JP2007181830A - Process and apparatus for cleaning or decontaminating object by ultraviolet laser beam - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a process and an apparatus for implementing surly and safely cleaning or decontaminating a plastic or metal surface without resoiling the surface after the treatment. <P>SOLUTION: Exfoliation is caused on an object 20 to be treated with the impact of an ultraviolet laser beam 22 emitted by a laser 21 by scanning the laser beam 22 with a mechanical or optical means 30, 31, 32, 41, 51 and 70 so that the surface of a material itself composing the object 20 is removed. It is favorable for a stainless steel that the process is performed in an atmosphere of a reducing gas, particularly an inert fluorinated gas. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a method for cleaning or decontaminating the surface of an object using an ultraviolet laser beam. It also relates to an apparatus for carrying out the above method.

  The use of laser beams to clean or decontaminate object surfaces has been described in many publications. The following documents can be cited from these publications:

WO 90/079888 discloses the use of a laser beam to clean the surface and guides this laser beam manually during the cleaning operation. The output of the laser beam is on the order of several tens of mW / cm ² at a wavelength on the order of microns. The technique disclosed in this document has drawbacks regarding repeatability, lack of accuracy, complexity, and inefficiencies for certain materials.

  FR-A-2525380 describes a decontamination method using a light beam emitted by a YAG laser at a wavelength of 1.06 μm. This method was developed to remove a thin oxide film from the surface of a metal object contaminated by radioactive elements. However, this method cannot be used to treat the substrate of the oxide layer, i.e. the pure metal part of the object. Moreover, the effect of the laser beam is inherently thermal and is therefore unsuitable for decontamination of plastic objects. In fact, this laser beam causes the surface melting of the plastic object, which has the effect of finally putting contaminants into the plastic material. This is the opposite effect that is required.

  In FR-A-27000882, decontamination of a surface contaminated with radioactive elements is contingent on maintaining the liquid on the surface to be treated. Using this liquid has many disadvantages, such as removal of the liquid after the end of the process.

  FR-A-2707877 proposes to use an ultraviolet laser beam and a reactive gas such as oxygen. Experiments have shown that the presence of oxygen during the decontamination operation leads to immediate re-formation of the oxide layer on the treated metal. These layers re-contain the contaminants, which is contrary to the desired result. This document also mentions the trapping area of particles released after removal from the treated surface. This capture area is between 2 and 10 mm from the treated surface. This short distance is a disadvantage for carrying out this method.

  WO 95/13618 describes a method of decontamination by using a laser beam to locally melt the surface of a metal object to be treated and removing this molten metal. The disadvantage of this method is that it needs to work in contact with the surface to be decontaminated. Furthermore, melting the metal is a major drawback because it damages the surface condition, creates slag, and recloses nearby contaminants.

  Thus, the above method using the prior art has a considerable number of drawbacks.

  They typically use laser beams with wavelengths in the infrared or visible portion of the spectrum. They therefore cause surface melting of the metallic material, which leads to serious drawbacks.

  When such techniques are performed manually, the scan of the surface to be treated is incomplete. This will leave a small unprocessed area or it will be necessary to carry out repeated treatment steps. Another significant drawback when related to radioactive decontamination is the risk of staff being exposed to ionizing radiation.

  One of the described procedures involves the use of a laser beam in combination with a spray of soot liquid having reinforcing properties. Although aqueous, the liquid in question is difficult to apply, remove and handle. Moreover, experiments conducted by the inventors of the present invention have shown that microcracks can be formed under each residual droplet. These microcracks make the surface porous and are very susceptible to recontamination.

  Plasma heat generated by laser beam bombardment or surface delamination caused by laser bombardment, in combination with the use of a reactive gas such as oxygen, immediately causes very significant surface reoxidation. Such reoxidation can cause recontamination.

