PACKAGING FOR TRANSPORT AND/OR STORAGE OF RADIOACTIVE MATERIALS, WHICH INCLUDE IMPROVED MEANS OF THERMAL CONDUCTION

TECHNICAL FIELD

The present invention relates to the field of packaging for the transport and/or storage of radioactive materials, preferably of the irradiated nuclear fuel assembly type.

STATE OF THE PRIOR ART

Traditionally, storage devices, also known as “baskets” or “racks”, are used for the transport and/or storage of radioactive materials. These storage devices, habitually cylindrical in shape and essentially circular or polygonal in section, are suitable for receiving radioactive materials. The storage device is intended to be housed in the cavity of a package in order to form jointly with the latter a container for the transport and/or storage of radioactive materials in which they are completely confined.

The aforementioned cavity is generally defined by a lateral body extending along a longitudinal axis of the package, together with a base and a packaging cover arranged at the opposing extremities of the body along the direction of the longitudinal axis. The lateral body includes an interior wall and an exterior wall, which generally assume the form of two concentric metal shells which jointly form an annular space inside which means of thermal conduction are housed, together with means of radiological protection, in particular in order to form a barrier against neutrons emitted by the radioactive material housed in the cavity.

The means of thermal conduction enable the heat released by the radioactive materials to be conducted towards the exterior of the container, in order to prevent any risk of overheating, which might cause damage to these materials, an impairment of the mechanical properties of the materials constituting the package, or again an abnormal rise in pressure in the cavity.

The means of thermal conductions have been the subject of numerous developments, which have led to various embodiments. One of the most widely used involves the placing of fins/ribs in the annular space between the two shells. These fins, which extend lengthways in the direction of the longitudinal axis of the packaging, therefore enable heat to be conducted from the interior shell towards the exterior shell. Furthermore, in this embodiment, traditionally there are radiological protection blocks inserted between the fins.

Despite being in widespread use, this solution using thermal conduction fins can prove to be problematical in that it can generate hot spots on the exterior shell of the lateral body of the package, at the junctions with these same fins.

Another solution, known in particular from document EP 1 355 320, partly addresses this problem of homogeneity of heat transfer by employing honeycomb structures. Nevertheless, the layout proposed in this document provides a less than perfect heat conduction capacity. Furthermore, it requires the use of thermal conduction fins in combination with the honeycomb structures, which complicates the design of the package.

DESCRIPTION OF THE INVENTION

The purpose of the invention is therefore to remedy, at least partially, the above-mentioned disadvantages, relating to the embodiments of the prior art.

In order to do this the object of the invention is a package for the transport and/or storage of radioactive materials, where said package includes a lateral body which defines a cavity for housing said radioactive materials which extends along a longitudinal axis of the package, where the body includes an interior wall together with an exterior wall which between them define a space which extends around said longitudinal axis, where said space houses means of radiological protection as well as means of thermal conduction. According to the invention, said means of thermal conduction include multiple thermal conduction elements each defining internally a void which extends lengthways in a direction of conduction which runs from the interior wall towards the exterior wall. Furthermore, at least a part of the thermal conduction elements, and preferably each of them, has a void filled at least partially by a radiological protection material, and preferably entirely filled with this material.

The specific orientation of the voids, together with the orientation of the thermal conduction elements running from it, confer improved thermal conduction capacity, in particular relative to the solution using honeycomb structures described in EP 1 355 320, in which the voids formed by the honeycomb cells are orientated parallel to the walls and to the longitudinal axis of the packaging. This means that thanks to these specific orientations in the present invention, the thermal conduction path defined by the conduction elements is shortened in relation to those encountered in the honeycomb structure in document EP 1 355 320, since it connects the two walls of the lateral body in a more direct manner. Furthermore, the thermal conduction path between the two walls does not suffer from multiple interruptions of the type encountered in the honeycomb structures of document EP 1 355 320, which result from the superimposition of the metal sheets which together form the cells of the honeycomb.

In addition, by its appropriate distribution and quantity of thermal conduction elements, the solution provided by the present invention enables the appearance of hot spots on the exterior wall of the lateral body to be easily prevented.

