Package comprising packaging for transporting and / or storing radioactive content comprising a compact internal shock-absorption system

Abstract

The invention relates to a package comprising radioactive content and to packaging for transporting and / or storing said content, the packaging comprising an internal shock-absorption system (22) housed in the confinement chamber between the radioactive content and an axial blocking member of the chamber, the radioactive content comprising a storage device and one or more radioactive elements. The internal shock-absorption system (22) comprises a first shock-absorption device (40) for absorption by plastic deformation in order to provide shock absorption for the storage device (30), and a second shock-absorption device (50a-50f) for absorption by plastic deformation associated with a radioactive assembly formed of one or more radioactive elements, the first and second shock-absorption devices being designed to operate independently of each other and to take account of the maximum decelerations that can be withstood by the storage device and the radioactive elements accommodated in said device.

Description

TECHNICAL FIELD

[0001] The present invention relates to the field of packages of radioactive materials, comprising packaging and radioactive contents housed in a containment enclosure defined by the packaging.

[0002] The packaging may be of the type comprising a removable lid, such as, for example, used for the transport of nuclear fuel assemblies. Alternatively, it may be of the carton or container type, in which the lid is preferably permanently fixed to the side of the packaging, for example by welding.

[0003] Within the containment area delimited by the packaging, the radioactive content includes a storage device as well as one or more radioactive elements, the latter being able to be nuclear fuel assemblies, preferably fresh, containers / casings of vitrified waste or technological waste, containers / casings of shells and end caps, or cases for the transport of powder (PuO2 powder for example).

[0004] The invention relates more specifically to an internal shock-absorbing system, housed axially within the containment structure between the radioactive contents and an axial sealing element of the packaging, namely its lid or base. In this regard, it is noted that such a shock-absorbing system is preferably associated with the packaging lid to protect the integrity of the containment structure in the event of an axial fall of the package. However, such an internal shock-absorbing system may also, or alternatively, be provided in association with the base of the packaging. PREVIOUS STATE OF THE ART

[0005] A radioactive material storage and / or transport package generally consists of a sealed outer casing with a side body, a base, and a lid. These parts of the package define a cavity, known as a containment chamber, for housing radioactive contents, such as a storage device containing nuclear fuel assemblies. The storage device is usually called a storage basket. It has one or more axial compartments, each designed to hold a radioactive element, namely a nuclear fuel assembly in the example given above. In another case, where the radioactive elements are casings and / or containers, several of them can be stacked axially in each compartment of the storage basket.

[0006] The safety demonstration of the packaging containing radioactive material relies in particular on regulatory drop tests. Thus, during a nine-meter axial drop onto a shock-absorbing head cover over the packaging lid, the radioactive material can move axially within the packaging cavity towards the lid due to a functional gap between the radioactive material and the lid. As the shock-absorbing head cover compresses, the radioactive material can then impact the lid itself during this axial drop. Significant forces are thus generated, with a delay, in the closing system of the removable packaging lid, due to the effect of the radioactive material contained within the enclosure. In particular, the lid fastening device is subjected to considerable stress; this device may, for example, be made using fixing screws or incorporate a bayonet fitting.

[0007] To ensure the packaging remains leak-proof after an axial drop, it may be necessary to limit the forces transmitted by the radioactive contents to the lid using an internal shock-absorbing system located within the containment structure, between the lid and the radioactive contents. This system is also known as an internal shock-absorbing system.

[0008] Generally, such a system includes one or more plastic deformation damping devices, such as metallic foam. To achieve optimal compression of each damping device, and thus to best dissipate mechanical energy through plastic deformation, numerous solutions have already been proposed in previous designs. Among the design criteria for the internal damping system, consideration is given to the maximum decelerations that can be withstood by both the storage basket and the radioactive materials housed within it.Usually, the lower of the two maximum decelerations is used for the design of the damper, in addition to other criteria such as the need to absorb all the potential energy of the radioactive content in the event of an axial fall, without risk of the damper system bottoming out, or the need to limit the forces transmitted by the radioactive content on the lid fixing device.

