RU2559291C2 - Method of removing firmly fixed radioactive contaminants - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: decontaminating solution is prepared directly on the contaminated surface by first depositing a layer of concentrated sulphuric acid with content of the basic substance of not less than 92%, placing a sheet porous material soaked in decontaminating reagent solutions, holding said material, removing said material and washing the surface with water. The decontaminating reagent used is hypophosphorous acid or potassium or sodium salts thereof with reagent concentration in the soaking solutions of 2-3 g/l and the porous material is held on the surface to be decontaminated for not less than 12 minutes.

EFFECT: reducing the hazard class and concentration of decontaminating reagents, lowering the required level of safety equipment at the workplace when carrying out decontamination while maintaining efficiency of decontamination.

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Description

The invention relates to nuclear technology, and in particular to methods of decontamination, and can be used to remove firmly fixed radioactive contaminants from structural materials.

It is known to use easily removable protective films (coatings) having deactivating properties. The following polymers can be used as such films: polyethylene, casein, polyvinyl chloride, polyvinyl alcohol (PVA) [Recommendations for decontamination and methods for assessing the surface decontamination in leading foreign countries. Analytical review of the laboratory DOR TSNIIATOMINFORM, Moscow, 2000, P.41, table 7].

For decontamination of metal, concrete and plastered surfaces, VNIPIET employees (St. Petersburg) developed film-forming compositions based on a LAN that contain (wt.%):

polyvinyl alcohol with a molecular weight of 3000-14000 12 ÷ 25, one of the acids of the series: HNO ₃ , H ₂ SO ₄ , HCl, HF 0.5 ÷ 2.0 water rest

The compositions are prepared by dissolving PVA in water at a temperature of 90 ÷ 95 ° C, followed by the introduction of these ingredients with vigorous stirring. The application to the surface of the compositions, depending on their viscosity, is usually carried out by airless sprayers or with brushes, spatulas, brushes, rubber rollers, rollers. The hardened film, together with a layer of radioactive contamination adhering to it, is removed only mechanically (manually). Removable films are usually used to decontaminate easily accessible surfaces, such as floors (coated and uncoated), walls, and reactor cavities. [Normalization of the radiation situation during the liquidation of the consequences of the Chernobyl disaster: Collection / Ed. E.A. Konstantinova. - Sosnovy Bor: printing house of Leningrad NPP, 2006. - S.176-178].

The advantages include ease of implementation, the possibility of fixing and localizing radioactive contaminants (RE), the absence of liquid radioactive waste (LRW), and the small amount of solid radioactive waste (SRW) generated.

The main disadvantages of the use of film-forming compositions are: low decontamination efficiency, the compositions are used mainly to remove poorly fixed RE; low productivity and high labor intensity; the need for manual labor, especially when removing films, which can lead to significant radiation doses to personnel at high levels of surface contamination; formation of secondary non-conditioned solid radioactive waste, requiring further processing and conditioning [NP-020-2000 “Collection, processing, storage and conditioning of solid radioactive waste. Safety requirements"].

In the process of eliminating the consequences of the Chernobyl disaster, dust suppression works were carried out using domestic and foreign latexes. As a result of these works, latex-based dust suppression compounds formed solid-state dust-resistant compounds on process equipment located on the territory of the material storage base near the emergency unit and intended for installation of the 5th and 6th Chernobyl power units. The composition obtained by mixing concentrated sulfuric acid (specific gravity 1.84 g / cm ³ ) with aerosil (highly dispersed amorphous silicon dioxide with a specific surface of 175-380 m ² / g, which turned out to be the most effective for removing such REs from the surface of stainless steel equipment from stainless steel equipment) used in a number of industries as a thickener for lubricants, adhesives, paints and fillers, for example, in the manufacture of rubber) [Chemical Encyclopedic Dictionary. Chief ed. I.L. Knunyants. - M .: Owls. Encyclopedia, 1983, S.284]. By adding small portions of Aerosil to the acid with stirring, the composition was adjusted to a paste-like consistency, and then it was applied using a spatula to the surface to be treated. After exposure for 1-5 hours, the composition together with the degraded latex film was removed mechanically, and the treated surface was washed with water.

