RU2501106C1 - Method of decontaminating materials - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: decontamination is performed in two steps: at the first step, steam activated with chemical reagents is fed into a decontamination chamber with contaminated materials heated to 110°C; at the second step, the decontamination chamber is cooled and the decontaminated material is treated with solutions of organic solvents and complexing agents in a medium of liquefied gases or low-boiling solvents. The method may involve successively using multiple treatment cycles, alternating the first and second steps.

EFFECT: high decontamination efficiency, increasing the decontamination factor 4-30 fold compared to a single-step decontamination method in supercritical fluids, low operating pressure and volume of liquid radioactive wastes and shorter decontamination time.

3 cl, 1 dwg

Description

The invention relates to nuclear engineering and technology, mainly to the decontamination of materials from radioactive contamination.

The decontamination of the surface of materials is usually based on the dissolution of surface contaminants and deposits in electrolytes (aqueous solutions of acids and alkalis), as well as on the processes of extraction of elements that pollute the surface with various extractants.

Known methods for supercritical extraction of various metals (Wai S.M., Smart NG, Phelps C. US Patent 5606724 A. Publ. 25 Feb., 1997; Beckman E.J., Russel AJ US Patent 5641887 A. Publ. June 24, 1997 g .; Wai CM Patent PCT International, WO 9533541 Al. Publ. December 14, 1995), allowing supercritical extraction of various metals such as uranium, rare earth elements, as well as iron, mercury and cobalt. According to the proposed methods, a matrix containing metals (sand, paper, stainless steel surface, etc.) is treated with a complexon dissolved in a supercritical fluid, usually in supercritical carbon dioxide. Various organic compounds were used as complexones; the best results were observed for fluorinated β-diketones.

There is a known low-waste method of supercritical fluid extraction of cesium and transuranic elements (TUE) using a mixture of crown ethers and di-2-ethylhexylphosphoric acid (Murzin A.A., Starchenko V.A., Smart NG etc. Report "Decontamination of Real World Contaminated Stainless Steel Using Supercritical CO ₂ "Spectrum′98, Denver, Colorado, USA, September 13-18, 1998, Proceedings, American Nuclear Society Inc., USA, 1998, p. 94-98), selected by us as a prototype. In this method, a matrix with a high specific surface area containing cesium (sand, paper, stainless steel surface, etc.) is treated with a mixture of crown ether and di-2-ethylhexylphosphoric acid dissolved in supercritical carbon dioxide. The method allows to extract TUE and partially cesium from various matrices.

Its disadvantage is the use of expensive and, in many cases, toxic crown ethers for extraction.

A common drawback of all the previously proposed methods is that they use solvents in a supercritical state at ultrahigh working pressures (for example, carbon dioxide at pressures above 71.6 MPa), and also with their help it is not possible to quantitatively extract metals such as cesium and strontium, especially from firmly fixed deposits. The extraction of these metals is very important, because their isotopes - cesium-137 and strontium-90 - make the main contribution to the radioactivity of spent nuclear fuel and the contamination of the surfaces of equipment that must be decontaminated. In contrast to classical liquid extraction, the use of selective extractants for strontium or cesium — various crown ethers — as modifiers in supercritical extraction does not bring the expected effect and does not provide effective extraction and deactivation. To increase efficiency and reduce the time of decontamination, it is often necessary to increase the temperature of the surface to be decontaminated, however, when using supercritical extraction and organic solvents, this is difficult to do, because there is a significant increase in working pressure and the destruction of organic complexing agents and solvents.

The closest in essence and purpose to the claimed is a method of decontamination of materials (patent 2168779 G21F 9/28), based on the fact that the matrix contaminated with radionuclides is kept in a high pressure chamber in a supercritical solvent in the presence of water, an organic acid and complexon. After exposure, the chamber is pumped with supercritical carbon dioxide to ensure complete extraction and the extracted metal is collected in solution.

The disadvantage of this prototype is the low efficiency of decontamination in relation to firmly fixed and "old" pollution, as well as the long duration of decontamination.

The purpose of the invention is the development of an efficient low-waste method that allows you to deactivate materials with a high specific surface and permanently fixed radioactive contaminants at pressures below critical for the solvents used in the decontamination process.

