JPH1075991A - Method and device for cleaning object or eliminating contamination by ultraviolet laser beam - Google Patents

Abstract

PROBLEM TO BE SOLVED: To easily, surely and safely clean plastic or metal surfaces or eliminate contaminations without re-contaminating the surface after a processing. SOLUTION: An ultraviolet laser beam 22 emitted by a laser 21 is scanned toward the surface of an object 20 to be processed by mechanical/optical means 30, 31, 32, 41, 51 and 1O, peeling is generated on the object 20 by the impact of the laser beam 22 and the surface of a material itself for constituting the object is eliminated. For stainless steel, it is advantageous when the work is performed by reducing gas, especially inert gas fluoride atmosphere.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

FIELD OF THE INVENTION 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 performing the method.

[0002]

BACKGROUND OF THE INVENTION The use of laser beams to clean or decontaminate the surface of objects has been described in many publications. The following documents can be cited from these publications:

[0003] WO 90/07988 discloses the use of a laser beam to clean a surface, which is guided manually during the cleaning operation. The output of the laser beam is several tens mW / cm ² at a wavelength on the order of microns.
Of the order. The technology described in this document is repetitive,
There are disadvantages related to lack of accuracy, complexity and inefficiencies for certain materials.

[0004] 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 oxide thin films from the surface of metallic objects that have been contaminated by radioactive elements. However, this method cannot be used to treat the substrate of the oxide layer, ie the pure metal part of the object. Moreover, the effect of the laser beam is inherently thermal, which makes it unsuitable for decontaminating plastic objects. In fact, this laser beam causes a surface melting of the plastic object, which has the effect that contaminants can eventually penetrate into the plastic material. This is the opposite effect that is required.

[0005] In FR-A-2700882, decontamination of surfaces contaminated by radioactive elements is contingent on maintaining a liquid on the surface to be treated. Using this liquid has many disadvantages, such as removal of the liquid after the end of the method.

[0006] 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 immediately leads to the reformation of the oxide layer on the treated metal. These layers retrap the contaminants, which is contrary to the desired result. The document also mentions a capture zone for particles released from the treated surface and released. This capture area is between 2 and 10 mm from the treated surface. This short distance is a disadvantage for implementing this method.

[0007] WO 95/13618 describes a method of performing decontamination by using a laser beam to locally melt the surface of a metal object to be treated and removing the molten metal. A disadvantage of this method is that it requires working in contact with the surface to be decontaminated. In addition, melting the metal damages the surface condition,
This is a major drawback because it creates slag and re-contains nearby contaminants.

[0008] Thus, the above method using the prior art,
It has a number of disadvantages.

They usually use laser beams at wavelengths in the infrared or visible part of the spectrum. Thus, they cause a surface melting of the metallic material, which leads to significant disadvantages.

When implementing such techniques manually,
Incomplete scanning of the surface to be treated. This will either leave a small unprocessed area or it will be necessary to perform repeated treatment steps. Another significant drawback when involved in radioactive decontamination is the risk of staff exposure to ionizing radiation.

One of the procedures described involves the use of a laser beam in combination with the application of a liquid which should have reinforcing properties. Although aqueous, the liquid in question is difficult to apply, remove and handle. In addition, experiments performed by the inventors of the present invention have shown that microcracks can form under each remaining droplet. These microcracks make the surface porous and are very susceptible to recontamination.

The surface peeling caused by the plasma heat generated by the laser beam impact or the laser impact,
Combined with the use of a reactive gas such as oxygen, a very pronounced reoxidation of the surface occurs immediately. Such re-oxidation
May cause recontamination.

[0013]

SUMMARY OF THE INVENTION The present invention is intended to overcome all disadvantages of the prior art. The inventor has performed a considerable number of investigations, which have unexpectedly led to methods for remote decontamination or cleaning of plastic or metal surfaces. The method can be easily automated and, depending on the circumstances, may be supplemented with a gas which favors the final condition of the surface to be treated, for example for polishing this surface without re-oxidation.

[0014]

SUMMARY OF THE INVENTION Accordingly, the present invention is a method for cleaning or decontaminating the surface of a metal object using the impact of an ultraviolet laser beam, depending on the material comprising the metal object. A method wherein the ultraviolet laser beam causes exfoliation of the metal object, using the ultraviolet laser beam under conditions selected to include surface removal of the material comprising the object itself.

