Electrochemical surface treatment

Abstract

A method and apparatus for electrochemically removing material from a surface in which two or more fluid jets or streams are arranged to impinge on the surface of an object, and an electric current is passed through one fluid flow path, through the object, and then through a second fluid flow path.

Description

Technical Field

[0001] The present invention relates to electrochemical surface treatment. Its main application is for surfaces contaminated by radionuclides.

Background Art

[0002] Decontamination of metal surfaces is a common problem in industry, including the nuclear power industry where metals are contaminated by contact with radionuclides. Contaminated metals may include mechanical parts such as ducts, pipes, glove boxes, storage containers, and stirring devices. When a medium containing radioactive chemical species comes into contact with a metal, some radioactivity remains on the surface that cannot be removed by simple rinsing or washing, because the radioactive elements have reacted with the surface or, alternatively, have penetrated slightly from the surface into the interior. Diffusion into the surface interior may occur directly from the surface of the metal into the interior and also along cracks propagating in the metal. As a result, radioactivity associated with the surface is present.

[0003] Therefore, it is desirable to remove radioactive contamination from the surface of an object. If such materials can be declassified, there are significant practical advantages in the decommissioning of nuclear power plants, as it allows handling of most of the contaminated materials while reducing the risk to workers and the need for storage of high-level radioactive waste.

[0004] Handling of contaminated materials is often a challenge as workers cannot approach the material or stay in its vicinity for long periods, as the degree of proximity affects the operator's allowable radiation exposure. Therefore, additional preventive measures, methods, and facilities are required to address this contamination, with the aim of removing the contamination, minimizing the risk to health, and recovering the decontaminated metal for reuse through conventional recycling processes.

[0005] A further problem is that surface contamination is not static and can change in response to surface treatment. In some instances, after removing the contaminated surface layer, it has been found that the contamination "sweats again", i.e., the surface radioactivity decreases after decontamination treatment and then increases. This is the result of chemical species diffusing from the subsurface layer to the newly formed surface. This indicates that it is necessary to be able to effectively control any decontamination process.

[0006] Conventional means of addressing this problem are the physical removal and disposal of the entire object. The obvious drawbacks of this method are the large amount of contaminated material that will be discarded or stored, and the complete lack of the possibility of recovery through recycling for any material intended for general use.

[0007] A second means is to use a smelter under operating conditions such that the radioactive contamination ends up in the slag, as described in U.S. Patent No. 5,268,128 (WESTINGHOUSE), December 7, 1993, "Method and apparatus for cleaning contaminated particulate material", and the slag is separated and stored indefinitely in combination with the treatment of radioactive metal waste using molten decontamination as described in U.S. Patent No. 2013 / 296629A (KEPCO NUCLEAR FUEL CO LIMITED), November 7, 2013, and most of the metal can be recovered and reused as a non-contaminated stream. This process is commercially operated. The disadvantage of this approach is that large-scale facilities are required, and the facilities themselves require extensive management measures.

[0008] Therefore, it is desirable to have a means of decontaminating materials so that most of the metallic objects can be recycled without further preventive measures. This can be applied in situ, for example, to containers so that the work of dismantling and decommissioning can be carried out while reducing risks, and can also be applied after dismantling for the purpose of recovering more materials for reuse.

[0009] One method is to chemically dissolve the contaminated metal layer, including oxide films and other deposited layers. The challenge is to completely dissolve this contaminated layer while ensuring that the uncontaminated base metal dissolves only in a limited and controllable amount. Acid treatments are used on mild steel, stainless steels including 304 stainless steel, and other materials. Nitric acid is widely used in the nuclear industry because of the high solubility of the target contaminants as nitrates and the good corrosion resistance of 304 stainless steel to nitric acid. The radioactive contaminants are recovered from nitric acid by standard means including precipitation and floc formation as used, for example, in the Enhanced Actinide Removal Plant (EARP) at Sellafield in the UK.

[0010] Other chemical treatments of metal surfaces are known in the metalworking industry, where heat treatment of the metal produces an oxide surface layer that must be removed before further processing steps can be carried out. Various chemical treatments are known, including the use of acetic acid (hence the term "pickling"), sulfuric acid, and other or additional agents such as hydrochloric acid for mild steel and hydrofluoric acid for stainless steel, or a hydrofluoric acid / nitric acid mixture. These treatments are not preferred for nuclear decontamination because they are incompatible with the stainless steel structures of downstream wastewater treatment plants.

