RU2288515C1 - Method for decontaminating metal surfaces - Google Patents

Abstract

FIELD: nuclear engineering.

SUBSTANCE: proposed method that can be used for decontaminating radionuclide-contaminated metal surfaces of various nuclear power installations, manufacturing and other pieces of equipment, including those having complex configuration and subject to disposal and burial, involves their scanning by contact-arc discharges from current supply having flat current-voltage characteristic of at least 280 A/V at voltage of 7 - 4 V across working electrodes; granules of contaminated metal brought out of decontamination zone are sized to minimum 0.05 mm.

EFFECT: enhanced productivity and personnel safety.

3 cl

Description

The invention relates to nuclear engineering and can be used to decontaminate metal surfaces of nuclear power plants contaminated with radionuclides, technological and other equipment, including those subject to disposal and disposal.

Currently, various methods for the decontamination of metal surfaces are known, for example, electrochemical, redox, shock wave, etc.

A known method of cleaning the surface from radioactive contamination by treating the contaminated surface with an aqueous decontamination solution, followed by removing the spent decontamination solution, in which the contaminated surface is treated in a closed volume of the decontamination solution by exposing the contaminated surface to a cavitating flow of decontamination solution, wherein the difference between the pressure of the decontamination solution and static pressure closed volume deactivate present solution is 40-60 atm. (0.4-0.6 MPa), and the treatment of the contaminated surface is carried out at a temperature of a decontaminating solution of 18-22 ° C (RF patent No. 2240613, class G 21 F 9/28, publ. 20.11.2004).

This method is intended only for confined volumes using a liquid decontaminating solution, which then goes into the category of liquid radioactive waste. Maintaining a certain temperature and pressure in a closed volume makes it difficult to widely use this method.

Most often, mechanical and chemical methods of decontamination are practiced.

The mechanical method of decontamination using scrapers, brushes, rags and washing liquids, as well as a jet flushing head, is ineffective and involves a large dose of personnel.

Chemical methods for the decontamination of metal surfaces, which implement the dissolution or neutralization of the fractions being cleaned in combination with the mechanical cleaning or immersion of decontaminated products in bathtubs, are acceptable only for small-sized products and require the neutralization of environmentally harmful liquids used.

A known method of decontamination of metal surfaces, in which the surface to be decontaminated is covered with a layer of water and melted by scanning it with a local high-temperature heat source (focused laser beam), while the outer metal layer boils, sprays and partially evaporates. The process of evaporation of the molten metal is accompanied by the release of gaseous and aerosol radioactive waste, which together with the particles of the molten metal are trapped in water (French patent No. 2700882, CL G 21 F 9/30, published on 07.29.94).

The reason that impedes the use of the known method is the significant amount of gaseous and aerosol radioactive waste leaving the treatment zone that is not trapped in water, which increases the radiation hazard for the maintenance staff. In addition, the disadvantage is the formation of a large amount of liquid radioactive waste, which must be disposed of, which leads to an increase in the cost of the known method.

A known method of decontamination of metal surfaces, in which the decontaminated surface is covered with a layer of water and fused with a local high-temperature heat source (high-temperature plasma) by scanning it with a plasmatron. Then the granulation of the molten metal is carried out by spraying it with a jet of high-temperature gas leaving the plasma torch and cooling the particles of molten metal in water (Japanese application No. 63-33116, class G 21 F 9/28, publ. 04.07.1988) .

An obstacle to using the known method is a significant amount of gaseous and aerosol radioactive wastes leaving the treatment zone that are not trapped by water, which is explained by the process of reflow and spraying in a practically dry place, since the stream of gas leaving the plasma torch not only forces water from the treated area, but also it evaporates due to its high temperature, while gaseous and aerosol wastes are mixed with steam and enter the atmosphere of the room. This disadvantage leads to an increase in radiation hazard for maintenance personnel. In addition, the disadvantage of this method is the large dispersion in the size of the granules, which is explained by the difference in the values of the gas flow rates along the cross section of the annular jet (in the center - the maximum speed, and at the periphery - the minimum). The central jet sprays molten metal onto smaller particles, and the peripheral jets spray onto larger particles. Larger particles drown in water under the influence of gravity, and small particles can remain in water for a long time in suspension and even partially dissolve in water, thereby increasing the amount of liquid radioactive waste, the disposal of which makes the entire decontamination process more expensive.