The disadvantages of this composition are: limited use (removes poorly fixed REs only from horizontal surfaces); long duration of the process (up to 5 hours); use of the composition only with the use of manual labor; receiving secondary non-conditioned SRW.

There is a method of removing firmly fixed REs from the surface of metals and alloys, which consists in the fact that a decontamination solution is prepared directly on the contaminated surface, for which a layer of concentrated sulfuric acid with a basic substance content of at least 92% is first applied to it, and then a porous material moistened aqueous solution of reagents. In order to reduce the consumption of reagents and ensure the optimum ratio of the mass of water and the mass of sulfuric acid, the porous material is used in the form of sheets (newsprint or filter paper), and to increase the decontamination efficiency, a hydroquinone deactivation reagent is introduced to create a concentration of 5- 10 g / l This method we have chosen for the prototype [USSR Author's Certificate No. 1565278 A1. A method for removing firmly fixed radioactive contaminants, issued January 15, 1990].

The main disadvantage of the proposed method is the use as a deactivating reagent of an organic compound of hydroquinone (1,4-dihydrobenzene-C ₆ H ₄ (OH) ₂ ), which is related to the degree of exposure to the body to hazard class 2 - highly hazardous harmful substances, the handling of which requires increased safety measures [Hygienic standards GN 2.2.5.1313-03. Maximum allowable concentration (MPC) of harmful substances in the air of the working area (as amended on July 12, 2011)]. According to this document, the maximum permissible concentration of hydroquinone aerosols is 1 mg / m ³ . Aerosols in the human body can lead to a decrease in hemoglobin in the blood. The presence of hydroquinone aerosols in the air contributes to eye damage, causing skin changes as the initial stage of dermatosis. The formation of hydroquinone aerosols occurs as a result of heating the solution, which occurs due to the hydration reaction of concentrated sulfuric acid, which is accompanied by intense heat. In connection with the evaporation from the surface of porous materials moistened with aqueous solutions of hydroquinone and the formation of toxic fumes, a local exhaust device and protection of the eyes and hands of personnel are necessary as a preventive measure.

Another significant disadvantage of the prototype method should be considered the formation of unconditioned combustible SRW. To reduce the amount of combustible waste and reduce the fire hazard during storage, transportation and disposal, they should be directed to incineration [see Clause 5.5 NP-020-2000]. To burn solid radioactive waste containing organic substances, including hydroquinone, additional equipment of existing solid waste burning furnaces will be required to increase the tightness of the combustion unit and the afterburner, absorption (neutralization) of hydroquinone thermolysis products and purification of the resulting flue gases, etc.

The problem to which the invention is directed, is to create a method for removing firmly fixed radioactive contaminants from the surface of structural materials, allowing:

- to achieve high removal efficiency of strongly fixed REs (at least 98-99%);

- reduce the hazard class used deactivating reagents from highly hazardous harmful substances to non-toxic;

- reduce the requirements for the level of safety during the organization of the decontamination process;

- eliminate the formation of SRW containing toxic organic reagents;

- reduce the likelihood of local types of corrosion (intergranular, crevice, corrosion cracking, etc.) on technological equipment made of stainless steel;

- use the existing technological equipment (without upgrading the incinerator) when processing and conditioning the resulting secondary SRW.

To solve the problem and achieve the technical result in a method of removing firmly fixed REs from the surface of structural materials, including preparing a decontamination solution on the surface by applying a layer of concentrated sulfuric acid containing at least 92% H ₂ S ₂ O ₄ , applying a porous sheet material (newspaper or filter paper) moistened with an aqueous solution of a deactivating reagent, soaking, removing material and washing the surface with water as a deactivating agent reagent is present hypophosphorous acid or its potassium or sodium salt with concentration of reactants in the wetting solution of 2 ÷ 3 g / l delayed porous material decontaminated surface for at least 12 min.

