RU2635202C2 - Method of processing metals containing principlined surface radioactive pollutions - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: electrochemical deactivation of the metal is carried out in the method with simultaneous exposure to ultrasonic vibrations. As a deactivating solution, an aqueous solution of an acid is used, the anion of which forms an insoluble compound with calcium. Then, the spent deactivating solution is neutralized. The solution separated from the suspension is then reinforced and reused. Liquid radioactive waste, which is a suspension of metal hydroxides and hardly soluble compounds formed by calcium and anion of the corresponding acid, is liquefied.

EFFECT: increasing the efficiency of decontamination.

4 tbl, 4 ex

Description

The invention relates to the field of processing materials with radioactive contamination, and in particular to methods for removing firmly fixed radioactive contaminants from metal surfaces, and can be used to decontaminate metal radioactive waste generated during decommissioning of nuclear and radiation hazardous facilities (NROO), for example nuclear power plants or radiochemical industries.

The massive formation of radioactively contaminated metal during the dismantling of equipment, pipelines and metal structures during the decommissioning of a nuclear power plant creates the problem of its further treatment as radioactive waste requiring final isolation. At the same time, radioactive contaminated metal, if it is not volumetric activated, can be decontaminated in order to reduce the hazard class or to remove it from regulatory control for subsequent unlimited use. The applied methods and means of removing contaminants can be quite aggressive, allowing etching, including of the metal itself, since the main task is to achieve the maximum possible decontamination factors. At the same time, the economic rationality of processing will be determined not only by the costs of removing contaminants, but also by the management of secondary radioactive waste. Thus, an integrated approach to the processing of radioactive contaminated metal is required, consisting in the use of a high-performance, efficient and cheap method of decontamination, while minimizing the volume and ease of conditioning of secondary radioactive waste.

To deactivate metal surfaces, a number of methods have been developed using mechanical, chemical and physico-chemical effects. Some of them are quite “soft”, since they are designed to remove poorly fixed contaminants from the surfaces of products during their operation without active etching of the metal. This requirement does not allow the use of “hard” methods that ensure the removal of firmly fixed (non-porous, polyoxide or deep) contaminants by removing not only the oxide layer, but also part of the metal surface. At the same time, the handling of radioactive contaminated metal during decommissioning allows the use of such technologies.

A known method in which the surface of the metal is blown by a stream of gas containing a powder of abrasive material [1]. The method has a fairly high efficiency, but among its shortcomings can be distinguished relatively low productivity, lack of efficiency in the processing of products with complex internal geometry, dust formation, as well as the unresolved problems of handling secondary waste. In addition, abrasive blowing sometimes leads to “driving” radioactive contaminants by powder particles into the treated surface, which leads to a decrease in the efficiency of the process.

There are also known methods of processing metal radioactive waste by remelting (for example, [2]). They allow to minimize the amount of metal radioactive waste sent to the final isolation. However, the disadvantages of these methods include high energy consumption, as well as low decontamination efficiency during the remelting process, which requires the preliminary removal of most of the radioactive contaminants by another method.

Known methods for the decontamination of radioactively contaminated metals by exposure to them with solutions of chemical reagents [3]. With their help, processing of products with complex geometry is possible. Among the disadvantages of these methods, it is worth highlighting low efficiency in relation to firmly fixed contaminants, low decontamination rate, and large volumes of secondary liquid radioactive waste.

These disadvantages can be partially or completely eliminated by applying intensifying effects on the solution or decontaminated metal.

For example, a method is known [4], in which the decontamination of equipment from surface radioactive contamination is carried out by applying ultrasound to the surface to be decontaminated through a decontamination solution containing clay, an abrasive component and phosphoric acid. Ultrasound accelerates the decontamination process, but one of the main drawbacks of this approach is still the low efficiency of the removal of fixed contaminants.

When an electric field is applied, the process of metal decontamination with solutions also occurs several times more intensively, and firmly fixed contaminants can be removed. In the known method [5] it is proposed to carry out the decontamination of metal waste after soaking first in sodium chloride solutions (50-100 hours), and then in acidic solutions (50-100 hours), followed by electrochemical decontamination in the anode mode in the same solutions. The obvious disadvantage of this method is the high duration of the operation.