[0009] Considering the lowest permissible maximum deceleration, this can lead to significant oversizing of the internal damping system and the entire packaging, particularly in the axial direction. Therefore, there remains a need to improve these internal damping systems to offer a better compromise between size, cost, and performance.

[0010] Finally, it should be noted that this issue also exists for an internal shock-absorbing system associated with a fixed packaging lid, such as a welded carton lid, or for an internal shock-absorbing system associated with the packaging base, which is also generally fixed. In both cases, even though the issue of the closure system's fastening mechanism does not exist as it does for a removable lid, the maximum decelerations in the event of an axial fall on the fixed lid / base side must also be taken into account in certain situations.

[0011] The relevant prior art is described in JP 2015 087324 A. DESCRIPTION OF THE INVENTION

[0012] To meet the need identified above, the invention relates to a package comprising radioactive content and packaging for the transport and / or storage of this content, the packaging comprising a side body extending around a longitudinal central axis of the packaging, and a base and a lid respectively arranged at the axial ends of the side body of packaging and forming two axial sealing elements delimiting, with the side body of packaging, a containment chamber in which the radioactive content is housed, the packaging also comprising an internal shock-absorbing system housed in the containment chamber and arranged axially between the radioactive content and one of the two axial sealing elements, referred to as the associated axial sealing element, the radioactive content comprising a storage device and one or more radioactive elements,The storage device delimits one or more axially open compartments facing the associated axial sealing element, and in each of which at least one radioactive element is arranged. According to the invention, the internal damping system comprises a first plastic deformation damping device to dampen the storage device, as well as a second plastic deformation damping device associated with a radioactive assembly formed of one or more radioactive elements. the first and second damping devices being arranged so as to operate independently of each other in the event of an axial fall of the package, the first damping device being associated with the following first parameters: σ 1 , corresponding to the crushing stress of this first damping device; S 1 , corresponding to the active surface of the first damping device intended to be axially impacted by the storage device in the event of an axial fall of the package; M 1 , corresponding to the mass of the storage device intended to be damped by the first damping device; γ 1 , corresponding to a first representative value of the deceleration of the storage device in the event of an axial fall of the package causing this storage device to plastically crush the first damping device, the first representative value of deceleration γ 1 being determined by the following formula: γ 1 = (σ 1 *S 1 ) / M 1 ,the second damping device being associated with the following second parameters: σ2, corresponding to the crushing stress of this second damping device; S2, corresponding to the active surface of the second damping device intended to be axially impacted by its associated radioactive assembly, in the event of an axial fall of the package; M2, corresponding to the mass of the radioactive assembly intended to be damped by the second damping device; γ2, corresponding to a second representative value of the deceleration of the associated radioactive assembly, in the event of an axial fall of the package causing this radioactive assembly to plastically crush the second damping device, the second representative value of deceleration γ2 being determined by the following formula: γ2 = (σ2 * S2 ) / M2 ,The first and second parameters are such that the second representative deceleration value γ2 is different from the first representative deceleration value γ1. The invention provides a high-performance internal damping system with satisfactory compactness. This compactness is increased compared to prior art designs, thanks to the consideration of a differentiated maximum permissible deceleration, firstly for the storage device, and secondly for the radioactive assembly formed of one or more radioactive elements, such as one or more nuclear fuel assemblies, and preferably a single assembly.

[0013] Each independent damping device can thus be precisely designed to improve the overall compactness. In other words, the invention is based on the observation that radioactive elements can withstand greater decelerations than the storage device that houses them, or vice versa. The associated second damping device can therefore exhibit a greater crushing force, or a larger active surface area, and thus adopt a lower axial height, resulting in a reduced footprint and greater compactness. These advantageous measures can be implemented while respecting the maximum permissible deceleration for the radioactive elements and being able to absorb all of their potential energy without risk of the second damping device bottoming out.

[0014] The invention preferably provides for the implementation of one or more of the following optional features, taken individually or in combination.

[0015] Preferably, the ratio |γ 2 - γ 1 | / min (γ 2 , γ 1 ) is greater than 1.1.