If a layer of oxide forms on the surface of the metal object, the operating conditions evaporate the oxide and produce a plasma, which results in the surface removal of the material comprising the object itself. It is supposed to.

Depending on the circumstances, these operating conditions are such that the impact of the laser beam on the metal object is an inert gas atmosphere, for example an argon atmosphere, which is inert to the material constituting the metal object. It may be advantageous to include what happens in.

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

The present invention is directed to an apparatus for cleaning or decontaminating the interior surface of a container by using the impact of an ultraviolet laser beam, wherein, depending on the material of which the object is made, the object is subject to delamination, A laser that emits the ultraviolet laser beam under conditions selected to include surface removal of the material that constitutes the object itself, transmitting the laser beam to the interior surface and directing the laser beam to an area of the interior surface of the container. And means for removing material resulting from delamination of said inner surface, said mirrors, under the action of the control device, scan all surfaces to be cleaned or decontaminated. Apparatus characterized in that they are arranged as described above.

[0019] 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.

[0020]

DETAILED DESCRIPTION OF THE INVENTION
A laser beam with a wavelength between 400 nm and 400 nm is used.

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 ones currently available, namely a 250 Hz Lambda Physik 400 mJ.

The apparatus of FIG. 1 directs a laser beam using direct image transfer techniques. The laser beam 2 is first reflected by the mirror 3 and then passes through the first focusing lens 4; then is reflected by the mirrors 5 and 6 sequentially and then through the second focusing lens 7 (focal length 0.5 m) and into the protective container 9 To the target object 8 arranged inside. The video camera 10 monitors the impact of the laser beam 2 on the target 8.

The output of the laser beam measured at the target using this apparatus is 300 mJ per pulse. Mirror 5
And 6 can rotate to direct the laser beam 2 to the entire surface of this 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 intend to apply the present invention to other materials will only need to test using available technology without having to show special inventiveness. These tests will give, for any given material, information on the required laser beam power rating at the surface of the target and the optimal conditions required to practice the invention.

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

The power of a single ultraviolet photon is sufficient to excite an electronic level at or near the dissociation limit of an organic bond. This process is photochemical and has poor thermal effects.

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

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

EXAMPLE II In a second example, the target is a copper mass with a naturally oxidized surface.

Since this oxide layer absorbs ultraviolet light very well, a sufficiently powerful laser beam ( ² J / cm ² )
Vaporizes this copper oxide layer and forms a plasma. Analysis of plasma light emission can be used to determine the elements that make up the material from which it is made.

Curve 11 in the graph of FIG. 2 shows the emission spectra of the first two shots of the laser. 510.5 of this curve
The three peaks at 5 nm, 515.32 nm and 521.82 nm actually correspond to the emission of copper atoms from the positively stripped area.

Beginning with the third laser, the emission spectrum (curve 12 in the graph of FIG. 2) shows that no more plasma is created and no more copper is emitted and the metal has a low absorption coefficient. In order for the plasma to recur, a fairly high laser beam is required.

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

It was noted that an acoustic signal was emitted during plasma formation. Thus, while the laser beam interacts with the oxidized surface, an acoustic signal may be recorded for the first two shots. The third laser emits a very low acoustic signal.

EXAMPLE III In a third example, the target is Fe / Cr / Ni consisting of 72% iron, 18% chromium and 10% nickel by weight.
It is a lump of stainless steel. Due to aging, an oxide layer having a thickness of several tens of microns is formed on the steel. In order to decontaminate such targets, it has been found that it is essential to destroy the oxide layer initially formed during aging.

The oxide layer is easily vaporized with a luminous flux of ² J / cm ² emitting a very bright plasma of about 8 mm in length. The sudden expansion of this plasma simultaneously makes a bang.

In order to completely remove this oxide layer,
A few shots are sufficient, indicating that this process is very efficient from this point of view. After that, even if it is peeled off and irradiated on a shiny metal substrate at 3 J / cm ² , it does not generate the sound of plasma or burn. But,
Decontamination as part of the demolition of nuclear facilities imposes high decontamination factors that cannot be achieved without material removal from the metal substrate itself. At the same time, care must be taken not to melt the metal surface and re-oxidize the treated surface, and both processes will trap contaminants. This discussion leads to precisely defining the good working conditions required for stainless steel.

