U.S. NUCLEAR 

REGULATORY 

COMMISSION

Transporting Spent Fuel 

Protection Provided Against Severe 

Highway and Railroad Accidents

.'Ii

!Lij
Transporting Spent Fuel 

Protection Provided Against 

Severe Highway and Railroad 

U.S. NUCLEAR Accidents

REGULATORY 

COMMISSION March 1987

Author: William R. Lahs 

Division of Regulatory Applications 

Office of Nuclear Regulatory Research 

U.S. Nuclear Regulatory Commission 

Washington, D.C. 20555

TABLE OF CONTENTS

Overview Page

Introduction ................................ 

Accidents With No Expected 

Radiological Hazards (illustration) .............. 

Summary of Objective and Results ............. 

Background ................................. 

How Is Safety Achieved? ..................... 

Standards for Spent Fuel Casks ............... 

Accident Scenarios .......................... 

Occurrence Rates for Truck 

Accident Scenarios (illustration) ............... 

Occurrence Rates for Railroad 

Accident Scenarios (illustration) ............... 

Cask Characteristics and Responses 

Can Cask Safety Be Characterized 

in Real-W orld Accidents? ..... . . . . . . . . . . . . . . . . 

Representative Designs for 

Truck and Rail Casks (illustration) .............. 

Schematic of Spent Fuel Cask (illustration) ..... 

Cask Safety Functions and 

Representative Cask Design Features ........... 

Establishing a Scale to 

Measure Cask Response ...................... 

Measure of Cask and Spent Fuel 

Response to: Mechanical Loads 

and Thermal Loads (2 illustrations) ............. 

Evaluating Cask and Spent Fuel 

Response to Accident Loads .................. 

Cask Response (Damage) 

Regions (illustration) ......................... 

Relationships Between Cask Response 

and Potential Radiological Hazards .............

1

2 

3 

3 

4 

5 

6

"7

F 

Typical Radioactive Material 

Release Pathway (illustration) ................. 

Typical Radiation Level Increase 

as a Result of Lead (Gamma Shield) 

Slumping (illustration) ........................ 

Cask Damage - What Accident 

Conditions Are Important 

and How Are They Defined? .... . . . . . . . . . . . . . .

Potential Hazards and Risk 

9 Fraction of Accidents Without 

Expected Radiological Hazards ................ 

Accident Scenarios Generating 

Mechanical Forces Incapable 

of Causing Functional Cask 

10 Damage (illustration) ......................... 

Accident Scenarios Generating 

10 Thermal Forces Incapable of Causing Functional Cask 

11 Damage (illustration) ......................... 

Potential Radiological Hazards 

11 Resulting From Functional 

Cask Dam age ............................... 13 Fraction of Truck and Rail Accidents 

Involving Spent Fuel Shipments That 

Cause Cask Responses Within 

14 Each Response Region (illustration) ............ 

Interpretation of the Relationship 

15 Between Potential Radiological Hazards 

and Real-World Severe Accidents ............. 

15 Risk Estimate for Spent Fuel Shipments ........ 

Risk Results - Comparison With 

16 Past FES Evaluation (illustration) ..............

'age 

17 

19 

20 

22 

23 

25 

26 

27 

28 

28 

30
OVERVIEW

INTRODUCTION 

This report summarizes the results 

of a study conducted for the 

Nuclear Regulatory Commission 

(NRC) to determine the level of 

safety provided during shipments 

of spent fuel from U.S commercial 

nuclear power plants. The study 

focuses on the protection provided 

for shipments that may be in

volved in truck or railroad 

accidents. 

During shipment, the cask and the 

form and structure of the spent 

fuel being shipped provide the 

primary physical means for con

taining radioactivity and for 

limiting radiation levels outside the 

cask. These functions must be 

maintained at acceptable levels 

even under the wide range of 

forces the cask and fuel could be 

subjected to during an accident. 

Spent fuel shipments are regu

lated by both the Department of 

Transportation (DOT) and the 

NRC. The NRC evaluates and cer

tifies the design of the shipping 

casks used to transport spent fuel, 

while DOT regulates vehicles and 

drivers. 

Current NRC regulations require 

that shipping casks meet certain 

performance standards. The per

formance standards include normal 

operating conditions and hypo

thetical accident conditions a cask 

must be capable of withstanding 

without exceeding specified ac

ceptance criteria that (1) limit 

releases of radioactive material and 

radiation levels outside the cask

Y I

and (2) assure that the spent fuel 

will remain subcritical (that is will 

not undergo a self-sustaining 

nuclear reaction). 

The study, conducted by 

Lawrence Livermore National 

Laboratory (LLNL),* began with 

an assessment of the possible 

mechanical and/or thermal forces 

generated by actual truck and 

railroad transportation accidents. 

The magnitudes of forces from ac

tual accidents were compared with 

forces attributed to the "regula

tory-defined" hypothetical accident 

conditions. The frequency of the 

accidents that can produce de

fined levels of thermal or mech

anical forces was also developed. 

With this information, the study 

results show that for certain broad 

classes of accidents, spent fuel 

casks provide essentially complete 

protection against radiological 

hazards. For extremely severe ac

cidents, those that could con

ceivably impose forces on the cask 

greater than those implied by the 

hypothetical accident conditions, 

the likelihood and magnitude of 

any radiological hazard were con

servatively calculated. The study 

also contains an evaluation of the 

"* "Shipping Container Response to Severe 

Highway and Railway Accidents," 

NUREG/CR-4829, February 1987. This 

report underwent peer review by the 

Denver Research Institute. The LLNL 

report and documentation resulting from 

peer review are available for inspection and 

copying at the NRC Public Document 

Room, 1717 H Street, NW, Washington, 

D.C. Formal NRC reports are available for 

purchase through the Superintendent of 

Documents, U.S. Government Printing Of

fice, Post Office Box 37082, Washington, 

D.C. 20013-7082.

radiological risk from transporta

tion accidents. Risk represents the 

summation of the products of the 

magnitude and likelihood of all ac

cident outcomes. The purpose for 

making the risk calculations was 

to compare the resulting values 

with those previously used by 

NRC in judging the adequacy of 

its regulations. 

The purpose of this summary, 

prepared by the NRC staff, is to 

present the results of the LLNL 

study to a broad range of readers 

who may possess varying degrees 

of knowledge on the technical 

subjects covered in the LLNL 

technical report. As a result, this 

summary focuses on the overall 

approach and major results of the 

study. Although this summary 

describes many important assump

tions and insights, a complete 

understanding of the scope and 

meaning of the LLNL work would 

require, as a minimum, frequent 

reference to the main LLNL report 

and its supporting appendices. 

For the reader interested solely in 

the results of the LLNL study, the 

figure on the next page, the foldout 

on page 29, and the discussion 

under "Summary of Objective and 

Results" should be consulted. 

Readers wishing to understand the 

logic of the approach and the basis 

for major assumptions should refer 

to the main body of this summary 

report, which presents a step-by

step explanation of the separate 

tasks required to meet the study's 

objectives.

1
OVERVIEW

Accidents With No 

Expected Radiological 

Hazards

" 950 of Every 1000 Accidents 

* "Soft" Target Vis-a-Vis 

Spent Fuel Cask 

* No Fire or Fire of 

Either Limited Extent, 

Temperature, or Duration

S44 of Remaining 50 

Accidents 

One or More of the 

Following Apply: 

* Velocity Too Low 

* Impact Angle Too 

Shallow 

* Object Surface Too Soft 

* No Fire or Fire of 

Either Limited Extent, 

Temperature, or Duration 

Conclusion: 

994 of Every 1000 Truck 

Accidents Generate Forces 

Incapable of Causing Cask 

Functional Damage.

Impact Velocity 

a = Impact Angle 

P = Cask Orientation Angle

A

2

SI i.
OVERVIEW

I.

Summary of Objective 

and Results 

The objective of this study was to 

characterize the level of safety for 

commercial spent nuclear fuel 

shipments should they become in

volved in severe transportation ac

cidents. Researchers evaluated a 

broad spectrum of severe, his

torically documented, truck and 

rail accidents that caused death, 

injury, or significant property 

damage and assessed the minimal 

level of performance that should 

be achieved by NRC-licensed 

spent fuel shipping casks. The 

results, illustrated in the figure on 

the opposite page, indicate that 

no radiological hazard would be 

expected in at least 994 of every 

1000 severe transportation acci

dents. In only about one acci

dent every 40 million shipment 

miles (or once every 13 years 

assuming 3 million shipment miles 

per year) would minor functional 

cask damage be expected. If any 

radiological hazards were created, 

their magnitude would be expected 

to be less than currently-defined 

compliance values in existing 

regulation. In only about one acci

dent every 80 million shipment 

miles could cask damage be 

significant enough to cause a 

radiological hazard which could 

equal or slightly exceed existing 

compliance values. 

The data from documented severe 

accidents had to be extrapolated 

to characterize extremely severe 

accidents for which experience 

provided no models. This process

led to the finding that in about 1 

in 100,000 truck accidents and 1 in 

10,000 rail accidents, extensive 

damage to cask and fuel could oc

cur. In these situations, engineer

ing judgment was used to conser

vatively estimate the resulting 

radiological hazard; however, 

predictions made under such 

unlikely accident conditions are 

subject to uncertainty. 

