1

3-1

3-2

3-5

3-5

3-8

3-8

3-11

3-14

3-14

3-15

3-17

3-17

3-18

3-19

4-1

4-1

4-2

vi
TABLE OF CONTENTS (continued)

Page

4.2.1 Structural Response Level, S1 .................... 4-4

4.2.2 Structural Response Level, S2 4-4**~********

4.2.3 Structural Response Level, S3 ................. 4-6

4.2.4 Application of Response States and Levels .................... 4-6

4.3 Response States and Levels for Thermal Loads ........................ 4-7

4.3.1 Thermal Response Level, T1 .............. 0........ 4-9

4.3.2 Thermal Response Level, T2 ...... .............. .......... ... 4-11

4.3.3 Thermal Response Level, 13 ....... ................ 4-11

4.3.4 Thermal Response Level, 14 .... 0..** .................... 4-12

4.3.5 Application of Response States and Levels .............. ..... 4-12

4.4 Cask Response Regions ................... *................ ...... ... ..... 4-14

5.2 Probabilistic Inputs ................................................ 5-4

5.2.1 Mechanical Loading Parameter Distributions ............ *....... 5-5

5.2.1.1 Object Hardness Distributions .......... .. *... .... .... 5-5

5.2.1.2 Impact Velocity Distributions ............*.......... 5-5

5.2.1.2.1 Cask Velocity .................. .......... 5-5

5.2.1.2.2 Impact Angle ..... .. .....*....*........ .... -

5.2.1.3 Cask Orientation Distributions ...................... 5-11

5.2.2 Thermal Loading Parameter Distributions ...................... 5-13

5.2.2.1 Fire Duration Distributions ......................... 5-13

5.2.2.2 Flame Temperature Distributions ..................... 5-15

5.2.2.3 Fire Location Distributions ......................... 5-18

5.3 Probability Calculation ............................................. 5-20

6. FIRST-STAGE SCREENING ANALYSIS ... .. . .. . ... .. . .. ... .. .. .. . ... .. .. . .. .. .. .. 6-1

6.2 Structural Response Analysis ......... ... *....... .. ... .. .... .. ... ..... 6-7

vii
TABLE OF CONTENTS (continued)

Page

6.2.1 Cask Response Analysis for Highway Accidents ................. 6-15

6.2.1.1 Response to Minor Accidents .......................... 6-15

6.2.1.2 Response to Other Accidents ......................... 6-16

6.2.1.2.1 Response for Impacts with Unyielding

Surfaces . .. . . . . . . .*............. * .** .... * 6-18

6.2.1.2.2 Response for Real Objects ................. 6-18

6.2.2 Cask Response Analysis for Railway Accidents ................. 6-21

6.2.2.1 Response to Minor Accidents.......................... 6-23

6.2.2.2 Response to Other Accidents ...................... 6-25

6.2.2.2.1 Response for Impacts with Unyielding

Surfaces ........... .. .............. .. .. 6-25

6.2.2.2.2 Response for Real Objects ........ ...... 6-27

6.2.3 Discussion of Structural Analysis Resul~ts................. 6-31

6.3 Thermnal Response Analysis . . ... .. .. .. . .. .. ... .. .. .. .. .. . ... .. . ... ... 6-32

6.3.1 Cask Response Analysis for Highway Fire Acci dets.......s. 6-36

6.3.2 Cask Response Analysis for Railway Fire Accidet e......s. 6-39

6.3.3 Discussion of Thermal Analysis Results ................ 6-43

6.4 Accident Screening Analysis.......... ... .. ..... .... .... .. ... .... .... 6-45

7. SECOND-STAGE SCREENING ANALYSIS .............................. .. 0 . . .. 7-1

7.2 Structural Response Analysis .......... ............ . .. .. .. .. . .. .. . .. 7-3

7.2.1 Cask Response Analysis for Highway Accidents ................. 7-4

7.2.1.1 Endwise Impacts .. . .. ... .. .. . ... . . .............. . .. 7-5

7.2.1.2 Sidewise Impacts . ... ...... t...................... 7-

7.2.1.3 Impact Response Surmmary ...... 0*........... 7-8

7.2.2 Cask Response Analysis for Railway Accidents ........... 7-12

7.2.2.1 Endwise Impacts ....... . ... ........... .. .. .. .. .. . ... 7-12

7.2.2.2 Sidewise Impacts ..... ...... ..... .................... 7-14

7.2.2.3 Impact Response Summary ............................. 7-14

vii i
TABLE OF CONTENTS (continued)

Page

7.2.3 Discussion of Structural Analysis Results .................... 7-14

7.3 Thermal Response Analysis .......................................... 7-18

7.3.1 Cask Response Analysis for Highway Fire Accidents ............ 7-19

7.3.2 Cask Response Analysis for Railway Fire Accidents ............ 7-21

7.3.3 Discussion of Thermal Analysis Results ....................... 7-22

7.4 Accident Screening Analysis ......................................... 7-24

8. POTENTIAL RADIOLOGICAL SIGNIFICANCE OF TRANSPORTATION ACCIDENTS .......... 8-1

8.1 Introduction ........................................................ 8-1

8.2 Description of Spent Fuel .......................................... 8-1

8.3 Measures of Radiological Significance ............................... 8-3

8.4 Estimates of Radiological Hazards ................................... 8-7

8.4.1 Potential Radioactive Material Releases to the Environment ... 8-7

8.4.2 Potential Radiation Increases from Shielding Reduction ........ 8-12

8.5 RadiologicalEffect Estimates for Response Regions .................. 8-18

9. RESULTS AND CONCLUSIONS ................................................... 9-1

9.1 Introduction .................. .. ........ ........... .......... . 9-1

9.2 Results .......................... ,................................... 9-2

9.2.1 First-Stage Screening ........................................ 9-2

9.2.2 Second-Stage Screening ........................................ 9-4

9.2.3 Comparison with Previous Risk Assessments: NUREG-0170 ....... 9-6

9.2.4 Estimated Responses for Sample Severe Accidents .............. 9-15

9.2.4.1 Caldecott Tunnel Fire ............................... 9-15

9.2.4.2 1-80 Bridge Accident ......................... ....... 9-16

9.2.4.3 Livingston Train Fire ............................... 9-17

9.2.4.4 Derailment into the Alabama River ................... 9-18

9.3 Uncertainties ....................................................... 9-19

9.3.1 Uncertainty in Cask Response ................................. 9-20

9.3.1.1 Selection of Representative Cask Designs ............ 9-20

9.3.1.2 Definition of Accident Loads ........................ 9-21

ix
TABLE OF CONTENTS (continued)

Page

9.3.1.3 Computer Code Applications and Modeling ............. 9-21

9.3.2 Uncertainty in Estimating an Accident's Potential

Radiological Hazard .. .. .. .. . .. .. .. . .. ... . .. ... .. .. .. .. .. . .... 9-23

9.3.2.1 Radioactive Releases from Fuel Rods . ........ 9-23

9.3.2.'2 Radioactive Releases from Casks ............... 9-24

9.3.2.3 Reduction in Radiation Shielding ........... 9-24

9.3.2.4 Reduction in Subcriticality Control............ 9-24

9.3.3 Uncertainty in Probability Models ............... 9-25

9.3.3.1 Accident Statistics ... ..... ................... * 9-25

9.3.3.2 Surveys of Structures and Features .......... 9-26

9.3.3.3 Past Analysis and Models ..................... 9-26

9.3.3.4 Engineering Judgment ...... ..... ................ * 9-27

9.3.4 Overall Statement of Uncertainty ................. 9-27

9.4 Concl usions . .............................. . ................. . 9-27

REFERENCES ............................................... *.*......... R-1

APPENDIX A: Severe Accident Data .. . ................ ...... .. ............... A-i

APPENDIX B: Truck Accident Data.................... ... . .. ....... . ........ B-i

APPENDIX C: Railroad Accident Data .......... .... ... ................ C-i

APPENDIX 0: Highway Survey Data and Bridge Column Properties ............. 0-1

APPENDIX E: Structural Analysis .. .. ............ . . .. .. .... .. . .. . .. . ........ E-1

APPENDIX F: Thermal Analysis ................... . ... ................. F-i1

APPENDIX G: Probability Estimation Techniques .................. G-1

APPENDIX H: Benchmarking for Computer Codes used in Impact Analysis .......... H-i

x
LIST OF FIGURES

1-1 Schematic of a typical spent fuel cask ....................... 1-6

1-2 Two-stage screening process used in evaluating the regulations ......... 1-11

1-3 Schematic representation of the report ................................. 1-13

2-1 Three impact loading parameters considered in the response

analysis for impacts on surfaces ....................................... 2-7

2-2 Three impact loading parameters considered in the response

analysis for impacts with objects such as train sills ................ 2-8

2-3 Truck collision accident scenarios and their percent

probabilities .*...***..... ... .... ...... .... .................... *......... 2-12

2-4 Truck non-collision accident scenarios and their percent

probabilities *......... ....... * ...... ... ........................ ....... 2-13

2-5 Train accident scenarios . ... . ... . ... .. .. . ............ .. .. . .. . ... . . . ... . 2-29

3-1 Spent fuel cask features important to safety ........................... 3-4

3-2 Typical closure designs for spent fuel casks ........................... 3-7

3-3 Typical cask penetration subsystems .................................... 3-9

3-4 Preliminary truck cask designs with three types of gammna

shielding, used for quasi-static loading response studies

only ................... *.............* ..... . ............... *.*.*...... 3-12

3-5 Preliminary rail cask designs with three types of gamma

shielding, used for quasi-static loading response studies

3-6 Representative truck cask design used for dynamic structural

and thermal response studies *.........*.... *...... ... * *.......... ........ 3-20

3-7 Representative rail cask design used for dynamic structural

and thermal response studies .................... *.............. 3-21

3-8 Force-deflection characteristics of the limiter design as a

function of cask orientation at impact .................... 3-22

xi
LIST OF FIGURES (continued)

Page

4-1 Schematic representation of cask response state for

mechanical load ........................................................ 4-5

4-2 Schematic representation of cask structural response for

various surface hardness and impact velocities ......................... 4-8

4-3 Schematic representation of cask response state for thermal

loads .................................................................. 4-10

4-4 Schematic representation of cask response for various fire

locations and fire durations ......................... 4-13

4-5 Matrix of cask response regions for combined mechanical and

thermal loads .......................................................... 4-15

5-1 Effect of cask orientation on the strain-impact velocity

relationship for. a truck cask impacting an unyielding

object ................................................................. 5-2

5-2 Effect of flame temperature and fire location on

lead-temperature-time relationship for a truck cask .................... 5-3

5-3 Distribution of vehicle velocities adjusted for braking ................ 5-7

5-4 Flow Chart of TASP computer code ....................................... 5-32

6-1 Identification of first-stage screening ................................ 6-2

6-2 Methods of analysis used in cask response determinations ............... 6-5

6-3 Three impact loading parameters considered in the response

analysis for impacts on surfaces ....................................... 6-8

6-4 Three impact loading parameters considered in the response

analysis for impacts with objects such as train sills .................. 6-10

6-5 Equivalent damage technique ............................................ 6-13

6-6 Strain versus impact velocity and cask orientation for the

representative truck cask impacting an unyielding surface .............. 6-19

6-7 Impact force for a rigid truck cask dropped endwise onto

real surfaces ........................................................... 6-20

xii
LIST OF FIGURES (continued)

Page

6-8 Rail car coupler override of spent fuel cask car ....................... 6-24

6-9 Strain versus impact velocity and cask orientation for the

representative rail cask impacting an unyielding surface ............... 6-28

6-10 Impact force versus impact velocity for a rigid rail cask

dropped endwise onto real surfaces ..................................... 6-29

6-11 Comparison of an engulfing hypothetical fire and a real fire ........... 6-34

6-12 Representative truck cask temperature response to a

hypothetical 1475 0F (equivalent to a real 17000F) fire

versus fire duration . ....... *...... **....................*.............. 6-37

6-13 Heat flux versus fire duration for the representative truck

cask exposed to the regulatory 14750 fire ................................. 6-38

6-14 Average heat flux factor versus temperature for the

representative truck cask *.............. ...... 6-40

6-15 Heat load factor for real fire versus location of,

representative truck cask ................... *................. o......... 6-41

6-16 Representative rail cask temperature response to a

hypothetical 14750F (equivalent to a real 17000F) fire

versus fire duration ........................... 6-42

6-17 Heat load factor for real fire versus location of

representative rail cask ........................ 6-44

7-1 Second-stage screening analysis relationship with response

7-2 Example showing strain response of the representative truck

cask for 45 mph endwise impact on an unyielding surface (2-

D model with impact limiters) without any truck cab

crushing included *.............. ........ **** ....... * ****   ** ** 7-6

7-3 Response of the representative truck cask to endwise

impacts on an unyielding surface (2-D model with impact

limiters and cab crush) . .. .. .................. ** .* **.  .. ** **. ***............... 7-7

xiii
LIST OF FIGURES (continued)

Page

7-4 Example showing strain response of the representative truck

cask for 60 mph sidewise impact on soil (2-D model without

limiters) with strain exceeding the 2% (S2) limit .................... 79

7-5 Response of the representative truck cask to sidewise

impacts on various surfaces ........ ... .. . ... ............ ... . .. .. .. . .. 7-10

7-6 Response of the representative rail cask to endwise impacts

on an unyielding surface (2-D model with impact l imiters

and railcar crush) ............... ........................... 7-13

7-7 Response of the representative rail cask to sidewise

impacts on various surfaces.......... ... .. .. .................. .. ... 7-15

7-8 Representative truck cask temperature response to a

hypothetical 14750F (equivalent to a real 17000F) fire

versus fire duration................. .............. *** ........ **. . . 7-20

7-9 Representative rail cask temperature response to a

hypothetical 1475*F (equivalent to a real 17000F) fire

versus fire duration .................................. ... * ... ..... 7-23

7-10 Fraction of truck accidents that could result in responses

within each response region, assuming an accident occurs ........ 7-25

7-11 Fraction of rail accidents that could result in responses

within each response region, assuming an accident occurs ............... 7-26

8-1 PWR fuel bundle ........................................ 8-4-

8-2 Three mechanisms required to establish a radioactive

material release path ............................... . ... . ....... 8-8 -

8-3 Percentage of fuel rods breached as a function of force for

endwise impacts ............................... .............. .. 8-9

8-4 Percentage of fuel rods breached per fuel assembly in each

cask response region.............. . .. .. ... .. ...................... 8-11

8-5 Lead voiding due to lead slump resulting from endwise

impact of cask . ......................................... . .. 8-15

xiv
LIST OF FIGURES (continued)

Page

8-6 Lead voiding due to high thermal loads and lead melting .............. ,. 8-17

8-7 Radiological hazards estimated for response regions for a

representative truck cask .............................................. 8-19

8-8 Radiological hazards estimated for response regions for a

representative rail cask ....................... ............ ...... 8-20

9-1 Two-stage screening process in the 20 response regions ................. 9-5

9-2 Probability-hazard estimates in curies for the 20 truck

cask response regions ................................................... 9-8

9-3 Probability-hazard estimates in curies for the 20 rail cask

response regions ............................................. . . . . . g -9

xv

LIST OF TABLES

Page

1.1 Correlation of NUREG-0170 Accident Fractional Occurrence

and Radiological Hazards as a Function of Accident Severity ............ 1-8

2.1 Accident Loads and Loading Parameters .................................. 2-5

2.2 Fractional Occurrence of Surface Types below Bridges on

Interstate 80 from Davis, California to.Nevada Border .................. 2-15

2.3 Distribution of Velocities for Trucks/Semitrailers Involved in Fatal and

Injury Accidents in California, 1958-1967 ........................... *. 2-19

2.4 Distribution of Bridge Heights along Interstate 5 through

Orange and Los Angeles Counties, California ........................... 2-20

2.5 Train Velocity Distribution for Rail-Highway Grade-Crossing

Accident/Incidents Involving Motor Vehicles, 1975-1982 ................. 2-22

2.6 Frequency of Fire for Truck Accident Types. ............................. 2-25

2.7 Railroad Accident Velocity Distribution, Collisions, Main

2.8 Railroad Accident Velocity Distribution, Derailments, Main

Line, 1979-1982 . .... *. . .****... & *.... . ...... *. ...... ..... ...* . 2-33

2.9 Train-Fire Accident Types ......... ........ .................... ....... 2-36

5.1

5.2

5.3

5.4

5.5

5.6

5.7

5.8

5.9

5.10

5.11

Cumul ative

Cumulative

Cumulative

Cumulative

Cumulative

Cumulative

Cumulative

Cumulative

Cask Velocity Distributions for Highway Analysis ......

Cask Velocity Distributions for Railway Analysis ......

Impact Angle Distributions ... .......... .... .. ............

Cask Orientation Angle Distributions ............

Fire Duration Distributions for Truck Cask Analysis ........

Fire Duration Distributions for Rail Cask Analysis .....

Flame Temperature Distribution .. .. ..... ... . . ... .. . ... .....

Fire Location Distributions .. .. ............ .. ... ..... ...

5-8

5-10

5-12

5-14

5-16

5-17

5-19

5-21

5-25

5-28

5-31

Probability Inputs for Highway Analysis ................

Heat Flux Factors for Flame Temperatures (Engulfing Fire) .......

Probability Inputs for-Railway Analysis . .....................

xvi i
LIST OF TABLES (continued)

Page

6.1 Material Parameters Selected for Real Surfaces ......................... 6-14

6.2 Evaluation of Quasi-Static Force for Minor Highway Accidents ........... 6-17

6.3 Impact Velocities Required to Reach the 0.2% Strain (S$)

Level for Objects Impacted in Highway Accidents ........................ 6-22

6.4 Evaluation Summary of Minor Railway Accidents .......................... 6-26

6.5 Impact Velocities Required to Reach the 0.2% Strain (SI)

Level for Objects Impacted in Railway Accidents ........................ 6-30

7.1 Impact Velocities Required to Attain 2% (S2) and 30% (S3)

Strain Levels for Objects Impacted in Highway Accidents ................ 7-11

7.2 Impact Velocities Required to Attain 2% (S2) and 30% (S3 )

Strain Levels for Objects Impacted in Railway Accidents ................ 7-16

8.1 PWR Fuel Assembly Decay Heat and Radioactivity ......................... 8-2

8.2 10 CFR 71 Release Limits for Radioisotopes .......... 8-6

8.3 Material Release Fractions from Breached Fuel Rods

Occurring over l Week Following Rod Burst .............................. 8-13

8.4 Gamma Dose Summary for Lead Slump in a Rail Cask for

Impacts on Closure Region .............................................. 8-16

9.1 Comparative Measure of Risk/Accident for Spent Fuel

Shipment by Truck ...................................................... 9-12

9.2 Comparison of Release Risk/Accident for Spent Fuel Shipment

by Rail ................................................................ 9-13

xviii
PREFACE

This report describes a study conducted to estimate the responses of

spent fuel casks to severe highway and railway accident conditions and to

assess the level of safety provided to the public during the shipment of spent

fuel. The study was performed by the Lawrence Livermore National Laboratory

for the U.S. Nuclear Regulatory Commission (NRC), Office of Nuclear Regulatory

Research.

This report is divided into two volumes: Volume I, the main report,

describes the study, the technical approach, the study results, and

conclusions; and Volume II, the Appendixes, provide supporting accident data

and engineering calculations. This report has been reviewed by the Denver

Research Institute at the University of Denver under a separate contract to

the NRC as the peer review. A companion summary report entitled "Transporting

Spent Fuel-Protection Provided Against Severe Highway and Railway Accidents"

(NUREG/BR-O11I) has been prepared by the NRC for wide distribution to federal

agencies, local governments, and interested citizens.

Commercial spent fuel shipments are regulated by both the Department of

Transportation (DOT) and the NRC. The NRC evaluates and certifies the design,

manufacture, operation, and maintenance of spent fuel casks, whereas the DOT

regulates the vehicles and drivers which transport the spent fuel.

Current NRC regulations require spent fuel casks to meet certain

performance standards. The performance standards include normal and

hypothetical accident conditions which a cask must be capable of withstanding

without exceeding established acceptance criteria that

(1) limit the release of radioactive material from the cask,

(2) limit the radiation levels external to the cask, and

(3) assure that the spent fuel remains subcritical.

This study evaluates the possible mechanical and thermal loads generated

by actual and potential truck and railroad transportation accidents. The

magnitudes of the loads from accidents are compared with the loads implied

from the hypothetical accident conditions. The frequency of the accidents

that can produce defined levels of mechanical and thermal loads are developed

from the accident data base. Using this information, it is determined that

xix
for certain broad classes of accidents, spent fuel casks provide essentially

complete protection against radiological hazards. For extremely severe

accidents--those which could impose loads on the cask greater than those

implied by the hypothetical accident conditions--the likelihood and magnitude

of any radiological hazards are conservatively estimated. The radiological

risk is then estimated and compared with risk estimates previously used by the

NRC in judging the adequacy of its regulations.

The results of this study depend primarily on the quality of the cask

response models, the radiation release models, and the probability models and

distributions used in the analysis. Models for cask responses, radioactive

releases, and distributions for the accident parameters are new developments

based on current computer codes, limited test data on radioactive releases,

and limited historical accident data. The results are derived using

representative spent fuel casks which use design principles and materials that

have been used in casks currently licensed by the NRC. The representative

casks are assumed to have been designed, manufactured, operated, and

maintained in accordance with national codes and standards (or equivalent)

which have adequate margins of safety embedded in them. The results of this

study are limited to spent fuel casks designed and fabricated under current

technologies and operated under current regulations. New designs using

alternative design principles and materials, or changes to regulations such as

the imposition of a 75 mph national speed limit, could affect the results and

conclusions of this study.

This study does not consider the effects which human factors can have on

the cask design, manufacture, operation, and maintenance. If further study is

conducted, human factors should be considered because they can contribute to

the overall risk in each phase of transporting spent fuel.

L. E. Fischer

xx
ACKNOWLEDGEMENTS

The authors wish to acknowledge the technical contributions made to this

report by R. C. Chun, L. L. George, T. E. McKone, and M. W. Schwartz of

Lawrence Livermore National Laboratory. The authors wish to thank 'G. E.

Cummings of Lawrence Livermore National Laboratory for his report review and

comments. The authors also wish to thank J. R. Cook, W. R. Lahs, and

W. H. Lake of the U.S. Nuclear Regulatory Commission for their support and

comments during the research and preparation of this report. Many thanks to

N. J. Barnes and E. A. Sturmer for report preparation and D. Bowden for report

editing.

In addition, the authors would particularly like to thank the following

organizations for providing information and counsel which were used in

preparing this report:

Anatech International Corporation

Association of American Railroads

Bureau of Motor Carrier Safety

California Department of Transportation

Central Electricity Generating Board, England

Denver Research Institute

Department of California Highway Patrol

Electric Power Research Institute

Engineering Computer Corporation

Federal Highway Administration

Federal Railroad Administration

Health and Safety Executive, England

Los Alamos National Laboratory

National Fire Protection Association

Oak Ridge National Laboratory

Ridihalgh, Eggers and Associates, Inc.

Sandia National Laboratories

Southern Pacific Transportation Company

xxi

1.0 INTRODUCTION

This report addresses the level of safety provided during the shipment of

spent fuel from nuclear power reactors. The number of shipments will increase

in the near future because of the need to transfer this fuel from the nuclear

power reactors to a waste repository. During the shipments the shipping

containers (casks) carrying the spent fuel could be exposed to severe highway

and railway accident conditions. At the request of the U.S. Nuclear

Regulatory Commission (NRC) the Lawrence Livermore National Laboratory (LLNL)

has performed studies to evaluate and document the response of spent fuel

casks exposed to severe highway and railway accident conditions.

1.1 Background

Nuclear fuel, contained in fuel rods, is used in nuclear power reactors

to generate useful heat for electric power generation. The fuel rods used in

most nuclear power reactors in the United States are made up of approximately

one-half-inch-diameter ceramic pellets of uranium oxide encased within a

cylindrical cladding. The fuel rods are approximately 15 feet in length. The

cladding is made from metallic materials such as zirconium. After being

capped, the cladding provides a contained environment for the uranium oxide

fuel pellets. Depending on the type of nuclear power reactor, square arrays

of the fuel rods numbering from about 50 to 300 are structurally assembled to

form a single fuel bundle.

When nuclear fuel burns or fissions, it not only generates useful heat,

but also creates radioactive fission products. Spent fuel is nuclear fuel

that has been burned to its specified limits and has served its useful

purpose. Spent fuel is highly radioactive when initially removed from a

nuclear power reactor. Before being transported to a waste repository, spent

fuel is usually stored five or more years in the spent fuel pool at the

reactor site to allow the fuel to cool or decay to lower radiation levels.

Because of its radioactive nature, spent fuel is shipped in specially

designed shipping containers called casks. These casks are massive,

cylindrically shaped objects weighing from 25 to more than 100 tons. The

1-1
designs of several currently used casks consist of steel shells enclosing a

dense metallic material (lead or depleted uranium) that is used to provide

radiation shielding. In the United States, these casks must be certified by

the U.S. NRC as being in compliance with the regulations contained in Title 10

of the Code of Federal Regulations, Part 71 (10 CFR 71).1 These regulations,

which are almost identical in substance to internationally accepted standards,

have been in effect for nearly 20 years. The regulations are intended to

assure that the public will be protected both during normal transportation or

in the event that a spent fuel shipment is involved in a transportation

accident.

Basically, the regulations state that each spent fuel cask must meet

certain containment, radiation control, and criticality control requirements

when it is subjected to specified normal transport conditions and also

hypothetical accident conditions. The hypothetical accident conditions are of

most interest to this discussion. They are specified in terms of regulation

defined test conditions that include a free drop (30 feet onto a flat

unyielding surface), a puncture (40-inch drop onto a vertical 6-inch-diameter

mild steel bar), thermal exposure (30 minutes to a defined 1475 0 F

environment), and immersion under specified depths of water. The test

conditions must be sequentially imposed on all casks in a manner that would

cause maximum damage. The resulting cask response must then be determined by

test or analysis.

The regulations do not define the allowable structural or thermal damage

a cask may sustain, but instead use radiological criteria, i.e., radioactivity

release (leakage) and radiation levels external to the cask as a measure of

the acceptability of the design. The cask response must be such that the cask

can (1) meet containment requirements (any radioactive material release must

be restricted within extremely small limits), (2) keep radiation levels

external to the cask within stated limits, and (3) ensure that a criticality

event cannot occur. In more practical terms, these compliance criteria

require the cask structural integrity to be effectively unimpaired.

1-2
Historically, the few shipments of spent fuel that have been involved in

transportation accidents have never created any significant radiological

hazard. However, the number of these events has been limited. To quantify

the radiological risk to the public from all shipments of radioactive

material, including spent fuel, the NRC published, NUREG-0170, in 1977

entitled, "Final Environmental Statement on the Transportation of Radioactive

Material by Air and Other Modes." 2 The study was primarily performed using

conservative engineering judgments. The analysis performed in that document

presumed that, in certain classes of accidents, transportation accident loads

could exceed those implied by the hypothetical accident conditions specified

in the regulations. The analysis further presumed that for these classes of

accident, releases of radioactive material could occur. Even under these

presumptions, the analysis indicated that the potential radiological hazards

from real transportation accident loadings on a spent fuel cask were most

often very small (i.e., limited to minor property contamination which required

only cleanup actions). Since no release of spent fuel material has ever

occurred, this assessment is consistent with historical events. Even though

NUREG-0170 presumed the release of radioactive material under certain severe

accident circumstances, the overall resulting radiological risk "from

transporting spent fuel under current regulations was calculated to be

acceptable.

Nevertheless, because of the lack of actual data on the real effects of

severe accidents on spent fuel casks, studies were initiated by the NRC prior

to this work to define more precisely (1) the variability of mechanical and

thermal loads which could be experienced by a cask in recorded severe railway

and highway accidents, and (2) the degree to which these loads might exceed

those implied by the hypothetical accident conditions. 3 In order to better

understand the effectiveness of current regulations, this recorded severe

accident information supplemented with other accident data has been used by

the LLNL, working under contract to the NRC, to evaluate the responses of

spent fuel casks exposed to severe highway and railway accident conditions.

This report documents the work performed under this contract.

1-3
1.2 Regulations and Past Assessments

1.2.1 Title 10, Code of Federal Regulations, Part 71

To protect the public health and safety, commercial shipments of spent

fuel are required to be made in spent fuel casks which are designed,

fabricated, and operated in accordance with provisions of 10 CFR 71. The

three basic safety requirements addressed by the regulations and which must be

met when transporting spent fuel are:

1. Adequate containment of radioactive material

2. Adequate shielding of the radiation emitted by the radioactive contents

3. Prevention of nuclear criticality.

The containment requirements, as they apply to spent fuel shipments,

impose a limit on radioactive material releases following the application of

certain mechanical and thermal loadings on a spent fuel cask. The loadings

are imposed by a series of test conditions called hypothetical accident

conditions. The radioactive material release limits include a value for the

relatively innocuous inert gas, 8 5Kr, ( s 10,000 curies in one week) and a

separate limit on other releases over a 1-week period (called an A2

quantity). These limits on specific radioactive material releases are such

that the doses to members of the public can be expected to be less than the

allowable annual dose to individuals whose occupation involves potential

exposure to radiation.

The shielding requirement following the application of the hypothetical

accident condition is stated in terms of an external radiation dose rate at

1 meter from the external surface of the cask. This radiation level must not

exceed one rem per hour.

The prevention of criticality under accident conditions is achieved by

cask design features which assure subcriticality. This subcriticality must be

achieved assuming (1) optimum (most reactive) configurations of the spent fuel

consistent with the cask damage imposed by the hypothetical accident

conditions and (2) most reactive conditions associated with the presence of

water. (Water or other materials which act as neutron moderators or

reflectors enhance criticality possibilities when in close contact with spent

fuel.)

1-4
The safety requirements of 10 CFR 71 play an important role in this study

because they provide a benchmark for relating a specific magnitude of

mechanical or thermal loading (implied by the hypothetical accident

conditions) to a specified level of cask response. For example, in practice,

the containment limits are usually met by demonstrating that the cask

containment experiences essentially no permanent deformations and the closure

seals and penetration remain essentially leak tight (Fig. 1-1). The external

dose rate limit is met by demonstrating that essentially no loss of the gamma

shield occurs under accident conditions. Finally, the prevention of

criticality requirement is typically met by demonstrating that essentially no

deformation occurs to the basket, the structure within the cask which holds

the spent fuel. These limits serve as benchmarks against which cask responses

in real accident conditions can be compared.

One particular cask design feature is especially significant in ensuring

that a spent fuel cask will meet the containment, shielding, and

subcriticality requirements when the cask is subjected to the 30-foot drop

onto the unyielding surface called for by 10 CFR 71. This feature is called

an impact limiter (Fig. 1-1). Impact limiters reduce the mechanical loads to

the main cask body under accident conditions.

Impact limiters are typically made of crushable material surrounding the

extremities of a cask, but designs can also include the use of crushable

exterior metal fins. In either case, the impact limiters are designed to

absorb most of the energy generated in the regulatory-defined 30-foot drop

onto the unyielding surface without causing any significant permanent damage

to the cask containment or closure features.

The significant point is that, through the response of this design

feature, a load level is defined which translates into no cask containment

damage and, therefore, essentially no radiological hazard. For those real

accidents which result in mechanical loads less than this limit, the

radiological hazard is insignificant.

Similarly, protection against the regulatory-defined thermal loading

conditions is typically provided by the use of thermal barriers. Thermal

1-5
-Lifting

trunnions

Impact

limiter

Figure 1-1 Schematic of a typical spent fuel cask.

1-6
barriers limit the heat transfer from a fire or thermal source external to the

cask, to the cask containment structure, and to the contained spent fuel.

Again, real world accidents involving fire can be compared with this defined

thermal loading. These types of comparisons form the essence of the

first-stage of a screening process used in this study.

1.2.2 Transportation of Radioactive Material - Environmental

Statement (NUREG-0170)

In December 1977, the NRC published NUREG-0170, "Final Environmental

Statement on the Transportation of Radioactive Material by Air and Other

Modes". 2 The report included an assessment of the likelihood and magnitude of

the radiological consequences associated with potential transportation

accidents for all shipments of radioactive material. Most shipments consisted

of medical and industrial isotopes, but spent fuel shipments were specifically

addressed. The assessment indicated that the radiological risk involved in

all shipments was small. This conclusion provided the technical basis for the

Commission's decision that the existing 10 CFR 71 regulations are adequate and

not in need of immediate change.

The NUREG-0170 analysis provides an additional benchmark for this

study. Specifically, the radiological risk from spent fuel shipments reported

in NUREG-0170 can be compared with the risk estimated in this study. In

NUREG-0170, accident severities were divided into eight categories. For each

category, the radiological hazards were assigned based on conservative

engineering judgments. These hazards were measured in terms of the fraction

of radioactive material released from the spent fuel and an equivalent

fraction caused by shine from any unshielded fuel. For truck and rail

accidents, the estimates in NUREG-0170 indicated that 91% of truck accidents

and 80% of train accidents would result in no significant radiological

hazard. In the remaining accidents, the radiological hazards increased as the

accident severity increased. The increase is indicated in Table 1.1. As a

point of reference, NUREG-0170 indicated that 0.4% of truck accidents and 0.2%

of train accidents could involve a complete release from the cask of certain

gaseous and volatile materials. These materials represent the radioactivity

1-7
Table 1.1

Correlation of NUREG-0170 Accident Fractional Occurrence and

Radiological Hazards as a Function of Accident Severity

Radiological Hazards

Fraction of Fraction of

Accident Truck Train Radioactive Equivalent

Severity Fractional Fractional Material Unshie~lded

Category Occurrences Occurrences Releaseda/ FuelbI

I 0.55 0.50 0 0

II 0.36 0.30 0 0

III 0.07 0.18 0.01 0

IV 0.016 0.018 0.10 0

V 0.0028 0.0018_4 1 0

VI 0.0011.5 1,3xI0-5 I 3.18x10-

VII 8.5x10- 6.OxlO• 1 3.18xl0 5

-

VIII 1.5x10- 5 1.Oxi0- 5 1 3.12xI0-3

a/ Radioactive gases and vapors

b/ Approximates the reduction in radiation shielding

1-8
which typically migrates from the fuel pellets to the fuel rod gap, the void

space between the fuel pellets and the surrounding fuel rod. In this small

percentage of accidents, all the fuel rods in the shipment were assumed to

fail and to release their radioactivity.

Also, for accidents in Category VI and greater, a reduction of shielding

was assumed. To provide a consistent measure of the radiological effects with

cask damage, the radiological hazard due to the reduction in shielding was

presented in terms of an equivalent fraction of unshielded fuel. The

equivalent fraction of unshielded fuel is the ratio of that portion of the

total spent fuel inventory that, if unshielded, would produce radiation levels

equivalent to those being emitted from a damaged cask with reduced shielding.

The results of NUREG-0170 rely in part on the presumption that spent fuel

casks have sufficient margins designed into them that major radioactive

hazards will not occur even at loading conditions which exceed those specified

in regulations. These margins of safety are. included in all licensed cask

designs through the use .of established codes and standards which have margins

of safety embedded in them.

The evaluation conducted in this study analyzes the response of

representative shipping casks in severe accident environments. This

evaluation uses representative cask designs that are likely to be licensed and

have margins of safety, included in their designs. The responses of the

representative casks to all possible accident conditions are analyzed and

categorized into cask response regions. For each cask response region,

assessments are made of the potential for release of radioactive material and

the potential for reducing the radiation shielding capabilities of the cask.

.This evaluation is the basis for a comparison with NUREG-O170; that is, what

accident classes result in radiological hazards and how do those hazards and

their likelihoods compare in terms of radiological risk to the public.

1.3 Objective and Approach

The objective of this study, the Shipping Container Response to Severe

Highway and Railway Accident Conditions, is to estimate the adequacy of

1-9
radiological protection offered the public by the current NRC regulations when

highway or railway accidents occur involving spent fuel shipments. The

estimates are performed using data from real accident histories of similar

types of vehicles and using models of cask designs that have a likelihood of

meeting requirements for spent fuel shipments.

A two-stage screening process is used. The screening process is

illustrated in Fig. 1-2. The first stage compares cask responses to accident

loading conditions with those associated with the accident test conditions

specified in 10 CFR 71. As an example of such a comparison, cask loadings

from a class of accidents involving impacts exceeding 30 mph (the velocity

reached in the 30-foot drop) are examined.

An example of such an accident class is the accident scenario involving a

60-mph collision with a highway sign pole. The cask loading ih this scenario

is such that no damage occurs to the containment, radiation shielding, or

subcriticality assurance features of the cask, even though the accident

velocity exceeds the regulatory-implied impact velocity. The reason is that

although the accident velocity is twice the regulatory defined velocity, the

loading imposed on the cask in the 30-foot drop test far exceeds the loading

achieved on impact with the sign pole. The pole failure essentially limits

the load to which the cask is exposed.

There are classes of accidents in which the loading can be conceived to

approach or exceed the values imposed by the accident test conditions.

Examples of these classes are high-speed impacts with massive bridge abutments

and falls from great heights onto hard rocks. Sophisticated analysis can be

used in many cases to demonstrate that the loadings on a cask are still less

than those imposed by the regulation-defined hypothetical accident

conditions. However, questions arise involving the specifics of a particular

cask design and the orientation of impact (i.e., does the orientation assumed

cause maximum damage). On the analysis side, the validity of analytical

methods used to predict the cask response can be questioned. A major part of

this report is directed toward demonstrating what broad classes of real-world

accidents and their associated loadings are enveloped by the loadings implied

1-10
Define accident

scenarios and

loading conditions

Estimate cask

responses

to loading

conditions

First-4tage screening

Within 1OCFR71 Compare estimates with

responses to

1OCFR71 tests

Exceed

IOCFR71

Second-stage screening

Evaluate and

categorize responses

I

Estimate radiological

hazards I I t r

Compare radiological

risks with those

in NUREG-0170 I Evaluate

and document

Figure 1-2 Two-stage screening process used in evaluating the regulations.

1-11
in the current regulatory standards. The first-stage screening envelopes

accident loading conditions whose magnitudes do not exceed those defined by

the accident test conditions and, therefore, the potential radiological

hazards are less than those implied by regulations.

For those accident scenarios with loads and cask responses greater than

those implied by the accident test conditions, a second-stage screening is

performed. This screening evaluates the likelihood of the cask responses.

The potential radiological hazards associated with the cask responses are

then determined. By summing all accident scenarios, the probability and

magnitude of the radiological hazards is estimated and then compared with the

risk evaluated in NUREG-0170.

Because of the numerous variables involved in defining cask loading and

response, and because of the broad range of possibilities and

interrelationships for each of the variables, a systematic scheme is developed

to accomplish the two-stage screening process and to assess the effectiveness

of 10 CFR 71 in assuring adequate radiological protection to the public. To

describe this systematic process, this report is arranged into several

sections. Many tasks are performed: model developments, data sources, data

development, analysis of models, classification, and comparison of results.

Although the tasks are described in the report by sections, the separate tasks

are not developed independently, and they cannot be described without

considering the interrelationship involved.

Figure 1-3 shows the interrelationship of the various tasks and how they

influence the performance of the analysis. The initial tasks in this study

involve developing models for casks and accident environments. Methods are

also developed for evaluating how the cask models respond to accidents and for

classifying their responses into response regions. The screening analyses are

performed by subjecting the casks to the accident events identified in the

accident scenarios, determining the predicted responses of the casks to these

events, and classifying these predicted responses into the response regions.

The cask physical responses are then related to any resulting radiological

hazards. Because the likelihood or probability associated with an accident

1-12
Identify and compile

accident data.

Develop accident scenarios

and loading conditions

(Section 2.0)

Define representative

casks1) truck, 2) rail

(Section 3.0)

Define cask

response regions

(Section 4.0)

T

Develop probabilistic model

1) Correlate and extend

accident data base

2) Calculate parameter

distributions to cover

all loading conditions

3) Relate cask responses to

response regions, loading

conditions and probability

of occurence

(Section 5.0)

III

Perform first-stage

screening.Calculate

cask responses and

their precentage within

1OCFR71 test

conditions

(Section 6.0)

Y

Perform second-stage

screening. Calculate

cask responses and

their percentage for

each response region

exceeding 10CFR71

test conditions

(Section 7.0)

Relate radiological

hazards to cask

damage and response

regions exceeding

10CFR71 test

conditions

(Section 8.0)

T

Compare with values

in NUREG 0170

1) Percentage of

accidents within

IOCFR71 loading

conditions

2) Radiological risk

for all accidents

(Section 9.0)

Figure 1-3 Schematic representation of the report.

1-13
event can be derived from accident data, the probability associated with the

response and radiological hazard can be estimated.

In Section 2.0, the mechanical and thermal loads associated with real

accidents are discussed. Also, accidents are classified into accident

scenarios to systematize the analysis. Statistical accident data have been

used and enhanced where necessary to establish likelihood estimates for the

occurrence of those mechanical and thermal accident loads determined to be

important to cask -response. The mechanical loads are described in terms of

parameters such as velocity of the cask, the hardness of the object that the

cask hits, and whether the crash is head-on, glancing, or at some intermediate

direction. The thermal loads are described in terms of location, temperature,

and duration of a fire.

In Section 3.0, two casks are defined as representative of those used for

ground transportation of spent fuel, one for highway and one for railway. The

details and justification for selecting the representative cask designs are

explained. The margins of safety included in their designs are discussed.

Cask response regions are specified in terms of the physical response of

the cask to accident events. The response regions are described in Section

4.0; they are represented as strain for mechanical loads, and as temperature

for thermal loads. The strains from mechanical loads and temperatures created

by thermal loads which define the response regions are related to deformations

and degradation of the cask's containment and shielding system. Deformation

and degradation of the cask's containment and shielding systems can result in

specific radiological hazards for each of the response regions. Details for

relating radiological hazards to the response regions are found in Section

8.0.

In Section 5.0, the probabilistic model used in the analysis is

described. The formulations used to relate cask responses to loading

conditions, response regions, and the probability of occurrence are

described. Techniques also are developed for calculating the probability for

combined loading conditions for each accident scenario.

1-14
In Section 6.0, the first-stage screening process is described. The

first step in this process is to subject the casks to each accident scenario

identified in Section 2.0 and to estimate the responses. The responses are

sorted into the response regions. The appropriate response region for the

first-stage screening is the lowest response region since it is defined to

encompass 10 CFR 7i accident test conditions. Since the accident rates are

known, the fraction of accidents falling into each response region can be

determined.

In Section 7.0, the second-stage screening process is described. The

accidents not falling into the lowest response region are analyzed and the

responses calculated. These responses are then categorized into the other

response regions.

