enriched bomb uranium isotope ratios from oak ridge oralloy

8/10/2019 Enriched bomb uranium isotope ratios from Oak Ridge Oralloy

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2/12

DISCLAIMER

This report was prepared as an account of work sponsored by anagency of the United States Government. Neither the United StatesGovernment nor any agency Thereof, nor any of their employees,makes any warranty, express or implied, or assumes any legalliability or responsibility for the accuracy, completeness, orusefulness of any information, apparatus, product, or processdisclosed, or represents that its use would not infringe privatelyowned rights. Reference herein to any specific commercial product,process, or service by trade name, trademark, manufacturer, orotherwise does not necessarily constitute or imply its endorsement,recommendation, or favoring by the United States Government or anyagency thereof. The views and opinions of authors expressed hereindo not necessarily state or reflect those of the United StatesGovernment or any agency thereof.


3/12

DISCLAIMER

Portions of this document may be illegible inelectronic image products. Images are producedfrom the best available original document.


4/12

2

TABLE

OF

CONTENTS

INTRODUCTION

...................

5

EXPERIMENTALLY

DETERMINED

DOSE KATE

3

CALCULATED

DOSE

RATE

4

ABSORBED

DOSE

IN

PLASTICS

6

GAI4A

DOSE RATE

..................

7

CONCLUSIONS

....................

9


5/12

IrRODUCTION

Oralloy

(enriched

uranium)

is

a

potential

source

of

radiation

damage

because

of

its

alpha

and

gamma

radioactivity.

The

radiation

effects

will

be

relatively

small

because

of

the

low

specific

activity.

However,

the

long

design

lifetimes

(many

years)

of

some

materials

could

lead

to

appreciable

damage.

To

estimate

this damage,

the

dose rate

from

the

surface

of

oralloy

must

be

determined.

This

has

been

done experimentally

and

theoretically.

The isotopic

composition

of oralloy

used

at

Rocky Flats,

as

determined by

mass spectrometry,

is

the

ollowing.

U-24

1.01

U-23S

93.13

U-236

0.32t

U-238

S.SSt

EXPERIMENTALLY

DETERMINED

DOSE

RATE

Radioactive counting

techniques

were

used

to

measure

the

dose

rate

from orallov.

A

counting

source

was fabricated

by machining

an

oralloy

disc

(diameter

1/4

inch

and thickness

1/16 inch)

which

fit

exactly

into

the

machined

depression

of

an

almainu

disc

(di--eter-

1

inch

and thickness

1/8 inch).

The

top

of

the

oralloy was flush with the aluminum surface

so

that

only

the

top

surface

Carea

0.316

cm

z

would

be

counted.

Since

the

range

of the

alpha particles

in

oralloy

is about

6

microns,

this

is an

infinite

source.


6/12

4

Results

obtained

from

a

machined

or

polished

surface

were

essentially

the

same.

The

total

alpha disintegration

rate,

measured

with

a

proportional

counter,

was

151,680

+/-225

counts

per

minute.

The average alpha

energy

was

determined

using

a

silicon

surface

barrier

detector

coupled

to

a

pulse

height

analyzer.

Integration

under

the

resulting

spectrum

gave

an

average energy

of

2.69

MeV,

which

is

56t

of the

maximum

energy.

s

Multiplying

the

count rate

by

the

average

energy

gives

an

alpha

dose rate of 2.15 x 101

eV

per

second

per

cm

z

CALCULATED

DOSE

RATE

The

following

properties

of

the

uranium

isotopes

were

used.

half life

alpha energy

ramie

in

alr

z

U-34

2.47xi0

years

4.76

MeV

4.00

ag/cm=

U-235

7.1

xlO

e

years

4.40

MeV

3.50

mg/cm

U-238

4.51x10

years

4.18 14eV

3.25

mg/cm

U-236 2.34x107

years

4.49

eV

The dose

contributions

from

uranium-2 8

and

-236

were

considered

and

found

to

be

negligible.