[0011] The limitation when using nitric acid as a solvent is that the dissolution reaction is slow, so a relatively large-scale plant is required to process the large amounts of acid reagent needed. The reaction rate can be increased by adding complexing agents such as chlorides, fluorides, and organic complexing agents such as citric acid, oxalic acid, and ethylenediaminetetraacetic acid. These agents increase the reaction rate with surface contaminants, but in return, they produce a more corrosive liquid that cannot be processed in conventional nuclear wastewater treatment plants, and this liquid is corrosive to the metals used in the structures of those plants.

[0012] Another method of surface decontamination is described in U.S. Patent No. 7,384,529 B (US ENERGY) on June 10, 2008, "Method for electrochemical decontamination of radioactive metal", in which a conductive electrolyte bath is used to pass an electric current through the contaminated article. The electrochemical descaling method (or "electrochemical pickling") is widely used in metal processing. This method has a significant advantage over chemical methods in that the surface removal rate is much higher. As a practical result, electrochemical treatment requires much less acid reagent than chemical treatment. A further advantage is that the electrochemical process can be easily controlled because the electrochemical process responds immediately to the level of current passing through, and this current is determined by the applied potential. However, the electrochemical process has a major drawback in that it is only effective in geometries where the counter electrode can be placed close to the workpiece. The reason for this is that in a liquid-immersed system, the current can flow through the electrolyte over the entire surface in contact with the fluid, and the electrochemical effect will either spread quickly over a uniform surface away from the counter electrode or concentrate at the points giving the lowest resistance path.

[0013] U.S. Patent No. 2003075456A (COLLINS ET AL) on April 24, 2003 shows that when using an AC waveform with a DC bias, a wide range of metals coated with an oxide film can be descaled more rapidly than when using an AC waveform without a DC bias. It has also been shown that it is advantageous to periodically reverse the polarity of the DC bias. The removal or purification of the oxide film on the surface of the metal has been shown to be faster when applying a DC bias to the AC waveform compared to using only the AC current. The purification mechanism involves some dissolution of the contaminated layer, some undercutting where the underlying metal dissolves, and some scrubbing action due to the generation of bubbles at the interface.

[0014] AC with a DC bias can accelerate the destruction of the oxide film because in the potential range where dissolution occurs, only the DC current causes surface passivation or either oxygen evolution or pitting, while only the AC current reduces the dissolution effect. It has been found that an AC current with a DC bias can obtain optimal dissolution while minimizing local pitting.

[0015] Electrochemistry has been used with non-metals. The results of Bradley et al (U.S. Patent No. 3075902) show that a steady jet in contact with the surface can be used for local measurements of the thickness and thinning of semiconductors. From this disclosure, it is taught that measuring the thickness of the remaining material is a basic feature in the process used to etch a material of a specific thickness. The measuring process is realized by monitoring the electrical resistance between two electrodes through the thickness of the semiconductor wafer. This measuring method is not suitable for conductive metals because in this case, the resistance of the electrolyte is one order of magnitude greater than that of the metallic conductor, making it impossible to determine the thickness through the change in resistance.

[0016] The electrochemical treatment of surfaces using a jet or stream of electrolyte that impinges on an object and conveys an electric current to that object is known. This method has applications in electroplating, electroforming, electrolytic etching, electrochemical machining, cutting, and electrolytic polishing.

[0017] When used in electrochemical machining, the use of a collimated jet of electrolyte enables the machining of shapes that are difficult to manufacture by other methods. Different configurations of the apparatus are known, including different nozzle structures, different means of introducing an electric current into the flowing liquid stream, different mechanical arrangements suitable for the relative support and movement of the head with respect to the object or vice versa, different means of collecting, recycling, and filtering the electrolyte, and the use of different electrode polarities and waveforms. What is common to these applications is the use of a single stream of electrolyte to the object and a current path that flows from the power supply through the electrolyte to the object and returns from the object through the wiring system to the power supply.

