IL H
DESIGN FOR KRYPTON-85 ENRICHMENT
BY THERMAL DIFFUSION
Roger A. Schwind and William M. Rutherford
Monsanto Research Corporation
Mound Laboratory*
Miamisburg, Ohio 45342
Substantial quantities*of krypton having a krypton-85 concentration
of less than 10% will become available if nuclear fuel-processing
plants are required to collect the gaseous fission products
rather than releasing them into the atmosphere. A modular
thermal diffusion unit was designed for the enrichment of the
krypton-85 to useful concentrations of greater than 45%. The
design emphasizes reliability and integrity by incorporating
no moving parts within the unit.
The modular design also offers
flexibility in the size of the enrichment facility that need
be constructed at any time.
*Mound Laboratory is operated by Monsanto Research Corporation for
the U. S. Atomic Energy Commission under Contract No. AT-33-1-GEN-53.
This document is
ILICLY RELEASABLE
f OJDQQJS
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INTRODUCTION
Krypton-85 is of value in scientific and technological applications.
It is also a potential source of environmental contamination
so that the recovery and retention of this noble gas may be
required in the near future. » » »
The nuclear power industry
is expanding at a rapid rate, and the nuclear fuel reprocessing
industry will also expand.
A direct result of this will be an
increased production rate of krypton-85 and its possible release
to the environment during nuclear fuel reprocessing.
Although
krypton-85 release rates at reprocessing plants are presently
deemed acceptable, release rates from larger plants under design
will probably have to be controlled.
In addition, krypton-85 has several uses based upon the fact
that it gives off a 0.72 MeV beta particle, a 0.54 MeV gamma,
and has a half-life of approximately 10 years.
It has been
used in radiation stimulated light sources, leak detection
equipment and thickness gages.
It has also been incorporated
into solid materials by preparation of kryptonates. '
Higher
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isotopic concentrations of krypton-85 than are available from
nuclear fuel reprocessing would be useful for preparing kryptonates
with greater specific activity.
At concentrations of 40-50%,
krypton-85 could possibly be used as a heat source fuel.
This paper describes a modular thermal diffusion system for
the isotopic enrichment of krypton-85 emanating from nuclear fuel
reprocessing plants.
CASCADE DESIGN
Krypton available from fuel reprocessing is expected to have
a
Kr concentration of less than 10%. For the purposes of this
discussion, it will be assumed that other techniques (such as
adsorption) will be applied to separate krypton from other
gaseous fission products.
The exact concentration of the fission product gas will vary
depending on the neutron flux of the reactor from which the
fuel was recovered and the time elapsed since the fuel was
removed from the reactor.
In order to be consistent with a
previous krypton-85 cascade design prepared by ORNL? and also
in order not to be too optimistic with regard to the krypton-85
concentration in the feed gas, the following isotope composition
3
was assumed as typical for the fission product krypton gas.
Volume °/o
Krypton-86
51
Krypton-85
4
Krypton-84
30
Krypton-83
15
Small variations from these concentrations would not affect
the results or conclusions of this article.
A double cascade of thermal diffusion columns was designed to
enrich the krypton-85 to an enrichment of greater than 45%.
In the first cascade the components lighter than
J
Kr are
removed; in the second cascade the heavier components are removed.
(Alternately, the roles of the two cascades can be reversed).
This design problem was approached in two steps.
The first
step was the application of multicomponent cascade theory to
determine the size and shape of the two idealized cascades.
The sepond step was to approximate these smooth cascade shapes
with three different types of thermal diffusion columns.
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The performance of the squared-off cascades was then calculated
by a computer program which describes the steady-state behavior
of each cascade, respectively.
The key weight or M* cascade design method originally described
Q
by De la Garza, 1963, was used to obtain the ideal double cascade
size and shape required to obtain 45% krypton-85.
Krypton-85
can be enriched in either of two separate double cascade systems.
In one double cascade system the krypton-85 would be enriched
at the top of the second cascade while in the other type the
krypton-85 is enriched at the bottom of the second cascade.
It
was decided to use the double cascade system in which the
krypton-85 is enriched at the top of the second cascade because
the highly enriched stream would be physically more accessible
and, therefore, easier operation would result.
The other type
of double cascade system ideally requires slightly less (2-3
per cent) separation capacity than the first arrangement.
However, it is believed that the operational convenience outweighs
the slightly smaller equipment that might be required in the
other system.
