ORNL-TM-3515.txt

 

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ORNL- TM- 3515
Cy 3§

   

 

5
: STATUS OF NOBLE GAS REMOVAL AND DISPOSAL
|
l& Ja Pl NiChDIS
| F. T. Binford
%
d
NOTICE This document contains information of o preliminary nature
and was prepared primarily for internal use at the Ock Ridge Nationsl
Laboratory. It is subject to revision or correction and therefore does
not represent o final report.
<

 
 

 

 

This report was prepared as an account of work sponsored by the United
States Gowvernment. Neither the United States nor the United States Atomic
Energy Commission, nor any of their employees, nor any of their contractors, -
subcontractors, or their employees, makes any warranty, express or implied, or
assumes any legal liability or responsibility for the accuracy, completeness or
usefulness of any information, apparatus, product or process disclosed, or
represents that its use would not infringe privately owned rights.

 

 
ORNL-TM-3515

Contract No. W-Th05-eng-26

CHEMICAL TECHNOLOGY DIVISION

STATUS OF NOBLE GAS REMOVAL AND DISPOSAL

J. P, Nichols
F. T. Binford*

 

* . R
Operations Division

AUGUST 971

OAK RIDGE NATIONAL LABORATORY
Oak Ridge, Tennessee
operated by
UNION CARBIDE CORPORATION
for the
U.S. ATOMIC ENERGY COMMISSION 1OSKHEED MARTIN ENERGY RESEARCH Limmamieg

T

3 Y456 0514533, 2
iii

CONTENTS

A S aC Tt . i et i et et et
N < e T b e s o) o
z. Effects of Releases of Noble Gases to the Atmosphere ...........
3. Review of Processes for Holdup and RECOVETY .. vvirerrnnerreennnn.
3.1 Charcoal Adsorption at Ambient Temperature ................
3.2 Cryogenic AQSOrption vuve it inoe ittt iee e eenesennenesenes
3.3 Cryogenic Distillation e i
3.4 Selective AbSOrPLiom wuvurvr ittt e
3.5 PermseleCtive MembDraes e e eeeeeeeeneeseesesnenoesens
3.6 Clathrate Precipitabion v.uue e eereeneeneeernennennennens
L. Cryogenic Distillation ProCESS . veuereeeeeenseneneneeneenenennes
5. Fluorocarbon AdsOrplion ProCESS v it ieteiteeertertnenenenereecens
6. Conceptual Systems for Recovery of NODLE GASES «vevervneernnnnn.
6.1 Boiling Water Reactors «vuveeeeeeeveeeeneenns et
6.2 Fuel ReprocessSing PLant v.uuveeeereeernenereereneennens e

6.3 Summary of Conceptual Disposition of Noble
Gases gt Nuclear Facilities ....vverernennnnnnnnes i
7. Shipment of Noble GaSES tiiirirtiettiressnneernennernonnesenenns
8. Methods for Long-TerM StOrBEE v v v e s eeeneeeneneeeneeeeeneenness
8.1 Storage of Cylinders in Surface Vaults ...vveeeverrenneneens
8.2 Burial Of Cylinders v uueeneenrnnoneeneeoenasoseseneeneneas
8.3 Injection into Porous Underground Formations ..............
8.4 Relatively Unfavorable Storage MethodsS ....vevvereernennnn.
O, ConCluSionS vueivieeineeeenennononeennes ettt et e
10, ReI I eN S vttt ittt ittt ts et et sennsneannssuoassssesseasennnes

Page

11
11
12
13
13
14
16
20
20

22
STATUS OF NOBLE GAS REMOVAL AND DISPOSAL

 

J. P. Nichols
F. T, Binford
ABSTRACT

Currently, noble gas fission products generated by
the nuclear power industry are discharged to the atmosphere
following interim holdup for decay of short-lived species.
The resultant off-site radiation exposures have been small
as compared with current guidelines for population exposure.
Recovery of these gases will begin in the near future,
however, as a result of stated poclicies of maintaining
radiocactive releases to the environment at the lowest
practical levels.

Several processes are available or under development
for recovery of these gases from off-gas streams of nuclear
reactors or fuel reprocessing plants. Processes that incor-
porate selective adsorption on low-temperature charcoal and
cryogenic distillation have been demonstrated at the Idaho
Chemical Processing Plant. A process utilizing selective
absorption in fluorocarbons is undergoing engineering
development at the Oak Ridge Gaseous Diffusion Plant.

Initially, it 1s probable that the recovered noble
fission product gases will be collected under high pressure
in gas cylinders. These cylinders probably will be shipped
to a large, remote site and stored under conditions that
will promote long-term integrity of the cylinders. A
longer-term possibility for encapsulation, which would
Tend to enhance containment during shipping and storage,
1s the sealing of gases as small bubbles or individual
molecules within solids. Longer-term possibilities for
storage include injection of the gases into deep under-
ground formations,

 

1. INTRODUCTION

The purpose of this paper will be to review the status of technology
of systems for minimizing the release to the atmosphere of radioisotopes
of krypton and xenon that are produced by fission. Isotopes of krypton
and xenon are relatively difficult to confine in the nuclear power industry
because they occur in gaseous form, are chemically inert, and are relatively

insoluble in water.
Currently, all of the noble gas fission products generated within
nuclear power reactors are discharged ultimately to the atmosphere
following interim holdup for decay of short-lived radionuclides. The
experience has been that greater than 99% of the gases are released
when the spent reactor fuel is chopped up and dissolved at a spent fuel
reprocessing plant. Since the fuel is sftored at least 150 days before
reprocessing, however, the only noble gas radionuclide of significance
in the effluent is 88Kr — which has a half life of 10.74 years. A small
fraction of the noble gases, generally less than 1% of the total, is
released at nuclear power plants as a result of fission of "tramp
uranium” on the surfaces of fuel elements and minor leakage from fuel
rods. In pressurized water reactors it has been feasible to provide
for holdup of these gases for one to two months before discharge; after
these times, only 8BKr and 123Xe — which has a half life of 5.27 days —
contribute significantly to the radiocactivity of the effluent. In boiling
water reactors it has been economically undesirable to provide for gas
holdup times greater than about 30 min because of the appreciable leakage
of gas into the system through the steam condenser; the gaseous effluent
from a BWR, therefore, contains short-lived isotopes of xenon and krypton

in addition to the 8BKr and 133Xe,

The radiation exposures resulting from the release of noble gas
fission products from reactors and reprocessing plants to the atmosphere
have been small as compared with current guidelines for population
exposure.l_LL However, in continuing observance of the policy that
radiation exposures should be maintained "as low as practicable,’ the
AEC has funded development of systems for minimizing the release of
noble gas nuclides to the atmosphere. Some of these systems, particu-
larly those for enhanced holdup of noble gases, have attained a status
of technology that is acceptable for commercial application. Other
systems, particularly those directed toward long-term isolation of 85Kr

from the biosphere, are in an advanced stage of development.