In an attempt to gauge this uncer

tainty, the study assessed the 

potential for a radiological hazard 

in extremely severe accidents by 

assuming that a spent fuel shipping 

cask with minimally acceptable 

capabilities was involved in the four 

documented severe accidents 

shown on page 29. The most likely 

outcome in three of these four 

accidents would be minor or super

ficial damage to the cask and no 

radiological hazard. In the fourth, 

and under some circumstances in 

two of the three previous accidents, 

a radiological hazard could occur. 

Its magnitude would be less than or 

comparable to the hazard implied 

by compliance values in existing 

NRC regulations. 

As a final point of reference, the 

risk of spent fuel shipments was 

evaluated and compared with 

previous estimates used in assess

ing the adequacy of existing 

regulations. The resulting risk level 

was less than one-third of past 

estimates.

BACKGROUND 

Over the last 10 years, thousands 

of shipments of commercially 

generated spent nuclear fuel have 

been made throughout the United 

States without causing any ad

verse radiological consequence to 

members of the public. In the near 

future, the number of these 

shipments is expected to increase. 

More than 40,000 spent fuel 

assemblies have been used at 

nuclear power plants in the United 

States and are currently being 

stored in underwater "fuel pools" 

at these sites. Under the terms of 

the Nuclear Waste Policy Act 

(NWPA) of 1982, these spent fuel 

assemblies will be placed in a 

Federal Repository for permanent 

storage beginning in 1998. 

Shipments from reactor sites to 

the Repository for ultimate disposi

tion will require increased rail and 

road movement of spent fuel. 

In part, because of the projected 

increase in the number of spent 

fuel shipments, the U.S. Nuclear 

Regulatory Commission (NRC) 

decided to reassess the level of 

safety provided by casks designed 

to existing regulations. 

In large measure, the safety 

associated with spent fuel 

shipments, especially in the event 

of a transportation accident, is 

provided by the casks that contain 

the spent fuel during shipment. 

These casks must meet perform

ance requirements specified in the 

Code of Federal Regulations (10 

CFR 71) and their design must be 

certified by the U.S. Nuclear 

Regulatory Commission.

3
OVERVDEW

Other elements of safety are pro

vided by the Department of 

Transportation's operating require

ments for vehicles and drivers. 

These operating requirements are 

defined under Title 49 of the Code 

of Federal Regulations. 

What Is Spent Nuclear 

Fuel? 

Spent nuclear fuel refers to 

uranium-bearing fuel elements that 

have been used at commercial 

nuclear power reactors. This spent 

(used) fuel contains radioactive 

material resulting from the fission 

process that takes place within the 

reactor. The radioactive material is 

formed within ceramic fuel pellets 

about the diameter of an aspirin 

tablet but twice as thick. These 

pellets are contained in 15-foot

long sealed metal tubes or rods-a 

few hundred per rod. From about 

50 to 400 of these rods are 

grouped in a square array to form 

a spent fuel assembly. 

When spent fuel is removed from 

the reactor, the self-sustaining 

fission process has stopped; 

however, spent fuel assemblies 

still generate significant amounts 

of radiation and heat. This heat 

and radiation are caused by the 

"radioactive decay" of the pro

ducts of the fission process. The 

actual material emitting the radia

tion is, for the most part, still con

tained within the ceramic fuel 

pellet. Some material, however, 

mainly in gaseous or volatile form,

can leave the pellet. This material 

is normally contained within the 

metal fuel rods that surround the 

pellets. 

The heat and radioactivity in spent 

fuel necessitates that any ship

ment be made in containers or 

casks that provide the necessary 

degree of public protection. In 

practice, this means a cask must 

shield and contain the radioactivity 

and dissipate the generated heat. 

How Is Safety 

Achieved? 

Safety in the shipment of spent 

nuclear fuel is achieved by a com

bination of factors including the 

physical properties of the spent 

fuel itself, the ruggedness of the 

container or cask containing the 

fuel, and the operating procedures 

and controls applicable to both the 

cask and the vehicle transporting 

the cask. If a transportation acci

dent should occur, safety is 

primarily assured by the integrity 

of the spent fuel shipping cask. 

The design of all casks used to 

ship commercially generated fuel 

in the United States must meet 

performance-oriented requirements 

specified in Federal and interna

tional regulations. The perform

ance requirements include the 

definition of a series of "hypo

thetical accident conditions," 

described on the opposite page. 

All licensed casks must be capable 

of withstanding the mechanical 

and thermal loadings imposed by 

these conditions and still meet 

specified acceptance criteria.

These acceptance criteria include: 

(1) stringent limits on both the 

maximum allowable release of 

radioactive material and the radia

tion levels outside of a cask and 

(2) requirements regarding cask 

configurations which assure that 

subcriticality of the spent fuel is 

maintained. 

In practice, NRC verifies con

formance with these acceptance 

criteria by analyses demonstrating 

that essentially no permanent 

deformations or excessive temp

eratures occur within a cask's con

tainment shell following the 

sequentially applied loadings im

posed by hypothetical accident 

conditions. Demonstrations that 

casks can withstand these condi

tions, coupled with information 

about cask designs and construc

tion materials, suggests that casks 

should be capable of withstanding 

far greater mechanical and thermal 

loadings during an accident than 

those caused by hypothetical acci

dent conditions without causing 

any significant radiological hazard. 

The LLNL quantifies this capability 

through two supporting analytical 

assessments. The first identifies 

actual documented accidents in 

which mechanical and thermal 

loads would be less than those im

plied by the hypothetical accident 

conditions. The second identifies 

accidents (and their likelihood of 

occurrence) in which loads could 

exceed those specified in the 

regulations and evaluates the 

capability of a cask to continue to 

function safely under such 

conditions.

4
OVERVIEW

Standards for Spent 

Fueý Casks 

For certification by the NRC, a 

cask must be shown by test or 

analysis to withstand a series of 

accident conditions. These condi

tions have been internationally ac

cepted as simulating damage to 

spent fuel casks that could occur 

in most severe credible accidents. 

The impact, fire, and water

immersion tests are considered in 

sequence to determine their 

cumulative effects on one pack

age. A separate cask is subjected 

to a deep water-immersion test. 

The details of the tests are as 

follows: 

Impact 

Free Drop (a) 

- The cask drops 30 

feet onto a flat, horizontal, unyielding 

surface so that it strikes at its 

weakest point. 

Puncture (b) 

- The cask drops 40 

inches onto a 6-inch-diameter steel 

bar at least 8 inches long; the bar 

strikes the cask at its most vulnera 

ble spot. 

Fire (c) 

After the impact tests, the cask is 

totally engulfed in a 1475°F thermal 

environment for 30 minutes. 

Water Immersion (d) 

The cask is completely submerged 

under at least 3 feet of water for 8 

hours. A separate cask is completely 

immersed under 50 feet of water for 

8 hours.

Insights on the Safety 

Provided by Typical Spent 

Fuel Shipping Casks 

Over the last decade, considerable 

experimental and analytical 

evidence has been gathered to 

provide insights into the safety 

provided by spent fuel shipping 

casks. The most dramatic 

evidence has involved full-scale 

crash tests carried out both in this 

country and in Great Britain. 

Trucks and rail cars carrying casks 

have been run head-on into 

massive concrete barriers at 

speeds from 60 to over 80 mph. 

Casks have also been struck by 

locomotives travelling at 100 mph 

and have been immersed in fires 

in which temperatures have been 

deliberately kept high. In all tests, 

the resulting cask damage ranged 

from superficial to very minor. 

These results certainly attest to 

the overall ruggedness of the 

casks tested and the general in

tegrity of their design. From an 

analytical standpoint, the most 

notable effort to provide insights 

into the safety of spent fuel 

shipments involved the preparation 

of a generic environmental state

ment on the shipment of all 

radioactive materials, including 

spent fuel.* This study included 

an evaluation of the risks from 

transportation accidents involving 

shipments of radioactive material. 

Risk is a measure that multiplies 

all potential radiological hazards by 

" "Transportation of Radioactive Material 

By Air and Other Modes," NUREG-0170, 

December 1977.

5

0 •0 

b.

C.
their individual likelihood of occur

rence and sums the results. The 

risk associated with all radioactive 

material shipments was so small 

that the Nuclear Regulatory Com

mission judged that its regulatiors 

regarding the packaging of these 

materials were adequate and not 

in need of immediate change. The 

Commission did call for continuing 

efforts to further understand the 

hazards and risks posed by the 

transportation of radioactive 

material. The LLNL study is one 

result of that effort. 

Accident 3 Scsnarios 

Spent fue! shipments could be 

subjected to a variety of trans

portation accident situations or 

scenarios. Identifying these poter 

tial scenarios began with the 

historical data from typical truck 

accidents that involved deaths and 

injuries or those that exceeded 

certain levels of property damage. 

Data from minor accidents (e.g., 

fender benders) were excluded. 

Hi gh 7va V 

Most of the information on the 

likelihood of single and multi

vehicle accidenis in the figure on 

the opposite page is based on 

historical data. The solid lines 

show accident scenarios derived 

from the historical data whereas 

the dashed extrapolations consider

the potential effects of cask im

pacts with a variety of hard ob

jects or surfaces. Impacts with 

these types of objects or surfaces 

have the greatest potential for 

causing damage. The extrapolation 

was made by merging docu

mented accident data with statis

tical data representing highway 

terrain and adjacent structures. 

This data was obtained from 

recorded information and by 

surveying hundreds of miles of 

typical irLerstate highway to deter

mine hovw, frequently surfaces and 

objects such as large bridge col

umns or hard rock surfaces occur. 

