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INVENTION
Patent of the Russian Federation RU2288514

SORBENT FOR CALCULATION OF RADIOACTIVE IODINE FROM THE GAS PHASE

The name of the inventor: Rovny Sergey Ivanovich (RU); Pyatin Nikolai Petrovich (RU); Istomin Igor Alexandrovich
The name of the patent holder: Federal State Unitary Enterprise "Production Association" Mayak "
Address for correspondence: 456780, Chelyabinsk Region, Ozersk, Lenin Avenue, 31, FSUE "PO Mayak", VET
Date of commencement of the patent: 2005.05.11

The invention relates to the processing and immobilization of gaseous radioactive waste from radiochemical enterprises of the nuclear industry, namely to the region of iodine capture from the gas-aerosol flow from the felling-dissolution unit of irradiated nuclear fuel. The sorbent for collecting radioactive iodine consists of a porous base impregnated with a salt of silver nitrate (AgNO ₃ ), porous silicon carbide with a porosity of 30 to 60% is used as a sorbent substrate. Technical and economic efficiency lies in the higher corrosive and mechanical stability of this material in corrosive environments.

DESCRIPTION OF THE INVENTION

The invention relates to the processing and immobilization of gaseous radioactive waste from radiochemical enterprises of the nuclear industry, namely to the region of iodine capture (in particular, iodine-129) from the gas-aerosol flow from the felling-dissolution unit of irradiated nuclear fuel.

The technological processes of the reprocessing of irradiated nuclear fuel (SNF) of power reactors and transport facilities are inextricably linked with the purification of gas-aerosol waste from long-lived iodine-129 (T _1/2 = 1.57 · 10 ⁷ years). As a sorbent for its capture, porous materials impregnated with silver nitrate are used.

A silver-containing sorbent "Siloxid" can serve as an analogue of the invention in accordance with the technical specifications "Siloxide sorbent for trapping radionuclides of iodine from gaseous media". ТУ ЛКВШ 94.373.00.000, Sosnovy Bor, NITI them. A.P. Alexandrova, 1995.

Sorbent "Siloxid" does not meet the requirements of iodine treatment during reprocessing of SNF, t. Has a small granule size (2 mm) and a low specific surface, which contributes to a significant increase in the aerodynamic resistance of the gas cleaning unit and is reflected in the absorbing capacity of the sorbent. There is a sorbent for iodine capture [S. Smooth, N.P. Pyatin, I.A. Istomin. Iodine-129 capture when reprocessing irradiated nuclear fuel power plants. An article in the journal "Atomic Energy", Vol. 92, No. 6, June 2002], which is taken as the prototype of the invention, where the base is impregnated with a silver salt (AgNO ₃ ), is used aluminum oxide grade A in the form of white granules cylindrical Shape with a base diameter from 3 mm to 4 mm and a height of 10 mm to 15 mm in accordance with GOST 8136-85 "Aluminum oxide active. Technical conditions ».

The disadvantages of aluminum oxide are:

- propensity to mechanical destruction and abrasion in the processes of operation and regeneration;

- low chemical resistance in alkaline and acidic media;

- dusting in the processes of re-packing.

The object of the invention is to increase the mechanical strength in the operation, regeneration and retaking processes, as well as the chemical stability of the sorbent in acidic and alkaline media while maintaining the basic characteristics required for the sorbent and its basis (effective saturation of the base with impregnant, high dynamic capacity for iodine, Sorbent and silver recovery for reuse). It is solved by using porous silicon carbide impregnated with a salt of silver nitrate (AgNO ₃ ) as a sorbent for trapping iodine from the gas phase.

Silicon carbide is a material that is a chemical compound of silicon with carbon (SiC); Mohs hardness 9.1; Microhardness is 3300-3600 kG / mm ² . It is obtained in electric resistance furnaces by silicizing carbon particles with silicic acid vapor. The raw materials are materials rich in silica: vein quartz, quartz sands and quartzites containing not less than 99.0-99.5%, SiO ₂ , and carbonaceous material - petroleum coke. Silicon carbide is a promising material for modern instrumentation due to its high radiation resistance and temperature stability.

The proposed material differs from aluminum oxide in that silicon carbide exhibits significantly higher corrosion resistance in corrosive media, since it does not react with inorganic acids and alkalis even at the boiling temperature. Laboratory studies have shown that corrosion of silicon carbide in groundwater with pH ~ 8 at a temperature of 170 ° C is 0.15 μm per year. The mechanical strength of silicon carbide significantly exceeds the strength of aluminum oxide, so this material is more stable in the processes of re-packaging and transport.

The use of porous silicon carbide for the manufacture of iodine sorbent is possible, both in the form of granules of various shapes and in the form of a filter cartridge (FP) in the case of a small-sized regenerable modular filter capable of purifying the gas phase from iodine.