The following will review briefly the processes that are available
or under development for holdup or recovery of noble gases and present
preliminary concepts of methods for use in handling and disposal of the

recovered gases.
2. EFFECTS CF RELEASES OF NOBLE GASES TO THE ATMOSPHERE -

A very simple model has been developed to provide a rough estimate
of the worldwide population exposure that would result from continued
release of noble gas fission products from multiple sources. This model
assumes that the exposure within relatively short times after release
can be described by the Gaussian plume dispersion model and that later
exposures would result from steady circulation of the fixed volume of

air in the northern hemisphere.

Assuming that the radiocactive gas is released at ground level (which
tends to overestimate exposures near a given source), the Gaussian plume
dispersion model predicts that the exposure in air at ground level under

constant meteorological conditions is:

2 2
Qe—lx/u e'y /20'y

T
O'yO'Z

 

X(X)Y) =

where
Q = quantity of curies released over a short term (or constant
source strength in Ci/yr)
u = wind speed, m/yr

Iy = horizontal dispersion coefficient, m
c, = vertical dispersion coefficient, m
x = downwind radial distance from source, m
y = distance crosswind from source, m
A = radioactive decay constant, yr-?
X(x,y) = ground level exposure, Ci- yr/m® (or concentration, Ci/md,

for a constant source)

The population exposure — considering a first phase of dispersion

and a second phase of transport in the troposphere — is estimated as

T

D e
E = 2K J j P(x,t)x(x,y)dydx + E%é r P(X,t)e_Kt
o o

dt (2)

+

D/u
where
K = dose rate (predominantly to skin) per unit of concentration
2.0 x 10 (rem/yr)/(Ci/m®) for 8BKr and 133Xe
P(x,t) = surface density of population as a function of radial distance
from the source and time since initial release of the gas
A = surface area of the northern hemisphere
= 2.5 x 1014 n?
V =

volume of alr in the troposphere of the northern hemisphere

converted to sea level pressure = 1,9 x 108 nS ’

D = radial downwind distance at which the initial dispersion is
essentially complete (to be determined), m

T = time since initial release of the gas, yr

E = population dose, man-rems, incurred within time T > D/u

following the initial release (or man-rems/yr for a constant

source )

At this point, we will make the further simplifying assumption that
the surface density of population in the northern hemisphere is spatially

uniform but increases with time.
+i~T
P(x,y,t) = P_e""P (3)

In early 1970, the estimated world population was 3,550,000,000
5,6

persons, of whom approximately 9#% reside in the northern hemisphere.

It has been projected that the world population will double in the next

4O years.
P, = (3.55 x 102)(0.94)/(2.5 x 10 )= 1.3 x 10"® persons/m?
A, = 1o 2/k0 = 0.01733 years~!

We will assume further that the dispersion under long-term averaged
conditions may be represented by the following formulations for dispersicon

coefficients.

0.05 x

Q
I

o, = 0.7 \x
By making the above substitutions in Eq. (2) and performing the

integrations, we obtain

AN
g - KPoe™® V2 . [(i-)p)D

0.7 u\x=n, - U

kqPoeto A [e-(x-xp >§ _ o)t ] )

+
7 i

where N is the time in years between 1970 and the initial release,

The first term represents exposure during initial dispersion; the

second term represents the subsequent long-term exposure.

If we temporarily assume that there is no appreciable change in
radiocactivity or population density through time T,this equation reduces
to

_ KQPOe)\PN 2\[2D . KQP~A e)\pN(T-D/U-) (5)
0.7 u \w v

We may now estimate the value of D on the basis that this is the
downwind distance such that the rate of exposure in the dispersion phase
becomes equal to the rate of exposure after lateral and vertical disper-
sion are complete.

4 [KQPgete" 2}[213 ] ?\P Kqpoetpia (6)

& 0.7 w \7

Utilizing aD _ u = D
- ad - - Tt

_V\E T _ .
[O Ta = 75 x 10° meters . (7)

Thus, D represents about 1.9 trips around the earth,and the corre-
sponding time is about 78 days for a typical global wind transport speed
of 40 km/hr.

Using the above value of D,we may now construct several simplifica-
tions of Eq. (4). The estimated population dose resulting from the
release of a quantity of 8%Kr (or the dose rate resulting from release

at a constant rate) is:
_ KQPoe ZV?" AeKPN-rl - e-(A_KP)Tj
0.7 u \n v _ k-kp -

 

_o.ou721I§j . (8)

0’ [0.0015 + 0.072 (1 - e

The total exposure from a "puff” release (or the maximum exposure

rate from a constant source) is:
= 0.074 Q Mol man-rem . (9)

For 132Xe, which decays essentially completely during the dispersion

phase, the exposure (or exposure rate) is:

T = KQPQeAPN \[2
0.7 \[ux

0.000405 g &PV | (10)

If W f Qe -\t dt is the number of curies that have accumulated in
the troposphere of the northern hemisphere through a given year and QN
is the incremental number of curies of 8BKr that are released in that

year, the population exposure in the year is:

r =
E = ehPN | 0.0015 @ + 0.0034% W_1 (11)
L n N_
Table 1 illustrates the application of these results to estimation
of the average population exposure that would result from quantitative
release of 123Xe and 8®Kr from hypothetical nuclear reactors and fuel

reprocessing plants.

A more pertinent example of the application of these results — the
primary purpose of this exercise — is in projecting the population
exposure rate that might result from continued release of all of the
88Kr that is formed in projected power reactors. These results are
shown in Table 2.

