EP0454584A1 - Process and apparatus for decontamination using ion etching - Google Patents

Abstract

According to the invention, in order to decontaminate an object (2) whose surface is contaminated by a contaminating material, at least a part of the surface is covered with an enclosure (6), this part, taken as a target, is etched by cathodic sputtering and the contaminating material thus removed is collected on a substrate (12) contained in the enclosure. Application to the decontamination of objects contaminated with radioactive materials. <IMAGE>

Description

The present invention relates to a decontamination process and device.

It applies in particular to the decontamination of objects which have been polluted by radioactive materials.

Techniques are already known for decontaminating such objects.

These known techniques consist in performing mechanical or electrolytic smears on objects.

• elles nécessitent l'utilisation d'effluents liquides,
• les produits contaminants se retrouvent dans ces effluents liquides ou sont dilués dans des gaz utilisés au cours de la mise en oeuvre de ces techniques,
• elles sont spécifiques des produits contaminants,
• elles dépendent de la nature des surfaces qui portent ces produits contaminants.

These known techniques have the following drawbacks:

• they require the use of liquid effluents,
• the contaminating products are found in these liquid effluents or are diluted in gases used during the implementation of these techniques,
• they are specific for contaminating products,
• they depend on the nature of the surfaces which carry these contaminating products.

The object of the present invention is to remedy the above drawbacks.

To this end, the invention uses a sputtering technique to decontaminate a polluted object, and a substrate, or collector, to collect the contaminating material.

Specifically, the present invention firstly relates to a method of decontaminating an object whose surface is polluted by a material contaminant, process characterized in that at least part of the surface is covered by an enclosure, in that this part taken for target is etched by cathode sputtering, and in that the contaminant material thus removed is collected, on a substrate contained in the enclosure.

• elle utilise une pulvérisation cathodique, phénomène physique qui peut s'appliquer à tout type de dépôt contaminant et à tout type de surface portant ce dépôt,
• dans la présente invention, le transfert de matière contaminante se fait sous pression partielle de gaz, atome par atome, en ligne droite, et il y a donc une absence totale d'effluent.

The present invention has the following advantages:

• it uses cathode sputtering, a physical phenomenon which can be applied to any type of contaminating deposit and to any type of surface carrying this deposit,
• in the present invention, the transfer of contaminating material takes place under partial gas pressure, atom by atom, in a straight line, and there is therefore a total absence of effluent.

The object can be, at least superficially, electrically conductive. In this case, according to a particular mode of implementation of the method which is the subject of the invention, the target is brought to a high voltage in absolute, continuous and negative value with respect to the earth.

The object can, on the contrary, be electrically insulating. In this case, according to another particular embodiment of the method which is the subject of the invention, the substrate is electrically conductive and an alternating electric voltage, of high maximum value and of high frequency (for example from the 'around 10 MHz). In this case, the substrate acts as an antenna.

According to a particular embodiment of the method which is the subject of the invention, a relative displacement of the substrate is also carried out relative to the object in order to successively strip a plurality of parts of the contaminated surface.

The use of sputtering is also advantageous because it is possible to deposit, after etching of the surface and over the residual contaminating material, a thin layer of a material, for example chromium, making it possible to ensure containment of this residual contaminating material and therefore biological protection.

More precisely, it is possible to complete the pickling carried out by covering the pickled surface with a protective layer allowing the confinement of the residual material, this covering being produced by sputtering from a target which is made of the material constituting the protective layer. .

• une enceinte prévue pour recouvrir au moins une partie de la surface à décontaminer,
• un substrat, destiné à recueillir le matériau contaminant enlevé par pulvérisation cathodique, ce substrat étant contenu dans l'enceinte et ayant une surface de collection supérieure ou égale à la surface de ladite partie,
• des moyens de polarisation prévus pour porter la cible à une tension élevée en valeur absolue, continue et négative par rapport à la terre,
• des moyens de pompage prévus pour mettre l'enceinte en dépression, et
• des moyens d'alimentation de l'enceinte en un gaz générateur de plasma.

