Abstract:
A container including a support having at least one first longitudinal support surface defining a longitudinal housing for receiving a nuclear fuel assembly, and a door having a second longitudinal support surface. The door is movable between a position of holding the nuclear fuel assembly between the two longitudinal surfaces and a released position in which the assembly is free with respect to the support. The container includes means for adjusting the transversal separation between the first and second surfaces in the holding position of the door.

Description:
Priority is claimed to French Application Serial No. FR 06 07424 filed Aug. 21, 2006 through International Patent Application Serial No. PCT/FR2007/001032, filed Jun. 21, 2007. 
     The invention relates in general to transport containers for nuclear fuel assemblies. 
     To be more precise, the invention relates, according to a first aspect, to a transport container for nuclear fuel assemblies of elongate shape in a longitudinal direction, of the type comprising a support having at least a first longitudinal bearing surface delimiting a longitudinal housing for receiving a nuclear fuel assembly, and a door having a second longitudinal bearing surface, the door being movable between a position holding the nuclear fuel assembly between the two longitudinal surfaces and a release position in which the assembly is free with respect to the support. 
     BACKGROUND 
     The document WO-99/41754 describes such a container. In this container, the second longitudinal surface rests on a nuclear fuel assembly located in the housing by means of bearing runners mounted movably on the door. These runners are distributed longitudinally so that they each rest on a grid of the skeleton of the nuclear fuel assembly. Since each type of assembly has a specific cross-section and specific grid positions, it is necessary to use a specific door for each type of assembly, which is complicated and expensive. 
     SUMMARY OF THE INVENTION 
     In this context, an object of the invention is to provide a container which is suitable for transporting several types of assembly and which is more readily adaptable to each of them. 
     To that end, the invention relates to a transport container of the above-mentioned type, characterized in that it comprises means for adjusting the spacing between the first and second surfaces in the holding position of the door. 
     The container may also have one or more of the following features, considered individually or in accordance with any technically possible combination:
         the first surface comprises a first pair of longitudinal faces arranged in the shape of a V, and the second surface comprises a second pair of longitudinal faces which are arranged in the shape of a V and which are parallel with and opposite the faces of the first pair when the door is in the holding position;   the first and second pairs of faces in a V shape converge towards first and second vertices, respectively, the adjusting means comprising means for adjusting the position of the door with respect to the support by translation of the door in a transverse adjusting direction passing via the first and second vertices when the door is in the holding position;   the support comprises parallel longitudinal surfaces for guiding the translation of the door in the direction of adjustment;   the container comprises means for displacing the door with respect to the support between its holding and release positions by translation in the direction of adjustment and then rotation about at least one longitudinal shaft;   the faces of the first pair form between them an angle substantially equal to that which the faces of the second pair form between them, this angle being from 60° to 135°;   the second longitudinal bearing surface is free from movable runners for resting on a nuclear fuel assembly; and   the second longitudinal bearing surface is suitable for resting directly on a nuclear fuel assembly.       

     According to a second aspect, the invention relates to the use of a container as defined above for transporting a nuclear fuel assembly. 