  The present invention is intended to overcome all the disadvantages of the prior art. The inventor has made a considerable number of investigations that have unexpectedly led to methods for remote decontamination or cleaning of plastic or metal surfaces. This method can be easily automated and, depending on the circumstances, a gas may be added which favors the final state of the surface to be treated, for example to polish this surface without reoxidation.

  Accordingly, the present invention provides a method for cleaning or decontaminating the surface of a metal object using the impact of an ultraviolet laser beam, and depending on the material comprising the metal object, the ultraviolet laser beam is peeled off the metal object. And a method characterized by using an ultraviolet laser beam in conditions selected such that this delamination involves the removal of the surface of the material that constitutes the object itself.

  If an oxide layer is formed on the surface of the metal object, the working conditions cause the oxide to evaporate and generate a plasma that removes the surface of the material that constitutes the object itself. It has become.

  Depending on the circumstances, these working conditions occur in an inert gas atmosphere, for example an argon atmosphere, in which the impact of the laser beam on the metal object is inert to the material comprising the metal object. It may be advantageous to include that.

  It may also be advantageous for this laser beam to impact the object in the reduced state. These reduced states may be obtained by the presence of at least one suitable additive for this inert gas atmosphere.

  The present invention relates to an apparatus for cleaning or decontaminating the inner surface of a container by using the impact of an ultraviolet laser beam, and depending on the material constituting the object, the object is peeled off. A laser that emits the ultraviolet laser beam under conditions selected to include removal of the surface of the material comprising the object itself, to transmit the laser beam to the inner surface and to direct the laser beam to a region on the inner surface of the container Means for removing material resulting from delamination of the inner surface, and these mirrors are arranged to scan the entire surface to be cleaned or decontaminated under the action of the control device It also relates to a device characterized in that

  Other features and advantages of the present invention will be better understood from the following description. This description is non-limiting and refers to the accompanying drawings.

  The present invention uses ultraviolet light, ie a laser beam with a wavelength between 10 nm and 400 nm.

  The experimental apparatus shown in FIG. 1 uses an XeCl laser 1 that emits a beam of light having a wavelength of 308 nm with a pulse of 28 ns. This type of laser is one of the most powerful currently available, namely a 250 Hz Lambda Physik 400 mJ.

  The apparatus of FIG. 1 uses a direct image transfer technique to direct the laser beam. The laser beam 2 is first reflected by the mirror 3 and then passed through the first focusing lens 4; then sequentially reflected by the mirrors 5 and 6 and then through the second focusing lens 7 (focal length 0.5 m), the protective container 9 To reach the target 8 placed inside. The video camera 10 is used to monitor the impact of the laser beam 2 on the target 8.

  Using this apparatus, the output of the laser beam measured with the target is 300 mJ per pulse. The mirrors 5 and 6 can rotate to direct the laser beam 2 to the entire surface of the target.

  Next, the processing of three different materials using the method of the present invention will be described. Using these examples, those skilled in the art who would like to apply the present invention to other materials would only have to test using available techniques without the need to demonstrate special inventiveness. These tests will give information about the laser beam power rating required on the surface of the target and the optimum conditions needed to carry out the invention for any given material.

Example I
The first example concerns the treatment of a plastic coating, in this example an epoxy paint.

  The force of a single ultraviolet photon is sufficient to excite the electronic level near or greater than the dissociation limit of the organic bond. This process is photochemical and has a poor thermal effect.

At 0.5 J / cm ² the coating does not peel; this value is therefore the peel limit. 0.7 J / cm ² in out effectiveness peeling, leading to maximum efficiency in the 1.8 J / cm ^2. With a repetition rate of 250 Hz, a peel rate of 0.5 m ² / h can be achieved for a depth of 30 μm. It should also be noted that fluctuations in the incident angle of the laser beam up to 45 ° have little effect on the stripping efficiency.

  For plastic materials, the maximum power rating of the laser beam is the one above which the plastic begins to melt or burn.

Example II
In the second example, the target is a lump of copper with a naturally oxidized surface.