In Section 8.0, the radiological hazards associated with each cask

response region are estimated. The radioactive material releases are

estimated from laboratory test data. The radiation increases caused by lead

slump are estimated from structural, thermal, and shielding calculations.

Finally, in Section 9.0, the results of the two-stage screening process

are presented with respect to NUREG-0170. The conclusion reached is that at

least 99.4% of truck and train accidents involving a spent fuel shipment will

result in negligible radiological hazards which are less than those implied by

the current 10 CFR 71 regulations. Of the remaining spent fuel shipment

accidents, the overall radiological risk is less than the risk estimated in

NUREG-0170.

1-15

2.0 ACCIDENT RATES, ACCIDENT SCENARIOS, AND LOADING PARAMETER DISTRIBUTIONS

2.1 Introduction

Severe accidents are typically characterized and reported by fatalities,

injuries, property damage, transportation equipment damage, or a combination

of these consequences. In this study, however, the characterization is in

terms of the magnitude and frequency of loads that could be experienced by a

spent fuel cask under accident conditions. Normally the higher the load on a

cask, the higher the cask response and the greater the potential for

radioactive release.

Both mechanical and thermal loads generate response states for a cask

which could result in damage to the cask. High mechanical loads caused by

impact can cause damage to the cask shielding or cause the cask containment to

leak. High thermal loads caused by fires can cause the cask containment seals

to deteriorate and leak or the lead shield to melt. In performing the twostage screening process of accidents discussed in Sections 6.0 and 7.0, all

possible accidents have to be included, especially those that could cause high

mechanical and thermal loads on a cask.

Mechanical and thermal loads depend on the magnitudes of the accident

loading parameters. Two examples of accident loading parameters and their

magnitudes are a velocity of 50 mph and a fire duration of one hour. The same

accident-caused load on a cask can occur for various combinations of loading

parameters and loading magnitudes. For example, the same impact force on the

cask can be generated by a low-velocity impact on a hard object or a highvelocity impact on a soft object. Also, the same heat load on a cask can

occur for a short duration high-temperature fire or a long duration lowtemperature fire. Consequently, specific mechanical and thermal loading

conditions on the cask can occur under a variety of accident conditions.

Accident loading conditions must take into account many loading

parameters and must include a wide range of values for each loading

parameter. Accident scenarios can be derived from historical records. An

accident scenario describes a sequence of events as they occur, allowing the

2-1
identification of possible loading conditions. For example, an accident

scenario can involve a truck running off the highway, going over an

embankment, and crashing into a rock. The loading conditions for this

scenario primarily depend on the hardness of the rock, the velocity of the

truck when it hits the rock, the direction of the truck velocity, and the

orientation of the truck with respect to the rock. By varying these four

parameters, thousands of loading conditions are possible for one accident

scenario.

In order to evaluate all possible accident loading conditions on a cask,

the following accident information is derived in this section:

(1) Accident rates for spent fuel shipments are estimated from historical

accident records for truck and train accidents for similar vehicles.

(2) Accident loads that dominate the accident loading conditions and the

structural and thermal responses of spent fuel casks are identified. The

significant loading parameters for the dominant accident loads are

identified.

(3) Accident scenarios, to include all possible accident loading conditions

for truck and train transport, are identified. Accident data, survey

results, and engineering judgment are used to establish accident loading

parameter distributions.

The accident information derived in this section is used with the

probabilistic computer code called TASP (Transportation Accident Scenario

Probabilities) described in Section 5.0 to calculate and screen the expected

magnitude and frequency of cask responses to accident conditions.

In Sections 2.2 and 2.3, the expected accident rates for spent fuel

shipments by highway and railway are estimated. In Section 2.4, the accident

data required to estimate the accident loads on a cask are identified. In

Sections 2.5 and 2.6, the accident scenarios and loading parameter

distributions are discussed.

2-2
2.2 Highway Accident Rates

Highway accident rates depend on many elements including road type,

vehicle type, regulations, and driving practices. The accident rate for all

vehicles on California highways during 1981 through 1983 ranged from 1xi0-6

accidents/vehicle-mile for freeways with limited access to 5x10-6 accidents/

vehicle-mile for conventional four-lane highways. 1 Studies by the U.S.

Department of Transportation (DOT) have indicated that accident rates are

significantly lower for interstate federal highways (usually freeways) than

for other road types. Routes for transport of spent fuel are selected in

accordance with the DOT regulations to minimize the radiological risk. In

general, the routes follow interstate federal highways. 2

As discussed in Appendix B, two sources are used for estimating a typical

accident rate for spent fuel transportation. An average accident rate of

2.5xi0- 6 accidents/vehicle-mile is derived from the data published by the

Bureau of Motor Carrier Safety (BMCS) for all roadways. 3 - 5 Their data covered

all truck and carrier type accidents from 1960 through 1972. The second data

source is the American Petroleum Institute (API) for the period of 1968

through 1981 for all roadways. 6 -1 0 The average accident rate is 6.4x10-6

accidents/vehicle-mile or approximately 2.5 times higher than that based on

the BMCS data. For this study the API accident rate is used as the estimate

for spent fuel truck accident rates because the data is judged to be more

reliable, and trucks which transport hazardous petroleum materials are similar

in size and weight to trucks that transport spent fuel casks. The use of the

more conservative API value is. not critical to the results of this study.

2.3 Railway Accident Rates

Train accident rates depend on many elements including the type of train,

the type of track, and the reporting requirements. Freight trains are used to

transport spent fuel over all track types and are subject to Federal Railroad

Administration (FRA) reporting requirements. Because over 90% of all train

mileage is attributed to freight trains, there is no significant difference in

applying data based on all trains to freight trains in order to estimate

accident rates, accident velocities, fire frequencies, etc.

2-3
Appendix C discusses the train accident rate selected for spent fuel

shipments by train. Based on the FRA data for all train and track types, an

accident rate of 1.2xi0- 5 accidents/train-mile is assumed for spent fuel rail

shipments.11-17

2.4 Accident Loading Data Requirements

Historical data bases on transportation accidents exist at all government

levels. These data bases range from local accident records to state and

national accident statistics. Typically, these records include many accident

conditions and consequences that are not pertinent to this study, including

weather conditions, fatalities, injuries, and property damage. However, some

of the data are pertinent to this study; namely, data pertaining to accident

loading conditions which could cause cask damage. Typical of such data are

estimations of accident velocities, descriptions of objects impacted, and

duration of fires. Most of these data bases are compiled to aid general

transportation safety with the main focus on reducing injuries, fatalities,

and property damage. They do not always include all the information necessary

to define the loading a cask might experience. Therefore, specific data

necessary to estimate accident loads on a cask are not always available.

Table 2.1 presents mechanical and thermal loads that can occur in an

accident. The accident loading parameters that cause the loads and affect the

response of the cask for various load types are also listed.

Mechanical loads include forces on the cask caused by impact with a

surface or hard object, puncture by strong objects, and crushing by heavy

objects. Based on the evaluation in Appendix E, it is concluded that impact

loads are the dominant mechanical loads and have the greatest potential for

causing significant structural damage to a spent fuel cask. Therefore, only

impact loads and their associated loading parameters are used to perform the

two-stage screening of accidents generating mechanical loads.

Mechanical loads from impacts can be analyzed using three loading

parameters that affect the cask response and potential damage: impact

velocity, orientation of the cask, and the hardness of the object impacted.

2-4
Table 2.1

Accident Loads and Loading Parameters

Accident Loads

Loading Mechanical Load Type Thermal Load Type

Parameter Impact Punch Crush Fire Torch Decay Heat-A/

Object

Hardness X X X

Impact Velocity X X

Cask

Orientation X X X

Object Weight X X X

Object

Impact Area X

Flame

Temperature X X

Fire Duration X X

Fire Location X X

Flame

Emissivity X X

Convection

Coefficient X X

Surrounding

Material X

a/ Decay heat from spent fuel cargo.

2-5
Figure 2-1 defines these three loading parameters. The impact velocity is the

cask velocity perpendicular to the surface impacted. The angle of impact, a,

represents the angle between the cask velocity vector and the object's

surface. When an accident occurs, the cask velocity vector can take any

direction. However, it can always be decomposed into two components: one

perpendicular to the impacted object surface and one parallel to it. The

accident velocity is a function of reported vehicle velocity, braking effects,

and fall heights from bridges or embankments. In the cask response

calculations, only the velocity component perpendicular to the object surface

is considered. The velocity component parallel to the object surface

introduces a sliding-friction effect to the cask structure. The slidingfriction effect will not induce any significant structural deformation in the

cask. In this study, the angle of impact is combined with the cask velocity

to produce the cask impact velocity, i.e., impact velocity equals cask

velocity times sine a where a is the angle of impact and the impact velocity

is treated as a single loading parameter.

The angle defining the cask orientation, B, is the angle between the cask

longitudinal axis and the object's surface. The cask orientation affects the

cask response, particularly for endwise impacts (0 - 900) where lead slump can

occur at high impact velocities.

Object hardness needs to be considered because casks can strike objects

such as concrete abutments, roadbeds, hard rock, soft rock, hard soil, and

water. The hardness of the objects and the associated impact responses vary

greatly. The weight of the object impacted can also affect the response of

the cask. However, only massive objects can cause significant mechanical

loads on a cask, hence the object hardness is the dominant parameter that is

considered for objects impacted.

In some accidents, such as rail grade-crossing accidents, the impact

limiters on the cask can be bypassed and the side of the cask can be struck

directly. Once again the mechanical loads depend on the impact velocity, the

orientation of the cask, and the hardness of the object struck. Figure 2-2

defines these three loading parameters for this type of accident. The impact

2-6
,C3 wX Je~oc%. iVqp ~ c ~ ~ o

Impact velocity = cask velocity X sina

-bjiect surface

o Object surface hardness

o Impact velocity: Cask velocity component perpendicular to the object surface

o Cask orientation is defined by angle 0, the angle between the cask longitudinal axis and the object's surface

Figure 2-1 Three impact loading parameters considered in the

response analysis for impacts on surfaces.

2-7
'- Accident velocity

(normal to cask axis)

Accident velocity

o Object hardness

o Impact velocity: Relative velocity component perpendicular to cask

surface.

o Cask orientation angle, 8: the angle between the accident velocity and

impact velocity.

Figure 2-2 Three impact loading parameters considered in the

response analysis for impacts with objects such as

train sills.

2-8
velocity is the component of the relative velocity of the cask and object that

is perpendicular to the task surface. The angle of impact, a, represents the

angle between the relative velocity direction and the cask axis. For the

purpose of this study, the impact angle is conservatively assumed to be 900,

that is, perpendicular to the cask axis in all cases. Also, it is assumed

that the impact occurs at the mid-plane of the cask to cause the most

damage. The. cask orientation angle, 0, is the angle at which the impact

occurs on the cask surface as shown in Fig. 2-2. In the worst case the cask

is hit at 00 or' head-on. For orientation angles near 900, the cask is

essentially not struck. The object hardness depends on the object hitting the

cask, such as a train sill or a small bridge column.

The thermal loads identified in Table 2.1 include the heating of a spent

fuel cask by large fires, both engulfing and non-engulfing; torch fires; and

decay heat from the spent fuel, particularly when the cask is accidentally

buried in debris. Based on the evaluation in Appendix F, it is concluded that

heat loads from large fires, both engulfing and non-engulfing, have the

greatest potential Ifor causing significant damage to a spent fuel cask.

Therefore, only heat loads from large fires and their associated loading

parameters are used in the screening of accidents generating thermal loads.

Thermal loads from large fires depend on three loading parameters that

affect the cask response and potential damage: fire duration, flame

temperature, and fire location. The fire duration affects the amount of heat

that is transferred into the c'ask--the longer the fire burns the greater the

amount of heat that is absorbed by the cask. Higher flame temperatures cause

greater amounts of heat to be transferred to the cask. As discussed in

Appendix F, the flame temperature, assuming a flame emissivity of 0.9, is the

single par-ameter used to characterize both radiation and convection heat

transfer over a wide range of conditions. The location of the fire with

respect to the cask affects the amount of heat that can be transferred to the

cask. An engulfing fire would transfer the most heat to the cask, given the

same flame temperature and fire durations, whereas less heat would be

transferred from non-engulfing fires.

2-9
Accident records typically classify accidents into broad categories or

types that describe, in general, the causes of the accidents. Examples are

ran-off-the-road, overturn, and derailment. Accident scenarios describe a

sequence of events and involve individual accidents that occur at specific

velocities, impact specific objects at specific angles, and perhaps include a

fire. For the purpose of this study, accident scenarios are specified and

typically identified by the object impacted. By interpreting accident data

bases in the context of these scenarios, the analysis is made manageable.

2.5 Highway Accident Loading Parameters

2.5.1 Mechanical Loading Parameters

Three mechanical loading parameters have been identified which can affect

the structural response of a cask in a severe accident: object hardness,

impact velocity, and cask orientation. The distribution functions for these

parameters can differ with each specific accident scenario. The object

hardness distribution is derived from the truck accident data base. For

accident scenarios that could cause high mechanical loads on a cask, impact

velocity distributions are estimated from truck and train accident velocity

data, bridge height data, and engineering models. No specific data is

available to estimate cask orientation on impacts; therefore, distributions

are estimated from engineering models.

2.5.1.1 Accident Scenarios and Object Hardness

Data from several sources are collected and combined in Appendix B to

estimate the frequency of specific accident scenarios and potential impacts on

specific objects of varying hardness. The accident scenarios are primarily

based on truck accident data documented in the BMCS annual reports for the

years 1973 through 1983.18-27 The BMCS accident data are for all truck sizes

and all roadways including city streets, county roads, state and interstate

highways.

2-10
Figures 2-3 and 2-4 list the truck collision and non-collision accident

scenarios used to categorize the response of spent fuel casks to accident

loads. Thirty-one scenarios, each identified with an accident index number,

are presented. By combining historical accident records with a survey of

highway roadside structures, the probability associated with each accident

scenario is estimated in percent. For example, a truck can be involved in a

collision accident, hit a bridge railing, run over the bridge, and drop into

water below (accident index 7 in Fig. 2-3). This scenario describes a

sequence of events involving many different accident features such as

collision objects, bridge railings, and water.

An example will be used to illustrate how this probability estimate is

made. Figure 2-3 shows that 74.12% of truck accidents are collisions. Of

these collision accidents, 11.95% involve hitting a roadside fixed object.

The probability that the roadside object is a bridge railing is 5.77%. The

probability that the truck, after hitting the bridge railing, breaks through

the bridge railing and lands in the water is 20.34%. Therefore, the

fractional occurrence for the example scenario is 0.104% given that a truck

accident occurs. Multiplying this fractional occurrence by the assumed truck

accident rate'of 6.4x10- 6 accidents/vehicle-mile gives the chance of this kind

of accident occurring per mile traveled as 6.7x0- 9 .

2.5.1.1.1 Collision Accident Hardness Data

Figure 2-3 summarizes collision accident scenarios and the frequencies of

collisions with moving objects such as trucks, autos, and trains as compiled

from the BMCS data. Over 56% of the truck accidents involve collisions with

another truck or auto. The BMCS accident data did not classify collisions

with fixed objects, even though they ranged from stop signs to bridge

columns. To classify fixed objects, highway accident data are obtained from

the California Department of Transportation (CALTRANS) reports of stationary

objects struck along state and interstate highways for the years 1975 through

1983.2836 Those objects in the CALTRANS survey are tabulated and a fraction

calculated for each type of fixed object. These fractions are then applied to

the fixed object collision accidents in the BMCS data to estimate the number

of accidents involving each type of object, such as a bridge rail or column.

2-11
'Soft objects' cones, animals, pedestrians

Non-fixed

object

10.8805

0.0521

Motorcycle

0.0124

Automobile

0.6612

Truck, bus

0.2041-

Train

0.0118

Other

Collision

0.0584

Water

0.20339

Railbed/roadbed

0.77965

Bridge railing Clay, silt

0.0577 0.015486

Hard soil, soft rock

0.001262

Hard rock

0.000199

Small

Colmn 0.8289

Colmn, abutment 0.9688 Large

0.0042 lAbutment 0.1711

Concr. obj, bottom str. 0.0382

Probability

percent"

3.4002

0.8093

43.1517

13.3201

0.7701

3.8113

0.1039

0.3986

0.0079

0.0006

0.0001

0.0299

0.0062

0.0011

0.0850

4.0079

0.5111

3:7050

Accident index

1

2

3

4

5*

6

7*

8*

9*

10*

11*

12"

13"

14"

15

16

17

18

On road

fixed obj.

0.1195

0.0096

Wall barrier, wall, post

0.4525

Signs, cushions

0.0577

Truck Curb, culvert

accident

6.4X10.6

per mi.

0.4183

Noncollision See figure 2-4 for

0.2588 non-collision accidents

* Potentially significant accident scenarios

Conditional probability which assumes an accident occurs

Figure 2-3 Truck collision accident scenarios and their percent

probabil ities.

2-12
Collision See figsre 2-3 for,

0.7412 collision accidents

Truck

accident

6.4X10-6

per mi. Clay, silt

0.91370

Into slope Hard soil/soft rock

0.2789 0.074b4

Hard rock

0.01176

Of f road Over embankment

Clay, silt

0.5654

Hard soillsoft rock

0.0461

Hard rock

0.007277

Drain ditch

0.381223

Probability

percent"

2.3063

0.1881

0.0297

1.3192

0.1076

0.0170

0.8894

0.9412

3.2517

8.3493

0.3497 0.2578

Accident index

19"

20*

21'

22*

23*

24*

25

26

27

28

29

30

31

!Non- ;cllsion

0.2588

Trees

0.1040

Other

0,3593

Overturn Imnpt

roadbed 0.6046

0.5336 Jackknife

0.3954

Other-involving mech. loading

5.4603

2.0497

0.9705

0.0792

Fire only

0.0375

* Potentially significant accident scenarios

" Conditional probability which assumes as accident occurs

Figure 2-4 Truck non-collision

percent probabilities.

accident scenarios and their

2-13
Based on the quasi-static screening analysis in Section 6.0 for

mechanical loads and responses of the representative truck cask, only three

significant accident scenarios can cause mechanical loads high enough to

damage a spent fuel cask: collisions with trains and columns, trucks running

off bridges and over embankments, and trucks running into slopes. Therefore,

detailed accident loading information is compiled only for these significant

scenarios.

Since collision accidents involving piers, columns, and abutments may

lead to significant damage to a spent fuel cask, a survey is performed to

differentiate among the various sizes of piers, columns, and abutments along

state and interstate highways.37 From the survey data, the fractional

occurrence is determined for each pier, column, and abutment size and is used

to estimate the probability of collision accidents involving piers, columns,

and abutments. For example, the expected probability of collisions with large

concrete abutments is estimated to be 0.0011% as given in Fig. 2-3.

In the event a truck runs off a bridge, the magnitude of the resulting

impact load depends not only on the bridge height, but also on the surface

being impacted below the bridge. A survey along Interstate 80 in California

is performed to identify the types and frequency distributions of surfaces

that could be impacted below the bridge. 3 8 These surfaces are classified into

four categories: roadbeds, railbeds, water, and earth. The earth category is

then subdivided into three sub-categories: soil, soft rock, and hard rock.

The earth sub-category distributions are determined by the survey performed

for "ran-off-the-road." Table 2.2 is a summary of the impact surface

distribution under bridges.

2.5.1.1.2 Non-Collision Accident Hardness Data

Non-collision accident scenarios include rollover, jackknifing, and

running off the road. The accident scenarios judged to have greatest damage

potential for a spent fuel cask are the ran-off-the-road scenarios. In these

accidents, the truck could impact a slope or go over an embankment, with the

possibility of hitting a hard rock such as granite.

2-14
Table 2.2

Fractional Occurrence of Surface Types below Bridges on

Interstate 80 from Davis, California to Nevada Border

Surface Type Fractional Occurrence

Water 0.2034

Roads/Railways 0.7797

Earth

Soil 0.0154

Soft Rock 0.0013

Hard Rock 0.0002

2-15
The hardness of earth surfaces adjacent to highways can vary over a wide

range. This variability can have a significant effect on the loadings that

could be imposed on a cask or any other impacting object. The water and land

(hard rock, soft rock/hard soil, and tillable soil) distribution along

proposed spent fuel shipment routes between the east coast and west coast is

initially estimated using agricultural soil survey data and geological highway

maps for the United States. 3 9 ,

4 0 The initial distributions estimated from

these sources are considered to be indicative of the types of surfaces which

could be impacted along highways in the various regions of the United

States. However, since highway construction and landscaping can greatly

affect the adjacent surroundings, the initial distributions are used to select

representative portions of Interstates 5 and 80 in California to perform

detailed highway surveys and to establish final distributions along highways.

The types of earth adjacent to 133 miles of Interstate 5 through Orange

and Los Angeles Counties in California are classified into three groups:

tillable soil, non-tillable soil, and hard rock (Appendix D, Table D.2). Only

tillable soil (92.8% fractional occurrence) and untillable soil, classified as

soft rock (7.2% fractional occurrence), are identified on a total mileage

basis. Although this survey included portions of the Santa Susana Mountain,

no hard rock is identified in the survey.

A highway survey of soil types adjacent to the roadway is then performed

on a section of Interstate 80 from Davis, California, to the Nevada

border. 38 This 122 mile section of Interstate 80 crosses the Sierra where

numerous outcroppings of granite rock occur. This survey (Appendix D, Table

D.3) indicates the following earth distribution : tillable 90.2%,

non-tillable 7.3%, hard rock 2.5%.

Based on the results of both highway surveys and the reviews of the

agricultural soil surveys, the geological highway maps, and proposed spent

fuel shipping routes, the representative earth distribution used in this study

is tillable soil 91.4%, soft rock/hard soil 7.4%, hard rock 1.2%.

2-16
2.5.1.2 Impact Velocity

The impact velocity depends on the relative velocity of the cask and the

angle of impact with respect to the object impacted. The distributions of

these two variables are estimated from truck accident records, train accident

records, highway surveys, and engineering judgments for the significant

accident scenarios.

2.5.1.2.1 Cask Velocity

The distribution of potential cask velocities can vary depending on the

specifics of the accident scenario. Each accident scenario may have a

different historically based velocity distribution. For example, the

distribution of accident velocities experienced in truck-truck collisions

differs from the distribution associated with accidents involving falls from

bridges. In the truck-truck accidents, the distribution depends on the

individual velocities of the trucks at collision. For accidents involving

falls from bridges, the accident velocity is determined by the fall height.

The accident velocity distribution for accident scenarios involving trucks

running over or off embankments could, at worst, be represented by the vector

sum of the vehicle velocity and the velocity attained in the resulting fall.

One of the following distributions of cask velocities at impact is

considered applicable to a particular truck accident scenario:

VI: A distribution based on truck accident velocities with braking

effects included,

V2: A distribution based on fall heights from bridges,

V3: A distribution based on truck accident velocities with braking

effects and fall heights from bridges, or

V4: A distribution based on train accident velocities at grade crossings.

2-17
Reports record accident velocity data in many different forms. Mos t

reports give the vehicle velocity prior to the accident. Therefore, it is

difficult to estimate the actual velocity of impact which a cask can

realistically experience.

Distribution V1 is determined by consideration of accident reports

involving trucks/ semitrail1ers. Table 2.3 gives the fraction of accidents

occurring in the State of California for 1958 through 1967 for trucks/

semitrailers as a function of truck velocity prior to the accident.41-

51 This

accident data is derived from the California Highway Patrol's (CHP) annual

report on fatal and injury motor vehicle traffic accidents. This data

represents a sample of truck/semitrailer drivers involved in fatal and injury

accidents and their estimated accident velocity without braking effects

included. Approximately half of truck accidents occur at velocities greater

than 30 mph. This velocity data is conservative because it does not include

non-injury accidents, which typically occur at lower velocities.

Accident velocities for the State of California are compared with those

in the states of Alabama, Texas, Virginia, and North Carolina.52-

56 The

comparison is made for all vehicles because not all of the states had

information on trucks. The comparison shows that the California accident

velocities are comparable for the same conditions. Therefore, it is concluded

that the accident velocities from California are representative of those in

the nation and that the truck/semitrailer accident velocities for California

provide a reasonable estimate of future accident velocities for spent fuel

transport trucks. Accident data from North Carolina is used to estimate the

effects of braking on the reduction of impact velocity. The method used to

estimate the velocity reduction is described in Subsection 5.2.1.2.

Distribution V2, the velocity attained in falls from bridges is developed

directly from a survey of bridge height data presented in Table 2..3 This

bridge height data is collected along Interstate 5 during the survey of bridge

column sizes and types of soil along the highway. The bridge height

distribution is reasonable for representing travel on interstate and state

highways.

2-18
Table 2.3

Distribution of Velocities for Trucks/Semitrailers

Involved in Fatal and Injury Accidents in California, 1958-1967a/

Number Fractional Cumulative

.Velocity of Percent Percent

(mph) Accidents (M) (%)

0 1,774 6.41 6.41

1 - 10 4,143 14.96 21.37

11 - 20 4,122 14.89 36.25

21 - 30 4,248 15.34 51.59

31 - 40 4,733 17.09 68.69

41 - 50 7,264 26.23 94.92

51 - 60 1,173 4.24 99.15

61 - 70 171 0.62 99.77

>70 63 0.23 100.00

Subtotal M 1 -

Not stated 2,834 -

Total 30,525

a/. Data derived from the 1958 to 1967 annual reports on fatal and injury

motor vehicle traffic accidents, California Highway Patrol

2-19
Table 2.4

Distribution of Bridge Heights along Interstate 5

through Orange and Los Angeles Counties, California

Number Fractional Cumulative

Bridge Height of Percent Percent

(ft) Bridges (%) (%)

0 - 10 5 4.13 4.13

11 - 20 22 18.18 22.31

21 - 30 74 61.16 83.47

31 - 40 14 11.57 95.04

41 - 50 3 2.48 97.57

51 - 60 1 0.83 98.34

61 - 70 1 0.83 99.17

71 - 80

81 - 90 1 0.83 100.00

Total MT M -

2-20
Distribution V3 is developed for those accident scenarios in which the

velocity is considered to be the vector sum of the accident velocity V1 and

the fall velocity V2. This distribution is used for accidents that involve

running off of embankments and into slopes.

Distribution V4 is used for accident scenarios involving train-truck

collisions at grade crossings. The magnitude and frequency of the cask

velocity is estimated from rail-highway grade-crossing accident velocity

data. This accident data is derived from the FRA annual report on railhighway grade-crossing accident/incident and inventory for the years 1975

through 1982.57-64 Table 2.5 gives the fraction of rail-highway gradecrossing accidents as a function of train velocity. Fewer than 30% of the

accidents occur at velocities greater than 30 mph.

2.5.1.2.2 Impact Angle

The impact angle is the angle between the cask velocity and the plane of

the surface struck. The damage caused in a transportation accident is not

controlled solely by the vehicle(s) velocity at impact. A head-on impact is

more severe than a sideswiping event, even though both accidents could involve

similar accident velocities. The reason is that accident severity is most

directly related to the vector component of the accident velocity

perpendicular to the object being struck. The orientation of the vehicle, or

in this case, cask motion relative to the plane or surface of the object

impacted, is established by a parameter called the impact angle, depicted

earlier as angle a in Fig. 2-1. A 90 0-impact angle defines the accident as

head-on; that is, the impact velocity and accident velocity at impact are the

same. An impact angle close to 00 defines the accident as a sideswiping

impact; that is, the impact velocity is only a small fraction of the accident

velocity. In mathematical terms the impact velocity is the accident velocity

multiplied by the sine of the impact angle.

The distribution of impact angles can be expected to be a function of the

accident scenario being considered. For example, if an accident involves a

collision with another vehicle on the road, any impact angle is equally

2-21
Table 2.5

Train Velocity Distribution for Rail-Highway Grade-Crossipg

Accident/Incidents Involving Motor Vehicles, 1975-198Za/

Number Fractional Cumulative

Velocity of Percent Percent

(mph) Accidents (%) (%)

0 - 9 27,553 33.79 33.79

10 - 19 16,765 20.56 54.35

20 - 29 14,611 17.92 72.47

30 - 39 10,788 13.23 85.50

40 - 49 7,617 9.34 94.84

50 - 59 2,879 3.53 98.37

60 - 69 824 1.01 99.38

70 - 79 461 0.57 99.94

80 - 89 29 0.04 99.98

>90 17 0.02 100.00

Subtotal 8,4 T -

Unknown 573

Total 82,117

a/ Data derived from the 1975 to 1982 annual inventory on rail-highway gradecrossing accidents/incidents, Federal Railroad Administration

2-22
likely. Information on impact angle distributions is not readily available;

however, three distributions are defined. The distributions include:

VVI: A uniform distribution in which any impact angle is equally likely,

VV2: A distribution which considers all impacts as 900 occurrences, and

VV3: A triangular distribution in which 900 impacts are most likely with

other orientations decreasing in likelihood as the impact angle

decreases.

2.5.1.3 Cask Orientation

Historical records do not contain significant information on the

orientation of the cask with respect to the object impacted. For impacts on a

surface 00 cask orientation defines a sidewise impact while a 900 cask

orientation defines an endwise impact of the cask. Alternatively for impacts

by train sills, a 00 cask orientation defines a head-on impact to the cask

side while a 900. cask orientation indicates a near miss. Again, since the

cask orientation distribution can be dependent on the accident scenario being

considered, three cask orientation distributions are defined. The

distributions include:

CTI: A uniform distribution in which all cask impact orientations are

equally likely,

CT2: A triangular distribution in which end-on impacts on surfaces or

head-on impacts to the side of the cask by train sills are most

likely, with other orientations decreasing linearly in likelihood as

the orientation angle approaches 00, and

CT3: A triangular distribution in which impacts at 450 are most likely,

with other orientations decreasing linearly in likelihood as the

orientation angle approaches either 00 or 900.

2-23
2.5.2 Thermal Loading Parameters

The thermal response of a cask, specifically the temperature reached

within the gamma shield, is determined by three major thermal loading

parameters: fire duration, flame temperature, and fire location with respect

to the cask. The distribution functions for these parameters can be a

function of the specific accident scenario being evaluated and can also vary

from accident to accident within the same accident scenario (e.g., variations

of fire locations with respect to the cask).

The BMCS reports and other sources provide information such as the

accident type, the cause of fire property damage, and method of

extinguishment. 6 5 This information is useful for defining actions to improve

public safety. The sources, however, do not provide data on thermal loading

parameters such as flame temperature and fire duration. Limited data on

thermal loading parameters are sometimes included in the National

Transportation Safety Board severe accident reports, but the data is not

sufficient to adequately define thermal loads and their fractional occurrence.

A truck-fire accident has many variables that affect the fire and thermal

loads. The variables include the involvement of the truck's fuel tank and its

contents; the possibilities of a collision with an auto, another truck or a

tanker truck; and the availability of fire fighting equipment. The many

variables and the lack of.-specific data lead to the use of the Monte Carlo

technique 6 6 and engineering models to determine the distribution functions for

the thermal loading parameters.

2.5.2.1 Accident Scenarios and Fire Frequency

The accident scenario in which a truck is involved can affect the thermal

loads on the truck and its cargo. Table 2.6 presents the accident type and

the frequency of fires. 6 6 In Subsection 5.3 these accident fire frequencies

are correlated with the accident scenarios in Figs. 2-3 and 2-4 to determine

the probabilities of fire for each of the scenarios.

2-24
Table 2.6

Frequency of Fire for Truck Accident Types

Fire Involved No Fire

Accident Type in Accident in Accident

S(%)

Collision with Auto 0.3 99.7

Collision with Truck 0.8 99.2

Collision with Fixed Object 0.4 99.6

Other Collision 0.9 99.1

Ran off Road 1.1 98.9

Overturns 1.2 98.8

Other Noncollision 13.0 87.0

2-25
2.5.2.2 Fire Duration

Since the available fire-accident data do not provide specific

information on fire duration, the Monte Carlo method is used to derive the

fire duration distribution for each accident scenario. 66 This method combines

data on accident types, cause of the fire, availability of combustibles, and

fire-fighting efforts with statistical engineering models on the burning of

combustibles for various types of accidents. A Monte Carlo computer code is

used as recommended 6 6 to analyze the interaction and probabilistic involvement

of fuel tanks, tires, cargo, brakes, and electrical systems, as well as the

effects of fire fighting efforts.

The Monte Carlo code is also used to predict fire duration distributions

for each accident scenario in Figs. 2-3 and 2-4. As might be expected, there

is a large variation in the fire duration distributions for the scenarios. In

general, the fire durations following high impact loads on hard surfaces are

shorter compared to those involving lower impact loads or collisions with

other trucks, particularly tanker trucks.

2.5.2.3 Flame Temperature

Flame temperature depends on the burning materials and the amount of

oxygen present in the flame. This study uses the flame temperature

probability distribution from Sandia. 6 6 The fire distribution is primarily

based on the open burning of hydrocarbon fuels such as diesel and gasoline in

the temperature range of 1400 to 24000 F, but also includes other materials

which tend to burn at lower temperatures.

The size of a fire affects both the radiation heat transfer capabilities

and the duration of the fire. Fires with a flame that is at least four feet

high radiate essentially as a blackbody with flame emissivity in the range of

0.9 to 1.0. Smaller fires have much lower emissivities and are usually of

short duration, and would have little effect on a cask.

The convection heat transfer from a fire to a truck and its cargo is

usually less than 10% of the radiation heat transfer. As discussed in

Appendix F, an equivalent flame temperature for specific cask configurations

2-26
can be used to estimate the thermal loads for various combinations of flame

temperatures, flame emissivities, and convection coefficients. In this study,

it is conservatively assumed that all fires will have an emissivity of 0.9.

2.5.2.4 Fire Location

The heat load to a cask varies with the location of the fire with respect

to the cask. The heat load to the cask can decrease by a factor of 4 for a

fire 20 feet from the cask compared with the heat load for an engulfing

fire. As with other fire parameters, insufficient historical accident data

exists to develop fire location distributions with respect to the cask. A

uniform distribution for cask-to-fire location is assumed for all fire

accident scenarios defined by:

LI: A uniform distribution in which any fire location relative to the

cask is equally likely, in the interval between 0 and 31.5 feet. The

cask is sidewise to the fire in all cases to maximize the heat load

to the cask.

2.6 Railway Accident Loading Parameters

2.6.1 Mechanical Loading Parameters

Types of train accidents are identified from FRA data, and supplemented

by other sources to define accident scenarios used in this study. For some of

the accident scenarios, loading parameter magnitudes and frequencies are

estimated from highway data. In other cases, loading parameter data is

derived from severe accident reports. In all cases, the selection of the data

is justified as being suitably conservative. As with highway accident

scenarios, the primary effort in obtaining railway accident data is placed on

collecting information on those accident scenarios that could result in high

loads to a cask. In this subsection the distribution functions are determined

for three mechanical loading parameters: object hardness, impact velocity,

and cask orientation.

2-27
2.6.1.1 Accident Scenarios and Object Hardness

Data is collated from several sources to derive accident scenarios and to

estimate the cask impact frequency with a particular object. The combined

data are presented in Fig. 2-4 for derailment, collision, and other accident

types. The fraction of train accidents due to each type is estimated from the

FRA data in Appendix C.1 1- 1 7 Derailment is the most common railway accident,

accounting for 77.1%. Derailment involves a section or all of the train

leaving the track. The section leaving the track separates from the preceding

car as it leaves the track, causing the braking system to activate for all

cars in the train. The lead car leaves the track at the highest speed, and

the other cars follow at successively slower speeds. The average derailment

involves approximately 10% of the cars in the train.

Collision accidents account for 13.4% of train accidents. The damage

during a collision is usually limited to the cars near the impact point and

involves less than 10% of the cars. For head-on collisions, damage is usually

limited to the locomotive and the few cars that follow. For rear-end

collisions, only the caboose and the few cars ahead of it are damaged.

Other accidents, including grade-crossing accidents, account for the

remaining 9.5% of the accidents. These accidents usually do not cause serious

impact forces to the train.

As shown in Fig. 2-5, collision accidents can result in derailments. In

64% of the collisions, the train remains on the tracks. In this case the cars

may impact each other, but the forces would be relatively low or else the cars

would have left the tracks. In 36% of the collisions, a derailment results

and the cars leave the tracks. When considering the percentage of derailments

occurring with collisions, the total percentage of train accidents that

involve derailment is 82%.

The severe accident data in Appendix A is used in conjunction with the

highway data to identify the objects and to estimate impact frequencies for

the derailment accidents. 6 7 Owing to the limited amount of severe accident

data and the nature of the reports, there is a high uncertainty in applying

the data to the continuous spectrum of accidents.

2-28
Rail-highway grade crossing

0.0304

Remain on track

Water

0.20339

Clay, silt

0.015486

Hard soil/soft rock Collision

0.1341

Derailment

0.3596

Derailment

0.7705

Over bridge

U.UuI1 & concr. U.MU1252

Hard rock

0.000199

Railbed, roadbed

0.77965

Drain ditch

" laer ilt

Iembankment !0.5654

0.0110 [Hard soillsoft rock_

0.04610

•Hard rock

0.007277

Clay, silt

0.91370

Into dope IH48rd soillsoft rock

Train

accident

1.19.\10"

per mile

Probability Accident

percent- index

3.0400 1

8.5878 2

0.1615 3"

0.0122 4"

0.0010 5"

0.0002 6"

0.6192 7"

0.3433 8

0.5092 9.

0.0415 10"

0.0066 1W"

1.4437 12*

0.1178 13"

0.0186 14"

0.0465 1W

0.0096 1W"

0.0017 17"

16.4477 18

3.2517 19"

10.0148 20

0.8408 21"

15.9981 22

31.9865 23

6.500 24

3erailmont I

).818722 0.0193 0.07454

LHaid rock •

0.01175

Small

Coun 0.8289

0•,,-003 [Large

D .1711I

Into structure I

0.2016 IAbutruent

0.0001

lother

0.9965

Locomotive

0.2305

coil. ICar

i0.2272 0.7099

0.7584 / Roadbed

Nr:.10.333ý4

0.77128 Lert

0.6666

Other

0.0t650U

"Potentially significant accident scenarios

-Conditional probability which assumes an accident occurs

Figure 2-5 Train accident scenarios.

2-29
If a derailment accident occurs, the train can go off a bridge or an

embankment, strike a slope, or rollover onto the adjacent ground. In this

study, the percentage of accidents that go off a bridge or an embankment or

onto a slope is estimated to be the same as those for highway accidents. For

these types of accidents, the frequencies of impacting different soils,

roadways, and water are also assumed to be the same as those used for highway

accidents. These estimates and assumptions are made because of the lack of

data on railway accidents and the fact that railways cross similar terrain as

highways for similar routings. The remaining derailment accidents are assumed

to be rollover-type accidents.

When a train derails in a rollover type of accident, it can (1) slide

along the adjacent railbed or earth with relatively low damage occurring;

(2) hit the superstructure of adjacent cars or locomotives; (3) strike

couplers from adjacent cars; or (4) impact structures adjacent to the track.

The severe accident data from Eggers 67 is used to estimate the frequencies for

impact on railbed, earth, car superstructure, locomotive superstructures, car

couplers, and adjacent structures. As shown in Fig. 2-5, it is estimated from

the Eggers database that 0.8% of the train derailment accidents involve train

couplers. The frequency for impacting large structures, such as columns and

abutments, is estimated to be the same as the frequencies obtained from the

CALTRANS highway data.

2.6.1.2 Impact Velocity

The impact velocity of a cask involved in a train accident depends on the

cask velocity and the impact angle. The cask velocity depends on the train

velocity prior to collision or derailment and the height of any fall that

might occur. The impact velocity distributions for a cask involved in train

accidents are estimated from train accident records, surveys, and engineering

judgments.

2-30
2.6.1.2.1 Cask Velocity

For potential accidents in which the rail cask impacts an object, the

magnitude and frequency of the impact velocity are estimated from the train

accident velocity provided in Appendix C. This estimate conservatively

disregards the fact that a reduction in impact velocity occurs because of

energy absorption by the transporting car or the rest of the train.

Tables 2.7 and 2.8 give the average frequencies of train collisions and

derailments as functions of accident velocities, respectively, for the years

1979 through 1982. This accident data is derived from the FRA reports on

train accidents. 13 -1 7 The velocities for other accidents include gradecrossing incidents which are included in the truck data.

In the absence of a statistical data base on distance fallen by trains

going off bridges and embankments in actual accidents, the highway survey

bridge distribution in Table 2.4 is used to estimate distances fallen in this

type of accident. Since specific train and truck routes for transporting

spent fuel traverse similar terrain, the use of the highway bridge data for

this study is reasonable.

In summary, the cask velocity distributions for each of the potentially

significant train accident scenarios are:

TV1: A distribution based on train collision accident velocities without

braking,

TV2: A distribution based on train derailment accident velocities without

braking,

TV3: A distribution based on fall heights from bridges, and

TV4: A distribution based on the vector sum of train derailment

velocities and fall heights from bridges.

2-31
Railroad Accident Velocity

Table 2.7

Distribution, Collisions, Main Line, 1979-1982-/

Number Fractional Cumulative

Velocity of Percent Percent

(mph) Accidents (%) (%)

1 - 10 392 46.12 46.12

11 - 20 182 21.41 67.53

21 - 30 117 13.76 81.29

31 - 40 92 10.82 92.12

41 - 50 47 5.53 96.65

51 - 60 14 1.65 99.29

61 - 70 3 0.35 99.65

71 - 80 2 0.24 99.88

81 - 90 0 0.00 99.88

>91 1 0.12 100.00

Subtotal MW -

Unknown 8

Total 858

a/ Data derived from Federal Railroad Administration reports

accidents, 1979 - 1982.

on train

2-32
Table 2.8

Railroad Accident Velocity Distribution, Derailments, Main Line, 1979-1982a/

Number Fractional Cumulative

Velocity of Percent Percent

(mph) Accidents (%) (%)

1 - 10 4,394 40.42 40.42

11 - 20 2,250 20.70 61.12

21 - 30 2,183 20.08 81.21

31 - 40 1,091 10.04 91.24

41 - 50 659 6.02 97.30

51 - 60 239 2.20 99.50

61 - 70 41 0.38 99.88

71 - 80 10 0.09 99.97

81 - 90 3 0.03 100.00

>91 0 0.00 -

Subtotal T16,8-

Unknown 76

Total 10,946

a/ Data derived from Federal Railroad Administration reports

accidents, 1979 - 1982.

on train

2-33
2.6.1.2.2 Impact Angle

As for highway accidents, there is insufficient historical accident data

available to define distribution functions for the impact angle of a spent

fuel cask onto an object. Three distribution functions for spent fuel cask

impacts are assumed for train accidents, namely: (1) uniform distribution, (2)

all impacts at 900, and (3) triangular distribution in which 900 impacts are

most likely.