     According to one variant, the container is used with the same support and the same door to transport nuclear fuel assemblies of at least two different types. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other features and advantages of the invention will emerge from the description thereof which is given hereinafter, by way of example which is in no way limiting, with reference to the appended Figures, in which: 
         FIG. 1  is a side view of a transport container according to the invention; 
         FIG. 2  is an end view of the container, in accordance with the arrow II of  FIG. 1 ; 
         FIG. 3A  is a side view of the rear portion of the internal structure of the container of  FIGS. 1 and 2 , the lower half-shell of the external casing being shown with a dot-dash line; 
         FIG. 3B  is a top view of the internal structure, viewed in accordance with the arrow IIIB of  FIG. 3A ; 
         FIG. 4  is a sectional view of the internal structure of  FIGS. 3A and 3B , in a transverse plane, taken in accordance with the arrows IV of  FIG. 3A , showing two housings for receiving nuclear fuel assemblies, the doors of which are shown in the holding position, the spacing between the two bearing surfaces of the housing on the left being suitable for an assembly having a small cross-section and the spacing between the two bearing surfaces of the housing on the right being suitable for an assembly having a large cross-section; 
         FIG. 5  is a view similar to that of  FIG. 4 , the door of the housing on the right being shown in the release position, the door of the housing on the left being shown in a position which is off-set to the maximum extent towards the top and which is intermediate between the holding and release positions; 
         FIG. 6  is an enlarged sectional view of the means for displacing a door, taken in accordance with the arrows VI of  FIG. 4 ; and 
         FIGS. 7A and 7B  are simplified schematic representations in cross-section of the bearing surfaces of the internal structure of the container, for two variants of the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1 and 2  show a container  1  for transporting fresh fuel assemblies for a pressurized water nuclear reactor. 
     The transport container  1 , which is intended to transport two fuel assemblies in the horizontal position, comprises an external casing  2  formed by a lower half-shell  2   a  and an upper half-shell  2   b  mounted one on top of the other in accordance with a junction plane. 
     Each of the half-shells  2   a  and  2   b  is produced from sheet-steel and comprises respective reinforcing bows  3   a ,  3   b  distributed along the length of the half-shell. 
     Sectional members  4  and  4 ′ forming support feet for the container are also secured to the lower portion of the lower half-shell  2   a . In addition, adjustable bearing members  5  and  5 ′ which comprise screw jacks and which are fixedly joined to a longitudinal end portion of the container enable the inclination of the container resting on a support surface to be adjusted, about the longitudinal axis of the container and about a transverse axis, respectively. 
     The two half-shells  2   a  and  2   b  are mounted one on top of the other by way of peripheral end-plates constituting an upper flat bearing portion of the lower half-shell  2   a  and a flat lower bearing portion of the upper half-shell  2   b  of the container. 
     In the closed position of the container shown in  FIGS. 1 and 2 , the end-plates of the two half-shells  2   a  and  2   b  are mounted and secured one on top of the other by screws and nuts and form an assembly flange  6 . 
       FIGS. 3A and 3B  show a portion of the container in the open state, that is to say, with the upper half-shell of the casing of the container separated from the lower half-shell and removed. 
     In  FIGS. 3A and 3B  it is possible to see the internal structure of the container which is indicated in a general manner by the reference  7  and which comprises, in particular, a cradle  8  which rests on supports  9  formed by damper studs, in the lower half-shell  2   a  of the external casing  2  of the container. A second portion of the internal structure of the container is formed by an assembly  10  for receiving and supporting two fuel assemblies placed side by side in a horizontal position. The assembly  10 , which rests on the cradle  8 , delimits two completely closed housings for two fuel assemblies, as will be explained hereinafter. 
     The cradle  8  comprises two longitudinal members  8   a ,  8   b  formed by angle beams which are secured to the support studs  9  and which are maintained in parallel arrangements with a spacing corresponding to the width of the receiving assembly  10  by cross-members  8   c . At one of its ends, the cradle comprises an assembly for stiffening and for pivot mounting, comprising two plates  11   a  and  11   b  which are parallel with each other, and two cross-members  12  formed by hollow sectional members secured below the longitudinal members of the cradle and to the plates  11   a  and  11   b.    
     The pivot mounting of the assembly  10  on the lower half-shell of the container, about a horizontal axis of transverse direction, is ensured by means of the stiffening and pivot-mounting assembly comprising the plates  11   a  and  11   b.    
     In addition, as will be explained hereinafter, a retaining plate  11   c  for the fuel assemblies is also mounted between the plates  11   a  and  11   b.    