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Three different size columns were used to fit the smooth curve
of the M* cascade shapes. The column dimensions, operating
conditions, and calculated transport coefficients for these
columns are given in Table 1. A schematic of the results of
this squaring off of the cascades is shown in Figure 1. The
steady-state flow rate and concentrations are also presented
in Figure 1. The logarithm of the separation factor used in
the calculations is equal to 80 percent of the theoretical value.
It can be seen that 0.042 liter/day of krypton containing greater
than 46% krypton-85 can be produced in a 16-column arrangement.
The total power input to this 16-column system would be 61
kilowatts.
EQUIPMENT CONCEPTS
As described in the section on cascade design, an efficient
separation system for krypton-85 necessarily requires two
coupled multiple column cascades.
Each cascade must be tapered
so that it has a high transport capacity at the feed point and
tapers to a low capacity and low holdup at the product ends.
It has been normal practice at Mound Laboratory and elsewhere
to assemble such cascades from a large number of identical
thermal diffusion columns.
Tapering is accomplished by variation
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of the number of columns in parallel along the length of the
cascade.
In principle this approach is effective; however,
in practice parasitic flows occur in the parallel sections
unless special, complex, timed valving arrangements are installed
to interrupt the flows.
Such cascades have been successfully operated at Mound Laboratory
Q
for a number of stable isotope separations, including krypton.
Theoretical, computational and experimental techniques are now
well established;
»
and the performance of these units can
be predicted with confidence.
In dealing with radioactive materials, it is undesirable to
use complicated valving systems.
In addition, it is undesirable
to have any components such as interstage circulation pumps
which would be subject to failure or which would require periodic
service.
A system which meets these severe limitations has been successfully
worked out.
1.
The proposed system has these major features:
Effective taper of the cascades is accomplished by
variation of the cold wall diameter.
There are no
columns in parallel.
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2.
The dual cascade is a single modular unit approximately
30 cm in diameter and 10 m long.
(A schematic of the
modular arrangement is shown in Figure 2.)
3.
There are no moving parts within the unit.
Interstage
circulation is established by thermosiphon convection.
4.
Production capacity can be increased indefinitely by
the addition of modular units.
5.
The unit can be completely sealed by welding or brazing
with the exception of valve outlets for feed, product
waste, and samples.
6.
For shielding, each unit can be placed in a small
diameter cased hole in the ground.
The proposed column design is quite similar to that used routinely
at Mound Laboratory. 12
The hot wall is the outer surface of a
4-mm OD tubular electric heater with a 7.3 m long active section.
The cold wall is a water cooled stainless steel tube, the inside
diameter of which depends on position of the column in the
cascade.
In keeping with standard practice at Mound Laboratory,
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spacers are spot welded to the heater at 40 cm intervals to
maintain axial alignment of the heater.
The columns comprising a cascade are connected in series by
thermosiphons.
well known.
The use of thermosiphons for this purpose is
The principal drawback of such devices, high gas
holdup, is offset by the advantage that there are no moving
parts and no seals. Therefore, maintenance is minimized.
The columns of the two cascades are placed in a circle around
a common thermosiphon heater, and the assembly is enclosed
in a single water jacket. With reasonable care in layout, it
should be possible to insert the assembled system inside an
18-in diameter cased hole in the ground.
Detailed operational tests with non-radioactive krypton would
be necessary to verify design calculations.
The non-radioactive
experiments would verify calculated column parameters and
interstage transport rates. With this experimental information
at hand, subsequent performance with krypton-85 could be quite
accurately predicted.
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At the completion of this testing program, a prototype unit
would be available for installation in a suitably shielded and
ventilated facility for production of krypton-85.
This design
is applicable to a wide range of production rates since the
choice of the number of modules in the facility is arbitrary.
SUMMARY
The above modular thermal diffusion unit is a reliable and
flexible design.
Reliability is a necessity to minimize the
probability of any release of krypton-85 into the environment.
Flexibility is desirable to allow for future increases in
production.
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Table 1.
Column Parameters
Column A
Column B
Column C
Effective Length, L, cm
731.5
731.5
731.5
Cold Wall Radius, cm.
0.687
0.629
0.549
Hot Wall Radius, cm.
0.4
0.4
0.4
Cold Wall, Temperature °C
44
44
44
•Hot Wall, Temperature °C
750
750
700
Operating Pressure, Torr
3146
3146
3146
Initial Transport Coeff.
H 0 , gram/sec.
2.959 x 10~ 5
1.389 x 10"5
0.338 x 10"5
Convective Remixing Coeff.
K c , gram cm/sec.
0.03189
5.861 x 10"3
2.816 x 10 - 4
Diffusive Remixing Coeff.