The projections of installed nuclear power in the world are basically

7

those of Spinrad,’ with the exception that his projections for Asia have

been increased in proportion to 1970 population (from 1075 to 1815 million
Table 1. Estimates of the Worldwide Population Exposure Resulting
from Unrestrained Release of Noble Gases from
Reactors and Spent Fuel Reprocessing Plants

 

 

1500-Tons/Year
1000 Mw(e) Reprocessing
Nuclear Reactors® Plant
Delay Time Before Release 30 min L5 days 150 days
Assumed Release Rate
86Ky, Ci/year 3000 3000 14,000,000
133%e, Ci/year 1,000,000 3000 -
Population Exposure Rate,
man-rems/year
1 year operation 420 P 16 67,000
15 year operation 520 115 530,000
30 year operation 570 170 780,000

 

®pssumes release of about 0.67% of the core inventory each year,
or about 1% of the total noble gas that is formed.

bThe estimated population exposure rate within 50 miles of the site is
approximately 50 man-rems/year. This estimate of close-in population
dose rate is probably low, however, since most reactors are sited in

regions that have higher population surface density than the assumed

average of 13 persons/km? (34 persons/square mile).
Table 2. Estimates of Population Dose in the Northern Hemisphere That Would Result
from Quantitative Release of 8%Kr Produced in Nuclear Reactors

 

 

1970 1975 1980 1985 1990 1995 2000

Installed Nuclear Capacity

U.S., GW(e) 6.1 63 149 281 481 788 1294

World, GW(e) 24 125 353 827 1660 2900 4500

Percent ILMFBR's 7.7 31.5 58.7

Thermal Efficiency 0.325 0.325 0.325 0.325 0.332 0. 353 0.378

Average Capacity Factor 0.71h 0.754 0.761 0.754 0.742 0.716 0.700

Ci of 8BKr/MWd(th) 0. 342 0.342 0.342 0.342 0. 340 0.332 0.323
8BKyr Produced Annually

U.S., megacuries 1.68 18.3 43.6 81.5 134 194 284

World, megacuries 6.59 36.3 103 240 Lol 713 988
Total 85Kr Accumulated in
Northern Hemisphere, MCi 55 116 339 901 2070 3870 6280
Relative Population 1.0 1.09 1.19 1.30 1.4 1.54 1.68
Population Dose, millions 0.20 0.49 1.6 L.L 11 22 38

of man-rem

Average Dose Rate, mrem/year 0.055 0.13 0.37 0.96 2.2 4,0 6.4

 
persons — a factor of 1.69) in order to account approximately for nuclear
power growth in Mainland China. All nuclear power 1is assumed to be gen-
erated in the northern hemisphere. The projections of nuclear power
generating capacity in the United States were based upon a recent study
(Case 43) with the Oak Ridge Systems Analysis Code,8 which allowed for
competition between fossil steam plants, light-water reactors, and Liquid
Metal Fast Breeder Reactors (assumed to be commercially available after
1985). The worldwide generation rate of 85Kr was based upon the fraction
of LMFBR's, average thermal efficiency, average capacity factor, and
curies/MWd(th) that were determined in the U.S. study.

These results indicate that continued release of 8BKr through the
year 2000 would cause skin dose rates from 8%Kr exposure that are about

5% of the dose rate that is a consequence of natural background radiation.

3. REVIEW OF PROCESSES FOR HOLDUP AND RECOVERY

Table 3 presents a summary of the development status and pertinent
features of processes that are potentially applicable for holdup or
8,9

recovery of krypton and xenon from gaseous effluents.

3.1 Charcoal Adsorption at Ambient Temperature

The adsorption of noble gases on charcoal or molecular sieves at
ambient temperatures is the process that has been studied most exten-

sively.lo’ll

This process is effective for interim holdup of xenon and
krypton because selective adsorption and desorption cause these gases
to move much more slowly through a packed bed than the air or other
carrier gas. This process is not suitable for recovery of krypton and
xenon since it does not provide for withdrawal of a concentrated product.
The primary disadvantage of room temperature adsorption is that very
large bed volumes are required to provide appreciable holdup. Also, a
fire hazard exists from the use of charcoal, which has low thermal con-
ductivity, in an environment that includes oxygen and heat production by

radiocactive decay. The use of molecular sieves, typically inorganic

zeolite-type (metal aluminosilicate) materials, avoids the fire problem
Table 3.

Processes for Holdup or Recovery of Noble Gases

 

Process

Development
Status

Comments

 

Ambient ~temperature adsorption
(charcoal or molecular sieves)

Cryogenic adsorption (charcoal
or silica gel)
Cryogenic distillation (liquid

nitrogen)

Selective absofption
(fluorocarbons)

Permselective membranes

Clathrate precipitation

Holdup systems
in reactors

Production recovery
system at ICPP
Production recovery

system at ICPP

"Cold" recovery
pilot plant

Laboratory studies

Laboratory studies

Simple flow system. Very large
beds. FIire hazard.

Small beds, batchwise. High refrig-
eration costs. Fire, explosion
hazards.

Small size, continuous. High concen-
tration factors. Explosion hazards.

Small size, continuous. Effects of
contaminants, radiation, corrosion?

Simple flow system. High operating
costs? Radiation damage?

Slow precipitation, high pressure.
Radiation degradation of solid.

 

—
O
11

but the materials are expensive and require the use of beds that are

two to four times larger than charcoal beds having comparable holdup.

The ambient-temperature adsorption process has been used in a
number of U.S. research reactors and has been in use since 1966 in the
KRB reactor in Germany, which is a GE-~designed BWR. Another power
reactor in Germany has used this system since 1968, and a third German
reactor using this system is due to come on line late this year. The
German company (AEG) that markets the system can furnish a charcoal
system that will reduce the radicactivity of BWR effluent by a factor
of 2000 by providing three days of holdup for krypton and 70 days for
xenon.ll such a system for an llOO-MW(e) BWR requires five charcoal
tanks, each 6 to 9 ft in diameter and 50 ft long. Somewhat smaller and
less bulky charcoal adsorption systems that provide radioactivity reduc-

tion factors up to 200 are offered by General Electric in this country.

