EP0507641B1 - Process and equipment for working with a laser in a contaminated area of a nuclear plant - Google Patents

Description

The present invention relates to a method and an installation for laser work on a surface contained in a contaminated area of a nuclear installation.

The invention applies in particular to the decontamination by laser beam, in aqueous or gaseous medium, of surfaces which have received a deposit of radioactive materials such as activated metal oxides, in order to reduce the level of radiation and thus allow the access or approach of response personnel.

The primary circuit of pressurized water nuclear power plants is concerned with this invention and more particularly the water box of the steam generators and the primary pipes.

Decontamination may be necessary during a check or repair to be carried out in the contaminated part of the plant, when replacing equipment such as a steam generator, and also when dismantling this plant.

• la projection de particules abrasives pour éliminer par abrasion la pellicule d'oxyde radioactive, ou la dissolution chimique de cette pellicule, qui ont pour inconvénient de produire des quantités importantes d'effluents coûteux à traiter;
• la décontamination par faisceau laser. Dans un procédé connu de ce type, décrit dans le EP-A-0 091 646, un faisceau laser est émis à l'entrée de la boîte à eau et renvoyé sur la paroi intérieure de celle-ci par des miroirs orientables fixés à la plaque tubulaire. Ce procédé, de par sa conception même, ne permet pas, même avec des impulsions laser à forte densité d'énergie, de traiter de façon uniforme toutes les surfaces à décontaminer.

Several decontamination processes are known:

• the projection of abrasive particles to remove the radioactive oxide film by abrasion, or the chemical dissolution of this film, which have the disadvantage of producing large quantities of effluents which are expensive to treat;
• laser beam decontamination. In a known method of this type, described in EP-A-0 091 646, a laser beam is emitted at the entrance to the water box and returned to the inner wall thereof by orientable mirrors fixed to the tubular plate. This process, by its very design, does not allow, even with high energy density laser pulses, to uniformly treat all surfaces to be decontaminated.

The object of the invention is to make it possible to work efficiently by means of a laser in a contaminated area.

To this end, the method according to the invention is in accordance with claim 1. According to the method, a pulsed laser beam is emitted outside the contaminated area, this beam is transported by means of an optical fiber to a location close to said surface, and, at this location, the beam is amplified and the amplified beam is sent onto said surface, possibly by means of a deflection mirror, the laser beam, after amplification, having pulses having an energy of 0 , 3 to 5 joules, or more, a duration of 10 to 30 ns, and an energy density of 1 to 15 J / cm2.

• on envoie un gaz protecteur ou actif dans la région de travail pendant le travail au laser;
• on confine la région de travail et, pendant le travail au laser, on aspire le gaz contenu dans la région confinée;
• on fait passer le faisceau amplifié à travers un orifice d'une électrode parallèle à ladite surface, et on crée un champ électrique entre cette électrode et ladite surface pendant le travail au laser;

According to other characteristics:

• a protective or active gas is sent into the working region during laser work;
• the working region is confined and, during the laser work, the gas contained in the confined region is sucked;
• passing the amplified beam through an orifice of an electrode parallel to said surface, and creating an electric field between this electrode and said surface during laser work;

• un générateur de faisceau laser pulsé, du type Nd-YAG, saphir ou excimère, eventuellement muni d'un miroir gaussien de renvoi, disposé en dehors de la zone contaminée;
• un amplificateur de faisceau laser;
• une fibre optique de transport du faisceau laser pulsé jusqu'à l'entrée de cet amplificateur; cette fibre optique ayant une longueur d'au moins 15 m environ;
• des moyens pour déplacer l'amplificateur en regard de ladite surface et au voisinage de celle-ci.

The invention also relates to equipment intended for the implementation of such a method. This equipment is characterized in that it includes:

• a pulsed laser beam generator, of the Nd-YAG type, sapphire or excimer, optionally provided with a Gaussian reflecting mirror, placed outside the contaminated area;
• a laser beam amplifier;
• an optical fiber for transporting the pulsed laser beam to the input of this amplifier; this optical fiber having a length of at least about 15 m;
• means for moving the amplifier opposite said surface and in the vicinity thereof.

• l'équipement comprend un miroir de renvoi monté en sortie de l'amplificateur et éventuellement mobile par rapport à celui-ci.

According to other characteristics:

• the equipment includes a deflection mirror mounted at the output of the amplifier and possibly movable relative to the latter.

• la Figure 1 représente schématiquement un équipement de décontamination au laser conforme à l'invention;
• la Figure 2 représente à plus grande échelle un détail de cet équipement;
• la Figure 3 est une vue analogue à la Figure 2 d'une variante; et
• la Figure 4 est une vue partielle d'une autre variante.

Examples of implementation of the invention will now be described with reference to the accompanying drawings in which:

• Figure 1 schematically shows a laser decontamination equipment according to the invention;
• Figure 2 shows on a larger scale a detail of this equipment;
• Figure 3 is a view similar to Figure 2 of a variant; and
• Figure 4 is a partial view of another variant.