The dose rate wa

calculated

by

multiplying

the

alpha particle

flux

by

the

average

energy

of

the

emergent particles. The

particle

flux

(n)

calculated

according

to:

*


7/12

5

n

sp

k

x

R/4

where

sp

A

specific

activity

(disintegrati

per

second

per

R

range

The

rahge

was calculated

according

to:

R

z/0.01

Z/z

R

a

[0.90/

0.0275

Z/

(0.06-

0.0086

Z) log

where

R

z

range,

mg/cm

2

R

a

range

in

air

Z

atomic number

of

element

z tomic

number

of

particle

E

particle

energy

in

MeV

M mass

number of

particle

The flux

and

range

(dividing

R

z

by

18.81,

the

density

of

oralloy,

gives

the

range in cm)

of

alpha

particles

from

uranium-254 and

-235

are:

n

R

z

R

U-254 7.48x10

/sec/cm

2

15.I

mg/cm

2

6.96x10

.4

cm

U-255

2.28x102

/sec/cm

2

11.4

mg/cm

2

6.08x10

.4

ca

The

average

energy

was

taken to

be

2/3

of the

maximum

energy.

This

value was derived

using

an

approximation

of

the

Bethe-Block

equation.

(s

1/2

E

11

where

E

energy

of

the

particle

E

o

initial

particle

energy

r distance

particle

travels

R

particle

range


8/12

6

The

average

energy

is defined

as

f

Edr

divided

by

f

dr.

o

The

dose

rates

fromU-Z34and

U-Z3

in

oralloy were

thereby

determined

to

be:

U-254

2.57x1012

eV/sec/cm

2

U-255

6.68x108

eV/sec/cm

The

dose rate from

oralloy

is the

weighted

average

rom these

two

isotopes.

2.37x10

(1.01t)

/

6.68x10

(95.15)

2.45xI0s

V/sec/cm

This

value

is

a little

larger

than

the

measured

dose

(2.15

x

10

I

eV/sec/cm

)

because

of

the

difference

in the

average

alpha

energies.

The measured

energy

was

6

of

the

maxim,,=

energy

and

the theoretical

energy

(using

an

approximate

calculation)

was

66.6t

of

the

maximum

energy.

ABSORBED DOSE

IN

PLASTICS

To

assess

the

radiation

damage upon

prolonged exposure

from

oralloy,

a calculation

has been

made

using

polyethylene as

a

representative

plastic.

The

first

step

was to calculate

the

range

of

the

alpha

particles

in

polyethylene,(CH2)x.

The

assumption was

made

that

the

plastic

is in

contact

with

the

oralloy

so

that

the

average

particle

energy

is

still

2.69

MeV.

Ranges

in

hydrogen

and

carbon

were

calculated

using equations

similar

to

the one

used

previously in

this

report,

r

The

ranges


9/12

7

are

0.51 mg/ca

and

1.75

ag/ca

2

for

hydrogen

and

carbon,

The

range

in

polyethylene

was calculated

espectively.

according

to:

-,1

0. S7

0.143

gC,)

x

/

*weight

fraction of element

in

CCH2)

x

The

range

was 1.30

ag/ca

2

or ..:x10

.3

ca.

The

absorbed dose

can

thereby

be

calculated

I

radl0

eY/gl( :3 lSxlO

secl

.15x101

eV/sec/ca

6 Z

4

ose

1. :30

x

10-

3

g/ca

x

-e

8.:3

x

10

s

fads/year

This is

the

approximate

dose at

which

polyethylene

and

uny

other

plastics

start

to

decompose.

The

damaging

dose

is

greatly variable

and

depends

on

the

plastic

as

well

as

the

property

of

interest.

Nevertheless,

it

appears

that

many

organic

materials w111

undergo

significant

decomposition

in

a

few

years

of

contact with

oralloy.

However, these

effects will

only occur to a

depth

of

approxi-

aately

1-2x10

.3

ca.