3.2 Cryogenic Adsorption

Adsorption on charccal at liquid nitrogen temperatures permits the
use of a small adsorption bed and is adaptable for recovery of krypton

and xenon by a process of temperature cycling.lz’l3

This process for
recovery of krypton and xenon was demonstrated on a large scale at the
TIdaho Chemical Processing Plant (ICPP) about 15 years ago. Because the
beds are cooled and heated alternatively, the refrigeration costs were
very high. Other disadvantages are the fire hazard and the possibility
of explosion of hydrocarbons, nitrogen oxides, and ozone (produced by
irradiation of oxygen). The system also requires prior removal of gases
that would freeze at liquid nitrogen temperatures and plug the adsorbers.
The disadvantages of this system are such that it cannot be recommended

for recovery of krypton and xenon, but the process does have potential

appilication for interim holdup of the effluent gases from a reactor.

3.3 Cryogenic Distillation

Cryogenic distillation provides an effective, continuous, small-size

system for separation of gases based upon their relative volatility.l3_l5

This type of process is used commercially for isolation of the components
1z

of air and is being used intermittently to remove radicactive xenon and
krypton from an off-gas stream at ICPP. The process is capable of
recovering krypton and xenon in a relatively pure form suitable for
direct bottling in gas cylinders. A serious concern in this process,
particularly when applied to a fuel reprocessing plant, is the explosion
hazard that results fraom the presence of ozone or mixtures of liquid

oxygen with hydrocarbons and nitrogen oxides.,

Union Carbide Corporation, Linde Division, presently has a contract
to supply cryogenic distillation systems to Philadelphia Electric for
99.9% recovery of noble gases from the effluent of three BWR units at
their proposed Limerick station. In addition, Air Products has designed
a cryogenic distillation system for use in the Newbold Island plant of

the New Jersey Public Service and Gas System.lu

Cryogenic distillation is considered to be one of the two most
promising processes for krypton and xenon recovery and will be discussed

in more detail later.

3.4 Selective Absorption

The study of the separation of noble gases from alr streams by
adsorption in (or extraction by) chlorofluorcmethanes has progressed to
the nonradiocactive pilot plant stage at the Oak Ridge Gaseous Diffusion
Plant.l6’17 The system is versatile, continuous, and adaptable to
scaleup. It also appears to be considerably less subject to fire and
explosion than the previous processes. Primary questions that remain
to be resolved in further development work relate to the tolerance of
the system to contaminants in the off-gas streams, the effects of
radiation damage on the solvent, and corrosion problems that may result
from the evolution of fluorine and chlorine. This is the other process
that is considered to be promising for recovery of krypton and xenon
from reactors and reprocessing plants and will, also, be discussed later

in greater detail.
13

3.5 Permselective Membranes

The permselective membrane process for recovery of krypton and xenon
from air has been investigated on the laboratory scale at ORNL.18 This
process, which is based upon selective permeation of gases through sili-
cone rubber membranes, operates at ambient temperatures but requires
differential pressures across the 1.7-mil-thick membranes of as much as
150 psi. The process requires many stages for effective separation. A
workable large-scale process would require the development of a method
for packaging the membranes to densities of several hundreds of square
feet of active membrane area per cubic foot of volume. The economic
viability of the process would require very large production of membranes
in order to substantially reduce the price of the membrane material below
the present value of $10 per square foot. Some questions with respect
to the radiation stability of the membranes for some reactor applications

also remain. The ORNL development work on this process has been curtailed

in favor of the fluorocarbon absorption process.

3.6 Clathrate Precipitation

The precipitation of noble gases from organic solvents as solid

9,19

clathrates has been investigated on a laboratory scale. The process
requires prior absorption of the krypton and xenon in an organic liquid
at a pressure of about 1000 psi. The solid clathrates form very slowly,
even at these high pressures, and are known to be decomposed by radiation
ahd temperature. These clathrates, as well as all of the known compounds
of krypton, are unstable at temperatures higher than about 50°C. At
present this process can be regarded as little more than a laboratory
phenomenon. In the future it may have gpplication to the solidification

of krypton and xenon for storage.
1L

4.  CRYOGENIC DISTILLATION PROCESS

Figure 1 presents a description and the basis for the cryogenic
distillation system that is used at the ICPP for recovery of krypton
and xenon from a dissclver off-gas stream. The equipment consists of
a gas pretreatment train, a regenerative heat exchanger, the primary
distillation column, and a batch distillation column that is used inter-
mittently for product purification. The off-gas, at a rate of about
20 scfm, is first passed through a catalytic converter at a temperature
of about 1000°F to decompose nitrogen oxides and hydrocarbons and to
convert H, to water. Most of the water is removed in a condenser. The
gas 1s then compressed to about 30 psig; remaining traces of water and
CO, are removed in a demister, absorber bed, and switching regenerative
heat exchanger; and the resultant gas is fed to the primary column.
The column has a diameter varying from 2.5 to 6 in. and an overall height
of 7 ft. Liquid nitrogen, introduced at the top of the column, flows
downward through the sieve plates, condensing and absorbing the higher
boiling components of the gas. The effluent gas cools the condenser of
the batch still, recycles through the regenerator, and passes to the
stack. Bottoms from the column are transferred several times per day
until there is sufficient volume to make a run for purification of the

contained krypton and xenon in the batch still.

The lower part of the figure presents relative boiling and freezing
points of the various gases that are present in the off-gas strean.
These particular data apply at atmospheric pressure; all of the boiling
points are displaced toward higher temperatures at the pressure of
30 psig. The still is operated at a temperature of about 80°K (about
—300°F),at which nitrogen is liquid and relatively volatile. - Hydrogen,
if present, is considerably more volatile and is distilled out the top
of the column. Argon and oxygen, if present, concentrate in the bottom
of the column as a liquid. All of the other gases, normally being solid,
hopefully occur in sufficiently low concentraticn that they are dissolved
in the liquid nitrogen —argon —oxygen solution at the bottom of the
column. Obviously, all significant quantities of water, CO;, nitrogen

oxides, and hydrocarbons must be removed from the inlet gas to prevent
 

  

 

 

RHQOIUM
CATALYTIC
CONVERTER

 

 

PROCESS
OFF-GAS

CONDENSER

Fig. 1.

 

ORNL DWG 7i- 6238

EFFLUENT
TO STACK

 

 

 
     
   

DRYER
Kr STORAGE
Xe STORAGE
DEMISTER RECYCLE
REGENERATOR
PRIMARY
COLUMN BATCH
STILL
BOILING POINT (FREEZING POINT) °K

H>0 373(273)
NO» 294(262)
CO> 194 (194)
CoH> 189 (192)
N,O 184 (182)

Xe
O3
NO
Kr

165(161)  CH4  111(90)
163(80) 0, 90(50)
121 (109) A 87(84)
120 (104) Ny 77(63)

H, 20(14)

Cryogenic Distillation System for Recovery of Kr and Xe.

