EP0877388B1 - Sealed closure system for multi-purpose confinement container for irradiated nuclear fuel assemblies of high activity waste material - Google Patents

Description

TECHNICAL AREA

The invention relates to a device for sealing a housing multi-use containment, for example for fuel assemblies irradiated nuclear or high-level waste, using seals.

STATE OF THE ART

A transport and storage technique for fuel assemblies irradiated nuclear power is under development.

• à introduire un boítier métallique à parois minces et à ouverture totale à une de ses extrémités, dans un suremballage métallique à parois épaisses également fermable hermétiquement; ledit boítier permet d'assurer le confinement de la matière radioactive tout en présentant une résistance mécanique suffisante. Le suremballage épais assure un blindage radioactif suffisant et l'essentiel de la tenue mécanique pour que l'ensemble du colis ainsi constitué par le suremballage, le boítier de confinement et le contenu de matières radioactives qu'il est prévu d'y placer, soit transportable, c'est-à-dire puisse répondre aux spécifications règlementaires en cas d'accident sévère (par exemple chute),
• à introduire l'ensemble conteneur, c'est-à-dire le suremballage et le boítier, dans la piscine où sont entreposés les assemblages combustibles nucléaires irradiés à transporter,
• à charger lesdits assemblages combustibles irradiés dans le boítier,
• à mettre en place le couvercle du boítier de façon à assurer le confinement de la matière radioactive et à vidanger le boítier,
• à effectuer le séchage de la cavité du boítier éventuellement hors piscine, et à fermer hermétiquement le colis résultant.

It consists schematically:

• to introduce a metal case with thin walls and full opening at one of its ends, in a metal overwrap with thick walls also hermetically sealable; said housing ensures the confinement of the radioactive material while having sufficient mechanical strength. The thick overwrap provides sufficient radioactive shielding and most of the mechanical strength so that the entire package thus constituted by the overwrap, the containment box and the content of radioactive materials that it is intended to place there, or transportable, i.e. can meet regulatory specifications in the event of a severe accident (e.g. fall),
• to introduce the container assembly, that is to say the overpack and the casing, into the swimming pool where the irradiated nuclear fuel assemblies to be transported are stored,
• loading said irradiated fuel assemblies into the housing,
• to set up the cover of the box so as to ensure the confinement of the radioactive material and to empty the box,
• to dry the housing cavity possibly outside the pool, and to seal the resulting package.

Said package can then be transported safely and brought to its destination. The containment box will then be removed from the overpack to be stored at the desired location.

The multi-purpose containment box is also known as Anglo-Saxon term of “multi-purpose canister” (MPC) and overpackaging under the term of “over-pack” (OP).

Usually the fully open end of the case is closed hermetically either by means of a welded cover or, reversibly, by using tight seals between a cover and a flange fixed at the end open said housing.

Figure 1 illustrates such a reversible closure device by seals.

We see in (1) the thin wall of the multi-use containment box (MPC), in (2) the interior cavity of the case in which the assemblies irradiated nuclear fuels are introduced, at (3) the flange fixed to the upper end with total opening of the case, in (4) the cover comprising two seals (5), in their housings, ensuring sealing after closing with bolts (6).

With such a closure device, we see that after introduction of the housing in the overpack, the flange (3) rests on the upper end of the overpack (7) (OP).

Thus the introduction as the extraction of said housing with tight closure reversible, respectively in and out of the overpack, cannot be than by said upper end of the overpack, which implies either a shielded cell or device, i.e. a swimming pool operation, in order to have the radiological protection that the case (MPC) does not have.

To facilitate the maneuver of removing the overpack in a vertical position and avoid the use of such a shielded device intermediate between the overpack (OP) and the desired storage location, it is advantageous to use an overpack having a total opening not only at its upper end, but also at its lower end; thus after having removed the bottom of the overpack and having brought the package directly above the storage place, one can extract the case loaded with fuel assemblies irradiated by the lower end of said overpack by sliding one with respect to the 'other.
It is obvious that the presence of a flange on the housing prevents this type of handling where the housing is introduced into the overpack by its upper end and is extracted from it by its lower end.

It is therefore essential to facilitate handling to have a housing having a perfectly smooth outer wall, in particular without flange. The plaintiff has thus sought to develop such a housing which has simultaneously a waterproof and reliable reversible closure, but also simple to to realize or to implement and containing no element exceeding the outer cylindrical wall of the housing.

