EP2074632B1 - Novel shielded container structure for the transport and storage of a radioactive source for medical use - Google Patents

Abstract

In a transport container (1) for a vessel (2) containing medical radioactive material, with a radiation-shielding body (3) consisting of an enclosure with side-walls (3a), a base (3b) and a top opening (4) sealable by a shielded lid (5) using a bolting system (11), the bolting system is activated by translational movement of the lid along the opening axis towards the base. The lid has a bolting system deactivator (16) to allow separation from the container body.

Description

The present invention relates to a new shielded container structure made of radioprotective material, for the transport and storage of a radioactive source for medical use.

In certain medical specialties and particularly in oncology, the patient is sometimes given a determined dose of a suitable radioactive substance for the purpose of diagnosis or treatment.

To comply with safety and radiation protection standards, these radioactive substances must be transported and stored in armored containers, which have the advantage of protecting any person likely to come into their immediate environment, whether patients or hospital staff, from radiation. , the individuals in charge of his transport, etc.

Current containers are generally composed of an armored body made of radioprotective material, provided with an opening for the entry and exit of the bottle containing the radioactive source.

A shielded cover, also made of radioprotective material, is used to close this container and to stop the radiation passing through its opening.

In some containers, the cover is simply placed on the opening of the shielded body, without particular locking means; this is of course a possible source of incident.

This is why, in most cases, the lid is held on the container body by means of locking means (in particular by screwing).

The document US 2005/107698 discloses a container having the features of the preamble of claim 1.

However, the current locking means are complex to implement; they further require the operator to firmly hold the container body during the installation and removal of the cover for their activation or deactivation. During these closing / opening operations, the person handling the lid and the container body eventually undergoes radioactive radiation which may not be negligible.

To overcome these drawbacks, the Applicant has developed a container in which the lid can be locked very simply and very quickly on the container body, without forcing the operator to hold this container body for a long time and firmly.

According to the invention, the locking means in question have been shaped to be activated by a translational movement of the cover at the level of the opening of the container body, called "locking movement", along the axis of said opening and towards the bottom element of the container, which cover is further equipped with means for deactivating said locking means, to allow the separation of said shielded lid with respect to said container body.

According to an interesting embodiment, the cover is provided with a plurality of retractable protruding lugs adapted to be housed by a snap-type phenomenon within at least one complementary housing formed at the opening of the body of the body. container.

• une position de verrouillage, dans laquelle ils pénètrent dans ledit logement par un phénomène de type encliquetage lors du mouvement en translation de verrouillage, et
• une position rétractée, mise en oeuvre par les moyens de désactivation, dans laquelle ils se situent hors de l'encombrement dudit logement, permettant une dissociation du couvercle blindé par rapport au corps de conteneur.

These protruding lugs are operable between two positions:

• a locking position, in which they penetrate into said housing by a snap-type phenomenon during the locking translation movement, and
• a retracted position, implemented by the deactivation means, in which they are outside the bulk of said housing, allowing dissociation of the shielded lid relative to the container body.

In addition, these projecting lugs cooperate, on the one hand, with spring means shaped to tend to maintain them in the locking position, and on the other hand, with a deactivating member controllable by an operator for manipulating said lugs protruding in their retracted position.

In this embodiment, the lugs are advantageously in the form of hooks articulated at one of their ends and whose other end is projecting inwards so as to cooperate with an annular groove open towards the outside and arranged at the opening of the container body. The projecting hooks cooperate with the operator-controllable deactivating member to pivot them outward in their retracted position.

• une position au repos, dans laquelle les crochets sont en position de verrouillage, et
• une position de désactivation, obtenue par le pilotage de l'opérateur, dans laquelle sa partie évasée vient pousser lesdits crochets de sorte à provoquer leur pivotement vers l'extérieur en position rétractée.

According to another characteristic, the deactivating member consists of a part comprising an outwardly flared portion forming a cam, said deactivating member being movable within the cover between two positions:

• a rest position, in which the hooks are in the locked position, and
• a deactivation position, obtained by the control of the operator, in which its flared portion pushes said hooks so as to cause their pivoting outwards in the retracted position.

In this case, the cover is advantageously equipped with a handle whose operation by the operator ensures the displacement of the deactivating member so as to control the displacement of the projecting members between the deployed positions and folded; alternatively, the deactivating member is operable in the deactivation position, corresponding to the retracted position of the hooks, by application by the operator of a magnetic field generated by means of a magnetic handle attached to said cover.