The present invention also relates to a device for decontaminating an object which is, at least superficially, electrically conductive and whose surface is polluted by a contaminating material, device characterized in that it comprises cathode sputtering means provided for scouring the surface taken for target, these sputtering means comprising:

• an enclosure intended to cover at least part of the surface to be decontaminated,
• a substrate, intended to collect the contaminating material removed by sputtering, this substrate being contained in the enclosure and having a collection surface greater than or equal to the surface of said part,
• polarization means provided for bringing the target to a high voltage in absolute, continuous and negative value relative to the earth,
• pumping means provided for placing the enclosure under vacuum, and
• means for supplying the enclosure with a plasma generating gas.

The enclosure can be provided to cover the entire surface to be decontaminated, the surface for decontaminating the substrate being greater than or equal to the surface to be decontaminated.

In this case, when the enclosure is made of an electrically conductive material, it is possible to use, as substrate, the enclosure itself or, more precisely, the internal wall of this enclosure. This makes it possible to decontaminate a large number of parts placed in the enclosure, the latter receiving the contaminating material from each of these parts.

On the contrary, the enclosure can be provided to be placed on the surface to be decontaminated, thus covering part of the latter for the decontamination of this covered part. In this case, the device which is the subject of the invention may further comprise means for relative displacement of the enclosure and the substrate with respect to the object in order to successively etch a plurality of parts of the contaminated surface.

It is thus possible to decontaminate planar structures of very large dimensions relative to the enclosure. Preferably, the surface to be decontaminated, taken as a target, is then grounded, the substrate is given a shape almost identical to that of the enclosure and a continuous and positive high voltage is applied to this substrate.

The substrate may include a support and an electrically conductive thin film which covers this support.

It can therefore be seen that the invention makes it possible to capture and fix the contaminating material which has been removed, on a thin film which forms a protective screen from simple shape and reduced surface area, which facilitates the subsequent conditioning of this contaminating material which has been removed, while the known techniques mentioned above do not make it possible to fix the contaminating material removed by these techniques.

The present invention makes it possible to decontaminate not only flat surfaces but also surfaces which are not completely flat, for example mechanical parts which include weld beads, and even non-flat surfaces, such as profiled parts.

The substrate can follow the shape of the enclosure and be close to it, that is to say have walls close to the walls of the enclosure, to avoid pollution of the latter during decontamination.

• une enceinte prévue pour recouvrir la surface à décontaminer,
• un substrat électriquement conducteur, destiné à recueillir le matériau contaminant enlevé par pulvérisation cathodique, ce substrat étant contenu dans l'enceinte et ayant une surface de collection supérieure ou égale à la surface à décontaminer,
• des moyens prévus pour appliquer au substrat une tension électrique alternative de valeur maximale élevée et de grande fréquence,
• des moyens de pompage prévus pour mettre l'enceinte en dépression, et
• des moyens d'alimentation de l'enceinte en un gaz générateur de plasma.

The present invention also relates to a device for decontaminating an object which is electrically insulating and whose surface is polluted by a contaminating material, device characterized in that it comprises cathode sputtering means provided for scouring the surface taken for target, these cathode sputtering means comprising:

• an enclosure intended to cover the surface to be decontaminated,
• an electrically conductive substrate intended to collect the contaminating material removed by sputtering, this substrate being contained in the enclosure and having a collection surface greater than or equal to the surface to be decontaminated,
• means provided for applying an alternating electric voltage of high maximum value and high frequency to the substrate,
• pumping means provided for putting the enclosure in depression, and
• means for supplying the enclosure with a plasma generating gas.

The device which is the subject of the present invention may further comprise means capable of creating a magnetic field which is perpendicular to the electric field generated in the space between the substrate and the surface to be decontaminated during sputtering and which increases the density. of ions in this space and therefore the speed of spraying.

When these means capable of creating the magnetic field are driven by an alternating translation movement, the decontamination of the surface is homogeneous.