For the production of iodine sorbent, porous silicon carbide is impregnated with a salt of silver nitrate (AgNO ₃ ). To this end, a porous sorbent substrate (silicon carbide with a porosity of 30 to 60% in the form of granules or in the form of a filter cartridge) is impregnated with a solution of a silver salt with the desired silver concentration and dried at a temperature of 100 to 150 ° C. The impregnation and drying operations are repeated until the solution is completely absorbed.

After the sorbent is saturated with radioactive iodine from the gas phase, it can be regenerated for reuse. If necessary, it is possible to extract silver from the proposed sorbent to reuse it.

Example

To carry out experiments to study the possibility of manufacturing a sorbent based on porous silicon carbide (porosity of 30%), a sample of the material (hereinafter referred to as FP-filter cartridge) was used in the form of a hollow cylinder with a base diameter of 70 mm, a height of 105 mm and a wall thickness of 5 mm. The initial mass of the OP was 184574 mg. The impregnation of the OP by silver nitrate was carried out with a nitric acid solution with a silver concentration of 2.6 g / l and nitric acid of 5 mol / l in several stages, each of which is a process of uniform moistening of the filter and its subsequent drying. Thus, complete absorption of the solution was achieved.

The OP was weighed before and after saturation. The saturation results of the AF by silver are shown in FIG.

After the sixth stage, the solution containing 2.2 g of silver was completely absorbed, while the silver concentration in the filter cartridge was 11.9 mg / g.

Experiments on the capture of iodine FP were carried out on a laboratory unit (Fig. 2).

2: 1 - sealed container, 2 - heat-resistant beaker, 3 - iodine-127, 4 - furnace, 5 - PT, 6 - bubbler, 7 - solution of NaOH.

Iodine-127 was added to the bottom of the heat-resistant beaker 2 in an amount exceeding the stoichiometric value with respect to the silver iodide formation reaction. FP 5 was placed on a metal mesh fixed above the level of iodine crystals. The AF glass was installed in a hermetically sealed container 1 equipped with a nozzle for diverting the gas phase to a bubbler 6 filled with a sodium hydroxide solution with a concentration of 2 mol / l. The saturation of the sorbent with iodine was carried out at a temperature of 200 ° C in the shaft furnace 4. The beginning and completion of the evaporation of iodine was monitored by the color change of the solution in the bubbler and the supply flues.

The amount of captured iodine was determined by weighing the OP before and after saturation. The results of the experiment are presented in Table 1.

The concentration of iodine-127 in the absorber was 18.1 mg / g, which is more than the amount of iodine in the form of silver iodide formed in accordance with the stoichiometry of the reaction

6AgNO ₃ + 3I ₂ <---> 4AgI + 2AgIO ₃ + 6NO ₂ .

This can be explained by the adsorption of molecular iodine on the surface of the PT pores.

Regeneration of the FP was carried out with an alkaline solution of hydrazine nitrate with an alkali concentration of 30 g / l and hydrazine of 15 g / l. The processing time of the sorbent solution was 30 minutes at a temperature of 80 ° C. After the regeneration, the filter cartridge was rinsed with hot distilled water to pH 5-7, dried and weighed. From the solutions formed during the regeneration of the FP (regenerates and washing), iodine was concentrated in the form of copper iodide. The results of the regeneration of FP and precipitation of iodine in the form of copper iodide are shown in Table 2.

In order to obtain silver from spent sorbent on the basis of porous silicon carbide, AF after extraction of iodine from it was treated with nitric acid at a concentration of 5 mol / l for 30 minutes at a temperature of 80 ° C, then the FP was dried and weighed. Uterine and wash solutions were analyzed for the content of silver in them (the initial silver content in OP is 2.2 g) in two ways. In the first case, the mass concentration of silver in the solutions was determined on the X-ray fluorescence energy dispersive analyzer ERA-03. In the second case, silver was precipitated from the solution in the form of chloride. The results of the analysis of solutions are presented in Table 3.

As can be seen from the data presented in Table 3, the efficiency of silver recovery from the filter cartridge was (99.4 ± 0.3)%, which is higher than the degree of silver recovery from the alumina based sorbent, where it is at the level of (97.4 ± 0.6)%.

The distinctive features of the proposed material are significantly higher corrosion and mechanical stability in corrosive environments. This allows to significantly extend the life of the iodine sorbent, to increase the efficiency of using expensive silver.

CLAIM

Sorbent for the capture of radioactive iodine, consisting of a porous base impregnated with a salt of silver nitrate (AgNO ₃ ), characterized in that porous silicon carbide with a porosity of 30 to 60% is used as a sorbent substrate.

print version
Published on February 19, 2007

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