System and method for decontamination of contaminated metal tubes

Abstract

The invention describes a cost-effective method for efficiently decontamination pipes that have been dismantled during the decommissioning of a nuclear power plant or removed from heat exchangers during repair. Another objective of this invention is to ensure control over the entire surface for radioactive contamination (cleanliness) and to reduce the amount of radioactive waste. A third goal is to provide a safer and more environmentally friendly process for decontamination, control, and preparation for the reuse of metal. The method described above includes the following steps: longitudinal cutting of the pipes with at least one cut; flattening (straightening) the pipes; cutting and arranging the straightened pipes in cassettes; soaking the cassettes filled with straightened metal strips; removing radionuclides from the surfaces of the metal plates by decontamination them in a pulsed electrochemical and ultrasonic bath filled with an acid solution; managing the secondary waste; removing the solution from the metal strips placed in cassettes; determining the level of radioactivity (cleanliness) of the metal strips; repeating the decontamination steps until a 100% safety level is achieved; and drying the metal strips placed in cassettes.

Description

FIELD OF THE INVENTION

[0001] The invention relates to nuclear energy and is intended for the mechanical preparation of pipelines and for the decontamination of radioactive contaminated pipe surfaces by using the synergistic effect of a pulse-reverse electrochemical method and ultrasonic treatment in a weakly acidic environment.BACKGROUND OF THE INVENTION

[0002] All technical equipment, technologies, and process parameters have been selected taking into account the latest trends and challenges faced by many countries in the field of nuclear energy, particularly in addressing complex issues related to the environmentally safe and economically efficient decommissioning of nuclear facilities and other radioactively contaminated industrial installations.

[0003] In nuclear power plants, metallic piping is widely used for coolant transfer and heat exchange processes. Most of these pipes have a diameter of less than 120 mm and a wall thickness of less than 4 mm. Various metals are used in pipe manufacturing, including stainless steel, carbon steel, copper-nickel, and zirconium alloys.

[0004] During use, the surfaces of the pipes become contaminated with radioactive nuclides. These radioactive nuclides accumulate in a thin oxide layer that is tightly adhered to the pipe surface. It is mechanically difficult to effectively remove 100% of these oxide layers. Another method of handling radioactive waste is chemical decontamination, especially of metals, down to a safe permissible level, enabling their reuse in industry.

[0005] A "safe level" means 100% absence of radioactive contamination.

[0006] The IAEA document "Decontamination Approaches During Nuclear Power Plant Failures - Lessons Learned and Experience Gained" (IAEA-TECDOC-1946, 2021) describes ultrasonic-based and electrochemical decontamination devices. These devices improve the decontamination process and preserve chemical reagents.

[0007] At Dukovany NPP, ultrasonic decontamination is performed in a 3.5 m 3< bath with 18 ultrasonic transducers, a heating device, and a basket for decontaminated parts. The drawbacks of this system include difficulty removing tightly adhered oxide films and a limited ultrasonic effect distance (up to 0.5 m).

[0008] The second device at Dukovany NPP is an electrochemical decontamination system in a stainless-steel bath filled with electrolyte. The anode is the part being decontaminated, from which the radioactive oxide layer is removed. The decontamination factor can reach up to 500, but there are limitations: the surface must be smooth and free of contaminants (oil, deposits, or coatings).

[0009] Russian patent application RU2008107119 (A) describes a method for decontaminating metal parts using electrochemical means (the treated part acts as an anode) and ultrasonic transducers firmly attached to the body of the decontamination bath.

[0010] JP2005337778 (A) proposes a method involving longitudinal cutting of a pipe and chemical decontamination of both its inner and outer surfaces, followed by radiological control of both surfaces.

[0011] Another patent application, JP2007085796 (A), describes a method for measuring contamination on the inner surface of a pipe and decontaminating it without using a longitudinal pipe cutting device. The pipe, taken from a heat exchanger, is cut into segments. Each segment is flattened into a strip, and the edges are trimmed using a knife-like tool. The resulting two metal strips are decontaminated and inspected for radiation. The trimmed pipe edges are treated as radioactive waste. These inventions are important as they provide solutions for safely decontaminating metal pipes.