Finally, since the invention provides excellent heat transfer, it no longer requires the use of thermal conduction fins of the type used in the prior art. It is therefore of a simpler design.

Preferably at least some of said thermal conduction elements each extend in an essentially radial direction from the lateral body of the package, which is in effect the direction of the most direct path for linking the two walls of the lateral body. In this respect the radial direction must be understood as being the direction which locally intercepts, orthogonally, each of the two walls of the lateral body. Nevertheless the invention is not restricted to such a conduction direction, and the latter could be, for example, inclined in relation to a radial plane and/or in relation to a transverse plane.

Preferably at least some of the said thermal conduction elements each exhibit an essentially cylindrical form. However, the cylindrical form could be replaced by a form which increases in size going from the interior wall to the exterior wall, in particular in order to allow for the difference of the mean diameters between these walls. In such a case the geometry of the cross section of the element preferentially remains the same, with only the magnitude of the cross-section therefore changing.

As an illustrative example, the cross-section of the thermal conduction element may be circular or polygonal, such as square or hexagonal.

Preferably at least some of the said thermal conduction elements each extend lying together over a length essentially equal to the distance separating the interior and exterior walls, along the direction of conduction. This provides an uninterrupted thermal conduction path between the two walls, which favours good removal of heat. However, at least some of the thermal conduction elements could be cut into sections along the direction of conduction, that is to say, made of several lengths arranged end-to-end. This offers a specific benefit when the thermal conduction elements are intimately connected to a radiological protection material, for example so as to form blocks, as is preferentially the case in the invention. In effect, when it is necessary to replace only a part of the means of thermal conduction and/or the means of radiological protection, the above mentioned sectioning means that replacement blocks of smaller dimensions can be used, often more appropriate for the size of defects, thus reducing the loss of materials caused during these replacement operations.

Preferably, said thermal conduction elements together form a network of voids which, in cross section along at least one plane parallel to the longitudinal axis and traversing this network, offers at least one zone whose density of voids has a value greater than or equal to 100 voids/m². Such a high minimum density, which is preferably encountered in all the means of thermal conduction, enables excellent homogeneity of heat conduction to be achieved. It is also stated that this density may change within the means of thermal conduction.

Furthermore, the walls of the thermal conduction elements which delimit the voids may thin, which is favourable to a reduction in the risk of radiological leaks. Preferably, the mean thickness of the walls of the thermal conduction elements which delimit the voids is between 0.02 and 0.5 mm.

Preferably, each void exhibits, in a cross section orthogonal to the direction of conduction, a maximum width of between 2 and 25 mm, where this maximum width naturally corresponds to the diameter in the specific case of a circular cross-section.

Also, the ratio of the length of the void along the direction of conduction to its maximum width is preferentially between 3 and 100.

The high density value stated above may be achieved by ensuring that at least some of the said thermal conduction elements are made using one or more honeycomb structures, with each honeycomb cell forming said void of a thermal conduction element. Here the cells may be of any form, for example polygonal, such as square or hexagonal. In addition they may be cylindrical or of a shape whose size increases going from the interior wall to the exterior wall, as stated above. The advantage of such a use resides in the fact that the honeycomb structures are widely available commercially, in highly varied forms. Furthermore it is stated that the high cell density offered by honeycomb structures is obtained thanks to the walls which each delimit several cells. This aspect furthermore ensures that there is an excellent ratio between the heat conduction capacity of the honeycomb structure and the mass of the structure. Relative to an equivalent structure mass, this ratio is increased even further when the structure includes cells of small cross-section, representing a high cell density, and whose walls are thin.

For the above, it is specified that a honeycomb structure must be made of a structure formed using a stack of sheets/strips which form the cells, where the stacking direction is orthogonal to the longitudinal direction of these cells.

With this solution which uses honeycomb structures, it is preferentially arranged that each structure is provided with holes linking the cells with one another. This facilitates the introduction of radiological protection material into cell when this material is introduced by casting, in particular when the casting is made directly between the two walls of the lateral package body, with the honeycomb structure already in place in the inter-wall space. The holes are preferably made in the direction of stacking of the sheets of the honeycomb structure. The number of these used depends on various parameters, such as the viscosity of the material being cast.

