FR2619953A1 - SYSTEM FOR DISMANTLING INSTALLATIONS PRESENT IN A LOCAL, ITS METHOD OF IMPLEMENTATION AND HOOD USED IN THIS SYSTEM - Google Patents

Abstract

The object of the present invention is a system for dismantling installations present in a room 10 maintained in depression by means of a ventilation system. After blocking the air inlet 15 to room 10, a hood 28 is brought above the installations to be dismantled. A fan 36 makes it possible to extract in the central part of the hood 28 the air laden with aerosols due to the dismantling operations. After purification, part of the gas is returned to the hood 28 and flows to the periphery of the latter, thus creating a dynamic confinement zone around said installations. Another part of the gas is sent into the extraction pipe 18 in order to maintain the desired depression inside the room 10. Application to the dismantling of nuclear installations.

Description

SYSTEM FOR REMOVAL OF INSTALLATIONS PRESENTED IN A

  LOCAL, ITS IMPLEMENTATION METHOD AND HOOD USED IN

THIS SYSTEM

DESCRIPTION

  The present invention relates to a system for dismantling installations, and in particular nuclear installations, located in a room that can be

  depression through a ventilation system.

  The dismantling of nuclear installations differs from the dismantling of conventional industrial installations in that the buildings or installations to be removed contain a large quantity of radioactive materials that are either deposited on the surfaces (volatile compounds or aerosols produced by the processes used). are present in the structures because of the irradiation that the latter have undergone (activation products). These initial conditions give the decommissioning operations of nuclear installations the following constraints: It is first necessary to optimize the packaging of the waste by evacuating the maximum of the activity in the minimum volume by a judicious choice of the cutting processes taking into account the specific characteristics of the installation to be dismantled (handling equipment, nature and size of the openings, means of evacuation, etc.) It is also important to reduce the doses received by the personnel involved to as low a level as possible.

(irradiation, contamination).

  The contamination caused by the dismantling operations themselves must be reduced as much as possible. Indeed, the destruction of machines or other components, especially when performed using cutting tools, causes a resuspension of contamination in the form of aerosols. This can lead to conditions

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  unacceptable procedures, for example by contamination of hitherto healthy areas and filter clogging by irradiating aerosols. Finally, the volume of secondary waste produced during the decommissioning operations themselves, especially those generated by

  collection of aerosols produced by cutting tools.

  The techniques currently used to carry out the dismantling operations give priority to the first two constraints without providing specific solutions to the

  problems posed by the last two.

  The operating conditions currently adopted make it possible to install, within the room to be dismantled, the cutting tools to be used (chain saw, saw, plasma arc torch, laser, etc.) as well as the teleoperation means necessary for their implementation (telemanipulable arms, cameras, ...). Handling and evacuation problems are usually dealt with by the means of the installation (remote manipulators, existing openings, ...). On the other hand, contaminations due to radioactive aerosols produced during cutting operations can not be specifically treated.

for two main reasons.

  First of all, in order to maintain in the room the depression essential for the containment of the radioactive products it contains, it is necessary to use the general ventilation of the installation at all times. This causes, by the design of ventilation systems, high volume renewal rates (of the order of 10 to 30 renewals per hour) that promote the dissemination of radioactive aerosols produced by the cutting tools. This causes significant surface contaminations in different

places of the premises to be dismantled.

  On the other hand, the purification of the radioactive aerosols thus suspended is carried out by means of filtration devices specific to the room in question. These filtration devices may therefore quickly reach clogging and / or irradiation levels such that they must be renewed often. Renewal operations have a number of difficulties, they are long, which lengthens the duration of the dismantling operations themselves. The present invention aims to overcome the above drawbacks by proposing a system for dismantling installations, including nuclear facilities, which can significantly reduce the contamination caused by the dismantling operations themselves and which can be

  used when the ventilation system of the room is at a standstill.