• la figure 1 est une vue schématique d'un mode de réalisation particulier du dispositif objet de l'invention,
• les figures 2 à 5 illustrent schématiquement d'autres modes de réalisation particuliers du dispositif objet de l'invention, et
• la figure 6 illustre schématiquement le recouvrement d'une surface décontaminée conformément à la présente invention, par une couche mince de protection.

The present invention will be better understood on reading the description of exemplary embodiments given below by way of purely indicative and in no way limitative, with reference to the appended drawings in which:

• FIG. 1 is a schematic view of a particular embodiment of the device which is the subject of the invention,
• FIGS. 2 to 5 schematically illustrate other particular embodiments of the device which is the subject of the invention, and
• FIG. 6 schematically illustrates the covering of a decontaminated surface in accordance with the present invention, with a thin protective layer.

The device according to the invention, which is schematically shown in Figure 1, is intended to decontaminate a metal part 2 whose surface 4 is flat and has been contaminated with a polluting material 5, for example a radioactive material.

This device allows sputtering of the surface 4 which, in the example shown, is grounded.

Of course, this device could also be used to decontaminate an electrically insulating object superficially covered by an electrically conductive layer which would have been polluted and it is this conductive layer that is grounded.

The device schematically represented in FIG. 1 comprises a bell-shaped enclosure 6 which can be made of an electrically insulating material such as for example glass or, on the contrary, of an electrically conductive material such as for example steel or l 'aluminum.

In the latter case, the enclosure 6 is also earthed (case of FIG. 1).

The device also includes pumping means 8 which are provided to create a vacuum in the enclosure 6.

This vacuum can be primary or secondary depending on the precise goal sought (spraying speed, heavy or light pickling products, vapor pressure of these products).

In the case where it is desired to achieve a primary vacuum, the pumping means can comprise a vane pump. In certain configurations, the vane pump can be supplemented by a turbomolecular pump for example.

The device also includes means 10 provided for introducing into the enclosure 6 a plasma generating gas, for example argon, under low pressure, of the order of 1 Pa for example.

The device also includes an electrically conductive subsrat 12, for example made of steel stainless. This substrate 12, which is placed in the enclosure 6, is intended to collect the polluting material and also plays the role of electrode. It substantially follows the shape of the enclosure and its walls are close to the walls of this enclosure: they are spaced a few centimeters, for example 2 to 3 cm for an enclosure whose diameter is of the order of 30 cm.

The device further comprises means 14 provided for bringing the substrate 12 to a positive positive high voltage.

The substrate 12 is of course not in contact with the surface 4.

The enclosure 6 rests against the surface 4 by means of sealing means 16, such as an O-ring for example, which make it possible to maintain the vacuum in the enclosure 6.

The means 14 are electrically connected to the substrate 12 via an electrical conductor 22 which is fixed to the substrate 6 by means not shown and which passes through the top of the enclosure 6 by a sealed passage 24, this passage also being electrically insulating when the enclosure is electrically conductive.

The conductor 22 is immobilized in this sealed passage and thus supports the substrate 12.

The use of the device schematically shown in Figure 1 will now be explained.

The substrate 12 is brought to a high positive potential with respect to the part of the surface 4 which is covered by the enclosure 6 and which here plays the role of target.

Then, an electric field is created between this substrate and the target (grounding).

By introducing the plasma generating gas, a plasma is created between the substrate and the target.

This target is bombarded with positive ions from the plasma.

By striking the target, these ions transfer their energy to the surface atoms of this target, which are then ejected and deposit on the internal wall of the substrate 12.

To successively decontaminate different parts of the surface 4, the device shown in FIG. 1 can be moved by placing it successively above these parts of the surface 4.

This is interesting in the case where the part 2 is large and not transportable.

For the movement of the device, it is possible to use a conventional remote control arm 28 which supports the enclosure 6.

As a variant, when the part 2 is mobile, the device is kept fixed and the part 2 is made to pass in front of this device in order to successively decontaminate the different parts of the surface 4.