[0018] The use of a collimated jet for the purpose of electrochemical surface decontamination is advantageous because it eliminates the need to immerse the surface to be treated in the electrolyte, enhances the degree of spatial controllability of the area being treated, and provides an effective means of rapidly removing heat from the electrolyte and gas from the object. When treating the inner surface of a large container, this means that there is no need to immerse the container in a liquid and a smaller amount of electrolyte can be used. Such use of a collimated jet is an advanced form of electrochemical surface decontamination in which the object and the counter electrode are immersed in an electrolyte bath and a current is passed between them.

[0019] Systems using a collimated jet for the purpose of etching metallic surfaces have applications in various fields, such as for the purpose of removing radioactive contamination from the surface layer of a nuclear power plant. In such a system, the object is the surface to be decontaminated and constitutes one electrode, and the second electrode is in contact with either the stream of electrolyte in a nozzle or tube structure or, alternatively, the stream of electrolyte before it exits the nozzle.

[0020] The disadvantage of this method is that the surface being processed forms part of an electrical circuit and a good electrical connection needs to be formed to the object to close the circuit. If the connection is insufficient, the effectiveness of the process is impaired. When scanning or moving an electrolyte jet along part of a contaminated container or pipe, for example, in situations where the area is difficult to reach and a remotely operated device has to be used, it is advantageous that this electrical connection does not need to be formed.

Summary of the Invention

Means for Solving the Problems

[0021] According to the present invention for electrochemically removing material from the surface of a conductive metallic object, two or more fluid jets or laminar flows are arranged to impinge on the surface, and an electric current flows through one fluid flow path in at least one jet or flow to the metallic object, through the object material, and away from the metallic object through a second fluid flow path in at least one second jet or laminar flow.

[0022] This arrangement has an advantage over previous systems in that it is not necessary to make a direct electrical connection to the object. The liquid jet device may be brought close to the object, and the treatment can be carried out without any need for an electrical connection.

[0023] In this new arrangement, the electrochemical removal of the material occurs through an anodic process at one or more points where one or more jets impinge, and the cathodic process occurs at points where one or more additional electrolyte jets impinge. The polarity of the power supply device with respect to the object may be selectively changed periodically so that the electrochemical effects occurring at the collision points of two or more jets alternate between anode and cathode. The waveforms of the applied current and voltage may be advantageously adjusted to optimize the electrochemical effects, such as by using an AC waveform with a DC bias or any other suitable waveform. Alternating currents of different frequencies may also be used.

[0024] The current density to be used at the surface of the object will depend on various factors and may be in the range from 0.1 ampere per square centimeter to over 100 amperes per square centimeter. An intermediate current density may be preferred that can achieve a suitable balance between maximizing the processing speed and minimizing heat in the electrolyte and gas generation at the surface of the object.

[0025] Two or more jets may be used. If the number of flow paths is more than two, one or more flow paths will be designated as flow paths for current flowing in one direction, and the remaining flow paths will be designated as flow paths for current flowing in the opposite direction.

[0026] The transformer is of an insulated type, and in the general way, no part of the secondary circuit is grounded. Its arrangement eliminates any electrical connection to the object other than through the liquid jet, and furthermore, there is no possible current path from the object other than through the fluid jet, thus meaning that there is no possibility of unwanted electrochemical effects occurring at locations remote from the intended working area.

[0027] The current density to be used at the surface of the object will depend on various factors and may be in the range from 0.1 ampere per square centimeter to exceeding 100 amperes per square centimeter. An intermediate current density may be preferred that can achieve a suitable balance between maximizing the processing speed and minimizing heat generation in the electrolyte and gas at the surface of the object.

[0028] In another aspect of the present invention, an apparatus for electrochemically removing a material containing a radionuclide from the surface of a conductive metallic object includes two or more coherent fluid jets or laminar flows of an electrolyte arranged to impinge on the surface of the metallic conductive object, the fluid jets or laminar flows including at least one fluid jet or laminar flow that provides a current path through the at least one fluid jet or laminar flow and through the object, and through the path of at least one other fluid jet or flow, and further includes a conduit of a flexible and electrically insulating material that supplies an electrolyte stream to an outlet proximate the surface, and an electrode within the conduit or outlet that introduces a current into the electrolyte.