  More precisely, the subject of the invention is a system for dismantling installations present in a room that can be maintained in a vacuum, or possibly under an overpressure, thanks to a ventilation system, the dismantling being carried out using at least a tool generating a pollution of the atmosphere prevailing in the vicinity of said facilities. According to the main characteristic of this dismantling system, it comprises: - means for creating a dynamic confinement zone around the tool using gas flow blown and extracted respectively, - gas extraction means polluted present in said confinement zone, thus creating said extracted gas stream, means for purifying the gases thus extracted, and - means for regulating said gas flows able on the one hand to maintain the room at said determined pressure - when the ventilation system is stopped, and, on the other hand, to ensure the flow of gas necessary to create said zone

confinement.

  The term "causing pollution of the atmosphere" means that the implementation of the dismantling tool (s) leads to the production and dissemination around facilities for the treatment of harmful substances such as radioactive aerosols, toxic vapors, or dangerous gases

and / or contaminants.

  According to another aspect of the invention, the means for creating a dynamic confinement zone comprise: a hood that can be placed above the tool, this hood having a periphery and a central part; gas blowing means at the periphery of this hood arranged so as to create a curtain of gas surrounding the tool, thus participating in the creation of said dynamic confinement zone, and gas extraction means present in the said dynamic confinement zone, the extraction being carried out by the

central part of the hood.

  According to a preferred embodiment, the hood comprises, all along its periphery, a double wall defining a space communicating with the interior of the room, said blowing means being arranged so as to inject

gas in this space.

  Advantageously, the purification means can be mounted on the hood. In this case, they comprise: at least one compartment inside the hood, this compartment communicating on the one hand with said confinement zone by an opening and, on the other hand, with a suction means mounted on the hood, and - at least one filter placed inside this compartment so as to filter all the gas passing through the compartment from said opening to the suction means. According to a preferred embodiment, this compartment comprises: - a first chamber communicating with the confinement zone by said opening, a second chamber separated from the first by a partition arranged so as to leave a passage for the gas between the two chambers, the second chamber communicating a

2619953

  part with said suction means and secondly with the first chamber through said passage, at least one of the chambers being provided with at least one filter placed so as to filter the

  all the gas that passes through this room.

  According to another aspect of the dismantling system which is the subject of the invention, the latter comprises at least one curtain fixed at the periphery of the hood and surrounding said dynamic confinement zone. Optionally, it is also possible to provide a second curtain fixed at the periphery of the hood so as to define a space between the two curtains, this space surrounding the dynamic confinement zone and the blowing means being arranged so as to blow gas into the the space thus created. These curtains

  can be made for example vinyl.

  According to another characteristic of the dismantling system which is the subject of the invention, the latter may further comprise a mechanical frame on which the hood, the extraction means and the purification means are mounted, thus constituting a machine. The latter can be provided mobile and movable using handling means, for example a bridge

rolling.

  In this case, it can further provide at least one heavy element mounted on this machine and intended to increase

  stability. This heavy element can be a shield.

  It is also possible to provide means for handling the tool mounted on this machine, as well as fluid supply means also mounted on this machine. Lastly, it is possible to provide on the latter an armored cell capable of accommodating an operator and comprising control apparatus

and control.

  The invention also relates to a method for implementing this dismantling system. When the ventilation system comprises a gas inlet in the room and a gas extraction network present inside the room, this method comprises the following steps: (a) - blocking said gas inlet, and b) - Connect said gas extraction means present in the dynamic confinement zone to the gas extraction network located inside the room. It is thus possible to pass from the normal ventilation of the Local to the clean ventilation of the dismantling system which is the subject of the invention, avoiding the phases of sudden transition which are

  generally sources of radioactive releases.

  Finally, another subject of the invention is a hood that can be used to create a dynamic confinement zone.

  hood having the characteristics indicated above.