Examples of specific performance.

Example 1. The experimental technique. The removal efficiency of REs was evaluated in laboratory conditions on samples of structural materials with a size of 40 × 40 × 1 mm. In each experiment, 3 samples were used, which were contaminated on one side with droplets of a solution containing cesium - 137, cobalt - 60, cerium - 144 and ruthenium - 106 in a ratio of 40: 5: 3: 2 with a total volumetric activity of 5 · 10 ⁷ Bq / l To evaporate moisture from the surface of the samples, they were placed in a fume hood, where they were kept at 18–20 ° C for 24 hours, then placed in an oven, where they were kept for 14 days at 230–250 ° C.

The effectiveness of RE removal methods was evaluated by the deactivation coefficients (Cd), representing the ratio of the initial contamination of the surface of the samples to the residual, as well as the degree of remote activity — the ratio of remote activity to the initial, expressed as a percentage. To determine these indicators, a counting rate (imp./min) created by the RE before and after processing was measured on a radiometric setup. A gas discharge counter was used as a radiation detector, which was located in a lead house, and the samples were installed in the holder, creating a constant geometry between the counter window and the sample surface.

To confirm the fact that solidly fixed REs were formed on the contaminated surface of the samples, three samples were deactivated with an aqueous solution containing 5 g / L H ₂ C ₂ O ₄ + 3 g / L sodium hexametaphosphate + 1 g / L sulfonol, which is widely used in domestic nuclear energy. Decontamination of the samples was carried out in dynamic mode in the following sequence: first, they were fixed in a holder, then placed in a glass container, filled with the above solution heated to 90 ° C, closed with a lid, and mounted on a shaker. Processing time - 15 minutes. As a result of the processing, the Cd did not exceed the value of 1.05, which indicated that the chosen method of sample contamination ensured the formation of firmly fixed contaminants on their surface.

Evaluation of the effectiveness of the removal of RE by the prototype method was carried out using samples prepared from the above method from stainless steel X18H10T. Concentrated Н ₂ SO ₄ with a density of 1.84 g / cm ^{2 was} applied onto the contaminated surface of the samples with a thin layer formed during free acid spreading. Then, filter paper with a density index of 73.5 g / cm ² (GOST 12026-76) was wetted with two solutions with a concentration of organic hydroquinone reducing agent of 5 and 10 g / l. The filter paper was kept in the wetting solution for the purpose of impregnation by immersion in the solution for 5 minutes. The impregnated filter paper was applied to the surface of the samples pre-coated with concentrated H ₂ SO ₄ , then after 5 min exposure the filter paper was removed, the surface was washed with a stream of water, and dried. Radiometric measurements of the samples showed that when the filter paper was wetted with a solution with a hydroquinone concentration of 5 g / l, the degree of RE removal from the surface was 97.8% (Cd = 45), and when using a solution with a concentration of 10 g / l of hydroquinone, it reached 98.3 % (Cd = 65). The average value of the deleted activity was equal to 98%.

When evaluating the efficiency of removing RE from stainless steel samples according to the proposed method, the filter paper was moistened with an aqueous solution containing hypophosphorous acid as well as its potassium salt with various concentrations as deactivating agents, see Fig. 1.

Wet filter paper was applied to the surface of the samples, held for 15 minutes, then removed, and the surface was washed with a stream of water, then dried. After measuring the residual activity of the samples, the Cd values were calculated; the experimental results are shown in Fig. 1. An analysis of the curves shown in the figure shows that the maximum efficiency of RE removal is observed when using aqueous solutions of hypophosphorous acid and its potassium salt with a concentration of 2-3 g / l and is 99% for H ₃ PO ₂ (Cd = 79), and for KH ₂ PO ₂ - 98.5% (Cd = 69).

