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MICROCOPY RESOLUTION TEST CHART
NATIONAL BUREAU OF STANDARDS – 1963
Opnu-p-1581
Conf.6508211
SEP21 1965
DESORPTION OF RADIOIODINE FROM CLAYS,
D. G. Jacobs
RELEASED POR ANNOINCEMENT
IN NUCLEAR SCIENCE ADSTRACTS
Health Physics Division
Oak Ridge National Laboratory
Oak Ridge, Tennessee
Abstract
In the event of a fuel meltdown incident in a nuclear power reactor,
large quantities of radionuclides in the gaseous state would be released
to the containment shell. To alleviate the hazards upon release to the
environment, it has been suggested that these highly radioactive gases
be discharged into permeable formations in the ground. Iodine isotopes
present the most serious hazard potential; hence, it is necessary to be
able to predict their behavior in the ground.
Clays and sediments were contaminated with lodine vapor from the
oxidation of tagged KI. When the iodine was displaced with dry air pre-
heated to various temperatures, the heat of desorption increased from
7.3 to 16.7 kcal/mol as the surface concentration decreased. These values
suggest that the adsorbed iodine is a mixture of HI, which has a heat of
vaporization of 4.9 kcal/mol, In, which has a heat of sublimation of
14.7 kcal/mol, and adsorbed iodine, which has an even higher energy of
desorption.
The rate of desorption decreases with decreased surface concentration,
and after 24 hours the residual concentration is proportional to the
Research sponsored by the U. S. Atomic Energy Commission under
contract with the Union Carbide Corporation.
"For publication in the Proceedings of the Clay Minerals Society Meeting,
Berkeley, California, August 23-26, 1965.
re
reciprocal of the absolute temperature of the desorbing air. When the
water vapor content of the desorbing air is increased, desorption occurs
more readily. Most of the lodine 1s easily removed with tap water, but
again it was noted that the desorption rate decreases with a decreased
surface concentration.
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Introduction
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In the event of a fuel meltdown incident in a nuclear power reactor,
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Large quantities of radionuclides in the gaseous state would be released
to the containment shelí. Much of the radioactive gas would adsorb rapidly
ou surfaces but a portion 18 likely to escape from the containment shell and
be deposited in the surrounding environment. The predicted areal extent of
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severe contamination of the environment in the event of such an incident is :
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used in developing siting criteria for power reactors. To alleviate the
hazards upon release to the environment, it has been suggested that these
highly radioactive gases be discharged into permeable formations in the
ground and held there until they have decayed to acceptable levels of radio-
activity. Iodine 1sotopes present the most serious hazard potential; hence,
it is necessary to be able to predict their behavior in the ground.
Desorption of Iodine with Air
Clinch River Floodplain Sediment
Sediment from the clinch River floodplain was contaminated with lodine
vapor from the oxidation of tagged KI with FeNH, (SOM), to a level of ap-
proximately 0.004 millimole 17/8 of sediment. The surface area of the
sediment sample, determined by the Analytical Chemistry Division, was about
4 m/8. Thus the original lodine loading of the sediment was about
10-7 millImole I, ema.
J. J. D1Nunno, F. D. Anderson, R. E. Baker, and R. L. Waterflela,
"Calculation of Distance Factors for Power and Test Reactor Siites," TID-24844,
March 23, 1962.
A. C. Chamberlain, A. E. J. Eggleton, W. J. Megaw and J. B. Morris,
"Physical Chemistry of Iodine and Removal of Iodine from Gas Streams,"
Nucl. Sci. and Tech. 17 519-550 (1963).
PA. C. Chamberlain, Aerodynamic Capture of Particles, pp. 63-88,
Pergamon Press, 1960.