If laser irradiation of the surface of the steel ingot is actually continued in an air atmosphere and the oxide layer is removed once, the surface treated by irradiation for a short period of time becomes black, and Not only will the irradiation of the period increase the darkening, but also the appearance of the brick oxide structure due to the different thermal stresses between the metal and its oxides. This new oxide layer is undesirable because it has the effect of absorbing laser photons and shielding subsequent delamination; it also tends to trap contaminant particles, reduce the decontamination factor and thereby degrade the surface condition.

A series of experiments were performed to overcome these phenomena. In these experiments, a large number of samples of this steel were used and surface analysis was performed using XPS photoelectron spectroscopy and ion bombardment. This method rubs the sample at a rate of a few nm / min (2 nm / min in this experiment) so that the deep physicochemical structure of the processed sample can be identified.

Several samples were examined using these techniques, starting with the untreated sample (ie, retaining the original oxide layer), followed by the samples treated under various conditions.

The untreated sample is constructed as follows: At a depth of 0.13 μm, iron and chromium are mainly present as oxides, while nickel is in its metallic state. The composition of this sample remains constant over a depth of 0.13 μm, with iron oxide present. 0.4
The relative concentrations of Fe, Cr and Ni measured at a depth of 8 μm are 70.9%, 21.9% and 7.
2%. Given the measurement accuracy of the technique used, these values are the certified composition of this sample, ie, 72% by weight.
%, 18% and 10%.

The sample treated in air as described above is composed as follows: up to a depth of 1.1 μm, apart from a small amount of metallic nickel, this sample is:
Almost all consist of oxygen and metal oxides. At a depth of 1.3 μm, the ratio of iron oxide to metallic iron is still about 50%. Therefore, there is an oxide and oxygen reforming.

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

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

It is therefore very advantageous to carry out the stripping according to the invention in an inert gas, of which argon is the most convenient. Nitrogen should be avoided because it can cause nitridation. It is preferable to work in the reduced state achieved by using additives, since it is difficult to completely eliminate oxygen under current decontamination conditions. For example, a jet of pure argon can be replaced with very little hydrogen (less than 4% by volume) or argon containing ethanol, low molecular weight sugars or hydrocarbon compounds such as polyalcohols. In these situations, the danger of recarbonization of the surface is, for example, as follows:
Decreased by the presence of carbon monoxide generated in situ: 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 trapped in a small, very effective filter changes color from light brown to very deep black.

The use of non-hazardous fluoride additives, such as sulfur hexafluoride or Freon,
Interesting results have been obtained. By replacing oxygen with a stronger oxidant such as fluorine, volatile metal compounds may also be created in the plasma. When SF ₆ is present, peeling was found to be faster by kinetic.

These advantageous conditions should be applicable to numerous potential contaminants, such as actinides.

The decontamination apparatus Figure 3 shows a decontamination apparatus. Reference numeral 20 is a sealed container to be decontaminated; this container may be any type of boiler, a steam generating unit with a tubular plate, and the like.

This device is a 250 Hz LambdaPhysik Exiplex.
chip (XeCl) 400 mJ laser 21. The laser 21 directs the light beam 22 to a vertical laser beam guide arm 23 which is used to direct the light beam 22 to the interior surface of the container to be decontaminated. The device further comprises a unit 24 for controlling the movement of the laser beam and a gas control wagon 25.

FIG. 4 shows details of the guide arm 23. This arm should operate in highly contaminated areas and must be reliable, ie simple and robust.
This is because we have decided to use only two axes of rotation to scan the entire inner surface (4πsr) of this container.

The laser beam 22 reaches the upper end of the device horizontally, where the translucent mirror 30 directs it to the guide arm 23.
Is directed vertically to mirrors 31 and 32. Mirror 3
1 and 32 are two coaxial tubes 41 and 51, respectively.
May be rotated.

The inner tube 41 includes a vertical portion, the upper end of which is outside the container 20. The rim at this end 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 this tube is
It carries a mirror 31 which lies within 0 and is inclined at 45 ° from the vertical. The lower end of this tube extends horizontally with a small portion 45. This small part 45 carries a small 90 ° elbow 55, which is free to rotate with respect to the part 45. The small tube 55 carries a mirror 32, which is inclined at 45 ° from the vertical and is arranged facing the mirror 31.

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

Therefore, the motors 44 and 54
0. 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 mounted thereon, 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 from the laser 21 is
The light is reflected in the direction of the mirror 31 by the translucent mirror 30 inside the guide arm 23, and the mirror 31 reflects the light toward the mirror 32,
This beam is then reflected back to the focusing lens 70.