When one of the parts of the surface 4 has been decontaminated, the vacuum is broken in the enclosure 6, the device is moved relative to the part to be decontaminated or this part relative to the device, so as to place this device in the -above the next part and we redo the vacuum in enclosure 6.

Another device according to the invention is schematically represented in FIG. 2. This other device is intended to decontaminate a metal part 2, the surface of which has been contaminated, for example, with a radioactive material. The part rests on a flat surface 30. If the latter is electrically conductive, it is grounded and the part 2 rests on this surface 30 by through an electrically insulating support 31.

The device of FIG. 2 comprises the enclosure which is earthed and which covers the part 2 to be decontaminated, the pumping means 8 and the means for introducing gas 10. The device also comprises the electrically conductive substrate 12 which , in the case of FIG. 2, no longer follows the shape of the enclosure 6 but has a planar shape and a surface greater than that of the part 2. The substrate 12 is placed opposite and close to the part 2 and it is suspended from the conductor 22 which, in the device of FIG. 2, is earthed.

Decontamination also takes place by sputtering, the part 2 being brought to a continuous voltage, high in absolute and negative value, by suitable means 32 which are connected to the part 2 via an electrical conductor 33 which passes through the enclosure 6 through a sealed and electrically insulating passage 34.

The substrate 12 that can be seen in FIG. 2 can optionally be covered, before starting the decontamination, with an electrically conductive protective film 26, intended to be compacted after having finished the decontamination.

In a variant of FIG. 2, not shown, the substrate 12 is electrically insulating and the part 2 is decontaminated by bringing it to a high voltage in absolute, continuous and negative value relative to the earth.

To increase the probability of ionization of the gaseous atoms and therefore increase the speed of tearing of material from the surface of the target (part 2), it is possible to use, as seen in FIG. 2, means 35 which are placed below the surface 30 and at the level of the target, and which are designed to create a magnetic field perpendicular to the electric field generated in the space between the target and the substrate.

As means 35, it is possible, for example, to use a set of magnetic bars positioned according to an arrangement suitable for inducing a wear zone forming a closed geometric circuit. So that this wear zone is homogeneous over the entire surface of the part 2, means 36 are provided for animating the means 35 and therefore the magnetic field with an alternating translation movement.

This of course assumes that the material constituting the surface 30 does not form a magnetic screen.

It is also possible to use the means 35 in the case of FIG. 1. These means 35 are then placed behind the part 2 (considering that the enclosure 6 is located in front of this part 2). This assumes that the material constituting the part 2 does not form a magnetic screen. The means 35 can be provided with means 36 of alternating translation, in order to have a homogeneous wear zone of the surface to be decontaminated.

In the case where the device of FIG. 1 is moved, the means 35 must also be moved so that they are always below the enclosure 6.

This is for example achievable by mounting the means 35 (or the means 36 and the means 35 when the means 36 are used) on displacement means 37 and using servo means not shown which allow the relative position of the means 35 and of the enclosure 6, when the latter is moved by the remote control arm 28, by acting on the displacement means 37.

In the case of the device of FIG. 1, if there are sufficiently powerful pumping means 8, one can be satisfied with weakly sealed means 16 and simply maintain a dynamic vacuum with a substantially constant leak rate, which makes it possible to move the device relative to surface 4 without stopping the decontamination process.

With sufficiently powerful pumping means 8, it is also possible to decontaminate, with the device schematically represented in FIG. 1, a part 2 comprising weld beads, by simply maintaining a dynamic vacuum when the sealing means 16 are located above a weld bead.

The device shown diagrammatically in FIG. 1 can also be used with parts whose surface 4 to be decontaminated is not flat but for example curved or "uneven" (case of profiled parts).

As can be seen in FIG. 3, a flexible skirt is then used as the sealing means 16 which borders the lower part of the enclosure 6 and which thus connects this enclosure 6 to the surface 4 to be decontaminated, this skirt allowing the maintenance of a dynamic vacuum (with sufficiently powerful pumping means).