Most spent fuel shipments will be 

made over such interstates. 

T'IUCK ACCIDENT SPEEDS 

1.0 

Applicable ao Vehicle- Highvway Object Impacts 

TS; 0.5 

LL Applicable\,

slmph)

S

WVE VIEV

The historical data also provided 

the basis for developing speed 

distributions typical of the acci

dents (see the figure on this page). 

The speed distributions were 

based on (1) estimated vehicle 

speeds at time of impact; 

(2) speeds attained in falls (where 

fall heights were calculated from a 

survey of bridge heights along in

terstate highways); or (3) combi

nations of these speeds. For the 

truck-train scenario, the train 

speed distribution reflects the 

historical data applicable to grade

crossing accidents. 

Historical data on accident-related 

fires was limited to statements of 

whether or not a fire occurred. In

formation on the duration and 

-temperatures of fires, and oi their 

location with respect to a vehicle's 

cargo was extremely sparse. As a 

result, the environments typicEl of 

accident-related fires had to be 

assessed through an engineering 

model. This model is discussed in 

the -following section on railroad 

accident scenarios.
OVERVIEW

Occurrence Rates for 

Truck Accident 

Scenarios

t Rounded values 

* Accident sequences subsequently 

shown to have the most likely 

possibility of causing cask damage 

mmr Developed extensions of historical 

scenario data

soft~ Ib.'co- ~ estrtaa 

<Motoroyvoldabos 005* " nlrel~ + p d o " ... 

0.06 

0,Ng-fied objeot Track, boo 

0.,20 

0.018 

0.000 

.+ + +o m +j;? ++;++++ ++ ++ ++++++ ++++ ++++++++ +++;W + r :++te +++ + ++ 

Resbodroodhe i~u~m~•'[+ :+ :" T-+'+ +: : ++?;0.70 ; 

++ +0 6 ++o +++ s+ ra: ls0 ; ;++++ C srlt+++ ++++ +;+ + + 

ST+'• :biJ: - r-d u tI. buf rft S, o ~ • : 2 :::... ..... ..... 

" ' "l~r~n +': : Herd f-+ oob 

+++o ;:;;m +++++ +++ ++++++ +++++. +++ ++]+: ++ 0 .0 6 0 2 ++ ++

0.83 Call.. 

Lay7 .r

Ot roa•o i-Xd objct C Io-n abutrcom 0. 12 0.004

0.04 

Conoct* Objac" o ct. tto ..... tra 

ftWoO battler. waO, prost 0.45 

C+++ url. t• lte-rt " - ;+ ++++ 

Inte tlepe Hard Sil/sott -1, 0.28 0.07 

/++0,+ il Herd rook 0.01 

D.0 

T-ee 

0.10

roa. -db.d S...... . .. ... .. ........ .. .... . J a c k k n ife 

0,40 

Other M-iaqfos rocealea odirsg) : ++++ ' 5: 64+ u + 0.00+++ 

Ryeoudy+~m .i=io ++

I

7

I.

0.74

OCCURRENCES 

PER 1000 ACCIDENTSt 

34 

8 

432 

133 

8 

38 

1 

4 

0.08 

0.006 

0.001 

0.3 

0.06 

0.01 

0.9

INDEX 

1 

2 

3 

4 

5* 

6 

7* 

8* 

9* 

10* 

11* 

12* 

13* 

14* 

15

0.35

40 16 

5 17 

37 18 

23 19* 

2 20* 

0.3 21" 

13 22* 

1 23* 

0.2 24* 

9 25 

9 26 

33 27 

83 28 

55 29 

20 30 

10 31
OVERVIEW

Railroad 

Railroad accident scenarios were 

also based on documented rail ac

cidents involving deaths, injuries, 

or property damage exceeding 

small thresholds. This historical 

data provided the bases for the 

likelihood of accident scenarios 

shown in the figure on the op

posite page. The dashed lines in

dicate accident scenarios derived 

mainly from route survey informa

tion. They were developed to 

more accurately determine the 

types of accidents with the poten

tial to cause functional damage to 

the cask. 

The available historical fire

accident data, pertinent to both 

rail and truck accidents, could not 

be used to determine potential 

thermal loadings on casks. 

Therefore, an existing computer 

code, previously developed to 

characterize transportation acci

dent fires, was used to estimate 

the likelihood of fire temperatures 

and durations. The code evaluated 

data on accident type, cause of 

fire, availability of combustibles, 

fire-fighting efforts, and combusti

ble burning rates to predict the 

likelihood that fire temperatures 

and durations would reach specific: 

values. The top graph on this 

page shows this evaluation for 

railroad collision and derailment 

accidents. The bottom graph gives 

the results of the evaluation ap

plicable to temperatures for both 

truck and rail accidents.

These results, which included 

several conservative assumptions, 

were used to represent transporta

tion accident fires. For example, 

for railroad accidents involving colY

lisions, about 10% of all fires were 

estimated to last longer than 

2 hours. Temperatures in over half 

of such accidents were estimated 

to exceed 1800 0F.

FIRE DURATIONS 

AND TEMPERATURES

0 

(n 

)CD 

C.2 

CU 

U-- Collisions

01

11

t(hrs)

1.0 

LL 

oI

C-I0') 

">co 

qi 0.5 

<U 0. 

U,'• L0 

.LL~

1400 1800 2200 

T(°F)

8
OVERVIEW

Occurrence Rates for 

Railroad Accident 

Scenarios

t Rounded values 

* Accident sequences subsequently 

shown to have the most likely 

possibility of causing cask damage 

mu Developed extensions of historical 

scenario data

RailI-hlghwaY gr-1e -- i-9 0.03 

SflRemain 00n trock 

0.00

0.13

Daraltirent (with C01ODtoo) 0.30

0-e -nb. G.01

Derailment tot0al) Into $lop. 0.02 0.02

002 

Over bridge Hard rok .oil/ofl • .rote 

0.01 0.00 ' 

Hard rock 

ArIIId. roadbed 

Drain diOch 038 

ankment 

Hard ooI/aoft m'ob 0.06 

Hord rook I0.027 

2Ci 9dy. outt 

Hard soo/osoft rook 

0,07: 

Hard rrok 0r 0t

Column 0,003

Into OREtrurO

Darailmnt )wthout colE o iobano 0W77

1, 7oor 1•76

SoreR 0.83

Abtment 

Other 0.00 

'Locomotive 

C.2w 

coll~obn Car... . 

0.23 0.71 

Eopart 

0.06 

N -oadbed 

0.33h 

Noo-o~boOo

0tbar 

0.07

9

Train l~TrIO ecldt par n-b

OCCURRENCE' PER 1000 ACCIDENTSt 

30 

86 

0.2 

0.1 

0.01 

0.002 

6 

3 

5 

0.4 

0.07 

14 

1 

0.2 

0.5 

0.1 

0.02 

164 

33 

100 

8 

160 

320 

65

INDEX 

1 

2 

3* 

4* 

5* 

6* 

7* 

8 

9* 

10* 

11" 

12* 

13* 

14* 

15 

16 

17 

18 

19 

20 

21" 

22 

23 

24

i I

i+ i
CASK CHARACTERIISTICS 

AND RESPONSES

Can Cask Safety Be 

Characterized 

in Real-World 

Accidents? 

This question was critical to the 

credibility of the LLNL study. The 

answer was "Yes." An approach 

to the problem could be followed 

to allow fair characterization of the 

minimal level of safety that would 

be meaningful to an assessment ol 

the adequacy of existing regulatory 

requirements. The first step taken 

in this approach was to define two 

representative cask designs-one 

for truck shipments and one for 

rail shipments. In both cases, the 

casks were designed to just meet "regulatory" acceptance criteria 

following an accident with mech

anical, thermal, and water

immersion accident conditions 

depicted on page 5. The cask 

designs included only those 

features absolutely necessary to 

determine a cask's ability to 

achieve its primary safety func

tions. (These safety functions and 

the cask features that achieve 

these functions are discussed briefly 

on pages 11 through 13.)

Representative 

Designs for Tru 

Rail Casks 

Impact Limiter-.

Item Weight, lbs 

Body 32,000 

Limiter 4,500, 

Contents 2,500 

39,000

Item Weight, lbs 

Body 122,500 

Limiter 22,500 

Contents 52,500 

197,000

All Dimensions in Inches 

ss - Stainless Steel 

Pb = Lead 

H2D 0 Water

ick and

Neutron Shield 13 (Diameter of Cavity)

TRUCK CASK

RAIL CASK

Note: 

The representative truck and rail casks 

consist of stainless steel cylindrical 

shells that enclose a ring of lead 

shielding material. A water jacket sur

rounds this cylindrical structure. At 

each end of the cask, an "impact 

limiter" is provided to protect the cask 

against impact forces.

--- I

10
CASK CHARACTERISTICS 

AND RESPONSES

Once these representative cask 

designs were defined, they were 

subjected to the most damaging 

accident scenarios identified on 

pages 6 through 9 to determine 

their structural response. By 

measuring structural response, 

researchers estimated their poten

tial for a radiological hazard. If the 

potential existed, the magnitude of 

the radiological hazard was con

servatively evaluated. Through this 

process, that fraction of severe rail 

and truck accidents capable of 

causing a specified radiological 

hazard was estimated. The 

radiological hazard was then com

pared with compliance criteria in 

existing NRC regulations. 