  The invention will appear better on reading the

  description which will follow, given as an example purely

  illustrative and not limiting, with reference to the accompanying drawings in which: - Figure 1 is a block diagram showing, in vertical section, the dismantling system object of the invention inside a locaL equipped with a system Figure 2 is a schematic vertical sectional view showing how the dismantling system object of the invention can be used for the dismantling of nuclear reactors, - Figure 3 is a schematic perspective view with partial section showing the hood used in the system object of the invention, and - Figure 4 is a schematic sectional folded view of the hood shown in Figure 3 showing the two chambers

of the same compartment.

  Figure 1 shows the local 10 containing nuclear facilities, the facilities to be dismantled in an area 12 shown schematically in the form of a hatched circle. The ventilation system of the room, intended to keep it in a vacuum, comprises an air intake duct 14 equipped with a fan 16 and an extraction duct 18 equipped with a fan 20. The ducts 14 and 18 are equipped with isolation valves 15 and 19 at the place where they open into the room 10. A control and control system 22 makes it possible to regulate the flow rate of the fans 16 and 20 as well as the position of the valves 15 and 19 The connections between the device 22 and the various devices it controls are schematically represented in the drawing by broken lines. A pipe 24 equipped with a valve 26 connects the inlet pipe 14 to the extraction pipe 18. The connection points of the pipe 24 on the pipes 14 and 18 are outside the room 10 and respectively , downstream of the fan 16 and upstream of the fan 20. The pipe 24 thus constitutes a bypass for circulating the air

  in the ventilation system without going through room 10.

  FIG. 1 again shows the dismantling system that is the subject of the invention placed inside the room 10. This first comprises a hood 28 having substantially the shape of a

  truncated cone, this hood being placed above zone 12.

  The hood 28 has at its periphery two concentric walls 30 and 32 defining between them a space 34. Thus, the space 34 communicates with the interior of the room by a circular slot located between the walls 30 and 32, at the base of these latest. An air circulation member (fan 36, compressor) extracts the air in zone 12 and in the vicinity thereof by means of a pipe 38 equipped with two filters 40 and 42. Filter 40 acts as a prefilter and can be, for example, an electrostatic filter or an electrostatic cyclone. The filter 42 is a very high efficiency filter or an absolute filter for stopping the finest dust or particles. Eventually, we can predict. in

  in addition to a charcoal filter 44 mounted on a pipe 46 itself

  even bypassed on line 38 through a three-way valve 48. The charcoal filter 44 can stop

any harmful vapors.

  The air extracted by the fan 36 is divided into two parts. A first part is returned to the upper part of the hood through a pipe 50. From there, it flows into

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  The space 34 and forms a curtain of air continuously surrounding the area 12 o are cutting tools and facilities to dismantle. This creates a dynamic containment zone

  around Area 12 and dust, aerosols, vapors etc ...

  products in this area are evacuated in the upper part of La

  hood with fan 36 and line 38.

  Optionally, part of the air returned to the top of the hood by the pipe 50 can be blown into the room through a valve 52 provided at the top.

of the hood.

  Another part of the air extracted by the fan 36 is sent into the extraction pipe 18 by means of a pipe 54 equipped with a valve 56. The pipe 54 makes it possible to maintain the desired depression inside the Local 10 at during dismantling operations. Indeed, before starting the dismantling, the valve 15 is closed in order to block the air inlet into the room. In this case, the valve 26 can be opened so that the air flows directly from the pipe 14 to the pipe 18 without passing through the room 10. However, there is always leakage and the outside air penetrates more or less to Inside the LocaL. The flow rate of the fan 36 is set to ensure, on the one hand, the flow rate necessary to create the zone of blown confinement at the periphery of the hood and, on the other hand, to evacuate in the ducts 54 and 18 an air flow rate.

  equal to the one who enters the room by the leaks.