Since this oxide layer absorbs ultraviolet rays very well, a sufficiently strong laser beam ( ² J / cm ² ) vaporizes this copper oxide layer and forms a plasma. Plasma light emission analysis can be used to determine the elements that make up the material from which it was made.

  Curve 11 in the graph of FIG. 2 shows the first two emission spectra of the laser. The three peaks at 510.55 nm, 515.32 nm and 521.82 nm in this curve actually correspond to the release of copper atoms from the positively debonded region.

  Starting with the third laser, the emission spectrum (curve 12 in the graph of FIG. 2) shows that the plasma is no longer created, so copper is no longer emitted and the metal absorption coefficient is low. In order to reoccur plasma, a considerably high laser beam is required.

  The process according to the invention is therefore very efficient and highly selective, since only two pulses are required to strip the copper oxide mass and leave it only on the substrate.

  I noticed that an acoustic signal was emitted during plasma formation. Thus, acoustic signals may be recorded for the first two shots while the laser beam and oxidized surface interact. The third laser emits a very low acoustic signal.

Example III
In a third example, the target is an ingot of Fe / Cr / Ni stainless steel consisting of 72% iron, 18% chrome and 10% nickel by weight. By aging, an oxide layer having a thickness of several tens of microns is formed on the steel. In order to decontaminate this type of target, it has been found that it is essential to first destroy the oxide layer formed during aging.

The oxide layer is easily vaporized while a ² J / cm ² luminous flux emits a very bright plasma having a length of about 8 mm. This sudden expansion of the plasma simultaneously makes a burning sound.

Several shots are sufficient to completely remove this oxide layer, indicating that this process is very efficient in this respect. Thereafter, even if the peeled and glossy metal substrate is irradiated at ³ J / cm ² , no sound of plasma or burn is generated. However, decontamination as part of the dismantling of a nuclear facility imposes a high decontamination factor that cannot be achieved without removing material from the metal substrate itself. At the same time, if care is taken not to melt the metal surface and reoxidize the treated surface, both processes will contain the contaminants. This discussion leads to an accurate definition of the good working conditions required for stainless steel.

  If the laser irradiation of the surface of the steel ingot is actually continued in an air atmosphere and the oxide layer is removed, the surface treated with short-term irradiation will become blackened, and long-term irradiation will occur. As well as increasing the blackening, it will also be seen that the appearance of the brick oxide structure due to the different thermal stresses between the metal and its oxide. This new oxide layer is undesirable because it has the effect of absorbing laser photons and shielding subsequent delamination; it also tends to confine contaminating particles, lower the decontamination factor, and thereby degrade the surface condition.

  A series of experiments were conducted to overcome these phenomena. These experiments used multiple samples of this steel and surface analysis was performed using XPS photoelectron spectroscopy and ion bombardment. This analysis method allows the sample to be rubbed at a rate of several nm / min (2 nm / min in this experiment) so that the deep physicochemical structure of the treated sample can be identified.

  Starting with an untreated sample (ie, one that retains its initial oxide layer), several samples were examined using these techniques, following samples treated at various conditions.

  The untreated sample is structured as follows: At a depth of 0.13 μm, iron and chromium are present mainly as oxides, while nickel is in its metallic state. The composition of this sample remains constant beyond the depth of 0.13 μm and iron oxide is present. The Fe, Cr, and Ni relative concentrations measured at a depth of 0.48 μm are 70.9%, 21.9%, and 7.2% by weight, respectively. Considering the measurement accuracy of the technique used, these values are in good agreement with the certified composition of this sample, ie 72%, 18% and 10% by weight.

  The sample treated in air as explained above is structured as follows: apart from a small amount of metallic nickel up to a depth of 1.1 μm, this sample is almost entirely oxygen and metal oxide. Consists of. At a depth of 1.3 μm, the ratio of iron oxide to metallic iron is still about 50%. Thus, there is oxide and oxygen modification.

