The localization of volatile radioactive iodine compounds by various sorbents from vapor-gas media is a vital issue for environmental protection during both irradiated nuclear fuel reprocessing and accidents at nuclear power enterprises, including nuclear power plants (NPPs). One of the most difficult-to-localize volatile organic forms of radioactive iodine is methyl iodide (CH3I). Sorbents based on activated carbon, silica gel, Al2O3, etc. are widely used today for its localization -. A study of the CH3131I sorption from the gas phase onto a wide range of sorbents has shown that inorganic sorbents containing 8 - 12 wt% silver ions are the most efficient at emergencies -. However, high cost of silver, one of the main components binding radioactive iodine, makes it topical to develop the cheaper CH3I localization systems.
Various methods have been suggested for converting CH3I to readily localizable forms. As noted in , under the action of UV radiation CH3I decomposes to form chemically active atomic iodine. At the same time, we suggested in  that decomposition of CH3131I under the action of UV radiation in air yields not only chemically active atomic iodine, but also finely dispersed IxOy aerosols. It is also suggested using for CH3I decomposition a method based on the action of ozone in a field of electric charge . The main product of the action of O3 on CH3I is I2O5 in the form of aerosols, which were deposited on the walls of the discharge chamber. The major drawback of these procedures is that the CH3I decomposition yields radioactive aerosols with a wide range of particle sizes, including nanometric size. Localization of nanoparticles requires the use of special filtration systems. One of alternative pathways of CH3I conversion without formation of radioactive aerosols is its thermal decomposition. Lorenz et al.  studied the thermal decomposition of CH3131I and found that 97.9% decomposition of CH3131I was observed only at 800˚C. At lower temperatures, the degree of CH3131I decomposition was lower: 13% at 500˚C and 83% at 600˚C. Published data on materials allowing thermal decomposition of CH3131I to be efficiently performed at lower temperatures are lacking. Therefore, the goal of this work was the development and study of the thermal decomposition of methyl iodide CH3131I in a gas flow in the presence of various modifications of “Fizkhmin”TM granulated materials based on silica gel impregnated with d-elements.
In our study we used carrier-free 131I supplied in the form of Na131I solution. The radionuclide activity was measured by γ-ray spectrometry using a multichannel analyzer with a semiconductor Ge?Li detector. 131I was used as radioactive tracer for weighable amounts of inactive iodine compounds. Therefore, the designation CH3131I refers to the labeled compound and not to the compound of pure 131I. In the course of experiments, CH3131I was introduced into the system by passing air at a definite rate through the vessel containing CH3131I. The required amounts of CH3131I were preliminarily frozen out from the helium flow with liquid nitrogen. CH3131I was obtained by the reaction of dimethyl sulfate with K131I .
All the salts, alkalis, and acids used in the study were of chemically pure grade. We used coarsely porous silica gel in the form of granules of size from 1.0 to 3.0 mm and the “Fizkhmin”TM composite materials based on it:
1) composites based on silica gel, obtained by impregnation of it with a 2 M NH4OH solution, storage of the wet sorbent for 24 h, and drying in air at a temperature increasing from room temperature to 300˚C at a rate of 20 deg/min, followed by conditioning at 300˚C for 4 h;
2) composites based on silica gel, obtained by impregnation of it with a 0.13 M NH4NO3 solution, followed by storage, drying, and conditioning under the same conditions;
3) composites based on silica gel impregnated with Cu by treatment with Cu(II) nitrate, followed by drying at 110˚C, treatment of the dry precursor with ammonia for 24 h, and conditioning in air at 300˚C for 4 h. Cu content was 8 wt% (designation KKM-Cu);
4) composites based on silica gel impregnated with Ni and Cu by treatment with a solution of Ni(II) and Cu(II) nitrates, followed by treatment similar to that of KKM-Cu. Total content of Cu and Ni was 10 wt%, Cu : Ni weight ratio 1:1 and 1:4 (designation KKM-CuNi);
5) composites based on silica gel impregnated with Ni by treatment with a solution of Ni(II) nitrate, followed by treatment similar to that of KKM-Cu. Ni content was 8 wt% (designation KKM-Ni).
The localization of CH3131I from a water vapor?air flow was studied on a setup schematically shown in the Figure 1. The set-up concluded the following basic parts: rotameters (1); a CH3131I generator (2); scrubber with water (3); the heating furnace of mine type (4); composite materials under study (5); the thermocouple (6); a column with SiO2-Cuo (7); scrubber with 0.05 M Na2SO3 solution (8); the heating furnace of tubular type (9); columns with SiO2-AgNO3 (10).
In this setup, the column with silica gel containing 10 wt% Cu0 was intended for trapping 131I2 released in thermal decomposition of CH3131I. It should be noted that CH3131I is not sorbed on this material. To trap the residual amounts of the unchanged CH3131I, we used two columns with silica gel impregnated with AgNO3 (30 mg/g silica gel).