Still in this case, the deactivating member is advantageously in the general form of a ring operable in axial translation, between the lower rest position and the upper deactivation position; this deactivating member is provided with a lower flange flared outwards to form the operating cam acting on the hooks by means of support rollers, each carried by one of said hooks, the latter being associated with elastic return means in the form of an elastic ring tending to keep them in the active locking position.

Still according to the invention, the container body consists of a shielded inner part fitted into an outer portion also shielded, these two shielded portions being each composed of a sidewall belt associated with a bottom member. The inner part, on the radioactive source side, is made of a radioprotective material having radioprotective characteristics greater than those of said outer part; the thickness of these inner and outer parts depends on the level of radiation protection sought.

According to an interesting embodiment, the container body comprises an inner portion of tungsten, nested in an outer portion made of lead; in this case, the closure cover is locked on the inner part, which has the advantage of being stronger and less malleable than lead.

According to an additional feature, the container body further comprises a peripheral shell of plastic material, covering the surface of its shielded outer portion.

This plastic shell advantageously consists of a belt of side walls whose lower edge is associated with a bottom element and whose upper edge is extended by an upper return covering at least the upper edge of the shielded outer portion; the bottom element and the lower edge of the sidewall belt of the plastic shell are provided with complementary structural means allowing their assembly by permanent fitting or removable.

This plastic shell has the advantage of being inexpensive, and thus to be replaced when it is degraded; it is also an envelope of protection for the outer lead part of the container, which is relatively sensitive to physical impacts such as shock or friction.

• la figure 1 est une vue en coupe transversale d'un conteneur selon l'invention, comprenant un corps de conteneur obturé par un couvercle du type déverrouillable au moyen d'une poignée magnétique ;
• les figures 2 et 3 sont des vues générales du couvercle d'obturation de la figure 1, avec des perspectives orientées respectivement par-dessous et par-dessus ;
• la figure 4 est une vue de dessus du couvercle d'obturation des figures 1 à 3 ;
• la figure 5 est une coupe brisée selon les plans sécants V-V de la figure 4 ;
• la figure 6 est une vue schématique partielle et en coupe plane du conteneur de la figure 1, dont le couvercle est représenté ici juste avant son verrouillage sur le corps blindé ;
• la figure 7 est encore une vue partielle et en coupe du conteneur conforme aux figures 1 et 6, dans laquelle les moyens de verrouillage du couvercle d'obturation sont manoeuvrés en position désactivée au moyen d'une poignée magnétique, pour autoriser la séparation dudit couvercle par rapport au corps blindé ;
• la figure 8 est une vue de dessus d'une autre forme possible de conteneur, dont le couvercle d'obturation est muni d'une poignée pour la manoeuvre par un opérateur de ses moyens de verrouillage ;
• la figure 9 est une vue en coupe brisée du conteneur de la figure 8, selon les plans sécants IX-IX, et dans laquelle le couvercle d'obturation est représenté juste avant son verrouillage sur le corps de conteneur ;
• la figure 10 est une vue partielle du conteneur de la figure 9, dans laquelle le couvercle d'obturation est verrouillé sur le corps de conteneur ;
• la figure 11 est encore une vue partielle du conteneur de la figure 9, dans laquelle les moyens de verrouillage sont pilotés en position rétractée par la manoeuvre de la poignée.

The invention will be further illustrated, without being limited in any way, by the following description of two embodiments given solely by way of example, and shown in the accompanying drawings in which:

• the figure 1 is a cross-sectional view of a container according to the invention, comprising a container body closed by a lid of the unlockable type by means of a magnetic handle;
• the Figures 2 and 3 are general views of the shutter cover of the figure 1 , with perspectives oriented respectively from below and from above;
• the figure 4 is a top view of the shutter cover Figures 1 to 3 ;
• the figure 5 is a broken section according to the secant planes VV of the figure 4 ;
• the figure 6 is a partial schematic and sectional view of the container of the figure 1 , whose cover is shown here just before locking it on the armored body;
• the figure 7 is still a partial view in section of the container according to figures 1 and 6 , in which the locking means of the shutter cover are operated in the deactivated position by means of a magnetic handle, to allow the separation of said cover relative to the shielded body;
• the figure 8 is a top view of another possible form of container, whose closure cover is provided with a handle for the operation by an operator of its locking means;
• the figure 9 is a broken section view of the container from the figure 8 , according to secant planes IX-IX, and wherein the closure cap is shown just before locking it on the container body;
• the figure 10 is a partial view of the container from the figure 9 in which the shutter cover is locked on the container body;
• the figure 11 is still a partial view of the container from the figure 9 , in which the locking means are controlled in the retracted position by the operation of the handle.