The closest analogue (prototype) of the claimed invention is a method for processing radioactively contaminated metals [6] by electrochemical deactivation in an aqueous solution of sulfuric acid (15-20 g / l), followed by purification of the spent solution by precipitation of calcium sulfate and metal hydroxides when calcium oxide is introduced, the strengthening of the clarified solution with acid and its reuse for metal deactivation. The disadvantages of the prototype include: the low speed and effectiveness of decontamination, especially when removing firmly fixed contaminants; limited set of deactivating agents (sulfuric acid); the formation of a relatively large amount of secondary liquid radioactive waste due to the low rate of decontamination; a decrease in the strength of the cement compound due to the excess content of unreacted calcium oxide in the suspension directed to cementing.

The problem solved by the claimed invention is to increase the effectiveness of decontamination during the implementation of a one-step process, accelerate the decontamination process, expand the number of decontamination agents used, as well as reduce the amount of waste sent to the final isolation.

The essence of the invention lies in the fact that in the method of processing radioactively contaminated metals by electrochemical decontamination in an aqueous solution of sulfuric acid (15-20 g / l), followed by purification of the spent solution by precipitation of calcium sulfate and metal hydroxides when calcium oxide is added, the clarified solution is supplemented with acid and its repeated use for metal decontamination, it is proposed that electrochemical deactivation be carried out while ultrasonic vibrations are applied to the solution and metal . In addition, it was proposed to use not only sulfuric acid as a deactivating agent, but also any acids containing an anion that forms sparingly soluble compounds with calcium, namely oxalic, citric, phosphoric, hydrofluoric, etc., at an acid concentration of 10- 50 g / l and bringing it to 5-10 g / l at the end of the process. In addition, it was proposed to neutralize and alkalize the spent solution using not only finely divided calcium oxide, but also limestone (calcium carbonate) with a particle size of 0.05-0.5 mm, lime milk, as well as alkali (sodium hydroxide or potassium hydroxide). In addition, it was proposed to cement liquid radioactive waste, which is a suspension of metal hydroxides and sparingly soluble compounds formed by calcium and the corresponding acid anion.

As follows from the essence of the invention, the problem is solved by the fact that in the method of processing metals containing firmly fixed surface radioactive contaminants, the process of electrochemical decontamination is intensified by the simultaneous action of ultrasonic vibrations on the solution and the metal being cleaned. Under the action of ultrasound, cavitation occurs with the formation of local shock waves and a local increase in temperature and pressure, intensive mixing of the liquid occurs not only in the volume of the solution, but also in the laminar layer at the metal surface. All this, apparently, has a significant effect on the kinetics of electrochemical processes: the anodic dissolution of the metal accelerates, the current efficiency increases at high current densities, the metal surface is cleaned of impurities and films (including those formed during electrochemical processing), the potential for hydrogen evolution at the cathode decreases. All of these factors together accelerate the decontamination process, which is directly related to the rate of anodic dissolution of the contaminated metal. At the same time, the electrochemical decontamination of the metal with the simultaneous action of ultrasonic vibrations makes it possible to remove even highly fixed impurities (for example, nonporous, polyoxide and deep ones) with high efficiency. Thus, the inventive method allows to achieve higher rates of decontamination in a single-stage process compared to the prototype (see example 1 and 2). Moreover, the decontamination rate at the same current density increases significantly (see example 1 and 2), which leads to the formation of smaller volumes of secondary liquid radioactive waste by reducing the processing time (see example 4).