In the analogs mentioned above, the disadvantage is that using a laser beam and high-temperature plasma, part of the surface radioactive contamination is melted into the crystal lattice of the deactivated metal during decontamination.

The closest set of essential features and the technical result achieved is a method of decontamination of metal surfaces, in which a decontaminated surface coated with a water layer is melted by scanning with a local high-temperature heat source and granulation of the molten metal is carried out, using a pulsed spark-arc discharge as a heat source, which is created with using a pulse generator in the gap between the treated surface and the electrode, and gr the melted metal is canceled by electro-hydraulic shock, which is created by quenching the discharge in water (RF patent No. 2171511, CL G 21 F 9/28, published on 08/10/2001).

The disadvantage of this method is the need to maintain a constant gap between the surface to be decontaminated and the electrode, which makes it impossible to deactivate a complex shape surface, reduce productivity, and also use special equipment (pulse generator), which leads to higher cost of equipment when using this method of decontamination. In addition, the reason that impedes the use of the known method is the use of high voltage within 50 ÷ 1500 volts, which is unacceptable according to the Electrical Safety Rules for portable equipment, as well as unreasonable energy losses due to breakdown of the gap between the electrode and the surface being decontaminated. In addition, the use of high speeds of rotation of the electrode-tool imposes a number of requirements for ensuring the safety of the environment and personnel (the use of protective covers, etc.). With high-speed decontamination in one pass of the electrode-tool, the scanned layer of surface contamination is no more than 1 mm (which does not allow deactivating corrugated surfaces), this increases the number of passes and reduces productivity. At the same time, because of the high speeds of small granules, more than large granules are formed, they settle longer, thereby polluting the water.

Patent No. 2171511, cl. G 21 F 9/28 is selected as a prototype.

The objective of the present invention is to provide a method for the decontamination of metal surfaces of any complexity, ensuring the safety of the environment and personnel, especially when working manually, and increasing the productivity of the decontamination process while reducing its cost.

The technical result of the present invention is the ability to decontaminate surfaces of any complexity by direct contact of the electrode tool and the decontaminated surface. In addition, due to the low scanning speed of the electrode, granules are formed, the size of which ensures rapid deposition and prevents the pollution of the technological environment.

The specified technical result is achieved by the fact that in the method of decontamination of metal surfaces, mainly in a liquid medium, in which the surface to be treated is scanned by pulsed electric discharges of the current source, and the contaminated metal is removed from the decontamination zone in the form of granules, scanning is performed by contact-arc discharges of the current source with stiffness current-voltage characteristics of at least 280 A / V at a voltage on the working electrodes of 7 ÷ 24 V, and granules removed from the decontamination zone of contaminated meta la is formed of at least 0.05 mm.

The best result is achieved by scanning a deactivated surface at a speed of 3-15 mm / s, and in one pulse of a contact-arc discharge, a layer of contaminated metal up to 5 mm thick is removed.

In one pulse of a contact-arc discharge, at least one hole up to 5 mm deep is formed on the surface, which helps to increase the productivity of the process.

The use of a current source with a rigid current-voltage characteristic (CVC) of at least 280 A / V is necessary in this method to maintain constant characteristics of electric arc discharges, since at a lower value the stability of the decontamination process is not ensured, and with a higher value of the CVC, the process is not economically feasible.

With a voltage of less than 7 V, the decontamination process will be ineffective, since the decontamination time and the number of passes will increase, and the use of a voltage of more than 24 V will lead to an unreasonable increase in energy consumption without increasing the technical result. In addition, the use of operating voltage in the range of 7 ÷ 24 V increases the safety of operating personnel, especially when using a hand tool.