6T
16

plugging of the column, It is particularly important that hydrogen and
solid forms of acetylene, other hydrocarbons, nitrogén oxides, and ozone
not be allowed to accumulgte in the still since these materials have been

the source of violent explosions in commercial air liquefaction plants.

At the ICPP these hazards are minimized by a high-quality system
for purification of the entering gas and frequent transfer to the batch
still to minimize the accumulation of objectional species. The accumu-
lation of potentially explosive concentrations of ozone from irradiation
of oxygen is a possibility, however. This problem would conceivably be

eliminated by removal of the oxygen in a pretreatment device.

Finally, it should be noted that the ICPP plant has not directly
demonstrated the attributes of sustained, and highly efficient, operation
of the type that is desired for recovery of krypton and xenon at a reactor
or reprocessing plant. The equipment has been operated in campaigns not
exceeding about 1.5 months in duration, and the Kr —Xe recovery has
generally been less than 90%. The reactor operator would prefer to have
a system that offers the potential of maintenance-free operation for
about one year, and Kr—Xe recovery of 99.9% or greater appears to be
a reasonable goal at both reactors and reprocessing plants. The system
does offer the potential of high recovery and reliability as evidenced
by the announcement that two utilities plan to incorporate such a system

in their power plants.

5. FLUOROCARBON ADSORPTION PROCESS

The fluorocarbon absorption process takes advantage of the relative
solubilities of gases in the solvent. Figure 2 shows the dependence of
the temperature on the Henry's law constant of several gases in dichlo-
rodiflucromethane, a typical solvent for use in the process that is
commonly known by the trade name of Freon-l2 or UCON-12, The constant
has units of atmospheres of overpressure of the gas per mole fraction
of gas dissolved in the solvent. The figure shows that both the solu-
bilities and the potential separation factors increase with decreasing

temperature.
17

 

 

HENRY'S LAW CONSTANT

 

 

 

oL L L Ll
-160 -120 -80  -40 0 40 80
TEMPERATURE, °F

 

Fig. 2. Relative Solubilities of Gases in Refrigerant-12.
18

There is very little experimental data on the solubility of the
variety of other gases that will be encountered in applications at nuclear
facilities. On the basis of theoretical considerations, however, it may
be estimated that Nz0, NO;, and CO, have solubilities similar to krypton
and xenon but that CH, , NO, and Hz will have solubilities in the range

of, or less than, those of Ar, Nz, and O,.

Figure 3 presents a schematic flow diagram of the process that is
used in the cold pilot plant. The entering gas is compressed to about
500 psia, cooled to about -L°F, and contacted countercurrently with the
liquid solvent in a packed absorber column. The least-soluble gases
(including nitrogen and oxygen) exit from the top of the column, exchange
heat with the feed gas, and are discharged to the atmosphere. The solvent
from this column — loaded with krypton, xenon, and other soluble gases —
is routed by pressure difference into a packed fractionating column that
is operated at about 30°F and 45 psia. At the lower pressure and higher
temperature, solvent vapor is recycled in the column between the reboiler
and condenser, driving the remaining slightly soluble gases in a recycle
back to the feed stream and concentrating the more soluble gases,
including krypton and xenon, in the liquid solvent flowing down the

column.

The enriched solvent is routed — again by pressure difference — from
the reboiler of the fractionator to a stripper column that is operated
at a temperature of about 12°F and a pressure of about 30 psig. Krypton,
xenon, and other soluble gases are vaporized in the stripper and may be
collected as a concentrated product. Essentially pure solvent, suitable
for recirculation to the absorber, is collected in the reboiler of the

stripper.

The pilot plant equipment is designed for an inlet air flow of
20 scfm. The absorber and fractionator columns have diametersof 3 ih.
and heightsof 10 ft. The stripper column is 6 in. in diameter by 8 ft
tall. In tests with various concentrations of natural krypton and xenon
in air, the system has recovered greater than 99.9% of these noble gases
in a gas stream that has less than 0.001 times the flow rate of the feed

stream.
19

DWG. NO. G-68-520

 

 

 

 

 

 

GAS RECYCLE T0 ABSORBER” FINAL CONDENSER -J---* GAS PRODUCT

 

 

 

 

PRIMARY
CONDENSER

Y

—P- >

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

LINE HEATER 3|$ FLASH FLASH
: UNIT UNIT
GAS
coo&s)\
PACKED —
== g ‘r ool |
LIQUID
CHILLER
GAS =] B
FEED —
COMPRESSOR
LIQUID COOLER
VENT @—
VWA VW
— ABSORBER——— REBOILER _ FRACTIONATOR _REBOILER
— STRIPPER
LIQUID
SOLVENT PUMP LIQUID PRECOOLER STORAGE
TANK

Fig. 3. Krypton-Xenon Absorption Process Pilot Plant Schematic Flow Diagram.
20

The preliminary results of the pilot plant tests with this system
sre very encouraging. The indications are that the process is safe from
the point of view of explosion and fire and can be operated for sustained
periods at high efficiency with low maintenance requirements. The process
is not yet ready for commercial applications, however. Further develop-
ment work is necessary to determine the disposition of contaminant gases
in the process and the effects of radiation degradation of the solvent.
Radiation damage of the solvent, particularly in the high radiation fields
of reactor applications, may generate fluorine and chlorine that could
cause corrosion problems. It is clear that the feed gas must be sub-
stantially free of water in order to avoid freezeout and the formation
of HF and HCl. The tolerance of the system to impurities such as NOg
will be a major consideration in developing the necessary devices for
pretreatment of gas streams in a reprocessing plant. Work directed

toward the resolution of all of these questions presently is 1n progress.

6. CONCEPTUAL SYSTEMS FOR RECOVERY OF NOBLE GASES

This section will present very preliminary concepts of flowsheets
that might be used for removal of krypton and xenon from the effluents

20,21 22

of power reactors and fuel reprocessing plants.