Brief Description of the Drawings

[0029]

Figure 1

Figure 2

Figure 3

Mode for Carrying Out the Invention

[0030] In FIG. 1, two coherent fluid jets or laminar flows 1A and 1B of an electrolyte (more than two may be provided, but additional jets are omitted for simplicity) are arranged to impinge on different points 1C on the contaminated surface 10 of an object 11 containing a conductive material. The jets issue from an outlet 7 of a duct 2 connected to a source of electrolyte through a tube 3, and the electrolyte is pumped through the tube at a pressure sufficient to maintain a suitable fluid flow. In this figure, the outlet 7 includes a nozzle, but may also be an orifice plate or slot. In one implementation, the outlet was a circular orifice plate having a radius between 5 mm and 100 mm. In another implementation, the coherent fluid jets were formed by an annular slot or a partially annular slot at each outlet 7.

[0031] The insulated transformer 4 is arranged to supply power using a current path that passes from the secondary winding 5 of the transformer through a fluid flow path of one jet stream 1A, through the object 11, through a second jet stream 1B, and returns to the secondary winding 5.

[0032] As a result of no part of the secondary circuit passing through the winding 5 being grounded, the transformer 4 is of the insulated type. This is in contrast to a common existing practical example where one side of the winding 5 of the transformer may be connected to the object 11. In contrast, in the present invention, the electrical connection to the object 11 is only through the jet streams 1A and 1B passing through the surface 10. As a result, there is no possible current path from the object 11 other than through the jet streams 1A and 1B, and there is no possibility of unwanted electrochemical effects occurring at locations away from the intended working area.

[0033] The use of the impinging jet streams 1A and 1B for the purpose of electrochemically removing material from a metallic surface has applications in various fields, such as for the purpose of removing radioactive contamination from the surface layer of a nuclear power plant.

[0034] By using the sponge 8 or other porous media, the flow of the fluid stream in the vicinity of the impingement point can be controlled to reduce unwanted splashing and distribution of the liquid away from the working area of the surface 10, and the flow can also be localized in the vicinity of the working area.

[0035] By arranging the flexible sealant 9 around part or all of the working area of the surface 10, the droplets can be at least partially contained, and it can also assist in guiding and collecting the electrolyte after it impinges on the surface 10.

[0036] The flow around the mechanism may be localized using formed openings such as annular or partially annular slots.

[0037] The electrolyte impinging on the surface 10 of the object 11 will flow to a suitable collection point 12 at the lowest point of the inner container or otherwise along the pipe.

[0038] The current may be introduced into the jets 1A and 1B by various means, for example, by the conductive nozzle 7, or by the electrodes in the duct 2, or by the electrodes in the electrolyte jet once the electrolyte jet exits the nozzle. In any case, the electrodes are connected to each side of the secondary coil 5 by the wiring 6. The material for applying the current to the electrolyte flow is preferably a material that is not consumed by the electrochemical process whether it is part of the nozzle or the pipe structure or outside the nozzle or the pipe structure.

[0039] The duct 2 and the pipes 3 that carry the electrolyte to the outlet 7 are made of an insulating material such as plastic. The pipes 3 are of sufficient length such that the electrical resistance of the flow path from one liquid stream to the other along the piping is much greater than the electrical resistance through the flow path of the jet and through the workpiece.

[0040] The duct 2 and the outlet 7 may be made of a flexible and electrically insulating material such as rubber so that no problem occurs even if the duct or the nozzle contacts the object. In this example, the electrodes for applying the current to the electrolyte flow must be inside the duct 2.

[0041] In the described embodiment, the material is typically removed from the metallic surface between 0.001 millimeter and 10 millimeters, including both end values. This can be done in a single pass or in more than one pass.

[0042] Typically, the time-averaged velocity of the impact point of the jet on the surface is between 0.01 times and 10 times the diameter of the fluid path per second, including both end values.

[0043] The applied waveform is an AC waveform with a DC bias, and its frequency was between 5 Hz and 2000 Hz, including both end values. The preferred current density in the fluid jet is between 100 amperes and 100,000 amperes per square meter, including both end values.

[0044] The transformer 4 is of the insulated type, and in the common way, no part of the secondary circuit is grounded. Its arrangement eliminates any electrical connection to the object 11 other than through a liquid jet, and furthermore, there is no possible current path from the object 11 other than through the fluid jet, meaning that there is no possibility of undesirable electrochemical effects occurring at locations remote from the intended working area.