     As shown in  FIG. 3B , in order to limit the effect of impact on the fuel assemblies, for example the effect of the container  1  falling, a buffer  43  is interposed between the longitudinal end of the internal structure  7  and the internal end wall of the external casing  2 , of circular shape. The buffer  43 , in the form of a disc, the cross-section of which is identical to the internal cross-section of the container casing, is formed by a disc of balsa wood surrounded by a casing of stainless sheet-steel. An identical buffer is located at the second longitudinal end of the container, between the second longitudinal end of the internal structure and the second end of the external casing. 
     As can be seen in  FIG. 4 , the assembly  10  comprises a parallelepipedal support  13  in which the housings  15 A and  15 B for receiving a nuclear fuel assembly are formed, and two doors  17 A and  17 B capable of closing the housings  15 A and  15 B. The support  13  is elongate longitudinally and has a rectangular cross-section which is constant over the entire longitudinal length of the support  13 . The two housings  15 A and  15 B extend longitudinally, parallel with each other, and open out in an upper face  19  of the support  13 . 
     The housings  15 A and  15 B are identical. Only one of them will be described below. Likewise, the doors  17 A and  17 B are identical, and only one of them will be described below. 
     The base of the housing  15 B is delimited by a first V-shaped bearing surface  21 , comprising a first pair of longitudinal faces  23  forming an angle of 90° between them. The first pair of faces  23 , viewed in cross-section, converges towards a vertex  25 , corresponding to the deepest point of the housing  15 B and where the faces  23  join. The two faces  23  continue towards the top of  FIG. 4 , that is to say, towards the upper face  19 , by way of two lower guide surfaces  27 , which are parallel with each other and perpendicular to the face  19 , then by way of two upper guide surfaces  29 , which are also parallel with each other and perpendicular to the face  19 . 
     The surfaces  27  have between them a transverse spacing less than that of the surfaces  29 , with the result that shoulders  31  are formed between the surfaces  27  and  29 . 
     The door  17 B extends over the entire longitudinal length of the housing  15 B. It is movable between a position, shown in  FIG. 4 , of holding the nuclear fuel assembly in the housing  15 B, and a release position in which the assembly is free with respect to the support  13  and which is shown in  FIG. 5 . These positions will be described in detail hereinafter. 
     The door  17 B comprises an upper portion  33  and a lower portion  35  of reduced width compared with the portion  33 , the width corresponding to the transverse direction when the door is in the holding position. The upper portion  33  therefore comprises two lateral edges  36  projecting one on each side of the portion  35 . 
     The respective widths of the portions  33  and  35  correspond to the transverse spacing between the upper guide surfaces  29  and the lower guide surfaces  27 , respectively, and are constant along the entire housing  15 B. 
     The upper portion  33  is delimited on the side remote from the portion  35  by a substantially flat upper surface  37 . The lower portion  35  is delimited on the side remote from the portion  33  by a second longitudinal bearing surface  39  having, in a transverse plane, the shape of a W. 
     The second bearing surface  39  comprises at the centre a second pair of longitudinal faces  41  arranged in the shape of a V and forming an angle of 90° between them. The faces  41  converge towards a second vertex  43  where they join. The second bearing face  39  also comprises two lateral faces  45  extending the faces  41  away from the vertex  43 . The faces  45  are substantially perpendicular to the faces  41 . 
     The faces  23  of the first pair are wider than the faces  41  of the second pair, viewed in a transverse plane. 
     For each door  17 A,  17 B, the assembly  10  also comprises means for displacing the door with respect to the support  13  between its holding and release positions, these means also enabling the spacing between the first and second surfaces  23  and  39  to be adjusted when the door occupies its holding position. Only the means for displacing the door  17 B will be described here, those of the door  17 A being identical. 
     The displacement means comprise, for example, two screws  47  mounted to rotate freely on the support  13 , a plurality of nuts  49  which are movable along the screws  47  and which are each provided with two shaft ends  51  ( FIG. 6 ), the door  17 B being mounted to be movable in rotation about the shaft ends  51  and being connected, in terms of translation along the screws  47 , to the nuts  49 . 