As a deactivating solution, it is proposed to use a mono-solution of one of the listed acids: sulfuric, oxalic, citric, phosphoric, hydrofluoric and others containing anion, which forms sparingly soluble compounds with calcium. These reagents are characterized by a relatively low cost, chemical activity, ease of neutralization and subsequent conditioning, which also reduces operating costs. It should be noted that anions of oxalic, phosphoric, and hydrofluoric acids form hardly soluble compounds with iron ions that can settle on the surface of the metal being treated and reduce the deactivation efficiency. However, ultrasonic treatment removes this limitation by preventing poorly soluble compounds from precipitating onto a decontaminated surface. Thus, the approach used in the present method, electrochemical decontamination with simultaneous ultrasonic exposure, allows you to expand the list of used deactivating agents (see example 2, 3 and 5).

In order to avoid overgrowth of the electrodes (cathode and metal being cleaned) and the occurrence of parasitic electrochemical processes, it was proposed to set the initial acid concentration, in contrast to the prototype, at a level of 10-50 g / l with a value of 5-10 g / l at the end of the process.

The spent solution is removed for cleaning and adjusting the composition. With intensive stirring, lime milk (calcium hydroxide), finely divided (powdered) calcium oxide and (or) limestone (calcium carbonate) are added to the spent decontamination solution. In this case, sparingly soluble compounds containing calcium and the anion of the corresponding acid are precipitated. The amount of added reagent is determined on the basis of one and a half excess compared with the amount required for the precipitation of all sparingly soluble compounds (calcium with the anion of the acid, as well as metal hydroxides). Then, to completely neutralize the residual acid and further increase the pH of the solution to a value of 10-11, an alkali (sodium or potassium hydroxide) is added. At the indicated pH values, the ions of iron, chromium, nickel, etc. present in the solution precipitate, with the formation of metal hydroxides practically insoluble in water. The resulting alkaline suspension is separated (by sedimentation, filtration or centrifugation), the clarified solution is separated from the precipitate and after acidification to a concentration of 10-50 g / l is sent to reuse for deactivation of a new portion of the metal. Precipitates formed during sedimentation of the solution are sent for cementing using Portland cement or other binders. Thus, to neutralize and alkalize the solution can be used not only finely dispersed calcium oxide, as proposed in the prototype, but also limestone (calcium carbonate) with a particle size of 0.05-0.5 mm, lime milk, as well as alkali (sodium hydroxide or potassium hydroxide).

In the inventive method, the following technological aspects are important:

- decontaminated products (fragments) should be located so that their entire surface is under the influence of an ultrasonic field. If this requirement is not met, then it is necessary to install additional sources of vibrations or move and rotate products (fragments) so that all parts of the surface are exposed to ultrasound. Alternatively, consider transmitting ultrasonic vibrations through hard contact between the emitter and the product (fragment);

- the reaction rate between the calcium-containing reagent and the acid anion can decrease to almost zero when the surface of the particle of the calcium-containing reagent is overgrown with a poorly soluble reaction product. To avoid this, it is recommended to carry out the process of precipitation of sparingly soluble compounds with vigorous stirring of the solution, for example, using ultrasonic treatment;

- for a more complete precipitation of iron ions in the form of hydroxide, it is recommended that before precipitation they are converted to the oxidation state (III), for example by sparging air through a solution;

- to further remove calcium ions and strontium radionuclides from the solution, a small amount of sodium fluoride can be added to the spent solution after neutralization and alkalization;

- Cesium radionuclides, unlike most other radioactive contaminants, will not noticeably co-precipitate with the resulting precipitates and may accumulate in the solution, therefore, it is necessary to provide additional purification of the solution using a cesium-selective (for example, ferrocyanide) sorbent.

The technical result of the claimed invention is confirmed by the following examples.

Example 1

Full-scale samples, which are pieces of steel pipes with an outer diameter of 60 mm, a wall thickness of 6.0 mm and a length of 100 mm, the inner surface of which contained radioactive contamination firmly fixed in the oxide layer, mainly represented by Cs-137, Co-60 and Sb -125, deactivated in 500 ml of a solution of sulfuric acid (30 g / l) at room temperature. The experimental design included four options for decontamination: 1) chemical with simultaneous mechanical stirring of the solution (X + M); 2) electrochemical with simultaneous mechanical stirring of the solution (EC + M) (prototype); 3) chemical with simultaneous exposure to ultrasonic vibrations (X + ultrasound); 4) electrochemical with simultaneous exposure to ultrasonic vibrations (EC + ultrasound) (the claimed method). The processing time for each option was 40 minutes, but the sample was removed from the solution every 5 minutes to measure the exposure dose rate.