  The composition of the sample treated in argon is roughly correct to a depth of 0.4 μm, ie 70% iron, 20% chromium and 11% nickel by weight. Between the surface of this sample and this depth, oxygen remains thinner for irradiation in air.

  Samples treated in argon in the presence of alcohol show a very clean surface because the correct base composition is obtained after a few minutes of analysis and the oxygen concentration is very low.

Therefore, it is very advantageous to carry out the stripping according to the invention in an inert gas, of which argon is the most convenient. Nitrogen should cause nitridation and should be avoided. Since it is difficult to eliminate oxygen completely under current decontamination conditions, it is preferred to work in the reduced state achieved by using additives. For example, a jet of pure argon can be replaced with a very small amount of hydrogen (less than 4% by volume) or argon containing hydrocarbon compounds such as ethanol, low molecular weight sugars or polyalcohols. In these situations, the risk of surface recarbonization is reduced by the presence of carbon monoxide generated in the field, for example:
CH ₃ —CH ₂ —OH → CO + CH ₄ + H ₂

  By using such an additive, a clean polished surface can be formed after irradiation with a laser beam. A dilute aerosol confined to a small, very effective filter changes in color from light brown to very deep black.

Interesting results have also been obtained by using fluoride additives such as sulfur hexafluoride or freon that do not pose a hazard to the operator. By replacing oxygen with a stronger oxidant such as fluorine, volatile metal compounds may also be formed in the plasma. It has been found that in the presence of SF ₆ , delamination is kinetically faster.

  These advantageous conditions should be applicable to a large number of potential contaminants, such as actinides.

Decontamination Device FIG. 3 shows a decontamination device. Reference numeral 20 is a sealed container to be decontaminated; this container may be any type of boiler, steam generating unit with a tubular plate, etc.

  The apparatus includes a 250 Hz Lambda Physik Exciplex (XeCl) 400 mJ laser 21. The laser 21 sends this beam to a vertical laser beam guide arm 23 that is used to direct the light beam 22 to the inner surface of the container to be decontaminated. The apparatus further includes a unit 24 and a gas control wagon 25 for controlling the movement of the laser beam.

  FIG. 4 shows the details of the guide arm 23. This arm should operate in a very contaminated area and should be reliable, i.e. simple and robust. This is because it was decided that only two rotation axes would be used to scan the entire inner surface (4πsr) of the container.

  The laser beam 22 reaches the upper end of the device horizontally, where a translucent mirror 30 directs it inside the guide arm 23 vertically to mirrors 31 and 32. The mirrors 31 and 32 may be rotated by two coaxial tubes 41 and 51, respectively.

  The inner tube 41 includes a vertical portion, and an upper end thereof is outside the container 20. This end rim is a gear 42 which cooperates with a gear 43 driven by a motor 44 to create a gear system. The lower end of the vertical part of the tube is in the container 20 and carries a mirror 31 inclined at 45 ° from the vertical direction. The lower end of this tube extends horizontally by a small portion 45. This small portion 45 carries a small 90 ° elbow tube 55 that rotates freely with respect to the portion 45. A small tube 55 carries the mirror 32, which is inclined 45 ° from the vertical direction and is arranged towards the mirror 31.

  The rim at the upper end of the outer tube 51 is a gear 52 that engages with a gear 53 driven by a motor 54 to form a gear system. The lower end of the tube 51 also has an edge, which is a gear 56, which acts in alignment with a gear 57 that is integral with the tube 55.

  Thus, the motors 44 and 54 are disposed outside the container 20. The guide arm 23 is supported by a dome of the container 20 pierced by the device 60 in advance. The device 60 has an O-ring 61 attached, which provides a hermetic seal for the dome of the container 20. A gland 62 is also provided to allow the outer tube 51 to rotate freely. A gland 58 arranged between the tubes 41 and 51 ensures that the tubes rotate freely with respect to each other.