Similar experiments were carried out using newsprint paper with a specific gravity of 45-65 g / m ² as a porous sheet material. As a result of processing, a decrease in CD was observed in comparison with filter paper by 1.2-1.4 times. However, despite the lower Cd, the percentage of removed activity from the samples decreased slightly - from 99 to 98%.

The experiments also showed that when using the sodium salt, a RE removal efficiency comparable to the potassium salt is achieved.

Example 2. To determine the minimum time required to obtain maximum efficiency in the removal of firmly fixed REs, a series of experiments was performed. The experimental procedure was identical to that given in Example 1; their results are presented in Fig. 2.

From the analysis of the curves shown in Fig. 2, it follows that the minimum contact time of the filter paper with the surface of the samples to obtain the maximum effect of the removal of RE is at least 12 minutes.

Example 3. To assess the possibility of using the proposed method for the decontamination of other structural materials, decontamination of samples made of titanium alloy VT 1-0 and brass grade L80 was carried out. The sizes of the samples, the method of contamination and decontamination were similar, as in example 1 for the proposed method. The concentration of potassium hypophosphite in an aqueous solution for impregnating filter paper with water was 3 g / l, and its contact time with a contaminated surface was 15 min.

The experiments showed that the efficiency of the removal of REs from the surface of the titanium alloy was comparable with the efficiency obtained for samples of stainless steel, Kd was 75. For samples of brass, Kd = 35, but the degree of removal of REs remained very high at 97%. This suggests that the proposed method can be used to decontaminate equipment made not only of stainless steels, but also of brass and titanium alloys.

Example 4. To assess the corrosive effects of the proposed method on structural materials from stainless steel, brass and a titanium alloy, experiments were conducted with samples of these materials that were not subjected to processing to obtain firmly fixed REs on their surface. First, the samples were degreased with acetone and dried. They were then weighed on an Adventurer analytical balance with a detection sensitivity of ± 0.2 mg. Samples of structural materials were processed according to the proposed method on both sides to obtain maximum losses using KH ₂ PO ₂ as a deactivating agent with a concentration of 3 g / l and a holding time of 12 minutes. After processing, the samples were dried and reweighed. No weight loss was detected. Based on the sensitivity of the balance, the maximum decrease in sample mass is 0.2 mg. With a sample area of 32 cm ^{2, the} specific corrosion loss per treatment by the proposed method will not exceed 0.006 mg / cm ² , which is a very small value.

The proposed method has a 98.5-99% efficiency in removing firmly fixed radioactive contaminants from the surface of structural materials, comparable to the efficiency of the prototype method (about 98%).

To implement the method, instead of a highly hazardous reagent (hazard class 2), reagents are used that are non-toxic, safe to use and are used to prepare drugs [Brief chemical encyclopedia in 5 volumes: v.5. - M .: Owls. Encyclopedia, 1967. S.514-515].

Reducing the hazard class and concentration of deactivating reagents (on average three times) will reduce the cost of ensuring the required level of safety at the workplace when carrying out decontamination works, for example, by reducing the frequency of air exchange in the work room or by not using an additional local ventilation system .

The final oxidation products used in the proposed method of deactivating reagents containing H ₂ PO ₂ ^- ions are PO ₄ ^-3 - ions, which are known to suppress the development of local types of corrosion (intergranular, crevice, corrosion cracking and some others) on the equipment made of stainless steel.

SRW generated during the implementation of the proposed method does not contain toxic organic reagents, and their disposal can be performed on existing incinerators operating at nuclear power plants and plants of the Radon type.

Claims (1)

The method of removing firmly fixed radioactive contaminants from the surface of structural materials, including the preparation of a decontamination solution on the surface by applying a layer of concentrated sulfuric acid, applying a porous sheet of material moistened with an aqueous solution of a deactivation reagent, exposure, removal of the material and surface washing with water, characterized in that as a decontamination hypophosphorous acid or potassium or sodium salts of this acid with a concentration of traction of reagents in wetting solutions of 2 ÷ 3 g / l when holding porous material on a decontaminated surface for at least 12 minutes.

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