2.6.1.3 Cask Orientation

Since there is insufficient historical railway accident data available to

define distribution functions for the cask orientation at the time of impact,

three distribution functions are assumed for train accidents. The

distribution functions are (1) uniform distribution, (2) all impacts endwise

or head-on to the cask, and (3) triangular distributions in which 450 impacts

are most likely.

2.6.2 Thermal Loading Parameters

As with truck accidents, every train accident does not necessarily result

in a fire. As indicated in Appendix C, approximately 1% of train collision

and derailment accidents involves a fire. As for truck accidents, the train

accidents have data on type of accident, frequency of fire, cause of fire, and

property damage estimates. However, the accident records do not provide data

on thermal loading parameters such as flame temperature and fire duration.

A train-fire accident has a large number of variables that affect the

thermal loads. Such variables are (1) type of accident (collision,

derailment, grade crossing, etc.), (2) type and amount of cargo (flammable or

nonflammable), (3) involvement of locomotive fuel, (4) types of cars involved

(box car, tanker, etc.), and (5) the availability of fire fighting equipment.

The same methods used in Subsection 2.5.2 to estimate the truck fire

duration distribution are used here to estimate the distribution functions for

the three thermal loading parameters: fire duration, flame temperature, and

fire location.

2-34
2.6.2.1 Accident Scenarios and Fire Frequency

The type of railway accident can affect the thermal load on a train and

its cargo. Table 2.9 presents the accident type and the frequency of fires,

modified to include grade-crossing accidents which were separately identified

beginning in 1978 (see Appendix C). 6 6 The fire frequency for "other"

accidents is judged to be too high, but owing to the lack of consistent data,

this conservative estimate is used. 6 6

2.6.2.2 Fire Duration

Since the available fire-accident data do not provide specific

information on fire duration for each of the railway accidents, the same

method used in Subsection 2.5.2.2 to estimate truck fire duration distribution

is used to estimate the fire duration distribution for trains. A Monte Carlo

scheme is used in analyzing a large number of variables and their

interactions. 6 6 The code can evaluate the interaction and involvement of

locomotive fuel tanks, different types of rail cars and their flammability,

and different types and amounts of flammable cargo, as well as the effects of

fire fighting efforts. The code is used to predict the fire distributions for

each of the accident types in Table 2.9 and the accident scenarios in

Fig. 2-5.

2.6.2.3 Flame Temperature

The thermal loads on atrain and its cargo are affected by the flame

temperature of the fire. They are primarily determined by the type of

material involved in the fire, the oxygen supply, and geometric

configuration. Train fires often include diesel fuel, flammable cargo, and

flammable parts of the cars. The flame temperature for train fires are the

same as those evaluated for truck fires in Subsection 2.5.2.3. For the

purpose of this study, it is assumed that all train fires will have an

'missivity of 0.9.

2-35
Table 2.9

Train-Fire Accident Types

Fire Involved No Fire

Accident Type in Accident in Accident

(%) (M)

Collision 1 99

Derailment I 99

Grade Crossing 1 99

Other 90 10

2-36
2.6.2.4 Fire Location

As with other fire parameters, insufficient historical accident data

exists to develop fire location distributions with respect to a spent fuel

cask. As is done for the truck cask accident scenarios, uniform distributions

(Li) are assumed for each of the fire accident scenarios for fire locations 0

to 43.0 feet from the cask.

2-37

3.0 SELECTION OF REPRESENTATIVE SPENT FUEL CASKS FOR EVALUATION

3.1 Introduction

Casks currently certified for shipment of spent fuel from nuclear power

reactors in the United States vary distinctly in design. 1-4 The most obvious

difference between these casks is that they are designed to carry differing

amounts of spent fuel. Casks weighing under 25 tons carry one or two fuel

assemblies and can be transported by truck. Other casks can carry three to

seven fuel assemblies and can also be carried by truck if appropriate highway

overweight permits are secured. Finally, because railroads can carry greater

loads, currently licensed rail casks can carry between 7 and 24 assemblies.

All of these casks must be designed to accomplish certain basic safety

functions which are defined by a set of performance-oriented regulatory

requirements. 5 In this regulatory approach, the cask design features which

accomplish a specific safety function can vary, but the functional result must

meet minimum specified requirements. In order to study the adequacy of the

regulations to provide radiological protection, representative casks are

defined which have design features likely to meet the regulations. Sufficient

features must be defined to evaluate the protection provided by spent fuel

casks involved in transportation accidents.

In addition, casks designed to meet regulatory requirements are usually

designed and manufactured to code and standards which have margins of safety

embedded in them. These margins of safety ensure that the spent fuel-cask not

only will meet the regulatory accident test conditions and radiation hazard

limits but will survive loading conditions beyond the regulatory conditions.

The purpose of this section is to define the representative casks which

are used in the accident response calculations described in later sections of

this report. These representative casks are developed from current cask

designs and technology. These representative casks include the necessary.

design features and safety margins for evaluating their response to accident

conditions.

3-1
In Section 3.2, general safety functions for the cask are defined. The

cask features needed to meet these functions are identified. Speci f ic

characteristics are determined for the various design features.

In Section 3.3, each design feature is evaluated from two standpoints:

(1) the feature's susceptibility to damage under transportation accident

conditions, and (2) the feature's ability to mitigate damage to other

important cask features. Some features, e.g., impact limiters, are

characteristically sacrificial and highly susceptible to damage, but are

effective in mitigating -further damage to the rest, of the system. At the

other extreme are features that are characteristically highly resistant to

damage, but transmit damaging forces into other parts of the system with

little mitigation.

In Section 3.4, six preliminary cask designs are evaluated on a

comparative basis. From this comparison the gammna shielding material for the

representative truck and rail cask designs is selected. The six designs

include three truck casks and three rail casks which use the candidate shield

materials: lead, depleted uranium, and steel.

Section 3.5 describes the two representative cask designs selected--one

for truck shipments and the other for rail shipments. The physical and

material specifications for the two designs are established. Those design

features which are necessary to perform the evaluations in this study are

identified. The rationale and the sensitivity studies used to define the

required design features are also described.

Section 3.6 describes the typical safety margins that are included in

licensed cask designs and the representative cask. These safety margins are

embedded in the codes and standards used in designing and manufacturing casks.

3.2 Cask Functions and Design Features

Casks currently certified for shipment of spent fuel are relatively

complex engineering structures designed to meet certain functional needs.1-4

Many of these functional needs are dictated by the characteristics of the

spent fuel being shipped. The spent fuel is a source of radioactivity and

3-2
heat, both originating w~ithin the fuel pellets which are contained within the

rods of a fuel assembly. The primary cask functions include (1) containment

of radioactive material, (2) shielding against the radiation emanating from

the spent fuel, and (3) the assurance that subcritical ity is maintained.

Containment is the retention of radioactive material within a closed

vessel. Containment is provided to preclude any contact between people and

radioactive material. Typically, containment is provided by the integrity of

the spent fuel and by a cylindrical steel vessel (Fig. 3-1). The vessel is

provided with a bolted end closure to accommodate spent fuel loading and

unloading operations. The closure contains a seal to inhibit leakage between

the cask containment and the environment. Piping penetrations of this

containment are needed for operating purposes, and the associated closure

valves are considered a part of the containment system. These penetrations

are in the containment vessel for draining, filling, testing, etc. The

containment cavity is filled with a non-oxidizing gas for shipments.

A radiation shield is a barrier which absorbs ionizing radiation or

subatomic particles emanating from a radioactive source. Two types of

radiation shielding are typically included in spent fuel cask design, gamma

and neutron*. The most important shielding provides protection against the

highly penetrating gamma radiation. This protection is achieved through the

use of dense materials such as lead, depleted uranium, or steel. These

materials surround the containment vessel (Fig. 3-1) and are, in turn,

enclosed within an outer steel shell. If steel is the shield material, this

shield can be an integral part of the containment vessel. The second type of

shielding is used to mitigate radiation caused by spent fuel emission of

neutrons. This source of radiation is typically less significant than gamma

radiation. Hydrogenous materials provide shielding against neutrons. The

neutron shield, usually a water jacket, surrounds the cask on its exterior

surfaces. The hazard associated with neutron radiation is such that loss of

neutron shielding does not result in radiation levels that exceed regulations

for accident situations. The regulations allow for higher external radiation

levels following an accident than during normal transport.

3-3
Closure

with bolts

Neutron shield

[0

uter shell

fG amma shield

/-Inner shell

-I I

, Valve/penetration -Impact

limiter

K.41 1 l- \ ri -'

\-Containment cavity

with neutron

poisons

Figure 3-1 Spent fuel cask features important to safety.

3-4
Criticality is a self-sustained nuclear chain reaction which might result

in high energy production and a radiation burst before self-termination.

Spent fuel casks are designed to maintain a condition of subcriticality. The

subcriticality assurance function, if not achieved by the physical limitation

on the amount of spent fuel being shipped, is assured by maintaining geometric

control of the spent fuel during shipment and by including neutron poisons in

appropriate cask structural materials. Neutron fission interactions with

spent fuel must attain a prescribed level before criticality can occur. The

neutron poisons, which are typically included in the basket holding the fuel

assemblies, absorb emitted neutrons to a sufficient degree to limit neutron

fission interaction and thus assure subcriticality.

As the above discussions of containment, shielding, and subcrlticality

assurance indicate, two fundamentally different concepts are applied in the

regulations: containment and shielding are limiting in nature while

subcriticality is absolute.

In all casks, the design features used to meet each of the specific

functional needs have many mutual dependencies. The containment shell, for

example, must be designed to structurally support the heavy surrounding gamma

shielding material. Also the geometry control achieved by internal cask

features is dependent on the protection against deformations provided by the

overall cask structure. These dependencies between specific design features

are further described in Section 3.3 which discusses the performance

requirements for the design features important to safety.

3.3 Cask Design Features Important to Safety

3.3.1 Containment

This subsection describes several design features which basically compose

the typical cask containment system: (1) the cylindrical steel containment

shell, (2) the bolted end-closure, (3) the closure seal, and (4) the piping

and valves associated with any containment system penetrations. The

containment system must be designed so that when subjected to the hypothetical

3-5
accident conditions specified in existing regulations, the regulatory limits

for radioactive material releases are met. In practice, the required function

of the containment vessel is achieved by a combination of three factors: (1)

the structural integrity of the individual containment system features, (2)

the provision of external features such as energy-absorbing structures

designed to protect the cask and its containment system against external

forces, and (3) the integration of the containment features into an overall

cask design which maximizes the protection provided against these external

forces.

The steel containment is designed as a system and must support itself and

the weight of the spent fuel and other internal structure under regulatorydefined normal and accident transport conditions. The steel containment shell

provides a substantial resistance to any externally applied forces. To

provide ensurance that this shell maintains its integrity under potential

transportation accident conditions, casks are designed with impact limiters.

Impact limiting devices can take the shape of large end-caps made of a

crushable material such as balsa wood or rigid foam, or they can be in the

form of bendable metal fins or tubes which protrude from the outer cask

body. In all cases, impact limiters are designed to limit, or reduce, the

mechanical loads imposed on the cask containment shell. The impact limiters

do this by deforming and sacrificially absorbing the energy of the accident.

The containment shell is designed for the impact-limited loads which arise

from the accident test conditions.

The bolted containment end closure and the closure seal are located

within the envelope of protection provided by the impact limiting devices

(Fig. 3-2). The bolted closure is typically recessed within the outer cask

shell, and the closure seal is located between the end closure and the

containment shell wall. These cask features are designed so that if the cask

is subjected to accident conditions, the containment function is not

compromised.

Piping and valves associated with subsystems that penetrate the

containment are also located in protected recessed areas within the outer cask

3-6
Bolt

: body

'--Cask body

b) Single closure system

Figure 3-2 Typical closure designs for spent fuel casks.

3-7
structure (Fig. 3-3). As a result, this piping system and its related valves

are also protected by the impact limiting devices. Again, this system is

designed to withstand the accident conditions without compromising containment

integrity.

3.3.2 Radiation Shielding

Shielding is provided in all cask designs to limit the gamma and neutron

radiation which emanates from the spent fuel. The gamma shield is typically a

dense metal, such as lead, depleted uranium, or steel. These materials

surround the cask containment vessel and, in the case of lead and depleted

uranium, are enclosed within an outer steel shell. The neutron radiation

shield typically consists of hydrogenous compounds such as water. The neutron

shield is generally located beyond the outer steel shell which encases the

gamma shield. When water is used for neutron shielding, it is contained

within a water jacket. The thicknesses of these shields are determined to

ensure that the radiation levels external to the cask are within regulatory

values which are specified for both normal transport and transportation

accident conditions, (i.e., 200 mrem/hr on the external surface and 41 rem/hr

at 1 meter from the external surface, respectively).

In practice, the dose rate of ;1 rem/hr at 1 meter from the external

surface can. be achieved by maintaining the integrity of the gamma shield. The

magnitude of neutron radiation is intrinsically limited to levels that allow

the loss of neutron shielding to be presumed in the event of a transportation

accident. The gamma shielding is protected by both the outer steel shell of

the cask and the cask's impact limiters. If the cask is subjected to the

accident test conditions, the cask gamma shield is designed to assure that

external radiation levels remain within regulatory limits.

3.3.3 Subcriticality Assurance

Subcriticality for one pressurized water reactor (PWR) assembly or two

boiling water reactor (BWR) assembly shipments (typically made by truck) is

assured because the amount of fissile material available in the U02 fuel form

3-8
a) Valve through closure b) Valve through flange

Impact fins

c) Exterior reinforced valve box

Figure 3-3 Typical cask penetration subsystems.

3-9
is insufficient to achieve criticality under any credible circumstances.

Larger shipments, however, which are generally made by rail, do contain enough

fissile material to make criticality a theoretical possibility if: (1) the

material can be optimally rearranged geometrically, (2) a neutron reflecting

material surrounds the fuel, and (3) a neutron moderating media such as water

can be interspersed between fuel rods and assemblies. For these shipments,

subcriticality assurance is achieved by geometry control features and the use

of neutron poisons, materials which preclude a self-sustaining fission

process.

A cask's capability to assure spent fuel subcriticality for these larger

shipments is evaluated in an extremely conservative manner. The effectiveness

of the geometry control provisions and the neutron poisons must be

demonstrated not only under the specified accident test conditions but also

under defined conditions which optimize the possibility for criticality.

Among these other conditions, the larger shipments must be demonstrated to be

subcritical when: (1) two similar casks are assumed to be stacked together in

an arrangement which optimizes criticality potential, (2) the stacked casks

are closely reflected on all sides by water, and (3) the fuel within each cask

is subjected to optimum, interspersed hydrogenous moderation.

The assumed presence of the reflecting and moderating materials increases

the possibility of achieving a critical configuration. The use of this

conservative approach to assure subcriticality highlights the importance of

cask features other than the spent fuel geometry control features and neutron

poisons previously described. For example, if containment integrity is

maintained, water or other hydrogenous material could not enter the cask

containment vessel and the possibility of criticality would be precluded.

Similarly, if the overall cask structure prevents gross internal distortions,

then spent fuel geometry control and neutron poisons would be sufficient to

assure subcriticality even if water or other hydrogenous material entered the

cask containment vessel.

3-10
3.4 Selection of Cask Shielding Material

Shielding provides protection from both the neutron and gamma radiation

emanating from spent fuel. The gamma shielding can be provided by several

different materials, each with a distinct capability to withstand the

mechanical and thermal loads associated with potential transportation

accidents. The selection of the gamma shield material for a representative

cask is based on an evaluation of the comparative performance of different

preliminary cask designs: three each for truck and rail. The six preliminary

designs shown schematically in Figs. 3-4 and 3-5 include consideration of

sizing differences typical to truck and rail casks and the use of each of the

three candidate gamma shield materials: lead, depleted uranium, and steel.

These six designs are evaluated against two quasi-static mechanical

loading conditions, i.e., end-on and side loads. Then the magnitude of loads

necessary to initiate yielding of the containment shell is determined. Static

loads are applied to the end and side of the casks for this evaluation. The

details of these evaluations are described in Appendix E. The results

indicate that the lead shielded casks--both the railway and highway

configurations--will begin to yield when subjected to a lower external force

than the casks with steel or depleted uranium shields. From a structural

standpoint, lead is the worst of the three candidate. gamma shield materials

and is, therefore, the material of choice.

The six preliminary designs also are compared in terms of their

capability to absorb thermal energy from potential fire environments. In

terms of thermal capacities, the steel-shielded designs are capable of

absorbing the most heat; the depleted uranium and lead designs have

essentially equal capabilities. Lead has a melting temperature below the

other cask shield materials, which is considered another factor significant to

safety. The thermal expansion effect is also the most significant for lead

shielded casks. From a thermal standpoint, lead is again the worst of the

three candidate shield materials and is the material of choice.

Based on these structural and thermal evaluations, lead is selected as

the gamma shield material for the representative cask designs.

3-11
I IV7

Dim Thickness

(in.)

Material

-- C

-Shield

A

B

C

-e---Cask E

centerline

I 193

u-B

A

B

C

E

D

E

Truck Cask 1

0.5

1.25

5.25

13.75

Truck Cask 2

0.5

1.25

4.25

12.75

Truck Cask 3

12.25

19.00

304SS

304SS

Lead

304SS

304SS

304SS

Depleted

uranium

304SS

Steel

Steel

17.0 I

E _

Figure 3-4 Preliminary truck cask designs with three types of gamma

shielding, used for quasi-static loading response studies

only.

3-12
7.0

Cask 7

centerline

A-40-

7.0

Dim Thickness

(in.)

.- Shield

0-C

ii '3

A

B

C

E

A

B

C

E

D

E

Material

Rail Cask 1

0.5

1.5

5.25

26.0

Rail Cask 2

0.5

1.5

4.0

24.8

Rail Cask 3

12.25

30.75

304SS

304SS

Lead

304SS

304SS

304SS

Depleted

uranium

304SS

Steel

Steel

Figure 3-5 Preliminary rail cask designs with three types of gamma

shielding, used for quasi-static loading response studies

only.

3-13
3.5 Definition of Representative Cask Designs

Previous sections discuss the functions of a spent fuel cask which are

important to safety in the event of a transportation accident. This section

presents the basis for the selection of the representative spent fuel casks

used in the response analyses. The response of these casks is evaluated when

subjected to the forces of real world accident environments in later sections

of this report. The definition of a representative cask involves the

accomplishment of two major tasks: (1) a determination of what cask features

important to safety require specific design definition, and (2) a selection of

a design definition which considers the variety of design features that can

accomplish a specific safety function.

The following subsections present the rationale for accomplishing these

two tasks. Separate subsections consider features which are important to the

containment, shielding, and subcriticality assurance functions of cask

designs. An additional subsection considers the definition of those cask

features whose principal purpose is to mitigate the damage to the cask caused

by accident forces (principally the impact limiters).

3.5.1 Shielding Features

Based on the evaluations in Section 3.4, lead is selected as the gamma

shield material for the representative cask designs. Under impact conditions,

lead is not self-supporting and can slump. A properly designed cask has

adequate thickness in each steel shell as well as a soft impact limiter to

prevent any significant lead slump from occurring under the 30-foot drop test

conditions. Bonding of the lead to the inner shell of the cask can provide

resistance to lead slump, but bonding varies significantly with the cask

design and the fabrication process. Lead slump effects and damage to the cask

are maximized when there is no bonding between the lead and the inner wall of

the cask. Therefore bonding of the lead is not assumed.

The neutron shield design will not be expected to significantly affect

cask response to the mechanical loads associated with severe transportation

3-14
accident environments. In fact, as indicated previously, the safety

evaluations performed on all current casks presume that the capabilities of

the neutron shield to reduce external radiation levels is lost as a result of

the effects of transportation accident forces. On this basis, specifying the

neutron shield design will not be necessary for the representative cask

designs. However, this neutron shield, whether lost or maintained, will

affect heat transfer. If a water neutron shield is maintained, it will

exhibit high heat capacity as well as good heat transfer characteristics. If

the water is lost, the empty tank containing air does not have high heat

capacity, but provides an effective thermal barrier against heat from a

fire. The post-fire effect of a neutron shield tank is to increase resistance

to dissipation of internal heat, thereby increasing internal temperatures.

Therefore, the volumetric characteristics of the neutron shield design must be

considered in the definition of the representative casks.

3.5.2 Containment Features

The containment system includes the steel containment shell, the closure

seal, the bolted-end closure, and the piping and valves in the containmentpenetrating subsystems.

The steel shell is the containment feature most likely to be subjected to

the full brunt of any severe transportation accident forces. The magnitude of

any accident damage sustained by the shell provides a broad indication of the

possibility and the magnitude of any resulting radiological hazard.

The containment seal can be subjected to damage by mechanical or thermal

accident loads transmitted through the cask body to the seal region. However,

the radiological hazard resulting from seal damage is limited to the spent

fuel material which can escape from the confines of the cask through the

damaged or deformed seal region. Rather than attempting to model one of

several possible seal designs, a worst-case evaluation of seal performance can

be made by presuming a loss of the seal functional capability and the release

of radioactive material. Specific levels of damage to the cask must be

exceeded as a result of accident forces.

3-15
The bolted cask end-closure can be subjected to damage by mechanical

loads transmitted through the cask body. Damage can also result from the

mechanical loads which can be caused by severe thermal environments associated

with certain transportation accidents. The end-closure, however, is a massive

structure highly resistant to mechanically imposed loads. Furthermore, the

closure bolts are designed with sufficient strength to resist tensile forces

from corner or end drops of the cask. The recessed characteristics of all

current closure designs provide significant protection against shearing of the

many large-diameter bolts typically used to secure the end-closure to the cask

body. Forces sufficient to cause significant damage to the cask containment

shell could occur in many of the conceived severe accident events without

compromising the gross integrity of the bolted end-closure. The converse,

that is, significant damage to the end-closure without similar containment

shell damage is certainly conceivable, but far from likely. From an

evaluation standpoint, the definition of a specific closure in a

representative cask design will add considerable complexity to the

calculations of cask response to severe accident environments. For the above

reasons, although the mass and configuration of the closure requires

definition, the details of the closure design are not included in the

representative cask design(s). Again, a specific level of damage to the cask

containment is used as a surrogate measure to indicate damage to, and the

occurrence of radioactive material leakage from, the cask closure region.

The penetration subsystems are typically located within the confines of

the cask body with exterior valves situated within heavily protected

enclosures. These subsystems are easily protected by design features. Unless

accident loads are highly localized, damage done to the cask shell will

dominate overall cask damage. Notwithstanding, a highly localized load can

violate the containment function by providing an opening from the cask

containment to the environment through a failed penetration subsystem. Such a

violation of containment will limit the escape of any spent fuel material to

that which can migrate or be driven out through the small-diameter, tortuous

passageways presented by the damaged penetration system.

3-16
As a result of the above considerations, the details of a penetration

subsystem are not included in any representative cask design. Damage to the

containment shell again is used to indicate the possibility of a failed or

damaged penetration subsystem.

3.5.3 Subcrittcality Assurance Features

Subcriticallty assurance features, are provided in casks used for the

shipment of larger numbers of spent fuel assemblies. The spent fuel geometry

control features and the neutron poisons can be subjected to transportationaccident-induced mechanical forces transmitted through the cask body. These

features form an integral part of the overall cask structure internal to the

containment shell. Significant damage to these features requires that

significant damage be incurred by the total cask structure including the

containment shell. Physical damage, taken alone however, does not affect the

cask's subcriticality assurance function. A hydrogenous material, such as

water, must surround the cask and be interspersed between the individual fuel

rods and fuel assemblies before criticality can become a credible possibility.

For these reasons, the subcriticality features are not specifically

modeled in the representative cask designs. Instead, a maximum estimate of

the likelihood of a criticality incident is provided in Section 9.0. This

estimate considers those transportation accident events in which the

structural damage is sufficiently severe to cause gross fuel assembly

damage. The estimate then evaluates the likelihood that such an event will

involve the intimate presence of hydrogenous material in the accident

scenario.

3.5.4 Damage-Mitigating Features

The principal damage-mitigating features provided in cask designs are the

impact limiters. These devices are designed to be sacrificial and can be of

two general types, hard and soft. In either case, they absorb some of the

energy of impact by deforming. The ratio of the energy absorbed by the impact

limiter to that transmitted to the cask depends on the accident severity and

3-17
the type of impact limiter. The choice of an impact limiter is strongly

affected by the choice of gamma shielding. If lead is the gamma shield

material, soft impact limiters of balsa wood or rigid foam are typically used

in cask designs. Soft impact limiters are designed to ensure that imposition

of the accident test condition loads will not produce forces sufficient to

cause lead slump.

Hard impact limiters in the form of bendable metal fins have been used in

casks using depleted uranium as the gamma shield material. In these designs,

the casks are more rigid. As a result, the forces transmitted through the

cask body when the cask is subjected to the accident test conditions

(specifically, the 30-foot cask drop onto an unyielding surface) are higher

than those associated with casks using soft impact limiters. In either case,

however, the cask design must meet the regulatory-defined post-test acceptance

criteria.

A soft impact limiter is selected for the representative cask design for

two major reasons. First, the soft impact limiter is consistent with the

selection of the lead gamma shield. Second, and more significant, casks with

soft impact limiters, if subjected to transportation accidents resulting in

severe mechanical and thermal loads, will be more likely to incur damage.

3.5.5 Representative Cask Design Description

Two representative cask designs are developed: one for truck shipments

and one for rail shipments of spent fuel. The representative truck cask

design uses the same dimensions as the preliminary lead truck cask design

(Fig. 3-4). The truck cask design allows transport of a single PWR fuel

assembly. The representative rail cask design dimensions differ from the

preliminary lead rail cask design (Fig. 3-5). The capacity of the rail cask

is 21 PWR fuel assemblies which reflects the greater capacities of anticipated

cask designs. Each design uses helium in the cask cavity.

Both designs include a lead gamma shield sized to meet current regulatory

requirements. The truck cask gamma shield of 5.25 inches is thicker than the

rail cask gamma shield of 4.00 inches to allow for the possibility of shipping

3-18
fuel decayed less than 5 years by truck. The neutron shield dimensions

reflect values typical of current cask designs. the cask shell structures,

including the containment shell, are sized to- support the lead shield.

Specifically, the thickness of each cask steel shell is selected based on

standard design practice; that is, the cask structure can withstand a force

level typically generated from the accident test conditions. The resultant

representative cask designs are indicative of current designs. 1-

4

The pertinent materials, weights, and dimensions of the representative

truck and rail casks are shown in Figs. 3-6 and 3-7, respectively. The

structural shell material is type 304 stainless steel. The lead shield is

assumed to be unbonded to the steel shells. This fabrication assumption

maximizes the potential for lead slump during transportation accidents

involving impacts. Cask resistance to accident forces is thereby minimized,

which Introduces an element of conservatism to the results of this study. The

impact limiters are made from balsa wood or rigid foam. Figure 3-8 shows the

force deflection characteristics of the representative limiter design as a

function of the presumed angle of impact between a cask and an impact

surface. The impact limiter is sized to transmit a force of approximately

40 g if the cask is subjected to the impact environment specified by the

accident test conditions.

3.6 Margins of Safety

The representative casks are designed to meet the 'regulatory accident

test conditions. However, before a cask is allowed to transport spent fuel,

it must be certified by the U.S. Nuclear Regulatory Commission (NRC). The

certification process requires that all activities related to the design,

manufacture, use, and maintenance of the cask be documented in a Safety

Analysis Report (SAR). The SAR is submitted to the NRC for review and

approval. The analyses and evaluations in the SAR must demonstrate that the

spent fuel cask meets all 10 CFR 71 requirements and has sufficient margins of

safety included to protect the public from undue risk. In general, margins of

safety are included by using established practices, codes, and standards such

as the American Society of Mechanical Engineers (ASME) Code and the American

3-19
-*-7.0

Neutron shield

13.5 dia. cavity-N\

7.0--

Imp

limi

'I

I'

Wý 1S7 -% -

lact

ter

I6. 27.5 179(cavity}

__. fIJLI C T -j

I~-~ 4~-.. I - 0 I IPK 2 55 SI5.25Pb I4.5tH2OL_

U.O sS 1.25s 0 5.Z $S

m I,• 1 - a DIa

971

All dimensions in inches

Item

Body

Limiter

Contents

Weight, lbs

32,000

4,500

2,500

39,000

Figure 3-6 Representative truck cask design used for dynamic

structural and thermal response studies.

3-20
All dimensions in inches

Item

Body.

Limiter

Contents

1e2ight2 l

122,500 22,500

52,000

197,000

Figure 3-7 Representative rail cask design used for dynamic

structural and thermal response studies.

3-21
180

160

140

120

= 100

0

C8o

,E

60

40

20

0

0 10 20 30 40 50"

Limiter deflection, inches

Figure 3-8 Force-deflection characteristics of the limiter design as

a function of cask orientation at impact.

3-22
National Standards Institute (ANSI) Standards, and Regulatory Guides, all of

which must be identified in the SAR.

Regulatory Guides are written by the NRC to provide guidance in many

areas of licensing that result in acceptable margins of safety. For example,

Regulatory Guide 7.6 adapts portions of the ASME Code, Section III to the

design of spent fuel casks and recommends that elastic methods of structural

analysis be used in the containment design. 6 Other Regulatory Guides relating

to spent fuel casks are 7.4 (Leak Testing), 7.8 (Load Combinations), 7.9 (SAR

Format), and 7.10 (Quality Assurance). 7 - 1 0

Although there is no specific section in the ASME Code applicable to

spent fuel casks, the ASME Code has been used extensively in designing,

manufacturing, using, and maintaining spent fuel casks. 1 1 In general,

materials adopted by the ASME Code provide a large margin of safety against

rupture because the materials have high ductility. Also the use of elastic

analysis for structural design usually results in a large margin of safety.

For example, cask containments using 304 stainless steel are designed for the

accident test conditions to ASME stress intensity limits that result only in

slight yielding of the cask structure. In most cases, depending on the

limiter design, the 304 stainless steel material can experience an off-set

strain less than 1% under accident test conditions but rupture of 304

stainless steel occurs at strains greater than 30%. Therefore, large amounts

of energy can be absorbed by the cask structure though large deformations

under loading conditions exceeding the accident test conditions without

catastrophic rupture occurring. To preclude brittle fracture failure from

occurring at low temperatures, only materials with adequate toughness can be

used in the structural design of spent fuel casks. 1 2 , 1 3 , 1 4

In this study it is assumed that the representative casks have been

properly designed and manufactured to appropriate codes and standards.15, 16

The representative cask designs are based on currently licensed cask designs

and are likely to be certified if a SAR were prepared and submitted to the

NRC. The margins of safety included in the cask design are representative of

those included in currently licensed casks.

3-23

4.0 REPRESENTATIVE CASK RESPONSE STATES, LEVELS, AND REGIONS

4.1 Introduction

If a shipping container is involved in an accident, a cask response is

generated and damage can occur. The response depends on many elements, such

as the magnitude of the loadings generated by the accident impact velocity,

the object struck, and if a fire is involved, the flame temperature and the

duration of the fire. The response can be different at different locations or

by various components within a cask. Different cask designs can have

different magnitudes and types of responses when subjected to the same

accident conditions. The actual response is a result of the combined effect

of all these factors. Normally, the higher the response, the greater the

damage to the cask and, therefore, the greater the potential for an event with

a radiological significance.

In order to determine the response, three methods are commonly used:

analytical, experimental, and a combination of the two. In this study, the

analytical method is used to estimate responses. Many different computer

codes are used to perform the analyses. These computer codes, as pointed out

throughout the report, are benchmarked against closed-form solutions and

experimental data. Appendix H discusses benchmarking for some of these codes.

In order to calculate response by analysis, a proper selection of

computer codes is essential. Every computer code has limitations. The proper

selection of a code requires a thorough understanding of its limitations.

Sections 6.0 and 7.0 discuss the method of analysis, including the assumptions

used in the analysis and the modeling technique by which the cask structures

are represented. Individual analyses and their results are also presented.

The purpose of estimating the response is to determine the degree of

structural damage. Certain types of damage, such as damage at specific

locations or to certain components within the cask structure, can result in

radiological hazards. Other types of damage may appear to be large, but

result in essentially no radiological hazards. In order to evaluate the

consequences resulting from structural damage, it is necessary to relate the

4-1
potential radiological hazard to the type of damage and cask response.

Sections 4.2 and 4.3 qualitatively discuss the association between the

structural and thermal damages and the potential radiation hazard. Section

8.0 provides a detailed discussion that relates the level of response to the

level of potential radiological hazard.

Defining a specific response state is a very complex problem because

response varies with different cask designs, severity of accidents, and

location within the cask structure. In order to evaluate the level of damage

between one response state and another, it is essential to establish some kind

of measurement scale.

Response can be expressed in terms of many parameters, such as force,

moment, displacement, stress, strain, and temperature. To establish a

measuring scale with too many different types of response parameters will make

any assessment unmanageable. The most effective approach is to identify one

response parameter which provides both an adequate indication of cask

structural damage and also an easy linkage to a radiological hazard

estimate. This section discusses the selection of the parameters to represent

the structural and thermal responses for the representative cask designs, the

justification of the selections, and the discretized levels of response states

used in this study.

4.2 Response States and Levels for Mechanical Loads

Various types of damage can occur to casks subjected to mechanical

loads. The most important types of damage to a lead shielded cask are

yielding, large dimensional changes, and rupture of the cask structure. Any

parameter selected to represent the structural response state of the

representative casks should indicate these types of structural damage.

Three engineering response parameters--stress, strain, and displacement--

are considered as candidates for the single parameter to represent the

response state for mechanical loads.

Stress is commonly used in structural analysis to represent the state of

response. Both the American Society of Mechanical Engineers and the American

4-2
Institute of Steel Construction use stress as the parameter to define

acceptance design limits in terms of yield and ultimate stress.1,2 It is a

good parameter for design within the elastic range of the material. When the

response is beyond the elastic range, however, large dimensional changes can

occur with only small changes in the stress level. The purpose of this study

is to estimate the damage and consequence to the representative casks when

subjected to severe accident conditions in which the response could exceed the

elastic range. Therefore, stress is not the best parameter to represent the

response state for mechanical loads applied to the representative casks.

Displacement is a parameter for measuring the dimensional change of

structural elements. It is capable of describing the deformation shape for

both small and large loading conditions. The deficiency in using displacement

is that it cannot provide direct comparison with the design acceptance

limits. Displacement cannot indicate directly when the structure has yielded

or ruptured.

Strain is the most appropriate single parameter to represent the response

state for mechanical loads. For a given material, dimensional changes

occurring with loading conditions are directly related to strain. Strain can

also indicate yielding and rupture when responses reach strain limits.

Therefore, strain is selected for mechanical load responses of the

representative casks.

Strain will most likely vary according to location within the

structure. Under one specific accident load, strain at the inner shell is

different from that at the outer shell, at the bolts, and at the enclosures.

Sensitivity studies are conducted using the representative casks to find out

the relationship between the strains at different locations or on different

components inside the cask structure. This relationship helps to estimate the

total cask damage level when strain at a particular component is identified.

The strain on the inner shell of the cask structure is selected as the best

single parameter to characterize mechanical load response states for the

representative casks.

4-3
Although the response of a cask is continuous over a loading range, three

discrete response levels are defined to relate ranges of response states and

mechanical loads to potential radiological hazards. The response levels are

defined as discrete levels of maximum effective strain on the inner shell of

the representative cask structure. The maximum effective strain of the

representative truck and rail cask impacting an unyielding surface can be

significantly different as shown schematically in Fig. 4-1. The three

discrete response levels or strain levels that bound the response state ranges

are identified on the figure.

4.2.1 Structural Response Level, Si

The first response level, S1, is defined to be 0.2% strain at the inner

shell. This level of strain is selected for the first response level because

the structural material of the representative casks is 304 stainless steel

which has a 0.2% offset yield point. For strains within the 0.2% yield strain

(S1), shown as range A in Fig. 4-1, the response of the structure is elastic

and there is no permanent dimensional change after the loading is removed.

This characteristic assures that little, if any, radiation release occurs when

the cask is subjected to accident loads that are within range A because the

seal and bolts remain functional. At 0.2% strain (SI), the representative

lead cask designs experience less than 40 g axial force on the lead for all

orientations of impact. No lead slump occurs. The fuel basket remains

functional. Up to 3% of the fuel rods can release limited amounts of

radioactive material into the cask cavity under these loading conditions.

Essentially all of the impact loads on the casks are absorbed by their impact

limiters. These loads and releases are within the regulatory design

conditions and release limits.

4.2.2 Structural Response Level, S2

The second response level, S2, is defined to be 2% plastic strain at the

inner shell. For strains between 0.2% (Sl) and 2% (S2), shown as range B in

Fig. 4-1, the response of the structure is plastic, and small permanent

dimensional changes occur. The dimensional changes can affect the cask

4-4
1.0 I III

Response level S3

~Cask response

C

*= 0.1

I.ID

4 ~X* Responsee level S

E

S0.01-

Response range B

Response level S

Response range A

0.0010 10 200I *I

0 100 200 300 400 500

Endwise impact load, g

Figure 4-1 Schematic representation of cask response state for

mechanical load.

4-5
closure seals and result in limited radioactive material releases. Also, a

small dimensional change can result from limited lead slump which can result

in an increase of radiation emanating from the cask. Up to 10% of the fuel

rods can leak into the cask cavity under these loading conditions. The

radiation hazards caused by seal leakage and lead slump in range B are near

regulatory limits. The loads that produce the second response state are near

the loads imposed by the accident test conditions. In this range, the impact

loads on the representative casks are absorbed mostly by their impact

limiters, but part of the loads are absorbed by the cask structure.

4.2.3 Structural Response Level, S3

The third response level, S3 , is defined to be 30% plastic strain at the

inner shell. The 30% strain (S3) level is below the fracture strain of 304

stainless steel, but the large distortions occurring with this strain level

can cause local cracking in the welded regions. For strains between 2% ($2)

and 30% (S3), shown as range C in Fig. 4-1, the response is plastic

deformation with large dimensional changes occurring, particularly for strain

near 30% (S3 ). Any large distortions of the cask will likely cause seal

leakage in the closure region, lead slump, localized weld cracking, and some

crushing of the cask contents. All of the fuel rods are expected to release

limited amounts of radioactive material into the cask cavity under these

extreme loading conditions. The radiological hazards associated with this

response can be outside of regulatory limits; however, there will not be any

failure that will result in release of solids from fuel rods, except very

small particles that may escape to the environment. In this response range,

an increasing amount of the impact force is absorbed by the cask compared to

the force absorbed by the limiter. In fact, at the 30% strain (S3 ) level, the

energy absorption by the representative casks may be eight times higher than

the energy absorbed by the limiter.

4.2.4 Application of Response States and Levels

Each response state implies a force on the cask as a result of impacts

upon various objects. The force is primarily determined by the impact

4-6
velocity and the hardness of the object, but various combinations of velocity

and object hardness can result in the same force. Consequently, the force

associated with each structural response state can be related to various

accident scenarios. Furthermore, the potential radiation hazard associated

with these response states can be related to these same accident conditions.

Figure 4-2 shows schematically the structural response state of a

representative cask in terms of strain as a function of both impact velocity

and surface hardness-for endwise impacts. The combination of impact velocity

and surface hardness for the strain levels 0.2% (Si), 2% (02), and 30% (03)

are also shown on the plot. For example, the impact velocities required to

reach the 0.2% strain ($I) level, will be 30 mph for an unyielding object, 60

mph for an object of medium hardness, and 90 mph for a soft object. For very

soft objects, the 0.2% strain (Sl) level can never be attained. Limiting the

velocities impacting various objects can similarly be obtained corresponding

to the 2% (S2) and 30% (S3) strain levels.

4.3 Response States and Levels for Thermal Loads

Various types of damage can occur to the representative casks subjected

to thermal loads. The most important types of damage are degradation of the

closure seal material, melting of the lead shield, dimensional changes to the

structure, and alloying of the lead with the nickel in the 304 stainless steel

structural material. Any parameter selected to represent the thermal response

state of the cask should indicate the various types of thermal damage that can

occur.

Two engineering response parameters, strain (thermally-induced) and

temperature, are considered as candidates for the single parameter to

represent the response state for thermal loads.

In Section 4.2, mechanical strain is selected as the single parameter to

represent the response state for mechanical loads. Thermally induced strain

provides a good indication of dimensional changes to the cask structure, but

does not provide any indication of seal deterioration, melting of lead, or

alloying of lead with the nickel in stainless steel. Therefore strain is

4-7
1.0

0.3

I Ouject

hardness

Response level S 3

12

E.

42

0.1 I-

/00ý

u•zUnyielding

Medium

Response level S2

0.02

0.01

Response levelS

Soft

0.002

0.001 (

)

very

or_- - - sOft 0 20 40 60 80 100 120

Endwise impact velocity, MPH

Figure 4-2 Schematic representation of cask structural response for various surface hardness and impact velocities.

4-8
determined not to be the best parameter to represent the response state for

thermal loads.

Temperature is the best single parameter to represent the response state

for thermal loads. Temperature provides an indication of seal deterioration,

melting of lead, and alloying of lead with the nickel in stainless steel. It

also provides an indirect measure of dimensional changes with lead melt.

Therefore, temperature is selected for thermal load responses.

Temperature varies from location to location within the cask. For any

specific fire-accident, the temperature at the inner shell is different from

that at the outer shell, at the bolts, and at the enclosures. Sensitivity

studies are conducted to find out the relationship between the temperatures at

different locations and on different components inside the cask structure.

This relationship provides a means to estimate the total cask damage level

when the temperature at a particular component is identified. The temperature

at the middle of the lead shield thickness is selected as the appropriate

single parameter to characterize thermal load response states.

Although the response of a cask is continuous over a loading range, four

discrete response levels are defined to relate ranges of response states and

thermal loads to radiological hazard. The response levels are defined in

terms of the temperature at the middle of the lead shield thickness. As an

illustration of a cask exposed to a regulatory fire, Fig. 4-3 shows

schematically the lead mid-thickness temperatures as a function of the thermal

loads to the cask. The four discrete response levels, or lead mid-thickness

temperatures, that bound the response state ranges are identified on the

figure.

4.3.1 Thermal Response Level, T1

The first response level, T1 , is defined as a temperature of 500°F at the

middle of the lead shield thickness. This temperature is selected because the

cask seals are below temperatures that can cause degradation of properties to

such materials as silicon and fluorocarbons. Also, there is a significant

margin between 500OF (T1 ) and the melting point of lead at 621 0 F. For

4-9
1200

1100 Cask

Response level T4response

1000 f

900

uL Response range D 0

1~*800-

E 

700 7 -Response 

Response _ _ __ 

level 

level _ 

T

T

_ 3__ _

1

2 Response range C

St 600 • 50•••400Respon range B

ResponseResponse range 500

E

-o 400- a

.j Response range A

200 - •i

100

0

0 5 10 15 20 25

Heat absorbed, KBtu/ft 2

Figure 4-3 Schematic representation of cask response state for thermal

1 oad.