  Can also be provided, attached to the periphery of the hood, a curtain 58, for example vinyl, shown schematically in phantom in Figure 1. This curtain contributes to directing the air flow out of the space 34 in a manner. vertical direction to reinforce the confinement of the zone 12. Optionally, there may be a second curtain 60, also in vinyl, and shown in phantom lines in the drawing, the air leaving the space 34 thus penetrating the space between these two curtains. Finally, the control and control device 22 also makes it possible to control the valves 15 and 19 as well as the valve 56 equipping the pipe 54, the valve 48 and the fan 36, and possibly the valve 52 whose adjustment makes it possible to optimize the dynamic confinement, adjusting the flows

blown and extracted.

  Figure 2 shows how the dismantling system object of the invention can be used in a large building, for example a building containing a nuclear reactor. This building, with the general reference 62, is limited by a double wall, namely an inner wall 64 and an outer wall 66. The latter are shown in phantom in FIG. 2, as are the support structures 68 on which the reactor 70, the heat exchangers 72, the pumps 74 and the pipes connecting these different elements are mounted. The dismantling system 76 which is the subject of the invention can be mounted on a traveling crane 78 flowing in the upper part of the building 62, the system 76 being hooked to this traveling crane by means of cables 80. In fact, the cables 80 are attached to a carriage 82 mounted on the traveling crane and movable relative thereto. It is thus possible to bring the device 76 to any point at

the interior of the building 62.

  The dismantling system 76 will now be

  described in more detail with reference to FIG.

  It can be seen in this figure that the dismantling system 76 which is the subject of the invention is in the form of a machine comprising firstly a mechanical frame 84 on which the hood 28 is mounted. In the example represented here,

  the hood 28 is mounted to the lower part of the frame 84.

  The latter supports, at its upper part, a cell 86 that can accommodate an operator and in which the control devices 88 are located. The cell 86 is surrounded by a protective shield 87. As this is of considerable weight it constitutes a heavy element capable of increasing the stability of the machine 76. Optionally, the cell 86 can be removed and heavy elements of another form can be used.

  to improve the stability of the machine.

  The tools used to dismantle the installations to be eliminated can be manipulated by means of remote manipulators 90. These can advantageously be mounted on the machine 76. In the example shown here, the casing 92 comprising all the mechanical and electrical controls of the manipulator 90 is attached directly on the one hand to the hood and on the other hand to the cell 86. However, this is only a preferred arrangement and it would not depart from the scope of the invention using another assembly, or even by not mounting remote manipulators or cutting tools on the

machine 76.

  The device illustrated in FIG. 3 has the advantage of being able to integrate in the dismantling machine 76 a certain number of accessories and, first of all, the tools of

  cutting and their control and manipulation system.

  However, it can also mount on this machine illumination and remote viewing means, temperature detectors, toxic gases or explosives, means for handling parts to be cut or movable aeraulic screens. We will now describe in more detail the hood 28 in

reference to Figures 3 and 4.

  These figures show the two sheets 30 and 32 placed at the periphery of the hood and defining between them a space 34. Each of these sheets has a portion in the form of a truncated cone diverging downwards followed by a vertical portion . The vertical portions of the plates 30 and 32 define two concentric cylinders communicating with the outside through a slot located at the bottom of the sheets. At the bottom of the outer sheet 32 is fixed a curtain 58, which may be vinyl. This curtain is fixed all along the sheet 32 and imposes a vertical direction to the air curtain that escapes from the space 34 to the lower part of the hood. This curtain 58 may consist of a set of vinyl blades which is

2619 9 53

  overlap partially. Indeed, it is not necessary to have a perfect seal because, even if a part of the air escaping at the periphery of the hood left the confinement zone, this would interest only a small part of the flow rate and would not interfere with the operation of the device. Optionally, it is possible to provide a second curtain 60 hooked to the lower part of the inner sheet 30, the air leaving the space 34 thus being sent into the space defined between the two curtains 58 and 60. the upper part of the hood, a number of fans 94 arranged to blow gas in the space 34 between the plates 30 and 32. It is still seen in Figures 3 and 4 that the hood is divided into a certain number of compartments 96 each of which is limited at its lower part by a sheet 98 and at its upper part by the sheet 30. The sheet 98 has the shape of a truncated cone of the same angle at the top as the sheets 30 and 32. Each compartment 96 is separated from neighboring compartments by partitions 100 arranged radially with respect to the hood. Each partition 100 is in sealing contact with the plates 30 and 98 and

  extends to the central part of the hood.