[0012] RU2008107119 (A) describes an electrolytic bath designed for ultrasonic pipe decontamination. The main disadvantage of this invention is the high-power consumption of the equipment since considerable energy is needed to affect the internal pipe surfaces. In addition, high-concentration acidic solutions are required, and the decontamination process is time-consuming. As a result, a larger amount of metal is dissolved from both inner and outer surfaces, leading to an increase in secondary waste and material loss. Moreover, radiological inspection of round pipes is difficult and does not guarantee 100% reliability.

[0013] Patent LT6682 describes a system and method for decontaminating the inner and outer surfaces of tubular metal elements. The system comprises a cutting line, flattening line, containers for holding straightened elements, and a washing line. The decontamination process includes the following steps: longitudinal cutting of one side of a tubular metal element; flattening the tubular metal element into a metal sheet; loading the sheet into holding containers; soaking the containers with metal sheets; removing contamination particles during the washing step; handling removed residues; rinsing off the second solution from the containers; drying the containers with metal sheets; determining the radiation level of the metal sheets; and repeating the decontamination process if the safety level is not achieved.

[0014] This invention differs in that it additionally introduces an electrochemical decontamination stage, which increases the process speed and improves the final result quality.SUMMARY OF THE INVENTION

[0015] The objective of this invention is to provide a cost-effective method for efficiently decontamination pipes that have been dismantled during the decommissioning of a nuclear power plant or removed from heat exchangers during maintenance. Another objective is to ensure full-surface control of radioactive contamination (cleanliness) and to reduce the amount of radioactive waste. A third objective is to achieve a safer and more environmentally friendly process for decontamination, inspection, and preparation of the metal for reuse.

[0016] This invention overcomes the aforementioned technical drawbacks and provides a system and method for decontamination of the inner and outer surfaces of contaminated heat exchanger pipes or other contaminated metal tubular elements. The main principle of this method is to flatten the pipe without generating radioactive waste such as chips or dust. The system includes a pipe cutting line, a pipe flattening line, a cleaning line, and specialized devices for radiological inspection of decontaminated materials. All decontamination stages are optimized and integrated into a single industrial process to ensure optimal results, guaranteeing the highest worker safety and minimal environmental impact.

[0017] The method for decontamination of the inner and outer surfaces of contaminated metallic elements, which may include pipes and plates, involves preparation of the metal elements for decontamination by longitudinal cutting and flattening them into metal strips. The metal strips are loaded into holding devices, soaked, and placed into a decontamination bath, where the decontamination is carried out by immersing the cassette into a prepared decontamination solution while simultaneously subjecting the plates to electric current and ultrasonic waves of the required frequency. Afterwards, the cassette containing the metal strips is washed, dried, and the radiation level of the metal elements is monitored.

[0018] Electrochemical ultrasonic treatment is performed using an electrochemical decontamination cassette containing the metal strips. The electrochemical decontamination cassette connects the metal strips as anode and cathode in a reverse-processing configuration. The electrochemical ultrasonic treatment is performed in a decontamination bath comprising an ultrasonic wave source and a cathode-anode power supply, using an acidic solution as the electrolyte, preferably a 0.5-1.5% nitric acid solution.

[0019] This invention also provides a system for decontamination of the inner and outer surfaces of contaminated tubular elements, comprising a cutting line, a flattening line, holding devices designed accommodate elements during the washing stage, and an electrochemical decontamination line. The electrochemical decontamination line includes metal strips, a cassette, a decontamination bath made of acid-resistant materials, an electrolyte, an ultrasonic wave source, and a cathode-anode power supply. The electrochemical decontamination line additionally includes fluid filling and draining equipment, auxiliary circulation means, filters, pumps, and controllers that enable the second bath to be filled with solution and ensure process control. The cathode-anode power supply is configured to generate direct current pulses and periodically reverse polarity between the metal strips.BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 shows a general view of the decontamination line. Figure 2 shows the longitudinal cutting device (2) with the layout of the tubular metal element (1) cutting line in the decontamination system. The cutting line includes rollers (15) and pipe cutting tools (16), which are installed in the machine so that the rollers (15) push the tubular metal element (1) along the cutting tools, thereby splitting the tubular metal element (1) longitudinally. Figure 3 illustrates a schematic diagram of the cutting line with multiple rollers (15) arranged along a sharp blade designed for cutting. Figure 4 shows the pipe flattening diagram (4). The flattening line (4) includes flattening tools (16) installed in the machine such that they gradually compress the pipe into a metal strip (6). Figure 5 shows the device for flattening cut metal tubular elements into metal strips. The flattening tools (16) have roll-type central regions with gradually increasing diameters to ensure a smooth and efficient flattening process. Figure 6 shows a partially flattened metal strip (6) which is processed in the flattening line (4). Figure 7 shows a cassette containing metal strips (6), which are placed in the cassette (7) with gaps to allow for effective washing. Figure 8 shows a work bar for pulse-reverse electrochemical and ultrasonic decontamination of metal strips (6) placed in cassettes (7) in an acidic solution. Figure 9 illustrates the washing stage scheme. Top view. Figure 10 shows the decontamination scheme of the metal strips (6). The decontamination stage includes the first (8), second (9), and third (10) baths, vapor neutralization means (11), drying means (12), and lifting means (13). Figure 11 shows the electrolyte filtration system and the schematic diagram of the washing line, including vapor collection devices. Figure 12 shows a detailed scheme of the pulse-reverse electrochemical decontamination process in the second bath (9).