6.1 Boiling Water Reactors

Figure 4 presents concepts of systems for holdup or recovery of
krypton and xenon in a 1000-MW(e) Boiling Water Reactor, assuming, very
pessimistically, that 1% of the total noble gas fission products produced
in the reactor are released to the off-gas. The effluent from the reactor
is assumed to consist of steam; hydrogen and oxygen from radiolytic
decomposition of the cooling water; and 50 cfm of ailr, which contributes
about three times as much krypton and xenon from natural sources as is
assumed to be released from the fuel. Pretreatment of this off-gas
stream would conceivably consist of catalytic recombination of Hy; and
Oz, condensation of the steam, a 30-min holdup for decay of the short-
lived isotopes, and filtration to remove particles — especially the solid

species of Sr, Rb, Cs, and Ba that are formed from decay of the Kr and Xe.
TURBINE

CONDENSATE

REACTOR

50 cfm
AlR
LEAKAGE

CONDENSER

 

Fig. L.

STEAM

| % OF Xe AND Kr

7
STEAM
JET

CONDENSATE

Conceptual
from a 1000-MW(e) BWR.

 

[CATALYTIC

 

RECOMBINER

 

 

30 min
HOLDUP

 

 

]

 

 

PARTICULATE

FILTERS

 

 

 

g——f

ORNL DWG 71-6243

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

CHARCOAL ADSORBERS TO STACK gs
f——= 3 DAY DELAY FOR Kr p——@= 3000 Ci/yr pad d
45 DAY DELAY FOR Xe 3000 Ci/yr Xe
REMOVE LIQUID
§——i 0,, H,0,CO, NITROGEN —
HYDROCARBONS DISTILLATION
TO STACK
l 3 Ci/yr 83kr
1000 Cizyr P33 xeo
. :E:Wch | FLUOROCARBON
! 27 ¥V2 ABSORPTION -
HYDROCARBONS

 

 

 

 

 

 

'

ANNUAL PRODUCT:
190 scf A
27 st K
T
3000 Ci %k
20000 Ci 133 xq
ONE SOLITER CYLINDER AT 1700 psia

Systems for Holdup or Recovery of Noble Gases

12
2z

A room-temperature charcoal holdup bed for this reactor, providing
3 days of decay for krypton and 45 days for xenon, would conceivably
reduce the radioactivity of the gas by a factor of more than 1000 and
would limit the annual release of 8BKr and 32Xe to about 3000 curies

each.

A liquid nitrogen distillation system for this reactor, assumed to
provide for 99.9% recovery of the krypton and xenon, would limit the annual
release to the atmosphere to about 3 curies of %Kr and 1000 curies of
183¥e. It is assumed that oxygen (together with H,O and COp) would be
removed from the feed gas such that the product accumulated over a year
would contein about 20% krypton plus xenon in argon. The annual product
from this system could be bottled in a 50-liter gas cylinder at a pressure
of about 1700 psia.

The fluorocarbon absorption system would also require prior removal
of HyO0 and CO; — but not oxygen — and could conceivably achieve the same
recovery of noble gases and produce a product that also contains about

20% noble gases in argon.

While we have no definitive estimates on the costs of the systems
pictured here, it is likely that the cryogenic distillation system would
have higher capital and operating costs than the fluorocarbon absorption
system because of the assumed requirement for removal of oxygen and the
higher refrigeration costs associated with operation at lower temperature.
On the other hand, the absorption system has the disadvantage that we

know relatively less about it.

6.2 Tuel Reprocessing Plant

Figure 5 presents a conceptual design of a system for 99.9% recovery
of 8%Kr from the off-gas of a fuel reprocessing plant that has capacity of
5 metric tons of spent fuel per day (i.e., capability of servicing about
fifty 1000-MW(e) reactors). Conceptually, the off-gas would first be
treated in a system that is being developed at ORNL, which would provide
for recovery of tritium and greatly enhance recovery of iodine. The

process equipment would also be tightly sealed to maintain the off-gas
 

 

 

 

 

ORNL DWG 71-8244

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

TO VESSEL
FU'EL OFF GAS
W N2,02 o
14,000 Ci/yr of "'xr
AIR—|{ SHEAR M REMOVAL PRODUCT
¢
al CATALYTIC NO
¢ OXIDIZER [~®|ORYER COs
Xe
O2 VOLOXIDIZER Kr
COLD TRAP
OR
AIR—» DISSOLVER| ADSORBER
Y I,H0
i SCRUBBER REMOVAL
W ABSORPTION
SYSTEM

2 % NOz
12 ppm Kr

100 CFM OF AIR}
HOO ppm Xe

 

 

ABSORBER

 

 

 

 

 

 

BOTTLING
Xe
K" 80 LITER CYLINDERS
AT 2200 PS|

8! CYLINDERS/ YEAR
180,000 Ci 98K¢/CYLINDER

Fig. 5. Fluorocarbon Absorption System for Noble Gas Recovery in a
5 Ton-Per-Day Reprocessing Plant.

¢e
24

rate from the dissolver and other head-end operations at the relatively
low rate of 100 scfm, About 99% of the tritium, together with most of

the iodine and noble gases, would be evolved during shearing of the fuel
and "voloxidation" — a process step in which the chopped fuel is contacted
with oxygen for about L hr at a temperature of about 500°C. Off-gases
from these steps would be oxidized in a catalyst bed, dried for removal

of HTO, and combined with the scrubbed gases from the dissolver that

would contain most of the rest of the radioiodine. These gases would
then be passed into a cold trap or adsorber bed for removal of iodine

and water.

Conceivably, the dried gases would then be suitable as feed to the
fluorocarbon absorption system. The discharge to the vessel off-gas for
the next hierarchy of iodine removal treatment (but no more removal of
®®Kr prior to discharge to the atmosphere) would contain N,, O,, and
0.1% of the 85Kr from the fuel — corresponding to an annual release of
about 14,000 Ci/year. The product from the absorption system, primarily
NO; and COp; with small concentrations of krypton and xenon, could be
scrubbed for removal of NO; and COz and then bottled to produce approxi-
mately 81 50-liter gas cylinders per year, each having a gas pressure of

about 2200 psi and a 8PKr content of 180,000 curies.