[0045] The distance between the points where a plurality of jets or laminar flows impinge on the surface 10 of the object 11 will be arranged such that the electrical resistance of the object 11 between those points provides a surface treatment rate that meets the operating requirements. Typically, this means that the electrical resistance of the object 11 between the points where the electrolyte jet or flow impinges is less than the electrical resistance of the electrolyte liquid present at the surface of the metal between those two points, although this does not necessarily have to always be the case. This may be the case, for example, in the treatment of particularly inaccessible locations, where the geometric convenience of accessibility is prioritized at the expense of the electrical efficiency of the process, and additional power loss due to conduction along the electrolyte on the surface 10 of the object is tolerated.

[0046] The spatial direction of the electrolyte flows 1A, 1B and the impinging points 1C may be optimized to suit the geometry of the object being treated. Arrayed jets or spreading jets may be used. For treating the outside of a tube, the jets may be arranged, for example, in an annular or partially annular shape. For treating the inside of a container, the jets may be arranged, for example, in a shape that spreads radially in a number of directions.

[0047] The movement of the point 1C where the jets or laminar flows 1A, 1B impinge on the surface 10 of the object 11 may be pre-programmed so as to obtain a predetermined profile of the surface treatment. The program for controlling the movement may take into account both the contamination level, or the depth of surface removal required, and the geometric shape effect of the application on the intensity of the treatment. The automatically controlled movement of the electrolyte jets or laminar flows 1A and 1B may be controlled by the measurement results of the properties of the surface being treated, which are made either in an investigation before the treatment is carried out or in real time, for example, the level of radioactivity present, or its light reflectivity, or other suitable measurable properties.

[0048] The movement of the multiple jets or laminar flows 1A and 1B on the surface to be treated may be controlled so as to achieve a predetermined level of surface treatment or removal. This level may be determined by a prior radiological assessment of the substrate in question.

[0049] High-pressure electrolyte jets or laminar flows or jets may advantageously be used to provide a mechanical surface treatment effect in addition to the described electrochemical effect, which substantially constitutes the effect of pressure washing. This is useful in the case of surface contaminants such as oil or grease or particulate matter or paint or other materials that need to be removed. Solid particulate matter may optionally be dispersed in the liquid electrolyte to obtain an additional abrasive cleaning effect.

[0050] The generation of a continuous and coherent fluid path from the electrodes housed in the electrode housing (in the exemplary embodiment, the housing is the duct 2) to the surface 10 of the object 11 is essential for the operation of the system. If the jet or flow is disrupted or droplets are formed between the outlet 7 and the surface 10 being treated, the resistance of the fluid path will change significantly. The generation of a coherent flow is achieved by using flow regulation within the electrode housing (duct 2) to reduce the velocity fluctuations originating from the conduit. This is preferably accomplished by reducing the flow velocity by expanding the cross-section of the duct 2 prior to the outlet 7, which is an orifice plate, nozzle, or slot that preferably generates a fluid jet with low velocity fluctuations, and by controlling the pressure drop and straightening the flow. The coherent jet preferably has a continuous path longer than 1 meter in free space, but due to practical voltage limitations, the preferred operating distance is limited to 0.5 meters or less.

[0051] The electrodes are preferably made of a stable material with good conductivity. Suitable ones are carbon-based conductors, metallic conductors, or metallic conductors coated with a surface coating. Preferably, the metallic conductor or metallic surface coating contains a metal selected from the group including platinum, gold, stainless steel, chromium, nickel, tantalum, osmium, iridium, palladium.

[0052] Ideally, the exposed surface area of the electrodes in contact with the fluid is 5% or more of the cross-sectional area of the fluid jet. The electrodes are in contact with the electrolyte before the fluid exits the outlet 7 and are preferably sized to reduce the localized current density within the housing (duct 2). Practical arrangements of the electrodes include a perforated surface or mesh attached to an electrically insulating housing (which is also the duct 2 in the exemplary embodiment). Electrodes that form the edge portion of the flow path, such as rings, within the electrically insulating housing. Electrodes inserted into the flow path, such as tubes or rods, within the electrically insulating housing. Electrode orifice plates at or near the jet outlet that also defines the flow. Combinations of electrodes and flow regulating devices made of materials that also function as electrodes.