[0021] To clearly and concisely present the invention, the drawings do not depict elements unrelated to the essence of the invention. It is also important to note that the dimensions of elements are enlarged or shown only schematically. The dimensions of each part may not reflect actual dimensions.DETAILED DESCRIPTION OF THE INVENTION

[0022] This invention describes a system and method for decontamination both the internal and external surfaces of metal pipe elements. The aim of this system and method is the implementation of a new electrochemical decontamination stage carried out in a bath using an electrochemical decontamination cassette that functions as electrodes. The invention is unique in its ability to safely cut, flatten, and deactivate metal tubular elements without generating hard-to-manage radioactive waste. Here and hereinafter, metal tubular elements refer to radial, preferably hollow, metal components and metal plates contaminated with radioactive particles.

[0023] The invention provides a system and method for deactivating the internal and external surfaces of metal tubular elements. It is unique in that it describes a fully safe method for cutting, flattening, and decontamination of metal tubular elements without generating expensive-to-dispose radioactive waste.

[0024] The system comprises the following equipment: * A longitudinal cutting device (2) for tubular metal elements (1); * A flattening device (4) for converting cut tubular metal elements (1) into metal strips; * A cutting device (5) for segmenting the flattened metal strips (6) to desired lengths; * A work bar (7) for packing metal strips into specialized cassettes (7); * A work bar for soaking cassettes containing metal strips (6) in a bath with an acidic solution for pulse-reverse electrochemical and ultrasonic decontamination (Figs. 8-9); * A work bar for washing the metal strips (6) (Fig. 10); * A work bar for drying (12) and radiological inspection (14) of decontaminated metal strips (6); * A device for removing mechanical particles from the decontamination solution (Fig. 11); * An air purification system for removing radioactive chemical aerosols from the decontamination chamber.

[0025] The longitudinal cutting stage of the tubular metal elements (1) is carried out in the cutting line (2), involving at least one longitudinal cut of the element. In the cutting stage of tubular metal elements (1) depending on the material, the cutting tool may be a plasma cutter, laser cutter, sharp blade, or high-pressure water jet.

[0026] The cutting device (5) for segmenting the flattened metal strips (6) to desired lengths ensures uniform exposure of the metal surface when subject to electric current and chemical solution within the cassette (7).

[0027] The metal strips (6) are cut into segments matching the cassette length and packed into the cassette (7) with spacing, between them to allow even access to the solution and ultrasonic treatment on all surfaces.

[0028] The decontamination line includes three baths (8, 9, 10), a vapor neutralization unit (11), a drying module (12), and lifting mechanisms (13). Additional auxiliary units such as support frames, power supplies, pipelines and channels for fluid, air, or vapor supply and removal are used. It is understood that standard elements such as supports, power sources, and pipeline connectors are subject to applicable safety regulations and are not described in detail here. Moreover, the number of baths does not limit the scope of the invention, soaking may be done in a single bath or multiple ones to achieve better results. Efficiency is improved when electrochemical, ultrasonic stages and soaking stages are conducted in separate baths, avoiding the need to change the bath contents (e.g., acid solution, water).