6.3 Swmary of Conceptual Disposition of
Noble Gases at Nuclear Facilities

Table 4 summarizes the conceptual disposition of noble gases at
power and reprocessing plants that are assumed to recover 99.9% of the
noble gases that enter the off-gas streams. BWRs and PWRs would con-
ceivably release approximately the same annual quantity of krypton and
xenon, while the reprocessing plant — which serves 50 reactors and
releases 99% of the total gas — would release about 5000 times as much
®%Kr as a single 1000-MW(e) reactor. The PWR would conceivably produce
only about one-fourth as many gas cylinders as the BWR because the noble

gases would not be mixed with krypton and xenon from natural sources.

There is relatively little incentive to produce a krypton-xenon

product at a reactor that is more concentrated than that illustrated
25

Table 4. Conceptual Disposition of Noble Gases for Planning of Storage
or Disposal Facilities

 

1300-metric ton/yr

 

1000-MW(e ) 1000 -MW(e ) Fuel Recovery
BWR PWR Plant

Release to Atmosphere

85Ky, curies 3 3 14,000

133Xe, curies 1,000 1,000 L
Gas Cylinders per Year 1 1/h 81
Content of a Cylinder

Pressure, psia 1,700 1,700 2,200

Argon, scf 190 190 -

Krypton, scf 27 L 56

Xenon, scf 11 35 >15

85Kr, curies 3,000 10,700 180,000

133Ye, curies 20,000 20,000 4o

Thermal power, watts 26 39 290

 
26

here because the generation of only about one cylinder per year is quite
manageable. The relatively pure Kr-Xe produced from the reprocessing
plant could conceivably be concentrated further by removal of the non-
radicactive xenon, but this would only serve to compound the problem of
heat removal and make the gas more hazardous to handle. The 50-liter
gas cylinder containing this concentrated Kr-Xe mixture would require

the shielding equivalent of about 2 in, of steel.

The cylinders might conceivably be stored at the reactor or reproc-
essing plant site for periocds of months to years in a sealed compartment
of a water-filled canal. The water would provide for isclation of
cylinders, shielding, and heat dissipation. Air from a small-volume
containment zone above the water could be recycled through the recovery

system in the event of a leak in one or more of the cylinders.

7. SHIPMENT OF NOBLE GASES

Since it is very unlikely that each power and reprocessing plant
would have suitable facllities to provide storage of the gas for the
period of 100 to 200 years that is required for decay of the 8BKr to
insignificant levels, it is probable that the gas would eventually be
shipped off-site to a repository that is either operated or licensed by
the AEC.

A variety of methods that would promote safety in shipment of
19,23

concentrated noble gases have been considered. The preliminary
conclusion of these studies was that the gas can be shipped safely,

using current technology, in cylinders within casks that are patterned
after those that currently exist for shipment of other nuclear materials.
It is probable that a few types of heavy-wall cylinders will become
standard for shipment of krypton and xenon in order to facilitate the
sharing of shipping containers and handling of cylinders at a storage
site. Methods for dispersing the gas in solid materials may be developed

later as a further means of enhancing their containment during shipment.19

A current concept of a container for shipment of six cylinders of

noble gas is depicted in Fig. 6. Cylinders inside the cask are surrounded
STEEL RASCHIG RINGS

FUSIBLE g
PLUG —
(TYP 16

PLACES)

i

| ~L
P
-

K5

 

£ COOLING

 

 

ek

~a

 

)
1 —INSULATION

P
Ly
N

 

 

 

 

 

 

 

 

250 ° k.

ORNL - DWG- 71-3839

 

lrfg
e
==
=l
L) AL NOTE:
// { EX747  INSULATION INSTALLED IN
X /] . SEGMENTS & SO MOUNTED SECTION
LA, AS TO REMOVE ITSELF WHEN
Sm— WALL TEMPERATURE REACHES

Fig. 6. Conceptual Design of Noble Gas Shipping Cask.

Lz
28

by water and steel raschig rings. The rings provide a cushion against
impact. The water, together with the self-removing insulation, would
lengthen the period of exposure to a fire required to cause release of

gas from the bottles.

The type of shipping container depicted here has very low potential
for release of noble gas in conceivable accidents. It should also be
noted that ®°Kr is among the least hazardous of the radioactive materials.
The type of accident that would be required to rupture a cylinder — severe
impact followed by a fire of long duration — would probably disperse the
gas sufficiently such that there would be no detectable radiation

exposures.

8. METHODS FOR LONG-TERM STORAGE

Methods that have been considered for storage of the noble gas for
the 100 to 200 years that are required for 8%Kr decay are summarized in
Table 5.

8.1 Storage of Cylinders in Surface Vaults

Based upon current technology, one practical method for storage of
the noble gas would involve the storage of gas cylinders in an air-or
water-cooled vault at a large and remote government-owned site. The
vault would contain provisions to protect the cylinders from corrosion
and to isolate them from credible internal and external forces. Eventually,
it would probably be necessary to provide a system for recovery of any
krypton that may leak from the cylinders into the ventilation system of
the vault.

This type of storage, although it requires a relatively high degree
of surveillance, is probably most acceptable for the near term. Cylinders
could be maintained in a fully retrievable condition for a relatively long
time while other storage procedures that require less surveillance activity

are being investigated.
29

Table 5. Methods for Long-Term Storage or
Disposal of Noble Gas Wastes

 

Favorable
Storage of cylinders in surface vaults.
Burial of cylinders.

Injection into porous underground formations.

Unfavorable
Controlled release to atmosphere at remote sites.
Nuclear transmutation.

Disposal in space.

 
30

8.2 Burial of Cylinders

The storage of cylinders within deep, grouted holes in a suitable
geologic formation is a method that should be considered for longer-term
application. This concept will require development work to assure that
the cylinders are protected from corrosion, that the holes are well-
sealed, and that the diffusion of any leaked gas will be sufficiently

slow to provide for appreciable decay of the B8BKr.

8.3 1Injection into Porous Underground Formations

A disposal method for noble gases that has been considered in some
detail involves injecting the gas into porous underground formations.zu_z?
An acceptable formation for this purpose would have to be overlain with

a capping formation of very low permesgbility; be free of cracks, fractures,
and unsealed drill holes; and be located in a zone of low seismic risk.
These considerations are sufficiently restrictive and the necessary
geologic proofs are sufficiently time-consuming and expensive to lead

to the conclusion that this method is of somewhat longer range applica-

tion than the storage of cylinders in vaults.