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One-gram samples of the sediment were weighed into glass sample tubes
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equipped with ground, tapered joints to accommodate inlet and outlet fittings.
The assemblies were placed in small ovens for temperature control, and air
was passed through the samples at a rate of 120 m2/min. The outlet lines
led into scrutbers containing 1 M NaOH. At various times the desorption
process was halted and the samples counted in an NaI(TI) well-type crystal
with a single-channel analyzer.
The results (Fig. 1) show that desorption occurs in two phases. The
Initial desorption is quite rapid with the rate dependent on temperature,
apparently because a portion of the lodine 18 only condensed in the pores
or is loosely bound. After the mobile iodine has been removed, the "bound"
lodine is desorbed very slowly. After 300 hr, more than 30% of the lodine
remained on the samples subjected to air flow at room temperature. Experiment-
ally, no relationship was found between temperature and the rate of desorption
of the bound lodine.
An attempt was made to determine the activation energy for the desorp-
tion process from the desorption velocities at various temperatures. The
data were fitted to the equation:
In u = a - E/RT,
where
a = a constant,
= the desorption velocity (moles cmsec),
= the activati.on energy (cal/mole),
R = the gas constant,
H
T = the absolute temperature.
"0. O. Hayward and B. M. W. Trapnell, Chemisorption, p. 141, Butterworth's,
Washington, D. C. 1.964.

The values of E and a were obtained from the fit of the data and are shown
in Table ... The value of "a" remains constant with loading, but the activa-
tion energy increases with a decrease in loading. This indicates, as
mentioned above, that at least two desorption processes are involved. By
extrapolation to zero loading, it was estimated that the maximum activation
energy was 16.7 kcal/mole. The apparent activation energy was assumed to be
the weighted average of the activation energy of the lodine held with an
energy of 16.7 kcal/mole and that held with energy. This treatment suggests
that about 9.3 x 10-11 mole cm-2 of the lodine is loosely bound, with an
apparent activation energy of 6.5 kcal/mole, and 0.7 x 10-22 mole/cm2 is
more tightly bound, with an apparent activation energy of 16.7 kcal/mole.
Recalculation of the desorption curves, using the extracted activation
energies, indicates that the desorption equations used are too simple. The
experimental curves have a greater initial desorption velocity and change
more abruptly than the recalculated curves. This suggests three components
are present--probably AI which has a heat of vaporization of .9 kcal/mole,
I, which has a heat of sublimation of 14.7 kcal/mole and adsorbed iodine
with an even higher energy of desorption. The resuīts further suggest that
the quantity of adsorbed lodine is a function of temperature.
Other Clay Materials
Several other clays were contaminated with lodine vapor from the oxida-
tion of KI with FeNH, (S04), and desorption studies were carried out. The initial
loading of these clays was lower than the loading of the floodplain sediment,
but the same desorption pattern was followed; as the temperature was raised,
the desorption velocity increased, in qualitative agreement with Equation (1),

