Sorbent for entrapping radioactive iodine from gas phase

FIELD: nuclear-industry radiochemical enterprises for recovering and immobilizing gaseous radioactive wastes.

SUBSTANCE: sorbent used for entrapping radioactive iodine from gas-aerosol stream running from irradiated nuclear fuel cutting-and-dissolving unit has porous base impregnated with nitric acid silver salt (AgNO3); used as sorbent base is porous silicon carbide of 30 to 60% porosity.

EFFECT: enhanced corrosion and mechanical resistance of material in chemically active media.

1 cl, 2 dwg, 3 tbl

 

The invention relates to the field of processing and immobilization of gaseous radioactive waste radiochemical enterprises of the nuclear industry, in particular to the field trapping of iodine (in particular, iodine-129) from the aerosol stream with a host of cutting-dissolution of irradiated nuclear fuel.

Processes for processing of irradiated nuclear fuel (SNF) power reactors and transport facilities are inextricably linked with cleaning aerosol waste from long-lived iodine-129 (T1/2=1,57·107years). As a sorbent for his capture uses porous material impregnated nitrate of silver.

Analogue of the invention can serve as a silver-containing sorbent Selexid" in accordance with the technical conditions "Siloxy-sorbent for the capture of radioactive iodine from the gas environments". LCVS 94.373.00.000, Pine forest, THREAD them. A.P. Alexandrov, 1995.

Sorbent Selexid" does not meet the requirements of iodine purification in the processing of spent fuel, since it has a small size pellets (2 mm) and low specific surface area, which contributes to a significant increase in aerodynamic resistance of the gas-cleaning apparatus and reflected on the absorptive capacity of the sorbent. Known sorbent to adsorb iodine [S.I. Smooth, I.E. Pyatin, I.A. Istomin. Capture Yoda in the processing of irradiated nuclear fuel power plants. Article in the journal "Nuclear energy", I. 92, issue 6, June 2002], which is taken as a prototype of the invention, where as the basis saturated salt of silver (AgNO3), is used alumina grade a in the form of white granules of cylindrical shape with a base diameter of 3 mm to 4 mm and a height of 10 mm to 15 mm according to GOST 8136-85 "active aluminium Oxide. Technical conditions".

The disadvantages of aluminum oxide are:

- prone to mechanical failure and attrition in the processes of exploitation and regeneration;

low chemical stability in alkaline and acidic media;

- dusting in the process of repackaging.

The objective of the invention is to increase the mechanical strength in operational processes, regeneration and repackaging, and chemical resistance of the sorbent in acidic and alkaline environments while maintaining the basic performance requirements of the sorbent and its basis (effective saturation of the base impregnator, high dynamic capacity for iodine, the possibility of regeneration of the sorbent and silver extraction for reuse). It is solved through the application as a sorbent for the capture of iodine from the gas phase porous silicon carbide, impregnated with a salt of silver nitrate (AgNO3).

Silicon carbide is a material, which is a chemical compound of silicon is carbon (SiC); the Mohs hardness of 9.1; microhardness 3300-3600 kgf/mm2. Get it in electric furnaces of the resistance sililirovanie carbon particles pairs of silicic acid. Raw materials are materials that are rich in silica: vein quartz, quartz sand and quartzite, containing not less than 99,0-99,5%, SiO2and the carbonaceous material is petroleum coke. Silicon carbide is a promising material modern instrumentation due to the high radiation resistance and thermal stability.

The proposed material different from aluminum oxide to the fact that silicon carbide has a high corrosion resistance in aggressive environments, so it does not react with inorganic acids and alkalis, even at the temperature of boiling. Laboratory studies have shown that the corrosion of silicon carbide in groundwater with pH˜8 at a temperature of 170°0.15 mm per year. The mechanical strength of the silicon carbide substantially exceeds the strength of aluminum oxide, accordingly, this material is more resistant in the process of repackaging and transportation.

The use of porous silicon carbide for the manufacture of iodine sorbent may be in the form of granules of various shapes, and in the form of a filter cartridge (AF) in the case of small recycled modular filter capable OS is out cleaning the gas phase from iodine.

For the manufacture of iodine sorbent porous silicon carbide integriruetsa salt of silver nitrate (AgNO3). For this porous sorbent substrate (silicon carbide with a porosity of from 30% to 60% in the form of granules or in the form of a filter cartridge) is impregnated with a solution of silver salts with the desired concentration of silver and dried at a temperature of from 100 to 150°C. the Operation of impregnation and drying is repeated until the complete absorption of the solution.