[0029] The first bath is a soaking bath (8), preferably made from corrosion-resistant materials such as stainless steel or appropriate plastics. The soaking bath (8) filled with a detergent solution is used for initial wetting of the metal strips (6). It should be clear to the person skilled in the art that the first bath is not necessary for this process to be implemented but only increases the efficiency of the decontamination process.

[0030] The second bath is the decontamination bath (9), preferably made of acid-resistant material. The decontamination bath (9) is filled with 0.5-1.5% nitric acid solution. In addition, the decontamination bath (9) is equipped with a cathode-anode power supply (21) for generating direct current pulses and periodic polarity reversal, as well as ultrasonic wave generators (17) are equipped in a such way that a vector of generated waves is oriented parallel to the planes of the metal strips (6a, 6b) (see Fig. 12). The ultrasonic wave generators (17) are located at the bottom of the bath. The decontamination bath (9) may include liquid filling and draining devices, auxiliary circulation means (19), filters (18), pumps and controllers (20) that allow the decontamination bath (9) to be filled with a solution and ensure process control by removing (filtering) all mechanical particles so that the cleaning process is effective. In addition, these measures ensure the safe collection of radioactive particles in filtration devices (18). The solution in the decontamination bath (9) is heated and evaporated by absorbing the energy of the ultrasonic waves produced by the generator, which is converted into internal energy of the solution. The ultrasonic transducer, the power of the transducer or the frequency are adjusted depending on the substances to be decontaminated.

[0031] Special cassettes (7), made of durable and chemically resistant material such as stainless steel, are used to hold the metal strips (6) for soaking. The cassette (7) contains metal strips (6), prepared for the soaking stage. The metal strips (6) in the cassette (7) are parallel to each other with a distance between them, so soaking is more efficient, and the flow of solutions evenly reaches the surfaces of all strips. In addition, the metal strips (6) are layered within the cassette (7). The cassette (7) structure allows uniform processing of all metal strips (6), saving time, reagents, and reducing dissolved metal waste.

[0032] Measures to neutralize released gases and aerosols include measures for ventilation of the surface space through the side openings of the decontamination bath, washing with a spray of alkaline solution in a scrubber, and subsequent air purification with aerosol filters before discharge into special ventilation.

[0033] The third bath is a washing bath (10), made of chemically and mechanically resistant plastic or stainless steel. The washing bath (10) contains means for supplying a high-pressure water jet. The high-pressure jet removes the acid solution and oxide film residues from the metal strips (6) without removing the strips from the cassette (7) with minimal water consumption.

[0034] The drying device (12) is a high-volume air flow generator that generates air flows towards the metal strips (6), drying the strips in the cassette (7).

[0035] Lifting devices (13) are one or more hoisting mechanisms mounted on auxiliary structures. The lifting devices (13) are used to transfer the cassette (7) with metal strips (6) from one bath to another.

[0036] The present invention also provides a method for decontamination of radioactive contaminated metal pipes (1) with a diameter of less than 120 mm or metal strips (6).

[0037] The step of cutting a tubular metal element (1) along one side of the element comprises feeding the contaminated tubular metal element to a cutting device (2), where the device cuts the tubular metal element (1) by making at least one longitudinal cut. The cutting means may be a sharp knife, a laser beam, a plasma stream or a very high-pressure water jet. All of these alternative cutting methods generate a certain amount of waste, and appropriate air filtration methods are applied to ensure the safety of the respective cutting process.

[0038] The flattening step of the tubular metal element (1) includes feeding the cut tubular metal element (1) with a longitudinal cut into a flattening device (3), where the device flattens the tubular metal element (1), gradually flattening it to a flat strip (6). When using radioactive contaminated plates, the cutting and flattening stages are not used.

[0039] The flattened metal strips (6) are cut into segments according to the length of the cassette (7) using a cutting device (5) and packed into a cassette (7), leaving gaps between the metal strips.

[0040] The electrochemical decontamination cassette (7) connects the metal strips (6) as anode (6a) and cathode (6b) with reverse processing and ensures uniform ultrasonic wave exposure. The electrochemical decontamination cassette (7) functions as electrodes and is electrically conductive. A weak acidic solution (21) serves as the electrolyte. The entire process is carried out under ultrasonic influence, providing a synergistic effect that accelerates processing and improves final quality. Key process parameters include voltage, current direction, switching frequency, electrolyte concentration, ultrasonic power and frequency, solution temperature, decontamination time, and electrolyte composition.