[0016] Preferably, the internal damping system includes one or more other second plastic deformation damping devices, each associated with a separate radioactive set consisting of one or more other radioactive elements, the first and second parameters of the first and second damping devices being such that the second representative deceleration value γ2 associated with each of at least several second damping devices, and preferably with each of all such second damping devices, is different from the first representative deceleration value γ1 associated with the first damping device.

[0017] Preferably, the radioactive assembly associated with the second damping device, or with each second damping device, consists of a single radioactive element.

[0018] Preferably, each radioactive element is a nuclear fuel assembly, preferably a fresh fuel assembly, even more preferably of the MOX type, or a container / casing for vitrified waste or technological waste, or a container / casing for casings and end caps, or a casing for transporting powder. Preferably, the second damping device(s) consist of a single block of damping material, preferably metallic foam, wood, honeycomb, or a tubular metallic structure. Alternatively, each second damping device may comprise several blocks spaced apart and functioning independently in the event of an axial fall of the package. In all cases, for the damping of the radioactive elements, all blocks preferably exhibit the same crushing stress.

[0019] Preferably, the first damping device consists of several spaced blocks of damping material, preferably made of foam, wood, honeycomb, or a tubular metal structure. All these damping blocks associated with the storage device preferably exhibit the same crushing stress. Alternatively, the first damping device could consist of a single block of damping material.

[0020] Preferably, the aforementioned blocks are cylindrical, or pyramidal in shape.

[0021] Preferably, the crushing stress σ₂ of the first or each second damping device is different from the crushing stress σ₁ of the first damping device. Alternatively, identical crushing stresses may be used for the first and second damping devices without departing from the scope of the invention. In this case, the differentiating decelerations of the entities are controlled by the extent of the active surfaces of the first and second damping devices, or by the masses of the storage device and each radioactive assembly.

[0022] Preferably, the first damping device and each subsequent damping device(s) have the same or nearly the same axial thickness. For example, a thickness variation of up to 10% between the largest and smallest values ​​may be tolerated.

[0023] Preferably, the first damping device and the second or each second damping device are arranged in the same transverse plane of the package.

[0024] Preferably, the first damping device and the second damping device(s) are arranged in the same envelope.

[0025] Preferably, the storage device has a perforated head plate, and the internal shock-absorbing system is preferably attached to this perforated head plate. Alternatively, the internal shock-absorbing system can be attached to the associated axial sealing element, for example, the packaging lid, or simply be freely positioned between the storage basket and this associated axial sealing element. Preferably, the associated axial sealing element is the packaging lid, preferably removably mounted on the side of the packaging. Alternatively, the associated axial sealing element is the base of the packaging. Indeed, even if there is no issue of securing the screws of the closure system on the base side, controlling deceleration in the event of an axial fall on the base side may also need to be considered in certain situations.

[0026] Other advantages and features of the invention will appear in the detailed, non-limiting description below. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] This description will be made with reference to the attached drawings, among which are; [ Fig 1 ] represents a schematic axial cross-sectional view of a package according to a preferred embodiment of the present invention; [ Fig 2 ] represents a perspective view of the radioactive content shown in the preceding figure, according to a preferred embodiment of the invention; [ Fig 3 ] is a perspective view of the internal shock-absorbing system fitted to the package shown on the figure 1 ; Fig 4 ] represents an exploded perspective view of the internal damping system shown in the previous figure; [ Fig 5 ] represents a perspective view of the internal damping system similar to that of the figure 3, with one of the two half-envelopes having been removed for clarity; [ Fig 6 ] is a cross-sectional view of the internal damping system, taken along plane P of the figure 5 , passing through the ends of the shock absorber blocks; and [ Fig 7 ] represents a cross-sectional view taken along line VII-VII of the figure 6 . DESCRIPTION OF IMPLEMENTATION METHODS

[0028] With reference first and foremost to the figure 1 , it is represented a package 100 for storage and / or transport of radioactive content 12, in the form of a preferred embodiment of the present invention.