4-10
temperatures less than 500OF (T1), shown as range A in Fig. 4-3, there is no

significant damage to the cask due to thermal loads. However, it is assumed

that the water in the neutron shield is released before the 500OF temperature

(Tj) is reached. The release of the water forms a thermal barrier between the

neutron shield wall and the. cask outer wall which protects the cask from any

fire. The release of the water also increases the neutron radiation

surrounding the cask; however, all radiological hazards are within regulatory

limits below this first level thermal response.

4.3.2 Thermal Response Level, Tq

The second response level, T2, is defined as a temperature of 600OF at

the middle of the lead shield thickness. Temperatures between 500OF (TI) and

600OF (T2) are shown to be in range B in Fig. 4-3. In this temperature range,

the lead at the outer stainless steel wall of the cask is still below 6210F,

the melting point of lead. Even though the lead does not melt, the cask

closure seals can degrade and potentially release limited radioactive

material. Any radiological hazards-caused by seal leakage and the loss of the

neutron shield are likely to be within regulatory limits.

4 3.3 Thermal Response Level,_T 3

The third response level, 13, is defined as a lead mid-thickness

temperature of 6500F. For temperatures between 600OF (T2) an .d 650OF (T3),

shown as range C in Fig. 4-3, melting of the lead shield occurs. Lead melt

results in a phase change with a lead density decrease of approximately 10%.

The density change results in an increase in the lead volume and significant

plastic straining of the inner cask wall. After the cask cools, the lead

returns to its original 'density, and voids can occur in the lead shield owing

to the increased volume from the plastic strain of the inner cask wall. The

cask closure seals are assumed to leak. The increase in radiation level from.

the lead shield reduction and any radioactive material releases will likely be

outside of regulatory limits.

4-11
4.3.4 Thermal Response Level, T4

The fourth response level, T4 , is defined as a lead mid-thickness

temperature of 10500 F. For temperatures in the range of 650°F (T3 ) to 1050OF

(T4 ), shown as range 0 in Fig. 4-3, the lead shield thickness is reduced

further due to differential thermal expansion between the liquid lead and

stainless steel structural material. The fuel rods can also increase in

temperature and begin to burst. For temperatures above 1050OF (T4 ), the

alloying of the lead with the nickel in the stainless steel structure can

become significant and result in stress corrosion cracking. 3 - 5 In this

response range, the further reduction in shielding and possible bursting of

fuel rods increases the radiological hazards.

4.3.5 Application of Response States and Levels

Each response state implies a thermal load applied to the cask as a

result of various fire conditions. The thermal load is determined by the fire

characteristics. However, various fire characteristics can result in the same

thermal load. Consequently, the thermal load associated with each thermal

response state can be related to various accident conditions involving

fires. Furthermore, the potential radioactive hazards associated with these

response states can also be related to the same accident conditions.

Figure 4-4 schematically presents the thermal response of a cask in terms

of the lead mid-thickness temperature as a function of both fire duration and

fire location. The combination of fire duration and location for the

temperature levels 500OF (TI), 600°F (T2 ), 650°F (T3 ), and 1050OF (T4 ) is also

shown on the plot. For example, for a fire with a flame temperature of

17000 F, the time duration to reach the 500OF temperature (T1 ) level, will be

1.3 hours for an engulfing fire, 2.3 hours for a fire tangent to the cask, and

3.6 hours for a fire 20 feet from the cask. For fires greater than 50 feet

away, the 500OF temperature (T1 ) level can never be attained. Fire durations

for the various fire locations can similarly be estimated corresponding to the

600°F (T2 ), 650°F (T3 ), and 1050OF (T4 ) temperature levels.

4-12
U0

SooTangent

~.700-

E Response level T3

Response level T2 20 ft

is 600 away

C

Response level I/

~50o

:2O

E

79 400

_j300-ay

200

17000 F flame temperature

100 -

00

0 1 2 3 4 5 6

Fire duration, hrs

Figure 4-4 Schematic representation of cask response for various fire

locations and fire durations.

4-13
4.4 Cask Response Regions

In some cases, a cask will be exposed to both mechanical and thermal

loads. A range of combined structural and thermal responses for a cask can be

represented by the response matrix shown in Fig. 4-5. The ordinate of the

response matrix represents the structural response states; the abscissa

represents the thermal response states; and the boundaries of the response

regions are defined by the structural and thermal response levels.

There are 20 response regions denoted by R(Si,Tj) where Si is the

structural response level and Tj is the thermal response level. Although only

three discrete structural response levels are defined, a fourth unbounded

level exists that consists of cask strain responses greater than 30% (S3).

Similarly, a fifth unbounded thermal response level exists which consists of

cask temperature responses greater than 1050OF (T4 ). The first region,

R(1,1), represents the cask response to combined mechanical and thermal loads

within the 0.2% strain (Si) and 500OF temperature (T1 ) levels. Radioactive

releases, if any, for cask responses in R(1,1) will be within regulatory

limits. The twentieth region, R(4,5), represents the most extreme combined

response state in which the potential radiological hazards will be a

maximum. In general, the probability of occurrence of a particular

combination of mechanical and thermal loadings decreases with the severity of

these loads. The probabilities associated with each region of the load matrix

are discussed in more detail in Section 5.0.

4-14
CC

C

0

E

.R

cc

E

'CO

R (4,1) R (4,2) R (4,3) R (4,4) R (4,5)

S3

(30)

S2

(2)

S1

(0.2)

R (3,1) R (3,2) R (3,3) R (3,4) R (3,5)

R (2,1) R (2,2) R (2,3) R (2,4) R (2,5)

R (1,1) R (1,2) R (1,3) R (1,4) R (1,5)

T1 T2 T3 T4

(500) (600) (650) (1050)

Thermal response (lead mid-thickness temperature, *F)

Figure 4-5 Matrix of cask response regions for combined mechanical and

thermal loads.

4-15

5.0 PROBABILITY ANALYSIS

5.1 Introduction

The emphasis of the discussion in Sections 6.0 and 7.0 is on the physical

loads, both mechanical and thermal, which a spent fuel cask can experience in

a transportation accident. Specifically, cask response states, evaluated in

terms of containment vessel strains and lead shield temperatures, are related

to basic accident parameters such as impact velocities and fire duration.

The relationships between cask responses to mechanical loads and the

impact velocity of the cask are derived for several cask impact orientations

involving interactions with objects of differing'hardness. The effect of cask

orientation on the strain-impact velocity relationship for an unyielding

object is shown in Fig. 5-1 for the truck cask. The impact velocity, defined

as the cask velocity in the direction perpendicular to the object impacted, is

determined by the velocity of the cask due to the accident and the impact

angle.

The thermal loading to a cask depends on the flame temperature and fire

location as well as the duration of a fire. Thus, the relationship between

cask response to thermal loads and the duration of a fire is affected by the

flame temperature and location of the fire with respect to the cask. The

effects of these parameters are illustrated in Fig. 5-2.

In summary, the following accident parameters, which affect the cask

response to mechanical and thermal loads, are identified and are considered in

the probability analysis:

o Mechanical loads

- impact velocity

cask velocity

impact angle

- cask orientation

- hardness of the impacted object

5-1
I II I I I I I I I I

30% I- PC = 450

1C = 00 (side)

10-1F

B

0

a

2%

0.2%

10-3

10-4

II

Ii

II

Ii

II

II

Ij

Ii

Ii

II

ii

ii

ii

II

II I

I lit

III

II I

III

Ii I

I I *I

Ii~

Ii

II ,

IIII I

I. ii

I

I

! I I I I I I

0 10 20 30 40 50 60 70 80 90 100 110 120

Impact velocity, v

Figure 5-1 Effect of cask orientation on the strain-impact velocity

relationship for a truck cask impacting an unyielding

object.

5-2
11

1300

IL

0

900-

800-

E

£ 700-

X~ 600- - R= Locations from cask :S 500-cne

50 t = Temperature

~400 2 = 0, engulfing fire

2 = 1.5, tangent fire

300- /2 = 4.5, fire centered 3 feet

300 from cask

200

100 -

0 1 I

0 1.0 2.0 3.0 4.0 5.0 6.0

Time, hrs.

Figure 5-2 Effect of flame temperature and fire location on leadtemperature-time relationship for a truck cask.

5-3
o Thermal loads

- fire duration

- flame temperature

- fire location with respect to the cask.

Because future accident conditions are unpredictable, i.e., random, the

response state of a spent fuel cask cannot be predicted deterministically.

Assessment of the response states and the subsequent damage and release of

radioactive materials due to transportation accidents can only be expressed

probabilistically.

The purpose of this section is to describe the probability analysis

developed to estimate the likelihood that a spent fuel cask will attain

various response states during a transportation accident. Section 5.2

catalogs the probability distributions used to describe the random variation

associated with the accident parameters. The probability calculations are

outlined in Section 5.3.

5.2 Probabilistic Inputs

Estimation of the likelihood of various cask response states, represented

by the containment vessel strain and the lead mid-thickness temperature, is

based on estimates of the distributions of the accident parameters which

affect the response of the cask during a transportation accident. The

distributions of the accident parameters are described in terms of a

cuznulative distribution function, F(x), if the parameter is quantitative, or a

probability function, h(O), if the parameter is qualitative, e.g., the object

impacted. The cumulative distribution function describes the likelihood that

the parameter value is less than or equal to x, the argument of F(.), i.e.,

F(x) = Pr(X < x) (5.1)

where X denotes the accident parameter. The probability function describes

the likelihood of each B or object, i.e.,

5-4
h(O) - P r())(.2 (5.2)

where e denotes the qualitative object.

The distributions of the accident parameters used to estimate the

likelihood of cask response states are presented in this section. Development

of these distributions was discussed in Section 2.0. The data used to

estimate accident rates and velocity distributions is summiarized in Appendixes

B and C. The method of estimation-is discussed in Appendix G.

5.2.1 Mechanical Loading Parameter Distributions

Object hardness, impact velocity, and cask orientation are three

mechanical loading parameters which have a significant influence on a cask's

structural response in a transportation accident.

5.2.1.1 ObetHrns itiuin

Each of the accident scenarios, described in Section 2.0 and shown in

Figs. 2-3, 2-4, and 2-5, identifies a type of accident, e.g., a collision, and

the object or surface which a cask could impact, e.g., a truck, bridge

abutment, or embankment. From these descriptions, object hardness is

estimated. Thus, the distribution of hardness of the impacted object is

described in terms of the probabilities of the accident scenarios. These are

included in Figs. 2-3, 2-4 and 2-5 for highway and railway accid ents.

5.2.1.2 Impact Velocity Distributions

5.2.1.2.1 Cask Velocity

The distribution of cask

between accident scenarios.

experienced in truck-truck

distribution associated with

truck-truck accidents, the

individual trucks at the time

from bridges, the cask impact

velocity during a transportation accident varies

For example, the distribution of cask velocity

collisions is expected to differ from the

accidents involving falls from bridges. In

distribution depends on the speeds of the

of the collision. For accidents involving falls

velocity is determined by the fall height.

5-5
The following distributions of cask velocities are applicable to highway

accidents:

Vi: The truck velocity, adjusted for braking, prior to an accident

V2: The velocity due to bridge heights

V3: The vector sum of truck velocity, adjusted for braking, and velocity

due to bridge heights

V4: The train velocities at grade crossing accidents.

As discussed in Subsection 2.5.1.2.1, the primary source of truck velocities

is based on accident reports that estimate velocities prior to an accident.

The observed data does not account for any reduction in velocity at impact due

to braking efforts by the drivers. However, a North Carolina study provides

data which allow for braking effects. 1 These results are used to adjust the

basic cumulative distribution function of truck velocities as shown in

Fig. 5-3. The adjustment is based on the identity

FvI(s) - FI[S/6(s)] (5.3)

where

+0.35 0 s s 78 (s 065 .0 +i 9 s 78 (5.4) 6(s) 1:0s 7

and FVl(.) and FI(.) denote the adjusted and initial truck velocity cumulative

distribution functions, respectively. At velocities greater than 78 mph no

credit for braking is assumed. As velocity decreased, the effect of braking

increased, e.g., a 40 mph velocity is reduced to 33 mph, whereas a 10 mph

velocity is reduced to 7 mph.

The four cumulative distribution functions used for the velocity of

highway accidents are presented in Table 5.1. They are estimated from

historical accident data using the method of estimation described in

Appendix G.

5-6
UzC

C

4.2

Ez

0Z

1.1 1

1.0-

estin

0.9 - velocity

0.8 adjustei

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0 1.L

0 10

Figure 5-3

Truck velocity, V, mph

Distribution of vehicle velocities adjusted for braking.

5-7
Table 5.1

Cumulative Cask Velocity Distributions for Highway Analysis

Distributions

Vi V2 V3 V4

Cask Cask Cask Cask

Velocity,s Velocity,s Velocity,s Velocity,s

(mph) F(sS) (mph) Fs(s) (mph) Fs(s) (mph) Fs(S)

0.

2.0

6.0

10.0

14.0

18.0

22.0

26.0

30.0

34.0

38.0

42.0

46.0

50.0

54.0

58.0

62.0

66.0

70.0

74.0

78.0

82.0

86.0

90.0

94.0

98.0

102.0

106.0

110.0

150.0

0.

0. 03834

0.12916

0.23508

0.34886

0.46237

0.56877

0.66345

0.74353

0.80877

0.86020

0. 89961

0. 92881

0.95009

0.96547

0.97634

0.98383

0.98908

0.99261

0.99503

0.99670

0. 99825

0.99910

0.99956

0.99979

0.99990

0. 99995

0.99998

0.99999

1.0

0.

7.74

10.94

15.48

18.95

21.89

24.47

26.81

28.95

30.95

32.83

34.61

36.29

37.91

39.46

41.67

43.08

44.45

56.86

0.

0.00621

0.01550

0.04754

0.1051

0. 1952

0.3178

0.4629

0.6124

0. 7464

0.8508

0.9217

0. 9635

0.9849

0.9945

0.9991

0.9998

0. 9999

1.0

0.

5.0

10.0

15.0

20.0

25.0

30.0

35.0

40.0

45.0

50.0

55.0

60.0

65.0

70.0

75.0

80.0

85.0

90.0

95.0

100.0

105.0

110.0

115.0

150.0

0.

0.

D.00141

0.00821

0.03387

0.11129

0.28292

0.51279

0.70110

0.81951

0.89168

0.93543

0.96178

0.97751

0.98680

0.99227

0.99547

0.99766

0.99901

0.99961

0.99985

0.99995

0.99998

0.99999

1.0

0.

2.0

6.0

10.0

14.0

18.0

22.0

26.0

30.0

34.0

38.0

42.0

46.0

50.0

54.0

58.0

62.0

66.0

70.0

74.0

78.0

82.0

86.0

90.0

94.0

98.0

102.0

106.0

110.0

114.0

118.0

150.0

0.

0.06014

0.17906

0.29398

0. 40255

0. 50280

0.59331

0.67319

0.74210

0.80022

0.84814

0.88676

0.91718

0.94062

0.95826

0. 97125

0.98060

0.98717

0.99169

0.99473

0.99672

0.99800

0.99881

0.99930

0.99960

0.99977

0. 99987

0.99993

0.99996

0.99998

0.99999

1.0

Fs(s)

VI:

V2:

V3:

V4:

= Probability that cask velocity is less than or equal to cask

velocity listed.

The truck velocity, adjusted for braking, prior to an accident

The velocity due to bridge heights

The vector sum of truck velocity adjusted for braking and

velocity due to bridge heights

The train velocities at grade crossing accidents

5-8
The following distributions of cask velocities are considered applicable

to railway accidents:

TV1: The train velocities in collision accidents without braking

TV2: The train velocities in derailment accidents without braking

TV3: The velocities due to bridge heights

TV4: The vector sum of train velocities in derailment accidents and

velocities due to bridge heights.

The cumulative distribution functions are presented in Table 5.2.

5.2.1.2.2 Impact Angle

The damage resulting from an accident is not controlled solely by the

cask velocity at impact. A head-on impact is more severe then a sideswiping

accident, even though both accidents can involve similar velocities. The

reason is that accident severity is most directly related to the impact

velocity, the component of the cask velocity vector perpendicular to the

object impacted. The orientation of the cask motion, relative to the surface

of the object impacted is called the impact angle, a. A 900 impact angle

defines a head-on impact, i.e., the impact velocity and cask velocity at

impact are the same. An impact angle close to 00 defines a sideswiping

impact. In this case the impact velocity is a small fraction of the cask

velocity. Mathematically, the impact velocity is the cask velocity times

sine a.

As for cask velocities, the distribution of impact angle can depend on

the accident scenario. For example, if the accident involves a collision with

another vehicle on the highway, any impact angle is likely. Three impact

angle distributions are used:

VVI: Uniform (00,900) - any impact angle is equally likely

F(x) - x/90 00 1 x g9o0 (5.5)

5-9
Table 5.2

Cumulative Cask Velocity Distributions for Railway Analysis

Distributions

TV1 TV2 TV3 TV4

Cask Cask- Cask CaskVelocity,s Velocity,s Velocity,s Velocity,s

(mph) Fs(s) (mph) Fs(s) (mph) Fs(s) (mph) FS(S)

0.

2.0

6.0

10.0

14.0

18.0

22.0

26.0

30.0

34.0

38.0

42.0

46.0

50.0

54.0

58.0

62.0

66.0

70.0

74.0

78.0

82.0

86.0

90.0

94.0

98.0

102.0

106.0

110.0

114.0

118.0

150.0

0.

0.09385

0.26286

0.40788

0. 53042

0.63240

0.71598

0.78345

0.83709

0.87908

0.91147

0.93606

0.95446

0. 96801

0.97784

0.98486

0.98980

0.99323

0.99557

0.99714

0.99818

0.99886

0.99929

0. 99957

0.99974

0.99985

0.99991

0.99995

0.99997

0.99998

0.99999

1.0

0.

2.0

6.0

10.0

14.0

18.;0

22.0

26.0

30.0

34.0

38.0

42.0

46.0

50.0

54.0

58.0

62.0

66.0

70.0

74.0

78.0

82.0

86.0

90.0

94.0

98.0

102.0

106.0

150.0

0.

0.07543

0.22036

0.35480

0.47634

0.58341

0.67534

0.75225

0.81495

0.86477

0.90385

0.93246

0.95386

0.96920

0.97991

0.98720

0.99204

0.99516

0.99713

0.99834

0.99906

0.99948

0.99972

0.99985

0.99992

0.99996

0.99998

0.99999

1.0

0.

7.74

10.94

15.48

18.95

21.89

24.47

26.81

28.95

30.95

32.83

34.61

36.29

37.91

39.46

41.67

43.08

44.45

56.86

0.

0.00621

0.01550

0.04754

0.1051

0. 1952

0.3178

0.4629

0.6124

0.7464

0.8508

0.9217

0. 9635

0.9849

0.9945

0.9991

0.9998

0.9999

1.0

0.

5.0

10.0

15.0

20.0

25.0

30.0

35.0

40.0

45.0

50.0

55.0

60.0

65.0

70.0

75.0

80.0

85.0

90.0

95.0

100.0

105.0

110.0

150.0

0.

0.

0.00232

0.01244

0.04814

0.14919

0.35837

0.60624

0.77834

0.87230

0.92649

0.95855

0.97727

0.98792

0.99379

0.99692

0.99852

0.99932

0.99970

0.99987

0.99995

0.99998

0.99999

1.0

Fs(S)

TV1:

TV2:

TV3:

TV4:

Probability that cask velocity is less than or equal to cask

velocity listed.

The train velocities in collision accidents without braking

The train velocities in derailment accidents without braking

The velocities due to bridge heights

The vector sum of train velocities in derailment accidents and

velocities due to bridge heights

5-10
VV2: Degenerate (900) - impact is head-on only

F(x) - [ [• x < -0 900 (5.6)

VV3: Triangular (00,900) - head-on impact is most likely

F(x) - x2 /90 2 00 x S 90 0 (5.7)

The cumulative distribution functions are presented in Table 5.3.

5.2.1.3 Cask Orientation Distributions

The orientation of the cask with respect to the object impacted is called

the orientation angle, 0. It affects the severity of the cask response to

mechanical loads. As described in Subsection 2.5.1.3 for impacts on surfaces,

a 00 cask orientation defines a sidewise impact while a 900 cask orientation

indicates impact of the cask on its end. Alternatively for impacts by train

sills, a 00 cask orientation defines a head-on impact to the cask side while a

900 cask orientation indicates a near miss. Again, the cask orientation

distribution can depend on the accident scenario, thus three distributions are

used:

CT1: Uniform (00,900) - all cask orientation angles equally likely

F(x) - x/90 00 9 x 9 900 (5.8)

CT2: Triangular (00,900) - end orientation impact on surfaces or head-on

impact to side of cask by train is most likely

F(x) - x2 /90 2 00 x 9 90 0 (5.9)

5-11
Table 5.3

Cumulative Impact Angle Distributions

Distributions

Impact VVI VV2 VV3

Angle,a FT) FA a) FA7-'-) (0)

0. 0. 0. 0.

5.0 0.05556 0. 0.00309

10.0 0.11111 0. 0.01235

15.0 0.16667 0. 0.02778

20.0 0.22222 0. 0.04938

25.0 0.27778 0. 0.07716

30.0 0.33333 0. 0.11111

35.0 0.38889 0. 0.15123

40.0 0.44444 0. 0.19753

45.0 0.50000 0. 0.25000

50.0 0.55556 0. 0.30864

55.0 0.61111 0. 0.37346

60.0 0.66667 0. 0.44444

65.0 0.72222 0. 0.52160

70.0 0.77778 0. 0.60494

75.0 0.83333 0. 0.69444

80.0 0.88889 0. 0.79012

85.0 0.94444 0. 0.89198

90.0 1.0 1.0 1.0

FA(a)

VMl:

VV2:

VV3:

Probability that impact angle is less than or equal

to impact angle stated in left-hand column.

Uniform (00,900) - any impact angle is equally

likely

Degenerate (900) - impact is head-on only

Triangular (00,900) - head-on impact is most likely

5-12
CT3: Triangular (00,900) - 450 orientation impact on surface or 450 impact

on side of cask by train is most likely

F . 1x (x2/2(45) 2 2 0 0 1 x 1450 F~x) 1-.[ (90-x)-/2(45)2 450 (.0 S x 1 900 (.0

The cumulative distribution functions are presented in Table 5.4.

5.2.2 Thermal Loading Parameter Distributions

The thermal response of a cask, represented by the temperature reached at

the middle of the lead shield thickness, is determined by three major thermal

loading parameters: fire duration, flame temperature, and fire location with

respect to the cask.

5.2.2.1 Fire Duration Distributions

The duration of a fire occurring during a transportation accident depends

on a number of factors including

o the amount and type of fuel, combustibles, and other volatile

materials available

o the availability and feasibility of fire fighting support.

The first factor is influenced by the type of accident. For example, a

single truck accident is likely to involve a different fire environment than

a truck-truck or truck-auto collision. Similarly, a truck hitting a bridge

abutment is likely to cause a different type of fire than a truck jackknifing

or overturning along the roadbed. To accomimodate these possibilities, several

fire duration distributions are considered in the analysis of both highway and

railway accident fires. These distributions are generated using the

simulation code developed at Sandia2

5-13
Table 5.4

Cumulative Cask Orientation Angle Distributions

Distributions

Cask Orientation CM1 CT2 CT3

Angle,B FBTO) F B ) FBT)

(0)

0. 0. 0. 0.

5.0 0.05556 0.00309 0.00617

10.0 0.11111 0.01235 0.02469

15.0 0.16667 0.02778 0.05556

20.0 0.22222 0.04938 0.09877

25.0 0.27778 0.07716 0.15432

30.0 0.33333 0.11111 0.22222

35.0 0.38889 0.15123 0.30246

40.0 0.44444 0.19753 0.39506

45.0 0.50000 0.25000 0.50000

50.0 0.55556 0.30864 0.60494

55.0 0.61111 0.37346 0.69753

60.0 0.66667 0.44444 0.77778

65.0 0.72222 0.52160 0.84568

70.0 0.77778 0.60494 0.90123

75.0 0.83333 0.69444 0.94444

80.0 0.88889 0.79012 0.97531

85.0 0.94444 0.89198 0.99383

90.0 1.0 1.0 1.0

FB(B)

CTI:

CT2:

CT3:

m = Probability that cask orientation angle is less than or equal to

cask orientation angle stated in left-hand column.

Uniform (00,900) - all cask orientation angles equally likely

Triangular (00,900) - end orientation impact is most likely

Triangular (00,900) - 450 orientation impact is most likely

5-14
The following fire duration distributions are used in the analysis of

highway accident fires:

Fl: Non-collision accident fires

F2: Off-road (or collision with fixed objects) accident fires

F3: Truck/truck collision accident fires

F4: Truck/automobile collision accident fires

F5: Truck/train collision accident fires.

These distributions are presented in Table 5.5. The distributions for

accidents involving a truck colliding with a fixed object and a truck running

off the highway are simulated separately but result in the same output.

The following train fire duration distributions are presented in

Table 5.6 for analyzing railway accident fires:

TFI: Collision accident fires

TF2: Derailment accident fires

TF3: Other accident fires.

5.2.2.2 Flame Temperature Distributions

Flame temperature and fire duration are often correlated. Highly

volatile and chemically reactive substances exhibit high reaction rates and

high intensity (temperature), while substances with low reaction rates are

consumed slowly and exhibit low intensity. However, information about the

joint probability distribution of temperature and duration is not available.

Also, the distribution of flame temperature can vary between accident

scenarios due to several factors, including the likely amount of fuel

available. This information is also not available, thus a simple flame

temperature distribution is used in the probability analyses. This

distribution, TI, is based on a Weibull function for flame temperatures

between 1400OF and 2400°F:

TI: Weibull (14 0 00F,24000 F)

5-15
Table 5.5

Cumulative Fire Duration Distributions for Truck Cask Analysis

Distributions

Fire F1 F2 F3 F4 F5

Duration,d G6U)d) G -- ) GDrd) GDd) Gd)

(hours)

0.

0.083

0.167

0.250

0.333

0.417

0.500

0.583

0.667

0.750

0.833

0.917

1.0

1.083

1.167

1.250

1.333

1.417

1.500

1.583

1.667

1.750

1.833

1.917

2.0

3.0

4.0

5.0

6.0

7.0

8.0

0.

0.3311

0.6596

0.8551

0.9625

0.9801

0.9897

0.9944

0.9970

0.9985

0.9992

0.9996

0.9998

0.99991

0.99996

0.99999

1.0

O.

0.0321

0. 2821

0. 5860

0.7754

0.8769

0.9358

0.9643

0.9800

0. 9902

0.9949

0.9973

0. 9989

0.9995

0.9998

0.99995

0.99998

0.99999

1.0

0.

0.0035

0.0451

0.1572

0.3488

0. 5001

0.6034

0.6771

0. 7322

0.7750

0.7960

0.8123

0.8257

0.8367

0.8459

0.8535

0.8596

0.8652

0.8696

0.8737

.0.8779

0.8812

0.8847

0.8882

0.8917

0.9287

0.9503

0.9641

0.9773

0. 9905

1.0

0.

0.0131

0.1653

0.4179

0.6516

0.7878

0.8725

0.9161

0.9456

0.9662

0.9761

0.9838

0.9898

0.9936

0.9964

0.9984

0.9993

0.9997

0.9999

0.99996

0.99997

0.99999

1.0

0.

0.00238

0.07222

0. 16427

0.31099

0.43757

0.54957

0.64690

0.73075

0.80265

0.86416

0. 87612

0.88589

0.89828

0.90934

0.91874

0.92730

0.93452

0.94126

0.96792

0.98247

0.99056

0.99643

1.0

GD (d)

Fl:

F2:

F3:

F4:

F5:

-Probability that fire duration is less

duration stated in left-hand column.

Non-collision accident fires

Off-road (or collision with fixed objects)

Truck/truck collision accident fires

Truck/automobile collision accident fires

Train collision accident fires

than or equal to fire

accident fires

5-16
Cumulative Fire Duration

Table 5.6

Distributions for Rail Cask Analysis

Distributions

Fire TFI TF2 TF3

Duration,d GD-"f) GOO) Gd) (hours)

0. 0. 0. 0.

0.083 0.00238 0.01009 0.00943

0.167 0.07222 0.09213 0.09180

0.250 0.16427 0.17603 0.17574

0.330 0.31099 0.29164 0.29183

0.417 0.43757 0.39717 0.39789

0.500 0.54957 0.49517 0.49648

0.583 0.64690 0.58120 0.58291

0.667 0.73075 0.65917 0.66075

0.750 0.80265 0.72958 0.73139

0.833 0.86416 0.79154 0.79373

0.917 0.87612 0.80544 0.80765

1.0 0.88589 0.81870 0.82036

1.167 0.89828 0.83308 0.83454

1.333 0.90934 0.84752 0.91874

1.500 0.91874 0.86071 0.86292

1.667 0.92730 0.87388 0.87564

1.833 0.93452 0.88537 0.88704

2.0 0.94126 0.89665 0.89792

3.0 0.96792 0.94290 0.94342

4.0 0.98247 0.96790 0.96821

5.0 0.99056 0.98166 0.98239

6.0 0.99643 0.98868 0.98941

7.0 1.0 0.99380 0.99403

8.0 0.99702 0.99754

9.0 0.99910 0.99928

10.0 0.99978 0.99985

11.0 1.0 1.0

GO (d)

TFI:

TF2:

TF3:

-Probability that fire duration is 'less than or

duration stated in left-hand column.

Collision accident fires

Derailment accident fires

Other accident fires

equal to fire

5-17
-(x-140 1083 1000 )1.*83

F(x) - [1-e I / [1-e ] 1400°F I x i 2400°F (5.11)

This distribution covers the range of flame temperature achievable in typical

hydrocarbon fires. 2 These types of fires constitute the majority of fires

which occur in transportation accidents. The cumulative distribution function

is presented in Table 5.7.

5.2.2.3 Fire Location Distributions

The location of a fire has a significant affect on the heat flux to which

a cask is exposed and hence on the temperature attained at the middle of the

lead shield thickness. An engulfing fire typically produces a greater heat

flux exposure to the cask and results in higher cask temperatures than a fire

of the same temperature, size, and duration that is adjacent to the cask. The

greater the distance of the fire from the cask, the less the thermal

interaction and effective exposure.

As with the other fire parameters, no historical data is available for

developing a distribution of fire location with respect to the cask. In lieu

of such information, a uniform distribution of cask to fire location is

assumed. The fire locations are varied between the truck and rail casks in

proportion to the size differences between the two casks. The fire location

distributions, L1 , used are:

Truck fires - Uniform (0 ft, 30.75 ft)

F(x) = x/30.75 0 ft I x 1 30.75 ft (5.12)

Train fires - Uniform (0 ft, 43 ft)

F(x) - x/43 0 ft I x 1 43 ft (5.13)

5-18
Table 5.7

Cumulative Flame .Temperature Distribution

Flame TemPFpraturet GT(t)

1400 0.

1500 0.04551

1600 0.15306

1700 0.29588

1800 0.45059

1900 0.59847

2000 0.72714

2100 0.83069

2200 0.90849

2300 0.96342

2400 1.0

GT(t) - Probability that flame temperature is less

temperature stated in left-hand column.

than or equal to

5-19
The cumulative distribution functions are presented in Table 5.8. A fire is

considered engulfing if it is within 1/4 foot of the center of a truck cask or

within one foot of the center of a rail cask.

5.3 Probability Calculation

The purpose of the probability calculation is to estimate the likelihood

that specified sets. of cask responses will be realized if an accident

occurs. The calculation is based on combining the probabilistic information

about the accident parameters with the probabilities of the various accident

scenarios. The probability estimate is then combined with an estimate of the

expected accident rate/truck or train-mile to estimate the expected

frequency/mile of cask response in specified response regions. Once the

radiological hazards for each cask response region are characterized, the

risk, i.e., probability times hazard, associated with transporting spent fuel

is estimated.

As described in Section 4.0, the potential cask response represented by

the containment vessel strain and the lead mid-thickness temperature due to a

transportation accident are partitioned into 20 response regions R(i,j),

i-1,...,4, j-1,...,5, consisting of the combination of 4 structural response

regions and 5 thermal response regions:

Structural

Response Region Condition

i-I Less than 0.2% strain (<Sl)

2 Between 0.2% (SI) and 2% (S2) strain

3 Between 2% (S2 ) and 30% (S3) strain

4 Greater than 30% strain (>S3 )

5-20
Table 5.8

Cumulative Fire Location Distributions

Distributions

Truck ]rain

Fire Locationj 9L(IT -DR)

(feet)

0. 0. 0.

1.0 0.03175 0.02326

2.0 0.06349 0.04651

6.0 0.19048 0.13953

10.0 0.31746 0.23256

14.0 0.44444 0.32558

18.0 0.57143 0.41860

22.0 0.69841 0.51163

26.0 0.8455 0.60465

30.00 0.9756 0.69767

30.75 1.0

34.0 0.79070

38.0 0.88372

42.0 0.97674

43.0 1.0

GL( I ) - Probability that fire location is less

location stated in left-hand column.

than or equal to fire

5-21
Thermal

Response Region

j-1

2

3

4

5

Condition

Less than 500OF lead mid-thickness temperature (<T1 )

Between 500OF (T1 ) and 600°F (T2 ) lead

mid-thickness temperature

Between 600°F (T2 ) and 650°F (T3 ) lead

mid-thickness temperature

Between 650°F (T3 ) and 1050OF (T4 ) lead

mid-thickness temperature

Greater than 1050°F lead mid-thickness

temperature (>T4 )

The probabilities estimated in the probability analysis are the likelihood of

the cask response being in each one of the response regions.

The initial step in modeling the probability calculations is to relate

the containment vessel strain to impact velocity and the lead mid-thickness

temperature to effective fire duration. The first part is done by developing

strain-impact velocity curves for several object hardnesses. Similarly, the

lead mid-thickness temperature-fire duration models are developed for several

fire locations and a 1700OF flame temperature.

Given a fixed impact angle and cask orientation, the probability that

containment vessel strain is within a given region is derived from the

distribution of the impact velocity via the strain-impact velocity curves.

For example, given a truck cask, using Fig. 7-3 and assuming an unyielding

object and an end-on cask orientation, a strain between 0.2% (S1) and 2% ($2)

corresponds to an impact velocity between 38 mph and 46 mph. Thus, assuming a

head-on impact, i.e., 900 impact angle, the probability of the containment

vessel strain being between 0.2% (Sl) and 2% (02 ), denoted P(O.2 I St s 2),

is equal to the probability that the cask velocity is between 38 mph and 46

mph. Recognizing the fact that the relationships between strain and cask

velocity are conditional on the impact angle, cask orientation, and object

hardness, the identity involving the strain and cask velocity probabilities

can be written mathematically as:

5-22
P(O.2<St 21head-on, end-on impact with unyielding object)-Fs(46)-Fs(38) (5.14)

where FS(.) denotes the appropriate cumulative distribution function of cask

velocity.

Taking into consideration the fact that the impact angle and cask

orientation are variable, and recognizing that the hardness of the object

impacted is identified by an accident scenario, the probability of the

containment vessel strain given a specific accident scenario is obtained by

averaging the probability in Equation 5.14 with respect to the appropriate

distributions for impact angle and cask orientation. Mathematically,

P(O. 2<St<2 1Ak) -I If [FS[s 2 (cl,,Ak) 0Ak]-FS[sO. 2 (a,0,Ak) IAk]1 aB

x dFA (alAk)dFB (OIAk) (5.15)

where Ak identifies an accident scenario and FS(.), FA (.) and FB (.) are the

cumulative distribution functions for cask velocity, impact angle, and cask

orientation, respectively. Equation 5.15 recognizes that the cask accident

velocity corresponding to 0.2% (Si) and 2% (S2 ) strain depends on the impact

angle, cask orientation, and hardness of the object impacted, i.e., the

accident scenario.

As illustrated in Fig. 5.1, changing the cask orientation corresponds to

varying the strain-impact velocity curve. This change is included in the

probability analysis by developing strain-impact velocity curves for 00,. 450,

and 900 cask orientation for each level of hardness of the impacted object.

It is assumed, given a fixed impact angle, that the impact velocities for

intermediate angles can be approximated by:

v%(O) - '450) - v%(0°)] 0° < 0 < 450

vY(O) - { v450o) + (045) [v%(900) - v%(450)] 450 < B < 900 (5.16)

5-23
That is, a linear interpolation is assumed between the 00 and 450 curves and

between the 450 and 900 curves. Notationally, v%(8) denotes the impact

velocity corresponding to strain percent, %, for cask orientation angle, $.

The corresponding strain-impact velocity curves for several a's are

illustrated in Fig. 5-1.

The impact anglea relates the cask impact velocity to the cask accident

velocity. If the impact is head-on, i.e., a-900 , then the impact velocity

equals the accident velocity. On the other hand, if a is less than 900, then

the impact velocity is less than the accident velocity. Since the velocity

distributions Vi through V4 and TVM through TV4 are distributions for accident

velocities, it is necessary to transform the impact velocity corresponding to

a strain level to an accident velocity. This transformation, for a fixed cask

orientation angle, 0, is given by

s%($,a) - v%($)/sina (5.17)

where v%(Y ) represents impact velocity and s%(8,a) is the corresponding

accident velocity for the given impact angle.

To illustrate how cask orientation and impact angle are handled in the

calculations, we consider structural response region i-2, i.e., between 0.2%

(Sl) and 2% (S2) strain, being attained when a cask hits a concrete object at

a 450 orientation angle and a 350 impact angle. From Table 5.9 for accident

scenario No. 8 the impact velocities for 0.2% (S$) and 2% (S2) strain are

v0 . 2%(450 ) = 35 mph and v2%(450 ) = 49 mph. (Note: for other orientation

angles 8, Equation 5.16 would be used to evaluate v%(O).) Using

Equation 5.17, the vehicle velocities necessary to result in impact velocities

of 35 mph and 49 mph, if the angle of impact is 350, are (since sin 350 =

0.57378):

s 0.2% (45,35) ' v0. 2%(450) / 0.57378

= 61 mph

5-24
Table 5.9

Probability Inputs for Highway Analysis

5-25
s2%(450,350) ' v2 %(450 ) / 0.57378

85.40 mph

Given a fire, the thermal response of the cask, represented by the lead

mid-thickness temperature is related to the duration of the fire. This

relationship, illustrated in Fig. 5-2, depends on both flame temperature and

fire location. Using an argument analogous to the development of the

probability corresponding to a structural response region, the probability

that the cask thermal response is in a specific region, for example, between

600°F (T2 ) and 650°F (T3 ) or thermal response region j-3, is given by

P(600<T<6 50IAk with a Fire)-If [GD[d 6 5 0 (t,1)IAk]-GD[d 6 0 0 (t,1)IAk]J

x dGT(t)dGL(l) (5.18)

where GD(.), GT(.), GL(.) denote the fire duration, flame temperature, and

fire location cumulative distribution functions, respectively. Again, the

fire duration, do (t,I), corresponding to a lead mid-thickness temperature,

OF, depends on t~e flame temperature and fire location. This is denoted in

the argument of the fire duration distribution function. Also, the fire

duration distribution varies with the accident scenario.

The basic mid-thickness temperature of the lead shield-fire duration

curve is based on a 1700OF real engulfing fire. The effects of the other fire

parameters are included in the analyses by adjusting this basic curve. For

fires that deviate from a 1700OF fire, the same temperature is reached within

the shield, but the time to reach this temperature is shorter or longer

depending on the flame temperature. If the flame temperature is greater than

1700°F, the same lead mid-thickness temperature is reached in a shorter time;

whereas if the flame temperature is below 1700°F, it takes longer to produce

the same temperature in the middle of the lead shield thickness. Thus, for a

given lead mid-thickness temperature, the effects of different flame

temperatures for an engulfing fire are modeled by the identity

5-26
d 0(t,O) - 6(t) d0 (17000,0 ft) (5.19)

F F

A list of the factors 6(t) is presented in Table 5.10.

For fire location, as the distance between the fire and the cask

increases, heat exposure decreases, and a longer duration fire is needed to

produce the same temperature in the middle of the lead shield thickness as an

engulfing fire. Thus, the effect of fire location on the lead shield

temperature-fire duration relationship is modeled by a multiplicative

factor. The model used is

doF (t,1) - 6(1) d F(t°,0 ft) (5.20)

- 6(t) 6(t) doF (17000,0 ft)

where the factor 6(1) is given by

6(1) - 0.78e( 0"7 7 3 2+0" 0 6 28 7 1 ) I > 1.5 ft (5.21)

for a truck cask and

6(1) - 0.78e( 0"6 28 7 4 + 0.084711) 1 > 4 ft (5.22)

for a rail cask. In both cases, location is measured from the center of the

cask, which is mathematically assumed to represent the location of an

engulfing fire. Development of the flame temperature and fire location models

in Equations 5.19 through 5.22 is discussed in Subsections 2.5.2.3 and

2.5.2.4. The effect on the basic lead mid-thickness temperature-fire duration

curve for a truck cask is shown in Fig. 5-2.

Equations 5.15 and 5.18 are expressions for estimating the probability

that the containment vessel strain is within a given structural response

region, e.g., between 0.2% (S$) and 2% (S2) strain, and the probability, given

a fire, that the lead mid-thickness temperature is within a given thermal

response region, e.g., between 600°F (T2 ) and 6500F (T3 ), respectively. Both

expressions are conditional on a given accident scenario. A cask response

5-27
Heat Flux

Table 5.10

Factors for Flame Temperatures

(Engulfing Fire)

Flame Tem eraturet 6(t)

1400 1.72

1500 1.43

1600 1.21

1700 1.0

1800 0.86

1900 0.73

2000 0.64

2100 0.56

2200 0.49

2300 0.44

2400 0.39

5-28
expressions are conditional on a given accident scenario. A cask response

region involves a combination of structural responses and thermal responses.

Assuming that strain is independent of the lead shield temperature, these

probabilities can be multiplied to estimate the probability associated with a

response region. For example, for response region R(2,3), i.e., strain

between 0.2% (SI) and 2% (S2) and lead mid-thickness temperature between 600°F

(T2 ) and 650°F (T3 ), the probability, given accident scenario Ak is:

P[R(2, 3)IAk]mP(FireIAk)[f f [FSls 2 (aO,,Ak) jAk]-Fs[so. 2 (aiB,Ak) JAkR]

x dFA (ajAk)dFB (0IAk)][0 jfGD[d650(t,)IAk]-GD[d6 00(tt)IAk])

x dGT(t)dGL(1)] (5.23)

where the probability of a fire is included in the expression. Similar

expressions hold for each of the response regions R(i,j).