[0041] The cassette (7) with metal strips (6) is placed into the first bath which is soaking bath (8), filled with a weak detergent solution, and soaked for approximately from 30 seconds to 2 minutes. The step of soaking metal plates using a weakly acidic detergent solution provides: a) surface preparation for uniform exposure to the decontamination solution by immersion in a decontamination bath; b) reduced processing time in the decontamination bath; c) removal of mechanical particles from the surface and preservation of the decontamination bath solution.

[0042] The cassette (7) is then immersed in the second bath (9), which is the decontamination bath, filled with 0.5-1.5% nitric acid solution. The bath is made of a special plastic that is chemically resistant and non-absorbent. The construction material of the decontamination bath (9) is a sound and electrical insulator.

[0043] The metal strips (6) are arranged vertically in cassettes (7) and immersed vertically in the decontamination bath (9). This configuration optimizes the use of the bath volume relative to the treated surface area.

[0044] The decontamination of the metal strips (6) is carried out by immersing the cassette (7) in the prepared decontamination solution and simultaneously applying an electric current to the plates (anode) and ultrasonic waves of the required frequency from a powerful ultrasonic transducer placed at the bottom of the bath. The electrochemical decontamination cassette (7) combines the metal strips as an anode and cathode with reverse (reversible) treatment and ensures uniform exposure to ultrasonic waves. The electrochemical decontamination cassette performs the function of electrodes, as it is an electrical conductor. The electrolyte function is performed by a weak acidic solution. The electric power of the ultrasonic transducer (16) allows the solution in the decontamination bath to be heated to the optimum temperature.

[0045] The metal strips (6) are simultaneously treated electrochemically and ultrasonically in a weak acid solution, the required treatment time is reduced due to the synergistic effect: a) overvoltage is eliminated due to better mixing of the water layer on the surface of the plates; b. better removal of electrolysis gases from the anode and cathode plates; c. removal of oxide film and deposits from the plates by cavitation due to ultrasound.

[0046] A high-power ultrasonic transducer (16) is placed on the bottom of the decontamination bath (9) without rigid mounting, allowing more effective use of ultrasonic energy. The ultrasonic transducer (16) due to its arrangement and shape allows it to act evenly on the plates throughout the entire volume of the decontamination bath (9). The adjustable frequency of the ultrasonic transducer allows for optimal selection of parameters depending on the material, thickness of the decontamination strip and type of contamination.

[0047] During electrochemical and ultrasonic treatment, undissolved oxide film particles are detached from the metal strip (6) surfaces and are deposited in the lower part of the decontamination bath (9), while acidic oxides, hydrogen and oxygen gases and aerosols due to the electrolysis process are removed from the water surface through the side openings of the decontamination bath (9).

[0048] Undissolved oxide film particles are collected using a circulating filtration system. The electrolyte is filtered through filter capsules (17). The used capsules are further stored as solid radioactive waste. After being cleaned of mechanical impurities, the acid solution is returned to the upper part of the decontamination bath (9) and, if necessary, the required concentration is restored.

[0049] A pulsed reverse (reversible) direct current source for electrochemical decontamination provides: a. regulation of current pulses and duration, which reduces electrode overvoltage and increases efficiency; b. switching the direction of current in the cassette allows alternating use of strips as anode and cathode; c. cleaning of the cathode from radioactive deposits (ALARA principle); d. reducing the volume of the decontamination bath and processing time, since decontaminate strips are used as the cathode instead of an additional inert cathode.

[0050] The cassette (7) is transferred to the third bath, which is a washing bath (10), which is equipped with means for supplying a high-pressure water jet. The high-pressure jet removes the remains of the acid solution and the oxide film from the metal strips without removing the metal strips (6) from the cassette (7) with minimal water consumption.

[0051] From the wash bath, the cassette is dried in a drying unit, which is a high-volume air flow generator that generates air flows towards the metal strips, drying the strips in the cassette.

[0052] The cassette (7) then is passed through drying means (12), which are high-volume air flow generators that generate air gusts towards the metal strips (6), drying them until the surface of the strips is completely dry.