[0029] The package 100 comprises, firstly, a package 1 having a side body 2, a base 4 and a removable lid 6 closing an opening in the package opposite the base 4. The package has a central longitudinal axis 8 around which the body extends

[0030] lateral 2, this axis 8 passing through the lid 6 and the base 4 respectively arranged at the front and rear ends of the lateral packaging body 2. As illustrated schematically on the figure 1 The base 4 can be made in one piece with the side body of the packaging 2. The lid 6 is attached to the front end of the side body 2, corresponding to the upper end in the vertical position of the packaging shown in the diagram. figure 1 . The fixing of this cover 6 is preferably achieved using screw elements 14, distributed around the periphery of the cover.

[0031] The lateral body 2, the cover 6 and the bottom 4 define a containment enclosure 10 used to house the radioactive content 12. The cover 6 and the bottom 4 thus form the two opposing axial sealing elements of the containment enclosure 10.

[0032] The radioactive content 12, also centered on axis 8, comprises a storage device and one or more radioactive elements, here nuclear fuel assemblies, preferably fresh such as MOX fuel. Instead of nuclear fuel assemblies, the radioactive elements as defined by the invention could alternatively be vitrified waste containers / casings, shell and end cap containers / casings, or powder transport casings. In these alternative examples, the containers / casings housed within the storage device are also leak-proof.

[0033] As shown in dotted lines on the figure 1 , at the ends of the package considered along the direction of axis 8, the packaging can be equipped with shock-absorbing covers 20 protecting respectively the removable cover 6 and the base 4 of the packaging.

[0034] Packaging 1 is also equipped with an internal shock-absorbing system 22 specific to the invention, shown only schematically on the figure 1 This internal damping system 22 is housed within the containment enclosure 12, axially between an internal surface 24 of the lid 6 and an axial end surface 26 of the radioactive content 12. Preferably, the internal damping system 22 is fixed to the axial end surface 26 of the radioactive content 12, for example, by means of screws or by welding. Alternatively, it could be fixed to the internal surface 24 of the lid 6, or freely arranged between the radioactive content 12 and the lid 6.

[0035] Thus, the internal damping system 22 can be arranged in several ways between the radioactive content 12 and its associated axial sealing element, formed by the removable cover 6.

[0036] Now, referring to the figure 2The figure represents the radioactive content 12, comprising the storage device / basket 30 and the nuclear fuel assemblies 32a-32f. Each of these assemblies 32a-32f is arranged in a compartment 34 defined by the basket 30. The compartments 34 are axially open at a perforated end plate 36 of the basket, to which the internal damping system is intended to be attached. The perforated end plate 36 thus forms the axial end of the compartments 34. The axial end surface 26 of the radioactive content 12 is therefore formed by the flat outer surface of the end plate 36 and by the axial end surface of the assemblies 32a-32f located near or in the same plane as the flat outer surface of the end plate 36.

[0037] One of the distinctive features of the invention lies in the separation and independence of the means used to cushion, on the one hand, the basket 30, and on the other hand, each of the assemblies 32a-32f, in the event of an axial fall of the package towards the lid. This feature will now be described with reference to figures 3 to 7 , showing the internal damping system 22 which comprises overall a first plastic deformation damping device for damping the basket 30, as well as a second plastic deformation damping device associated with each of the assemblies 32a-32f. In this preferred embodiment, each nuclear fuel assembly 32a-32f thus forms, within the meaning of the claimed invention, a radioactive assembly composed of a single radioactive element (the assembly as such).

[0038] The first damping device 40 is formed here of several cylindrical blocks 40a-40e distributed around the periphery of the generally cylindrical damping system 22, centered on the axis 8. Each of the blocks 40a-40e is thus cylindrical with its axis orthogonal or substantially orthogonal to the flat outer surface 26 of the head plate 36, in the direction of which these blocks are oriented. These blocks are spaced apart from each other and are preferably made of foam, wood, honeycomb, or a tubular metal structure, preferably with the same crushing stress σ1 (in MPa).

[0039] The active surface S1 (in m²) of the first damping device 40 corresponds to the surface area of ​​blocks 40a-40e intended to be axially impacted by the flat outer surface 26 of the head plate 36, in the event of an axial fall of the package. More precisely, this active surface S1 corresponds to the sum of the active surfaces S1a-S1e of all the blocks 40a-40e forming the first damping device 40. These surfaces S1a-S1e correspond to the lower axial ends of blocks 40a-40e, and they are very preferably all flat or substantially flat, orthogonal or substantially orthogonal to the axis 8, and coplanar. In the event of non-planarity of these lower axial ends of blocks 40a-40e, each active surface S 1a -S 1e then corresponds to the projected surface of the lower axial end of the block concerned, in a plane orthogonal to axis 8, and according to the direction of this same axis.

