Oxidative hydrolysis in water vapor-air phase of CsI radioaerosols produced by CsI sublimation from metallic surface

Article abstract of DOI:10.1023/B:RACH.0000024639.33411.68

Behavior of CsI radioaerosols produced by CsI sublimation from a platinum support in argon, air, and water vapor-air mixture was studied. During 10-12 min of the vaporization at 900-1570 K, CsI radioaerosols undergo oxidative hydrolysis with atmospheric oxygen and water vapor to form CsOH aerosols and I₂. The cesium-to-iodine ratio determined in various fractions shows that oxidation of CsI in argon is minimal and is caused by the presence of oxygen and water traces. Oxidative hydrolysis of CsI strongly increases with increasing water vapor content in the vapor-gas flow. The degree of oxidative hydrolysis of CsI in the gas flow depends not only on the content of water vapor and oxygen but also on the initial CsI/O₂ molar ratio.

Full text of DOI:10.1023/B:RACH.0000024639.33411.68

Radiochemistry, Vol. 46, No. 1, 2004, pp. 63 66. Translated from Radiokhimiya, Vol. 46, No. 1, 2004, pp. 59 62.
Original Russian Text Copyright 2004 by Kulyukhin, Mikheev, Kamenskaya, Rumer, Konovalova, Novichenko.
Oxidative Hydrolysis in Water Vapor Air Phase
of CsI Radioaerosols Produced by CsI Sublimation
from Metallic Surface
S. A. Kulyukhin, N. B. Mikheev, A. N. Kamenskaya, I. A. Rumer,
N. A. Konovalova, and V. L. Novichenko
Institute of Physical Chemistry, Russian Academy of Sciences, Moscow, Russia
Received January 9, 2003
Abstract Behavior of CsI radioaerosols produced by CsI sublimation from a platinum support in argon, air,
and water vapor air mixture was studied. During 10 12 min of the vaporization at 900 1570 K, CsI radio-
aerosols undergo oxidative hydrolysis with atmospheric oxygen and water vapor to form CsOH aerosols and
I2. The cesium-to-iodine ratio determined in various fractions shows that oxidation of CsI in argon is minimal
and is caused by the presence of oxygen and water traces. Oxidative hydrolysis of CsI strongly increases with
increasing water vapor content in the vapor gas flow. The degree of oxidative hydrolysis of CsI in the gas
flow depends not only on the content of water vapor and oxygen but also on the initial CsI/O molar ratio.
2
Accidents at plants of atomic industry make urgent
the problem of localization and recovery of volatile
radionuclides and radioaerosols. CsI aerosol is one of
the most biologically hazardous volatile compounds
of fission products of nuclear fuel, penetrating into the
environment at accidents at atomic power plants. The
behavior of CsI radioaerosols in water vapor gas
phase was extensively studied [1 4]. Previously we
found that radioaerosols with a wide particle-size dis-
tribution (from hundreds of micrometers to 5 10 nano-
meters) are formed in the course of CsI vaporization
from a platinum surface under argon [5, 6]. Under
these conditions, the chemical form of CsI does not
change [7].
In this case, CsI can be completely oxidized to
form large amounts of molecular iodine.
Since the experimental data on CsI behavior in
these systems are scarce, the study of the behavior of
CsI radioaerosols in the water vapor air system is an
urgent problem. In this work we studied oxidative
hydrolysis of CsI radioaerosols formed in the water
vapor gas phase by sublimation of cesium iodide
from the metal surface at 900 1570 K.
EXPERIMENTAL
We used carrier-free 1 Cs and
37
131
I in the form
of a nitric acid solution of 1 Cs and an Na I solu-
tion, purchased from the Izotop Joint-Stock Company.
The -ray radiation of these radionuclides was meas-
ured on a -ray spectrometer with a Ge Li detector
and a multichannel analyzer (Nokia). Nonradioactive
CsI used in weighable amounts was of chemically
pure grade. Felt made from Bekipor WB stainless
steel, with the fiber thickness of 12 m and density
37
131
However, CsI can transform chemically in the pres-
ence of oxygen and water vapor in the gas phase.
When CsI is sublimed at 630 660 C and 1200
1
250 C in dry air, it is partially oxidized with atmos-
pheric oxygen to form I and IO [3]. The conversion
of 100 mg of CsI is 10%. The conversion of CsI in the
course of its sublimation at 1000 1200 C from the
quartz surface into a small volume ( 3.7 cm ) contain-
ing 0.15 cm of O is 3% [8]. It is suggested, how-
ever [8], that the conversion should increase with
increasing oxygen content in the gas phase.
2
3
3
3
of about 50 mg cm , was purchased from Bekaert
3
2
Fibre Technologies Company (Belgium) [9].
Oxidative hydrolysis of CsI radioaerosols was
studied on a unit schematically shown in the figure.
At high concentrations of water vapors and oxygen
in the gas phase and at elevated temperature, oxidative
hydrolysis of CsI is possible:
3
The unit consists of a 450 cm reaction chamber
and a stainless steel column 2.5 cm in diameter and
2
5.0 cm high. The reaction chamber hermetically con-
nected with the column was placed in a chamber
which can be heated to a required temperature. The
column was packed with steel felt (20.0 g) to the bed
4
CsI + 2H O + O
4CsOH + 2I2.
(1)
2
2
Deceased.
1
066-3622/04/4601-0063 2004 MAIK Nauka/Interperiodica