The experiment was performed as follows. The compressor was switched on, and air was passed at a definite rate through the CH3131I generator. The air with CH3131I vapor was fed to the bubbler with water, where it was saturated with water vapor. After that, the water vapor-air flow containing CH3131I was fed to the reaction column with the test materials heated to the required temperature. When passing through these materials, CH3131I decomposed to one or another extent with the formation of violet vapor of molecular iodine. After passing through the reaction column, the gas flow containing the CH3131I decomposition products was fed to a column packed with silica gel containing 10 wt% Cu0. The major amount of molecular iodine was sorbed on this material. Then, the gas flow was passed through a bubbler with a 0.05 M Na2SO3 solution to remove residual traces of molecular iodine and through two columns with silica gel impregnated with AgNO3 (30 mg/g silica gel). After the experiment completion, the compressor was switched off, the columns with various sorbents were cooled,
Figure 1. The scheme of the set-up for study of CH3131I thermal decomposition in a gas flow.
and the setup was disassembled. All parts of the setup were treated with a 0.1 M Na2S2O3 solution to avoid the loss of radioactive iodine and then were washed two times with water. The content of 131I in various parts of the setup was determined by γ-ray spectrometry. After determining the 131I content in various parts of the setup, we calculated the degree of decomposition of CH3131I.
3. Results and Discussion
Data on thermal decomposition of 10 mg of CH3131I in an air flow without composite materials are given in Table 1. As can be seen, the degree of decomposition of CH3131I increases with temperature and reaches a maximum at ~760˚C. For example, with an increase in temperature from ~540˚C to ~760˚C the degree of decomposition of CH3131I increases from ~9.5% to ~98.0%. In all the cases, 131I was virtually fully localized on the column packed with silica gel containing 10 wt% Cu0. This fact suggests that the major decomposition product of CH3131I is atomic iodine which instantaneously transforms into molecular iodine. Our data on thermal decomposition of CH3131I are well consistent with the data of Lorenz et al. , who showed that the degree of thermal decomposition of CH3131I at ~800˚C was 97.9%.
As follows from Table 1, an increase in the linear velocity of the gas flow by a factor of approximately 2 does not affect strongly the degree of decomposition of CH3131I. For example, with an increase in the linear velocity of the gas flow from ~4.5 to 8.9 cm/s the degree of decomposition of CH3131I decreases by only ~2.0%.
To stabilize the flow in the heating zone, we placed glass cylinders ~2 mm in diameter and ~4 mm high into the reaction column. As seen from Table 1, introduction of the glass cylinders into the heating zone leads to an increase in the degree of decomposition of CH3131I at ~550˚C and exerts virtually no effect at ~660˚C. The observed effect is probably associated with the residence time of the gas flow containing CH3131I in the heating zone. At ~550˚C, introduction of glass cylinders led not only to stabilization of the flow in the heating zone, but also to a relative increase in the residence time of CH3131I in the heating zone, which increases the degree of decomposition of CH3131I. At ~660˚C, the glass cylinders underwent sintering owing to fusion of their surface. The monolithic mass formed in the processes decreased the effective cross section of the reaction column and the free volume in the heating zone. Therefore, simultaneously with stabilization of the gas flow in the heating zone, its linear velocity increased and hence the residence time of CH3131I in the heating zone decreased. Owing to simultaneous occurrence in the heating zone of two processes exerting opposite effects on the thermal decomposition of CH3131I, the degree of decomposition of CH3131I at ~660˚C remained unchanged.
Because of fusion of glass cylinders at ~660˚C, it was interesting to study the thermal decomposition of CH3131I using other materials as gas flow stabilizers. The best of them is coarsely porous silica gel.
Data on thermal decomposition of CH3131I in the presence of silica gel and of certain “Fizkhmin”TM composite materials based on it are given in Table 2.
As can be seen, in the presence of silica gel the degree of decomposition of CH3131I is ~96.8% even at ~540˚C. Therefore, it was interesting to study the thermal decomposition of CH3131I at still lower temperatures. Table 2
Table 1. Thermal decomposition of CH3131I (10 mg) in air flow. (temperature of steam-air flow ~23˚C, linear rate of gas flow ~4.0 - 5.0 cm/s, steam content in steam-air mixture ~3 - 4 vol.%, volume rate of gas flow (17˚C - 20˚C)~0.8 dm3/min, time of the air flow presence in the heating zone with length 5.2 cm -1.1 - 1.3 s, Scolumn~3.5 cm2, experiment time-4 h).
*-linear rate of gas flow~8.9 cm/s, time of the air flow presence in the heating zone with length 5.2 cm - 0.6 s.