[0040] Moreover, due to the cylindrical nature of the block, each active surface S1a-S1e also corresponds, in terms of size and shape, to any cross-section of its corresponding block 40a-40e. Alternatively, the blocks could have a pyramidal shape, in which case the active surfaces could vary according to the block's thickness, thus generating lower decelerations at the beginning of the impact. In this scenario, the general principle of the invention, according to which the first and second parameters are such that the second representative deceleration value γ2 is different from the first representative deceleration value γ1, is verified at every instant during an axial fall of the package, causing the radioactive assembly to plastically crush the first and second damping devices.

[0041] These active surfaces are schematically represented on the cross-section of the figure 6taken in the plane of these surfaces or in close proximity, orthogonally to axis 8, and also partly visible on the figure 7 .

[0042] The mass M 1 (in kg) corresponds to the mass of the basket 30 intended to be damped by the first damping device 40.

[0043] Thus, using the aforementioned parameters relating to this first damping device 40, the value of γ 1 is determined, corresponding to a first representative value of the deceleration of the basket 30 in the event of an axial fall of the package leading this basket to plastically crush the first damping device 40. This first representative value of deceleration γ 1 is determined by the following formula: γ 1 = (σ 1 *S 1 ) / M 1 .

[0044] As previously mentioned, the internal damping system 22 also includes several secondary damping devices 50a-50f, each associated with one of the assemblies 32a-32g to dampen it in the event of an axial fall of the package. Thus, the secondary damping devices 50a-50f are independent of each other, and also independent of the blocks of the first damping device 40. Consequently, in the event of an axial fall of the package, each of these elements deforms plastically and freely, without being hindered by the deformation of other elements located at a distance. This independence of the blocks during crushing is achieved in particular due to the absence of a load distribution plate, which is found in some prior art solutions and is usually placed between the damping systems and the radioactive contents.

[0045] In this preferred embodiment of the invention, all the second damping devices 50 are identical, so only one of them will be described in detail below. Each second damping device is formed here of a single cylindrical block 50a-50f, the blocks being distributed around the periphery and center of the damping system 22, in the same distribution as that of the assemblies 32a-32f. The blocks 50a-50f are thus cylindrical with axes orthogonal or substantially orthogonal to the flat outer surface 26 of the head plate 36, as well as to the axial end surface of the nuclear fuel assemblies toward which these blocks 50a-50f are oriented, respectively. The blocks 50a-50f are spaced apart from each other, and equally spaced from the blocks of the first damping device 40.They are preferably made of foam, wood or honeycomb, preferably with the same crushing stress σ 2 (in MPa), itself preferably different from the aforementioned crushing stress σ 1.

[0046] The active surface S2 (in m²) of each second damping device 50a-50f corresponds to the surface of the block intended to bear axially against the axial end surface of the associated assembly 32a-32f in the event of an axial fall of the package. This surface S2 is therefore formed by the lower axial end of the damping block 50a-50f, which is very preferably planar or substantially planar, orthogonal or substantially orthogonal to axis 8. All active surfaces S2 are, moreover, preferably coplanar. If this lower axial end of the damping block is not planar, the active surface S2 then corresponds to the projected surface of the lower axial end of this block, in a plane orthogonal to axis 8, and along the direction of this same axis.

[0047] Moreover, due to the cylindrical nature of the block, the active surface S 2 also corresponds, in terms of size and shape, to any cross-section of its corresponding block 50a-50f. In axial view, this active surface S 2 also preferably corresponds, in size and shape, to that of the axial end surface of the associated assembly 32a-32f, a perfect or near-perfect correspondence being sought between these two surfaces, in the direction of the axis 8. Nevertheless, the active surface S 2 could be smaller or larger, without departing from the scope of the invention.