64
KULYUKHIN et al.
9
05 K). Preliminary experiments showed that, under
these conditions, the isotope exchange in cesium
iodide was complete. Even at 900 K, a CsI vapor is
visually observed. Then generation of CsI aerosols
was continued at 1470 1570 K (CsI boiling point
1
553 K) for 10 min. The process was monitored with
a pyrometer through a special quartz window in the
wall of the reaction chamber. After sublimation com-
pletion, the water vapor gas mixture was passed for
3
0 min at the same temperature in the heated chamber.
Then the reaction chamber and column were cooled to
room temperature and the system was disassembled.
The unit was dismantled and its internal parts were
washed with water. The wash waters from the column
and the trapping system were combined. The cesium
and iodine content in the wash waters from the cham-
ber and in the combined fraction was determined by
Unit for studying oxidative hydrolysis of CsI radioaerosols:
1) reaction chamber, (2) platinum heater, (3) heated cham-
ber, (4) column packed with metal felt, (5) bubbler with
.05 M Na SO , (6) condenser, (7) condensate receiver,
(
0
2
3
(
8) combined filter, and (9) rotameter.
the activity of 1 Cs and
37
131
I, respectively. The Cs/I
height of about 25.0 cm. The top of the column was
connected with the system for trapping the aerosols
broken through and the products of oxidative hydroly-
sis. The system consisted of two condensers, two bub-
blers with 0.05 M Na SO , and a combined filter (Pet-
weight ratio was calculated. The deviation of this ratio
from that in CsI (1.04) indicated enrichment of a frac-
tion with cesium or iodine, which, in turn, depended
on the degree of oxidative hydrolysis in the system.
2
3
ryanov’s filter and white ribbon paper filter). A rotam-
eter and a roughing pump were arranged at the outlet.
RESULTS AND DISCUSSION
To choose the material of the heater for CsI sub-
limation, we performed special experiments. It was
found that stainless steel and nickel strongly corroded
under the action of CsI even at 973 K. To exclude the
possible effect of the corrosion products on the oxida-
tive hydrolysis, we used a platinum heater.
To determine the resistance of CsI to water and
oxygen, we studied the behavior of CsI radioaerosols
in argon, air, and water vapor air mixture containing
up to 93 vol % water. The results of these experiments
at the temperature of the reaction chamber (hereinafter,
chamber) from 403 to 423 K are presented in Table 1.
The oxidative hydrolysis was performed as follows.
As seen from Table 1, in the course of vaporization of
137
137
A solution containing 1 mg of CsI, 5 Ci of
Cs,
and 50 Ci of 1 I was evaporated to dryness at 50
0 C on a heater made in the form of a 6 36-mm
1 mg of CsI in argon flow, about 30% of
Cs and
31
131
I are localized on the heater, walls, and bottom of
the chamber. The remaining 70% of the radionuclides
are present in the column and the system trapping the
oxidative hydrolysis products. The Cs/I weight ratio
in the chamber is close to 1.04. These data show that
the degree of oxidative hydrolysis in argon is neglig-
ible and is due to the presence of traces of water and
oxygen. The Cs/I ratio in the combined fraction is
below 1, suggesting oxidation of iodide ion to I2.
7
boat from 0.1-mm platinum foil. If necessary, 100 mg
of CsI was placed in the boat. Then the heater was
fixed in the reaction chamber. The chamber was her-
metically sealed and connected with the column and
the system for trapping the radioaerosols. Argon or a
water vapor air mixture was passed through the reac-
tion chamber and the column either at room tempera-
ture or on heating with a high-performance HABE-
The Cs/I ratio in the chamber after sublimation of
CsI in air was substantially higher than that in CsI.
An increase in the cesium content in the chamber as
compared to that in the experiment in argon is due to
formation of hydrophilic CsOH aerosols [10]. Thus,
in the course of CsI vaporization in air, unlike the
2
000 thermofan (Atlas Copco Electric Tools, Ger-
many). The water vapor gas mixture was evacuated
from the system with a roughing pump. The system
was purged at a required temperature for 15 min with
the water vapor gas mixture being studied. Then CsI
was evaporated from the platinum boat by gradual
heating to 473 K over a period of 2 min to remove
adsorbed moisture and to prevent spraying of the melt
and then to 923 K over a period of 2 min to produce
experiments in Ar flow, CsOH aerosols and I are
2
formed in large amounts along with CsI aerosols.
The cesium and iodine content in the chamber in
the course of CsI vaporization in the water vapor air
homogeneous 1 Cs I melt (melting point of CsI
37
131
RADIOCHEMISTRY Vol. 46 No. 1 2004