Alternatively to or simultaneously with the honeycomb structure solution, it may be arranged that certain of said thermal conduction elements are made using independent elements, spaced apart from one another, where these elements then preferentially each take the form of a tube, cylindrical or flared towards the exterior wall of the lateral body, and of any cross-sectional form whatsoever. In yet another different solution, which may be combined with the previous ones, the independent thermal conduction elements may be placed in contact with each other, and possibly fixed together. This leads to a configuration which approaches that of a honeycomb structure.

Preferably at least one of the thermal conduction elements, and preferably every one of them, fits externally against said radiological protection material, and also internally, at its void. Thus it is the same solid material which fits externally and internally against at least one of the thermal conduction elements.

In general, it is stated that the void defined by each thermal conduction element is not necessarily of closed cross-section in a plane which is orthogonal to the direction of conductions, although the closed character of the voids represents a preferred solution. Furthermore, the void extends preferentially in a continuous manner along its associated thermal conduction element, in the direction of conduction, whilst remaining open at its two opposite extremities, considered along this same direction of conduction.

Finally the lateral body of the package preferably exhibits a conventional cylindrical form, for example of a circular or polygonal cross-section. In all the cases the interior and exterior walls which adopt this same form are generally referred to as shells, and are concentric, centred on said longitudinal axis around which the inter-shell space is located.

The invention also relates to a container for the transport and/or storage of radioactive materials which includes a package as described above.

Other advantages and characteristics of the invention will appear in the non-restrictive detailed disclosure below.

BRIEF DESCRIPTION OF THE DRAWINGS

This description will be made with reference to the attached illustrations, among which;

FIG. 1 represents a transverse section view of a container for the transport and/or storage of assemblies of nuclear fuel, according to a preferred embodiment of the invention;

FIG. 2 represents a partial section view taken along line II-II of FIG. 1;

FIG. 3 represents a view similar to the one shown in FIG. 2, with the means of thermal conduction being shown in the form of an alternative embodiment; and

FIG. 4 represents a partial view in perspective of a block which forms a part of the means of thermal conduction and of the means of radiological protection, intended to be arranged in the inter-shell space of the lateral package body.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Firstly, with reference to FIG. 1, a container 1 for the transport and/or storage of assemblies of nuclear fuel can be seen.

Container 1 comprises globally a package 2 forming the subject of the present invention, inside which is a storage device 4, also called a storage basket. Device 4 is designed to be positioned in a receptacle cavity 6 of package 2, as is shown schematically in FIG. 1, where it is also possible to note the longitudinal axis 8 of this package, merged with the longitudinal axes of the storage device and of the receptacle cavity.

Throughout the disclosure the term “longitudinal” must be understood as being parallel to the longitudinal axis 8 and to the longitudinal direction of the package.

Container 1 and device 4 forming reception receptacles for the fuel assemblies are shown here in a horizontal/lying position, which is habitually adopted during the transport of the assemblies, and which differs from the vertical position for loading/unloading of the fuel assemblies.

Generally, package 2 essentially has a base (not represented) on which the device 4 is intended to rest in its vertical position, a cover (not represented) positioned at the other longitudinal end of the package, and a lateral body 10 which extends around and along longitudinal axis 8, i.e. in the longitudinal direction of container 1.

It is this lateral body 10 which defines the receptacle cavity 6, using a lateral inner surface 12, of essentially cylindrical shape and circular section, and having an axis merged with axis 8.

The base of the package, which defines the base of cavity 6 which is open at the cover, may be manufactured to form a single part with a part of lateral body 10, without going beyond the scope of the invention.

Again with reference to FIG. 1, it is possible to see in detailed fashion the design of lateral body 10, which firstly presents two concentric metal walls/shells forming jointly an annular space 14 centred on the longitudinal axis 8 of the package. This involves, in effect, an inner shell 20 centred on axis 8, and an outer shell 22, which is also centred on axis 8.