After saturation of the sorbent radioactive iodine from the gas phase may conduct its regeneration with the aim of re-use. If necessary, you can extract the silver from the proposed sorbent for re-use.

Example

For experiments to study the possibility of manufacturing a sorbent based on porous silicon carbide (porosity 30%) was used, the sample material (hereinafter OP - filter cartridge) 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 Original mass of the OP was 184574 mg Impregnation OP nitrate of silver was carried out by nitrate solution with a concentration of silver of 2.6 g/l and nitric acid 5 mol/l, for several stages, each of which represents a process of uniform wetting of the filter and subsequent drying. Thus, achieved full of pohlad is the solution.

The OP was weighed before feeding and after him. The results of saturation AF silver presented in figure 1.

After the sixth stage, there was a complete absorption of the solution containing 2.2 g of silver, and the concentration of silver in the filter cartridge was 11.9 mg/g

Experiments on the trapping of iodine OP conducted in a laboratory setup (Figure 2).

Figure 2: 1 - tight tank, 2 - heat resistant glass, 3 - iodine-127, 4 - oven, 5 - MP, 6 - bubbler, 7 - NaOH solution.

At the bottom of the heat-resistant glass 2 was poured by the addition of iodine-127 in excess of the stoichiometric value relative to the formation of silver iodide. FP 5 was placed on a metal grid attached above the level of the filling of iodine crystals. A glass of FP was installed in the hermetic container 1, provided with a socket for removal of the gas phase in the bubbler 6, filled with a solution of sodium hydroxide with a concentration of 2 mol/l, the Saturation of the sorbent with iodine was carried out at a temperature of 200°in a shaft furnace 4. The beginning and the end of the evaporation process of iodine was controlled by changing the colour of the solution in the bubbler and inlet ducts.

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

Table 1

Rez is ltati saturation OP iodine-127
The mass of the filter cartridge with AgNO3mgThe mass of the filter cartridge after saturation mgThe mass of iodine mgThe concentration of iodine, mg/g
189894193394350018,1

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

6AgNO3+3I2↔4AgI+2AgIO3+6NO2.

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

The regeneration of the OP spent alkaline solution of hydrazine-nitrate with a concentration of alkali 30 g/l and hydrazine 15 g/L. treatment of the sorbent solution was 30 minutes at a temperature of 80°C. After carrying out the regeneration of the filter cartridge was washed with hot distilled water until pH=5-7, dried and weighed. From solutions formed during regeneration of the PF (regenerated and rinsed) iodine is concentrated in the form of copper iodide. Results regeneration of AF and deposition of iodine in iodide of copper are shown in table 2.

Table 2

Removing iodine-127 of the silver-containing filter cartridge
The mass of the filter-holder mgThe mass of th is Yes in the sediment, mgThe efficiency of extraction of iodine, %
before regenerationafter regeneration
193394186720342098

With the aim of obtaining silver from the spent sorbent on the basis of porous silicon carbide OP after removing from it the iodine was treated with nitric acid with a concentration of 5 mol/l for 30 minutes at a temperature of 80°With, then the OP was dried and weighed. Uterine and washing solutions were analyzed for their content of silver (the original content of silver in the OP - 2.2 g) in two ways. In the first case, the mass concentration of silver in solution was determined by x-ray fluorescence energy dispersive analyzer ERA-03. The second silver besieged from solution in the form of chloride. The results of the analyses of the solutions presented in table 3.

Table 3

Extraction of silver from the regenerated filter cartridge
The mass of the filter-holder mgWeight of silver mgThe efficiency of extraction of silver, %
To retrieveafter extraction, thein solutionin draft
186720184575 2180219399,4±0,3

As can be seen from the data presented in table 3, the efficiency of extraction of silver from the filter cartridge was (99,4±0,3)%, which is higher than the degree of extraction of silver from the sorbent on the basis of aluminum oxide, where she is on the level (97,4±0,6)%.

Distinctive features of the proposed material are significantly higher corrosion and mechanical resistance in aggressive environments. This can significantly prolong the life of iodine sorbent, to increase efficiency in the use of expensive silver.

Sorbent for the capture of radioactive iodine, consisting of a porous base, impregnated with salt of silver nitrate (AgNO3), characterized in that as the basis of sorbent used porous silicon carbide with a porosity of from 30%to 60%.



 

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