As a purely indicative and in no way limitative, it is possible, in the case of FIG. 1, to bring the substrate to a direct voltage of the order of +2000 V relative to the target, by supplying this substrate with a current of the order of 60 mA, and establish an argon pressure in the enclosure of the order of 5 Pa; one then obtains, without magnetic aid, a decontamination speed of the order of 0.06 nm / s and a decontamination rate of the order of 1/1000.

Also purely by way of indication and in no way limiting, it is possible, in the case of FIG. 2, to apply to target 2 an electric power of the order of 10 W / cm², and to establish in the enclosure an argon pressure of around 5 Pa; we then obtain, with magnetic aid, a decontamination speed of the order of 2 nanometers per second and a decontamination rate of the order of 1/10000.

In the particular embodiment which is schematically represented in FIG. 4, the enclosure 6, which is electrically conductive, is grounded and acts as a substrate. It covers the part 2 to be decontaminated which still rests on the surface 30 by means of the insulating support 31. The part 2 is brought to a direct and negative voltage, high in absolute value, by the biasing means 32, by the intermediate of the conductor 33 which, in the case of FIG. 4, crosses the surface 30 through a sealed and electrically insulating passage 34a. The pumping means 8 and the gas supply means 10 communicate with the interior of the enclosure 6 through the surface 30. By sputtering, the contaminating material torn from the surface of the part 2 is deposited on the wall internal speaker 6.

In the particular embodiment schematically represented in FIG. 5, the part 2a which one wishes to decontaminate is electrically insulating. It also rests on the surface 30 (grounded) via the insulating support 31.

The enclosure 6 grounded covers the part 2a and rests on the surface 30 by means of the sealing means 16. The substrate 12 is identical to that which is used in the device of FIG. 1 and therefore marries the made of enclosure 6. Pumping means and means for supplying plasma gas are also provided in the device of FIG. 5. In this device, the substrate 12 is brought to an alternating voltage of radio frequency type, of the 'order of 10 MHz for example, and high maximum value, of the order of 1000 V for example. This voltage is supplied by biasing means 14a and applied to the substrate 12 by the conductor 22 which passes through the passage 24 and which supports the substrate 12. In this device, it is also possible to use the magnetic means 35 and the displacement means 36 of which it was discussed above.

After decontaminating the surface 4 of a contaminated part 2 (FIG. 6), it is possible, if necessary, to cover this surface with a biological protection layer 38 which thus covers the contaminating material which may still be on the surface 4.

To do this, use is still made of a device of the kind described above, but in this case, the substrate 12 is replaced by a conductive disc 12a which is brought to a negative high direct voltage using means. appropriate 40 which are connected to it via the electrical conductor 22.

In this case, the surface 4 and the enclosure 6, when the latter is electrically conductive, are still grounded.

In addition, the disc 12a which then plays the role of target during sputtering, is made of the material of which the layer 38 is to be formed, for example chromium.

This material is then torn from the disk 12a and projected onto the part of the surface 4 which is delimited by the enclosure 6, this part then playing the role of substrate.

If necessary, the device can still be moved relative to the part (or the part relative to the device) to successively cover different parts of the surface 4 with a protective layer.

The disc 12a can be covered by an electrically insulating cover 44 intended to isolate this disc from the enclosure.

To obtain a suitable covering of the surface 4 by the protective layer, it is preferable that the disk 12a playing the role of target is kept at a distance of the order of 2 to 3 cm from the surface 4 which acts as a substrate. .