[0053] The electrically insulating housing can include fluid flow regulating means such as a number of parallel tubes or perforated plates. The parallel tubes and / or perforated plates within the electrically insulating housing can also form part or all of the electrodes within the housing.

[0054] The electrodes are in contact with the electrolyte before the fluid exits from outlet 7 and are preferably sized to reduce the localized current density within the housing (duct 2). The electrodes can be formed as rings or cylinders located within the electrode housing (duct 2), or preferably as a mesh or perforated plate, or the outlet 7 can also be the electrode.

[0055] The described method and apparatus are applicable for treating a wide range of geometries of contaminated surfaces. This includes the inside and outside of pipes, the inside and outside of containers, various types of structures such as valves, pipe manifolds, support structures, individual components, or any surface requiring surface treatment. This method is suitable for treating local hot spots of contamination.

[0056] Figure 2 illustrates the treatment of a cylindrical container 21 using two electrolyte jets 22 directed towards the inner surface of the container 21. The two electrolyte jets 22 are shown as arrows and have a diverging path. The nozzles are shown as 23 (which in this embodiment perform the function of the outlet 7 in Figure 1), and the supply of electrolyte to the nozzles is shown as 24 (which performs the function of the duct 2 in Figure 1).

[0057] For the purpose of treating surfaces of components larger than the cross-section, the point of impact 1C of the fluid jets on the surface is moved. The speed of movement is proportional to the size of the region of jet impact, the applied current density, and indirectly proportional to the depth of the material to be removed. The movement can potentially be continuous, stepwise, or in a raster pattern depending on the surface features and the control method employed.

[0058] In addition, although two jets or laminar flows 1A and 1B, 22 are shown in FIGS. 1 and 2 respectively, a large number of jets or laminar flows can also be used.

[0059] In FIG. 1, the jets or laminar flows 1A and 1B are shown as being parallel, but the inventors have found that when the diverging jets as shown in FIG. 2 are directed towards a concave surface such as the inside of a cylindrical container and a duct, the surface is accelerated. Similarly, the converging jets can accelerate the purification of the convex surface such as the outside of the cylindrical container.

[0060] The best flow velocity of the fluid forming the coherent jets or laminar flows has been found to be between 0.15 m / s and 50 m / s.

[0061] The electrolyte is preferably a conductive fluid, and as the conductivity increases, the required voltage decreases. The electrical resistivity of the fluid is desirably less than 1 Ω meter, preferably less than 0.2 Ω meter.

[0062] In order to minimize the amount of liquid required to treat a given area and prevent precipitation of the removed metal, it is preferable that the dissolved metal has some solubility in the electrolyte.

[0063] It is preferable that the electrolyte does not dissolve the substrate chemically to a large extent or cause significant local damage, and that corrosion is minimized throughout the electrochemical treatment. For stainless steel and most nickel alloys, nitric acid is a preferred option because the chemical corrosion rate is not high and this acid is suitably conductive.

[0064] For the treatment of radioactive contamination derived from the nuclear industry, it is advantageous to use nitric acid or nitrates because many radioactive nucleotides are soluble and often compatible with known waste treatment routes.

[0065] FIG. 3 illustrates one arrangement for the outlet 7 (in FIG. 1).

[0066] In FIG. 3, the fluid enters an electrically insulating housing 32 (such as the duct 2 in FIG. 1) through a conduit 31 (such as the pipe 3 in FIG. 1). There are arrayed flow adjustment tubes 33 facing the exit surface of the electrically insulating housing. Then the electrolyte fluid passes through the porous plate electrode 34. This is connected to a power supply device through an electrical connection 35. Then the fluid exits through the holes of an orifice plate 37 after passing through a second electrically insulating housing 36. The orifice plate also has an optional connection 38 to the power supply device.