Two response regions correspond to accidents involving either no fire or

fire only. In these cases, it is assumed that there is no cask thermal

response and no cask structural response. For no fire, the response regions

are denoted R(i,O), and the probabilities are

P[R(i,O)IAk]'[1-P(FirelAk)] P[st,i9S•su,iIAk] (5.24)

where s ,i and Su,i denote the lower and upper strain limit for the ith

region, respectively. For fire only, the response regions are denoted R(Oj),

and the probabilities are

P[R(O,J)IFire only]-P[d Ij1`9d ,jJFire only] (5.25)

where d ,l and du,j denote the lower and upper shield temperature for the jth

region, respectively.

5-29
The final step in the probability calculation is to combine the

probabilities over all accident scenarios. Thus, for response region R(ij),

P[R(iJ)] - A I P(Ak) P[R(ij)IAk)] (5.26) k

where P(Ak) is the likelihood of accident scenario Ak given an accident.

Tables 5.9 and 5.11 summarize the value of P(Ak); P(FirejAk); choice of

distributions for each accident scenario; and the structural response region

limits for 00, 450, and 900 cask orientation for a truck cask and rail cask,

respectively.

The actual probability calculations described in Equations 5.23 and 5.26

are done by a computer code, called TASP (Transportation Accident Scenario

Probabilities). The inputs into the code are appropriate distributions for

the accident parameters. These are combined for each accident scenario using

Equation 5.23 and averaged over accident scenarios using Equation 5.26. The

integration in Equation 5.23 is based on approximating the integrals by

sums. Details of the integration are discussed in Appendix G. A flow chart

of TASP is given in Fig. 5-4.

The results of the probability calculations are presented and discussed

in Section 9.0.

5-30
Table 5.11

Probability Inputs for Railway Analysis

5-31
Input and catalog

distributions of

accident parameters

Adjust velocity distributions for

braking

-I

For each

accident scenario

II

Evaluate probability in each

mechanical response region

- Adjust cask accident

velocity for impact

angle and cask

orientation

a Estimate probabilities

using Eqn. 5.15

Evaluate probability in each

thermal response region

" Adjust fire duration

for flame temperature

and fire location

" Estimate probabilities

using Eqn. 5.18

Sum probabilities

over all accident

scenarios using

Eqn. 5.26

Combine probabilities

. P[R(1, j)) = P[R(1, j)] + P[R(O, j)]

* P[R(i, 1)] = P[R(i, 1)1 + P[R(i, 0)]

I

Radiological

hazard

estimates,

a(i, j)

Evaluate probability -

hazard estimates

o(i,j) P[R(i,j)]

and radiological risk Y..•(zdi, j) P[R(i, j)]

ij I .

I

Evaluate probability in

each response region

using Eqn. 5.23 I

-I

Output

" Probabilities, P[R(ij)]

" Probability - hazard estimates

Evaluate probability given " Radiological risk

" No fire

" Fire only I

I -No -All accident -YesFigure 5-4 Flow chart of TASP computer code.

5-32
6.0 FIRST-STAGE SCREENING ANALYSIS

6.1 Introduction

A two-stage screening process is used to evaluate the level of protection

provided by licensed fuel casks against real accident loading conditions.

Response regions are developed on the basis of cask performance and are

described in terms of damage. The response regions are used to sort or screen

accident events in terms of the analytically predicted performance of the

representative casks. Figure 6-1 shows the 20 response regions defined in

Section 4.0. In the first-stage screening, the intent is to determine by

analysis which accident-caused loading conditions can result in cask responses

that will fall within the first response region R(1,1). Cask responses in

this region are less than or equal to responses implied by the 10 CFR 71

accident test conditions. 1 The second-stage screening analysis identifies

accidents which produce loading 'conditions that can cause cask responses

outside the R(1,1) region. The first-stage screening analysis is discussed in

this section; the second-stage screening analysis is discussed in Section 7.0.

Within the R(1,1) region, the cask structural response does not exceed a

strain level of 0.2% (S$) on the inner shell of the cask. The cask thermal

response does not exceed a temperature level of 500OF (T1 ) at the middle of

the lead shield thickness. Within the R(1,1) region, all the major cask

components important to safety during transportation accidents are expected to

remain fully functional, and the cask meets regulatory requirements. The cask

responses within the R(1,1) region do not exceed the responses that would be

expected if the cask were subjected to the accident test conditions of 10 CFR

71. Since cask responses within the R(1,I) region do not result in any

significant damage to the cask, no radiological release beyond the regulatory

limit is expected from the accident causing this level of damage. In fact, in

most cases, releases, if any, would be much less than regulatory limits.

The first-stage screening analysis follows this procedure:

o For each representative cask, dynamic structural and transient thermal

analyses are performed to calculate responses to a range of loading

6-1
4.. 'a

E

E

C

0

CL

Ii

T4 (500) (600) (1050) (650)

Thermal response (lead mid-thickness temperature, *F)

Note: o The radiological hazard of cask responses falling in region R(1,1)

are negligible and less than limits specified in existing regulations (10 CFR 71).

o The radiological hazard of cask responses falling outside region R(1,1) can exceed the limits specified in existing regulations (10 CFR 71).

Figure 6-1 Identification of first-stage screening.

6-2
conditions for the accident scenarios identified in Section 2.0. The

loading conditions for the accident scenarios are defined by three

mechanical loading parameters and three thermal loading parameters. The

mechanical loading parameters are impact velocity, object hardness, and

cask orientation. The thermal loading parameters are fire duration,

flame temperature, and fire location with respect to the cask.

o The structural response is calculated for various impact velocities. The

impact velocity is equal to the component of the accident velocity

perpendicular to the surface impacted. It is arrived at by multiplying

the accident velocity by the sine of the impact angle. Since the impact

angle is not precisely known, it is assumed to follow selected

probability distributions depending on the accident scenario under

study.

o For each accident scenario, the loading conditions that result in cask

responses within the R(1,1) region are determined by comparing the cask

response with the response levels of 0.2% strain ($I) on the inner shell

for mechanical loads and 500OF (T1 ) at the mid-thickness of the lead for

thermal loads.

o For each accident scenario, the probability of occurrence of the specific

loading conditions that could result in cask responses within the R(1,1)

region is estimated as described in Section 5.0, using the data bases

identified in Section 2.0.

0 The fraction of accidents with loading conditions that could result in

cask responses within the R(1,1) region is calculated by summing the

individual occurrence probabilities associated with each accident

scenario.

The major differences between the first-stage and second-stage screening

analyses involve the methods used in the structural and thermal analyses. For

6-3
the first-stage screening analysis, less sophisticated methods of analysis can

be reliably used. For structural responses below the 0.2% strain (Sj) level,

dynamic linear elastic analysis can be used with high confidence to evaluate

mechanically induced structural responses. For responses beyond the 0.2%

strain (Sj) level, 2% strain (S2) and 30% strain (S3 ), dynamic nonlinear

analysis is required. For thermal responses below the 600°F temperature (T2 )

level, standard transient heat transfer analysis methods can be used. These

methods include transient heat transfer by conduction, radiation, and

convection. Responses beyond the 600OF temperature (T2 ) level include melting

of the lead shield, which requires that the transient analysis method include

the consideration of phase changes of materials. Figure 6-2 is a schematic

diagram showing the general methods of analysis used in the cask response

calculations for each of the response regions. Analyses are not performed to

calculate responses beyond the 30% strain (S3) and 1050OF temperature (T4 )

levels since the uncertainties in calculational results would be large.

However, in Section 8.0, the potential radiological significance is estimated

for responses beyond these levels.

In order to consolidate the many variables and analyses required to cover

the wide range of potential accident situations, the following approaches and

assumptions are used in this study.

(1) Casks used for spent fuel shipments are assumed to be properly designed,

fabricated, maintained, and operated in accordance with regulations. The

intent of this evaluation is not to assess the probability and potential

effects of cask defects or deficient or misapplied operational

procedures.

(2) The accident loading parameter distributions in Section 5.0 are generated

from the accident data identified in Section 2.0 and are assumed to

represent loadings which could be experienced by a spent fuel cask.

These accident data are derived from several broad data bases and are

independent of any specific transportation route. The frequency of

occurrence of certain accident scenarios and their loading conditions can

6-4
no

a.-

C

S

0C

0

CL

no -4 0. 9

analysis

S3

(30)

nonlinear

large

deform.

S2

(2)

nonlinear small

deform. $1

.2)

linear

elastic constant phase phase no

constant phase change change analysis

phase

(0

T(

(500)

T2 -

(600)

T3

(650)

T4

(1050)

Thermal response (lead mid-thickness temperature, 'F)

*Combined analysis

Figure 6-2 Methods of analysis used in cask response determinations.

6-5
experience some variations depending on the specific routing selected.

These variations are considered minor for purposes of this study.

(3) In evaluating highway and railway accidents involving impacts, any damage

done to the cask is assumed to result from striking a single object.

Real accidents can involve impact with multiple objects; however, for

impacts into the harder objects of interest, almost all of the energy

involved in the accident is associated with the initial impact. In

certain cases, such as accident scenarios involving impacts with bridge

railings, conservative assumptions are made. In this scenario, it is

assumed that the bridge railing does not cause the transport vehicle to

stop but instead allows the cask to fall off the bridge and onto the

surface below. The cask response is calculated for falling off the

bridge and striking the surface below. Damage to the cask caused by

hitting the bridge railing is not significant to the overall

evaluation. Conservatism is further introduced in the probability

portion of the evaluation because a cask is assumed to fall off a bridge

whenever the truck hits the bridge railing.

(4) The representative truck and rail casks selected for this study and

described in Section 3.0 are defined to meet regulatory requirements and

generally reflect the designs of casks on the roads and railways today.

In actual shipments there will be a variety of cask designs. For all of

the accident conditions analyzed, most, if not all, would be expected to

exhibit degrees of damage equal to or less than those calculated for the

representative casks. Ideally the screening analyses would have used a

variety of cask designs with their commensurate variety of potential

responses. The results of using a representative cask design for the

screening process undoubtedly results in an underestimate of the fraction

of accidents leading to cask responses in the R(1,1) region. Conversely,

the fraction of accidents leading to cask responses in the other regions

is most likely overstated.

6-6
(5) If there is a lack of data or any unknown factors involved in the

structural and thermal analyses or in the accident definition, one of two

approaches is followed. Either reasonable assumptions are made from

sensitivity study results, or conservative assumptions are made. This

approach reduces the need to significantly expand the current data base

or unnecessarily complicate the analysis.

Section 6.2 discusses the structural response analysis for impact loads

on the representative truck casks. The response analysis determines which

accident loading conditions can result in responses that are less than the

0.2% strain ($I) level within the inner shell of the cask. Section 6.3

discusses the thermal response analysis for thermal loads on the cask that

result in responses within the 500OF temperature (T1 ) level at the midthickness of the lead shield. In Section 6.4 the probabilities of occurrence

are estimated for highway and railway accident loading conditions that could

result in cask responses falling in the R(1,1) region.

6.2 Structural Response Analysis

Impact loads dominate the structural evaluation. Other loads such as

crushing and projectile loads are determined to have little effect on the

structural screening analysis. The significance of these loads is discussed

in Appendix E. Many accident loads are easily screened out. Minor accidents

involving low impact loads, like a rollover or impact with low-resistance

objects such as a cask hitting a tree, motorcycle, or automobile, are screened

out because the maximum forces generated in these impacts cannot cause

significant damage to the cask.

The structural response of a cask to loads generated by potentially

significant accidents involving impacts with harder objects at high velocities

are calculated. There are-three parameters that are considered in estimating

structural response. These are shown in Figure 6-3 for impacts on surfaces as

impact velocity, cask orientation angle, 0, and object hardness. Response

calculations are made for various impact velocities and cask orientation

angles. The impact velocity is the component of the cask velocity vector

6-7
.4eloci "p c

impact velocity cask velocity X sina

Object surface

o Object surface hardness

o Impact velocity: Cask velocity component perpendicular

to the object surface is defined as impact velocity.

o Cask orientation is defined by angle -, *the angle between the cask

longitudinal axis and the surface of the object struck

Figure 6-3 Three impact loading parameters considered in the

response analysis for impacts on surfaces.

6-8
perpendicular to the surface impacted. The angle of impact, a, represents the

angle between the cask velocity direction and the surface of the impacted

object. When an accident occurs, the cask velocity vector can be in any

direction. However, it can always be decomposed into two components: one

perpendicular to the impacted object surface and one parallel to it. In the

cask response calculation, only the velocity component perpendicular to the

object surface is considered. The velocity component parallel to the object

surface introduces a sliding-friction effect to the cask structure. The

sliding-friction effect will not produce any significant structural

deformation to the cask; therefore, it is ignored. The angle defining the

cask orientation is the angle between the cask longitudinal axis and the

surface of the object struck. Object hardness needs to be considered because

casks can strike objects such as concrete abutments, roadbeds, hard rock, soft

rock, hard soil, and water. The hardness of the objects and their responses

to impact vary over a wide range.

In some accidents, such as rail grade crossing accidents, the impact

limiters on the cask can be bypassed and the side of the cask can be struck

directly. Once again the mechanical loads depend on the impact velocity, the

orientation of the cask and the hardness of the object struck. Figure 6-4

defines these three loading parameters for this type of accident. The impact

velocity is the component of the relative velocity of the cask and object that

is perpendicular to the cask axis. The angle of impact, a, represents the

angle between the relative velocity direction and the cask axis. For the

purposes of this study, the impact angle is conservatively assumed to be 900,

that is perpendicular to the cask axis in all cases. Also, it is assumed that

the impact occurs at the mid-plane of the cask to cause the most damage. The

cask orientation angle, 0, is the angle at which the impact occurs on the cask

surface. In the worst case the cask is hit at 00 or head-on. For orientation

angles near 900. the cask is essentially not struck. The object hardness

depends on the object hitting the cask, such as a train sill or a small bridge

column.

Two methods of analysis are used in performing the first-stage screening:

quasi-static and linear elastic dynamic. The quasi-static method is used to

screen out minor accidents involving low-resistance objects such as poles and

6-9
Accident velocity

(normal to cask axis)

Accident velocity

o Object hardness

o Impact velocity: Relative velocity component perpendicular to cask

surface.

o Cask orientation angle, 0: the angle between the accident velocity and

impact velocity.

Figure 6-4 Three impact loading parameters considered in the

response analysis for impacts with objects such as

train sills.

6-10
automobiles. A variety of tools are used to accomplish the quasi-static

evaluation, including engineering formulas, impact test data, and a computer

code called NIKE 2-D, the 2-D designation indicating the two-dimensional

modeling option. 2 The linear elastic method is used to perform a dynamic

response analysis of the cask structure for accidents involving impacts with

hard, massive objects in which cask damage cannot be ruled out by the quasistatic evaluations.

The IMPASC code is a linear elastic dynamic code within the SCANS

computer program that can be operated on a personal computer. 3 IMPASC is

developed specifically for analyzing dynamic impacts of shipping casks when

the casks are subjected to loadings generated as a result of imposition of 10

CFR 71 accident test conditions. The code which is inexpensive to run can be

used to analyze oblique impacts and to analyze non-linear behavior of an

impact limiter. The deficiency is that IMPASC can model only collisions with

unyielding surfaces and cannot handle real surfaces, such as soil or

concrete. Also, IMPASC cannot assess lead slump.

In order to perform the dynamic response calculations, the IMPASC code is

used in conjunction with two other codes called NIKE 2-D/3-D and DYNA 2-D/3-D;

the 2D/3-0 designation indicating that either two- or three-dimensional

modeling can be performed. 2 , 4 The NIKE 2-D/3-D and DYNA 2-0/3-0 codes are

powerful finite element codes suitable for dynamic impact analysis. IMPASC is

used to evaluate cask responses for impacts on an unyielding surface for

various cask orientations. DYNA and NIKE are used to evaluate cask responses

for endwise and sidewise impacts on unyielding and real surfaces. IMPASC is

benchmarked against NIKE as discussed in Appendix E.

A cost-effective equivalent damage technique is used to estimate the

response of the representative casks impacting real surfaces. The basic

assumption in the equivalent damage technique involves conservation of energy;

that is, the total energy of the falling cask is absorbed by deformation of

the cask and the surface that it hits. In order to estimate the energy

absorbed by the surface, the cask is first modeled as a rigid body and the

impact surface as deformable and energy-absorbent. This model is used to

6-11
establish the force on a rigid cask generated by a real surface and the

deformation of the real surface for several impact velocities. Next,

calculations are made with the representative cask impacting an unyielding

surface at different impact velocities. This establishes the impact forces on

the cask and the corresponding cask deformations.

In order to account for the energy absorbed by an actual surface, the

force determined from the first analysis, i.e., a rigid cask hitting a

deformable surface, is applied to the representative cask to determine a

corresponding cask deflection and an associated velocity. By summing both the

cask and surface deflections and again considering the defined force level, an

equivalent impact velocity on an unyielding surface can be estimated for a

representative cask impacting a real surface. Figure 6-5 illustrates this

analysis process for the case of a vertical end-drop of a cask without impact

limiters. The process is discussed in detail, including the benchmark

calculation, in Appendix E.

Three surfaces are used to represent the range of credible impact

surfaces. These surfaces simulate hard rock, soft rock/hard soil, and

tillable soil. Soft rock and hard soils are similar for impact and are

represented as a single surface. Real surfaces exhibit complex response

characteristics but can be considered to deform elastically during the early

part of an impact, with a subsequent energy dissipation phase. The exact

nature of the energy dissipation mechanism is not well known; therefore, for

simplicity, an elastic-plastic formulation is used. The parameters used in

this formulation, namely, the initial elastic modulus, the poisson ratio, and

the yield stress are calibrated to approximate an equivalent energy-absorbent

medium. To provide the calibration, penetration data 5 are used as discussed

in Appendix E. Reasonable predictions of penetration are possible using the

approximate elastic-plastic formulation. The resulting calibrated parameters

are listed in Table 6.1 for each surface.

Subsection 6.2.1 describes the structural response analysis for highway

accidents. The 31 accident scenarios identified in Section 2.0 are

individually analyzed to determine the loading conditions that could cause

6-12
-H

(a) Rigid cask

impacts real

surface

V2

(b) Representative cask

impacts unyielding

surface

I~ F V2

VWW = 2SI

(c) Representative cask

impacts real

surface

g = 2(dsI + dU )

Figure 6-5 Equivalent damage technique.

6-13
Table 6.1

Material Parameters Selected for Real Surfaces

Surface type Young's Modulus Poisson Ratio Yield Stress

(psi) (psi)

Hard Rock 7,000,000 0.28 25,000

Soft Rock/Hard Soil 3,640,000 0.2 4,000

Tillable Soil 6,000 0.4 1,000

6-14
cask responses of 0.2% strain (SI) or less. Subsection 6.2.2 describes a

similar response analysis performed for 24 railway accident scenarios.

Subsection 6.2.3 discusses the structural response results..

6.2.1 Cask Response Analysis for Highway Accidents

The representative truck cask described in Section 3.5 is used to perform

the highway accident response analysis. Appendix E discusses the computer

models of the cask and the detailed structural calculations used in the

analysis. The structural evaluations use the highway accident scenarios

presented in Figs. 2-3 and 2-4. The results of the response evaluations are

described in Subsection 6.2.1.1 for accidents involving minor forces and in

Subsection 6.2.1.2 for accidents in which the forces are potentially

significant.

6.2.1.1 Response to Minor Accidents

Accident scenarios which result in minor forces are determined with an

evaluation of cask performance under static loads. A static crushing force of

1.6 million pounds is applied to the cask side. The resulting strain

calculated at the inner shell is less than 0.2% (S1). When crushing the cask

from the end, 3.2 million pounds of force generates a strain of less than 0.2%

(SI). Assuming that the sidewise impact force is linearly applied, the

force/unit length that could cause local deformation can be estimated. The

representative cask can resist a linear force of 100,000 pounds/foot,

generating a strain of less than 0.2% (S1). The linear force required to

crush objects in many accidents is much less than 100,000 pounds/foot, and

thus these accidents are screened out (placed in the R(1,1) response region).

The maximum force that an object generates during a high velocity impact

can be estimated using -quasi-static methods. By substituting equivalent

static forces for inertial forces due to deceleration, calculations indicate

that objects such as automobiles or truck trailers cannot generate forces

greater than 100,000 pounds/ foot-of- contact, even at high impact velocities.

The automobile, as this calculation indicates, is a relatively soft object

6-15
when compared with the massive steel cask and is severely damaged. The energy

generated by the high-velocity impact of the automobile is almost totally

absorbed in the destruction of the automobile, and no impact force greater

than 10,000 pounds/foot is applied to the shipping cask. Also, for such a

relatively light object (<5,000 pounds), the massive cask (50,000 pounds) will

accelerate the object, hence reducing the impact forces to values

significantly less than the 10,000 pounds/foot.

Many other low-resistance objects, such as trees, road signs, utility

poles, motorcycles, trailers, and trucks, are also in this relatively soft

object category. All these objects pose no threat to the cask and require no

further analysis. Table 6.2 identifies all objects that can generate a

maximum quasi-static force less than 100,000 pounds/foot at any velocity. The

percentage of accidents involving these objects is 94.7%.

The remaining highway accidents involve stronger and more massive

objects, such as trains, bridge columns, abutments, and certain real surfaces

such as roadbeds. The analysis of these accidents is described in the next

subsection.

6.2.1.2. Response to Other Accidents

Truck accident scenarios involving impacts with trains, running off

bridges or over embankments, and running into slopes or massive concrete

structures require dynamic structural analysis. The cask dynamic response is

analyzed for impacts with the principal objects involved in these accidents.

Figure 6-3 shows the variables considered in the dynamic response

analysis: cask orientation, object hardness, and impact velocity. The IMPASC

code is used only for unyielding targets. Different methods of analysis are

used for soft objects, depending on their hardness. Hard objects are

considered unyielding surfaces. The impact analysis application for these

objects is presented in Subsection 6.2.1.2.1. Cask responses for relatively

soft objects are discussed in Subsection 6.2.1.2.2

6-16
Table 6.2

Evaluation of Quasi-Static Force for Minor Highway Accidents=a

Total Linear

Accident Scenario Frequency Force Force

(lb) (lb/ft)

1. Soft objects (cones,

animals, etc.) 0.034 <1,000 < 1,000

2. Motorcycle 0.008 <20,000 <10,000

3. Automobile 0.432 <50,000 <10,000

4. Truck, bus 0.133 <400,000 <70,000

5. Train 0.008 b/

6. Other (rocks, furniture, etc.) 0.038 <50,000 710,000

7-11 Bridge railing 0.005 b/

12-14 Columns, abutments <0.001 F/

15. Bridge bottom structure <0.001 <100,000 730,000

16. Wall barrier, post 0.040 <50,000 <50,000

17. Signs, cushions 0.005 <10,000 <10,000

18. Curb, culvert 0.037 <10,000 <10,000

19-21 Into slope 0.025 b/

22-24 Over embankment 0.014 5/

25. Over embankment (draining ditch) 0.009

26. Trees 0.009 <100,000 <70,000

27. Other (fences, bushes, etc.) 0.033 <50,000 <10,000

28. Overturn 0.083 c/

29. Jackknife 0.055 V/

30. Other (cargo shift, etc.) 0.020 <1,000 71000

31. Fire only 0.010 No load

1.000

a_/ Accident scenarios are screened out as minor except those designated for

dynamic analysis.

b/ Linear force may exceed 100,000 lb/ft. Dynamic analysis is required.

c/ Fall impact distance is <15 ft.; therefore the linear force is <100,000

lb/ft.

6-17
6.2.1.2.1 Response for Impacts with Unyielding Surfaces

This subsection assesses cask response during impact with objects such as

hard rock, which have a hardness close to the unyielding surface specified in

regulations. The analysis considers variations in two parameters: cask

orientation angle and impact velocity. IMPASC is used to calculate the cask

response for cask orientation angles, 0, of 00, 100, 300, 500, 700, and 900

and impact velocities of 30 mph, 38 mph and 45 mph. The 00 cask orientation

angle represents an impact to the side of the cask, whereas the 900 cask

orientation angle is an impact to the end of the cask.

For the 900 angle case, the effects of truck cab crushing and lead slump

pressures are considered. The sensitivity study results are given in Fig.

6-6. The results indicate that, for the representative truck cask, a line

connecting the endwise and sidewise strain responses conservatively bounds the

strain responses for all other cask impact orientations. Therefore, for cask

orientations from 0-900, the structural strain responses can be linearly

interpolated between the sidewise and endwise strain responses. The strain in

the inner cask shell can reach 0.2% (S$) at an impact velocity of 32 mph for

sidewise impacts and an impact velocity of 38 mph for endwise impacts.

6.2.1.2.2 Response for Real Objects

The equivalent damage technique estimates the representative truck cask

response for endwise impacts on real surfaces. A rigid body with the outer

dimensions and weight of the truck cask is dropped onto various surfaces from

heights up to 480 feet and with equivalent velocities up to 120 mph. Figure

6-7 plots the interface forces for endwise impacts of the rigid body on

tillable soil, soft rock/hard soil, and hard rock.

The impact force exceeds 1000 g for hard rock and 200 g for soft

rock/hard soil. By comparison, an impact force of 40 g is presumed to cause a

0.2% strain (SI) at the inner shell of the representative truck cask. For

impact forces up to 40 g, the kinetic energy of the representative cask will

be almost entirely absorbed by the cask's impact limiter. Above this force

level, cask deformation will begin. Because 40 g << 200 g, soft and hard rock

6-18
4..

30 mph (0.

Cask velocity

4~~Zi-Impact velocity

"AYMAyM• Object surface

0 I I I

00 100 300 500 700 900

Side End

impact Cask orientation angle 1 impact

Figure 6-6 Strain versus impact velocity and cask orientation for

the representative truck cask impacting an unyielding

surface.

6-19
1000 Fa0

€.4

4.'

E

100 -

' Hard rack

Soft rock/

hard soil

Tillable

soil

10-

1

0 30 60

Impact velocity, MPH

90 120

Figure 6-7 Impact force for a rigid truck cask dropped endwise onto

real surfaces.

6-20
are treated as an unyielding surface. For impacts on tillable soils, the

results shown in Fig. 6-7 indicate that significant energy can be absorbed by

the soil at an impact force of 40 g. In this case, the representative cask

can impact soil surfaces endwise at velocities up to 84 mph without exceeding

the 0.2% strain (SI) level.

A similar equivalent damage evaluation is performed for sidewise drops

onto various surfaces. To evaluate grade-crossing accidents, sidewise impacts

by train sills are also analyzed to determine conditions which could cause

0.2% strain (SI) at the inner shell. Table 6.3 summarizes the impact

velocities needed to attain the 0.2% strain (SI) level for sidewise and

endwise impacts on various surfaces, including water.

At the 0.2% strain (S1 ) level and below, representative truck cask

responses to impacts on hard or soft rocks are essentially equivalent to

impacts on an unyielding surface for all orientation angles. Endwise and

sidewise impact velocities of 38 mph and 32 mph respectively produce 0.2%

strain (Sj) levels. For endwise impacts on soil, significant energy is

absorbed by the soil, which allows the maximum impact velocity to increase to

84 mph.

For cask impacts on water at a 450 orientation, an impact velocity of 150

mph will not cause the strain to exceed the 0.2% (Sl) level. One-hundredfifty mph is defined as the maximum credible impact velocity that can be

*attained based on review of the historical data base. This velocity

corresponds to a drop height of 750 feet.

Head-on impact by locomotive sills at velocities greater than 9 mph can

cause the 0.2% strain (SI) level to be exceeded. The train sill goes between

the impact limiters and strikes the side of the cask.

6.2.2 Cask Response Analysis for Railway Accidents

The representative rail cask described in Section 3.5 is used to perform

the railway accident response analysis. The computer model of the cask and

the detailed structural calculations used in the response analysis are

discussed in Appendix E. The railway accident scenarios in Fig. 2-5 are used

6-21
Table 6.3

Impact Velocities Required to Reach the 0.2% Strain (SI) Level

for Objects Impacted in Highway Accidents

Impact Velocity at 0.2% Strain

(mph)

Cask Orientation Angle

(0)

Object Impacted 0 45 90

Hard Rock 32 35a/ 38

Soft Rock 32 35a/ 38

Tillable Soil 32 5R/ 84

Water 42 150 38

Train Sill 9 14 150

a_ Impact velocities at these orientation angles are linearly interpolated

between the two bounding values.

6-22
as the basis for the structural evaluations. The results of the response

evaluations are provided in Subsection 6.2.2.1 for minor accidents and in

Subsection 6.2.2.2 for accidents in which the damage to a cask could be

significant.

6.2.2.1 Response to Minor Accidents

Train accidents are primarily derailments or collisions with other

trains. Collisions not involving derailment are usually minor. In nonderailment cases, the only events that must be considered are those in which

the coupler of one rail car can override the impacted car and cause damage to

a rail car or cask. Rail cars specially designed for casks place the cask in

the center of the car. In general, collisions not involving derailment do not

generate enough force for the coupler of an adjacent car to penetrate a rail

cask because the coupler is too short, as shown in Fig. 6-8. In those cases

where the force is great enough for the coupler to strike the cask, it is

assumed that the cars derail and the coupler strikes the side of the cask.

Impacts with small structures such as poles and retaining walls or impacts

with the superstructure of locomotives or other cars cannot significantly

damage a cask.

A rail cask is larger than a truck *cask and requires greater forces to

damage it. A 1.6-million-pound static crush (100,000 pounds/foot) is required

on the side of the representative rail cask to cause a 0.2% strain ($1) at the

inner shell; whereas a 13.0-million-pound static force is required on the end

of the cask to cause a similar level of strain. Based on the first-stage

screening of the truck cask, dynamic impact analysis of the rail cask has to

be considered only for derailment-caused impacts with massive objects or

surfaces adjacent to railroad right-of-ways. Derailments that result in

rollovers onto the adjacent railbed involve falls that are less than 15 feet

and impact velocities less than 22 mph. These impact velocities can partially

crush the rail cask impact limiters but cannot cause any significant damage to

the cask.

" 6-23
Figure 6-8 Rail car coupler override of spent fuel cask car.

6-24
Table 6.4 summarizes the 24 railway accident scenarios with their

frequencies of occurrence. Those accident scenarios that can cause only minor

cask damage are identified. The total fraction of minor accidents is

calculated by summing the individual frequencies. The percentage of accidents

screened out as minor is 96.1%. The remaining accidents involve derailments

and impacts with massive objects such as train couplers, bridge columns, and

abutments, and with surfaces such as rock. Subsection 6.2.2.2 discusses the

analyses performed on the representative rail cask for these accidents.

6.2.2.2 Response to Other Accidents

Railway accident scenarios involving derailments and falls off bridges or

run-offs over embankments or into slopes or massive concrete structures

require dynamic analysis. These accidents may involve impacts with a variety

of surfaces: hard rock, soft rock/hard soil, and tillable soil. The dynamic

response of the cask for impacts with each of these objects is analyzed.

Three parameters are considered significant in the dynamic response

analysis as shown in Figs. 6-3 and 6-4: cask orientation, object hardness,

and impact velocity. Again, different methods of analysis are used to analyze

objects of different hardness. Hard objects are considered unyielding

surfaces and the impact analysis applicable for these objects is presented in

Subsection 6.2.2.2.1 below. Cask responses for relatively soft objects are

discussed in Subsection 6.2.2.2.2.

6.2.2.2.1 Response for Impacts with Unyielding Surfaces

This subsection assesses cask response during impact with objects such as

rock that has a hardness close to the unyielding surface specified in

regulations. The analysis considered variations in two parameters: cask

orientation angle and impact velocity. IMPASC is used to calculate the cask

response for cask orientation angles, B, of 00, 100, 300, 500, 700, and 900

and impact velocities of 30 mph, 45 mph, and 60 mph. The 00 cask orientation

angle represents an impact to the side of the cask, whereas the 900 cask

orientation angle is an impact to the end of the cask.

6-25
Table 6.4

Evaluation of Minor Railway Accidentsa/ Summary

Total Linear

Accident Scenario Frequency Force Force

(lb) (lb/ft)

1. Grade crossing 0.030 <400,000 <70,000

2. Non-derailment 0.086 <500,000 <62,500

3-7 Over bridge 0.008 b/

8. Over embankment - ditch 0.003 El

9-11 Over embankment - other 0.006 5/

12-14 Into slope 0.016 F/

15-17 Columns, abutments <0.001 5/

18. Other structures 0.164 <500,000 762,500

19. Locomotive superstructure 0.033 <500,000 <62,500

20. Rail car superstructure 0.100 <500,000 <62,500

21. Coupler/sill 0.008 b/

22. Roadbed 0.160 Vl

23. Earth 0.320 El

24. Other, fire cargo shift 0.065 <10,000 710,000

Total 1.

Accident scenarios are screened out as

significant for dynamic analysis.

minor except those designated as

c/

Linear force may exceed 100,000 lb/ft. Dynamic Analysis is required.

Fall impact distance is <15 ft; therefore the linear force is <100,000

lb/ft.

6-26
The sensitivity study results are given in Fig. 6-9. For the 900 angle

case, the effects of lead slump pressure and the crushing of the front end of

the rail car are included. The results indicate that, for the representative

rail cask, a line connecting the endwise and sidewise strain responses

conservatively bounds the strain responses for all other cask orientations.

Therefore, for cask orientations from 0-900, the structural strain responses

can be linearly interpolated between the sidewise and endwise strain

responses. The strain in the inner cask shell can reach 0.2% (SI) at an

impact velocity of 55 mph for sidewise impacts and an impact velocity of 38

mph for endwise impacts.

6.2.2.2.2 Response for Real Objects

The equivalent damage technique estimates the representative rail cask

response for endwise impacts on real surfaces. A rigid body with the outer

dimensions and weight of the rail cask is dropped onto various surfaces from

heights up to 480 feet and with equivalent velocities up to 120 mph. Figure

6-10 plots the interface forces for'endwise impacts on tillable soil and soft

rock/hard soil. Calculations are not performed for impacts on hard rock. It

is apparent from the soft rock/hard soil cask results that a hard rock surface

is essentially an unyielding surface with respect to the representative rail

cask.

The impact force exceeds 400 g for soft rock/hard soil. The impact

forces required for significant energy absorption by tillable soil exceed 40 g

at velocities above 40 mph. Since the cask is designed to withstand an impact

force of 40 g, it is presumed that such a force causes less than a 0.2% strain

(S1 ) at the inner shell of the representative rail cask. For impact forces up

to 40 g on hard or soft rock surfaces, the kinetic energy of the

representative cask will be almost entirely absorbed by the cask's impact

limiter. For soil impacts, the kinetic energy will be absorbed by both the

soil and the cask impact limiter.

A similar equivalent damage evaluation is performed for sidewise drops

onto various surfaces. Table 6.5 summarizes the impact velocities needed to

6-27
0.3

0.2 velocity Impact velocity

-t Object surface

4..

0.1 60mph

~45 mph

30mh (Impact velocity)

0III I

0 10 30 50 70 90

Side Cask orientation angleo End

impact impact

Figure 6-9 Strain versus impact velocity and cask orientation for

the representative rail cask impacting an unyielding

surface.

6-28
10,000

1000

C!

0

*0LE

100[-

I ' I I ' I

Soft rock/hard soil

Tillable soil

I I , i I , I , i

10[--

1

0 30 60

Impact velocity, MPH

90 120

Figure 6-10 Impact force versus impact velocity for a rigid rail cask

dropped endwise onto real surfaces.

6-29
Table 6.5

Impact Velocities Required to Reach the 0.2% Strain

for Objects Impacted in Railway Accidents

({I) Level

Impact Velocity at 0.2% Strain

(mph)

Cask Orientation Angle

(0)

Object Impacted 0,45 90

Hard Rock 55 47 38

Soft Rock 55 47 38

Tillable Soil 55 47 40

Water 55 150 38

Train Sill 11 16 150

Impact velocities at these orientation angles are linearly interpolated

between the two bounding values.

6-30
attain the 0.2% strain (Sl) level for sidewise and endwise impacts on various

surfaces, including water.

At the 0.2% strain (SI) level and below, the representative rail cask

responses to impacts on hard rock, soft rock, or soil are essentially

equivalent to impacts on an unyielding surface for all orientation angles.

For cask impacts on water at a 450 orientation, an impact velocity of 150 mph

can be reached without exceeding the 0.2% strain {Sj) level.

Head-on impacts by locomotive sills at velocities greater than 11 mph

could cause the 0.2% strain (SI) level to be exceeded. The train sill goes

between the impact limiters and strikes the side of the cask.

6.2.3 Discussion of Structural Analysis Results

This section has thus far addressed highway and railway accidents that

can generate structural cask responses less than the 0.2% strain (S$) level.

Cask structural responses within the 0.2% strain (S$) level are in the elastic

range and would not lead to any significant radiological releases. Cask

response within these constraints will meet requirements imposed by existing

regulations.

For those accidents requiring a dynamic structural calculation, the

dynamic structural response of the cask is calculated using primarily elastic

analysis methods. Dynamic elastic response methods are routinely used to

analyze structures, and the results can be used with confidence.

Current and future cask designs are expected to be stronger than the

selected representative cask designs and would be able to withstand higher

mechanical loads before the 0.2% strain (S$) level is reached. If a higher

mechanical loading is required to cause the cask containment shell to reach

the elastic limit, then a higher fraction of accidents will be screened out or

shown to result in radiological hazards less than those in current

regulations.

In July 1984, in Old Dalby, England, the United Kingdom Central

Electricity Generating Board performed a train crash test with a steel spent

6-31
fuel cask. 6 The 100-mph train crash subjected the cask to a force greater

than 8 million pounds but caused only minor deformation to the outside of the

cask. The primary response of the cask structure was elastic. In fact, the

force the train applied to the steel cask was less than 40% of the

International Atomic Energy Agency test condition loads, 7 which are similar to

the test conditions specified in 10 CFR 71. Therefore, the actual percentage

of highway and railway accidents that are within the envelope of current

accident test conditions and radiation hazard limits specified in regulations,

are likely to be higher than the percentages indicated in Section 6.4.

6.3 Thermal Response Analysis

Thermal loads due to large fires dominate the thermal evaluation. Other

thermal loads due to torch fires or cask burial in debris that result from

self-heating are insignificant and are eliminated in the thermal screening

analysis. Each type of accident is evaluated for its potential for causing

damage to a spent fuel cask, such as melting of the lead shield or damage to

the cask seal. Even accidents involving only impact of a spent fuel transport

truck with small objects or the adjacent roadbed can result in a fire that

could burn up to an hour because of the diesel fuel being carried by the

truck. Other accidents involving impacts with tanker trucks, locomotives, and

tank cars, each of which carry considerable amounts of fuel, can cause fires

that could last for a few hours.

The intent of this section is to determine the fraction of accidents that

will not cause a temperature exceeding 500OF (T1 ) at the middle of the lead

shield thickness of the representative casks. Heating the cask structure to

500OF (T1 ) does not result in any significant deterioration of the cask

components. This statement applies to cask seals, which are the component

whose failure could signify the earliest onset of a potential radioactive

rel ease.

A finite element computer code called TACO 2-D is used to perform the

thermal analysis of the cask. 8 Sensitivity studies indicate that a onedimensional (1-D) heat transfer model can be used, which simplifies the

6-32
analysis, reduces computing time, and provides suitably conservative

results. In all of the analyses, the representative casks neutron shield tank

water is lost prior to the fire. The thin outer shell of the remaining

neutron shield tank provides a thermal barrier to the fire. Loss of the

shield water reduces heat transfer into the cask; it also removes a

significant heat absorber, water.

Currently licensed cask designs are reviewed to relate the temperatures

at the mid-plane of the cask to the temperatures at other locations,

particularly the closure seals. Valve boxes located where they can be exposed

to heat loads and temperatures approaching those in the middle portion of the

cask are also considered. These sensitivity studies confirm the selection of

the lead shield temperature as the most appropriate and conservative measure

of cask thermal response.

Fire accidents have three loading parameters that can affect the response

of a spent fuel cask: fire duration, flame temperature, and fire location.

These loading parameters vary widely when considering all fire accidents.

Longer fire durations and higher flame temperatures increase the thermal loads

to the cask and affect temperature responses. The proximity of the cask to a

fire is also important. The closer a cask is to a fire, the higher the

thermal load; the worst case is a cask being engulfed by a fire.

In order to reduce the large amount of analysis otherwise required to

cover a wide range of fire accidents, a simplified calculational method is

developed. The method includes the following steps:

1. A reference fire condition is established to perform the thermal response

analysis for the representative truck and rail casks. The first step in

accomplishing this task is to relate the thermal condition specified in

10 CFR 71 to real fire conditions. As shown in Fig. 6-11, a cask is

completely surrounded by fire in the accident test conditions used to

guide design; whereas the cask would most likely be only partially

surrounded by a fire in a real situation because of the shielding effects

of the ground, transport vehicle, or other cask-supporting surfaces. For

6-33
(a) Hypothetical fire

surrounds cask

(b) Real fire partially

surrounds cask

Figure 6-11 Comparison of an engulfing hypothetical fire and a real fire.

6-34
the same flame temperature, the average heat flux into the cask in a real

engulfing fire is 0.78 of the heat flux on the cask in the hypothetical

engulfing fire. A flame temperature of 1700OF is required for a real

engulfing fire, including ground or transport vehicle shielding effects,

to provide the same average heat flux and temperature response as the

1475 0 F hypothetical engulfing fire. The 1700OF real engulfing fire is

the reference fire condition.

2. The heat fluxes and temperature responses of the truck and rail casks are

calculated as a function of fire duration. These evaluations are

performed using a 1-D model and the thermal parameters for the accident

test conditions.

3. Based on sensitivity studies in Appendix F, the time to reach a specific

temperature is approximately proportional to the incident heat flux on

the cask caused by the fire. A fire that causes a heat flux twice that

of the reference fire can heat a cask to a specified temperature in onehalf the time. Conversely, a fire that causes a heat flux one-half the

amount takes twice as long to heat the cask to a specified temperature.

Using this correlation and the results from step 2, the fire durations

required to reach the 500OF temperature (T1 ) level are calculated for a

range of heat fluxes that cover a wide range of real fire conditions.

4. The variation of heat loads on the representative casks is determined as

a function of the flame temperature and location. The heat load

variations are normalized to the engulfing real fire condition and

defined as flux factors for flame temperature and load factors for fire

location.

5. Using the fire duration results from step 3 and the heat flux factors

from step 4, the fire duration required to reach the 500OF temperature

(Tj) level is derived for a wide range of flame temperatures and

locations.