The speed of movement of the electrode should be maintained within 3 ÷ 15 mm / s, depending on the degree of radioactive contamination of the decontaminated surface and the voltage on the electrodes. The higher the pollution level, the lower the scanning speed and the higher the voltage. Burning at a speed of less than 3 mm / s can cause burnouts in a decontaminated surface, and at a speed of electrode movement of more than 15 mm / s there are no contact jumpers between the electrode and the deactivated surface, which leads to a sharp decrease in the efficiency of the decontamination process.

In addition, the speed of movement of the electrode, within the above limits, ensures the removal of deep fixed contamination in fewer passes; in this case, large at least 0.05 mm granules of contaminated metal are formed, which quickly precipitate and do not pollute the liquid and the environment in small fractions. The indicated granule size is provided by the working methods of the location of the electrode.

Scanning the surface by contact-arc discharges provides the ability to deactivate the surface of metal products of any configuration, since there is no need to strictly maintain the gap between the electrode and the surface being decontaminated.

The characteristics indicated in the claims are necessary and sufficient to ensure the specified technical result, that is, they are significant.

The present invention is not known from available sources of information, does not explicitly follow from the prior art and is industrially applicable, that is, it meets all the eligibility criteria under current law.

The method is as follows.

Metal products, the surfaces of which are contaminated with radionuclides, are placed in a working tank with a liquid process medium (working medium).

The electrode and hardware are connected to a current source. Depending on the level of pollution, the voltage value is selected.

By connecting the electrode and the surface to be decontaminated, voltage is applied, the electrode is scanned when they are in direct contact, and contact-arc discharges are thus formed, and contact jumpers arise between the electrode and the surface being decontaminated, which is accompanied by intense melting and ejection of metal granules from the treatment zone.

Unlike analogs and prototypes, this method melts and eliminates contaminated metal, which results in no fusion of pollution and the metal remains cleaner, excluding secondary pollution.

The method can be used in any liquid non-combustible media. Liquid technological environment, falling on the molten metal, contributes to the granulation of the melt and its removal from the decontaminated surface from the holes formed by arc discharges. Further, the flows of a liquid medium wash granular particles from the interelectrode gap, eliminating the secondary effect of discharges on them.

The liquid technological environment creates films on granulated metal particles (spherical granules, drops), prevents their adhesion and adhesion to the decontaminated surface.

In this method, the decontamination products do not dissolve in the technological environment and are metal powders consisting of spherical particles ranging in size from 0.05 to 5 mm, which are captured by gravity settlers.

If necessary, it is possible to reuse the process medium after separation of the mechanical component of the pollution.

Description of the method of decontamination.

As samples for testing this method, metal plates with a size of 100 mm × 100 mm and a thickness of 5-8 mm were selected. Material - X18H10T stainless steel. One side of the plates was contaminated with radionuclides: a mixture of Cs ¹³⁷ and Ce ¹⁴⁴ isotopes. The pollution is fixed, with the initial pollution of 3700 ÷ 4400 β-part / cm ² * min. The initial and residual contamination of the samples was determined using a KRAB-3 radiometer. The effectiveness of decontamination was evaluated using the coefficient of deactivation:

K _d = A _ref / A _ost ,

where A _ref is the initial contamination of the samples;

And _ost - residual pollution after decontamination.

A current source with a hard current-voltage characteristic of 280 A / V was used. The voltage at the working electrodes is from 7 to 24V. When using a current source for electric arc welding (TIR-200), which has a current-voltage characteristic of 50 A / V, the process was not feasible.

For intermediate values of the current-voltage characteristics, research power sources developed by the applicant were used.

A graphite plate (grade of graphite EG-0) with a working surface of 100 mm ^{2 was} chosen as the electrode of the tool.