[0048] The active surfaces S2 are schematically represented on the cross-section of the figure 6 taken in the plane of these surfaces or in close proximity, orthogonally to axis 8, and one of them is also visible on the figure 7 .

[0049] The mass M 2 (in kg) corresponds to the mass of each nuclear fuel assembly 32a-32f, intended to be damped by the second damping device 50a-50f located axially opposite it.

[0050] Thus, using the aforementioned parameters relating to each of these second damping devices 40, the value of γ 2 corresponding to a second representative value of the deceleration of each assembly 32a-32f is determined, in the event of an axial fall of the package leading this assembly to plastically crush its associated second damping device, the second representative value of deceleration γ 2 being determined by the following formula: γ 2 = (σ 2 *S 2 ) / M 2 .

[0051] Thanks to the proposed design, the first and second parameters mentioned above are advantageously chosen such that the second representative value of deceleration γ2 is different from the first representative value of deceleration γ1. In this respect, the ratio |γ2 - γ1| / min(γ2, γ1) is preferably greater than 1.1.

[0052] This leads to a reduction in the overall size of the internal damping system 22, since it specifically takes into account the maximum permissible deceleration for the basket 30 and that of each of the assemblies 32a-32f, which is often greater than that of the basket. Each of the independent damping devices 40, 50a-50f can thus be designed with minimal bulk, to improve the overall compactness.

[0053] The design is also retained by adapting the extent of the active surfaces S1 and S2 so that all blocks 40a-40e and 50a-50f lie in the same transverse plane of the package, with an identical or substantially identical axial thickness. However, blocks 50a-50f could alternatively have a greater axial thickness than blocks 40a-40e by extending axially into the basket housings through the openings in the perforated head plate 36.

[0054] In the case of the first hypothesis, this notably facilitates the enclosure of these blocks 40a-40e, 50a-50f within a single, generally cylindrical envelope 54, shown on the figure 3and corresponding to an outer casing of the internal damping system 22. This casing 54 can be made using two half-casings 54a, 54b welded at a mid-axial portion of the system 22 and closed at its two axial ends by two disc-shaped plates 58a, 58b, these half-casings also being visible on the figures 4 And 5It is noted that on this envelope 54, the closure plate 58b, positioned opposite / in contact with the radioactive contents, remains thin in order to maintain the independent operation of the various plastic deformation damping blocks 40a-40e, 50a-50f. In other words, this plate 58b does not function as a load distribution plate in the event of an axial fall of the package. Inside the envelope, the active surfaces S1, S2 of the blocks 40a-40e, 50a-50f are designed to be in contact with the inner surface of the plate 58b, even though axial clearances have been maintained in the representation of the figure 7 , for the sake of clarity.

[0055] Of course, various modifications can be made by a person skilled in the art to the invention just described, solely by way of non-limiting examples, the scope of which is defined by the appended claims. For example, the packaging 1 could include a fixed lid, for example welded to the side of the packaging, so as to form a sealed casing enclosing the basket and radioactive elements, this casing also being referred to as a "canister." Furthermore, the internal shock-absorbing system according to the invention could be associated with the base 4 of the packaging, without departing from the scope of the invention.