GA}@4A

DOSE

RATE

Instrumentation

was

not available

to

measure

the

gam

dose

rate

from

oralloy,

but

a

rough

approximation

was

calculated for

coapari-

son with

the

alpha

dose

rate.

Decay schemes

for

uranium-2:34

and

-2:35

show the

following:


10/12

U-234

specific

activity

4.30xI0

dis/sec/cm

s

72

of

decays

to

ground

28

of

decays

0.053

MeV gamma ray

U-235

specific activity

i. 0x10

dis/sec/cm

2

75.8

o

decays

a

spectrum

of

gamma

rays

with

an

average energy

of 0.18 MeV

The

absorption

coeficients

(

and

half

thickness for these

gamma

rays in

oralloy

are:

ulP

Ccm21e)

Cc

)

0.053

MeV

9.86

185

0.18 MeV

1.79

33.6

Half thickness

(ca)

3.74

x

I0

2.06

x

10

.2

For the

dose

rate

approximation,

assume

that

one-half

(g

ometry

factor)

of 811

the

gamma

energy

is

released

from

a

half thickness

o

oralloy.

U-234 4.30x10

dis

13.74x10

ec

ca

Dose 1.19

x

I0

eV/sec/cm

2

(0.28)(0.50)(5.3x10

s

/

dis

U-235

1.SO

x

10

[2.06

x

10

sec

Dose

2.19xl0

s

eV/sec/cn

2

c= w/

(0.788) (0.

so)

1.8xlO

s


11/12

-9-

The total dose

rate from

oralloy

is

the

weighted

average.

11

t

U-234

1.01t

(1.19

x

10

1.20

x

10

37.0t of

energy

9

9

U-235 95.13t (2.19

x

10

2.04

x

10

63.0t

of

energy

Total dose

rate 5.24

x

10

eY/sec/cm

a

Not

only

is

the

gamma

dose

rate

almost an order

of

magnitude

less

than

the

alpha

dose

rate,

but

the

gamma

penetration

(half

thickness)

in

plastics

is

about

a

thousand

times

that

of

alpha

penetration,

so

that

the

absorbed

dose

from

gamma

radiation is

negligible

compared to

that

from

alpha

radiation.

CONCLUSIONS

The

alpha

dose

rate

from

the

surface

of

oralloy

was

determined

10

10

to

be 2.15

x 10

eV/sec/cm

2

(or

2.45

x 10

eV/sec/cm

2

clculated

value). Almost all

of

the

dose

emanates

from

uranlua-234, whose

isotopic

concentration

is

1.01t. The

absorbed

dose

in

poly-

ethylene

was

calculated

to

be

8.5

x

10

fads/year

indicating

that

most

plastics

would

radiolytically

decompose

appreciably

in

a

few

years.

However,

the

decomposition

would

only

occur

to

a

depth

of

1-2 x

10

s

cm,

the

alpha

partlcle

range.

The

gma

dose

rate

was

also

estimated

and

found to

be

negligible

compared

to

the

alpha dose


12/12

10

REFERENCES

I.

The

counting

was done

by

Richard Murri

of

the

Rocky

Flats

Environmental

Science

Group

G.

Friedlander,

J.

W.

Kennedy,

and

J.

M.

Miller,

Nuclear

and

Radiochemistry,

John

Wiley

and

Sons,

New

York,

1964,

page

96.

5. G.

D.

Finney

and R.

D.

Evans,

Physical Review, 48,

505

(1935).

G.

Friedlander,

J. W.

Kennedy,

and

J.

M.

Miller,

Nuclear

and

Radiochemistry,

John

Wiley

and

Sons,

New

York, 1964,

page

95.

S.

M.

McD.

Baker,

R.

Hughes,

J. A.

Spooner,

ACO/UK-2325,

October 1964.

J.

H.

Hubbell,

Photon Cross

Sections,

Attenuation

Coefficients,

and

Energy

Absorption

Coefficients

from

10 keV to 100

GeV,

NSRDS-NBS

29, August

1969.

enriched bomb uranium isotope ratios from oak ridge oralloy

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