As an additional point of 

reference, the radiological risk of 

shipping commercial spent fuel 

was compared to documented 

estimates used by the NRC in 

making its past judgment on the 

adequacy of existing regulations 

(see insert on page 5). 

Schematic of Spent 

Fuel Cask

- S.

Cask Safety Functions 

and Representative 

Cask Design Features 

The primary cask safety functions 

include: (1) containment of 

radioactive material, (2) shielding 

against the radiation emanating 

from the spent fuel, and (3) as

surance that subcriticality of the 

fuel is maintained. 

Containment is achieved by retain

ing the radioactive material within 

a closed vessel. Typically, contain

ment is provided by the integrity 

of the spent fuel cladding and by 

the cylindrical steel containment 

vessel or inner cask shell (see 

figure below). The vessel is pro

vided with a bolted-end closure to 

permit loading and unloading. The 

closure contains a seal between 

Seal Area-,

the cask cavity and the environ

ment that prevents leakage. Piping 

penetrations, which terminate in 

protected enclosures, are also pro

vided for operational purposes. 

The required containment safety 

function is achieved by these 

features. Furthermore, the suc

cessful functioning of these 

features is promoted by (1) an ex

ternally located, energy-absorbing 

structure designed to protect the 

cask against impacts, and (2) the 

integration of the containment 

features into an overall cask 

design that maximizes protection 

provided against outside forces. In 

defining a representative cask, the 

complexities associated with 

various designs for containment 

closures, penetrations, and seals 

were not modeled. The failure of 

these features was assumed if the 

containment or inner shell was 

calculated to incur any significant 

permanent structural damage.

11
CASK CHARACTERII:STICS 

AND RESPONSES

Shielding is provided against 

gamma and neutron radiation. 

Protection against gamma radia

tion which is very penetrating is 

most important and is achieved 

through use of heavy materials 

such as lead, uranium, or steel 

that reduce the radiation level. 

This material surrounds the con

tainment vessel as seen in the 

schematic on page 11. Protection 

against neutron radiation is often 

provided by water, which typically

llpnV

fills a jacket surrounding the main 

cask body. Loss of the neutron 

shield normally results in a small 

increase in external radiation 

levels, but to a value that is within

regulatory limits for transportation 

accidents. Failure of the neutron 

shield was assumed to occur for 

all accidents considered in this 

study. As a result, only the lead 

gamma shield was modeled in 

some detail in the representative 

cask designs.

structural materials. The "poisons" 

are typically included in the solid 

structure or "basket" holding the 

fuel assemblies and absorb emitted 

neutrons, thereby making a "chain 

reaction" impossible and thus 

assuring subcriticality. Before the 

fuel basket can incur any signifi

cant damage, the total cask struc

ture, including the containment 

Er 1 0

Subcriticality is assured by either 

limiting the amount of spent fuel 

being shipped or by maintaining 

control of the spent fuel confi

guration during shipment and in

cluding "neutron poisons" in cask

A spent fuel cask being loaded on a 

truck-front end impact-limiter shown 

at right of truck.

Note: 

Actual spent fuel casks like the one 

shown in this figure are expected to 

perform their intended safety functions 

during an accident better than the 

representative cask designs assessed in 

this study.

12
CASK CXARACTEMSTDCS 

APD RESPONSES

shell, would have to be severely 

damaged. However, physical 

damage alone does not affect a 

cask's ability to maintain sub

critical conditions. A material like 

water must surround the cask and 

fill the area between individual fuel 

rods and fuel assemblies before 

criticality would be possible. For 

these reasons, the features to 

assure subcriticality are not speci

fically modeled in the represen

tative cask designs. Instead, an 

upper-bound estimate of the 

likelihood of criticality is provided 

in the LLNL report. The estimate 

is based on the type of accident 

that could substantially deform a 

cask in the presence of a material, 

like water, that would promote 

criticality. A brief discussion of 

this estimate is presented in the 

section on potential hazards and 

risks on page 26. 

What Constitutes a 

Severe Transportation 

Accident? 

In this study, a severe accident is 

one that could compromise one of 

three basic cask safety functions: 

(1) any loss of containment of 

spent fuel material, (2) a degrada

tion or reduction in cask shielding 

capability, or (3) a loss of sub

criticality control. Any of these oc

currences could potentially create 

a radiological hazard.

V

Severe accidents typically involve 

impacts with massive and hard ob

jects or surfaces or exposure to 

high-temperature fires of long 

duration. The scenarios shown on 

pages 7 and 9 are those that 

could compromise a cask's safety 

functions and potentially cause a 

radiological hazard. 

Given the ruggedness and massive

ness of spent fuel casks, a severe 

accident in this study would not 

include tragedies involving collisions 

between the vehicle transporting 

the cask and an automobile or bus 

in which several people might be 

killed or injured. Although potential

ly serious to the occupants of such 

vehicles, collisions with automobiles 

and buses at any speed involve 

forces that would not seriously 

compromise cask safety functions. 

Any deaths or injuries from such 

accidents would not be caused by 

the radioactivity of the spent fuel 

cargo. 

Estabtlshing a Scala to 

Measuire Cask 

Response 

Mechanical Loads

Measure of Cask 

Response 

A cask and the nuclear fuel it con

tains can undergo various types of 

damage when subjected to mech

anical loads. The most significant 

damage would include material 

yielding, dimensional changes, and

rupture of the cask. The most 

common engineering guidelines 

used to characterize structural 

damage are stress, strain and 

displacement. Strain, particularly 

on the inner "containment" shell 

of the cask, was selected as the 

best single indicator to charac

terize cask damage following a 

transportation accident. Sensitivity 

studies established a relationship 

between the strains at different 

cask locations and the maximum 

strains experienced in the cask 

containment shell. As a result, it 

was possible to use a specific 

strain in the cask shell to estimate 

damage to cask components such 

as seals, closures, and 

penetrations. 

Three discrete levels of strain were 

defined to encompass four broad 

ranges of cask and fuel damage, 

as shown in the figure on the 

following page. The significance of 

the 0.2-, 2-, and 30-percent strain 

values, in terms generally indicative 

of cask and fuel damage, is also 

illustrated on page 14.

13
CASK CHARACTERIISTICS 

AND RESPONSES

Thermal Loads-Measure 

of Cask Response 

Heat from a fire can conceivably 

damage cask components, the 

cask structure itself, or the spent 

fuel. The more important types of 

damage can involve degradation of 

cask seals, melting of the lead 

gamma shield, or structural failures. 

The significance of high tempera

tures on spent fuel is that it can 

eventually cause the fuel rods to 

rupture and release radioactive 

material into the cask. 

The temperature aT the centerline 

of the cask's gamma radiation 

shield is the indicator most likely 

to reveal the extent of cask 

damage from fires associated with 

transportation accidents. Four 

temperature levels are defined to 

categorize five ranges of cask and 

fuel damage. These response 

ranges are indicated in the next 

column at the bottom of the page 

What Does Strain 

Measure? 

When subjected to a force, the 

steel used in the cask containment 

shell Gan change dimension. The 

change in dimension of any seg

ment of the steel shell along a 

given direction, when divided by 

the original length of the segment 

in that direction, is termed 

",strain." Strains experienced by 

materials under design loads are 

typically small, except for a few

Measures of Cask and 

Spent Fuel Response 

to Mechanical Loads

Q) 

-C) 

co 

C 

0 

U 

U 

U) 

E 

5) 

Er 

S

30 

2

0.2 I

0

Damage Exceeds 

Quantifiable Range: 

"* Fracture or Ruptures Possible 

"* Large Cask Deformations 

Significant to Major 

Damage: 

"" Large Deformations 

Possible "* Strain Below Material 

Rupture Levels 

Minor Damage: 

* Some Small 

Permanent Yielding 

of Containment Shell

Superficial Damage, 

If Any: 

* No Permanent 

Yielding of Containment Shell 

CASK DAMAGE 

DESCRIPTOR

E 

C 

5) 

5)

30 

2

0.2

0

All Fuel Rod 

Caddings Assumed 

to Be Breached 

All Fuel Rod 

Claddings Assumed 

to Be Breached 

Claddfing Failure. 

Assumed in 10%ý 

of Fuel Rods 

No Fuel Cladding Failures Beyond 3% 

Assumed to Result 

From Normal 

Oper~ations 

FUEL DAMAGE 

DESCRIPTOR

E 

M

Measures of Cask and 

Spent Fuel Response 

to Thermal Loads

Cask 

Damage 

Descriptor 

Fuel 

Damage 

Descriptor

No Functional No Grosa Localized Shield Complete Shield Gross Structural 

|Damage Structural Effects; Melting Dependent Melting Dependent Damage Possible 

Seal Degradation on Shield Material; on ShieldMaterial; Due to Thermally 

Possible Seat Degradation Functional Loss Generated 

Likely of Cask Seal Structural Forces 

0 500 600 650 1050 

Increasing Damage 

No Fuel Cladding Failures Beyond 3% Assumed -10 All Fuiel Claddlings Assumed 

toResult From Normal Operations to Fail Due to Overpiressure 

0 500 600 650 1050 

TEMPERATURE ('F) AT CENTERLINE OF CASK GAMMA SHIELD

--- I

14
CASK CHARACTERISTICS

materials such as rubber. For a 

ýgiven material, the measure of 

strain can indicate whether a 

material wil remain elastic and not 

deform or permanently yield or 

fracture and result in a rupture. 

What Does Temperature 

IMeasure? 