     As can be seen in  FIG. 4 , the screws  47  extend in a vertical direction in  FIG. 4 , perpendicularly to the upper face  19 . They are engaged by their free ends in bearings  53  formed in the shoulder  31  of the support  13 . The screws  47  are fixed in terms of translation vertically in the bearings  53  and are free to rotate in these bearings. The bearings  53  are located in the shoulder  31  furthest away from the housing  15 A. 
     The screws  47  are distributed longitudinally along the door  17 B. 
     The vertical length of the screws  47  is such that their heads  55  are located outside the support  13 , projecting above the upper face  19 . 
     As can be seen in  FIGS. 4 to 6 , the door  17 B comprises, in the region of each screw  47 , a recess  57  formed in the edge  36  of the upper portion  33 . 
     The recesses  57  are formed through the entire vertical thickness of the edge  36 , the screws  47  passing through the recesses  57 . The nuts  49  are located in the recesses  57 . 
     The door  17 B also comprises blind holes  59  formed longitudinally in the thickness of the edge  36  and opening out in each recess  57 . 
     As shown in  FIG. 6 , the shaft ends  51  are fixedly joined to the nuts  49  and extend longitudinally from the nuts  49 . They are engaged in the blind holes  59  and can rotate freely in these holes. 
     The assembly  10  also comprises for each housing  15 A,  15 B a plurality of threaded orifices  61  formed in the shoulder  31  remote from the screws  47 , and a plurality of screws  63  for securing the door  17 A,  17 B in the holding position, which screws  63  can be screwed into the orifices  61 . The number of screws  63  may be, for example, from ten to fifteen. Only the means for securing the door  17 B will be described here. 
     As shown in  FIG. 4 , each of the securing screws  63  comprises a threaded end portion  65 , a head  67  remote from the portion  65 , and a smooth portion  69  interposed between the head  67  and the threaded portion  65 . The door  17 B comprises a plurality of smooth holes  71  ( FIG. 5 ) formed in the edge  36  located on the same side as the housing  15 A in the holding position. The screws  63  are engaged in the smooth holes  71 , the smooth portion  69  being located in the smooth hole  71 , the head  67  bearing against the upper face  37  of the door  17 B, and the threaded portion  65  being screwed into the threaded orifice  61  of the support. The orifices  61  and  71  and the securing screws  63  are distributed regularly along the housing  15 B. 
     In addition, each of the doors  17 A,  17 B comprises two handles  73  projecting towards the top relative to the face  37 . These handles  73  are located in the vicinity of the longitudinal ends of the doors. 
     When the door  17 A,  17 B is in the holding position, its upper portion  33  is engaged between the upper guide surfaces  29  and the lower portion  35  is engaged between the lower guide surfaces  27 . The second bearing surface  39  faces the base of the housing  15 A,  15 B, and the faces  41  of the second pair are parallel with and opposite the faces  23  of the first pair. The first and second vertices  23  and  43  are then aligned vertically in  FIG. 4 , that is to say, in a direction perpendicular to the face  19 , and the upper surface  37  is parallel with the face  19 . 
     The release position of the door  17 B is illustrated in  FIG. 5 . In this position, the door  17 B is mounted to the maximum extent along the screws  47  and is swung towards the outside of the housing  15 B about the shafts  51 . The nuts  49  are in abutment with the heads  55  of the screws  47 . The upper surface  37  of the door  17 B extends substantially horizontally, at the level of the upper face  19 , the second bearing surface  39  facing the top of  FIG. 5  and away from the housing  15 B. The release position of the door  17 A is symmetrical with the release position of the door  17 B relative to a centre longitudinal plane of the housings  15 A and  15 B. 
     The operation of the container described above will now be explained. 