8.4 Relatively Unfavorable Storage Methods

The controlled release of the recovered noble gases to the atmos-
phere at a remote site is not considered an acceptable method of disposal
since it would not resolve the envisaged problem of long-term buildup of

8Ky in the troposphere.

The transmutation of 8®Kr to nonradioactive 8%Kr by neutron capture
1s technically infeasible at present because of the very low neutron
capture cross section of B®Kr. Pure 8%Kr would be destroyed by neutron
capture in a power reactor at a rate of less than 1% per year. This is

considerably less than the rate of transmutation by radiocactive decay.

It is theoretically possible, through the introduction of a 0.3-MeV
gamma ray, to convert 88Kr to its short-lived isomer 25WKr, which decays

principally to stable 8°Rb. Such a process, which would require an
31

environment at near absolute zero temperatures and copious production
of gamma rays with precisely the correct energy to effect the transition,

is not feasible within the current limits of technology.

Finally, disposal into the sun would require the development of
booster systems that are considerably more reliable and inexpensive

than those that presently exist or can be foreseen.

9. CONCLUSIONS

In conclusion, there are two processes in an advanced state of
development that appear to be capable of reliably recovering up to 99.9%
of xenon and krypton from effluents of reactors and fuel processing
plants. Commercial companies who have experience with liquid air plants
and are familiar with the work at ICPP can now design a cryogenic dis-
tillation system, particularly for use on a reactor, that has high
probability of success. It may be advisable to incorporate features
such as O, removal to minimize the possibility of ozone explosions.

Much of the required development work and proof testing on the system
could be done within the year or more that would be required to place

the system in operation.

A fluorocarbon absorption system can also be designed today,
particularly for use on a reactor, that has good probability of success.
A system designed today, however, would probably incorporate much con-
servatism in order to compensate for unknowns with respect to the required
pretreatment of gas, radiation damage, and corrosion effects. Many of
the uncertainties will be resolved in the continuing development program

at the ORGDP.

It will probably be worthwhile to wait a few years before installing
a system for recovery of xenon and krypton from a reprocessing plant,
pending the development of low flow off-gas systems that would also have
capability for recovery of tritium, and greatly enhanced recovery of

iodine.
32

Initially, most of the recovered noble gas may be shipped within

cylinders to a remote site. The cylinders would probably be stored

there in vaults in a retrievable fashion,pending development work on

storage systems that require less surveillance.

10.

11.

1z,

13.

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U.S. Joint Congressional Committee on Atomic Energy, Environmental
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Lester Rogers and Carl C. Gemertsfelder, U.S. Regulations for the
Control of Releases of Radioactivity to the Environment in Effluents
from Nuclear Facilities, IAEA-SM-14618 (Aug. 10-1L, 1970).

 

 

 

ORNL Staff, Siting of Fuel Reprocessing Plants and Waste Management
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"AEC Formulates 'As Low As Practicable’ Numbers ," Nuéleonics Week
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The 1971 World Almanac (1971).

 

"Population Crisis,” Hearings Before the Subcommittee on Foreign Aid
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B. I. Spinrad, "The Role of Nuclear Power in Meeting World Energy
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F. G. Welfare et al., The Oak Ridge Systems Analysis Code (ORSAC)

 

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"A Radiation Decay Factor of 2000 for Charcoal Filters," Nucleonics
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C. L. Bendixsen and G. F. Offutt, Rare Gas Recovery Facility at
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R. H. Rainey, W. L. Carter, and S. Blumkin, Completion Report —
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W. E. Clark and R. E. Blanco, Encapsulation of Noble Fission
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R. A. Loose and J. A. Battaglia, "Design Considerations in PWR Waste
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R. H. Jason and J. E. Ward, "Nuclear Waste Management Practices and
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0. 0. Yarbro, D. J. Crouse, and G. I. Cathers, "Iodine Behavior and
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J. 0. Blomeke and J. J. Perona, Management of Noble-Gas Fission-
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1969 ).

 

 

D. G. Jacobs, "Behavior of Radioactive Gases Discharged to the Ground,"
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J. B. Robertson, Behavior of Xenon 133 Gas After Injection Underground,
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K. E. Cowser et al., Health Phys. Div. Ann. Progr. Rept. July 31,
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62
63

O CO~1 0w Fwo

35

INTERNAL DISTRIBUTION

R. G. Affel 13-32., J. P. Nichols

F. T, Binford 33. D. B. Trauger

R. E. Blanco 34. W. E. Unger

J. 0. Blomeke 35. A. M. Weinberg

A. L. Boch 36. 0. 0. Yarbro

R. S. Carlsmith 37-38. Central Research Library
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D. E. Ferguson 58. Laboratory Records-RC

J. M. Holmes 59. ORNL Patent Office

W. H. Jordan 60-61. DTIE

W. C. McClain

EXTERNAL DISTRIBUTION

R. L. Philippone, AEC-ORNL RDT Site Office.
W. M. Culkowski, AEC-ORO ATDL.

AFEC Washington

76.

7.
78.

79.
80.

81.

82,

83.
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85.

Bartlett, WSM
Belter, OEA
Chitwood, DML
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H. McVey, RDT

F. Perge, WSM

H. Regan, RDT
schneider, RDT

F. Soule, WSM

R. Workinger, IML

Crwew

HESREPE=2000=20

R. I. Newman, Allied Chemical Nuclear Products, Inc., P.0. Box 35,
Florham Park, New Jersey 07932.

R. G. Barnes, General Electric Company, 175 Curtner Avenue,

San Jose, California 95125,

W. H. Lewis, Vice President, Nuclear Fuel Services, Inc.,

6000 Executive Blvd., Suite 600, Rockville, Maryland 20852,

Carl R. Cooley, WADCO, Inc., P.0. Box 1970, Richland, Washington 99352,
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Laboratory, Aiken, South Carolina 2980L.

R. E. Tomlinson, Atlantic Richfield Hanford Co., Attention: Document
Control, P.O. Box 250, Richland, Washington 99352.

D. S. Webster, Argonne National Laboratory, 9700 South Cass Avenue,
Argonne, Tllinois 60L439.

Eugene V. Benton, Research Professor, Univ. of San Francisco,

San Francisco, California 94117.

R. Johnson, Environmental Radiation Branch-NERHL, 109 Holton St.,
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95-96.

97.

36

John Brandt, Ruder & Finn, Inc., 110 East Fifty-Ninth Street,

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