There is shown in Figure 1, in axial section, one 1 of the two compartments of the water box 2 of a steam generator of a pressurized water nuclear reactor. This compartment 1 is delimited upwards by the tubular plate 3, on one side by the middle vertical partition 4 of the water box, and on the other side and downwards by the hemispherical bottom 5 of the water box , which is crossed by a manhole 6.

Also shown in Figure 1 is a equipment 7 adapted to allow the decontamination by laser beam of the surfaces which delimit compartment 1. This equipment comprises an external apparatus 8 arranged outside the water box, in an appropriate room protected from radiation, and an internal apparatus 9 arranged inside compartment 1 and which can be introduced therein through the manhole.

The apparatus 8 comprises a control console 10, a generator of electrical energy and fluids 11, a pulsed laser beam generator 12, constituted by an oscillator possibly followed by a preamplifier, and a suction pump 13 at the inlet which is provided with a filter 14.

The apparatus 9 comprises a laser beam amplifier 15 and a confinement enclosure 16 carried by a support 17. The input of the amplifier 15 is connected to the output of the generator 12 by an optical fiber 18 of multimode type having a length at least about 15 m. The enclosure 16 is connected on the one hand, via a line 19, to a source of protective (neutral or reducing) or active gas contained in the generator 11, and on the other hand, via a line 20, to the filter 14 and at the pump 13. The support 17 constitutes the end of an articulated robot, shown diagrammatically at 21, remotely controlled from the console 10 and making it possible to arrange the apparatus 9 facing any region of the surfaces 3, 4, 5 to decontaminate and in the vicinity thereof.

The apparatus 9 is shown in more detail in Figure 2. As can be seen in this figure, the amplifier 15 is housed in a housing 22 fixed to the support 17 and provided with supply lines 23 and power supply 24 and 25 for cooling water discharge. Lines 23 to 25 are connected via a line 26 (Figure 1) to the generator 11. An inlet face of the housing 22 is pierced with an orifice in which the distal end of the optical fiber 18 is fixed, and an inlet optic 27 makes it possible to introduce at the input of the amplifier 15 a parallel beam of diameter equal to that of the bar of the amplifier. This amplified spring beam at the other end of the amplifier 15, and its diameter is reduced by an exit optic 28, then leaves the housing 22 in the form of a parallel pulsed beam through an exit orifice 29.

At its distal end, the support 17 carries a frame 30 in which several columns 31 parallel to the axis X-X of the amplifier 15, biased by springs 32 in the opposite direction to this amplifier, are slidably mounted. The enclosure 16, which has a cup shape, has a bottom 33 perpendicular to the axis XX which is fixed to the distal end of the balusters 31, and a side wall 34 whose free edge is provided with rollers 35. The bottom 33 has an orifice 36 of axis XX whose diameter is slightly greater than that of the amplified beam 37.

The laser generator 12 is of a type allowing the transport of the beam by optical fiber. It can in particular be of the Nd-YAG type (wavelength 1.06 μm), of the sapphire type (wavelength centered on 0.78 μm) or of the excimer type (wavelength 0.3 μm) . It emits pulses with a duration of 10 to 30 ns. This generator 12 and the amplifier 15 are adjusted to provide an amplified beam 37 whose pulses have an energy of 0.3 to 5 joules or more and an energy density (or fluence) of 1 to 15 J / cm².

In operation, the rollers 35 are applied, with a force determined by the springs 32, to the surface to be decontaminated, which is the partition 4 in the example shown. A protective or active gas scans the enclosure 16, and the pulsed beam emitted by the generator 12, transported by the optical fiber 18 and amplified at 15, is sent directly, in the form of the parallel beam 37, to the surface to be treated, perpendicular to it. All the surfaces to be decontaminated are scanned in this way by moving the support 17 by means of the robot 21.

The above-mentioned energy density is chosen so as to allow thermal penetration corresponding to the thickness, or part of the thickness, of the layer of radioactive oxide to be eliminated, each pulse creating a shock wave on this layer. . The use of a neutral or sweeping gas reduces the oxidation of the etched surface, while the use of an active gas, in particular oxygen, makes it possible to increase the thickness of the oxide layer. interested in laser pulses. The choice of sweep gas will therefore be established according to the specific conditions of each application.

The use of a multimode optical fiber for the transport of the unamplified laser beam, provides a considerable advantage linked to the energy distribution in the beam leaving said fiber, and therefore at the level of the beam impact spot. on the wall. Indeed, in this case, the energy distribution is substantially constant over the entire surface of the spot; it is in the form of a slot instead of having a distribution comprising a central peak as is the case with transmission of the beam by air. However, the fiber must be long enough for the energy homogenization to be correct, for example at least 15 m. With a shorter optical fiber, it would be appropriate in certain cases to use in the generator 12 a so-called "Gaussian" mirror, known per se, providing a homogeneous, niche distribution of energy.