OXIDATIVE HYDROLYSIS IN WATER VAPOR AIR PHASE OF CsI RADIOAEROSOLS
65
flow containing 93 vol % water was 50 and 39%,
respectively. The Cs/I weight ratio was 1.30. Sub-
stoichiometric increase of the Cs/I ratio by 25 % is
due to hydrolysis of CsI in water vapor to form CsOH
aerosols. Aerosols of cesium hydroxide are readily
hydrated and are deposited in the chamber. At the
same time, the Cs/I ratio in the combined fraction
was lower than the stoichiometric value by 23%.
Table 1. Content of 137Cs and 131I in various fractions*
formed in the course of oxidative hydrolysis of CsI (1 mg)
evaporated from a platinum heater (initial temperature in
the chamber 403 423 K) at different compositions of
the gas phase
Content, %
Frac-
tion no.
Cs/I weight
ratio
137Cs
131I
137
131
The
Cs and
I content in the combined fraction
indicates that the content of molecular iodine formed
during CsI sublimation in the water vapor air mixture
containing up to 93 vol % water is larger compared to
sublimation in pure air. The Cs/I weight ratio is 0.81.
This is due to the fact that the oxygen content in the
water vapor air mixture is lower by an order of mag-
nitude than that in air ( 1.5 and 20 vol %, respec-
tively).
Argon
1
2
30.71
69.29
30.15
69.85
1.02
0.99
Air (3 4 vol % H O)
2
1
2
32.76
67.24
29.65
70.35
1.10
0.95
Water vapor air mixture (93 vol % H O)
We studied the behavior of CsI aerosols as in-
fluenced by the temperature in the reaction chamber.
First, we sublimed CsI in air without heating of the
chamber. It should be noted that the temperature in
the chamber in the course of CsI sublimation from
the heated platinum boat was lower than 373 K. The
temperature of the column bottom being in the direct
contact with the reaction chamber did not exceed
2
1
2
50.56
49.44
38.97
61.03
1.30
0.81
* Fraction 1 is wash waters of the reaction chamber; fraction 2
is combined wash waters from the column, receivers, bub-
blers, and filter; the same for Tables 2 4.
Table 2. Content of 137Cs and 131I in various fractions
formed in the course of oxidative hydrolysis of CsI (1 mg)
evaporated from a platinum heater in air at different tem-
peratures of the reaction chamber
313 K. As seen from Table 2, under these conditions,
the cesium and iodine content in the chamber in-
creased by 10 and 3%, respectively, as compared to
the experiment at 423 K. The Cs/I ratio in the cham-
ber increased by 20%. This confirms our assumption
on formation of CsOH aerosols in the course of CsI
sublimation in air at 900 1570 K. Since the tempera-
ture in the chamber is low, CsOH particles are rapidly
agglomerated and are deposited on the internal parts
of the chamber. At the same time, radioactive I2
passes through the column and is localized in the
trapping system. The Cs/I weight ratio in the com-
bined fraction decreases to 0.85.
Content, %
Frac-
tion no.
Cs/I weight
ratio
137Cs
131I
Initial temperature of the chamber 292 K
1
2
42.98
57.02
32.91
67.09
1.31
0.85
Initial temperature of the chamber 403 423 K
We also studied the behavior of CsI aerosols with
the initial amount of CsI increased from 1 to 100 mg.
1
2
32.76
67.24
29.65
70.35
1.10
0.95
137
131
The distribution of
Cs and
I after CsI
sublimation in air and argon at different water vapor
content in the gas phase is presented in Table 3. As
seen from Table 3, the oxidative hydrolysis in argon
is negligible even at a 52 vol % concentration of water
vapor. In this case and in an air flow containing 3
tion of CsOH CsI aerosols, more volatile than those
of CsOH and CsI taken separately [11].
The behavior of the oxidative hydrolysis products
formed at different initial molar ratios of CsI and
oxygen in the gas flow is shown in Table 4. In these
experiments, CsI was evaporated from the platinum
boat without heating of the chamber. After sublima-
4
vol % water, the Cs/I ratio in the chamber is con-
stant [ (3 7)% of the theoretical value]. When the
content of water vapor increases to 33 50 vol %, large
amounts of radioactive iodine, probably in the form of
HI and HIO (CsI < 1), are deposited in the chamber.
tion of 1 mg of CsI (CsI : O molar ratio 1 : 1100),
3
2
The cesium content in the combined fraction notice-
ably increases (Cs/I > 1), which can be due to forma-
the Cs/I ratio in the chamber and in the combined
fraction considerably differes from the theory. When
RADIOCHEMISTRY Vol. 46 No. 1 2004