The temperature at points within a 

massive spent fuel cask can indi

cate the amount of heat absorbed 

from external sources such as fires 

and can also indicate potential 

cask or fuel damage. The cask 

.can be damaged by the degrada

tion of seals or the melting of the 

,gamma radiation shield. For the 

"spent fuel, the pressure of gases 

within the fuel rods, and the 

strength of the fuel cladding is 

strongly influenced by tempera

ture. If temperatures become high 

enough, fuel rods could rupture 

and release radioactive material in

side the cask. 

Evaluating Cask and 

Spent Fuel Response 

to Accident Loads 

On the previous two pages, cask 

containment strains and centerline 

shield temperatures were defined 

separately to characterize broad 

categories of cask and fuel 

damage. In real transportation ac

cidents, however, a cask could 

undergo a combination of mech

anical and thermal loads. The 

"11cask response matrix" shown on 

this page therefore combines the

Cask Response 

(Damage) Regions

0, 

O-c 

•o 

U) 2 

tc E 

S0.2

R (4, 1) R (4,2)

Note: 

The size of each region or group of 

regions has no relationship to the 

likelihood of accidents causing the 

described damage level.

R (4,3)> R (4,4) R (4, 5)

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)

500 600 650 1050

Thermal Response, 'F 

(Centerline Lead Temperature)

SUPERFICIAL DAMAGE-No perma

nent deformation to containment 

vessel. Temperatures too low to 

degrade material. Strains and 

temperatures less than or equal to 

values considered acceptable following 

imposition of "regulation-defined" 

hypothetical accident conditions. 

MINOR DAMAGE-Limited permanent 

containment vessel deformations. 

Temperatures approaching the range 

where the lead shield could melt and 

the seals could degrade.

MAJOR DAMAGE-Large containment 

vessel deformations without gross frac

tures or ruptures. Temperatures high 

enough to melt lead shielding. 

DAMAGE EXCEEDING DEFINABLE 

RANGES--Fractures or ruptures possi

ble. Temperatures sufficiently high to 

affect cask and spent fuel integrity.

15
CASK CHARACTERISTICS 

AND RESPONSES

structural and thermal responses to 

categorize cask damage from all 

possible combinations of mechani

cal and thermal loads. 

The process of categorizing cask 

response for a specific accident 

scenario is best described by an 

example. From the figure on 

page 7, scenario 20 indicates that 

about 2 of every 1000 truck acci

dents are expected to result in an 

impact into a slope consisting of 

hard soil or soft rock. Cask 

damage from this type of accident 

can be estimated (in terms of 

maximum containment vessel 

strain) if truck velocity, angle of 

impact, and cask orientation at im

pact are specified. Similarly, if a 

fire occurs during this accident (an 

event expected in about 1 of every 

100 slope-impact accidents), 

damage to the cask can be 

estimated in terms of temperature 

at the centerline of the lead shield 

if the fire temperature, duration, 

and cask location relative to the 

fire are specified. The overall cask 

damage for the entire spectrum of 

transportation accidents charac

terized by cask impact with a soft 

rock slope can be calculated and 

placed into one of the response 

regions shown on page 15. 

Two further steps are then re

quired to complete the evaluation 

of the level of safety provided for 

spent fuel shipments. First, each 

response region must be con

sidered in terms of the radiological 

hazard that could result from the 

specified level of cask damage.

This relationship is described on 

pages 16 through 19. Second, the 

likelihood that the specific acci

dent scenario (for example, impact 

into soft rock slope) can lead to a 

cask response within a particular 

region must be evaulated. This 

part of the evaluative process is 

further described on pages 20 

through 27. 

Relationship Between 

Cask Response and 

Potential Radiological 

Hazards 

For most cask responses to 

transportation accident loads, any 

resulting radiological hazards can 

be conservatively estimated with a 

high degree of confidence. 

Relationships of 

Mechanical Loads, Cask 

Response, and 

Radiological Hazards 

For accidents causing small struc

tural strains in the cask contain

ment shell, no radiological hazards 

would be expected since, for less 

than 0.2 percent strain, no signifi

cant permanent deformation 

would occur in the containment 

shell. 

Strains in the 0.2- to 2-percent or 

the 2- to 30-percent ranges were

presumed to cause containment 

functional failure, but without 

gross rupture of the containment 

(see figure on opposite page). The 

lack of any gross rupture is a 

reasonable expectation based on 

the known ductility (that is, the 

ability to stretch without fractur

ing) of the stainless steel material 

typically used in cask containment 

shells. At these strain levels, 

however, the impact loads could 

cause the lead gamma shield 

material to "slump." Where voids 

or gaps in the shield occur, 

radioactivity inside the cask could 

increase radiation levels outside 

the cask (see figure on page 19). 

The major difference between ac

cidents causing 0.2- to 2-percent 

strain as opposed to 2- to 

30-percent strain involves the 

behavior of the fuel rod cladding 

that contains the spent fuel within 

the cask. The lower range was 

assumed to cause failure of up to 

10 percent of the fuel rod clad

ding, whereas at the higher range, 

all rod claddings are assumed to 

fail. In either case, experimental 

information on radioactive releases 

from failed fuel rods is used to 

establish the fraction of gaseous, 

volatile, and solid radioactive 

material that could escape from 

each fuel rod. For the purpose of 

this study, all of this material was 

assumed to be released from the 

cask, although in reality, a large 

but undefinable fraction would "plate out" or adhere to surfaces 

within the cask.

_ L _ _ _ _ _ _ _ I__ _ _ _ _ _ _ _ _ _

16
CASK CHARACTERISTICS 

AND RESPONSES

What Types of 

Radiological Hazards 

Could Be Possible in 

Transportation Accidents? 

The fuel assemblies used in com

mercial power reactors contain 

solid ceramic uranium oxide (UO2 ) 

fuel pellets. During reactor opera

tion, the uranium fuel fissions 

creating radioactive fission and ac

tivation products. Physically, most 

of the radioactive material remains 

in solid form within the pellets, 

although the pellets may exhibit 

some degree of fracturing. 

However, a small fraction of the 

fission products are gases or are 

in volatile form (the amount of 

volatiles being dependent on 

temperature). The radiological 

hazards that could conceivably be 

created by this material can occur 

through two distinct cask-damage 

mechanisms: (1) a release of 

material from a damaged cask or 

(2) an increase in the external 

radiation level emanating from 

material within the cask. 

Material releases can occur in 

gaseous, volatile, or in solid form. 

The solids can be small airborne 

particles or larger pieces. Solid 

particles that could be inhaled can 

pose a significant hazard to 

people. 

Increased radiation levels from 

material still within the cask could 

occur as the result of voids in the 

cask shielding due to mechanical 

forces or temperatures high 

enough to cause shield materials 

to melt.

Typical Radioactive 

Material Release 

Pathway

Presumed if either: 

(1) Cask containment vessel strain 

between 0.2 and 30 percent, 

or 

(2) Centerline gamma shield 

temperature between 500OF 

and 1050OF

Inner Shell 

(Containment)

17
CASK CHARACTER ISTICS 

AND RESPONSES

*1

The radiological hazards from 

accidents causing cask strains 

greater than 30 percent could not 

be precisely predicted because of 

the extensive and potentially 

varied nature of cask and spent 

fuel damage. In these situations, 

all gaseous material was presumed 

to be released while radioactive 

material in volatile and solid form 

was arbitrarily assumed to increase 

by a factor of 10 over the values 

predicted for accidents causing 

strain in the range of 2 to 30 per

cent. Only a very small fraction of 

truck or rail accidents, beyond any 

known accidents, could be severe 

enough to cause strains greater 

than 30 percent in the cask con

tainment shell. 

Relationships of Thermal 

Loads, Cask Response, 

and Radiological Hazards 

Fires resulting from transportation 

accidents can affect a spent fuel 

cask and its contents. If the fire 

does not cause 5001F temper

atures at the cask shield cen

terline, no radiological hazard 

would be expected since cask 

structural components are not 

susceptible to thermal deteriora

tion or damage at temperatures 

below this level. 

If temperatures at the shield 

centerline should reach between 

500OF and 6000 F, certain cask seal 

materials could degrade and lose 

their capacity to function. The

spent fuel within the cask, how

ever, would not reach tempera

tures high enough to fail the fuel 

rod cladding material. As a result, 

any potential radiological hazard 

created by a release of radioactive 

material from a cask would be 

limited to gaseous and volatile 

materials that have escaped from 

fuel rods whose cladding has 

failed during or before the acci

dent for reasons other than the 

fire. Based on past experience, 

3 percent of the fuel rods in a 

shipment were assumed to have 

cracks or breaks as a result of 

their use in the reactor, handling 

and storage before shipment, or 

vibrational loads during normal 

shipment. 

At centerline shield temperatures 

between 600OF and 6500 F, two 

types of radiological hazard could 

be created if lead is used as the 

gamma shield material (as is the 

case for the representative cask 

designs). Lead melts at 6211F and 

expands in volume during the 

melting process. This expansion 

can cause structural stresses that 

can result in loss of the cask's 

containment function. When the 

lead cools and resolidifies, its con

traction can cause voids or gaps 

to form in the gamma shield. 

These gaps degrade cask shielding 

capabilities and so increase radia

tion levels outside the cask, as 

shown in the figure on the op

posite page. In this study, a cask's 

loss of shielding capability was 

calculated as a function of 

temperature. A cask configuration 

that maximizes lead slump and 

subsequent voids, thereby max

imizing radiation levels outside the 

casks, was also assumed.