Claims (16)

Method for decontaminating an object (2, 2a), the surface of which is polluted by a contaminating material, method characterized in that at least part of the surface is covered by an enclosure (6), in that it is stripped by sputtering this part taken as a target, and in that the contaminating material thus removed is collected, on a substrate (12) contained in the enclosure. Method according to claim 1, characterized in that the object (2) is, at least superficially, electrically conductive and in that the target is brought to a high voltage in absolute, continuous and negative value with respect to the earth. Method according to claim 1, characterized in that the object is, at least superficially, electrically conductive, in that the substrate is electrically conductive and in that the substrate is brought to a high, continuous and positive voltage with respect to the 'object. Method according to claim 1, characterized in that the object (2a) is electrically insulating, in that the substrate (12) is electrically conductive and in that an alternating electrical voltage of high maximum value is applied to this substrate and very frequent. Method according to any one of Claims 1 to 4, characterized in that a relative displacement of the substrate (12) relative to the object (2) is also carried out in order to successively etch a plurality of parts of the contaminated surface . Process according to any one of Claims 1 to 5, characterized in that the stripped surface is covered with a protective layer (38) allowing the confinement of the residual material, this covering being produced by sputtering from a target (12a) which is made of the material constituting the protective layer. Device for decontaminating an object (2) which is, at least superficially, electrically conductive and the surface of which is polluted by a contaminating material, device characterized in that it comprises cathode sputtering means provided for scouring the surface taken for target, these cathode sputtering means comprising: - an enclosure (6) designed to cover at least part of the surface to be decontaminated, - a substrate (12), intended to collect the contaminating material removed by sputtering, this substrate being contained in the enclosure and having a collection surface greater than or equal to the surface of said part, - polarization means (32) provided for bringing the target to a high voltage in absolute, continuous and negative value relative to the earth, - pumping means (8) provided for placing the enclosure under vacuum, and - Means (10) for supplying the enclosure with a plasma generating gas. Device for decontaminating an object (2) which is, at least superficially, electrically conductive and the surface of which is polluted by a contaminating material, device characterized in that it comprises sputtering means provided for scouring the surface taken for target, these cathode sputtering means comprising: - an enclosure (6) designed to cover at least part of the surface to be decontaminated, - an electrically conductive substrate (12), intended to collect the contaminating material removed by sputtering, this substrate being contained in the enclosure and having a collection surface greater than or equal to the surface of said part, - polarization means (14) provided for bringing the substrate to a high, continuous and positive voltage with respect to the object, - pumping means (8) provided for placing the enclosure under vacuum, and - Means (10) for supplying the enclosure with a plasma generating gas. Device according to any one of claims 7 and 8, characterized in that the enclosure (6) is provided to cover the entire surface to be decontaminated, the surface for decontaminating the substrate being greater than or equal to the surface to be decontaminated. Device according to claim 9, characterized in that the enclosure (6) is electrically conductive and in that the substrate is formed by the internal wall of this enclosure. Device according to any one of claims 7 and 8, characterized in that the enclosure (6) is designed to be placed on the surface to be decontaminated, thus covering part of the latter for the decontamination of this covered part. Device according to claim 11, characterized in that it further comprises means (28) for relative displacement of the enclosure (6) and the substrate (12) relative to the object (2) so as to successively etch a plurality of parts of the contaminated surface. Device according to any one of Claims 7 to 12, characterized in that the substrate comprises a support (12) and an electrically conductive thin film (26) which covers this support. Device according to any one of claims 7, 8, 9, and 11, characterized in that the substrate (12) follows the shape of the enclosure (6) and is close to this enclosure. Device for decontaminating an object (2a) which is electrically insulating and whose surface is polluted by a contaminating material, device characterized in that it comprises cathode sputtering means provided for scouring the surface taken for target, these means of cathode sputtering comprising: - an enclosure (6) provided to cover the surface to be decontaminated, - an electrically conductive substrate (12), intended to collect the contaminating material removed by sputtering, this substrate being contained in the enclosure and having a collection surface greater than or equal to the surface to be decontaminated, - means (14a) provided for applying to the substrate an alternating electric voltage of high maximum value and of high frequency, - pumping means (8) provided for placing the enclosure under vacuum, and - Means (10) for supplying the enclosure with a plasma generating gas. Device according to any one of Claims 7 to 15, characterized in that it further comprises means (35) capable of creating a magnetic field which is perpendicular to the electric field generated in the space between the substrate (12) and the surface to be decontaminated during sputtering and which increases the density of ions in this space.

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