  In the embodiment illustrated here, each compartment comprises two chambers, namely a first chamber 102 and a second chamber 104. The chambers 102 and 104 are separated by a partition 106 which is arranged radially inside the hood, as the partition 100, but unlike the latter, does not extend to the vertical portion of the inner sheet 30. Thus, a passage for air is formed between the two chambers inside the hood and at

'the periphery of it.

  Each chamber 102 communicates with the outside through an opening 108 located in the vicinity of the axis of the cone defined by the hood. As for the second room 104, it communicates

  share with the first and, on the other hand, with the fan 94.

  The first chamber 102 is equipped with prefilters 40 which may be electrostatic filters or electrostatic cyclones. As for the second chamber, it is equipped with one or more very high efficiency filters or absolute filters 42. Optionally, it is also possible to provide a carbon filter placed between the filters 40 and 42 relative to the direction

  flow of air through chambers 102 and 104.

  Thus, the fan 94 makes it possible to suck up the air that is in the confinement zone, that is to say the zone surrounded by the air curtain flowing from the space 34. The air thus sucked up penetrates into the hood through the openings 108, circulates in the first chamber 102 from the center of the hood to the periphery passing through the filters 40, then enters the second chamber 104 after having bypassed the partition 106 and flows through the second chamber from the periphery of the hood to the center of it by passing through the filters 42. So it is purified air that is sucked

  by the fan 94 and returned to the space 34.

  Thus, the dismantling system object of the invention has particularly interesting advantages, the main purpose of which is to allow effective removal of pollution due to cutting tools. This is made possible by the fact that a dynamic confinement zone is created at the location where the installations to be dismantled and the air in the zone is drawn, this air passing immediately through front filters. to be recycled to create this dynamic containment zone. In addition, the system can be implemented when the normal ventilation system of the room is stopped. This avoids circulating very large amounts of air which may disperse the contamination since the flow of the fans 36 (Figure 1) or 94 (Figures 3 and 4) is set to ensure that the flow required to create the the containment zone increased the flow required to evacuate the air or gases that enter the room through unavoidable leaks in the walls of the latter. These leaks are thus

  used to improve containment.

  The dismantling system that is the subject of the invention also has a great flexibility of use since it is possible to make models in an inactive zone in order to study the difficulties of implementation and implementation before reproducing the operations in a active area. In addition, the dimensions of the system, as well as the number and type of equipment that must be fitted to it, can be adapted

  depending on the type of installation to dismantle.

  This system also has interesting advantages in the field of security. Indeed, one can supply this system with neutral gas or oxygen rarefied air, which limits the risk of fire and explosion. The system can be supplied with refrigerant gas, for example nitrogen gas extracted from a bottle of liquid nitrogen. In addition, the capture and filtration installation mounted on the dismantling system object of the invention is designed to recycle the room air. This allows, after a dismantling operation, to capture all aerosols suspended in the air before starting again the ventilation of the

  local (the time of the operation is related to the volume of the room).

  In addition, the system object of the invention allows a longer life of the very high efficiency or absolute filters of the ventilation system since during dismantling operations, they are not the ones that stop the aerosols

  products, but the filters of the dismantling system itself.

  The filters of the ventilation system are crossed only by purified air, which improves the confinement of the room. On the other hand, as the capture, filtration and recycling of the room air is systematically carried out for each dismantling operation, it avoids the increase in the activity of the local which necessarily occurs with the dismantling processes. current. As a result, there is a decrease in decontamination time and staff exposure time. Finally, the significant reduction in gas flows facilitates the detection of rare gases and their capture by appropriate traps: indeed,

14 2619953

  the flow rate of the fans, such as 36 or 94, even if there are several on the hood, is much lower than the normal flow rate

  crossing the room under the effect of the ventilation system.