  The laser beam 22 emitted by the laser 21 is reflected in the direction of the mirror 31 by the semi-transparent mirror 30 inside the guide arm 23, and the mirror 31 reflects toward the mirror 32, where it is reflected again and converged. Head toward lens 70.

  Since plasma radiation is noisy, it is advantageous to place a microphone on the dome of the container to monitor the removal of the oxide layer.

  Since the mirror 30 is translucent, the video camera 75 is placed on the guide arm 23 and the impact of the laser beam on the container wall is observed along the same optical path. Light from the plasma emitted near the processing surface is also sent along the same optical path for spectrophotometric analysis. Therefore, it is possible to determine which ions are removed from this surface and present in the plasma.

  The control unit 24 (see FIG. 3) includes a stepping motor control module, various power supply units, and connectors. Signal exchange between these control modules and the microcomputer managing the implementation of this process may occur via an RS232C serial link. The interaction between these modules is not subject to electromagnetic disturbances and is performed using a predetermined digital communication protocol. The scanning of the surface to be processed by the laser beam may be combined with a three-dimensional image preprogrammed in the control computer of the apparatus.

  The gas control wagon 25 includes a bottle of gas to be injected, such as argon, and an additive as described above. This gas is guided to the laser exposure area by a nozzle. It also includes two flow control pumps, one of which recirculates the ambient gas after fines filtration through an electromagnetic filter and a high performance filter. The recirculated gas is also used to blow off the lens 70. The second pump keeps the container 20 at a negative pressure using techniques known in the field of radioactive decontamination.

  This device can be used to carry out the method according to the invention automatically and thus without disturbing the operator. It also performs monitoring and atmospheric gas filtration.

It is a schematic diagram of the experimental apparatus used for this invention. 2 shows the emission spectrum of a plasma produced on a copper surface oxidized by a laser beam. 1 illustrates an apparatus that uses the principles of the present invention to decontaminate a container. FIG. 4 is a detailed view of the apparatus shown in FIG. 3.

Explanation of symbols

2 Ultraviolet laser beam 8 Metal object 20 Metal object 21 Laser 22 Ultraviolet laser beam 30 Mirror 31 Mirror 32 Mirror 41 Coaxial tube 51 Coaxial tube 75 Observation camera

Claims (7)

  In an apparatus for cleaning or decontaminating the inner surface of the container (20) by using the impact of an ultraviolet laser beam, depending on the material comprising the object, the object is peeled off, A laser (21) that emits the ultraviolet laser beam (22) under conditions selected to include surface removal of the material that comprises the object itself, transmits the laser beam to the inner surface, and transmits the laser beam (22) to the inner surface; Means including a mirror (30, 31, 32) for directing to a region of the inner surface of the container (20), and means for removing material resulting from delamination of the inner surface, the mirror (30, 31, 32) Is arranged to scan the entire surface to be cleaned or decontaminated under the action of a control device.   2. The apparatus according to claim 1, wherein the means for transmitting the laser beam to the inner surface comprises two mirrors (31, 32) and two coaxial tubes (41, 51), each coaxial tube (41, 51). , Which penetrates the container and can be rotated on their axes, moving one of the mirrors (31, 32).   3. An apparatus according to claim 1 or 2, further comprising means for injecting an inert gas onto the inner surface of the container which is subjected to the impact of the laser beam (22).   4. The apparatus according to claim 1, further comprising means for injecting a reducing additive onto the inner surface of the container which is subjected to the impact of the laser beam (22). apparatus.   5. The apparatus according to claim 1, further comprising means for transmitting an image of the cleaned or decontaminated surface to the observation camera (75).   6. An apparatus according to claim 1, wherein when the laser beam (22) bombards the inner surface (20) generates a plasma, the light from the plasma is analyzed spectrophotometrically. A device comprising means for communicating to the device.   In an apparatus according to any one of claims 1 to 6, in order to notify the generation of the plasma when the impact of the laser beam (22) on the inner surface (20) generates the plasma. An apparatus comprising an acoustic sensor.

Images