Between 6501F and 10500 F, re

lease of radioactive material from 

the cask or increased radiation 

levels outside the cask from con

tained material are more likely to 

occur and the magnitude of the 

resulting hazard could become 

larger. The major factor affecting 

the potential radiological hazard is 

the fraction of fuel rods experien

cing cladding failures. For shield 

temperatures in this range, fuel 

rod temperatures can cause clad

ding failures; therefore, any radio

active material in mobile form 

could be released from the fuel to 

the cask. If cask containment is 

compromised, this material could 

reach the environment. Experimen

tal information on the release of 

radioactivity from spent fuel has 

been used to estimate the magni

tude of the potential radiological 

hazard. The conservative assump

tion was made that any material 

released inside the cask would 

escape from the cask to the 

environment. 

If centerline shield temperatures 

exceed 1050 0F, a cask's functional 

capabilities could be affected by 

several complex chemical, thermal, 

and structural processes that can

not be precisely predicted. In 

these situations, all gaseous radio

active material was presumed 

released to the environment 

whereas the release of radioactive 

material in volatile or solid form 

was arbitrarily assumed to increase 

by a factor of 10 over values

18
CASK CHARACTERISTICS 

AND RESPONSES

Typical Radiation 

Level Increase as a 

Result of Lead 

(Gamma Shield) 

Slumping

Fuel Bundle----\

r"

Centerline

1AIAI I E~I I r I

00 0 00 0 0° O 0 

0 oO 

o 0 000 0 0 

00 0 0 0 00 000 0 000 0 0 00 

0 

000 /o O Areas of o° 0o 

o V 0 0Increased 00o 

o o o Radiation 0 o 

0 0 0 0 

00 0 0 ~0 0 0 

S00 00,0 

D o°o °o eve oo°o 

0 

-- Lead Gamma 

Shield 

,--Cask Containment 

Shell

(, 

00 

.Cu 

Cu

I.

Presumed if either: 

(1) Cask containment vessel strain 

exceeds 0.2 percent, or 

(2) Centerline gamma shield (lead) 

temperature exceeds 600OF

Allowable Under 

Accident Conditions

Increasing 

Radiation 

Levels As Void 

(Lead Slump) 

Increases

Distance From Cask 

(log)

19

I

!
CASK CHARACTERlISTICS 

AND RESPONSES

assigned for temperatures in the 

650OF to 1050OF range. As is the 

case for accidents causing ex

tremely large structural strains, no 

historical truck or rail accident 

could be specifically identified that 

would have the potential to cause 

shield temperatures above 1050 0 F. 

Cask Damage-What 

Accident Conditions 

Are Important and 

How Are They 

Defined? 

Damage Caused by 

Mechanical Loads 

The most important accident con

ditions used to define the 

mechanical loads imposed on a 

cask during an accident are those 

associated with various impacts. 

Because of the large weight, hard

ness, and rigidity of spent fuel 

casks, loads caused by crushing, 

by projectiles, or by other mecha

nisms have been demonstrated to 

be far less damaging than loads 

caused by impacts with hard, 

massive objects. As in any impact 

involving a motor vehicle or train, 

the damage sustained would de

pend on vehicle speed, the angle 

of impact (a head-on or a side

swiping impact), the hardness and

massiveness of the object struck, 

and the orientation of the vehicle 

or object at the time of impact 

(front, rear, or side impact). 

*Velocity at Impact 

Potential cask velocities on impact 

were principally based on records 

of truck and rail accidents. The 

truck information shown on 

page 6 was derived from a sample 

of truck accidents causing fatal

ities or injuries reported by the 

California Highway Patrol. The rail 

information was derived from 

mainline accident data available 

from the Federal Railroad Ad

ministration. For accidents involv

ing falls, the velocity of impact 

was based principally on a survey 

of bridge heights along a typical 

section of interstate highway. The 

velocity of trains involved in truck 

impacts was derived from rail

highway grade-crossing accident 

information. 

* Angle of Impact 

The angle of impact between a 

cask-carrying truck or rail car and 

the object or surface hit was 

estimated for each of the accident 

scenarios shown on pages 7 and 9. 

For example, head-on impacts 

with objects such as bridge 

abutments and columns were 

estimated to be far more likely 

than a side-swiping impact. 

Specifically, about 40 percent of 

all impacts with columns or

abutments were assumed to occur 

at an angle less than 200 from 

head on. About 21 percent were 

estimated to occur within 100 of 

head on. 

0 Hardness of Object Struck 

The hardness and massiveness of 

the object struck was determined, 

for the most part, by the informa

tion from the accident scenarios 

described on pages 7and 9. Sur

faces, such as hard rock, soft 

rock, and clay/silt, were modeled 

to provide a conservative represen

tation of the variety of possible 

surfaces occurring within these 

three "earth" classifications. 

*QOrientation at Impact 

Cask orientation on impact was 

estimated for each accident 

scenario similar to the process 

used to determine the possible 

angles of impact. For impacts with 

slopes or in impacts with other 

vehicles, any orientation was con

sidered equally likely. For impacts 

with bridge columns and abut

ments, all orientations were con

sidered possible, but the most 

likely orientation was estimated to 

involve an impact with the front 

end of the cask.

20
CASK CHARACTERISTICS 

AND RESPONSES

Damage Caused by 

Thermal Loads 

The temperature of an accident

generated fire is the most impor

tant consideration in assessing 

potential cask functional degrada

tion. The cumulative heat affecting 

a cask depends not only on the 

temperature and duration of the 

fire but also on the extent to 

which the cask is exposed. Data 

on fire temperatures and durations 

are not readily available in 

accident records; however, con

servative estimates of fire tem

peratures and duration can be 

calculated based on pertinent in

formation about the accident. For

example, the thermal loading to a 

cask involved in a collision with a 

tanker carrying flammable cargo 

can be estimated by knowing the 

maximum volume carried by a 

typical tank truck and the nature 

of the product being shipped (for 

example, gasoline). For accidents 

involving trucks or trains carrying 

nonflammable cargo, knowledge 

of fuel tank volumes and the types 

and amounts of combustible 

material typical of truck or rail car 

construction is sufficient to allow 

similar conservative estimates to 

be made. 

The only accident condition that 

could not be based, even quali

tatively, on recorded accident data

was the location of a cask relative 

to a fire resulting from a transpor

tation accident. In the absence of 

recorded data, the researchers 

provided estimates that would be 

prudently conservative. The result 

was a presumption that in all ac

cidents involving fires, a truck 

cask would be located at or within 

31.5 feet of the fire center, the 

chance of any specific location 

within this range being equally 

likely. For rail casks, this location 

parameter was broadened slightly 

to encompass a range of 0 to 43 

feet. Beyond these ranges, the 

thermal loads were not significant.

21
POTENTIAL HAZARDS 

AND RISK

Fraction of Accidents 

Without Any Expected 

Radiological Hazards 

For every 1000 truck or rail ac

cidents involving spent fuel 

shipments that are capable of 

causing injury, death, or signifi

cant property damage, 994 would 

be expected to cause no signifi

cant radiological hazard. This 

estimate took into consideration 

cask responses to both mechanical 

and thermal accident loadings. 

Mechanical Forces 

* Responses to "Non-Severe" 

Transportation Accidents 

How the cask responded to 

mechanical forces was first con

sidered for the objects identified in 

the accident scenarios described 

on pages 7 and 9. Estimates were 

made of the maximum forces that 

could be generated by each object 

or surface when struck at any im

pact velocity. These estimates 

were compared to the force 

necessary to cause a cask's con

tainment structure to begin to per

manently yield or deform. Througlý 

this comparison, many scenarios 

involving impacts with "soft"

targets are shown to cause no 

functional damage to a cask (see 

opposite page). (These scenarios 

are shown without an asterisk on 

pages 7 and 9.) To illustrate this 

process, consider damage to a 

truck caused by a variety of colli

sions with animals and pedestri

ans; motorcycles; automobiles; 

other trucks; and, finally, fixed ob

jects. Collisions with animals, 

pedestrians, motorcycles, and, to 

some degree, with automobiles 

typically cause little truck damage. 

These objects are "soft" relative 

to the truck, and as a result incur 

most of the damage sustained in 

the accident. Shipping casks are 

massive, heavy structures so that 

the objects so indicated on pages 

7 and 9 are indeed "soft" relative 

to the cask. 

Summing the accident rates for 

truck accident scenarios involving 

impacts with a "soft" object pro

vides a basis for concluding that 

these accidents describe about 950 

out of every 1000 truck accidents. 

Such accidents would be unlikely 

to cause any functional cask 

damage. For the railroad accident 

scenarios, "soft" object impacts 

would occur in about 960 of every 

1000 railroad accidents. 