The shielded container 1, shown schematically and in section on the figure 1 , is suitable for the transport of a container 2 containing a radioactive substance, intended for example for a medical use.

This container 1 comprises a cylindrical body 3, made of radioprotective materials (namely the combination of a thickness of tungsten and a lead thickness). This container body 3 is composed of a cylindrical side wall 3 is associated with a bottom element surface 3 b; the upper edge 3 c of the cylindrical wall 3a delimits a circular top opening 4, closed on this figure 1 by a shutter cover 5 also made of radioprotective material. According to the invention and as detailed below, the shutter cover 5 comprises locking means with the container body 3, shaped to be activated by a translational movement of the cover 5 at the opening 4, said "locking movement", this along the axis of this opening 4 and towards the bottom element 3b facing; this cover 5 is further equipped with means for deactivating its locking means to allow separation of said shielded cover 5 with respect to said container body 1 (always by translation).

In this case, the closure cover 5, shown in isolation on the Figures 2 to 5 , is mainly made of tungsten.

This cover 5 has a generally circular shape, of central axis 6; it comprises a main body 7 comprises a lower portion 7 has a generally cylindrical shape and an upper part 7b of frustoconical general shape surmounted by a head 8 consists of a circular central portion 8a, the edge is extended by an annular skirt 8 b extending opposite and away from the main body 7.

The lower part 7a of the main body 7 comprises an O-ring 7 c, for sealing the lid 5 when locked to the container body 3.

This lid 1 is equipped with means for locking it on the container body 3.

These locking means are composed here of four mobile lugs 11 in the form of hooks, regularly distributed in the space between the main body 7 and the skirt 8b facing.

• une extrémité supérieure 11a, portée par un anneau 12 (visible figure 5), avec une possibilité d'articulation en rotation autour d'un axe 13 (perpendiculaire à l'axe longitudinal 6 du couvercle 5), et
• une extrémité inférieure 11b, présentant d'une portion saillante vers l'intérieur (c'est-à-dire ici orientée en direction de l'axe longitudinal 6), destinée à venir s'insérer dans un logement du corps de conteneur 3 pour assurer le verrouillage du couvercle 5.

The locking hooks 11 are each in the form of an elongated piece having two ends:

• an upper end 11a, carried by a ring 12 (visible figure 5 ), with a possibility of articulation in rotation about an axis 13 (perpendicular to the longitudinal axis 6 of the cover 5), and
• a lower end 11b having an inwardly protruding portion (i.e., oriented in the direction of the longitudinal axis 6) for insertion into a housing of the container body 3 to secure the lid 5.

These hooks 11 also each carry a support roller 14 mounted to rotate freely about an axis 14a parallel to its axis of rotation 13, for reducing friction during the operation of the hooks 11.

• une position de verrouillage, correspondant aux figures 1 et 5, dans laquelle lesdits crochets 11 sont aptes à assurer le verrouillage du couvercle 5 sur le corps de conteneur 3, et
• une position rétractée, détaillée par la suite en relation avec la figure 7, dans laquelle lesdits crochets 11 sont désactivés pour permettre la séparation du couvercle 5 par rapport au corps de conteneur 3.

These locking hooks 11 are operable around their axis of rotation 13 between two positions:

• a locking position corresponding to the figures 1 and 5 wherein said hooks 11 are adapted to lock the lid 5 on the container body 3, and
• a retracted position, further detailed in connection with the figure 7 in which said hooks 11 are deactivated to allow separation of the lid 5 with respect to the container body 3.

Elastic return means 15 are associated with the hooks 11 to tend to keep them in the locking position.

These return means 15 are here in the general form of an elastic ring.

The operation of the hooks 11 from their locking position to their retracted position is implemented by the movement of a movable part 16 fitted to the cover 5, called "deactivating member", controllable by the operator.