The annular space 14 is filled by means of thermal conduction 16, and also by means of radiological protection 18 essentially designed to form a barrier against the neutrons emitted by the fuel assemblies housed in the storage device 4. Thus, these elements are housed between inner shell 20, whose inner surface corresponds to the lateral inner surface 12 of cavity 6, and outer shell 22.

The radiological protection device 18 is made using a solid material, which is known per se, such as a polymer matrix composite material, and more specifically whose matrix is a resin, preferably highly hydrogenated, for example of the vinyl ester resin type. This neutron protection material is also known by the name of “resin concrete”.

Additives intended to render the composite self-extinguishing may be added in addition.

The means of thermal conduction 16 are for example made using an alloy which offers good thermal conduction characteristics, of the aluminium alloy or copper alloy type. It may also be a ceramic or carbon-based material, such as silicon carbide.

Furthermore, boron made be included in the means of radiological protection and/or the means of thermal conduction, in order to reinforce the neutron protection function.

In the embodiment shown in FIGS. 1 and 2, the means of radiological protection 18 take the form of a single block of material cast between the two shells 20, 22 which penetrates inside the means of thermal conduction 16, as will be described in detail below.

First of all the means of thermal conduction are here formed using several honeycomb structures 30, which are placed circumferentially next to one another in the inter-shell space 14. Each structure 30 exhibits, for example, a form of ring angular section, which extends along an angle which is preferably between 5 and 60°. Each structure 30 also extends over the entire length of the space 14 in the direction of the axis 8, as well as over essentially the entire radial length of this space, or alternatively could even be cut into sections in one and/or the other of these two directions.

Each structure 30 forms thermal conduction elements 31 which each define internally a void 32 which corresponds to a cell/compartment of the structure. Thus because of the “honeycomb” design, the walls of the voids/cells 34 which form the elements 31 each define several voids/cells 32.

One of the particular features of the invention resides in the fact that the voids 32 each extend lengthways in a direction of conduction 36 going from the interior shell 20 towards the exterior shell 22, where this direction corresponds to the longitudinal axis of the honeycomb cell concerned. As shown in FIG. 1, this direction 36 is preferentially radial or essentially radial. In this respect, on the left-most honeycomb structure 30 in FIG. 1, the conduction elements 31 are essentially cylindrical and parallel with each other, just like the voids 32 that they define. The directions of conduction 36 are here very close to the radial direction, even though they may be inclined by a few degrees relative to this same radial direction. In this configuration, several voids 32 may nevertheless exhibit a conduction direction 36 which corresponds precisely to the radial direction of the body 10, that is, orthogonally intercepting the axis 8. On the other hand, in the right-most honeycomb structure 30 in FIG. 1, the conduction elements 31 are no longer cylindrical, but each exhibit a shape of increasing size going from the interior shell 20 to the exterior shell 22, in particular in order to allow for the difference in diameters between these two shells. The geometry of the cross-section of each element 31 preferentially remains identical, with only the size of this section then increasing in going towards the exterior shell 22.

Here the direction of conduction 36 of each of the elements 31 corresponds to the radial direction of the body 10, by orthogonally intercepting the axis 8.

As stated above, the thermal conduction elements 31 and the voids 32 that they define each extend over a length which is essentially the same as the distance separating the two shells in the direction of conduction 36 of the element 31 involved. For indication purposes, it is noted that only an assembly gap is preferentially left, in order to allow the structures 30 to be introduced into the inter-shell space 14.

In the preferred embodiment described and represented in FIGS. 1 and 2, the honeycomb structures 30 define thermal conduction elements 31 of hexagonal cross-section, although any other form may be envisaged, without going beyond the scope of the invention. This hexagonal form is achieved in a conventional manner using a stack of embossed sheets/strips 40 which form voids/cells 32, where the stacking direction 42 of these sheets is orthogonal to the longitudinal direction 36 of the cells.