* Responses to "Hard" Object 

Impact Accidents 

In accidents involving cask im

pacts with potentially massive 

and/or hard objects (see the

scenarios marked with asterisks on 

pages 7 and 9), the possibility of 

cask functional damage is con

trolled by accident-specific 

parameters. For example, a truck 

carrying a spent fuel cask could 

hit a bridge column at 60 miles 

per hour. If the truck and cask 

side-swipe the column, however, 

the effective impact velocity (cask

vehicle velocity perpendicular to 

the column) could be only a few 

miles per hour and the resulting 

forces would be insufficient to 

damage the cask functionally. A 

second possibility is that the truck 

hits the bridge column or abut

ment head on but the truck and 

cask are traveling at less than 

30 mph. Because current regula

tions require that a cask be sub

jected to a 30-mph impact on an 

unyielding surface without sustain

ing unacceptable damage, any im

pact of less than 30 mph on a 

generally flat surface would not be 

expected to cause functional 

damage. When these combina

tions of possible accident 

parameters are taken into account, 

at least 44 out of every 50 ac

cidents involving impacts with "non-soft" objects or surfaces 

would be expected to cause no 

functional damage to a cask. The 

same outcome is anticipated for 

railroad accidents: conversely 

stated, a maximum of about 6 ac

cidents out of every 1000 have the 

potential to cause some degree of 

cask functional damage.

- a

22
POTENTIAL HAZARDS 

AND RISK

Accident Scenarios 

Generating Mechanical 

Forces Incapable of 

Causing Functional 

Cask Damage

r\ 950 of Every 1000 Accidents

* "Soft" Target Vis-a-Vis 

Spent Fuel Cask

r44 of Remaining 50 

Accidents

One or More of the 

Following Apply:

* Velocity Too Low 

* Impact Angle Too Shallow

Conclusion:

994 of Every 1000 Truck 

Accidents Generate 

Mechanical Forces 

Incapable of Causing 

Cask Functional Damage.

Impact Velocity

a = Impact Angle

fl = Cask Orientation Angle

23

I

Q !E
POTENTIAL HAZARDS 

AND RISK

---- I Y

Thermal Forces 

Cask damage from fires could 

cause melting of the lead shield or 

degradation of the closure seal. 

Either form of damage requires 

that the affected component reach 

temperatures in excess of 5000 F. 

The mass and heat capacity of 

spent fuel casks are large. For a 

truck cask to reach such a tem

perature, it would have to be 

engulfed in a 1700OF fire for over 

an hour. For the larger represen

tative rail cask to sustain equiv

alent damage, it would have to be 

engulfed for an estimated 

1.35 hours. With few exceptions, 

only about 1% of the accidents in 

the truck and rail accident 

scenarios listed on pages 7 and 9 

involve fires. Many of these fires 

would be fed by diesel or gasoline 

fuel from the truck or other vehi

cle involved in a highway acci

dent, or from diesel fuel, 

lubricants, and rail car structural 

materials in railroad accident 

scenarios. These types of fires 

would not be expected to 

generate the heat necessary to 

cause functional cask damage. 

Furthermore, these types of fires 

are generally localized and not

likely to completely engulf a cask 

over 16 feet long and 5 feet in 

diameter. The potential for func

tional cask damage from fires is 

therefore limited to accidents in

volving tanker trucks, locomotives, 

and tank cars with large quantities 

of flammable materials. 

The approach taken to calculate 

cask responses to fires was to 

determine the likelihood that a fire 

would occur given a specific truck 

or train accident scenario defined 

on pages 7 and 9. Each scenario 

was assigned one of eight fire 

duration estimates (five for truck 

and three for rail accidents), two 

of which are shown on the upper 

figure on page 8. For rail acci

dents, a significant fraction of 

fires were assumed to have long 

durations (1 of 8 for the accident 

scenarios illustrated on page 9 

were assumed to last longer than 

1 hour). For truck accidents with 

other trucks or with trains, a 

similar fraction of fires exceeded 

1 hour. Only for truck accidents 

involving no collision, a collision 

with a fixed object or a collision 

with an automobile were the fire 

durations limited so that only 

about 1 percent or less exceeded 

1 hour. This assessment reflects 

the likelihood that fire durations

would be limited by the amount of 

fuel in the fuel tanks of the vehi

cle involved in the accident. 

These estimates were chosen con

servatively because of the lack of 

actual accident data. The likeli

hood distribution applicable to fire 

temperatures is shown in the bot

tom figure on page 8. A large 

fraction of fires were assigned 

temperatures in excess of those 

typical in such accidents. 

The fire temperatures and duration 

parameters, when considered with 

the potential for cask involvement 

in any accident-caused fire, 

resulted in the prediction that less 

than 1 of every 1000 truck or rail 

accidents has the potential to 

cause a fire capable of com

promising cask safety. This con

clusion is illustrated on the op

posite page.

I _____________________________________________________________

24
POTENTIAL HAZARDS 

AND RISK

Accident Scenarios 

Generating Thermal 

Forces Incapable of 

Causing Functional 

Cask Damage

Fire Not Hot Enough or 

of Long Enough Duration 

to Affect Cask

In Greater Than 

999 of Every 1000 Accidents 

- No Cask Function Damage 

from Thermal Forces

Cask Location Not Close 

Enough to Fire

25
POTENTIAL HAZAIFI DS 

AND RISK

Potential Radiological 

Hazards Resulting 

From Functional Cask 

Damage 

The evaluations described on 

pages 22 through 25 indicate that 

less than 6 of every 1000 truck ac 

cidents and 6 of every 1000 rail 

accidents could cause some func

tional cask damage. Damage to 

the cask could lead in turn to 

radiological hazards caused by 

either M11 the release of radioactive 

material from the cask's contain 

ment, or (2) an increased level of 

radiation emanating from the 

spent fuel within the cask caused 

by a degradation in a cask's 

shielding. The magnitude of any 

radiological hazard will vary 

depending on the extent of the 

cask's damage the hazard tend

ing to increase in magnitude as 

cask damage increases. In order Ia 

evaluate this variability in the 

potential hazard, three broad areas 

of cask response were charac

terized (see the figure on the op

posite page). 

Most of the accidents capable of 

causing any functional cask 

damage produce the limited 

responses shown within the gray 

area of the figure. In fact, of the 6 

truck accidents out of every 1000 

capable of causing any functional 

damage, about 4 are estimated to

result in a cask response within 

this region. Similarly, 4 of the 6 

damage-producing rail accidents 

are estimated to generate similar 

levels of damage. In this gray 

area, containment vessel structural 

damage is limited (to strains of 

less than 2 percent) and cask 

gamma radiation shield tempera

tures within the body of the cask 

are typically below melting 

temperatures (less than 600OF 

compared with the lead-melt 

temperature of 621 0 F). Note that 

other casks which do not use lead 

as a shield material would be ex

pected to experience little, if any, 

shield damage. At this level of 

response, any radioactive materials 

released from the cask would exist 

as a gas and only a small fraction 

would occur either in volatile form 

or as small solid particles in an 

aerosol. Furthermore, little 

degradation of the cask's shielding 

would be expected since the 

mechanical and thermal forces im

posed on the cask are insufficient 

to cause significant shield "slump" 

or voiding. In quantifying the 

potential magnitude of any radio

logical release created by 

responses in this area, researchers 

estimated that the magnitude of 

any release was likely to be less 

than compliance values applied to 

casks after they have been sub

jected to the hypothetical accident 

conditions described on page 5. 

In the large open area, structural 

damage to a cask's containment 

could be significant, although

gross rupture of the cask's con 

tainment shell would not be ex 

pected. The heat could melt lead 

in the shield, resulting in voids 

and increased external radiation 

levels. For cask responses in the 

large open area, radioactive material 

releases and/or external radiation 

levels potentially could slightly ex

ceed existing regulatory com

pliance values. Just about 2 of 

every 1000 truck and rail accidents 

involving a spent fuel shipment are 

conservatively predicted to be 

capable of causing this level of 

radiological hazard. 

Finally, only about 1 in every 

100,000 truck accidents and 1 in 

every 10,000 rail accidents are 

calculated to lead to cask damage 

as described in the outer ring of 

response regions. No documented 

accident can be specifically iden

tified that can cause this degree of 

cask damage. As indicated on 

pages 16 through 20, the 

radiological consequences of 

events in the outer ring were 

hypothesized because of the ex

tensive and potentially varied 

nature of cask and spent fuel 

damage. Similarly, the potential 

for a loss of the cask's subcriti

cality function would be expected 

to be restricted to a small fraction 

of the "outer ring-type events" in 

which sufficient quantities of 

water were physically present.

26
POTENTIAL HAZARDS 

AND RISK

Fraction of Truck 

and Rail Accidents 

Involving Spent Fuel 

Shipments that Cause 

Cask Responses 

Within Each Response 

Region

_____________________________________________________________________I

TRUCK

S3 (30)

UJ-) 0? 

C: 

ErC 

_.j 0 

< E

•E 

X- _

4 ss'Than 

"-u2 per 

Thousand

'0j /

4 4 4

i<' 0

- Less Than 1 per Million 

(All Boxes)

(2 (2)

T , 

(500)

j1 per Million i -- 0 

(All Boxes) 

per 100 per 100 per 100 

Thousand Thousand Thousand

T3 

(650)

T4 

(1050) T2 

(600)

THERMAL RESPONSE, OF 

(Centerline Lead Temperature)

RAIL

(/) co 

U 08 

u o 

<•C: 

c: o 

() I 

CD

S3 

(30) 

12 (2) 

S1 

(0.2

0/

per 10 

Thousand

/ 0v rQ 0

I I - Less Than 3 per 10 Million

(All Boxes) 

I I

"--'2 per Millionr..

(All Boxes) 

"'1 per ' 1 per per 10 

Thousand Thousand Thousand

11 

(500) (600)

T3 

(650)

T4 

(1050)

THERMAL RESPONSE, OF 

(Centerline Lead Temperature)

Note: Numbers have been rounded off.