In the first embodiment, the sample was mounted in a glass on the end and a spiral nozzle of the overhead mixer (Heidolph RZR 1) was lowered into the pipe, which ensured mechanical stirring at a speed of 300 rpm.

The second option differed from the first in that the sample was connected by the anode circuit, and a titanium cathode was placed along its axis. A potential difference of 10 V from the IPT-10 direct current source was applied to the electrodes; the current in the circuit was controlled by an M-201 type ammeter. The current density during the experiments was 0.6-0.7 A / dm ² .

During decontamination according to the third variant, the sample was placed in a glass on the end, and the glass was placed in an MO-283 ultrasonic bath (Alexandra-Plus LLC, Vologda) above an ultrasonic vibrating emitter (22 kHz, 0.1 kW).

The fourth option (the claimed method) differed from the third in that the sample was connected by the anode circuit, and a titanium cathode was placed along its axis. A potential difference of 10 V from the IPT-10 direct current source was applied to the electrodes; the current in the circuit was controlled by an M-201 type ammeter. The current density during the experiments was 0.6-0.7 A / dm ² .

The deactivation coefficient K _{D was} determined as the ratio of the exposure dose rate (minus the background) at a certain point in the sample before and after treatment.

The generalized experimental results are shown in table 1.

It is obvious that the claimed one-stage method (electrochemical decontamination with simultaneous exposure to ultrasonic vibrations) is characterized by a significantly higher speed and efficiency not only in comparison with the prototype (electrochemical decontamination), but also in comparison with ultrasonic cleaning.

Example 2

Samples of stainless steel 12X18H10T in the form of plates 40 × 15 × 1 mm with firmly fixed contamination ( ¹³⁷ Cs, ²⁴¹ Am, ¹⁵² Eu, ¹³⁴ Cs, ⁹⁵ Zr) obtained by heat curing at 400 ° C for 24 hours were subjected to processing according to the options shown in example 1 with the following differences: solutions of sulfuric and oxalic acids (10 g / l) were used, the current density was 3 A / dm ² , the processing time was 5 minutes.

The deactivation coefficient K _{D was} determined as the ratio of the count rates of alpha or beta particles (minus the background) from the sample before and after processing.

Summarized experimental results are shown in table 2.

The data presented in table 2 confirm the conclusions about a significant acceleration of the process of electrochemical decontamination with simultaneous exposure to ultrasonic vibrations (the inventive method) compared with electrochemical processing (prototype). In addition, the higher efficiency of the sulfuric acid solution during ordinary chemical, electrochemical and ultrasonic deactivation, apparently associated with its significant etching effect, during electrochemical decontamination with simultaneous exposure to ultrasonic vibrations is not detected, which confirms the possibility of using a solution of oxalic acid in the claimed way.

Example 3

The samples from example 2 were subjected to electrochemical deactivation under the influence of ultrasonic vibrations in mono-solutions of sulfuric, oxalic, citric, phosphoric and hydrofluoric acids. The concentration of each acid was 10 g / L. The experimental conditions, see example 2.

The deactivation coefficient K _{D was} determined as the ratio of the count rates of alpha or beta particles (minus the background) from the sample before and after processing.

The generalized experimental results are shown in table 3.

The data shown in table 3 data demonstrate the possibility of expanding the list of used deactivating agents in comparison with the prototype. This result is achieved by preventing the deposition of sparingly soluble iron compounds and anions of some of the above acids on the surface of the metal subjected to electrochemical deactivation, due to the simultaneous action of ultrasonic vibrations on the solution and the metal.