  Finally, the dismantling system which is the subject of the invention has advantageous economic advantages. First, the costs of decontaminating and changing filters in the normal ventilation network are reduced. On the other hand, in the case of a PWR plant, we go from a flow of a few hundred thousand cubic meters per hour, which is the flow of the normal ventilation system, to only a few thousand cubic meters per hour which represents the flow rate of the capture and filtration system associated with the dismantling system, while having a much higher aerosol capture and purification efficiency than with traditional methods. This reduction in flow rates facilitates the capture of rare gases, reduces the size of the purification means (prefilters, carbon filters, very high efficiency or absolute filters) and recycles the air from the premises. It can also maintain it in depression by evacuating a flow equivalent to the leakage rate after filtration, which avoids further pollute the very high efficiency filters of the ventilation system. The system object of the invention is easy to adjust and has great flexibility of use. It also reduces implementation costs, reduces space congestion due to integrated design, reduces the volume of waste, and finally, it presents the possibility of changing the filters with remote manipulators used for dismantling. . The system which is the subject of the invention also makes it possible to optimize the efficiency of the tools and that of the collection of the aerosols produced because the position of the hood, and therefore the position of the tools it carries, is well defined in space. and can be adjusted precisely to

coincide with that of the task.

  As for the applications, they are not limited to the dismantling of nuclear installations but the system object of the invention can be used by any operation which causes a more or less significant pollution of the atmosphere at the place of intervention (production of aerosols, dust, toxic gases, etc ...). As for the hood, it can be used for any operation requiring the creation of a dynamic containment zone.

Claims (25)