66
KULYUKHIN et al.
Table 3. Content C of 137Cs and 131I in various fractions formed in the course of oxidative hydrolysis of CsI (100 mg)
evaporated from a platinum heater (initial temperature in the chamber 393 403 K) at different water vapor concentrations
in the gas flow
Content, %
Atmosphere
C, vol %
Fraction no.
Cs/I weight ratio
137Cs
131I
Air
3 4
1
2
47.89
52.11
44.62
55.38
1.07
0.94
3
5
2.7
0.5
1
2
46.03
53.97
47.27
52.73
0.97
1.02
1
2
40.55
59.45
42.68
57.32
0.95
1.04
Argon
52.3
1
2
35.44
64.56
32.55
67.45
1.09
0.96
1
00 mg of CsI is sublimed (CsI : O molar ratio 1:11),
ACKNOWLEDGMENTS
2
the ratios in these fractions are similar and are close
to that in CsI.
This work was financially supported by the Rus-
sian Foundation for Basic Researche (project no. 00-
Thus, CsI aerosols formed in the course of CsI
evaporation from the platinum heater at 900 1570 K
in water vapor air phase undergo oxidative hydrolysis
with water vapor and atmospheric oxygen to form
0
3-32692).
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RADIOCHEMISTRY Vol. 46 No. 1 2004