Since the 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 to observe the impact of the laser beam on the container wall 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 which are 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 that manages the implementation of this process may take place over an RS232C serial link. Interaction between these modules is performed using a predetermined digital communication protocol that is not easily affected by electromagnetic disturbance. 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 device.

The gas control wagon 25 controls the gas to be injected,
For example, a bottle of argon, and additives as described above. This gas is guided to the laser exposure area by the nozzle. It also includes two flow control pumps, one of which recirculates atmospheric gas after fine 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 container 20 at a negative pressure using techniques known in the art 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 filtering of atmospheric gases.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of an experimental apparatus used for the present invention.

FIG. 2 shows the emission spectrum of a plasma created on a copper surface oxidized by a laser beam.

FIG. 3 illustrates an apparatus that uses the principles of the present invention to decontaminate containers.

FIG. 4 is a detailed view of the apparatus shown in FIG.

[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 (16)

[Claims] 1. A method for cleaning or decontaminating a surface of a metal object (8, 20) using the impact of an ultraviolet laser beam (2, 22) depending on the material comprising said metal object. Wherein the ultraviolet laser beam causes exfoliation of the metal object, and the ultraviolet laser beam is used under conditions selected such that the exfoliation includes surface removal of a material constituting the metal object itself. Method. 2. The method according to claim 1, wherein an oxide layer is formed on the surface of the metal object, the operating conditions evaporating the oxide and producing a plasma, which A method comprising removing a surface of a material constituting a metal object itself. 3. The method according to claim 2, wherein the power of the laser beam is of the order of ² J / cm ² when applying the method according to claim 2 for cleaning or decontaminating copper objects. 4. The method according to claim 2, wherein, during the working conditions, the impact of the laser beam on the metal object is an inert gas which is inert with respect to the material comprising the metal object. A method characterized by including the fact that it occurs in an atmosphere. 5. The method according to claim 4, wherein said inert gas is argon. 6. The method according to claim 4, wherein, during the working conditions, the impact of the laser beam on the metal object is in the reduced state or the addition of a gas comprising hydrogen or a hydrocarbon compound. A method characterized by including the fact that it occurs below. 7. The method according to claim 6, wherein said reducing state is obtained by the presence of at least one suitable additive in said inert gas atmosphere. 8. The method according to claim 7, wherein said additive is a fluorinated compound, said compound evaporating metal atoms and eroding said metal object under the action of a laser beam. A method characterized by the following. 9. The method according to claim 6, wherein the laser beam output is ² J / cm ² when applying the method according to claim 6 to cleaning or decontamination of a stainless steel object.
A method characterized by the above order. 10. An apparatus for cleaning or decontaminating the interior surface of a container (20) by using the impact of an ultraviolet laser beam, depending on the material comprising the object,
A laser (21) that emits the ultraviolet laser beam (22) under conditions selected to cause exfoliation of the object and to include surface removal of the material comprising the object itself; To the laser beam (2
2) means including a mirror (30, 31, 32) for guiding the inner surface area of the container (20), and means for removing material resulting from delamination of the inner surface, wherein the mirror (30, 31, 32) are arranged to scan, under the action of the control device, all surfaces to be cleaned or decontaminated. 11. The apparatus according to claim 10, wherein the means for transmitting the laser beam to the inner surface comprises two mirrors (3).
1, 32) and two coaxial tubes (41, 51), each of said coaxial tubes (41, 51) penetrating said container and being rotatable on their axis, and of said mirror (31, 32). A device characterized by moving one of them. 12. Apparatus according to claim 10 or 11, further comprising means for injecting an inert gas onto the inner surface of the container which is impacted by the laser beam (22). . 13. An apparatus according to claim 10, wherein said laser beam (22).
A means for injecting a reducing additive onto the inner surface of the container which is subjected to the impact of (i). 14. The apparatus according to claim 10, further comprising means for transmitting an image of the cleaned or decontaminated surface to an observation camera (75). Equipment to do. 15. An apparatus according to claim 10, wherein the laser beam (22).
A means for transmitting light from said plasma to a spectrophotometric analyzer when the impact on said inner surface (20) produces a plasma. 16. An apparatus according to claim 10, wherein said laser beam (22).
An acoustic sensor for notifying the generation of plasma when an impact applied to the inner surface (20) generates plasma.