DESCRIPTION OF THE INVENTION

• au moins deux disques pleins métalliques amovibles entièrement situés à l'intérieur dudit corps tubulaire, prenant appui directement ou par l'intermédiaire d'un disque ou couronne d'appui, sur un épaulement pratiqué dans la paroi intérieure du corps tubulaire sur tout ou partie de sa périphérie, à proximité de son extrémité supérieure, chacun desdits disques pleins métalliques comportant des moyens d'étanchéité situés à leur périphérie coopérant avec une partie verticale de la paroi intérieure du corps tubulaire,
• un dispositif de serrage, situé également à l'intérieur du corps tubulaire au-dessus desdits disques pleins métalliques et agissant sur eux, permettant de réaliser l'étanchéité.

The invention is a removable leaktight closure device for a multi-use metallic containment box (MPC) for irradiated nuclear fuel assemblies according to claim 1. The box essentially consists of a tubular or cylindrical body, generally of circular section. , having a major axis, with a lower end closed off by a welded bottom and with a fully open upper end, said case being intended to be entirely included in fully open, said case being intended to be entirely included in a metallic overpack with thick walls, said sealed closure device comprising:

• at least two removable solid metal discs entirely located inside said tubular body, bearing directly or by means of a bearing disc or crown, on a shoulder formed in the interior wall of the tubular body over all or part from its periphery, near its upper end, each of said solid metal discs comprising sealing means situated at their periphery cooperating with a vertical part of the inner wall of the tubular body,
• a clamping device, also located inside the tubular body above said solid metal discs and acting on them, making it possible to achieve sealing.

The solid discs are thus stacked on top of each other and sealing takes place in particular between the sealing means situated on the periphery of the solid metal discs and the vertical wall of the tubular body as will be seen in more detail below.
The support disc can also be a crown. In addition, the lowest bottom solid disc comprising sealing means can bear directly on said shoulder without the intermediary of the support disc or crown.

Generally, and as will be seen below in detail, the means seals include O-rings which are compressed at using the clamping device which may be of known type.

The thickness of the tubular body of the housing (MPC) is thin, generally understood between 1 and 10 cm, but most often between 2 to 5 cm and despite this the device according to the invention ensures the tightness of the housing in the time. In comparison, the thick walls of the overpack (OP) are generally of the order of 10 to 30 cm, but most often between 15 and 25 cm. On the other hand, the order of magnitude of the diameter of the case (MPC) is between 0.5 m and 2.5 m (most often between 1.5 m and 2 m) and its length, linked to that of the fuel assemblies contained, can vary between 3 and 5 m.

Solid discs and the supporting disc or crown generally have a thickness of at least 15 mm which is linked to the thickness of the joint and allows avoid any risk of deformation during tightening ensuring tightness. Their thickness is usually less than 50 mm to avoid any harmful weighting of the container or to leave more space available to combustible elements introduced into the cavity.

The closure device according to the invention may additionally comprise devices for testing for leaks. In this regard, employment according to the invention of at least two solid metal discs with their means sealing makes it possible to control and therefore to guarantee said sealing.

Such a closure device made up of at least two elements (i.e. a metal disc and a clamping device) can be crossed by a orifice with its own sealing system comprising for example two controllable seals.

Figure 2 illustrates the invention. It appears particularly simple to perform or to be implemented and the entire container has an outer wall perfectly cylindrical, smooth and without roughness over its entire length.

In Figure 2 we see in (1) the thin wall of the tubular body of the housing multiple-use containment (MPC), in (2) the interior cavity intended for receive irradiated fuel assemblies stored in a basket appropriate. The upper end (10) of the tubular body may likewise be thickness than the body, but here it was reported by welding and presents a slight allowance towards the inside of said tubular body; it presents, of same as the tubular body, a circular cross section except possibly in places, for example at the point of specific cuts or when the support shoulder (11) does not occupy the entire periphery of the inner wall of the tubular body, as will be seen later; the walls external here vertical of the tubular body and its end possibly reported (10) are aligned so that they have no part protruding outside the housing may interfere. The internal allowance, possible, gives the upper end (10) of the housing a greater rigidity so that the guarantee of tightness of the device according to the invention is finds improved without decreasing the capacity of said housing.