27

I

I

I
POTENTIAL HAZARDS 

AND RISK

Interpretation of the 

Relationship Between 

Potential Radiological 

Hazards and Real-World 

Severe Accidents 

Predicting the likelihood and 

magnitude of any radiological 

hazard in a severe transportation 

accident is not an exact science. 

The forces applied to the spent 

fuel shipment in extremely severe 

accidents are based on extrapola

tions of historical accident data, 

the evidence from physical tests, 

and predictions from engineering 

models using conservative as

sumptions. What is clear is that E 

the severity of accidents increases, 

the extent of possible damage to 

casks and spent fuel also 

increases. 

This summary report has described 

the processes and results used to 

assess the level of safety for spent 

fuel shipments. To better under 

stand the results, two further in

terpretations of the level of safety 

can be made. First, an illustration 

of the relationship between poten

tial radiological hazards and some 

understandable accident param 

eters is provided in the illustration 

on the opposite page. The 

illustration applies to truck ship

ments of spent fuel subjected to 

mechanical forces. The expected 

yearly accident event frequencies, 

indicated on the figure, include 

consideration of predicted spent 

fuel shipment activity and a truck 

accident rate of 6.4 accidents per 

million truck miles. It is important

to remember that the statements 

on event likelihoods apply to the 

performance of the defined 

representative cask designs real 

cask designs are expected to pro

vide a greater level of safety in 

transportation accidents. 

The second interpretation involves 

the prediction of the performance 

of the representative cask designs 

if they had been involved in cer

tain historically documented, 

severe transportation accidents. 

Four specific events were selected 

from about 400 severe accidents 

that, in turn, were selected from a 

much broader DOT data base. The 

description of the four events and 

the predictions of cask response 

are illus-rated on a portion of the 

figure on the opposite page. 

Together, these results are be 

lieved to present a fair picture of 

the minimum level of safety pro

vided during shipments of spent 

fuel. The reader is encouraged to 

refer to the LLNL report for a 

complete interpretation of the 

studies approach and results. 

Risk Estimate for 

Spent Fuel Shipments 

"Risk" and "expected value" are 

two of several measures used to 

predict future occurrences based 

on past experience in fields rang

ing from safety to sports. In this 

study, historical information on 

truck and rail accidents was sup

plemented by route survey data to 

predict the occurrence frequency 

of severe transportation accidents.

Engineering models were then 

used to predict how a spent fuel 

shipment would respond in these 

accidents and what magnitude of 

radiological hazard might be 

created. A risk measure was deter

mined by multiplying the magni

tude of each potential hazard by 

its occurrence frequency and sum

ming all the resulting values. 

This type of risk measure has a 

regulatory precedent applicable to 

this study. In December 1977, a 

study that evaluated the risk for all 

radioactive material shipments, in

cluding spent fuel, was published 

as a Final Environmental State

ment (FES).* The evaluations con

tained in the FES indicated a 

radiological risk from transporta

tion accidents of one latent cancer 

fatality every 59 years for all pro

jected 1985 radioactive material 

shipments. Most of this risk was 

associated with shipments of 

medical radioisotopes. The con

tribution from spent fuel 

shipments was 2.5 percent of this 

estimate. 

"Transportation of Radioactive Material 

by Air and Other Modes," NUREG 0170, 

December 1977.

28
POTENTIAL HAZARDS 

AND RISK

Accident Scenarios Generating Mechanical 

Forces Potentially Capable of Causing a 

Radiological Hazard

Predicted Cask Response to 

Selected Historical Accident 

Events

Occurrence Rate 

= 6 Events per 1000 Accidents 

= One Accident Expected Every 

10 Years (Assuming ru3 Million 

Shipment Miles Per Year) Cask Velocity.Normal to Surface or 

Object - Between 32mph and 50 mph 

4 Events per 1000 Accidents or 

1 Expeeted Event Every 14 Years

-ý2 Events per 1000 Accidents or* 

1 Expected Event Every 35 Years 

Cask Velocity Normal to Surface or 

Object - Between 50 mph and 75 mph

{

POTENTIAL MAGNITUDE OF 

RADIOLOGICAL HAZARD 

"* Material Releases (Primarily Gases 

and Volatiles) Less Than 

Compliance Values* 

"* No Significant Increase in 

External Radiation Levels

* Material Releases (Primarily Gases 

and Volatiles) Could Exceed 

Compliance Values* by a Small 

Factor (i.e., 2 or 3 Times) 

* External Radiation Levels Could 

Equal or Slightly Exceed (by a 

Factor ofr\- ,3) Compliance Values*

"/ 

"Non-Soft" Object

Less Than 1 Event per 10 Million 

Accidents or No Expected Events 

During Repository Shipments 

Cask Velocity Normal to Surface or 

Object - Exceeds 75 mph

" Material Releases Estimated to 

Exceed Compliance Values* by 

About a Factor of 20 Dependent on 

the Specifics of the Accident 

" External Radiation Levels Estimated 

to Exceed Compliance Values* by 

a Factor of 30 Dependent on the 

Specifics of the Accident 

*Compliance Values as Defined in 

Current Regulations

CALDECOTT TUNNEL FIRE - 4/82 

* 3-Vehicle Collision - Gasoline 

Truck-Trailer, Bus and 

Automobile 

* 8,800 Gallons of Gasoline 

* Fire of 2 Hours and 42 Minutes 

40 Minutes 9 19001F

30 

Cc 2 

Co 

"LHARD SURFA 

:3 1/792 

E 

x SFT SURFI 

0 50 

No Radiological Hazard

Predicted Cask Response 

* No Significant Impact Damage 

"Soft" Objects 

* 45 Minutes I 1900OF Causes 500OF 

Centerline Temperature

Centerline Lead Temperature (OF)

I-I 

Radiological Hazard 

Approximates 

Compliance Values*

Radiological Hazard 

Exceeds Compliance 

Values By Up to a 

Factor of 4

1-80 BRIDGE ACCIDENT - 3/81 

e Collision With Pickup Truck 

and Fall from 64-Foot High 

Bridge Onto Soil 

Predicted Cask Response 

* 44 mph Impact 

* No Significant Impact Damage

LIVINGSTON TRAIN FIRE - 9/82 

* Derailment of Vinyl Chloride/ 

Petroleum Tank Cars 

* Large Fires for Several Days 

Moved Over Large Area 

* 2 Explosions 

Predicted Cask Response 

"* Maximum Probable Cask Exposure 

to Petroleum Fire - Between 

82 Hours and 4 Days 

"* No Significant Damage from 

Explosion 

"* Centerline Shield Temperature 

Between 600°F and 7201F 

Dependent on Degree of 

Cask Involvement 

DERAILMENT ON ALABAMA 

RIVER BRIDGE -1/79 

"* Plunge Off 75-Foot High 

Bridge 

"* Railcar Impacts Into Water 

and Mud 

Predicted Cask Response 

* 47 mph Impact in Soft Target 

* No Significant Impact Damage

29
POTENTIAL HAZARDS 

AND RISK

In the FES, the predicted per

formance of radioactive material 

packages was based, for the most 

part, on engineering models and 

conservative engineering judg

ments. The LLNL study, on the 

other hand, focused entirely on 

spent fuel shipments and provided 

a detailed engineering analysis of 

package or cask performance 

under severe transportation acci

dent conditions. The table on this 

page compares the results from the 

two studies.

The LLNL study included a more 

detailed approach to the calcula

tion of radiological hazards that in

volved the consideration of re

leases of radioactive material as 

small inhalable particles. Any solid 

material release from a cask would 

require the creation of a direct 

release pathway from both the 

containment provided by the fuel 

rod and the cask (that is, a 

pathway much more direct than 

one needed for gaseous or volatile 

material releases). With the 

assumption of such a pathway

and the presumed release of solid 

material,* the risk, as calculated in 

the LLNL study, is shown in the 

following table to be less than 

one-third of the values estimated 

in the FES. Therefore, to the ex

tent that the Commission's conclu

sion on the adequacy of NRC 

regulations were initially valid and 

were dependent on the FES risk 

estimates, the LLNL study has not 

identified any increase in risk that 

would change the Commission's 

conclusion. 

*A shipping cask has been subjected to at

tack by explosive to evaluate cask and 

spent fuel response to a device 30 times 

larger in explosive weight than a typical 

anti-tank weapon. This device would carve 

an approximately 3-inch-diameter hole 

through the cask wall and contained spent 

fuel and is estimated to cause the release 

of 2/100,000 of the total fuel weight ("-10 

grams of fuel) in an inhalable form. No 

transportation accident can be identified 

that would impose anywhere near the 

energy per unit volume caused by this ex

plosive attack.

30

RISK RESULTS - COMPARISON 

WITH PAST FES EVALUATION 

FES LLNL STUDY 

(NUREG-0170) RESULTS 

ESTI MATES 

Fraction of Transportation 

Accidents Involving Spent 0.09 (Truck) 0.006 (Truck) 

Fuel Shipments Causing 0.20 (Rail) 0.006 (Rail) 

Any Radiological Hazard 

Fraction of Transportation 

Accidents Involving Spent 0.004 (Truck) 0.00001 (Truck) 

Fuel Shipments Causing 0.002 (Rail) 0.00013 (Rail) 

Largest Estimated 

Radiological Hazard 

Overall Annual Risk 0.0004 Less Than 1/3 

From Transportation Latent Cancer of FES Value 

Accidents Involving Fatalities Per Year 

Spent Fuel Shipments
Notes