Example 4

Samples of carbon steel St20 in the form of plates with a total area of 25 cm ² (each), contaminated with ⁶⁰ Co, were successively treated in a solution of 30 g / l sulfuric acid with a volume of 500 ml. In the first series of experiments, electrochemical decontamination was performed with simultaneous exposure to ultrasonic vibrations (the claimed method), and in the second, a simple electrochemical decontamination was performed (prototype). The samples were processed in the anode mode, the current density was 5 A / DM ² ; the frequency of ultrasonic vibrations is 22 kHz. Each of the samples was purified until K _d reached about 100. A series of experiments was continued until a residual acid concentration of 10 g / L was reached. In the first series, it was possible to deactivate eight samples, while in the second series - only three with the same volumes and composition of the spent decontamination solution. As a result, decontamination by the present method allows to reduce the amount of waste sent to the final isolation, compared with the prototype.

Example 5

112 g of carbon steel were electrochemically dissolved in 5000 ml of a 50 g / l solution of phosphoric acid (an iron (II) phosphate precipitate formed). Then, air was bubbled through the solution for 2 hours at a flow rate of 3 l / h using a fine-porous Schott glass filter; upon completion of the process, the absence of iron (II) ions in the solution was checked by the permanganate method. After that, 65 g of powdered (particle size about (0.05 mm) calcium oxide was gradually added with vigorous stirring. Then, adding vigorously stirred concentrated sodium hydroxide solution, dropwise, the pH was adjusted to 11. The solution was stirred for another hour, then centrifuged by draining 4500 ml of clarified solution and leaving 500 ml of pulp.

The resulting pulp containing iron (II) phosphate, calcium phosphate and iron (III) hydroxide was mixed in various proportions with Portland cement grade M-400. The maximum degree of inclusion was 25%. After 28 days, the mechanical strength of the obtained samples of cement compounds and the leachability of calcium from the compounds were determined. The results are presented in table 4.

From the data given in the table it follows that with a degree of filling of up to 25%, the value of the compressive strength exceeds 50 kgf / cm ² (5.0 MPa), and the rate of leaching of calcium from the cement block is no more than 1⋅10 ^-3 g / ( cm ² days). Thus, the possibility of using phosphoric acid in the inventive method is confirmed.

Therefore, the technical result achieved by using the claimed invention is to increase the effectiveness of decontamination when implementing a one-step process, accelerate the decontamination process, expand the number of decontamination agents used, and also reduce the amount of waste sent to the final isolation, in comparison with the prototype.

Literature

1. New reference chemist and technologist. Radioactive substances. Harmful substances. Hygienic standards. - SPb .: ANO NPO Professional, 2004. - p. 192.

2. RF patent №2249056. "A method for processing radioactive contaminated equipment and a method for the production of steels and alloys using scrap metal radioactive waste."

3. Ampelogova, N.I. Decontamination in nuclear energy / N.I. Ampelogova, Yu.M. Simanovsky, A.A. Trapeznikov; Ed. V.M. Sedova. - M.: Energoizdat, 1982. - p. 123.

4. RF patent No. 2416833. "Method of decontamination."

5. RF patent No. 2417467. "Method for the decontamination of radioactive metal waste."

6. RF patent No. 2560083. "A method of processing radioactive contaminated metals."

Claims (1)

A method of processing metals containing strongly fixed surface radioactive contaminants, including electrochemical metal deactivation in aqueous acid solutions with the formation of a spent decontamination solution and cementing secondary radioactive waste, characterized in that the electrochemical metal decontamination is carried out under the influence of ultrasonic vibrations using an acid solution, the anion of which forms insoluble compound with calcium, at an initial concentration of acid levels of 10-50 g / l and bringing its concentration at the end of the process to 5-10 g / l, after decontamination, neutralize and alkalize to pH 10-11 the spent decontamination solution using milk of lime, finely divided calcium oxide and (or) limestone and alkalis in the form of sodium hydroxide or potassium hydroxide, the clarified solution is separated from the resulting suspension, supplemented with an appropriate acid to a concentration of 10-50 g / l and reused to deactivate the metal, and the resulting separated suspension, soda Holding metal hydroxides and sparingly soluble compounds formed by calcium with the anion of the corresponding acid are sent for cementing.