The deactivation member 16 in question is in the general shape of a ring having a lower edge 16 has flared outwardly and on which the roller 14 of the hooks 11 is kept pressed by the action of their means of elastic return 15.

The lower edge 16a of the deactivation member 16 forms an operating cam hooks 11 between their locking position and their unlocking position.

• l'une inférieure de repos (figures 1 et 5), maintenue par des moyens de rappel élastique 17, dans laquelle les galets 14 des crochets 11 coopèrent avec le petit diamètre de la bordure inférieure 16a de sorte que ces crochets 11 soient en position de verrouillage,
• l'autre supérieure de désactivation (détaillée par la suite en relation avec la figure 7), obtenue par l'action de l'opérateur, dans laquelle la partie de grand diamètre de sa bordure inférieure 16a vient pousser les galets 14 de sorte à provoquer le pivotement vers l'extérieur des crochets 11 en position rétractée.

For this, this deactivating member 16 is operable in axial translation within the cover 5, between two positions:

• the lower one of rest ( figures 1 and 5 ) Held by elastic return means 17, in which the rollers 14 of the hooks 11 cooperate with the smaller diameter of the lower edge 16a so that the hooks 11 are in the locking position,
• the other superior of deactivation (detailed later in connection with the figure 7 ), obtained by the action of the operator, wherein the large diameter portion of its lower edge 16 has push the rollers 14 so as to cause the pivoting outward hooks 11 in the retracted position.

The elastic return means 17 may consist of spiral springs inserted in housings formed in the ring 16, interposed between said ring and the cover head 8.

Moreover, still on the figure 1 it can be seen that the cover 5 is locked on a particular structure of the container body 3.

• une surface inclinée externe 20a, dans laquelle est ménagée une gorge annulaire 21 ouverte vers l'extérieur et conformée pour recevoir l'extrémité inférieure 11b des crochets 11 ;
• une surface inclinée interne 20b, destinée à épouser la partie tronconique 7b du couvercle d'obturation 5.

In this case, the upper edge 3c of the container 1 here mainly comprises an annular extension 20 with a generally triangular section, with:

• an outer inclined surface 20a, in which is formed an annular groove 21 open towards the outside and shaped to receive the lower end 11b of the hooks 11;
• an inner inclined surface 20b intended to fit the frustoconical portion 7b of the closure cover 5.

The locking and unlocking operations of the closure cover 5 on the container body 3 are described below in connection with the figures 1 , 6 and 7 .

First, in relation to the figure 6 , the locking procedure consists in properly positioning the closure cover 5 opposite the opening 4, and resting on the upper edge 20.

In this situation which precedes the locking, it can be seen that the hooks 11 are in the locking position (as described previously in relation to the figure 5 ), And that their lower end 11b is supported on the outer inclined surface 20a of the upper edge 20 above.

Then, the operator exerts a vertical pressure in the axis of the opening 4, in the direction of the bottom element 3b of the container body 3 (symbolized on the figure 6 by the arrow 22).

With the advancement of the cover 5 in the opening 4, the hooks 11 slide on the outer inclined surface 20a, thereby causing them to pivot outwards. Then, the hooks 11 automatically return to the initial position under the effect of their elastic return means 15, that when their lower end 11b comes opposite the housing 21; this particular movement of the hooks 11 for their establishment is a snap type movement.

The lid 5 is then properly locked at the top edge 3c of the container 3, and effectively closes the opening 4 of the container body 3 ( figure 1 ).

In this position, the main part 7 and the skirt 8b of the cover 5 are positioned on either side of the extension 20 of the body 3; thus, these two baffle elements are interlocked, which further contributes to optimizing the radioprotective efficiency of the container 1.

To unlock the cover 5 ( figure 7 ), the operator uses an attached means 24 emitting a magnetic field, shaped for example magnetic handle, for causing the control of the deactivating member 16 described above in its deactivation position.

For this, in practice, the operator positions his magnetic handle 24 in contact with the surface of the cover 5.

Once in place, the magnetic field emitted by this handle 24 causes the displacement of the deactivating member 16 upwards, in its upper position called deactivation. The lower edge 16a of the deactivation member 16 then pushes the rollers 14 outwardly, thereby obtaining the distance in the unlocked position of the hooks 11 associated.

The magnetic handle 24, secured to the cover 5 by magnetization, then allows the separation of this cover with respect to the container body 3.