6-35
The thermal response analysis of highway fire accidents is performed

based on the above calculatlonal method. The analysis appears in Subsection

6.3.1. The 31 highway accident scenarios are analyzed to detemine the

thermal loading conditions that can cause a temperature response of 500OF (T1 )

or less at the mid-thickness of the lead shield of the representative truck

cask. Subsection 6.3.2 describes a similar response analysis performed for 24

railway fire accident scenarios that could involve the representative rail

cask. The thermal response results are discussed in Subsection 6.3.3.

6.3.1 Cask Response Analysis for Highway Fire Accidents

The representative truck cask described in Section 3.5 is used to perform

the highway accident response analysis. Appendix F discusses the cask model

and the detailed thermal calculations used in the response analysis.

The temperature response of the representative truck cask is calculated

for a hypothetical engulfing fire with a 1475 0 F flame temperature. A flame

emissivity of 0.9 is assumed. The temperature at the middle of the lead

shield thickness is plotted in Fig. 6-12 as a function of fire duration. The

lead mid-thickness temperature reaches 500OF (T1 ) in 1.08 hours which is twice

the regulatory fire duration. The total heat absorbed by the cask in reaching

the 500OF temperature (T1) is 5,000 Btu/ft 2 which results in an average

thermal flux of approximately 4,630 Btu/hr-ft 2 compared with the initial rate

of 17,000 Btu/hr-ft 2 . The average thermal flux is lower because the thermal

barrier formed by the water jacket rapidly reduces the heat flow into the cask

during the first 10 minutes as shown in Fig. 6-13. These heat fluxes are

equivalent to those on a cask in a real engulfing fire with a flame

temperature of 17000 F.

For engulfing fires, the heat flux from the fire onto the surface of the

truck cask depends on radiation heat transfer caused by the flame

temperature. The average heat flux on the representative truck cask is

calculated as a function of flame temperature for a hypothetical engulfing

fire. The heat flux is then reduced by a factor of 0.78 to adjust the results

6-36
700

600

LL 500 -- T1 level _

0

400

E

• 300

-J 200

100

OR I

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

Time, hours

Figure 6-12 Representative truck cask temperature response to a

hypothetical 1475 0 F (equivalent to a real 17000 F) fire

versus fire duration.

6-37
|

14-

C0 0.5 1.0 1.5 2.0

Time, hours

flux versus fire duration for the representative

cask exposed to the regulatory 14750F fire.

Figure 6-13 Heat

truck

6-38
for real fire conditions. This normalized heat flux factor is plotted in

Fig. 6-14 as a function of flame temperature. For a 1700OF real f ire, the

average thermal flux on the representative cask is 4,630 Btu/hr-ft2 and the

heat flux factor is 1.0. As the flame temperature increases, the thermal flux

increases, and the fire duration required to reach the 500OF temperature (T1)

level decreases.

The heat load to the truck cask also depends on the location of the fire

with respect to the cask. In terms of location, an engulfing fire provides

the maximum heat load to the cask. The heat load decreases rapidly as the

distance between the fire and the cask increases. Figure 6-15 shows the

effect of distance between cask and fire for the truck cask where the heat

load factor is normalized with respect to a real engulfing fire.

The heat flux and load factors are used to calculate the change required

in the 1.08 hour reference fire to reach the 500OF temperature (TI) level for

a variety of flame temperatures and durations. The fire durations for the

wide range of fire conditions are calculated using the probabilistic code

described in Section 5.0.

6.3.2 Cask Response Analysis for Railway Fire Accidents

The representative rail cask in Section 3.5 is used to perform the

railway fire accident response analysis. The computer analysis of the cask

and the detailed thermal calculations are provided in Appendix F.

The temperature response of the representative rail cask is calculated

for a hypothetical engulfing fire with a flame temperature of 14750F and flame

emissivity of 0.9. The temperature at the middle of the lead shield thickness

is plotted in Fig. 6-16. The lead mid-thickness temperature reaches 500OF

(Tj) in 1.35 hours which is more than twice the regulatory fire duration. The

total heat absorbed by the cask in reaching the 500OF (171) level is

approximately 6,000 Btu/ft2 which results in an average heat flux of

approximately 4,445 Btu/hr-ft2. These heat fluxes are equivalent to those on

a cask in a real engulfing fire with a flame temperature of 17000F.

6-39
2.0 I I I I I

1.81-

1.6[- Reference 17000 F

real engulfing fire

1.4k-

*

1~0

.4-

x

.4-

1.2

1.0

0.8 -

0.6 ý0.4 F0.21-

*Heat flux factor = multiple

of heat flux at given fire

temperature to equal heat

flux from 1700' F fire

0 I I I

1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6

Flame temperature, thousands OF

Figure 6-14 Average heat

representative

flux factor versus temperature for the

truck cask.

6-40
7.0

0

40-

25

Distance, feet

Figure 6-15 Heat load factor for real fire versus location of

representative truck cask.

6-41
LL

0

4.. w1~

a)

0.

E

S

4..

4..

-o

S

*0S

0)

-J

Time, hours

Figure 6-16 Representative rail cask temperature response to a

hypothetical 1475 0 F (equivalent to a real 1700 0 F) fire

versus fire duration.

6-42
For engulfing fires, the heat flux from a fire to the surface of the rail

cask depends primarily on radiation heat transfer and is determined by the

flame temperature. The heat flux dependency on the flame temperature is

essentially the same as that for the truck cask discussed in Subsection

6.3.1. The average heat flux factors in Fig. 6-14 are used to adjust for

flame temperature for the rail cask.

The heat load to the rail cask also depends on the location of the fire

with respect to the cask. An engulfing fire provides the maximum heat load to

the cask. The heat load decreases rapidly as the distance between the fire

and the cask increases. Figure 6-17 shows the effect of distance between the

cask and fire for the representative rail cask. The heat load factor is

normalized with respect to an engulfing fire.

The heat flux and load factors are used to calculate the change required

in the 1.35 hour reference fire to reach the 500OF temperature CT1) level for

a variety of flame temperatures and durations. The fire durations are

calculated using the probabilistic code described in Section 5.0 for the wide

range of fire conditions.

6.3.3 Discussion of Thermal Analyi Results

This section addresses highway and railway fire accidents which generate

cask temperature responses less than or equ~al to the 500OF temperature (171)

level. These accidents result in, heating the cask structure to temperatures

at which no significant deterioration of the cask components is expected. As

a result, the radiological significance of such events is negligible.

The results indicate that the representative truck and rail casks can be

exposed to a regulatory fire (14750F, engulfing, etc.) for over 1 hour before

the 500OF temperature (T1) limit is reached. This fire duration is

approximately twice as long as that specified in the regulations for the

accident test conditions; hence, the representative cask designs have

considerable margin with respect to the fire duration. This margin is due to

the high heat capacity and thermal resistance inherent in the casks. The

massiveness of spent fuel casks due to shielding and mechanical strength

contributes significantly to the thermal response characteristics.

6-43
7.0

L 4.0

0

*4-

'U

XIn

0 5 10 15 20 25

Distance, feet

Figure 6-17 Heat load factor for real fire versus location of

representative rail cask.

6-44
In reality, many currently licensed casks use components and seals that

can reliably function at temperatures exceeding those associated with a 500OF

(Tj) lead mid-thickness temperature for long periods of time without being

damaged. Therefore, the actual percentage of highway and railway accidents

that are within the thermal loading envelope of the accident test conditions

is significantly higher than those documented in this study. The radiological

hazards for these events are expected to be negligible.

6.4 Accident Screening Analysis

Section 5.0 provides the detailed probabilistic calculations performed in

the accident screening analysis. From that analysis, approximately 99.4% of

both highway and railway accidents leads to cask responses within the R(1,1)

response region. At this level of damage, no radiological hazards of

significance are expected; therefore, all are within the stated regulatory

limits for radioactive releases and direct exposures. These results are

discussed in detail in Subsection 9.2.1.

6-45

7.0 SECOND-STAGE SCREENING ANALYSIS

7.1 Introduction

The first-stage screening analyses identify classes of accidents in which

the responses generated by the mechanical and thermal loadings are within the

R(1,1) response region. At levels of response within the R(1,1) region, the

accident event would not be expected to have any radiological significance.

Approximately 99.4% of highway and railway accidents are expected to cause

cask response states within the R(1,1) region.

The residual 0.6% of highway and railway accidents which could cause cask

responses outside the R(1,1) region are addressed in this section. The intent

of the second-stage screening is to determine what fractions of these residual

accidents can be categorized into regions bounded by 30% strain (S3) in the

inner cask containment shell and by a lead mid-thickness temperature of 1050OF

(T4

O. These regions are shown in Fig. 7-1.

The light stippled area in Fig. 7-1, which covers regions R(1,2), R(2,1),

and R(2,2), represents cask responses between the 0.2% (SI) and 2% (S2) strain

levels and between the 500OF (TI) and 600°F (T2 ) temperature levels. These

cask responses exceed the responses that would be generated If a shipping cask

were subjected to the 10 CFR 71 accident test conditions. 1 Responses in this

light stippled area can result in minor damage to the cask and could result in

small radioactive releases or small increases in the direct radiation levels

external to the cask. The radiological hazards associated with these cask

responses could approach or slightly exceed the regulatory limits specified in

10 CFR 71 for transportation accidents.

The remaining eight regions, beyond the 2% strain (S2) and 600OF

temperature (T2 ) levels, represent individual or combined cask responses

between the 2% (S2) and 30% (S3 ) strain levels and between the 600OF (T2 ) and

1050OF (T4 ) temperature levels. For clarity, they are darkly shaded in

Fig. 7-1. These responses are significantly greater than the responses

expected after exposing the representative casks to the accident test

conditions. Responses within the darkly shaded area in Fig. 7-1 can result in

7-1
$33 (30)

T1 T2;'s. T4

'R..

E((200....6........650............

o~~~ ~~~ Second-te screening anlssrdoota aad a els hn

.S -%

e **to,**.. or g **** r17 . i

gh t e a n d a t s w h n u

Figure se n Sn.:.:.:..v.:.v:g s

rs n Xegio

4o First-tg screening aayi ailgclhzrsaengiil n

lesta 1 F 1 iis

S - Dark shadedreal -esmorse subsanidthiacanesk dampeague, wihrailoia

lsthazadn rbbl 10 xedn CFR 71 limits.

Fiue0- Second-stage screening analysis relooiclhzad atinbelshi wthan

resoneirgins

7-2
significant permanent deformation of the cask structure and melting of the

lead shielding. Any radioactive material releases or increase in the direct

radiation levels that could result from these cask responses are probably

greater than the regulatory limits specified in 10 CFR 71 for transportation

accidents.

The second-stage screening analysis involves calculations similar to

those performed in the first-stage screening. The major difference between

the two screening evaluations involves the calculational methods used.

Nonlinear small-deformation analysis methods are needed to analyze the cask

structure for deformations having strain levels within the 2% strain (S2)

limit. For strain levels beyond the 2% (S2) limit, nonlinear, large

deformation methods are needed. Thermal analysis methods account for the

.melting of the lead shield in the 600°F (T2 ) to 1050OF (T4 ) temperature range.

Section 7.2 discusses the structural response of the representative casks

to mechanical loads; Section 7.3 addresses response to thermal loads. In

Section 7.4, the results of both structural and thermal response are combined

to estimate the fraction of accidents that fall within each of the response

regions.

7.2 Structural Response Analysis

The classes of accidents requiring structural analysis in the secondstage screening typically involve impacts with massive objects or hard

surfaces. In these accidents, dynamic forces greater than 400,000 pounds can

be generated. The computer codes selected to perform the required analysis

include two established codes called DYNA 2-D/3-D and NIKE 2-D/3-D; the 2-D/3-

D designation indicating that either two- or three-dimensional modeling can be

performed. 2 ,3 Two-dimensional calculations are much simpler and faster to run

and are used whenever possible. The applicability of the 2-D modeling is

verified through the performance of sensitivity studies which compared results

of 2-D and 3-rD modeling. The calculation methods and assumptions used in the

2-D modeling are discussed in further detail in Appendix E. The most

significant aspects include the following:

7-3
1. For cask orientations between sidewise and endwise in the range of

00 < 0< go , the structural strain responses for the representative

casks impacting solid surfaces are linearly interpolated from the results

of sidewise, 0-00, and endwise, 8-900, impacts.

2. Two-dimensional plane strain analyses without impact limiters or end

enclosures are performed for high velocity sidewise impacts, -0 0, on

hard rock, soft rock, and soil surfaces. This elimination of impact

limiters overestimates strain responses of the representative casks,

particularly for impact velocities less than 60 mph and for impacts on

soft surfaces such as soil. The 2-D method is benchmarked with a 3-D

impact analysis that modeled the representative truck cask with the

inclusion of the impact limiters and end closures.

3. The strain responses of the representative casks impacting real surfaces

are estimated using the equivalent damage technique discussed in

Section 6.2 and in Appendix E.

The structural response analysis of highway accidents is in Subsection

7.2.1. Highway accident scenarios, in which the first-stage screening

indicates the possibility of cask response outside the R(1,1) region, are

evaluated. The fraction of these accidents causing responses within the 0.2%

(SI) to 2% (S2) and 2% (S3 ) to 30% (S3 ) strain levels on the inner shell of

the representative truck cask is determined. Subsection 7.2.2 describes a

similar structural response analysis performed for the railway accidents. In

Subsection 7.2.3 the overall structural analysis results are discussed.

7.2.1 Cask Response Analysis for Highway Accidents

The representative truck cask described in Section 3.5 is used in the

second stage screening analysis for highway accidents. Appendix E discusses

the computer models of the cask, material properties, and the detailed

structural evaluations used in the response analysis.

7-4
The highway accident scenarios involve impacts by train sills and impacts

occurring as a result of a truck running off a bridge, over an embankment,

into a slope, or into a massive concrete structure.

In this evaluation, the maximum strain at the inner wall of the

representative truck cask is calculated as a function of the impact velocity

for both endwise and sidewise impacts with real surfaces.

7.2.1.1 Endwise Impacts

Since the representative truck cask is axl-symmetric along its length, a

2-D cask model with impact limiters is used to evaluate the response of the

representative truck cask for endwise impacts on an unyielding surface.

Figure 7-2 shows the strain response for the representative truck cask

impacting an unyielding surface at 45 mph. The maximum strain of 3.63% occurs

on the inner shell of the cask at t~he bottom junction with the end-cap, near

the point of impact. The lead slumps to the impacted end of the cask, causing

a 4-inch gap in the lead shield at the opposite end.

The cask impact calculations are performed, assuming impacts on an

unyielding surface, over a range of velocities from 30 to 90 mph. As

discussed in Appendix E, the energy absorption effects of crushing the

transport truck cab are included in the analysis. The resultant impact force,

maximum plastic strain at the inner shell of the cask, and the amount of lead

slump are plotted as functions of impact velocity in Fig. 7-3. The 2% strain

(SO) level occurs when a cask impacts an unyielding surface at a velocity of

46 mph. At this velocity the impact force is 80 g, and the lead slump is

about 3 inches. The 30% strain (S3) level occurs when a cask impacts an

unyielding surface at a velocity of 76 mph. The resultant impact force is

300 g and the lead slump is 16 inches. In both cases, the maximum strains

occur because of lead slump at the bottom of the cask on the inner shell.

The equivalent damage technique, discussed in Section 6.2 and Appendix E,

is used to estimate the cask response for endwise impacts on real surfaces. A

rigid body with the outer dimensions and weight of the truck cask impacts

varying surfaces at velocities up to 120 mph. The resultant interface forces

7-5
Figure 7-2 Example showing strain response of the representative truck cask for 45 mph endwise impact on an unyielding surface (2-D model with. impact limiters) without any truck cab crushing included.

7-6
C

E 10

1

0.1

0.01

0 30 60 90 120

Impact velocity, mph

Figure 7-3 Response of the representative

impacts on an unyielding surface

limiters and cab crush).

truck cask to endwise

(2-D model with impact

7-7
were calculated in the first-stage screening and are plotted in Fig. 6-7.

Using the equivalent damage technique, the 2% strain (S2 ) level is reached at

impact velocities of 46 mph for impacts on hard and soft rocks but is never

reached for impacts on soil. The 30% strain (S3) level is reached only for

impacts on hard rocks at impact velocities exceeding 76 mph.

7.2.1.2 Sidewise Impacts

An approximate 2-D plane strain model is used to calculate the response

for high-velocity sidewise impacts on soil, soft rock, and hard rock.

Figure 7-4 shows the strain response for the representative truck cask without

an impact limiter impacting tillable soil at 60 mph. The maximum strain of

8.47% occurs at the inner shell. During impact, the cask inner diameter

decreases by 50% in the impact direction and collapses onto any spent fuel

being transported.

In Fig. 7-5, the maximum plastic strain at the inner wall is plotted as a

function of impact velocity for impacts on hard rock, soft rock, and tillable

soil. In the approximate 2-D model, the strains calculated for a specific

impact velocity are essentially the same for sidewise impacts regardless of

the surface impacted. The 2% strain (S2) level occurs at a velocity of 51 mph

for impacts on all of the surfaces considered. The 30% strain (S3 ) level does

not occur because the representative cask walls collapse together or onto the

spent fuel contents before the limit is reached.

7.2.1.3 Impact Response Summary

Table 7.1 summarizes the impact velocities at the 2% (S2) and 30% (S3 )

strain levels for sidewise, $ - 00, and endwise, 0 - 900, impacts on hard

rock, soft rock, and soil surfaces. Impacts of the cask on water and by a

train sill are also included. In general, the endwise impacts result in

higher strains to the cask than sidewise impacts for the same impact

conditions on surfaces. The cask attains the 30% strain (S3 ) level only at

high-velocity endwise impacts on hard rock.

7-8
Figure 7-4 Example showing strain response of the representative

truck cask for 60 mph sidewise impact on soil (2-D model

without impact limiters) with strain exceeding the 2%

(S2) limit.

7-9
0

S

E

x

Mu

120

Impact velocity, mph

Figure 7-5 Response of the representative truck cask to sidewise

impacts on various surfaces.

7-10
Table 7.1

Impact Velocities Required to Attain 2% (S,) and 30% (S3 ) Strain Levels

for Objects Impacted in Highway Accidents

Impact Velocity A/ Impact Velocity Al

at 2% Strain (S2) at 30% Strain (S3)

(mph) (mph)

Cask Orientation Angle Cask Orientation Angle (o) (0)

Object Impacted U45 9 45 90

Hard Rock 51 49 b/ 46 >150 113 b/ 76

Soft Rock 51 495/ 46 >150 >150 150

Tillable Soil 51 101 6/ >150 >150 >150 >150

Water 59 >150 64 >150 >150 >150

Train Sill 20 27 >150 >150 >150 >150

b/

Impact velocity of >150 mph means that the strain level is not reached.

Impact velocities at these orientation angles are linearly interpolated

between the two bounding values.

7-11
7.2.2 Cask Response Analysis for Railway Accidents

The representative rail cask described in Section 3.5 is used in the

railway accident response analysis. Appendix E discusses the computer models

of the cask, the material properties, and the detailed structural evaluations

use in the cask response analysis.

The railway accident scenarios of interest are those involving falls from

bridges; drops over embankments; and impacts into slopes, train couplers, or

massive concrete structures. Again, the maximum strain at the inner wall of

the representative rail cask is calculated as a function of the impact

velocity for both endwise and sidewise impacts with real surfaces.

7.2.2.1 Endwise Impacts

As was done in the truck cask analysis, a 2-D model is used to evaluate

the response of the representative rail cask for endwise impacts on an

unyielding surface. The cask impact calculations cover a range of velocities

from 30 to 90 mph. Figure 7-6 shows the resultant impact force, maximum

plastic strain at the inner shell of the cask, and the amount of lead slump as

functions of impact velocity. The 2% strain (S2 ) level occurs when'a cask

impacts an unyielding surface at a velocity of 48 mph. At this velocity the

impact force is 102 g, and the lead slump is 6 inches. The 30% strain (S3)

level occurs when a cask impacts an unyielding surface at a velocity of 105

mph. The resultant impact force at this velocity is 500 g and the lead slump

is 28 inches. In both cases the maximum strain occurs at the bottom of the

cask on the inner shell.

The equivalent damage technique is used to estimate the cask response for

endwise impacts on real surfaces. A rigid body with the outer dimensions and

weight of the rail cask impacts various surfaces at velocities up to 120

mph. The resultant interface forces for these impacts are calculated in the

first-stage screening and are plotted in Fig. 6-10. Using the equivalent

damage technique, the 2% strain (S2 ) level is reached at impact velocities of

48 mph for impacts on hard and soft rocks, and 65 mph for impacts on soil.

The 30% strain (S3 ) level is reached only for impacts on hard and soft rocks

at an impact velocity of 105 mph.

7-12
1000

/ Force

I T

1001-

CL

E

CD

a,

CL

E,

S3 strain level

10--

1

S2 strain level

0.11-

Strain

0.01 I I

0 30 60 90 120

Impact velocity, mph

Figure 7-6 Response of the representative

impacts on an unyielding surface

limiters and railcar crush).

rail cask to endwise

(2-D model with impact

7-13
7.2.2.2 Sidewise Impacts

As done in the truck cask, a 2-D model of the rail cask is used to

calculate the response for high-velocity sidewise impacts on soil, soft rock,

and hard rock. In Fig. 7-7, the maximum plastic strain at the inner wall is

plotted as a function of impact velocity. The 2% strain (S2 ) level occurs at

a velocity of 72 mph for impacts on hard and soft rock and on soil. The 30%

strain (S3 ) level can never occur because the representative cask walls

collapse together or onto the spent fuel contents before the limit is reached.

7.2.2.3 Impact Response Summary

Table 7.2 summarizes the impact velocities at the 2% (S2 ) and 30% (S3 )

strain level for sidewise, $ - 00, and endwise, 0 - 900, impacts on hard rock,

soft rock, and soil surfaces. Impacts of the cask on water and by a train

sill are also included. In general, the endwise impacts result in higher

strains to the cask than sidewise impacts for the same impact conditions.

7.2.3 Discussion of Structural Analysis Results

This section has thus far addressed highway and railway accidents that

can generate cask responses within the 2% (S2) and 30% (S3 ) strain levels.

Cask structural responses at these levels result in permanent deformations to

the cask and potential radioactive material releases or increases in direct

radiation exposure levels which could approach or exceed the limits specified

in 10 CFR 71.

The dynamic response of the cask is calculated using the DYNA and NIKE

families of elastic-plastic finite element computer codes. 2 , 3 These codes

were developed at the Lawrence Livermore National Laboratory (LLNL) around

1979, and their predicted results were extensively benchmarked. Appendix H,

for example, discusses the capability of these computer codes to calculate the

dynamic responses of a cylinder impacting a rail, a nose cone impacting a

rigid wall, and a rod impacting a rigid wall obliquely.

7-14
1000 I I I

100 [-

S3 strain level

Iall

C

C

0

C

E

E

x

10

1_-

0.1 F

0.01,0

I i I

30 60

Impact velocity, mph

90 120

Figure 7-7 Response of the representative rail cask to sidewise

impacts on various surfaces.

7-15
Impact Velocities

Table 7.2

Required to Attain 2% (SI) and 30% (S3 ) Strain Levels for

Objects Impacted in Railway Accidents

Impact Velocity a/ Impact Velocity a/

for 2% Strain for 30% Strain

(mph) (mph)

Cask Orientation Angle Cask Orientation Angle

. (0)(0)

Object Impacted 45 9u 0 45 90

Hard Rock 72 60 b/ 48 150 128 b/ 105

Soft Rock 72 606/ 48 150 128-/ 105

Tillable Soil 72 69 -/ 65 150 150 E/ 150

Water 7? 150 60 150 150 150

Train Sill 27 49 150 150 150 150

a_ Impact velocity of 150 mph means that the strain level is not reached.

b_/ Impact velocities at these orientation angles are linearly interpolated

between the two bounding values.

7-16
These benchmark cases demonstrate the capabilities of the codes to

calculate the dynamic response of objects which, when subjected to impact, can

experience large permanent deformations. -In all three cases, the computer

predictions were within a few percent of the deformations measured in the

tests.

Benchmark tests of DYNA 3-D have also been performed in the United

Kingdom. Excellent agreement was obtained in predicting the dynamic response

of a missile impacting a pipe. 4 DYNA 3-D was also used to predict the high

deformation characteristics and response of a metal fin on the MAGNOX spent

fuel cask when subjected to a 30-foot drop onto an unyielding surface. 5 Again

there was a good comparison between the test results and the computer

predictions.

The Sandia National Laboratory (SNL) used scale model tests and a

computer code similar to DYNA 2-D to predict the dynamic response and

deformations of full-scale casks used in a series of crash tests. The fullscale tests included a 25-ton truck cask being struck by a 100-ton locomotive

at 80 mph. 6 Following the high-velocity impact, the cask was dented at the

points of impact on the side, was slightly bowed along the length, and had a

small leak at the closure. In another test, a similar truck cask was carried

at 80 mph on a truck which crashed into a huge unyielding concrete

abutment. 7 The endwise impact resulted in some lead slump and a small leak at

the closure. The results of both of these tests were in good agreement with

the computer predictions.

These benchmark tests of the computer codes support their use in

conservatively predicting the damage to a spent fuel cask which is subjected

to severe accident conditions. In many cases in this study, conservative

modeling assumptions are made to simplify the cask response evaluation over a

wide range of accident conditions. Examples include the 2-D modeling of 3-D

sidewise impacts, the use of elastic-plastic soil modeling, the use of the

equivalent damage technique for estimating strain, and the assumption of no

bonding between the lead shield and the inner shell of the cask. All these

assumptions result in overpredicting the cask damage response to real accident

7-17
conditions. In addition, the representative cask is structurally weaker than

current casks. Again, for the same impact conditions, damage to the

representative casks will be greater than that which would be incurred by real

casks.

7.3 Thermal Response Analysis

Many of the accident scenarios involving fire led to a cask response well

within the R(I,1) region associated with the first-stage screening. This

observation is true for both truck and rail casks, but more prevalent for

truck casks.

The accidents of interest in this section involve fires of approximately

1-hour duration and longer. These fire accidents have three loading

parameters that can affect the response of a spent fuel cask: fire duration,

flame temperature, and fire location. Longer fire durations and higher flame

temperatures increase the thermal loads to the cask and increase its

temperature responses. Also, the closer the cask is to the fire, the better

the thermal interaction and the higher the thermal load. In the worst case,

the cask is submerged or engulfed by the fire.

The thermal screening analysis in this section compares the truck and

rail cask responses to the three temperature response levels of 600°F (T2),

650°F (T3 ), and 1050OF (T4) at the middle of the lead shield thickness. Since

lead melts at 6210 F, the calculation of the responses between 600°F (T2 ) and

650°F (T3 ) has to include the melting of the lead shield. The computer code

TACO 2-D used in the first-stage screening has the capability of handling lead

melt. TACO 2-D is used with the same one-dimensional (1-D) thermal models to

perform the second-stage screening. 8 In other words, the thermal analysis is

a continuation of the analysis performed for the first-stage screening, but

includes consideration of lead melt.

The calculational method relies on the concept that the time to reach a

specific cask temperature is approximately proportional to the incident heat

flux on the cask caused by the fire. A fire that causes a heat flux twice the

heat flux of a reference fire can heat a cask to a specified temperature in

7-18
one-half the time it takes the reference fire. Conversely, a fire that causes

one-half the heat flux takes twice as long to heat the cask in comparison to a

reference fire. For details on the calculational method, refer to Section

6.3.

The thermal response analysis of highway fire accidents is provided in

Subsection 7.3.1. Subsection 7.3.2 describes a similar response analysis

performed for the railway fire accidents. In Subsection 7.3.3, the overall

thermal screening rosults are discussed.

7.3.1 Cask Response Analysis for Highway Fire Accidents

The representative truck cask described in Section 3.5 is used in the

highway fire accident response analysis. Appendix F discusses the computer

analysis model, the cask material properties, and the detailed thermal

calculations. All highway accident scenarios are evaluated for cask responses

to fire because in all scenarios, possibilities exist that a fire can occur

and last longer than I hour.

The temperature response of the representative truck cask is calculated

for a hypothetical engulfing fire with a flame temperature of 14750 F and flame

emissivity of 0.9. This hypothetical fire approximates a real engulfing fire

with a 1700OF flame temperature. The temperature at the middle of the lead

shield thickness is plotted in Fig. 7-8 as a function of time. The lead midthickness temperature reaches 600°F (T2 ) in 1.35 hours for the specified heat

flux conditions. The total heat absorbed by the cask in reaching the 600°F

temperature (T2) level is approximately 6,000 Btu/ft 2 which results in an

average thermal flux of approximately 4,450 Btu/hr-ft 2 . As the lead midthickness temperature increases beyond the 600°F (T2 ) level, the lead at the

outer shell starts to melt. The lead melts at the inner shell in 2.1 hours as

the mid-thickness temperature reaches 650°F (T3 ). The 1050OF temperature (T4 )

level is reached in 3.3 hours.

These temperature response and heat flux results from the hypothetical

fire are used to evaluate real fires. For an engulfing fire, the heat flux

from the fire onto the surface of the truck cask depends on radiation heat

7-19
LL

0

h.

'U

'U

2

S

S

C,

4..

-o

S

-J

1.2 1.4 1.6 1.8 2.0 2.2 2.4

Time, hours

Figure 7-8 Representative truck cask temperature response to a

hypothetical 1475 0 F (equivalent to a real 1700 0 F) fire

versus fire duration.

7-20
transfer which is strongly dependent on the flame temperature. The average

heat flux on the representative truck cask is calculated as a function of

flame temperature for a hypothetical engulfing fire. The heat flux is then

reduced by a factor of 0.78 to adjust the results to real engulfing fire

conditions. The heat flux factors are derived in the first-stage screening

evaluations, and the results are plotted in Fig. 6-14 as a function of flame

temperature. For a 1700OF fire, the average thermal flux on the

representative cask is 5,000 Btu/hr-ft 2 and the heat flux factor is 1.0.

The heat flux to the truck cask also depends on the location of the fire

with respect to the cask. An engulfing fire provides the maximum heat flux to

the cask. The heat flux decreases rapidly as the distance between the fire

and the cask increases. As discussed in Subsection 6.3.2 and plotted in

Fig. 6-15, the heat load factor is normalized with respect to a real engulfing

fire.

As the flame temperature increases, the thermal flux to the cask

increases, and the fire duration required to reach the 600°F (T2 ), 650°F (T3 ),

and 1050OF (T4 ) temperature levels decreases proportionally. On the other

hand, as the cask distance from the fire increases, the thermal flux decreases

and the duration time increases proportionally to reach the same temperature

levels.

The heat flux and load factors are used to determine the amounts of

increase or decrease required in each of the fire duration times to reach the

600°F (T2 ), 650°F (T3 ), and 1050OF (T4 ) temperature levels for a variety of

flame temperatures and fire locations.

7.3.2 Cask Response Analysis for Railway Fire Accidents

The representative rail cask described in Section 3.5 is used in the

railway fire accident response analysis. Appendix F discusses the computer

analysis model of the cask, the material properties, and the detailed thermal

calculations used in the response analysis.

All railway accident scenarios are evaluated for cask responses to fire

betause in all scenarios, possibilities exist that a fire can occur and can

last longer than 1 hour.

7-21
The temperature response of the representative rail cask is calculated

for a hypothetical engulfing fire with a flame temperature of 1475°F and flame

emissivity of 0.9. The temperature at the middle of the lead shield thickness

is plotted in Fig. 7-9 as a function of fire duration. The lead mid-thickness

temperature reaches 600°F temperature (T2 ) in 1.8 hours for the specific

thermal flux conditions. The total heat absorbed by the cask in reaching the

600°F temperature (T2 ) level is approximately 7,900 Btu/ft 2 which results in

an average thermal flux of approximately 4,400 Btu/hr-ft 2 . As the lead midthickness temperature increases beyond the 600°F (T2 ) level, the lead at the

outer shell starts to melt. The lead melts at the inner shell in 2.6 hours as

the lead mid-thickness temperature reaches 650°F (T3 ). The 1050OF temperature

(T4 ) level is reached in 5.1 hours. These final temperature response and heat

flux results are used to evaluate real fires.

As is done for the truck cask, heat flux and load factors are calculated

for the rail cask, as plotted in Figs. 6-14 and 6-17. These factors are used

to determine the amounts of increase or decrease in each of the fire duration

times necessary to reach the 600°F (T2 ), 650°F (T3 ), and 1050OF (T4 )

temperature levels for a variety of flame temperatures and fire locations.

7.3.3 Discussion of Thermal Analysis Results

Cask responses at the 600OF (T2 ), 6500 F (T3 ), and 1050OF (T4 ) temperature

levels can involve deterioration of safety components and melting of the lead

shield. Consequently, radioactive material releases and increases in direct

radiation exposures are possible and could equal or exceed regulatory limits

specified in 10 CFR 71 for transportation accidents.

The TACO 2-0 code used to perform the thermal analysis was developed

about 1978 at the LLNL and was benchmarked against proven engineering

solutions for various thermal conditions. 8 The benchmark cases demonstrate

the code's capability to calculate the temperature response for objects heated

under steady state and transient conditions. 8 In all cases, TACO 20D

calculates temperature results, which are within a few percent of the exact

solution.

7-22
U.

0

E

C

V

IM

a)

Time, hours

Figure 7-9 Representative rail cask temperature responsp to a

hypothetical 1475 F (equivalent to a real 1700'F) fire

versus fire duration.

7-23
In 1978, the SNL used similar computer codes to analyze temperatures from

a test involving a spent fuel cask suspended over a pit filled with burning

jet fuel. 9 , 1 0 Under these test conditions, the temperature measurement

instruments and the code predictions showed that the environment in a real

fire varies significantly along the length of the cask and as a function of

time. The thermal flux varies with the wind and ventilation conditions

surrounding the cask. Sandia- concluded that the regulatory thermal test

conditions (1475 0 F hypothetical engulfing fire) are equivalent to a real

engulfing fire with much higher flame temperatures.

Both the benchmark computer code calculations and the Sandia fire test

support the use of computer code modeling to evaluate the temperature response

of a cask to a real fire accident. In this study, conservative modeling

techniques are introduced to simplify the cask response evaluation over a wide

range of fire accident conditions. A I-D model of a hypothetical engulfing

fire with a nominal flame temperature of 1475 0 F is used in lieu of a 2-D model

with variable flame temperatures. In addition, no inclusion of heat loss with

cask location is considered. These modeling assumptions overpredict the cask

temperature response to fires. In addition, casks that use shielding material

other than lead cannot incur damage due to melting.

7.4 Accident Screening Analysis

Section 5.0 discusses how the detailed probabilistic calculations are

performed by the (Transportation Accident Scenario Probabilities) TASP code in

the accident screening analysis. The fraction of accidents calculated by the

TASP code is summarized in Figs. 7-10 and 7-11 respectively for the truck and

train accidents for each response region. Assuming that an accident occurs,

the percentage of both truck and train accidents within the 2% strain (S2) and

600°F temperature (T2 ) levels is about 99.8%. Fewer than 0.001% of truck

accidents and 0.013% of rail accidents fall outside of the 30% strain (S3 ) and

105O°F temperature (T4 ) levels for which the cask structural and thermal

analyses are performed.

7-24
.0

S

4U

4-

E

0

0.

C

CL

1.532E-7 3.926E-14 1'.495E-14 7.681E-16 <E-16

S3

(30)

S2

(2)

S1

(0.2)

1.7984E-3 1.574E-7 2.034E-7 1.076E-7 4.873E-8

3.8192E-3 2.330E-7 3.008E-7 1.592E-7 7.201 E-8

0.994316 1.687E-5 2.362E-5 1.525E-5 9.570E-6

T( (500) T2 (600)

T3

(650)

T4 (1050)

Thermal response (lead mid-thickness temperature, OF)

Note:

E+x= 10x

Figure 7-10 Fraction of truck accidents that could result

responses within each response region, assuming

accident occurs.

in

an

7-25
(A

0

C

4-

U,

E

9E

0

C.

U,

1.786E-9 3.290E-13 2.137E-13 1.644E-13 3.459E-14

S3

(30)

82

(2)

Sl

(0.2)

A L 1. 4.

5.545E-4 1.021E-7 6.634E-8 5.162E-8

4 4. 4.

5.296E-8

2.7204E-3 5.011E-7 3.255E-7 2.531E-7 1.075E-8

.993962 1.2275E-3 7.9511E-4 6.140E-4 1.249E-4

T( (500) T2 (600)

T3 (650) T4 (1050)

Thermal response (lead mid-thickness temperature, 'F)

Note:

E+x= 10x

Figure 7-11 Fraction of rail accidents that could result in

responses within each response region, assuming an

accident occurs.

7-26
The significance of these screening results is discussed in detail in

Section 9.0 with respect to the existing regulatory requirements and the risk

evaluations performed in NUREG-0170.11

7-27

8.0 POTENTIAL RADIOLOGICAL SIGNIFICANCE OF TRANSPORTATION ACCIDENTS

8.1 Introduction

The purpose of this section is to estimate the potential radiological

hazards of various classes of transportation accidents involving a spent fuel

shipment. Any significant radioactive material release or increase in the

radiation levels from a cask following an accident will originate with the

spent fuel. As the cask damage and response increase, the radiation hazard

will also increase.

In the previous section, the specific levels of damage that a cask might

experience in transportation accidents are categorized in terms of cask

response regions. In this section, the potential radiological hazards from

accident effects are estimated for each cask response region in terms of: (1)

releases of spent fuel material, and (2) levels of radiation from the cask

contents. Comparisons are then made in Section 9.0 with the release and

radiation limits defined in 10 CFR 711 and the radiological risk estimates

evaluated in the NUREG-0170, "Final Environmental Statement on the

Transportation of Radioactive Material by Air and other Modes". 2

8.2 Description of Spent Fuel

The characteristics of spent fuel strongly influences the potential,

radiological hazards of transportation accidents involving shipment. The

level of radioactivity and the heat generated within the spent fuel depend on

the amount of fission energy extracted from the fuel during its use in a power

reactor. However, after the fuel is removed from the reactor, the total

radioactivity decays or drops about 80 fold within 1 year and about 340 fold

within 5 years. The radioactivity and thermal power in the spent fuel is

produced, for the most part, by decay of radioactive isotopes residing within

the solid fuel pellets. However, a small amount of gaseous and volatile

radioactive material also migrates from the fuel pellets to the fuel rod

gap. The radioactive inventory and thermal power of a typical spent fuel

assembly is shown in Table 8.1 as a function of decay time. 3 The table

8-1
Table 8.1

PWR Fuel Assembly Decay Heat and Radioactivity-4

Radioactivity

(Ci)

Decay Time

(years)

Radioisotopes 1 5 10

6 0Co b/ 3.57x101 2.11x101 1.09x101

8 5Kr 3.99x10 3 3.08x10 3 2.23x10 3

9 0Sr 3.42x10 4 3.10x10 4 2.75x10 4

90y 3.41x10 4 3.09x10 4 2.73x104

106Ru 1.21x10 5 7.79x10 3 2.52x10 2

129, 1.48x10-2 1.48x10- 2 1.48x10-2

134Cs 1.00x10 5 2.60x10 4 4.85x10 3

137Cs 4.73x10 4 4.32x104 3.85x10 4

13 7mBa 4.47x10 4 4.07x104 3.62x10 4

144Ce 2.19x,0 5 6.86x10 3 9.01x10 1

2 3 8 pu 1.46x103 1.41x10 3 1.36x103

239pu 1.67x10 2 1.67x10 2 1.67x10 2

240pu 2.06x10 2 2 2.06x10 2 2.06x10 2

241pu 6.64x10 4 5.49x10 4 4.32x10 4

244cm 9.72x40 2 8.34x10 2 6.90xi02

Total Activityc/ 1.12x10 6 2.66x10 5 1.82x10 5

Decay Heat, KBtu-c. 16.42 3.02 1.93

a/ Assumed burnup is 33,000 megawatt-days/metric ton of uranium3 .

The 6 0 Co source is not a direct result of the fission process. It is

produced from neutron activation of non-radioactive elements contained

in struitural materials and appears as crud on the fuel assembly

surfaces .

c/ Includes all radioisotopes.

Note: Boxed column represents decay heat and radioactivity levels assumed for

the fuel in this study.

8-2
identifies only the specific isotopes that are important in performing a

radioactive release evalOation.

Different fuel assembly designs are used in nuclear power reactors.

There are two major types of fuel assemblies used for the two principal

reactor design concepts currently operating in this country--pressurized water

reactors (PWRs) and boiling water reactors (BWRs). For purposes of this

study, a typical PWR fuel assembly, shown in Fig. 8-1, is considered most

representative for the following reasons. First, this assembly is most

prevalent and typically contains the highest levels of radioactivity. Second,

in terms of resistance to transportation accident loads, no significant

difference can be identified in the gross structural response of the various

fuel assembly designs. Finally, previous studies indicate that PWR fuel rods

may be more susceptible to creep rupture than BWR fuel rods if subjected to

high temperatures (12000 F) for a long period of time (e.g., a 11 hours). 5

The radioactive inventory of the reference PWR fuel assembly is based on

an assumed burnup of 33,000 megawatt-days/metric ton of uranium and a decay

time of 5 years. This burnup level is typical of current PWR fuel.

Variations in burnup occur and increases in burnup are expected in future

reactor operations. The effect of burnup level on the potential radiological

significance of transportation accidents will not be large, i.e., less than a

factor of 2. The 5-year decay time is selected because the vast majority of

all spent fuel shipments, namely those expected to be made to the Federal

repository, will have experienced at least this period of decay. 6 Spent fuel

with minimum decay times of 4 to 5 months can be shipped in licensed casks;

however, such shipments are expected to be rare. The boxed column in

Table 8.1 shows the general radioactive characteristics of the spent fuel

assembly considered representative for this study.

8.3 Measures of Radiological Significance

The general description of the reference spent fuel assembly, shown in

Table 8.1, identifies the radioactivity level in terms of curie content. The

curie content is important to radiological si gni fi cance from two

8-3
Head

Figure 8-1 PWR fuel bundle.

8-4
standpoints. First, the curie content provides a starting point for

establishing which isotopes should be evaluated for potential release from the

containment barriers provided during the spent fuel shipment. These

containment barriers include the fuel pellet itself, the fuel rod cladding,

and the cask containment shell. Second, the curie content of each isotope

indicates the magnitude of the radioactive source for determining the direct

radiation level.

The potential for release of radioactive material from a cask depends

heavily on the physical form of the radioactive material. Certain

radioisotopes, such as 8 5Kr, are in gaseous form. Elements such as cesium,

ruthenium, iodine, and their compounds may be volatile at temperatures that

can be achieved by the fuel during transportation accidents but will condense

to solids at ambient temperatures. However, the vast majority of the

radioactive material is in solid and relatively immobile form. The material

release estimates made in the next section take into account the physical form

of specific isotopes.