Claims

1. A package (100) comprising radioactive content (12) as well as a packaging (1) for transporting and / or storing this content (12), the packaging comprising a lateral body (2) extending around a longitudinal central axis (8) of the packaging, as well as a bottom (4) and a cover (6) respectively arranged at the axial ends of the packaging lateral body (2) and forming two axial blocking members delimiting, with the packaging lateral body, a confinement chamber (10) in which the radioactive content (12) is accommodated, the packaging also including an internal shock-absorption system (22) accommodated in the confinement chamber (10) and arranged axially between the radioactive content (12) and one of the two axial blocking members, so-called the associated axial blocking member, the radioactive content (12) comprising a storage device (30) as well as one or more radioactive element(s) (32a-32f), the storage device delimiting one or more axial compartment(s) (34) axially open in the direction of the associated axial blocking member (6), and in each of which at least one radioactive element (32a-32f) is arranged, the internal shock-absorption system (22) includes a first damping device (40) by plastic deformation for damping the storage device (30), and a second damping device (50a-50f) by plastic deformation associated with a radioactive assembly formed of one or more radioactive element(s) (32a-32f), the first and second damping devices (40, 50a-50f) being arranged so as to operate independently of each other in the event of an axial fall of the package, characterised in that the first damping device (40) being associated with the following first parameters: - σ1, corresponding to the crushing stress of this first damping device; - S1, corresponding to the active surface of the first damping device intended to be axially impacted by the storage device (30) in the event of an axial fall of the package; - M1, corresponding to the mass of the storage device (30) intended to be damped by the first damping device (40); - γ1, corresponding to a first value representative of the deceleration of the storage device in the event of an axial fall of the package leading to this storage device (30) plastically crushing the first damping device (40), the first deceleration representative value γ1 being determined by the following formula: γ1 = (σ1*S1) / M1, the second damping device (50a-50f) being associated with the following second parameters: - σ2, corresponding to the crushing stress of this second damping device; - S2, corresponding to the active surface of the second damping device intended to be axially impacted by its associated radioactive assembly (32a-32f), in the event of an axial fall of the package; - M2, corresponding to the mass of the radioactive assembly (32a-32f) intended to be damped by the second damping device (50a-50f); - γ2, corresponding to a second value representative of the deceleration of the associated radioactive assembly, in the event of an axial fall of the package leading to this radioactive assembly (32a-32f) plastically crushing the second damping device (50a-50f), the second deceleration representative value γ2 being determined by the following formula: γ2 = (σ2*S2) / M2, and in that the first and second parameters are such that the second deceleration representative value γ2 is different from the first deceleration representative value γ1.

2. The package according to claim 1, characterised in that the ratio |γ2 - γ1| / min (γ2, γ1) is higher than 1.1.

3. The package according to any one of the preceding claims, characterised in that the internal shock-absorption system (22) has one or more other second damping device(s) (50a-50f) by plastic deformation, each associated with a distinct radioactive assembly formed of one or more other radioactive element(s) (32a-32f), and in that the first and second parameters of the first and second damping devices (40, 50a-50f) are such that the second deceleration representative value γ2 associated with each of at least several second devices damping device (50a-50f), and preferably with each of all of these second damping devices, is different from the first deceleration representative value γ1 associated with the first damping device (40).

4. The package according to any one of the preceding claims, characterised in that the radioactive assembly associated with the second damping device (50a-50f), or with each second damping device, is formed of one single radioactive element (32a-32f).

5. The package according to any one of the preceding claims, characterised in that each radioactive element (32a-32f) is a nuclear fuel assembly, preferably a fresh fuel assembly, still more preferably of the MOX type, or a vitrified wastes container / case, or a shells and nozzles container / case, or a case for transporting powder.

6. The package according to any one of the preceding claims, characterised in that the or each second damping device (50a-50f) is formed of one single damping material block, preferably made of metal foam, wood, honeycomb, or using a metal tubular structure.

7. The package according to any one of the preceding claims, characterised in that the first damping device (40) is formed of several damping material blocks (40a-40e) spaced apart from one another, preferably made of foam, wood, honeycomb or using a metal tubular structure.

8. The package according to any one of the preceding claims, characterised in that the crushing stress σ2 of the or each second damping device (50a-50f) is different from the crushing stress σ1 of the first damping device (40).

9. The package according to any one of the preceding claims, characterised in that the first damping device (40) and the or each second damping device (50a-50f) all have an identical or substantially identical axial thickness.

10. The package according to any one of the preceding claims, characterised in that the first damping device (40) and the or each second damping device (50a-50f) are arranged in the same transverse plane of the package.

11. The package according to any one of the preceding claims, characterised in that the first damping device (40) and the or each second damping device (50a-50f) are arranged in the same casing (54).

12. The package according to any one of the preceding claims, characterised in that the storage device (30) includes an apertured head plate (36), and in that the internal shock-absorption system (22) is preferably fastened on said apertured head plate.

13. The package according to any one of the preceding claims, characterised in that the associated axial blocking member is the cover (6) of the packaging, preferably removably mounted on the packaging lateral body.

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