[0053] The radiation level measurement stage involves measuring the radiation level on the surfaces of the metal strips (6) at a designated station (14), using industrial radiation measuring devices such as a Geiger sensor and a beta radiation counter of suitable sensitivity.

[0054] The advantage of this decontamination method is that the metal strips (6) remain fully straightened during the process, and full surface access of the metal strips (6) during measurement ensures efficient and reliable beta particle detection.

[0055] The decontamination process is repeated if the safety level is not achieved. If the measured value exceeds the required threshold, the following steps are repeated: removal of radioactive particles from the metal strips in the decontamination line and re-measurement of the radiation level.

[0056] Additionally, the decontamination method includes cleaning the decontamination solution of mechanical particles and purifying the air environment of the decontamination chamber from radioactive and chemical aerosols. Based on radiological characterization results, re-decontamination can be performed. Additional decontamination of specific metal strips is carried out in the second bath, then washed in the third bath and dried only for those strips that did not reach the safety level.

[0057] The proposed technology is energy-efficient due to the synergistic operation of the two methods simultaneously in a single decontamination reservoir. Simultaneous electrochemical and ultrasonic treatment significantly reduces decontamination time compared to separate processes.

[0058] By applying the invention described above, full decontamination of contaminated metal surfaces and complete removal of contaminated waste in a compact filter unit is achieved. Decontaminate metal can be reused after radiological inspection.

[0059] This integrated system enables the decontamination of piping materials made from stainless steel, carbon steel, copper-nickel, and zirconium alloys. Gases and aerosols released during decontamination are continuously removed from the solution surface via side ventilation openings and cleaned in a high-efficiency alkaline gas scrubber. This prevents the accumulation of hydrogen in dangerous concentrations.

[0060] The equipment enables the decontamination of pipes made from various materials using ultrasonic and / or electrochemical and / or pulse-reverse electrochemical methods in acidic decontamination solution media. The design and dimensions of the equipment, as well as the methods for preparing and processing the pipes, prevent the formation of radioactive dust and aerosol spread, comply with industrial safety requirements, and adhere to ALARA principles.

Claims

1. A method for decontamination internal and external surfaces of contaminated metal elements, comprising cutting the metal elements, flattening them, loading them into holding means, soaking, washing, drying the metal elements, and controlling their radioactivity level, characterized in that the method further comprises an electrochemical-ultrasonic treatment step, wherein the metal elements are decontaminated by immersing a cassette loaded with the metal elements into a prepared decontamination solution while simultaneously applying an electric current and ultrasonic waves of a required frequency.

2. The method according to claim 1, characterized in that the metal elements are flattened metal strips.

3. The method according to claim 1 or 2, characterized in that the electrochemical-ultrasonic treatment is carried out in a decontamination bath comprising an ultrasonic wave source and a cathode-anode power supply. <b>4. The method according to any one of claims 1-3, characterized in that the electrochemical-ultrasonic treatment is carried out using an electrochemical decontamination cassette with metal strips.

5. The method according to any one of claims 1-4, characterized in that the electrolyte is an acidic solution.

6. The method according to claim 5, characterized in that the acidic solution is a 0.5-1.5% nitric acid solution.

7. The method according to any one of claims 1-6, characterized in that the electrochemical decontamination cassette connects the metal strips as an anode and cathode with reverse (reversible) treatment.

6. The method according to any one of claims 1-5, characterized in that the metal elements are pipes or strips.

8. A system for decontamination internal and external surfaces of contaminated tubular elements, comprising a washing line, a cutting line, a flattening line, and holding means for accommodating the elements during the washing stage, characterized in that the system comprises an electrochemical decontamination line comprising metal strips, a cassette, a decontamination bath, an electrolyte, an ultrasonic wave source, and a cathode-anode power supply.

9. The system according to claim 8, characterized in that the electrolyte is an acidic solution.

10. The system according to claim 8 or 9, characterized in that the electrochemical decontamination line additionally includes liquid filling and discharge devices, auxiliary circulation means, filters, pumps and controllers allowing the second bath to be filled with the solution and ensuring process control.

11. The system according to any one of claims 8-10, characterized in that the decontamination bath is made of materials that do not react with acids.

12. The system according to any one of claims 8-11, characterized in that the cathode-anode power supply is installed to generate direct current pulses and provide periodic polarity reversal between the metal strips.

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