The radiological significance of any release is dependent not only on the

total radioactivity or number of curies released but also on the hazard posed

by a particular isotope. Krypton-85, for example, does not present a

significant health hazard. On the other hand, particles of plutonium can be

extremely hazardous. The potential radiological hazard of a particular

radioisotope is implied by the release limits specified in 10 CFR 71. The

10 CFR 71 release limits for the radioisotopes of interest are listed in

Table 8.2. The relative hazards of any two radioisotopes are roughly

estimated by comparing their release limits. For example, the release limits

for 85Kr and 13 4Cs are 10,000 and 10 curies, respectively; therefore, 1 34Cs

releases, on a radioactivity basis, are approximately 1,000 times as hazardous

as 8 5Kr. This report differs from other reports 2 ,7- 1 0 in that it does not

include detailed discussion of the radiological consequences or public health

impacts created by the release of specific isotopes. Rather, the releases

associated with each cask response region are compared to those releases

estimated in NUREG-0170. 2 Each cask response region, therefore, requires a

separate estimate of the release of gaseous, volatile, and solid radioactivity

8-5
Table 8.2

10 CFR 71 Release Limits for Radioisotopes

Radioisotope Release Limit

(Ci)

6 0Co

8 5Kr

9 0Sr

90y

10 6Ru

129I

134Cs

137Cs

137mBa

144Ce

238pu

237pu

240pu

241pu

244Cm

7

10,000

0.4

10

7

2

10

10

40

7

0.003

0.002

0.002

0.1

0.01

8-6
from the shipping cask to the environment. Section 9.0 uses the results of

NUREG-0170 to relate the release magnitudes to potential public health impacts

which can be associated with these releases.2

The radiological significance of the direct radiation emanating from a

cask as a result of shield degradation is typically controlled by those

isotopes emitting high energy gamma rays. The potential for direct radiation

exposure is presented for each cask response region in terms of an equivalent

unshielded spent fuel radioactivity. This radioactivity represents the amount

of material which, if no shielding were present, will lead to external

radiation levels equal to those resulting from the calculated degradation in

shielding associated with a specific cask response region.

8.4 Estimates of Radiological Hazards

8.4.1 Potential Radioactive Material Releases to the Environment

The potential for release of radioactive material to the environment from

a spent fuel shipment requires consideration of three mechanisms for

establishing a release path. These mechanisms are shown schematically in

Fig. 8-2.

Under normal conditions, certain radioactive material contained in the

ceramic fuel matrix migrates to the fuel rod gap. The migration involves

radioactive gas and vapors formed during the fission process in the reactor.

Since the claddings of most of the fuel rods are intact before the fuel is

shipped, this cladding must be breached during transport before radioactive

material is released into the cask cavity. A fuel rod's cladding can be

breached by high impact forces or high thermal loads. The number of rods

breached by mechanical forces is estimated by considering the rod responses

over the range of impact forces that the cask might experience in a

transportation accident. End-on impact is conservatively assumed since the

almost 15-foot-long (0.4 inch diameter) rods are most susceptible to breaching

by buckling. Figure 8-3 shows the percentage of fuel rods breached due to

end-on impacts as a function of impact force on a cask.

8-7
.4

Figure 8-2 Three mechanisms required to establish

material release path.

a radioactive

8-8
100 i i I

el

Z

a1

T

101-

31

I I I I I I

0 20 40 60 80 100 120 140

Deceleration force, g

Figure 8-3 Percentage of fuel rods breached as a function of force

for endwise impacts.

8-9
Cask impact forces are related to forces on the fuel rods necessary to

achieve 0.2% (S1), 2% 0S2), and 30% (S3) maximum effective strain on the inner

containment shell of the cask. Three percent of the rods are assumed to be

breached if the cask containment shell experiences maximum effective strains

equal to or less than 0.2% (S1). Similarly, 10% of the rods are assumed to be

breached for any transportation accident situation in which the containment

shell experiences between 0.2% (Si) and 2% (S2) effective strain. Beyond 2%

(S2) effective strain, all rods. are presumed to be breached. These results

are shown in Fig. 8-4.

Fuel rod cladding response to thermal loads is also evaluated. In

transportation accidents involving fires, heat may be absorbed by the cask and

its spent fuel contents. The resulting temperature increase of the fuel rod

cladding can cause an effect called thermal creep. This effect coupled with

pressures generated within the rods can cause a breaching of the cladding. If

the cask temperature level is 650OF (173) or less at the mid-thickness of the

lead shield, no breaching is expected to occur because the fuel rod

temperatures are too low to cause creep rupture. Beyond this thermal response

level, temperatures at the center of the cask and at the center of the fuel

assemblies are conservatively estimated to reach values which can breach up to

100% of the fuel rods for both the representative truck and rail casks. The

results presented in Fig. 8-4 include response to both mechanical and thermal

loads.

If a rod is breached, radioactive gases, volatiles, and solids can

potentially escape from the fuel rods into the cask containment. Experimental

information indicates that this escape involves three release mechanisms. The

first mechanism is associated with the actual breaching of the rod and is

referred to as the rod burst phenomenon. Pressure generated inside the fuel

rods by both non-radioactive and radioactive gases and vapors cause an

ejection of material to occur when the rod is breached. A temperaturecontrolled diffusion process is the second mechanism. Third, a chemical

oxidation process involving the uranium fuel takes place if fuel temperatures

exceed 400OF and air enters the cask cavity, thus replacing the normally inert

codtainment vessel atmosphere. This process, which involves a change in the

8-10
V,

0

.S

E

C

0

CLIA

4-. U

CO)

100% 100% 100% 100% 100%

S3

(3I)

100% 100% 100% 100% 100%/o

S2

12)

10% 10% 10% 100% 100%

S1

.2)

3% 3% 3% 100% 100%

(0

T1

(500)

T2

(600)

T3

(650) T4 (1050)

Thermal response (lead mid-thickness temperature, *F)

Figure 8-4 Percentage of fuel rods breached per fuel assembly in

each cask response region.

8-11
chemical form or phase of the uranium oxide, causes further radioactive

material releases.

Material release fractions for the significant radioisotopes are

estimated using the results of experiments conducted at the Oak Ridge National

Laboratories. 1 1 Table 8.3 summarizes these release fractions for the truck

and rail cask response regions. The rod burst and oxidation mechanisms are

the dominant mechanisms which control radioactive material release

fractions. Thus, Qnly the releases occurring as a result of these mechanisms

are tabulated.

Once the radioactive material has entered the cask containment volume, a

release to the environment can occur only through a leak or accident-caused

breach of the cask containment boundary. Several processes, which are

difficult to quantitatively analyze, will be expected to mitigate radioactive

material releases. Particles released from the rods will tend to settle

within containment without the presence of some driving force to promote their

release. Even if such a force exists, particles can become lodged in leak

passageways. Vapors released from the fuel rods will be cooled as they move

to the cask walls in most accident events, and the vaporous material will tend

to plate-out on all cask interior surfaces. These processes are expected to

limit essentially all environmental releases to those materials existing in

gaseous forms. In this study, however, because of the difficulty in

quantifying these processes, any radioactive material released from the fuel

rods is presumed to be released from the spent fuel cask if a leak path exists

in the containment vessel. This leak path is presumed to exist for any

transportation accident event resulting in (1) a maximum strain in the inner

containment shell greater than 0.2% (S$), or (2) lead mid-thickness

temperatures exceeding 500OF (T1 ).

8.4.2 Potential Radiation Increases from Shielding Reduction

Under accident conditions, a reduction can occur in the radiation

shielding provided by the shipping cask. Both neutron and gamma radiation

shielding can be affected. Typical cask designs can lose the effectiveness of

8-12
Table 8.3

Material Release Fractions from Breached Fuel Rods

Occurring over 1 Week Following Rod Bursa

Cask Release Fraction to Cask Cavity

Response Release Gas Vapors Particles

Regions Mechanism --Yr I Cs Ru

R(1,1)-R(3,1) Rod Burst 2.0x10-1 3.0x10- 4 2x10-4 2.0x10- 5 2x10-6

R(1,2)-R(3,2) Oxidation 1 .3 2.2x0- 3 1x10- 6 6.7x10- 6 0

R(1,3)-R(3,3) 3.3x10- 1 2.5x10- 3 2x10-4 2.7x10- 5 2x10-6

R(1,4)-R(3,4) Rod Burst 2.0x10- 1 3.0x10-4 2.0x10- 4 2.0x10- 5 2xi0-6

Oxidation 1.7x0-1 4.0x10- 3 8.0x10- 6 2.8x10- 5 0

3.9x10- 1 4.3x10- 3 2.Ox10- 4 4.8x10- 5 2x10-6

A_ Approximately the same fractional release for truck and rail cask.

8-13
neutron shields and still meet existing standards for allowable external

radiation levels. In this study, the neutron shielding is presumed lost in

all transportation accidents. Of greater concern is the effectiveness of the

gamma radiation shield. This type of shielding is provided by dense

materials, with lead being the material of choice in the representative cask

designs.

High-impact loads can cause the lead shielding to slump towards the

impacting side of the cask, e.g., to the bottom of the cask for the impact

orientation illustrated in Fig. 8-5. Shielding voids can be created and, in

Fig. 8-5, this void is shown near the top of the cask.

The gamma dose versus lead slump is calculated for the rail cask for

endwise impacts. The highest radiation increase occurs when the top of the

rail cask impacts a surface and the lead slumps towards the cask closure

region. In Table 8.4, the gamma dose is tabulated for various amounts of lead

slump as a function of distance from the cask surface to the receptor. The

dose from a truck cask with similar amounts of lead slump will be

approximately 21 times lower than the rail cask, because the truck cask

contains only 1 PWR assembly in comparison to 21 assemblies for the rail cask.

High thermal loads can cause the lead shield to melt and expand. The

lead expansion can cause the inner wall. of the cask to move inward. Upon

cooling, the lead shrinks and creates a void along the length of the cask as

illustrated in Fig. 8-6, causing the radiation level external to the cask to

increase. As it turns out, thermal loads can cause only minor lead voids and

increases in the local radiation.

To provide a consistent measure of radiological effects with cask damage,

the radiological hazard created by a gamma shielding reduction is presented in

terms of an equivalent inventory of unshielded spent fuel. This amount of

spent fuel, if unshielded, will produce radiation levels equivalent to those

emanating from the damaged cask. As an example of the calculation process, a

transportation accident which leads to 2% maximum effective strain (S2) in the

cask shell is presumed. The measure of the resulting radiation level is

calculated through the following steps: (1) the deceleration force necessary

8-14
T Radiation level

decreases exponentially

with distance

Fuel

00 oO 00

/0 000 0 (

0 00 o0 0 0 0 0? 

00000 00 0

c

0 ',00 JO o0o s

ofArOaofooooo

g4 o,increased oo0 o 5 0 0 0

00:0000

0000000

ýýZO.Area oO of o 0 +0

0Ol a

center

line

I

Bottom impact

Figure 8-5 Lead voiding due to lead slump resulting from endwise

impact of cask

8-15
Table 8.4

Ganmma Dose Summary for Lead Slump in a Rail Caska/

for Impacts on Closure Region

Dose Rate

(mrem/hr)

Gap at Cask Bottom Distance from Cask Surface to Receptor

Caused by (ft)

Lead Slump 3 10 30 300 3000

(inches)

5.0 1.02x103 1.93x10 2 2.38x10' 1.65x10- 1 8.03x10-6

10.0 8.64x40 3 1.30x10 3 1.53x102 9.13x10- 1 2.71x10-5

15.0 1.65x10 4 2.80x10 3 2.88x10 2 1.70x100 4.72x10-5

Truck cask dose is reduced by approximately the ratio of fuel assemblies

or a factor of 21.

I

8-16
Fire

Figure 8-6 Lead voiding due to high thermal loads and lead melting.

8-17
to achieve the 2% maximum effective strain (S2 ) level is determined, (2) the

lead slump level caused by this deceleration force is evaluated, (3) the

radiation level resulting from the lead slump is calculated, and (4) the

amount of unshielded spent fuel contents which will result in equivalent

radiation levels is determined.

8.5 Radiological Effect Estimates for Response Regions

The preceding evaluation provides the information necessary to estimate

the radiological effects in each response region. The measures involve four

parameters that can result in radiological hazards. The first three relate to

potential releases of radioactive material from the cask to the environment,

expressed in curies, and include: (1) the amount of radioactive gases, (2)

the amount of volatiles (isotopes weighted for health hazards), and (3) the

amount of solids (isotopes weighted for health hazards). The fourth measure

relates to the potential for increased external cask radiation levels

occurring as a result of losses or degradations in the cask shielding

capabilities. This measure is the equivalent amount of the total spent fuel

contents which, without shielding, will produce the calculated level of

external cask radiation. These measures are shown in Figs. 8-7 and 8-8 and

indicate the four types of radiological hazards estimated for the truck and

rail cask response regions.

Radiological hazards beyond the 30% strain (S3 ) and 1050°F temperature

(T4 ) levels are not calculated. They are assigned values 10 times those for

region R(3,4) except for 8 5Kr gas. The values assigned for 85Kr gas are 1.62

times the region R(3,4) values because a high percentage of the gas is already

released for states in the R(3,4) region.

8-18
C

0

C

0

CL

(G)1.95E+3

(V) 1.41E+2

(P)7.22E-2

(E)8.60E+l

1.95E+3

1 A1E+2

7.22E-2

8.60E+1

1.95E+3

1.41 E+2

7.22E-2

8.60E+1

1.95E+3

1.41E+2

7.22E-2

8.60E+1

1 .95E+3

1.41E+2

7.22E-2

8.60E+1

S

(3M

;3

S

(2

(G)1.02E+3 1.02E+3 1.02E+3 1.20iE+3 1.95 E+3

(V) 1.41 E+1 1.41E+1 1.41 E+1 1 42E+1 1.41E+2

(P)7.22E-3 7.22E-3 7.22E-3 7.22E-3 7.22E-2

(E)8.40E+0 8.40E+O 8.60E+0 8.60E+0 8.60E+1

2 ________________

(G)1.02E+2 1.02E+2 1.02E+2 1.20E+3 1.95E+3

(V)1.41E+0 1A1E+O 1.41E+0 1.42E+1 1.41E+2

(P)7.22E-4 7.22E-4 7.22E-4 7.22E-3 7.22E-2

(E)3.60E-1 3.60E-1 5.60E-1 5.60E-1 8.60E+1

(GF-O 3.05E+1 3.05E+1 1.20E+3 1.95E+3

(VFO 4.23E-1 4.23E-1 1.42E+1 1A1E+2

(P)-'o 2.17E-4 2.1 7E-4 7.22E-3 7.22E-2

(EPO _0 2.OOE-1 2.OOE-1 8.60E+1

S

(0.

T1

(500)

T2

(600)

T3

(650) (4 (1050)

Thermal response (lead mid-thickness temperature, *F)

(G)=Noble gases, curies

(V)=Vapors, curies

(P)=Particles, curies

(E)=Exposure, curies

E+x=10x

Figure 8-7 Radiological hazards

representative truck

estimated for response regions for a

cask.

8-19
aC

C

Co

x

M

E

CL

(0.

P'

w

Co

3

'2

1

Ti T2 T3 T4

(500) (600) (650) (1050)

Thermal response (lead mid-thickness temperature, OF)

(G)=Noble gases, curies

(V)=Vapors, curies

(P)=Particles, curies

(E)=Exposure, curies

E+x=10x

Figure 8-8 Radiolor;cal hazards estimated for response regions for a

representative rail cask.

8-20
9.0 RESULTS AND CONCLUSIONS

9.1 Introduction

In previous sections, a detailed evaluation is made of how spent fuel

casks designed to current regulations would respond in railway and highway

accident environments. The loading conditions that could conceivably affect

the response of a spent fuel cask are determined from surveys of accident

records. The responses of the representative truck and rail casks to a wide

variety of accident conditions are calculated and categorized into 20 cask

response regions. These response regions define specific levels of damage

that could be experienced by the cask during an accident. The boundaries of

these regions are defined in terms of structural strain experienced by the

cask containment shell and by material temperatures attained within the cask's

lead shield. The potential for radioactive material releases or increased

levels of external radiation are estimated for each of the 20 response regions

for both the representative truck and rail casks.

The first response region is defined by structural and thermal response

limits which would be within acceptable bounds implied by current regulations. 1 A major objective of this study is to determine the fraction of

accidents causing responses within this region. This process is called the

first-stage screening. For accidents which cause responses outside this

region, a second-stage screening is conducted. This screening involves

calculating cask responses to a wide variety of accident conditions *and

subsequently classifying the responses into the remaining 19 response

regions. The expected fraction of transportation accidents resulting in

responses in each region is then determined based on historical accident data

using probabilistic analysis.

In Section 9.2 the results of both the first- and second-stage screenings

are discussed. These results are compared with estimates made in the "Final

Environmental Statement on the Transportation of Radioactive Material by Air

or other Modes", NUREG-0170. 2 Several historical accidents are also

categorized into response regions in order to provide a perspective on the

9-1
meaning of severe accidents as used in this study. Uncertainties in the study

are discussed in Section 9.3. Conclusions and recommendations are provided in

Section 9.4.

9.2 Results

9.2.1 First-Stage Screening

In the first-stage screening, accidents are characterized which will

result in spent fuel cask responses that fall within the R(1,1) response

region. Within the R(1,1) region, the cask structural response is elastic,

and the strain on the inner shell of the cask does not exceed 0.2% (S1). The

cask thermal response does not exceed 500OF (T1 ) at the middle of the lead

shield thickness. Cask responses within the R(1,1) region are typically less

than the response generated on real casks by the accident test conditions

specified in 10 CFR 71.1 Accidents which produce loading conditions that

result in cask responses in the R(1,1) region do not result in significant

damage to a spent fuel cask; therefore, no radiological significance is

associated with these accident events.

Over 99.43% of all highway accidents result in a cask structural response

falling within the 0.2% strain (SI) level. Making up the largest algebraic

segment are the 94.7% of highway accidents which involve minor mechanical

loads resulting from rollovers of the transporting vehicle or impacts with

low-resistance objects. The remaining 5.3% of highway accidents have the

potential for generating significant loads, e.g., impacts with bridge columns,

abutments, or trains. The cask response to these potentially significant

accidents is dynamically evaluated. The calculations consider variations in

the impact velocity, the cask orientation, and the hardness of the object

struck. When all the factors for mechanical loads are considered, an

additional 4.7% of all highway accidents cause responses within the 0.2%

strain (Sl) level.

A similar evaluation is performed for railway accidents. The results

indicate that over 99.67% of railway accidents cause structural responses not

9-2
exceeding 0.2% strain (Sj) on the inner shell of the cask. As with truck

accidents, a large percentage (96.1%) of railway accidents are minor and would

not cause any significant cask damage.

The thermal loadings from accidents involving fires are analyzed to

determine the response of the truck and rail casks. The evaluations consider

the effects of fire duration, flame temperature, and cask location relative to

the fire. Given a fire accident, 99.97% of the truck and 99.04% of the train

accidents will generate heat loads on the casks less than those that can occur

for a half-hour regulatory 14750F engulfing fire. However, as calculated in

Section 6.3, a half-hour regulatory 1475 0F engulfing fire can only heat the

massive truck and rail casks to lead mid-thickness temperatures of 280OF and

3200F, respectively, which are well below the 500OF temperature (Tj) level

where radiological hazards could be generated. Therefore, the fire must burn

longer than one half-hour to reach the 500OF temperature (Tj) level and

consequently, a higher percentage of accidents is included. For the truck

cask, 99.99% or more of the accidents involving fire result in a lead midthickness temperature not exceeding 500OF (T1). For the rail cask, 99.72% or

more of the accidents involving fire result in temperature responses falling

within similar bounds.

The number of all accidents that included either mechanical or thermal

loads or both is estimated. These estimates are used to determine the

percentage of all highway and railway accidents causing cask responses within

the R(1,1) region. For the representative truck and rail casks, 99.43% and

99.40%, respectively, of the highway and railway accidents are estimated to

cause cask responses within the R(1,1) region as shown in Figs. 7-10' and

7-11. In those areas when the thermal load is expected not to exceed the

regulatory 1475OF engulfing fire, the percentage of accident conditions within

the 10 CFR 71 mechanical and thermal loading conditions is 99.41% for the

truck cask and 98.70% for the rail cask.

The structural and thermal responses within the R(1,1) response region

are evaluated with standard engineering methods of analysis. The structural

response limit for this region is selected such that the inner shell of the

representative cask will behave elastically and will experience no permanent

deformations. The thermal response limit is selected such that no thermal

9-3
degradation will occur to the seals or other parts of the cask. Responses

within these bounds will be within limits typically accepted when casks are

subjected to the regulatory accident test conditions. At this level of cask

damage, the radiological hazards are negligible and less than the 10 CFR 71

limits for radioactive material releases or for external radiation levels.

9.2.2 Second-Stage Screening

In the second-stage screening, accidents causing cask responses greater

than the 0.2% strain (S$) and 500OF temperature (T1 ) levels are evaluated. At

these higher levels of cask response, the potential exists for radioactive

material releases and external radiation levels equal to or greater than the

regulatory limits specified in 10 CFR 71. The highway and railway accident

loading conditions not eliminated by the first-stage screening are included.

A cask can be struck by a moving train or can fall off a bridge, plunge over

an embankment, run into a slope, or strike a massive concrete structure. The

thermal events include accidents involving high-temperature, long-duration,

engulfing fires that can cause high (a 500°F) temperature responses.

The second-stage screening considers response outside the R(1,1) region

as shown in Fig. 9-1. The fraction of accidents having cask responses within

each of these individual regions is summarized in Figs. 7-10 and 7-11 for the

truck and rail casks, respectively.

In most cases, the radiological hazard associated with accidents in the

response regions immediately adjacent to R(1,1) is limited and can be

negligible. The rationale for this judgment is that 2% strain (S2) will not

cause extensive structural damage to the cask containment, and temperatures up

to 600°F (T2 ) will not significantly degrade shield or seal materials

currently in use. For accidents causing cask responses within regions R(1,2),

R(2,2), and R(2,1), the occurrence of even a limited radiological hazard will

be dependent on the actual cask design, the amount of fuel being shipped, and

the specifics of the accident--especially with respect to how mechanical and

thermal loads are applied to the cask. In this study, the radiological

hazards estimated for these three regions are based on the performance of the

9-4
~'S 3

( (30)

C

0

E $2 Second screening

.E (2)

_ ..........

o

.. . ....... ..~........ ........

First --.-...

screening f f 4

R (1,11)

........ . .

T1 T2 T3 T4

(500) (600) (650) (1050)

Thermal response (lead mid-thickness temperature, *F)

Note: o First-stage screening radiological hazards are negligible and less

than 10 CFR 71 limits.

o Second-stage screening radiological hazards can be equal to or

greater than 10 CFR 71 limits.

Figure 9-1 Two-stage screening process in the 20 response regions.

9-5
representative truck and rail casks. The estimated radioactive material

releases and radiation levels are then compared with the regulatory limits

applicable to casks which have been subjected to the accident test conditions.

The result of this comparison indicates that the estimated radioactive

material releases and radiation levels are generally lower than the regulatory

limits as specified in 10 CFR 71. Compared with the representative truck and

rail cask designs, most existing cask designs can withstand higher mechanical

and thermal loads without significant damage. Approximately 0.39% of highway

and railway accidents that could involve spent fuel casks could result in

radiological hazards approaching or slightly exceeding those implied by

regulatory limits. The stated percentages of accidents are those which

produce cask responses less than the 2% strain (S2) and 600OF temperature (TO)

levels and represent the sum of the percentages determined by regions R(1,1),

R(2,1), R(1,2) and R(2,2).

Cask responses between the 2% (SO) and 30% (S3) strain levels and between

the 600OF (T72) and 1050OF (TO) temperature levels indicate both the

possibility of significant, permanent deformation to'the cask structure and

melting of the lead shield. The radiological hazard associated with this

degree of cask damage, will likely exceed the hazard implied by the regulatory

limits as specified in 10 CFR 71. Less than 0.001% of the truck shipment

accidents and 0.012% of the rail shipment accidents are estimated to cause

strains beyond 30% (S3) and temperatures beyond 1050OF (TO) in the casks.

9.2.3 Comparison with Previous Risk Assessments: NUREG-0170

In the second-stage screening, accidents are identified in which the

mechanical and thermal loading conditions on a cask can result in radioactive

releases beyond the regulatory limits. To assess the radiological risk of

these potential releases, a comparison is made between the probabilities of

specific radiological hazards calculated in this study and similar estimates

made in NUREG-0170.

The NUREG-0170 assessment indicates that the risk involved in spent fuel

shipments is small. This conclusion provided part of the technical

9-6
justification necessary for the Nuclear Regulatory Commission (NRC) to make

the judgment that the existing 10 CFR 71 regulations are adequate and not in

need of immediate change.

The comparison with NUREG-0170 begins by establishing the probability of

occurrence for accidents that can result in spent fuel cask responses in each

of the 20 regions; that is, the probabilities presented in Figs. 7-10 and

7-11. The probabilities are multiplied by the radiological hazards applicable

to each region, which are presented in Figs. 8-7 and 8-8. This product is

called a probability-hazard estimate. These probability-hazard estimates are

calculated for each of the three types of radioactive material releases

assessed in Section 8.0 (gas, vapor, and particle) and the cask external

radiation levels.

Figure 9-2 shows the truck cask probability-hazard estimates for each of

the 20 response regions. Estimates are given for the releases of radioactive

gases (8 5Kr), radioactive vapors which include 13 4 , 1 3 7Cs and 10 6Ru, and

radioactive solid particles which include 2 38 , 2 3 9 ,2 4 0 , 2 4 1 pu. The bottom

estimate in each region applies to the external radiation level. This process

provides numerical values which can be used for comparison. For instance, the

maximum values calculated for these probability-hazard estimates occurs in

region R(3,1). This region typically includes accidents involving highvelocity impacts which cause cask containment strain levels between 2% (S2 )

and 30% (S3 ) and a lead mid-thickness temperature of less than 500OF (T,).

Figure 9-3 presents similar probability-hazard estimates for spent fuel

shipments made by rail. For rail shipments, region R(1,5) has the maximum

estimates with the single exception of radioactive gas release. Region R(1,5)

includes accidents involving hot, long-duration fires resulting in lead midthickness temperatures beyond 1050OF (T4). The region which has the highest

radioactive gas release is R(1,4). The consequence of radioactive gas

release, however, is extremely small in comparison to the significance implied

by the other hazards.

The calculational methods and presentation of results in NUREG-0170

differ from those used in this study. In the NUREG-0170 evaluation, accident

9-7
C

C

0

E

E

0

(G) 2.98E-4

(V) 2.16E-5

(P) 1.11E-8

(E) 1.29E-5

7.65E-1 1

5.54E-12

2.83E-1 5

3.31E-12

2.91 E-11

2.11E-12

1.08E-15

1.26E-12

1.49E-12

1.08E-13

5.54E-17

6.46E-14

-0

-0

-0

S3

(30)

S2

(2)

S1

(0.2)

(G) 1.83E+0 1.61E-4 2.07E-4 1.10E-4 9.50E-4

(V) 2.54E-2 2.22E-6 2.87E-6 1.52E-6 6.87E-6

(P) 1.30E-5 1.14E-9 1.47E-9 7.78E-10 3.52E-9

(E) 1.51E-2 1.32E-6 1.71 E-6 9.07E-7 4.10E-6

(G) 3.90E-1 2.38E-5 3.06E-5 1.90E-4 1.40E-4

(V) 5.36E-3 3.29E-7 4.24E-7 2.24E-6 1.02E-5

(P) 2.76E-6 1.68E-10 2.17E-10 1.15E-9 5.20E-9

(E) 1.37E-3 8.39E-8 1.14E-7 6.05E-8 6.60E-6

(G0-0 5.15E-4 7.20E-4 1.83E-2 1.87E-2

(V)-0 7.14E-6 9.99E-6 2.15E-4 1.35E-3

(P)-O 3.66E-9 5.13E-9 1.10E-7 6.90E-7

(EM-0 -0 4.72E-7 3.05E-7 8.06E-4

T2

(600)

T3

(650)

T4 (1050) (500)

Thermal response (lead mid-thickness temperature, OF)

(G) = Noble gases, curies

(V) = Vapors, curies

(P) = Particles, curies

(E) = Exposure, curies

E+x= 10x

Figure 9-2 Probability-hazard estimates in curies for the 20 truck

cask response regions.

9-8
~S"$3

r (30)

C

0

E S2

. 121

C

C

0

C.

" .

S(0.2)

1..

CO

T1 T2 T3 T4

(500) (600) (650) (1050)

Thermal response (lead mid-thickness temperature, *F)

(G) = Noble gases, curies

(V) = Vapors, curies

(P) = Particles, curies

(E) = Exposure, curies (equivalent)

E+x= 10x

Figure 9-3 Probability-hazard estimates in curies for the 20 rail

cask response regions.

9-9
probability estimates, radioactive release fractions, and radiation levels

were class if ied into eight categories of accident severity. The

classification process was accomplished, in large part, through the use of

conservative engineering judgments. The first two accident categories in

NUREG-0170 were defined to include accidents with severities and radiological

hazards less than the 10 CFR 71 hypothetical accident conditions and, release

limits. These two accident categories generally correspond to accidents

causing responses within the R(1,1) region defined in this study. There is no

direct correspondence between the other 6 N4UREG-0170 categories and the

remaining 19 response regions in this study. Therefore, only two direct

comparisons can be made with NUREG-0170. The first involves a comparison of

the fraction of transportation accidents which generate cask responses that

cause no significant radiological hazards. The second point of comparison

involves the average radiological risk calculated in this report and 'the

average radiological risk estimate given in NUREG-0170.

In this study, the estimated percentage of accidents within region R(1,1)

is 99.4% for both truck and rail shipments. The radiological significance of

accidents involving R(1,1) cask responses is negligible. As a result, the

estimated percentages of accidents that could create a radiological hazard to

the public are 0.6% for both truck and rail shipments.

In contrast, the percentages of accidents estimated in NUREG-0170 to

result in negligible radiological hazard is 91% for truck shipments and 80%

for rail shipments. By subtraction, the estimated percentage of accidents

that could result in radioactive releases is 9% for truck shipment and 20% for

rail shipment. In comparing the estimated percentage of accidents that could

have a radiological significance, the more detailed estimates in this study

indicate that significantly fewer accidents are of radiological concern.

The second comparison between this study and NUREG-0170 essentially

involves measures of radiological risk, given that an accident occurs. In

this study, such a measure can be obtained by sunmming the probability-hazard

values for all of the 20 response regions. The summnation is performed for

gas, vapor, and particle releases and for direct radiation level effects.

9-10
The components of the summation are shown for each type of release listed

in Figs. 9-2 and 9-3. The same calculatlonal method is used in summing the

probability-hazard estimates for the eight accident categories in NUREG0170. Since NUREG-0170 did not evaluate particle releases, a direct

comparison is not possible. The comparative measures of radiological

risk/accident from both studies are presented in Tables 9.1 and 9.2 for truck

and rail shipments of spent fuel, respectively.

The expected gas, vapor, and direct radiation risks/accident in this

study for truck shipment are at least 3 times lower than those documented in

NUREG-0170. The estimated risk/accident for vapor releases (Cs) is at least

25 times lower in this study than in the NUREG-0170 evaluation. If

radioactive particle releases are not considered, it is the vapor release that

dominates the public health hazard.

In this study, the representative rail cask is designed to carry 21 fuel

assemblies compared with 7 fuel assemblies for the rail cask assumed in NUREG0170. When the differences in the carrying capacities are adjusted for

comparison, the gas, vapor, and direct radiation risks/accident estimated in

this study for rail shipment are at least 3 times lower than those documented

in NUREG-0170. As with truck shipments, risk/accident from vapor releases

(Cs) is at least 25 times lower in this study than that in NUREG-0170.

The release of aerosolized radioactive particles is considered in this

study but not in NUREG-0170. The release of small quantities of aerosolized

particles is important because the radiological hazard associated with

particles containing transuranic isotopes such as 2 3 8Pu can be 3,330 times

higher on a curie-for-curie basis than the hazard from 1 3 4Cs or 137Cs. As

Figs. 8-7 and 8-8 indicate, the estimated curie release of particles is about

a factor of 1,950 less than the release of cesium vapors.

Further perspective on the significance of the particle releases

predicted in this study can be gained by recalling that the cesium

releases/accident in this study are at least a factor of 25 less than those

that were predicted in NUREG-0170. As a result, the predictions of particle

release made in this study produce an overall public health hazard less than

one-tenth of the cesium hazard estimated in NUREG-0170.

9-11
Table 9.1

Comparative Measure of Risk/Accident

Shipment by Truck

for Spent Fuel

Twenty Response Regions NUREG-0170

(Ci) (Ci)

Gas 2.26 10.7

Vapors 3.24xi0-2 1.26

Particles 1.65Ex1O- 5 --

Direct radiation 1.73Ex1O-2 6.93ExlO-2

9-12
Table 9.2

Comparison of Release Risk/Accident for Spent Fuel

Shipment by Rail

Twenty Response Regions

(21 Fuel Assemblies)

(Cl)

NUREG-0170

(7 Fuel Assemblies)

(Ci)

NUREG-0170

(21 Fuel Assemblies)

(Cl)

Gas 39.6 61.0 183.

Vapors 0.651 7.17 21.9

Particles 4.16Ex10- 4 -- --

Direct radiation 0.276 0.300 0.900

9-13
The radiological risk on a per accident basis can be expanded into

risk/year estimates by considering highway and railway accident rates and by

estimating the number of cask-miles traveled in a year. NUREG-0170 assumed

that 3,000 metric tons of spent fuel would be transported annually in future

shipments (1,530 truck, 652 rail). Spent fuel, when shipped by truck, was

assumed to travel 1,525 miles/trip and, when shipped by rail, to travel 735

miles/trip. Based on current information, these assumptions on spent fuel

shipments made in NUREG-0170 are reasonable and are used in this study, except

that the rail mileage/trip is presumed to equal that for trucks, that is,

1,525 miles.

The estimated truck accident rate used in this study is 6.4x10- 6 truck

accidents/truck-mile compared to 1.7x10- 6 truck accidents/truck-mile used in

NUREG-0170. The truck accident risk/year associated with releases of

radioactive material in gaseous, volatile, or particulate form or external

radiation levels are estimated in this study as follows:

Annual risk - 6.4x10-6 truck accident 1525 truck-miles shipments trAua -rix x shipment x 1530 year x

release or external radiation level

truck accident

Values for the last term (release or external radiation level/truck

accident) appear in the first column (Twenty Response Regions) of Table 9.1.

The risk/year is calculated in a similar manner using values from NUREG0170. Comparing the results of this study with NUREG-0170 values indicates

that the estimated risks/year are smaller in this study, with the exception of

particle releases which are not considered in NUREG-0170. If the risk/year

for vapors and particles are combined after being weighted to account for

their relative public health hazard, the total risk calculated in this study

will be at least 3 times lower than the risk/year of vapor releases derived

from the NUREG-0170 report. The risk/year from rail shipments can be compared

in a manner similar to that used for truck transport.

9-14
The estimated train accident rate in this study is 1.19x0- 5 train

accidents/train-mile compared with the 1.05x1O-S train accidents/train-mile

figure used in NUREG-0170. Assuming, as was done in NUREG-0170, that an

average train length is 70 cars and an average of 10 cars are involved in each

accident, the overall estimated accident rate in this study is 1.7xi0- 6 rail

car accidents/rail car mile. Again, as with the truck shipments, the

comparison indicates that the risks/year in this study are within those

calculated for NUREG-0170, except for the particle release consideration.

Combining the risk/year in this study for vapors and particles after

appropriate weighting of the public health impacts results in a risk at least

4 times lower than the risk/year from vapor release calculated in NUREG-0170.

9.2.4 Estimated Responses for Sample Severe Accidents

In the previous section, emphasis is placed on compiling and analyzing a

broad range of accident loading data. This data is used to estimate the

probability of representative cask responses to accident loading conditions.

In this section, estimates are made regarding representative cask responses to

certain historic severe accidents.

From an extensive literature survey of historical accidents,

approximately 400 truck and train accidents are selected as having high

loading conditions. The selected accidents are summarized in Appendix A. For

each accident, the following information is provided: report source, date of

accident, type of accident, number of vehicles involved, velocity prior to the

accident, height of any fall involved, object struck, and duration of any fire

involved.

The loading conditions associated with four severe accidents are

evaluated to identify the response region into which each accident would be

categorized.

9.2.4.1 Caldecott Tunnel Fire3

A truck fire accident occurred in the Caldecott Tunnel near Oakland,

California, on April 7, 1982. The fire was caused by collisions of a gasoline

truck-trailer, a bus, and an automobile. The fire resulting from

9-15
approximately 8,800 gallons of gasoline had a peak flame temperature of

19000 F. Although it took 2 hours 42 minutes to completely extinguish the

fire, the peak flame temperature and the burning of most of the gasoline

occurred in less than 40 minutes, after which protected personnel entered the

tunnel to search for survivors and to extinguish the fire.

The probable response of the representative truck cask to the mechanical

and thermal loading conditions that occurred in the Caldecott Tunnel fire is

estimated using the accident information and the cask response information in

Section 6.0 and Appendix F.

The primary objects involved in the collisions were an automobile, a

truck-trailer, and a bus. Accidents involving these relatively soft objects

(i.e., when compared to a truck cask) are minor from a structural response

standpoint. These objects cause low levels of force to be imposed on a truck

cask regardless of the impact velocity and the cask orientation. Such impact

forces cannot cause a strain at the inner cask shell to exceed 0.2% (Sl).

If the representative truck cask were exposed to an engulfing 1900OF fire

such as the one in the Caldecott Tunnel, the fire duration required to reach

500OF (T1 ) at the middle of the lead shield thickness is 45 minutes. The hot,

engulfing fire lasted less than 40 minutes in the Caldecott Tunnel; therefore,

a 500OF temperature (TI) at the middle of the lead shield thickness would not

be reached during this accident.

From this evaluation, the response of the representative truck cask to a

Caldecott Tunnel fire accident environment is in region R(1,1) near the border

with region R(1,2). The containment inner shell of the cask, the closure

shell, and the lead shield would provide their safety functions without any

significant degradation during and following the accident. No radioactive

release or increase in radiation level is expected under these accident

conditions.

9.2.4.2 1-80 Bridge Accident 4

In March, 1981, a truck-tractor-trailer was struck by a pickup truck

while on an overpass bridge on Interstate 1-80 near San Francisco,

9-16
California. The truck-tractor-trailer veered into the bridge railing, broke

through the railing and fell 64 feet to the soil surface below.

The probable response of the representative truck cask to the mechanical

loading conditions that occurred with the drop onto the soil is estimated,

assuming that the truck struck the ground at an orientation angle between

200-700 and an impact angle of 900 for free fall onto a flat surface.

In this accident, the cask impact velocity would be approximately 44 mph

as determined by the fall of 64 feet. An impact velocity of at least 44 mph

is required to reach a 0.2% strain (SI) at the inner wall of the cask.

Therefore, this accident is also just within the R(1,1) response region.

9.2.4.3 Livingston Train Fire5

On September 28, 1982, 43 railroad cars derailed near Livingston,

Louisiana. Following the derailment, a fire started to burn various materials

which included plastic pellets, vinyl chloride, and petroleum products. The

fire which covered a wide area was allowed to burn for several days because of

the toxic chemicals and explosions involved. A railroad car carrying motor

fuel anti-knock compound (tetra-ethyl lead) exploded about 19 hours after the

derailment. A second thermally induced explosion occurred on October 1,

82 hours after the derailment, involving a car carrying vinyl chloride. The

fire cooled down sufficiently on the fifth day to permit fire-fighting

operations. Six cars carrying vinyl chloride materials were purposely

detonated on October 11 to dispose of the remaining unvented materials within

them.

The probable response of the representative rail cask to the thermal

loading conditions in the Livingston train fire accident is estimated by using

the accident information and the cask response information in Subsections

6.3.2, 7.3.2, and Appendix F.

The representative rail cask could have been located anywhere in the

derailed train wreckage and fire. The worst place for the cask would have

been in the environment of the seven cars burning vinyl chloride, where one of

the cars exploded and rocketed over 400 feet to the north of the derailment.

9-17
A maximum thermal loading condition on the cask can be calculated, assuming

that the cask was in the position of the rocketing car. First, it is

conservatively assumed that sufficient heat was absorbed by the vinyl chloride

in the car to vaporize all of the compressed gas, thus causing the

explosion. This would take no more than 3.5xlO7 Btu. It took approximately 82

hours to heat the vinyl chloride and cause the explosion. The cask area

exposed to the fire is estimated to be no greater than 1370 ft. 2 The average

heat transfer to the car during the entire period is then

- 3.5x107 - 3.1x10 2 Btu

(1.3x10 3 ) (8.2x101 ) hr-ft "

This is the average heat flux to which the neutron shield or thermal barrier

on the cask would have been exposed. The average heat transfer to the cask

lead shield would have been approximately a factor of 3 lower due to the

thermal shield. Assuming an average heat transfer rate of 103 Btu/hr-ft 2 , the

lead mid-thickness temperature would reach 500OF (T1 ) in 62 hours. At 82

hours, when the railroad car carrying the vinyl chloride exploded, the lead

mid-thickness temperature would have reached just over 600°F (T2 ), with some

lead melt occurring.

Assuming that the thermal conditions continued until the fifth day when

cool down started, the lead mid-thickness temperature would have reached a

temperature of 720 0 F, which is above 650°F (T3 ), but lower than 1050OF (T4 ).

This assumption of thermal conditions is very conservative, particularly

considering that the other six cars carrying vinyl chloride did not explode.

From this evaluation, the response of the representative rail cask to the

environment of the Livingston derailment fire accident is in the R(1,1),

R(1,2), R(1,3) or R(1,4) region depending on the location of the cask. Even

using the worst assumptions, the lead mid-thickness temperature would not

exceed 7200 F. Any radioactive releases would be much less than those

estimated in Section 8.0 for the R(1,4) region.

9-18
9.2.4.4 Derailment into the Alabama River 6

On January 19, 1979, a train derailed off a bridge into the Alabama River

near Hunter, Alabama. 6 One of the rail cars was carrying a pipe which struck

the bridge and caused the derailment. Five rail cars fell into the mud of the

river 75 feet below.

The probable response of the representative rail cask to the mechanical

loading condition caused by impact on the water was estimated by assuming that

the cask would strike the water at an orientation angle between 200-700.

In the accident, the rail cask impact velocity would be approximately

47 mph as determined by the fall of 75 feet onto the water surface. An impact

velocity of at least 90 mph is required to reach a 0.2% strain (SI) at the

inner wall for the cask impact; therefore, this accident is placed well within

the R(1,1) response region.