The upper end (10) has a shoulder (11) formed in the interior wall; this shoulder can be practiced on all or part only from the periphery of said interior wall. It is such that the disc or support ring (12) can rest stably there; the disc or crown support is circular and has an upper face, a lower face and a cylindrical side wall.

On the upper face of the disc or circular support crown (12) takes places a first circular full disc (13), which solid disc (13) comprises at its periphery sealing means comprising a chamfer (14) made in place of its lower peripheral edge and a most often metallic O-ring (15) taking place in the housing created by said chamfer (14). The metal seal can be of the Helicoflex type (registered trademark) or any other metallic seal with properties similar.

Another embodiment would consist in that the lower disc (13) bears directly on the shoulder (11).
Sealing is ensured by the metal seal (15) between the cylindrical vertical inner wall with circular section of the end piece (10) [or of the end of the tubular body (1) in the case where there is would have no specific end piece added] and either of the surface of the chamfer or of the horizontal upper face of the support disc.

This embodiment with a support disc is particularly interesting because it makes it possible to compress the seal (15) and to guarantee sealing when the shoulder (11) is not produced over the entire periphery of the wall interior, thanks in particular to the fact that said seal is obtained at less in part by pressing the seal on the vertical part of the inner wall.

To have a better guarantee of tightness and to be able to better control it, a second full disc (13 ') identical to the disc (13), with its chamfer (14') and its metal seal (15 '), is placed on the face upper disc (13); the seal (15 ′) is likewise compressed by reaction on the upper face of the lower solid disc (13) on which it rests.
The clamping device, not shown, is located entirely inside the end piece (10) [or, where appropriate, the tubular body (1)]. Generally it is of known type and comprises clamping means cooperating with the inner wall which allow to obtain the tightness of the closure of the housing by compression of the seals (15, 15 ') by pressing vertically on the upper disc.

We see that by tightening we will compress the joints (15, 15 ') to obtain a sealing between the chamfers of the solid discs (13) and (13 ') and the wall inside of the end piece (10) [or where appropriate the end of the tubular body (1)].

In a simple and modular way the number of full discs can be increased on demand to further improve the degree of guarantee of sealing.

It appears that the reversible closure device according to the invention is simple to realize, authorizes the passage of the entire housing (MPC) through a overwrap (OP) from one end to the other and allows to reach simultaneously high sealing and mechanical resistance meeting regulatory safety requirements.

Claims (4)

1. Removable sealed closing apparatus for a metal, multi-purpose confinement canister for irradiated nuclear fuel assemblies, essentially constituted by a tubular body (1) having a major vertical axis, a lower end sealed by a welded base, a completely open upper end, which can be sealed by the removable sealed closing apparatus, and a bearing shoulder (11) formed on at least part of the periphery of the inner wall of the tubular body (1), said canister being intended for complete inclusion in a thick-walled, metal over-pack, said closing apparatus comprising:

   a first, removable, solid, metal disk (13) which can rest directly on said bearing shoulder or on an intermediate bearing collar (12) interposed between said bearing shoulder and the first disk, a first chamfer (14) being formed on the periphery of one face of the first disk (13) turned towards said shoulder,

   a first O-ring seal (15) placed in a first space defined between said chamfer (14), said inner wall and said bearing shoulder (11) or said intermediate bearing collar (12),

   a second, removable, solid, metal disk (13') able to rest directly on said first disk (13), a second chamfer (14') being formed on the periphery of a face of the second disk (13') turned towards said first disk,

   a second O-ring seal (15') located in a second space defined by said first disk (13), said second shoulder (14') and said inner wall and

   a gripping apparatus entirely located in the interior of the open end of the tubular body (1) and bearing on the second disk (13') so as to compress the second O-ring seal (15') and create a seal in said second space and so as to compress the first O-ring seal (15) and create a seal in said first space.
2. Apparatus according to claim 1, wherein the first O-ring seal (15) and the second O-ring seal (15') are metal seals.
3. Apparatus according to either of the claims 1 and 2, wherein the thickness of the tubular body (1) is between 1 and 10 cm and its diameter between 0.5 and 2.5 m and the thickness of the first disk (13) and the second disk (13') is at least 15 mm.
4. Apparatus according to any one of the preceding claims, wherein the tubular body has an outer wall with no projecting portion.

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