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ORNL-D

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22°C
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FRACTION OF THE IODINE REMAINING ON
THE CLINCH RIVER FLOODPLAIN SEDIMENT
40°C
60°C
70°C
-80°C
100°C
- 120°C-0
0 2 4 6 8 10 12 14 16
TIME (hr)
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Effect of Temperature on the Desorption of lodine (13) from
Clinch River Floodplain Sediment Using Air.
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Table 1. The Desorption of Iodine from Clinch River Floodplain Sediment
Q(moles/cm?)
a
E(kcal/mole)
B(moles/cm)*
9 x 10-11
-21.0
-21.0
-21.1
-21.1
7.31
7.45
7.54
7.67
7.98
-21.9
on now fwano
6.85 x 10-12
7.16
6.90
6.72
7.12
7.30
7.13
7.80
6.85
8.39
8.95
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-20.1
-21.4
-20.8
10.5
13.5
(16.7) est
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*B 18 an estimate of the quantity of the bound lodine in the system.
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As the surface concentration of iodine is depleted, the remaining iodine
becomes more difficult to displace. Finally, the desorption velocity
becomes so slow that further changes in I activity levels are masked
by counting errors and the radioactive decay of the isotope. The activa-
tion energies for desorption of the iodine for all of the clays checked
were corrparable to those obtained for the floodplu in sediment, further
suggesting that three forms of iodine are present. The energy and the
rate of desorption are a functior. of surface concentration but do not
appear to be related to the nature of the adsorbing mineral; however, all
of these minerals have surfaces composed primarily of oxygen atoms. The
surface concentration of slowly desorbable lodine seems to be dependent
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on the temperature and not on the original surface concentration or the
nature of the mineral (Table 2).
A series of air moisture studies were made using kaolinite (lable 2)
and it was found that the desorption velocity was higher as the moisture
content of the air was raised. This may be due to one of two phenomena,
or both; that is, adsorption competition between the two adsorbates or
chemical reaction of the lodine with the passing water vapor. As the
moisture content of the air was increased it was also noted that the surface
concentration of the slowly desorbable lodine decreased.
Desorption of Iodine with Water
Two 10-g samples of the contaminated Clinch River floodplain sediment
were weighed into centrifuge tubes and leached with 25-01 increments of tap
water for time intervals of 30 min. The supernatant was decanted and an:
aliquot removed for counting.
Most of the iodine was readily removed by the tap water (Fig. 2), but
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it was noted that about 10% of the icdine is removed at a much slower rate:
than the initial 90%.
In order to determine whether there is a finite retention of iodine
against the leaching action of water, montmorillonite was contaminateå with
iodine (I.) vapor. Air was passed over the samples at a rate of 120 ml/min
at a number of constant temperatures for 16 br. At the end of this time
the samples were assayed for -I activity, and it was found, as previously,
that the amount of thI remaining on the clay was a function of temperature
(Table 4). These samples were then leached with five portions of tap water
at room temperature (25°c) as above. After the five water leaches there
was no significant difference in the amount of +-I retained.
These results present further evidence that a finite concentration
of iodine is actively adsorbed by wineral surface and that this portion
of the iodine can be considered immobile.
Behavior of Methyl Iodide
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A Berea sandstone core 12. inches long by 1 3/4 inches in diameter was
coated with epoxy resin; and end plates were attached with stainless steel
fittings. Methyl iodide vapor, tagged with I, was passed through the
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core at various rates of flow. Little, if any, of the methyl iodide was
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adsorbed onto the sandstone surface. The apparent adsorption amounted
to less than 5% of the pore volume of the core. Subsequent tests witia
clays also showed no appreciable adsorption of methyl iodide.
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Table 2. Slowly Desorbable Iodine on Different Clays Under Different Experimental Conditions
(10-8 mmoles/cm²;
Condition:
Floodplain
Sediment
Vermiculite
Nite
Montmorillonite
Kaolinite
Illite
1.69
1.69
0.906
0.783
1.69
0.879
1.32
70°C
0.867
0.783
5
Original Loading
10.0
Slowly desorbable at:
25°C
4.67
40°C
2.83
60°c
2.32
1.77
80°c
1.78
100°C
1.25
120°C
0.329
160°C
200°C
Slowly desorbable at 120°C
oven temperature with
air saturated at:
25°C (v.p. 8,0 = 24 mmHg or
1.5% R. 8.)
60°C (v.p. 1,0 = 149 mmHg or
10% R. H.)
100°C (v.p. 1,0 = 760 maig or
50% R. 8.)
0.463
0.393
0.312
0.904
0.288
0.797
0.459
0.322
0.504
0.154
0.155
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0.780
0.336
R. 8. = relative humidity
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ORNL-DWG 65-920
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FRACTION OF IODINE REMAINING ON THE CLINCH RIVER. FLOODPLAIN SEDIMENT
2
8 10
NUMBER OF LEACHES
длер -
Desorption of lodine From Clinch River Floodplain Sediment
With Tap Water.

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Table 3. Effect of Loading of Iodine on Montmorillonite on the Amount
of Iodine not Leached by Water
Temperature of Air Desorption
Time of Assay
Nonea
25°C
60°C
80°C
120°c:
160°c
200°C
4.12
4.11. - 4.11
4.13
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4.12
4.12
Initial loading
(10 mmoles/cm2)
After 16-hr air leach
(10-8 mmoles/cm²)
After five water leaches
(10 8 mmoles/cm)
4.03
3.79
3.64
3.53
1.38
0.798
0.570 -..
0.129
0.127
0.120
0.133
0.154
0.141:- 0.140
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This sample was capped for 16 hr with no air passing over it.
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Summary
The tendency for iodine to be adsorbed onto surfaces suggests that it
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would be immobllized in the ground as condensed HI and I, and as adsorbed
lodine if it is injected in the form of I, vapor. Even the leaching action
of water would not be expected to cause widespread movement of adsorbed
lodine before radioactive decay will have brought activity levels within
tolerable limits. Preliminary studies with methyl iodide, however, indi-
cate that this chemical form would move readily through the ground with-
out being adsorbed. In this case, reliance would have to be placed on"
physical containment of the released gases.
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END


DATE FILMED
11/ 15 /65






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