9.3 Uncertainties

This study evaluates the safety provided through current regulations for

the transport of spent fuel. Structural and thermal responses of a

representative shipping casks to a range of loading conditions which could

occur in potential transportation accidents are evaluated. These evaluations

are performed using realistic methods and assumptions. In many cases a range

of values is possible for a specific parameter. However, when the realism of

the assumption or method can be questioned or when an otherwise complex

analysis can be simplified, elements of conservatism are introduced into the

evaluations. These conservatisms, typically identified from sensitivity

studies, are discussed individually in previous sections of this report. They

are discussed collectively in this section, because an understanding of the

collective uncertainties is crucial to any judgment made on the overall

quality of study results.

Basically, the uncertainties can be classified under three headings:

(1) cask response, (2) radiological significance of cask response, and

(3) likelihood of accident events, cask response, and resulting radiological

hazard.

9-19
9.3.1 Uncertainty in Cask Response

The calculated responses of a spent fuel cask subjected to mechanical and

thermal loading conditions primarily depend on: (1) selection of the

representative cask designs, (2) definition of accident loads, and (3)

computer code applicabilities and modeling techniques used to estimate the

cask response to accident loads.

9.3.1.1 Selection of Representative Cask Designs

The accident resistance of the representative cask determined the

percentage of accidents causing specific cask response levels. The

representative truck and rail casks are purposely defined to meet existing

regulations. That is, the casks, if subjected to the accident test conditions

in the regulations will respond in an acceptable manner. The representative

truck cask is selected to have a capacity of 1 pressurized water reactor (PWR)

fuel assembly while the rail cask capacity is 21 PWR fuel assemblies. Both

cask designs use lead as a gamma shield material, and the fuel assemblies in

both cases are presumed to have experienced a 5-year decay period prior to

shipment.

The representative lead-shielded cask designs are selected by considering

currently licensed cask designs and the purported design capabilities of

future casks. Future casks are primarily being designed to transport existing

spent fuel to the planned geologic waste repositories. These repositories are

being designed to accept fuel which has experienced a decay time of 5 years or

more. Because of this lengthy decay time, the gamma radiation emanating from

the spent fuel is far less than the current casks are designed to

accommodate. As a result, shielding for casks can be accomplished by allsteel containment shells, particularly for rail casks. Also, uranium-shielded

casks may be used to transport spent fuel to the repositories. Any all-steel

or uranium shielded cask design will be intrinsically more resistant to

accident forces than either of the study's two representative cask designs.

Therefore, the results of this study underpredict the performance of the total

population of current and future cask designs.

9-20
The single element capacity chosen for the truck cask is typical for

casks whose shipment does not require highway overweight permits. If capacity

should be increased to two or, at most, three PWR assemblies, the minimum cask

resistance to accident forces, assumed in this study, will not change

significantly. Since the amount of radioactive material would be increased

by, at most, a factor of 3, the radiological hazard associated with a specific

accident sequence could conceivably increase by a similar factor. On the

other hand, a larger capacity cask would require fewer shipments, by a factor

of 3, hence the annual radiological risk would be unchanged.

An increase or decrease in the capacity of the rail cask would have a

similar effect in increasing or decreasing the potential radiological hazard

for a specific accident, but would not change the annual radiological risk.

9.3.1.2 Definition of Accident Loads

Real accidents can involve many different types of loading conditions

such as impact, crush, torch fires, engulfing fires, and burial. In this

study, the focus is on the response of representative casks to impact and

large fire loadings. Three loading parameters are used to determine the

impact loads: impact velocity, object hardness, and cask orientation. Three

loading parameters are also used to determine the fire loads: fire duration,

flame temperature, and fire location. Based on the reviews and sensitivity

studies included in this study, the conclusion is that impact collisions and

large fires impose loads which generally exceed those which can be achieved by

other loading conditions. When the massiveness of the cask is considered, the

loading magnitudes imposed by high-velocity impacts and large engulfing fires

conservatively bound all values which can be achieved by other loading

conditions.

9.3.1.3 Computer Code Applications and Modeling

The response of the representative casks to mechanical and thermal loads

are calculated with computer codes. Where possible the computer models and

results are benchmarked against existing test data. An elastic-plastic

9-21
material model is selected for performing response calculations for cask

impacts on soil, soft rocks, and hard rocks. Although an elastic-plastic

model oversimplifies real soil and rock characteristics, the shortcomings in

the model are accommodated by benchmarking against penetration test data (see

Appendix E).

The cask model is developed using, where possible, standard finite

element model techniques, bounding conditions, and material properties. The

modeling areas with the most uncertainty are the lead properties and the lead

interfaces with the stainless steel shell of the cask. In the absence of

reliable test data, the lead properties and boundary conditions are

conservatively selected to estimate lead slump and resulting strain on the

inner shell of the cask.

Ideally, three-dimensional (3-D) soil and cask models could be developed

and benchmarked against test data on representative casks impacting wellcharacterized soil and rock surfaces. This benchmarking approach could reduce

the uncertainties in the modeling and would improve the accuracy of

calculating the structural response of the cask. However, the benefits

derived from the improvement of modeling accuracy and the reduction of

modeling uncertainty cannot be fully realized unless the soil distributions

and soil uncertainties are better defined.

The thermal modeling of the cask and the fire are idealized. The

modeling depend strongly on the use of structural engineering material

properties, bounding conditions, and finite element techniques. A

one-dimensional (1-D) fire and cask model is used to predict the cask

response. The fire is represented by a homogeneous constant temperature and

constant location. In reality, fires are a 3-0 phenomenon in which the

temperature and location can vary significantly in any given accident.

Several conservative assumptions are made to accommodate the simplifications

in modeling the fire and cask. For example, the mid-plane of the cask, which

would be the hottest portion, is selected for the 1-D model. The temperature

response levels representing thermal degradation are selected to exist at the

mid-plane of the cask and over-predict the realistic response of the cooler

9-22
portions. For non-engulfing fires, heat absorption effects are included in

the modeling, but heat loss effects, such as thermal radiation to the

environment, are conservatively excluded.

Ideally, 3-D fire and cask models could be developed and benchmarked

against test data on representative casks involved in well-characterized

,fires. This approach could reduce the uncertainties in the modeling and would

improve the accuracy in calculating the thermal response of the cask.

However, the benefits derived from the improvement of the thermal model cannot

be fully realized unless the fire duration and flame temperature at fire

locations are better defined.

9.3.2 Uncertainty in Estimating an Accident's Potential Radiological Hazard

A damaged spent fuel cask could potentially cause a radiological hazard

with a magnitude dependent on (1) release of radioactive material from failed

fuel rods, (2) release of radioactive material from the cask, (3) reduction in

radiation shielding, and (4) reduction in subcritlcality control.

9.3.2.1 Radioactive Releases from Fuel Rods

Endwise impact of the fuel rods is assumed to determine the fraction of

rods which fail. This assumption is conservative in estimating the impact

failure of fuel rods for all other cask orientations. The release of

radioactive material from the fuel rods into the cask is estimated using Oak

Ridge National Laboratory (ORNL) test data. The ORNL tests were performed by

heating the rods to failure at high temperatures (greater than 13000 F). The

radioactive release is through a single leakage path caused by high internal

pressure bursting the rod. Under accident conditions, rod failure is more

likely during high impacts where multiple fractures to the rods can occur. In

contrast to test conditions, the fuel rods will likely be at relatively low

pressures and temperatures when impact occurs. Thus, the ORNL test data may

or may not. overestimate the actual releases under high-impact conditions. The

radiological hazards could be better estimated with pertinent tests performed

at high-impact conditions for the spent fuel rods.

9-23
9.3.2.2 Radioactive Releases from Casks

Radioactive releases from a cask depend on many factors which include

failure of the fuel cladding, the temperature and pressure in the cask cavity,

and a leakage path that could be through a closure seal. In this study, the

assumption is made that all of the radioactive material released from failed

fuel rods will be released from the cask. In reality, only a portion of the

radioactive material from the failed fuel rods will be released from the cask

cavity. Radioactive vaporous materials like cesium and its compounds will

deposit on the cooler inner shell of the cask and the cooler flange areas.

Radioactive particles will also be deposited on the walls and within the

leakage paths. In some cases, the particles may plug the leakage path. Thus

the estimates of the radioactive releases are higher than can be expected.

9.3.2.3 Reduction in Radiation Shielding

The external radiation from the representative cask is estimated by using

lead slump calculations. These lead slump calculations assume boundary

conditions that maximize the lead slump, hence the amount of external

radiation. In reality, the lead slump will be less. Also, the use of

depleted uranium or steel shielding will not allow shield slump and will

exhibit lower external radiation for the same accident loading conditions.

9.3.2.4 Reduction in Subcriticality Control

For large casks containing more than three PWR bundles, the effectiveness

of measures used to prevent a criticality event can be reduced under extreme

loading conditions. Any reduction in criticality safety depends on both the

cask and fuel basket design. However, since the margins used to prevent

criticality are very high, and since careful evaluations of the criticality

analysis and the design features are performed during cask licensing, the

possibility of a criticality event is small even under extreme loading

conditions. Using the probabilistic methods in Section 5.0, the probability

of a rail cask's having a structural response greater than 2% strain (S2) and

9-24
becoming submerged in water is estimated to be 0.00000078%, given an

accident. Using the accident and rail shipment rates in this study, this type

of accident is estimated to occur approximately once every ten million years.

9.3.3 Uncertainty in Probability Models

There is uncertainty associated with the probability distributions used

in this study. However, two points should be emphasized. First, although

direct experience with events involving the transport of casks would be the

best source of information, very little, if any, such information is

available. Thus, it is necessary to use data derived from similar types of

experiences--results which can be considered to be a sample of what

potentially will be experienced in the transport of spent fuel casks. Second,

similar types of probabilistic analyses have been done based on sparse data

similar to that used in this study. The important paint for those other

studies, and for this study as well, is the need to recognize that the

uncertainty exists and to consider this uncertainty in the use of the results.

The estimated probabilities and probability distributions used in the

probabilistic analyses are based on (1) accident statistics, (2) surveys of

physical structures/features, (3) past analyses and models, and

(4) engineering judgment, when no data is available.

9.3.3.1 Accident Statistics

The estimated accident rate for highway accidents is based on the number

of accidents experienced by trucks transporting petroleum products during

1973-1981. The extent to which the past experience of trucks transporting

petroleum products can be considered to be a random sample of the future

experiences of trucks transporting fuel casks determines the quality of the

estimate of the highway accident rate used in the analysis.

Similarly, the distributions of truck and train velocities in an accident

are based on statistics compiled from actual accidents. The train velocities

are derived from recorders in the locomotives, and are likely to represent a

good sample. That velocity is directly attributed to the cask upon impact,

9-25
but does not include braking effects. Truck velocities are based on estimates

by law enforcement officers in their investigation of accidents. The subset

of accidents used in this study is based only on data accumulated in

California. These accidents involved injury or fatality events that occur at

higher velocities than non-injury accidents. It is assumed that the accident

report data from 1973-1981 represent a sample of future incidents involving

cask transports. Also, the experiences in North Carolina are used to

empirically adjust for breaking. Overall, the distributions of train and

truck velocities used in this study are conservative.

9.3.3.2 Surveys of Structures and Features

The hardness of earth surfaces adjacent to highways can vary over a wide

range. This variability can have a significant effect on the loadings that

could be imposed on a cask or any other impacting object. The water and land

(hard rock, soft rock, and soil) distribution along proposed spent fuel

shipment routes between the east coast and west coast is initially estimated

using agricultural soil survey data and geological highway maps for the United

States. The initial distribution indicates the types of surfaces which can be

impacted along highways in the various regions of the United States. The

initial distribution is adjusted to an expected highway distribution by

performing highway surveys along representative portions of Federal

Interstates 5 and 80 in California. Also, these highway surveys are used to

estimate the distributions of bridge heights and column sizes along Federal

Interstates.

Improved distribution estimates could be made if the highway surveys were

actually performed along proposed spent fuel routes. However, for evaluating

the risk for cross-country transportation of spent fuel, the representative

distributions are reasonable.

9.3.3.3 Past Analysis and Models

Information on the occurrence of fires is very limited. Thus the thermal

evaluations rely on the models developed in a previous analysis of severe

9-26
accidents 7 . As mathematical models, the flame temperature and fire duration

distributions are only approximations of reality. Little or no information

has been compiled which directly models the fire accident environment. The

fire parameters, duration, temperature, and location, jointly affect the

thermal loading on the cask and hence its response.

9.3.3.4 Engineering Judgment

Finally, engineering judgment is used to model the distributions of some

accident parameters--impact angle and fire location. Distribution on these

important parameters could not. be found in actual data. For instance, a

uniform distribution is assumed for impact angle and a linear model for fire

location. In general, where judgment is used, conservative assumptions are

made.

9.3.4 Overall Statement of Uncertainty

As discussed, there are numerous uncertainties associated with the

analysis of the risks from transport of spent nuclear fuel. Related highway

and railway accident data is limited, and what is reported is often

insufficient or not applicable to developing the appropriate distributions and

models necessary to estimate risk. Similarly, mathematical models of the fire

environment in an accident and the structural and thermal responses of a cask

given the corresponding accident loadings are limited in their ability to

approximate the actual physical processes that occur during an accident.

Thus, the estimated probabilities and risks have uncertainty associated with

them.

However, recognizing the limited data and information on past accidents,

the limitations of using mathematical models to model complex physical

phenomena, and the limitations on the resources and time to do this analysis,

it is felt that a reasonably conservative estimate of risk is. provided.

9.4 Conclusions

The focus of this report is on the integrity of casks used for

U.S. shipments of commercially generated spent fuel, specifically on the level

9-27
of safety provided in the event of a transportation accident. Since all

shipping casks are designed to meet an existing set of regulatory standards,

the report evaluates the level of safety being provided by current

regulations.

The response of representative spent fuel casks are assessed under a

range of transportation accident conditions. The accident conditions are

derived from historical accident data applicable to truck and rail

shipments. The responses of the casks are categorized by a two-stage

screening process and compared with two benchmarks: 10 CFR 71 regulations,

and NUREG-0170.

The first benchmark is chosen to evaluate cask responses to accident

loading conditions which fall within the 10 CFR 71 accident test conditions.

As discussed in Subsection 9.2.1, approximately 99.4% of the truck accidents

and 98.7% of the rail accidents have both mechanical and thermal loading

conditions less than those implied by 10 CFR 71 regulations. The 10 CFR 71

benchmark is also chosen to represent a level of radiological hazard currently

reflected in existing regulations. This benchmark specifies limits for both

radioactive material releases and the magnitude of the radiation level

external to a cask. The limits are chosen to provide high assurance that

public radiation exposures would be less than permissible annual limits

established for workers in occupations involving the use of radioactive

materials. When considering real cask capabilities to withstand thermal

loading conditions beyond the regulatory ones, approximately 99.4% of the

truck and rail accidents would result in negligible radiological hazards which

are less than those implied by 10 CFR 71 regulations. As discussed in

Subsection 9.2.2, an additional 0.4% of both highway accidents and railway

accidents could result in radiological hazards near the regulatory limits.

The second benchmark value is chosen to provide a risk perspective; that

is, a benchmark which includes probabilistic consideration of all possible

levels of public radiological hazard. The probabilistic consideration was

originally presented in NUREG-0170, an environmental impact statement which

considered radiological risk from all shipments of radioactive material in the

9-28
U.S., including spent fuel. The significance of this particular document is

that based, in part, on the overall assessment of risk which it provided, the

NRC made a judgment on the adequacy of its transportation regulations. The

judgment was made that the regulations were adequate and not in need of

immediate change.

The benchmark taken from NUREG-0170 is the risk calculated specifically

for spent fuel shipments. The evaluations in Subsection 9.2.3 indicate that

the risks from spent fuel shipments derived in this study, are less than those

previously estimated in the NUREG-0170 document. The evaluations in NUREG0170 indicate that the expected radiological consequences from the shipment of

3000 metric tons of spent fuel per year is less than 1 latent cancer fatality

every 2300 years.

The results of this study depend primarily on the quality of the cask

response models, the radiation release models, and the probability models and

distributions used in the analysis. Models for cask responses, radioactive

releases, and distributions for the accident parameters are new developments

based on current computer codes, limited test data on radioactive releases,

and limited historical accident data. The results of this 'study apply to

s pent fuel casks which can be licensed by the NRC and are designed.,

manufactured, operated, and maintained in accordance with national codes and

standards (or equivalent) which have adequate margins of safety embedded in

them.

If the objective of this study is to precisely define spent fuel

transportation risks, many improvements need to be made to these models to

calculate the probability and radioactive release estimates and to quantify

the uncertainties in the estimates. For example, tests could be performed to

benchmark the DYNA/NIKE computer co des for predicting lead slump for a variety

of realistic boundary conditions which would provide nominal values with

uncertainty bounds. Similarly, more sophisticated modeling of rock surfaces,

which includes cracking, could be developed and benchmarked for improving the

prediction of cask responses to a variety of rock properties and impact

conditions. Finally, the probability distributions for all the accident

parameters, e.g., velocity, fire duration, impact angle, could be improved

9-29
with further research, data analysis and sensitivity studies. Human factors

which affect the cask design, manufacture, operation, and maintenance could

also be considered because they affect the cask response and contribute to the

overall risk in transporting spent fuel.

None of these improvements are being considered at this point for two

reasons: (1) the objective of this study is to estimate the level of safety

provided to the shipment of spent fuel using casks licensed to current

regulatory standards (a conservatively estimated measure of safety), and (2)

the radiological risk in current and future commercial spent fuel shipments is

a small component of the total risks applicable for all radioactive material

shipments.

The attempt is made in this study to use realistic, yet conservative when

appropriate, models and probabilistic distributions. Thus, the estimates

derived from the analysis are usable to achieve the study's objective.

9-30
REFERENCES

All references are listed numerically by sections. Appendix references

are at the end of each appendix.

Section 1.0

1. Office of the Federal Register, Title 10, Code of Federal Regulations,

Part 71, Office of the Federal Register, Washington, DC, January 1984.

2. U.S. Nuclear Regulatory Commission, Final Environmental Statement on the

Transportation of Radioactive Material by Air and Other Modes, U.S.

Nuclear Regulatory Commission, Washington, DC, NUREG-0170,

December 1977.

3. P. Eggers, Severe Rail and Truck Accidents: Toward a Definition of

Bounding Environments for Transportation Packages, U.S. Nuclear

Regulatory Commission, Washington, DC, NUREG/CR-3499, 1983.

Section 2.0

1. Accident Data on California State Highways, State of California

Business, Transportation and Housing Agency, Department of

Transportation, Division of Traffic Engineering, Sacramento, CA. Report

prepared in cooperation with the U.S. Department of Transportation,

Federal Highway Administration, 1983.

2. Office of the Federal Register, Title 49, Code of Federal Regulations,

Part 177, Office of the Federal Register, Washington, DC, January 1983.

3. 1969 Accidents of Large Motor Carriers of Property, Bureau of Motor

Carrier Safety, Federal Highway Administration, U.S. Department of

Transportation, Washington, DC, December 1970.

R-1
4. 1970 Accidents of Large Motor Carriers of Property, Bureau of Motor

Carrier Safety, Federal Highway Administration, U.S. Department of

Transportation, Washington, DC, March 1972.

5. 1971-1972 Accidents of Large Motor Carriers of Property,

Carrier Safety, Federal Highway Administration, U.S.

Transportation, Washington, DC, May 1974

Bureau of Motor

Department of

6. Summary of Motor Vehicle Accidents in the Petroleum Industry for 1977,

American Petroleum Institute, Washington, DC, May 1978.

7. Summary of Motor Vehicle Accidents in the Petroleum Industry for 1978,

American Petroleum Institute, Washington, DC, August 1979.

8. Summary of Motor Vehicle Accidents in the Petroleum Industry for 1979,

9.

American Petroleum Institute, Washington, DC, June 1980.

Summary of Motor Vehicle Accidents in the Petroleum Industry for 1980,

American Petroleum Institute, Washington, DC, September 1981.

10. Summary of Motor Vehicle Accidents in the Petroleum Industry for 1981,

American Petroleum Institute, Washington, DC, August 1982.

11. Accident/Incident Bulletin No. 145, Calendar Year 1976, Office

Safety, Federal Railroad Administration, U.S. Department

Transportation, Washington, DC, December 1977.

of

of

of

of

12. Accident/Incident Bulletin No. 146, Calendar Year 1

Safety, Federal Railroad Administration, U.S.

Transportation, Washington, DC, August 1978.

13. Accident/Incident Bulletin No. 147, Calendar Year 1

Safety, Federal Railroad Administration, U.S.

Transportation, Washington, DC, October 1979.

977, Office

Department

978, Office of

Department of

R-2
14. Accident/ Incident Bulletin No. 148, Calendar Year 1979, Office

Safety, Federal Railroad Administration, U.S. Department

Transportation, Washington, DC, July 1980.

15. Accident/ Inci dent* Bulletin No. 149, Calendar Year 1980, Office

Safety, Federal Railroad Administration, U.S. Department

Transportation, Washington, DC, June 1981.

16. Accident/Incident Bulletin No. 150, Calendar Year 1981, Office

Safety, Federal Railroad Administration, U.S. Department

Transportation, Washington, DC, June 1982.

17. Accident/Incident Bulletin No. 151, Calendar Year 1982,' Office

Safety, Federal Railroad Administration, U.S. Department

Transportation, Washington, DC, June 1983.

of

of

of

of

of

of

of

of

18. 1973 Accidents of Motor Carriers of Property,

Safety, Federal Highway Administration,

Transportation, Washington, DC, July 1975.

19. 1974 Accidents of Motor Carriers of Property,

Safety, Federal Highway Administration,

Transportation, Washington, D~C, 1975.

20. 1975 Accidents of Motor Carriers of Property,

Safety, Federal Highway Administration,

Transportation, Washington, DC, 1976..

21. 1976 Accidents of Motor Carriers of Property,

Safety, Federal Highway Administration,

Transportation., Washington, OC, October 1977.

Bureau

U. S.

Bureau

U. S.

Bureau

U. S.

Bureau

U. S.

of Motor Carrier

Department of

of Motor Carrier

Department of

of Motor Carrier

Department of

of Motor Carrier

Department of

R-3
22. 1977 Accidents of Motor Carriers of Property, Bureau of Motor Carrier

Safety, Federal Highway Administration, U.S. Department of

Transportation, Washington, DC, May 1980.

23. 1978 Accidents of Motor Carriers of Property, Bureau of Motor Carrier

Safety, Federal Highway Administration, U.S. Department of

Transportation, Washington, DC, May 1980.

24. 1979 Accidents of Motor Carriers of Property, Bureau of Motor Carrier

Safety, Federal Highway Administration, U.S. Department of

Transportation, Washington, DC, 1980.

25. 1980-1981 Accidents of Motor Carriers of Property, Bureau of Motor

Carrier Safety, Federal Highway Administration, U.S. Department of

Transportation, Washington, DC, August 1982.

26. 1982 Accidents of Motor Carriers of Property, Bureau of Motor Carrier

Safety, Federal Highway Administration, U.S. Department of

Transportation, Washington, DC, May 1983.

27. 1983 Accidents of Motor Carriers of Property, Bureau of Motor Carrier

Safety, Federal Highway Administration, U.S. Department of

Transportation, Washington, DC, October 1984.

28. TASAS Selective Record Retrieval Statewide Accident Summary for Year

1975, State of California Department of Transportation, Sacramento, CA,

October 1979.

29. TASAS Selective Record Retrieval Statewide Accident Summary for Year

1976, State of California Department of Transportation, Sacramento, CA,

October 1979.

R-4
30. TASAS Selective Record Retrieval Total Statewide Accidents for Year

1977, State of California Department of Transportation, Sacramento, CA,

May 1978.

31. TASAS Selective Record Retrieval Statewide Accident Summary for Year

1978, State of California Department of Transportation, Sacramento, CA,

Apri 11979.

32. TASAS Selective Record Retrieval, Summary Only, All Accidents for the

Year 1978, State of California Department of Transportation, Sacramento,

CA, August 1984.

33. TASAS Selective Record Retrieval, Summary Only, All Accidents for the

Year 1979, State of California Department of Transportation, Sacramento,

CA, August 1984

34. TASAS Selective Record Retrieval Statewide Accidents for Year 1981,

State of California Department of Transportation, Sacramento, CA., April

1982.

35. TASAS Selective Record Retrieval Statewide Summary 1982, State of

California Department of Transportation, Sacramento, CA, May 1983.

36. TASAS Selective Record Retrieval Statewide Summary 1983, State of

California Department of Transportation, Sacramento, CA, April 1984.

37. Assessment of the Stiffness Characteristics of Bridge Substructure

Components Encountered along a Section of Interstate 5, Engineering

Computer Corporation, Sacramento, CA, February 1985. A contractor

report to the Lawrence Livermore National Laboratory.

38. R. Imbsen, et al., Soil and Terrain Surveys, Engineering Computer

Corporation, Sacramento, CA, January 1985. A contractor report to the

Lawrence Livermore National Laboratory.

R-5
39. Soil Survey, United States Department of Agriculture, Bureau of

Chemistry and Soils, Superintendent of Documents, Washington, DC.

40. Geological Highway Map, American Association of Petroleum Geologist,

Tulsa, OK.

41. 1957 Annual

Sacramento,

42. 1958 Annual

Sacramento,

43. 1959 Annual

Sacramento,

44. 1960 Annual

Sacramento,

45. 1961 Annual

Sacramento,

Statistical Report,

CA, May 1958.

Statistical Report,

CA, May 1959.

Statistical Report,

CA, May 1960.

Statistical Report,

CA, May 1961.

Statistical Report,

CA, May 1962.

Department of California Highway Patrol,

Department of California Highway Patrol,

Department of California Highway Patrol,

Department of California Highway Patrol,

Department of California Highway Patrol,

46. 1962 Traffic Accident

Patrol, Sacramento, CA,

47. 1963 Traffic Accident

Patrol, Sacramento, CA,

48. 1964 Traffic Accident

Patrol, Sacramento, CA,

49. 1965 Traffic Accident

Patrol, Sacramento, CA,

Statistics,

May 1963.

Statistics,

May 1964.

Statistics,

May 1965.

Statistics,

April 1966.

Department

Department

Department

Department

of California

of California

of California

of California

Highway

Highway

Highway

Highway

R-6
50. 1966 Report of Fatal and Injury Motor Vehicle Traffic Accidents,

Department of California Highway Patrol, Sacramento, CA, July 1967.

51. 1967 Report of Fatal and Injury Motor Vehicle Traffic Accidents,

Department of California Highway Patrol, Sacramento, CA, July 1968.

52. E. G. Hamilton, Single, Variable Tabulations for 1979-1981 North

Carolina Accidents, University of North Carolina Highway Safety Research

Center, Chapel Hill, NC, September 1977.

53. R. K. Clarke, et al., Severities

National Laboratories, Albuquerque,

54. 1979 Virginia Traffic Crash Facts,

Richmond, VA, April 1980.

55. 1982 Virginia Traffic Crash Facts,

Richmond, VA, April 1983.

56. 1983 Virginia Traffic Crash Facts,

Richmond, VA, April 1984.

of Transportation Accidents, Sandia

NM, SLA-74-0001, July 1976.

Virginia Department of State Police,

Virginia Department of State Police,

Virginia Department of State Police,

57. Rail-Highway Grade-Crossing Accidents/Incidents Bulletin for the Year

Ended December 31, 1975, Office of Safety, Federal Railroad

Administration, U.S. Department of Transportation, Washington, DC.

58. Rail-Highway Grade-Crossing Accidents/Incidents Bulletin for the Year

Ended December 31, 1976, Office of Safety, Federal Railroad

Administration, U.S. Department of Transportation, Washington, DC,

December 1977.

59. Rail-Highway Grade Crossing Accident/Incident Bulletin No. 43, Calendar

Year 1977, Office of Safety, Federal Railroad Administration, U.S.

Department of Transportation, Washington, DC, July 1978.

R-7
60. Rail-Highway Crossing Accident/Incident and Inventory Bulletin No. I,

Calendar Year 1978, Office of Safety, Federal Railroad Administration,

U.S. Department of Transportation, Washington, DC, October 1979.

61. Rail-Highway Crossing Accident/Incident and Inventory Bulletin No. 2,

Calendar Year 1979, Office of Safety, Federal Railroad Administration,

U.S. Department of Transportation, Washington, DC, September 1980.

62. Rail-Highway Crossing Accident/Incident and Inventory Bulletin No. 3,

Calendar Year 1980, Office of Safety, Federal Railroad Administration,

U.S. Department of Transportation, Washington, DC, June 1981.

63. Rail-Highway Crossing Accident/Incident and Inventory Bulletin No. 4,

Calendar Year 1981, Office of Safety, Federal Railroad Administration,

U.S. Department of Transportation, Washington, DC, June 1982.

64. Rail-Highway Crossing Accident/Incident and Inventory Bulletin No. 5,

Calendar Year 1982, Office of Safety, Federal Railroad Administration,

U.S. Department of Transportation, Washington, DC, June 1983.

65. 1969 Accidents of Large Motor Carriers of Property, Bureau of Motor

Carrier Safety, Federal Highway Administration, U.S. Department of

Transportation, Washington, DC, December 1970.

66. R. K. Clarke, et al., Severities of Transportation Accidents, Sandia

National Laboratories, Albuquerque, NM, SLA-74-0001, July 1976.

67. P. Eggers, Severe Rail and Truck Accidents: Toward a Definition of

Bounding Environments for Transportation Packages, U.S. Nuclear

Regulatory Commission, Washington, DC, NUREG/CR-3499, 1983.

R-8
Section 3.0

1. Safety Analysis Report for NLI 1/1 Truck Cask, Docket No. 71-9010,

National Lead Industries, Inc., Wilmington, DE, November 1972.

2. Safety Analysis Report for TN-8/TN9 Shipping Casks, Docket

No. 71-9015/16, Transnuclear, Inc., White Plains, NY, October 1976.

3. Design and Analysis Report IF-300 Shipping Cask, Docket No. 70-1220,

General Electric Company, San Jose, CA, January 1971.

4. Safety Analysis Report for the NLI-10-24 Shipping Cask, Docket No. 70-

9023, National Lead Industries, Inc., Wilmington, DE, February 1976.

5. Office of the Federal Register, Title 10, Code of Federal Regulations,

Part 71, Office of the Federal Register, Washington, DC, January 1984.

6. U.S. Nuclear Regulatory Commission, Regulatory Guide 7.6: Design

Criteria for the Structural Analysis of Shipping Cask Containment

Vessels, U.S. Nuclear Regulatory Commission, Washington, DC, March

1978.

7. U.S. Nuclear Regulatory Commission, Regulatory Guide 7.4: Leakage Tests

on Packages for Shipment of Radioactive Materials, U.S. Nuclear

Regulatory Commission, Washington DC, June 1975.

8. U.S. Nuclear Regulatory Commission Regulatory Guide 7.8: Load

Combinations for the Structural Analysis of Shipping Casks, U.S. Nuclear

Regulatory Commission, Washington, DC, May 1977.

9. U.S. Nuclear Regulatoty Commission, Regulatory Guide 7.9: Standard

Format and Content of Part 71 Applications for Approval of Packaging of

Type.B, Large Quantity, and Fissile Radioactive Material, U.S. Nuclear

Regulatory Commission, Office of Standards Development, Washington, DC,

January 1980, Rev-1.

R-9
I0. U.S. Nuclear Regulatory Commission, Regulatory Guide 7.10: Establishing

Quality Assurance Programs for Packaging Used in the Transport of

Radioactive Material, U.S. Nuclear Regulatory Commission, Washington,

DC, January 1983.

11. American Society of Mechanical Engineers, ASME Boiler and Pressure

Vessel Code, the American Society of Mechanical Engineers, United

Engineering Center, 345 East 47th Street, New York, NY 10017, July 1983.

12. U.S. Nuclear Regulatory Commission, (Draft) Regulatory Guide, Fracture

Toughness Criteria for Ferritic Steel Shipping Cask Containment Vessels

with a Maximum Wall Thickness of Four Inches (0.1m), U.S. Nuclear

Regulatory Commission, Office of Nuclear Regulatory Research,

Washington, DC, June 1986.

13. W. R. Holman, and R. T. Langland, Recommendations for Protecting Against

Failure by Brittle Fracture in Ferritic Steel Shipping Containers Up to

Four Inches Thick, Lawrence Livermore National Laboratory, Livermore,

CA, UCRL-53013, NUREG/CR-1815, June 1981.

14. M. W. Schwartz, Recommendations for Protecting Against Brittle Fractures

in Ferritic Steel Shipping Containers Greater Than Four Inches Thick,

Lawrence Livermore National Laboratory, Livermore, CA, UCRL-53538,

NUREG/CR-3826, April 1984.

15. L. E. Fischer, and W. Lai, Fabrication for Shipping Containers, Lawrence

Livermore National Laboratory, Livermore, CA, UCRL-53544, NUREG/CR-3854,

March 1985.

16. R. E. Monroe, H. H. Woo, and R. G. Sears, Recommended Welding Criteria

for use in the Fabrication of Shipping Containers for Radioactive

Materials, Lawrence Livermore National Laboratory, Livermore, CA,

UCRL-53044, NUREG/CR-3019, March 1984.

R-1O
Section 4.0

1. American Society of Mechanical Engineers, ASME Boiler and Pressure

Vessel Code, Section III, Division 1, The American Society of Mechanical

Engineers, United Engineering Center, 345 East 47th Street, New York, NY

10017, July 1983.

2. Manual of Steel Construction, 7th ed., American Institute of Steel

Construction, Inc., 101 Park Avenue, New York, NY 10017, 1970.

3. M. G. Fontana and N. 0. Greene, Corrosion Engineering, 2nd ed., McGrawHill Book Company, New York, NY, 1978.

4. A. J. Romano, et al., The Investigation of Container Materials for Bi

and Pb A11oys, Part I., Thermal Convection Loops, Brookhaven National

Laboratory, Upton, NY, BNL 811(T313), July 1963.

5. G. M Tolson and A. Taboada, A Study of Lead and Lead-Salt Corrosion in

Thermal Convection Loops, Oak Ridge National Laboratory, Oak Ridge, TN,

1966.

Section 5.0

1. E. G. Hamilton, Single, Variable Tabulations for 1979-1981 North

Carolina Accidents, University of North Carolina Highway Safety Research

Center, Chapel Hill, NC, September 1977.

2. R. K. Clarke, et al., Severities of Transportation Accidents, Sandia

National Laboratories, Albuquerque, NM, SLA-74-0001, July 1976.

Section 6.0

1. Office of the Federal Register, Title 10, Code of Federal Regulation,

Part 71, Office of the Federal Register, Washington, DC, January 1984.

R-1 1
2. J. 0. Hallquist, NIKE-2D: An Implicit, Finite-Deformation, Finite

Element Code for Analyzing the Static and Dynamic Response of TwoDimensional Solids, Lawrence Livermore National Laboratory, Livermore,

CA, UCRL-52678, 1979, and Revision 1, NIKE-2D: An Implicit, FiniteDeformation, Finite Element Code for Analyzing the Static and Dynamic

Response of Two-Dimensional Solids, Lawrence Livermore National

Laboratory, Livermore, CA, UCID-18822, 1981.

3. T. A. Nelson, et al., SCANS - Shipping Cask Analysis System, Vol. 1,

Impact Analysis Code User's Manual, Lawrence Livermore National

Laboratory, Livermore, CA, UCID-20674/Vol. 1, Draft Report to be

published, U.S. Nuclear Regulatory Commission, Washington, DC,

NUREG/CR-4554, 1986.

4. J. 0. Hallquist, User's Manual for Dyna-2D--An Explicit Two-Dimensional

Hydrodynamic Finite Element Code with Interactive Rezoning, Lawrence

Livermore National Laboratory, Livermore, CA, UCID-18756, Rev. 2, 1984.

5. C. W. Young, "Depth Prediction for Earth-Penetrating Projectiles", Journal of the Soil Mechanics and Foundations Division, Proceedings of the

American Society of Civil Engineers, Vol. 95, No. SM3, Proceedings Paper

6558, American Society of Civil Engineers, New York, NY, May 1969.

6. Institute of Mechanical Engineers, "The Resistance to Impact of Spent

Magnox Fuel Transport Flasks", Power Industries Division of the

Institute of Mechanical Engineers and the British Nuclear Energy

Society, Seminar Paper, Mechanical Engineering Publications Ltd.,

London, England, April 30 and May 1, 1985.

7. International Atomic Energy Agency, Regulations for the Safe Transport

of Radioactive Materials, Safety Series No. 6, International Atomic

Energy Agency, Vienna, Austria, 1985.

R-12
8. P. J. Burns, TACO-2D-A Finite Element Heat Transfer Code, Lawrence

Livermore National Laboratory, Livermore, CA, UCID-17980, Rev. 2,

January 1982.

Section 7.0

1. Office of the Federal Register, Title 10, Code of Federal Regulations,

Part 71, Office of the Federal Register, Washington, DC, January 1984.

2. J. 0. Hallquist, User's Manual for Dyna-2D--An Explicit Two-Dimensional

Hydrodynamic Finite Element Code with Interactive Rezoning, Lawrence

Livermore National Laboratory, Livermore, CA, UCID-18756, Rev. 2, 1984.

3. J. 0. Hallquist, NIKE-2D: An Implicit, Finite-Deformation, Finite

Element Code for Analyzing the Static and Dynamic Response of TwoDimensional Solids, Lawrence Livermore National Laboratory, Livermore,

CA, UCRL-52678, 1979, and Revision 1, NIKE-2D: An Implicit, FiniteDeformation, Finite Element Code for Analyzing the Static and Dynamic

Response of Two-Dimensional Solids, -" Lawrence Livermore National

Laboratory, Livermore, CA, UCID-18822, 1981.

4. A. J. Neilson, A Dyna-3D Calculation for Impact on a Pipe Target, Safety

and Engineering Science Division, Winfrith, AEEW-M2058, United Kingdom

Atomic Energy Authority, Winfrith, England, 1981982, November 1983.

5. Institute of Mechanical Engineers, "The Resistance to Impact of Spent

Magnox Fuel Transport Flasks', Power Industries Division of the

Institute of Mechanical Engineers and the British Nuclear Energy

Society, Seminar Paper, Mechanical Engineering Publications Ltd.,

London, England, April 30 and May 1, 1985.

6. M. Huerta and H. R. Yoshimura, A Study and Full-Scale Test of a HighVelocity Grade-Crossing Simulated Accident of a Locomotive and a

R-13
Nuclear-Spent-Fuel Shipping Cask, Sandia National Laboratories,

Albuquerque, NM, SAND79-2291, TTC308, February 1983.

7. M. Huerta, Analysis, Scale Modeling, and Full-Scale Tests of a Truck

Spent-Nuclear -Fuel Shipping System in High Velocity Impacts Against a

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0270, 1978.

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a Fire-Exposed Spent Fuel Shipping Cask, Sandia National Laboratories,

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Transportation of Radioactive Material by Air and Other Modes, U.S.

Nuclear Regulatory Commission, Washington, DC, NUREG-0170,

December 1977.

Section 8.0

1. Office of the Federal Register, Title 10, Code of Federal Regulations,

Part 71, Office of the Federal Register, Washington, DC, January 1984.

R-14
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1977.

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R-15
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Section 9.0

1. Office of the Federal Register, Title 10, Code of Federal Regulations,

Part 71, Office of the Federal Register, Washington, DC, January 1984.

2. U.S. Nuclear Regulatory Commission, Final Environmental Statement on the

Transportation of Radioactive Material by Air and Other Modes, U.S.

Nuclear Regulatory Commission, Washington, DC, NUREG-0170,

December 1977.

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R-16
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R-17

NRC FORM 335 U.S. NUCLEAR REGULATORY COMMISSION 1, REPORT NUMBER fAssgnhed b TiDC. add Voat No. ifaarl

" NRCM 110NUREG/CR-4829, 1102* AT Vol. 1

3201.3202 BIBLIOGRAPHIC DATA SHEET UCID-20733 SEE INSTRUCTIONS ON THE REVERSE

2. TITLE AND SUBTITLE 3. LEAVE BLANK

Shipping Container Response to Severe Highway and

Railway Accident Conditions 4. DATE REPORT COMPLETED

Main Report MONTH YEAR

5. AUTHOR IS) I IqP,

L.E. Fischer, C.K. Chou, M.A. Gerhard, CY. Kimura, S. DATE REPORT ISSUED

R.W. Martin, R.W. Mensing, M.E. Mount, M.C. Witte MONTH YEAR

February 1987 7. PERFORMING ORGANIZATION NAME AND MAILING ADDRESS (Iclu Z49 CodeW 8. PROJECTITASK/WORK UNIT NUMBER

Lawrence Livermore National Laboratory

P. 0. Box 808, L-197 9. FIN OR GRANT NUMBER

Livermore, California 94550 A0397

10. SPONSORING ORGANIZATION NAME AND MAILING ADORESS finclude ZOp Codel Ale. TYPE OF REPORT

Division of Reactor System Safety Technical

Office of Nuclear Regulatory Research b. PERIOD COVERED lio,,usv,, 0k,

U.S. Nuclear Regulatory Commission

Washington, D.C. 20555 12. SUPPLEMENTARY NOTES

13. ABSTRACT O20 word or IAes)

This report describes a study performed by the Lawrence Livermore National Laboratory

to evaluate the level of safety provided under severe accident conditions during the

shipment of spent fuel from nuclear power reactors. The evaluation is performed using

data from real accident histories and using representative truck and rail cask models

that likely meet 10 CFR 71 regulations. The responses of the representative casks are

calculated for structural and thermal loads generated by severe highway and railway

accident conditions. The cask responses are compared with those responses calculated

for the 10 CFR 71 hypothetical accident conditions. By comparing the responses it is

determined that most highway and railway accident conditions fall within the 10 CFR 71

hypothetical accident conditions. For those accidents that have higher responses, the

probabilities and potential radiation exposures of the accidents are compared with those

identified by the assessments made in the "Final Environmental Statement on the Transportation of Radioactive Material by Air and other Modes," NUREU-0170. Based on this

comparison, it is concluded that the radiological risks from spent fuel under severe

highway and railway accident conditions as derived in this study are less than risks

previously estimated in the NURED-0170 document.

14. DOCUMENT ANALYSIS - 0. KEYWORDSIDESCRITORS

spent fuel casks

severe highway and railway accident conditions

b. IDENTIFIERS/OPEN.ENDED TERMS

15. AVAILABILITY

15. AVAILABILITY

STATEMENT

Unlimited

16. SECUR ITY CLASSIF ICATION

Unclassified

Irlim repor, JUnclassified 17. NUMBER Of PAGES