A Study of the Thermal Decomposition of CH3131I in a Gas Flow in the Presence of “Fizkhmin”TM Granulated Materials

Thermal decomposition of a volatile organic compound of radioactive iodine, 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 was studied. Under comparable experimental conditions, 97% - 99% decomposition of CH3131I is achieved at ~770?C without sorbents and at ~540?C and ~465?C in the presence of straight silica gel and of the material based on it, impregnated with compounds of Ni or its mixture with Cu (8 - 10 wt%), respectively.

• Iodine-131 Trapping
• Methyl Iodide
• Thermal Decomposition
• Sorption

1. Introduction

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 [1]-[6]. 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 [7]-[9]. 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 [10], under the action of UV radiation CH₃I decomposes to form chemically active atomic iodine. At the same time, we suggested in [11] that decomposition of CH₃¹³¹I under the action of UV radiation in air yields not only chemically active atomic iodine, but also finely dispersed I_xO_y aerosols. It is also suggested using for CH₃I decomposition a method based on the action of ozone in a field of electric charge [12]. 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. [13] 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.

2. Experimental

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 [7].

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-Cu^o (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 CH 3 131 I 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. [13], 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 CH 3 131 I (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 dm 3 /min, time of the air flow presence in the heating zone with length 5.2 cm -1.1 - 1.3 s, S column ~3.5 cm 2 , experiment time-4 h)

Material	T, ˚C	Degree of 131I sorption, %			The total degree of 131I sorption, %
		Column with material	Column with SiO2-Cu0	Scrubber with 0.05 М Na2SO3	Without accounting of thermal decomposition	With accounting of thermal decomposition
-*	540	-	~7.49	~0.004	~0	~7.50
-	550	-	~9.45	~0.004	~0	~9.50
	640	-	~74.97	~0.004	~0	~75.00
	760	-	~97.95	~0.004	~0	~98.00
Glass cylinders	550	~0.004	~19.51	~0.003	~0	~19.51
	660	~0.03	~73.94	~0.015	~0	~73.99

^*-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 CH 3 131 I (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 (17-20 o С) ~ 0.8 dm 3 /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, S column ~3.5 cm 2 , experiment time-4 h)

Material under study	T, ˚C	Degree of 131I sorption, %			The total degree of 131I sorption, %
		Column with material under study	Column with SiO2-Cu0	Scrubber with 0.05 М Na2SO3	Without accounting of thermal decomposition	With accounting of thermal decomposition
Silica gel	240	~2.12	~0.03	~0.006	~2.12	~2.16
	350	~1.15	~8.99	~0.007	~1.15	~10.15
	460	~0.54	~76.20	~0.009	~0.54	~76.74
	540	~0.31	~96.75	~0.012	~0.31	~97.07
Silica gel-NH4OH	340	~0.45	~7.99	~0.002	~0.45	~8.44
Silica gel-NH4NO3	340	~0.23	~3.09	~0.002	~0.23	~3.32

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 [14]. 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 [15], 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 [16].

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 CH 3 131 I (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 dm 3 /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, S column ~3.0 cm 2 , experiment time-4 h)

Composite “Fizkhimin”TM	T, ˚C	Degree of 131I sorption, %			The total degree of 131I sorption, %
		Column with composite Fizkhimin”TM	Column with SiO2-Cu0	Scrubber with 0.05 М Na2SO3	Without accounting of thermal decomposition	With accounting of thermal decomposition
KKM-8Cu	240	~9.31	~4.40	~0.002	~9.31	~13.72
	340	~3.96	~58.14	~0.002	~3.96	~62.10
	450	~1.33	~91.80	~0.013	~1.33	~93.15
KKM-5Cu5Ni	235	~6.93	~1.60	~0.002	~6.93	~8.52
	345	~15.97	~51.19	~0.003	~15.97	~67.15
	445	~2.89	~94.96	~0.009	~2.89	~97.85
KKM-2Cu8Ni	250	~12.48	~7.15	~0.006	~12.48	~19.64
	350	~37.59	~51.62	~0.026	~37.59	~89.24
	480	~2.26	~96.45	~0.039	~2.26	~98.75
KKM-8Ni	260	~15.86	~12.68	~0.016	~15.86	~28.56
	340	~48.75	~38.88	~0.016	~48.75	~87.79
	450	~7.63	~91.82	~0.026	~7.63	~99.48

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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