Of course, when the handle 24 is separated from the cover 5, the deactivating member 16 returns to its rest position under the effect of its resilient return means 17; the hooks 11 return to the locking position under the action of their associated spring 15.

Thus, this container structure allows a simple, fast and effective locking of the lid on the container body.

In addition, this container is interesting in terms of safety, since the operator must report additional resources to open the lid, thus limiting the risk of unintentional opening.

On the figures 1 , 6 and 7 the particular structure of the container body 3 is still noted. This body 3 consists of an inner part 29 made of tungsten, nested in an outer portion 30 made of lead; these two armored parts are each composed of a sidewall belt associated with a bottom element. Thus, the inner portion 29, on the radioactive source side, is made of a radioprotective material having radioprotective characteristics greater than those of the outer portion 30, the thickness of these inner and outer portions 29 being a function of the radiation protection level. research.

The two parts 29 and 30 are here held together by means of two cylindrical through-tungsten inserts 33; the outer free end 33 a of these inserts also serves to hold, by appropriate means, the two free ends with a handle 34 for transporting the container 1.

The extension 20 of the container body, on which the closure cap 5 is locked, is integral and constituent of said inner tungsten portion 29.

The locking hooks 11 on the inner portion 29 allows to use its characteristics of resistance to wear and friction; this locking also allows a continuity of tungsten shielding, which further optimizes the quality of radiation protection obtained.

In addition, it is noted that the reinforced portion of lead 30 includes an upper extension just surround the outer surface of the skirt cover 8 b.

To be complete, the body 3 further comprises a peripheral shell 31 of plastic material, covering the surface of its shielded outer portion 30.

This plastic shell 31 consists of a cylindrical wall 31a whose lower border is associated with a bottom element 31b and whose upper edge is extended by an upper return 31c covering the upper edge of the shielded outer part 30.

The base member 31b and the lower edge of the side wall 31a of the plastic shell 31 are provided with complementary structural means 32, type grooves / ribs, allowing their assembly by interlocking (removable or not). The possibility of disassembly in question may be useful to facilitate the replacement of this outer shell when it is worn or damaged.

In general, this particular structure of the container body 3, combining a tungsten shield and a lead shield, can be used with a cover not having the particular locking means described above.

The Figures 8 to 11 show another possible form of a container according to the invention, wherein the closure cover is provided with locking means deactivatable by mechanical means, in this case a handle provided for this purpose.

This container 35 is similar to that described above in relation to the Figures 1 to 7 .

Thus, there is a container body 3 identical to that already described above. Concerning the cover 36, there are movable hooks 11, each carrying a roller 14, and operable by the deactivating member 16; this cover 36 differs only from the previous one in that it is provided with an articulated handle 37 allowing the mechanical operation of the deactivating member 16.

More specifically, the handle 37 carries a transverse axis 38 from which extend two vertical rods 39 fixed to the deactivating member 16.

It is the operation of the handle 37 which here allows the displacement of the deactivating member 16, and the movement of the hooks 11 between the locking and unlocking positions.

In practice, for the closure of the body 3, the handle 37 is folded against the cover 36, which corresponds to the lower rest position of the deactivating member 16 and the locking position of the hooks 11 ( Figures 8 and 9 ). Then the lid 36 is positioned facing the opening 4 of the container body 3 ( figure 9 ), then the operator pushes the cover 36 towards the bottom element 3b until the hooks 11 are housed by snapping into the housing 21 of the body 3 ( figure 10 ). The lid 36 is thus suitably locked to the body 3.

Conversely, to separate the lid 36 from its body 3, it is sufficient for the operator to maneuver the handle 37 upwards, as shown in FIG. figure 11 .

During this maneuver, the transverse axis 38 moves upwards, under the leverage effect generated by the support of the two lateral extensions 40 against the upper face of the cover 36, causing axial traction on the vertical rods 39 which ensure in turn a movement of the deactivating member 16 in its upper disabling position.

Finally, the hooks 11 end in the unlocked position, and disassociate housing 21; the operator can then separate the lid 36 with respect to the container body 3.

This latter embodiment has the advantage of being relatively simple and does not require the use of added means to unlock the cover 36.