&d> gram cm/sec
6.217 x 10' 4
4.752 x 10~ 4
2.816 x 10~ 4
K c /K d
51.3
12.3
1.0
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CASCADE II
CASCADE I
Top Product I I
Top Product I
R
B
Feed I
0.737 liter/day,51% Kr-86
4% Kr-85
30% Kr-84
15% Kr-83
Kr-86
Kr-85
Kr-84
Kr-83
B
B
A
B
A
B
B
Bottom Product I ,
Feed I I
0.419 liter/day/
89.45%
5.94%
4.56%
0.05%
Kr-86
Kr-85
Kr.84
Kr-83
0.042 liter/dayf
7.69% Kr-86
46.42% Kr-85
45.38% Kr-84
0.51% Kr-83
C
A
A
*
0.37%
1.44%
63.50%
34.69%
_
A
A
B
B
^
Bottom Product I I
0.377 liter/day r98.47% Kr-86
1.47% Kr-85
0.06% Kr-84
i-1
Figure 1 - Double cascade design and
steady-state performance for krypton-85 enrichment.
A, B, and C designate three thermal diffusion columns
having different operating characteristics.
Common, central heated
Feed I
chamber for thermosiphon
lines
Top product I
Bottom product I
• » Cooling water
Top product II
Bottom product II
Feed II
Cooling
water
Cooling
\mm„.£
water
MM
, n j i , n „ n . , o.ti-r o_
Water j a c k e t (thermosiphon
return l i n e s are i n s i d e
jacket)
irai
Figure 2 - Modular thermal diffusion unit for krypton-85
enrichment. (Top and frontal views are shown.)
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REFERENCES
1.
C. A. Rohrmann, "Fission Product Xenon and Krypton - An
Opportunity for Large-Scale Utilization", Isotop. Radiat.
Technol., 8(3): 253-260, (Spring 1971).
2.
M. J. Stephenson, J. R. Merriman, and D. I. Dunthorn,
Application of the Selective Absorption Process to the
Removal of Krypton and Xenon from Reactor Off-Gas,
Union Carbide Corporation, Nuclear Division, Oak Ridge
Gaseous Diffusion Plant, February 14, 1972 (K-L-6288).
3.
0. 0. Yarbro, J. P. Nichols, W. E. Unger, "Environmental
Protection During Fuel Processing", Oak Ridge National
Laboratory, paper presented at 72nd National AIChE Meeting,
St. Louis, May, 1972.
4.
C. L. Bendixsen, G. F. Offutt, B. R. Wheeler, "Rare Gas
Recovery Facility at the Idaho Chemical Processing Plant",
Idaho Nuclear Corporation, paper presented at winter meeting
of American Nuclear Society, San Francisco, November 30December 4, 1969.
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REFERENCES (Contd.)
5.
J. E. Carden, "Preparation, Properties, and Uses of Kryptonates
in Chemical Analyses", Isotop. Radiat. Techno1. 3(3): 206-14
(Spring 1966).
6.
J. E. Carden, "Applications of the Kryptonates in Materials
Research", Isotop. Radiat. Technol. 3(4): 318-28 (Summer
1966).
7.
K. H. Lin, A Conceptual Design of a Continuous Thermal
Diffusion Plant for
5
Kr Enrichment, Union Carbide Corporation,
Nuclear Division, Oak Ridge National Laboratory, February 1969,
(ORNL-4372).
8.
A. De la Garza,
The Variable Key Weight Cascade for
Multi-Component Isotope Separation, Union Carbide Corporation,
Nuclear Division, Oak Ridge Gaseous Diffusion Plant, 1963,
(K-1571).
9.
Stable Gaseous Isotope Separation and Purification: AprilJune 1972, MLM-1943 (August 18, 1972), pp. 9-10.
15
10.
Stable
Gaseous Isotope Separation and Purification:
October-December 1969, MLM-1614 (May 8, 1970), pp. 42-50.
11.
Stable
Gaseous Isotope Separation and Purification:
January-March 1972, MLM-1904 (July 10, 1972), pp. 8-15.
12.
W. M. Rutherford, F. W. Weyler, and C. F. Eck, "Apparatus
for the Thermal Diffusion Separation of Stable Gaseous
Isotopes", Rev. Sci. Inst., 39 (1), pp. 94-100, (1968).
13.
W. J. Roos and W. M. Rutherford, "Experimental Verification,
with Krypton, of the Theory of the Thermal-Diffusion Column
for Multicomponent Systems", J. Chem. Phys., 50(1),
pp. 424-429, (1969).
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