Each void 32, considered in cross-section orthogonal to the direction of conduction 36 as is the case in FIG. 2, exhibits a maximum width “1” of between 2 and 25 mm. Furthermore, the walls of thermal conduction elements 31 which delimit the hollows 32 are thin, for example of mean thickness between 0.02 and 0.5. Here certain parts of the walls are formed by a single sheet, 40, whereas other parts are formed by the superimposition of two sheets 40. Thus the mean thickness stated above is defined as being equivalent to 1.5 times the thickness of the superimposed sheets 40 which make up the honeycomb structures 30.

In addition, the ratio of the length “L” of each void 32 along the direction of conduction 36 to its maximum width “1” stated above is preferentially between 3 and 100. Here the length “L” is preferentially between 75 and 200 mm.

The advantage of using honeycomb structures 30 is due to the high density of conduction elements 31 and of voids 32 that can be achieved. This means that the thermal conduction elements 31 together form a network of voids 32 which, in cross section along at least one plane parallel to the axis 8 and traversing this network, offers at least one zone whose void density 32 has a value greater than or equal to 100 voids/m². FIG. 2 shows such a section along the plane of line II-II shown in FIG. 1. Naturally, in a given zone of means 16, several planes of cross-section may exist in which this density value is observed. Furthermore, arrangements are preferably made to ensure that this minimum density value if encountered in all zones of the means of conduction 16, even though it may change value within these same means 16.

In the preferred embodiment described, the radiological protection material 18 preferably entirely fills the voids 32 in the honeycomb structures 30. Given that the casting of this material is made directly in the inter-shell space 14, with the structures 30 already in place in the packaging in a vertical position, it is envisaged that holes 46 be made in the sheets 40 in order to ensure the voids are 32 are linked together. During the gravity casting of the material 18, the latter can therefore use the holes 46 in order to be distributed as widely as possible in each of the voids 32 in the structures 30. The holes 46 are here made in the direction of stacking 42 of the sheets 40, as FIG. 2 shows. The number used depends on various parameters, such as the viscosity of the material being cast.

According to one alternative embodiment shown in FIG. 3, the thermal conduction elements are no longer made using honeycomb structures, but by independent elements 31 spaced apart from each other. Each of them therefore has, unlike the previous embodiment, a wall of their own, that is, which is not shared with other elements 31. They may be tubes, for example of circular section, as has been shown in FIG. 3. Alternatively, the elements may take a form which gets bigger going from the interior wall to the exterior wall, such as a tapered form. The geometry of the cross-section of the element preferentially remains the same, with only the magnitude of the cross-section therefore changing.

Their form, dimensions and arrangement in the inter-shell space 14 are identical or similar to those described for the solution using honeycomb structures. Furthermore, these tubes 31 which internally define the voids 32 can also be provided with holes, so that they might be filled more easily with the neutron protection material 18.

Finally with reference to FIG. 4, a block 100 is shown in the form of an angular sector of the shell, intended to be introduced into the inter-shell space 14. This solution, also envisaged for the present invention, therefore contrasts with the previous solution in that it involves making several shell sectors 100 outside the space 14 before introducing them into this space 14, so that they may be arranged circumferentially next to each other.

Each block 100 incorporates the neutron protection material 18 as well as multiple thermal conduction elements 31, filled with this material which defines more or less the entire peripheral surface of the block. Nevertheless, it is arranged that the ends of the conduction elements 31 remain exposed at the two concentric surfaces 110, 112 of the block, respectively intended to be facing/in contact with the surfaces of the shells 20, 22, which delimit the space 14. This means that better heat transfer can be achieved between the shells 20, 22 and the thermal conduction elements of the block 100. It should be noted that although the thermal conduction elements of the block 100 are here of the same type as those shown in FIG. 3, they may nevertheless take any form that conforms with the present invention, and in particular that shown in FIGS. 1 and 2.

Naturally, various modifications can be made, by those skilled in the art, to the invention which has just been described solely as non-restrictive examples.

PACKAGING FOR TRANSPORT AND/OR STORAGE OF RADIOACTIVE MATERIALS, WHICH INCLUDE IMPROVED MEANS OF THERMAL CONDUCTION

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