Claims (11)

1. A container for transporting a receptacle (2) containing a radioactive substance for medical use, said container (1) comprises on the one hand a body (3) shielded against the emission of radiation, made up of a belt of side walls (3a) connected to a base element (3b) and defining a top opening (4), and on the other hand a shielded closing lid (5, 36) attachable to said top opening (4) by locking means (11), said locking means (11) being shaped so as to be activated by a translational movement (22) of said lid (5, 36) in said top opening (4), this being termed the "locking movement" and taking place along the axis of said opening (4) and towards said opposing base element (3b), characterized in that said lid (5, 36) is further equipped with means (16) for deactivating said locking means (11) in order to enable said shielded lid (5, 36) to be separated from said container body (3).
2. The shielded container according to claim 1, characterized in that the lid (5, 36) has a plurality of retractable projecting studs (11) designed to fit by a snap-action type phenomenon into at least one complementary housing (21) formed at the level of the opening (4) of the container body (3), said projecting studs (11) being operable between two positions:

   - a locking position, in which said projecting studs (11) enter said housing (21) by a snap-action type phenomenon during the locking translational movement, and

   - a retracted position, actuated by the deactivation means (16) and in which said projecting studs (11) are located outside the bulk of said housing (21), enabling the shielded lid (5, 36) to be dissociated from the container body (3),

   and in that said projecting studs (11) cooperate on the one hand with spring means (15) shaped so as to tend to keep the former in their locking position, and on the other hand with a deactivation member (16) controllable by an operator to operate said projecting studs (11) to their retracted position.
3. The shielded container according to claim 2, characterized in that the studs (11) are in the form of hooks hinged at one of their ends (11a), and the other end (11b) of which projects inwardly so as to be able to cooperate with an annular groove (21) opened outwardly and formed at the level of the opening (4) of the container body, said projecting hooks (11) cooperating with the deactivation member (16), controllable by the operator, to make the former pivot outwardly to the retracted position.
4. The shielded container according to claim 3, characterized in that the deactivation member (16) consists of one piece having an outwardly flared part (16a) forming a cam, said deactivation member (16) being movable inside the lid (5, 36), between two positions:

   - a rest position, in which the hooks (11) are in the locking position, and

   - a deactivation position, obtained through an operator control action and in which said flared part (16a) pushes said hooks (11) so as to make them pivot outwardly to the retracted position.
5. The shielded container according to any one of claims 2 to 4, characterized in that the lid (36) is equipped with a handle (37) whose operation ensures the displacement of the deactivation member (16) so as to control the displacement of the protruding members (11) between the unfolded and folded positions.
6. The shielded container according to any one of claims 2 to 4, characterized in that the deactivation member (16) is operable to the deactivation position, corresponding to a retracted position of the hooks (11), through application by the operator of a magnetic field generated by means of a magnetic handle (24) added on the lid (5).
7. The shielded container according to any one of claims 3 to 6, characterized in that the deactivation member (16) is in the general form of a ring operable in an axial translational movement between the lower rest position and the upper deactivation position, said deactivation member (16) being provided with an outwardly flared lower edge (16a) to form the operating cam acting on the hooks (11) through pressing rollers (14), each of which is carried by one of said hooks (11), the latter being also connected to elastic return means (15), in the form of an elastic ring tending to keep them in the active locking position.
8. The shielded container according to any one of claims 1 to 7, characterized in that its body (3) is made up of a shielded inner part (29) nested into an also-shielded outer part (30), said shielded parts (29, 30) being each made up of a belt of side walls connected to a base element, and in that said inner part (29), on the side of the radioactive source, is made of radioprotective material having radioprotective characteristics higher than those of said outer part (30), the thickness of these inner (29) and outer (30) parts being a function of the required level of radioprotection.
9. The shielded container according to claim 8, characterized in that the body (3) comprises a tungsten inner part (29) nested into a lead outer part (30), the closing lid (5, 36) locking onto said inner part (29).
10. The shielded container according to any one of claims 8 or 9, characterized in that the body (3) also comprises a peripheral shell (31) made of a plastic material, covering the surface of the shielded outer part (30) thereof.
11. The shielded container according to claim 10, characterized in that the plastic shell (31) is made up of a belt of side walls (31a), the edges of which are connected to a base element (31b) and to an upper return (31c) covering at least the upper edge of the shielded outer part (30), and in that said base element (31b) and the lower edge of the belt of side walls (31a) of the plastic shell (31) are provided with complementary structural means (32) enabling them to be assembled by permanent or detachable nesting.

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