The power of the discharges between the electrode and the surface to be decontaminated was changed by a voltage change in the range from 7 to 24 V.

The deactivated surface was scanned with an electrode at speeds of 3–15 mm / s. Moreover, at a scanning speed of less than 3 mm / s, burn-through of the plates was observed. At scanning speeds above 15 mm / s, the decontamination efficiency is reduced.

The thickness of the removed metal layer was checked using a micrometer.

The contamination of the process water averaged 1.0-4.0 * 10 ^-8 Ci / l, the contamination of the removed metal (metal granules) was 4000 ÷ 5000 β-part / cm ² * min.

Moreover, it should be noted that with the formation of a fraction of particles of a metal powder with a particle size of less than 0.05 mm, the decontamination efficiency is reduced, and the contamination of the powder is 1500 ÷ 2000 β-part / cm ² * min, process water 1.7 ÷ 3.0 * 10 ^-8 Ki / l.

The table shows the modes and results obtained on the samples during testing.

In examples 4, 5, 8, metal plates with a corrugated working surface were used as test samples, in the remaining examples with a smooth working surface.

In examples 2, 4, 7, the tests were carried out in an environment of silicone oil brand SJ-101 K, in the remaining examples in an aqueous medium.

Example 1

A contaminated metal plate (A _ref = 4300 β-part / cm ² * min) with a thickness of 6 mm was connected to a power source and put in a cuvette with water.

The graphite electrode was connected to a source with a rigid current-voltage characteristic of 280 A / V, the voltage at the working electrodes was set to 20 V.

The plate was processed with an electrode scanning speed of 10 mm / s.

The plate was removed from the cuvette, dried by blotting with filter paper, the residual contamination and thickness of the removed metal were measured.

Residual pollution amounted to background values (A _rem = 4-5 β-part / cm ² * min). K _d = 970.

The thickness of the removed metal layer was 3 mm.

Process water was poured from the cuvette and filtered.

On the UMF-2000 radiometer, the contamination of the process water was determined. The contamination of the process water was 3.7 * 10 ^-8 Ci / l.

The metal precipitate was dried and its contamination was determined. The contamination of the removed metal (metal granules) was 4000 β-part / cm ² * min.

Using a set of standard sieves, the granule size was determined, which was 0.05-0.35 mm.

The decontamination method in examples 2-17 was carried out similarly, the test parameters are shown in the table.

Thus, in comparison with the prototype, in the proposed method, the following advantages are achieved:

- decontamination of complex, corrugated surfaces;

- the ability to remove deep radioactive contamination through the use of low surface scanning speeds;

- the possibility of decontamination in the proposed method in any liquid non-combustible environments;

- the use of low voltage, as well as low processing speeds for the safety of the environment and personnel;

- reducing the cost of the decontamination process through the use of cheaper and simpler technological equipment.

Such results are achieved by combining scanning the deactivating surface of metal surfaces of various configurations with contact-arc discharges of a current source with a hard current-voltage characteristic of at least 280 A / V with a specific voltage range of 7 ÷ 24 V at the working electrodes, with the formation of granules of contaminated output metal with a size of at least 0 , 05 mm.

Thus, the proposed invention is aimed at solving the problem and meets all the criteria for eligibility under applicable law.

Claims (3)

1. The method of decontamination of metal surfaces, mainly in a liquid medium, in which the treated surface is scanned by pulsed electric discharges of the current source, and the contaminated metal is removed from the decontamination zone in the form of granules, characterized in that the scanning is performed by contact-arc discharges of the current source with the stiffness of the current-voltage characteristics not less than 280 A / V at a voltage on working electrodes of 7 ÷ 24 V, and granules discharged from the decontamination zone of contaminated metal form a size of at least 0,0 5 mm. 2. The method according to claim 1, characterized in that the treated surface is scanned at a speed of 3-15 mm / s. 3. The method according to claim 1, characterized in that for one pulse of a contact-arc discharge, a layer of contaminated metal up to 5 mm thick is removed.