Table 2. Thermal decomposition of CH3131I (10 mg) on silica gel in air flow. (temperature of steam-air flow ~23˚C, linear rate of gas flow~4.8 - 5.2 cm/s, steam content in steam-air mixture ~3 - 4 vol.%, volume rate of gas flow (17-20oС) ~ 0.8 dm3/min, time of the contact between material under study and steam-air mixture - 1.0-1.1 s, weight of material under study-10 g, size of granules of material under study-3.0 mm, height of layer of material under study-5.2 cm, Scolumn ~3.5 cm2, experiment time-4 h).
shows that the degree of decomposition drastically decreases with a decrease in temperature. For example, as the temperature is decreased from ~540˚C to ~240˚C, the degree of CH3131I decomposition decreases from ~96.8 to ~0.03%. It should be noted that, under the experimental conditions studied, silica gel absorbs 131I weakly (0.3% - 2.1%), with the degree of absorption decreasing with an increase in temperature. Thus, the use of silica gel allows the temperature required for the CH3131I decomposition to be decreased by more than 200˚C. The observed effect is probably associated both with high porosity of silica gel and with the presence of microimpurities of various elements in silica gel, catalyzing the decomposition of CH3131I at high temperatures.
It is known that the heat treatment of silica gel with an ammonia solution leads to significant changes in the silica gel structure, associated with its porosity . In this connection, it was interesting to study the thermal decomposition of CH3131I in the presence of composite materials based on silica gel and containing products of thermal reaction of ammonia with silica gel. As seen from Table 2, the degree of decomposition of CH3131I in the presence of these composite materials not only did not increase but even decreased by a factor of ~3 in the case of silica gel with NH4NO3. Because the drying of the material was performed on heating from room temperature to 300˚C and the melting and decomposition points of NH4NO3 are 170 and 239˚C, respectively , in the course of conditioning the ammonium salt, probably, initially melted, impregnated the whole silica gel structure, and only then decomposed. Apparently, during the conditioning time the ammonium salt decomposed incompletely; therefore, a part of the volume and especially silica gel micropores were filled with NH4NO3, which prevented the access of CH3131I to active sites on the silica gel surface. As a result, the degree of decomposition of CH3131I in the presence of the silica gel with NH4NO3 composite material decreased relative to straight silica gel.
As noted above, another possible factor decreasing the CH3131I decomposition temperature in the presence of silica gel is the presence of impurities of various metals. In this connection, it was interesting to study the thermal decomposition of CH3131I in the presence of “Fizkhmin”TM composite materials based on silica gel impregnated with Cu and Ni compounds. These d-elements are widely used as base components of catalysts in various chemical processes .
Data on thermal decomposition of CH3131I in an air flow in the presence of “Fizkhmin”TM composite materials based on silica gel impregnated with Cu and Ni compounds are given in Table 3. As seen from Table 3, the use of “Fizkhmin”TM composite materials containing d-elements allowed the decomposition temperature of CH3131I to be decreased from ~540˚C to ~450˚C, i.e., by almost 100˚C more. For example, with KKM-8Ni the degree of decomposition of CH3131I increased from ~39% to ~92% with an increase in temperature from ~340˚C to ~450˚C.
Similar effect was observed with the other composite materials containing Cu and Ni compounds. It should be noted that the composite materials under consideration sorb from ~7.0% to ~49% of 131I at relatively low temperatures (~240˚C - 350˚C). As a result, the total degree of 131I localization in the systems with KKM-8Ni and KKM-2Cu8Ni at ~350˚C is close to 90%. The observed effect is probably associated with simultaneous occurrence of several processes in the system: (1) catalytic decomposition of CH3131I with the formation of atomic iodine; (2) reaction of iodine atoms with each other to form molecular iodine and with d-elements to form iodides; (3) thermal decomposition of d-element iodides with the formation of finely dispersed particles of their oxides or metals. It should be noted that the prevalence of one or another process depends on temperature. Therefore, increased 131I uptake will be observed either in the column with the composite material or in the column with silica gel containing 10 wt% Cu0.
Table 3. Thermal decomposition of CH3131I (10 mg) on composite material “Fizkhimin”TM in air flow. (temperature of steam-air flow ~23˚C, linear rate of gas flow ~6.0 - 7.0 cm/s, steam content in steam-air mixture ~ 3 - 4 vol.%, volume rate of gas flow (17˚C - 20˚C) ~ 0.8 dm3/min, time of the contact between composite “Fizkhimin”TM and steam-air mixture - 0.7 - 0.9 s, weight of composite “Fizkhimin”TM-10 g, size of composite granules-1.0 mm, height of composite “Fizkhimin”TM layer-4.8 cm, Scolumn ~3.0 cm2, experiment time-4 h).
Thus, the use of “Fizkhmin”TM composite materials based on silica gel impregnated with d-elements allows the CH3131I decomposition temperature to be decreased by more than 300˚C. The revealed features of thermal decomposition of CH3131I in the presence of the composite materials studied can be taken into account in the development of new and improvement of existing systems for environment protection at enterprises of nuclear industry.
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