  1. System for dismantling installations present in a room (10) that can be maintained in depression, or possibly in overpressure due to a ventilation system (16, 14, 18, 20), the dismantling being effected at the using at least one tool generating a pollution of the atmosphere prevailing in the vicinity of said installations, characterized in that it comprises: means (28, 36) for creating a dynamic confinement zone around the tool using gas flow, blown and extracted respectively, - means (36) for extracting the polluted gases present in said confinement zone, thereby creating said extracted gas stream, - means (40, 42) for purifying the gases thus extracted, and means (22) for regulating said gas flows able on the one hand to maintain the room (10) at said determined pressure when the ventilation system is stopped, and, secondly, to ensure the flow of gas needed to create said confinement zone.   2. System according to claim 1, characterized in that said means for creating a dynamic containment zone comprise: - a hood (28) can be placed above the tool, this hood (28) having a periphery and a central part; gas blowing means (36) at the periphery of this hood (28) arranged to create a curtain of gas surrounding the tool, thereby contributing to the creation of said dynamic confinement zone, and - means ( 36) for extracting the gases present in said dynamic containment zone, the extraction being carried out by   the central part of the hood (28). 2619 9 53   3. System according to claim 2, characterized in that the hood (28) comprises a double wall (30, 32) defining a space (34) communicating with the interior of the room (10), said blowing means (36) being arranged to blow gas into this space (34). 4. System according to claim 2, characterized in that said purification means (40, 42) are mounted on the hood (28). 5. System according to claim 4, characterized in that said purification means comprise: - at least one compartment (96) inside the hood (28), this compartment (96) communicating on the one hand with said confinement zone by an opening (108) and, on the other hand, with a suction means (94) mounted on the hood (28), and - at least one filter (42) placed therein compartment (96) so as to filter all the gas passing through this compartment (96) from said opening (108) to the suction means (94).   6. System according to claim 5, characterized in that said compartment (96) comprises: - a first chamber (102) communicating with the confinement zone by said opening (108), - a second chamber (104) separated from the first (102) by a partition (106) arranged to leave a passage for the gas between the two chambers (102, 104), the second chamber (104) communicating on the one hand with said suction means (94) and on the other hand with the first chamber (102) through said passage, at least one of the chambers (102, 104) being equipped with at least one filter (40, 42) placed so as to filter all of the gas running through this room (102).   7. System according to claim 2, characterized in that it comprises at least one curtain (58) attached to the periphery of the   hood (28) and surrounding said dynamic containment zone.   8. System according to claim 7, characterized in that 2619 9 53   it further comprises a second curtain (60) attached to the periphery of the hood (28), so as to define a space between the two curtains (58, 60), this space (34) surrounding the dynamic confinement zone and the blowing means (36) being arranged to blow gas into the space (34) thus created. 9. System according to claim 7 or 8, characterized   in that at least one of the curtains (58, 60) is made of vinyl.   10. System according to claim 2, characterized in that it further comprises a mechanical frame (84) on which are mounted the hood (28), the extraction means (94) and the   purification means (40, 42) thereby constituting a machine (76).   11. System according to claim 10, characterized in that said machine (76) is movable using means of handling (78).   12. System according to claim 10, characterized in that it further comprises at least one weighing element (87) mounted   on said machine (76) to increase the stability of the machine   this. 13. System according to claim 12, characterized in   said weighing element (87) is a shield.   14. System according to claim 10, characterized in that it further comprises means (90) for manipulating the   the tool mounted on said machine (76).   15. System according to claim 10, characterized in that it comprises means for supplying fluids mounted on said machine (76).   16. System according to claim 10, characterized in that it comprises an armored cell (86), able to accommodate a   operator, mounted on said machine (76).   17. A method of implementing the dismantling system according to claim 1, said ventilation system comprising a gas inlet (15) in the room (10) and a gas extraction network (18, 20) located at the interior of this room (10), characterized in that it comprises the following steps: (a) - blocking said gas inlet (15), and (b) - connecting said extraction means (36) gases present in the dynamic confinement zone to the gas extraction network (18, 20) present in the room (10). 18. Hood that can be used to create a dynamic confinement zone, this hood (28) having a periphery and a central part and being able to be placed above a determined location, characterized in that it comprises: - means (36) ) blowing gas at its periphery arranged to create a curtain of gas around said location, thereby participating in the creation of a dynamic confinement zone; and means for extracting the gases present in said dynamic confinement zone, the extraction being carried out by the central part of the hood (28).   19. Hood according to claim 18, characterized in that it comprises, all along its periphery, a double wall (30, 32) defining a space (34) communicating with the outside, the blowing means (36). being willing to   so as to inject gas into this space (34).   20. Hood according to claim 18, characterized in that it further comprises means for purifying gases thus extracted. 21. Hood according to claim 20, characterized in that said purification means comprise: - at least one compartment (96) inside the hood (28), this compartment (96) communicating on the one hand with said confinement zone by an opening (108) and secondly with a gas suction means (94) mounted on the hood (28), and - at least one filter (40, 42) placed inside the this compartment (96) so as to filter all the gas that passes through this compartment (96) from said opening (108) to the suction means (94).   22. Hood according to claim 21, characterized in that said compartment (96) comprises: - a first chamber (102) communicating with the confinement zone by said opening (108), and - a second chamber (104) separated from the first (102) by a partition (106) arranged to leave a passage for the gas between the two chambers (102, 104), the second chamber (104) communicating on the one hand with said suction means (94) and on the other hand with the first chamber (102) through said passage, at least one of the chambers (102) being provided with at least one filter (40) positioned to filter all of the gas crosses this room (102).   23. Hood according to claim 18, characterized in that it comprises at least one curtain (58) fixed at its periphery and   surrounding said dynamic containment zone.   24. hood according to claim 23, characterized in that it further comprises a second curtain (60) fixed at its periphery so as to define a space between the two curtains (58, 60), this space surrounding said confinement area dynamic and said blowing means (36) being arranged   to inject gas into the created space.   Hood according to claim 23 or 24, characterized in that   at least one of the curtains (58, 60) is vinyl.