The goal is achieved by the fact that the surface of the decontaminated material is pre-treated with steam activated by chemical reagents, and then with organic solvents and complexing agents dissolved in liquefied gases or low boiling solvents at pressures below critical.

The invention differs from the prototype in that the surface to be decontaminated is pre-treated with steam activated by chemical reagents in this case:

1. The efficiency of the decontamination process is increased due to the simultaneous exposure of heated chemicals to the surface to be decontaminated and dissolution of deposits in the condensed vapor-gas phase;

2. It becomes possible to constantly maintain the elevated temperature of the decontamination solution on the decontamination surface, up to its boiling point;

3. continuous change of the decontamination solution is carried out on the surface of the decontamination material;

4. increases the chemical activity of the vapor-gas phase and the decontaminating solution formed during condensation of the vapor-gas phase;

5. increase the coefficient of decontamination by 4-30 times in comparison with the prototype;

6. Reduces decontamination time by 1.5 times

7. The requirements are simplified and the safety of the application of the method in production is improved.

The use for deactivation of first activated steam, and then organic solvents and complexing agents in liquefied gases or low boiling solvents allows not only to increase the decontamination efficiency, but also to remove and concentrate in a small volume the radionuclides removed from the treated surface.

Figure 1 shows a diagram of a device for decontamination of various materials.

A device for decontamination of various materials contaminated with radionuclides includes a decontamination chamber 1, a mixer 2, a container for decontamination solutions 3, 6, a compressed air receiver 4, a condenser 5, a viewing window 7, an evaporator 8, an adsorber 9.

The procedure for using the proposed method is as follows: in a steam mixer 2, where the chemical reagent from the tank 3 in the form of a concentrated aqueous solution enters, it is simultaneously dispersed and diluted by condensation of steam on the surface of the dispersions. The combined mixture enters the chamber-decontamination 1, preheated to 110 ° C, on the decontaminated material. The surface of the material is heated by steam, and the dispersions of the decontamination solution condense on the decontamination surface. The decontamination chamber is cooled to 8-10 ° C and filled with a mixture of liquefied gas from a condenser 5 mixed with organic solvents and / or complexing agents from a tank 6. Visual observation of the process of filling the mixture and circulation of the process is carried out through the inspection window 7. Spent decontaminated solutions and CO ₂ (for regeneration) enter the evaporator 8 and then for regeneration. The discharge of the gaseous phase is carried out through the adsorber 9.

Examples of specific performance of the decontamination method.

1 Decontamination of samples of stainless steel grade 12X18H10T according to the prototype method and the proposed method

The characteristics of the radioactive contamination of stainless steel samples with strongly fixed contamination are presented in table 1.

Table 1 Characteristics of radioactive contamination of stainless steel samples Sample The specific activity, Bq / cm ² Pu ²³⁹ Am ²⁴¹ Sb ¹²⁵ Eu ¹⁵⁵ Cs ¹³⁷ Ce ¹⁴⁴ Co ⁶⁰ Ru ¹⁰⁶ Steel grade 12X18H10T 3300 8110 15100 654 347000 110000 750 265000

Decontamination by the prototype method was carried out by a complexing agent - polyethylene glycol with a content of 8 g per liter of liquid carbon dioxide. Decontamination according to the proposed method was carried out by treating a stainless steel sample at the stage of deactivation with activated solutions of potassium permanganate and nitric acid with steam containing 0.79 g / l of steam and 0.35 g / l of steam, respectively. Decontamination in a medium of liquefied gases and low boiling solvents was carried out by the composition of HFA - pyridine - water, dissolved in carbon dioxide. The values of the coefficients of decontamination of the controlled elements for decontamination by the method of the prototype and the proposed method are presented in table 2.

table 2 Results of a stainless steel sample decontamination experiment Sample Sample decontamination factor Pu ²³⁹ Am ²⁴¹ Sb ¹²⁵ Eu ¹⁵⁵ Cs ¹³⁷ Ce ¹⁴⁴ Co ⁶⁰ Ru ¹⁰⁶ Steel grade 12X18H10T (according to the prototype method) 960 20,4 - - 7.3 36.9 45,2 5.9 Steel grade 12X18H10T (by the proposed method) 29000 612 - - 29.3 147.8 226 59

It was shown that deactivation according to the proposed method proceeds most effectively for rare earth elements, as well as for Pu ²³⁹ , Am ²⁴¹ , Co ⁶⁰ . Moreover, the values of the decontamination coefficients from all controlled radionuclides are 4-30 times higher than by the prototype method.