As will be understood, a distribution of energy by niche makes it possible to work without loss of efficiency with reduced laser powers, which is advantageous.

• le générateur laser 12 est disposé en dehors de la zone contaminée;
• le faisceau laser peut être transporté par fibre optique jusqu'au voisinage de la surface à traiter, avec les avantages précités, ce qui ne serait pas le cas si toute l'énergie du faisceau 37 était fournie par le générateur 12, à cause des possibilités limitées de transport de puissance laser des fibres optiques;
• le faisceau 37 étant un faisceau parallèle qui arrive perpendiculairement sur la surface à traiter, la distance entre cette surface et l'orifice de sortie de faisceau 29 de l'amplificateur n'est pas critique, et il n'est pas nécessaire de la maintenir constante.

The use of an amplifier 15 near the surface to be decontaminated has many advantages:

• the laser generator 12 is disposed outside the contaminated area;
• the laser beam can be transported by optical fiber to the vicinity of the surface to be treated, with the aforementioned advantages, which would not be the case if all the energy of the beam 37 was supplied by the generator 12, because of the possibilities limited laser power transport of optical fibers;
• the beam 37 being a parallel beam which arrives perpendicularly on the surface to be treated, the distance between this surface and the beam exit orifice 29 of the amplifier is not critical, and it is not necessary to maintain it constant.

The apparatus 9A shown in Figure 3 differs from that of Figure 2 in that the support 17 is arranged so that the axis X-X of the amplifier 15 is parallel to the surface to be treated. The posts 31 are perpendicular to this axis X-X, and a deflection mirror 38 inclined at 45 ° is fixed opposite the orifice 36 of the enclosure 16. The operation of this variant is the same as that described above. This variant is particularly applicable to laser work in reduced spaces, for example to decontaminate the wall of primary pipes.

The variant of Figure 3 can be modified as follows: the 16-column 31-mirror enclosure assembly 38 is connected to the support 17 by means of another support mounted mobile on the latter, in translation and / or in rotation about the axis of the amplifier 15. It is thus possible, for each position of the amplifier , effectively sweep a relatively large region to be treated, regardless of the shape of this region.

Figure 4 illustrates means other than suction means for capturing the oxide particles detached from the surface by the impact of the laser beam. It is in this case an electrode 39 held parallel to the surface to be treated by spacers not shown and pierced with an orifice 40 allowing the passage of the laser beam 37. This electrode is carried, thanks to a power supply 41 , at a high potential with respect to the treated surface, so that the detached oxide particles, ionized by the laser beam, are attracted to the electrode 39.

Claims (8)

1. Process for decontaminating a surface (3, 4, 5) contained in a contaminated zone (2) of a nuclear installation, using a pulsed laser beam (12) emitted outside the contaminated zone, characterised in that this beam is transported by means of an optical fibre (18) to a site close to said surface, situated in the contaminated zone, and at this site the beam is amplified (15) and the amplified beam (37) is directed onto said surface, optionally via a reflecting mirror (38), the laser beam after amplification having pulses with an energy of from 0.3 to 5 Joules or more, a duration of 10 to 30 ns and an energy density of 1 to 15 J/cm².
2. Process according to claim 1, characterised in that a protective or active gas is delivered into the working area during the laser operation.
3. Process according to claim 1 or 2, characterised in that the working area is confined and, during the laser operation, the gas contained in the confined area is aspirated.
4. Process according to any one of claims 1 to 3, characterised in that the amplified beam (37) is passed through an orifice (40) in an electrode (39) parallel to said surface, and an electrical field is created between said electrode and said surface during the laser operation.
5. Laser equipment for decontaminating a surface (3, 4, 5) contained in a contaminated zone (2) of a nuclear installation, for carrying out a process according to any one of claims 1 to 4, characterised in that it comprises:
      a pulsed laser beam generator (12) of the Nd-YAG, sapphire or excimer type, optionally fitted with a Gauss reflecting mirror arranged outside the contaminated zone (2);
      a laser beam amplifier (15);
      an optical fibre (18) for transporting the pulsed laser beam as far as the entrance to the amplifier, said optical fibre (18) being at least about 15 m in length; and
      means (21) for moving the amplifier (15) with respect to said surface and in the vicinity thereof.
6. Epuipment according to claim 5, characterised in that it comprises a reflecting mirror (38) mounted at the exit from the amplifier (15) and optionally movable relative to the latter.
7. Equipment according to claim 5 or 6, characterised in that it comprises a confining enclosure (16) which is movable together with the amplifier (15) or with the mirror (38) and equipped with suction means (13, 14, 20) and optionally with means (19) for introducing a protective or active gas.
8. Equipment according to one of claims 5 to 7, characterised in that it comprises an electrode (39) parallel to said surface, drilled with an orifice (40) for the amplified laser beam to pass through and movable together with the amplifier (15), and means (41) for creating an electrical field between said electrode and said surface.

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