Example

[0067] Usage example using the exemplary embodiment Example 1 A sample of 304 stainless steel sheet was processed using two jets arranged 50 mm away from the sheet sample surface and 90 mm apart. Nitric acid (30% w / w) was pumped through each nozzle with a diameter of 25 mm to form jets at a speed of 3200 liters / hour. When the electrodes in each nozzle were energized at 160 V by an insulated power supply device, a current of 25 A flowed through the current passing through the stainless steel object and the circuit formed by the two jets, without any direct contact from the insulated power supply device to the sample, and also when the stainless steel sample was grounded, no current flowed to the ground. The sample was processed for 15 minutes, during which 3.2 g of mass was lost from the contact area of the two jets / sample. This corresponds to a current efficiency of 50% for metal dissolution.

[0068] Example 2 A sample of 304 stainless steel sheet was treated using two jets placed 50 mm away from the surface of the sheet sample. Nitric acid (30% w / w) was pumped from each nozzle with a diameter of 25 mm to form jets at a speed of 3200 liters per hour. When each nozzle's electrode was energized at 160 V by an insulated power supply, a current of 25 A flowed through the current passing through the stainless steel object and the circuit formed by the two jets, without any direct contact from the insulated power supply to the sample, and also when the stainless steel sample was grounded, no current flowed to the ground. One of the jets traversed the surface horizontally at a constant speed, removing 20 microns of material from the surface along the path where the jet crossed the surface.

Claims

1. Two coherent fluid jets or laminar flows of an electrolyte arranged to impinge on a surface, including a fluid jet or laminar flow through which a current path passes through one of said fluid jets or laminar flows and a conductive metallic object, and through a second of said fluid jets or laminar flows, further comprising a flexible and electrically insulating material conduit for supplying said electrolyte flow to an outlet proximate to said surface, and an electrode within said conduit or outlet for introducing a current into said electrolyte, in an apparatus for electrochemically removing a material containing a radionuclide from the surface of said conductive metallic object, wherein said two jets or laminar flows are formed by said outlet spaced from said surface during use, and each of said two jets or laminar flows moves through free space before impinging on said surface at different impact points. Apparatus.

2. The apparatus according to claim 1, wherein at least one electrode is constituted by a perforated surface or a mesh.

3. The apparatus according to claim 1 or 2, wherein said electrode is supported within an electrically insulating housing, said insulating housing accommodating means for adjusting fluid flow, said means for adjusting fluid flow including a plurality of parallel tubes or perforated plates.

4. The apparatus according to claim 3, wherein said means for adjusting fluid flow forms part or all of said electrode.

5. The apparatus according to any one of claims 1 to 4, wherein one or more of said coherent fluid jets are formed by an orifice plate, or an annular slot or a partially annular slot.

6. The apparatus according to any one of claims 1 to 5, wherein the current density in said fluid jet is between 100 amperes and 100,000 amperes per square meter.

7. The method according to any one of claims 1 to 6, wherein the velocity of the fluid forming said coherent jet is between 0.15 m / s and 50 m / s.

8. A method for electrochemically removing a material containing a radionuclide from the surface of a conductive metallic object, wherein two coherent fluid jets or laminar flows of an electrolyte are arranged to impinge on the surface, and a current flows through one fluid flow path in one jet or flow to said conductive metallic object, through said conductive metallic object, and through a second fluid flow path in a second jet or laminar flow away from said conductive metallic object. Method. The two jets or laminar flows are, in use, formed by an outlet spaced from the surface, and each of the two jets or laminar flows moves through free space before impinging on the surface at different impingement points. A method characterized by this.

9. The method according to claim 8, wherein the treatment of a surface larger than the cross-sectional area of the fluid flow path is achieved by moving the impingement points of one or more of the jets over the surface.

10. The method according to claim 9, wherein the average moving speed of the impingement points on the surface is between 0.01 times and 10 times the diameter of the fluid flow path per second, including the values at both ends.

11. The method according to claim 9 or 10, wherein the movement of the impingement points of the electrolyte flow is determined in real time by measuring the characteristics of the surface being treated and can be determined by measuring the existing radiation level.

12. The method according to any one of claims 8 to 11, characterized in that the electrical resistivity of the electrolyte is less than 1 Ω·m and may be less than 0.2 Ω·m.

13. The method according to any one of claims 8 to 12, wherein the electrolyte is formed from an aqueous solution containing nitric acid.

14. The method according to any one of claims 8 to 13, wherein the electrical resistance of the conductive metallic object between the impingement points of the electrolyte jets or flows is less than the electrical resistance of the electrolyte liquid present on the metal surface between those two points.

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