2 Decontamination by the proposed method of samples of brass contaminated with radionuclides

The characteristics of contaminated samples are given in table.3.

Table 3 Characterization of radioactive contamination of brass samples Sample The specific activity, Bq / cm ² Pu ²³⁹ Am ²⁴¹ Sb ¹²⁵ Eu ¹⁵⁵ Cs ¹³⁷ Ce ¹⁴⁴ From ⁶⁰ Ri ¹⁰⁶ brass 460 11300 831 509 20300 64200 14.6 16000

Brass samples were treated with solutions containing oxalic acid 0.72 g / l steam and citric acid 1.73 g / l steam. Decontamination in a medium of liquefied gases and low boiling solvents was carried out with the composition of HFA - Pyridine - Water, dissolved in carbon dioxide. The values of the total decontamination coefficients for the controlled elements for decontamination by the proposed method are shown in table 4.

Table 4 The results of the experience on the decontamination of brass samples Sample Sample decontamination factor Pu ²³⁹ Am ²⁴¹ Sb ¹²⁵ Eu ¹⁵⁵ Cs ¹³⁷ Ce ¹⁴⁴ From ⁶⁰ Ru ¹⁰⁶ brass 5.2 5.5 2,3 3,7 3.2 4.4 16,2 9.3

3 Decontamination of porous materials

As a highly porous material, actually contaminated dacron tissue samples were used. The values of the radioactive contamination of the sample are presented in table 5.

Table 5 Values of radioactive contamination of a tissue sample Sample The specific activity of the sample, Bq / cm ² Pu ²³⁹ Am ²⁴¹ Sb ¹²⁵ Eu ¹⁵⁵ Cs ¹³⁷ Ce ¹⁴⁴ Co ⁶⁰ Ru ¹⁰⁶ tissue 473 11600 621 385 19100 58900 not defined 14500

Processing of lavsan fabric at the stage of deactivation with activated steam was carried out with solutions of oxalic acid with a content of 0.72 g / l of steam and citric acid - 0.35 g / l of steam. Decontamination of samples in a liquefied gas medium was carried out with a mixture of HFA-pyridine complexones — water dissolved in CO ₂ .

Table 6 Deactivation of mylar samples Sample Sample decontamination factor Pu ²³⁹ Am ²⁴¹ Sb ¹²⁵ Eu ¹⁵⁵ Cs ¹³⁷ Ce ¹⁴⁴ Co ⁶⁰ Ru ¹⁰⁶ tissue 185 39.1 1.9 73.3 4.1 34.6 6.1 3.2

To increase the coefficients of deactivation, it is possible to treat the surface to be decontaminated in an environment of activated steam and in solutions of complexing agents in liquefied gases and solvents cyclically: alternating the treatment in steam and in liquefied solvents.

The total amount of liquid waste generated during decontamination by the combined method was 0.5-0.8 ml per 1 dm ^{2 of} the surface area of the decontaminated material. The amount of waste generated at the stage of steam decontamination amounted to 8-19% of the total amount of waste. The organic and aqueous phases in liquid waste are approximately 1: 3. Organic waste is generated only at the stage of decontamination in liquid carbon dioxide. The resulting amount of liquid radioactive waste allows us to attribute the combined decontamination method to low-water technologies.

Claims (3)

1. The method of decontamination of various materials from radioactive contamination, including exposure of the deactivated material in a high pressure chamber in a liquefied gas or low boiling solvent medium, characterized in that the decontamination is carried out in two stages, where the first is deactivated by steam activated by chemical reagents, and the second - in an environment of organic solvents and complexing agents dissolved in liquefied gases or low boiling solvents. 2. The method according to claim 1, characterized in that the operating pressure during decontamination is lower than critical for the solvents used in decontamination. 3. The method according to claim 1, characterized in that the increase in the coefficient of deactivation is achieved through the cyclical use of both stages.