     In order to load nuclear fuel assemblies into the container, the two half-shells  2   a  and  2   b  are first of all detached from each other by unscrewing the screws of the flange  6 , and the upper half-shell  2   b  is removed. The assembly  10  is then detached from the cradle  8  and the assembly  10  is swung into a substantially vertical position about the transverse axis located at one of the ends of the cradle. 
     The doors  17 A and  17 B are then placed in the release position in order to give access to the housings  15 A and  15 B. 
     A nuclear fuel assembly can then be placed in each of the housings  15 A and  15 B, by a fuel assembly lifting tool, such as the winch of a travelling crane, by displacing the assembly horizontally, in accordance with the arrow F 1  in  FIG. 5 . The fuel assemblies come to rest, by way of their lower ends, on the fuel assembly support plate  11 C secured between the two plates  11 A and  11 B of the assembly  10 . 
     In the case of fuel assemblies having a square cross-section, an assembly is placed in each housing  15 A,  15 B in such a manner that two adjacent lateral sides of this assembly rest on the faces  23  of the first bearing surface  21 , as illustrated on the left in  FIG. 5 . The edge separating the two adjacent sides of the fuel assembly is located along the vertex  25 . 
     Once the assemblies are in place in the housings  15 A,  15 B, the doors  17 A,  17 B are closed. For that purpose, each door  17 A,  17 B is caused to pivot about the shafts  51 , through approximately 180°, the door then occupying an intermediate position illustrated on the left in  FIG. 5 . In this intermediate position, the lower portion  35  of the door is engaged in the housing, the faces  41  of the second bearing surface  39  being separated from the fuel assembly by a free space. 
     The screws  47  are then caused to turn in the bearings  53  by means of suitable tools in order to cause the nuts  49  to descend along the screws  47 , the shaft ends  51  driving the door towards the assembly located in the housing. 
     When the faces  41  of the second bearing surface  39  come into contact with the nuclear fuel assembly, the translational movement of the cover  17 A,  17 B is interrupted. It will be noted that the second bearing surface  39  comes into direct contact with the nuclear fuel assembly. In particular, the door  17 B, just like the door  17 A, is free from bearing runners, such as those provided in the prior art at right-angles to each of the grids of a nuclear fuel assembly to be transported. 
     It will be appreciated that the translational movement is effected in a direction symbolized by the arrow F 1  in  FIG. 5  and passing via the vertices  25  and  43  of the two bearing surfaces  21  and  39 . 
     This second part of the movement of the door  17 A,  17 B enables the spacing between the first and second bearing surfaces  21  and  39  to be adjusted in the holding position of the door, in accordance with the size of the fuel. 
     For, as shown in  FIG. 4 , for fuel having a square cross-section of large size, the translational movement of the door  17 A,  17 B will be interrupted earlier. The cross-section of such fuel is symbolized by the line marked CG on the right in  FIG. 4 . In that case, the faces  41  of the second bearing surface  39  bear against the two adjacent sides of the fuel that face the top of  FIG. 4 , but cover only a portion of those sides. A band  74  of those sides remains free, between the first and second surfaces  21  and  39 . 
     For fuel having a cross-section of intermediate size, symbolized by the dot-dash line CM in  FIG. 4 , the translational movement of the door  17 A,  17 B is stopped further away than in the case of fuel having a cross-section CG. The free band  74  is reduced. 
     Finally, for fuel having a cross-section of small size, symbolized by the dot-dash line CP in  FIG. 4 , the descent movement of the door  17 A,  17 B is stopped even further away than for the cross-sections of size CG and CM, the door coming into contact with the faces  23  by way of the lateral faces  45  of the second bearing face. The sides of the assembly that face the top of  FIG. 4  are completely covered by the faces  41  of the second bearing surface  39 . There is no longer a free band  74 . 
     In its translational movement towards the vertex  25  of the housing  15 A,  15 B, the upper portion  33  of the door is guided by the upper guide surfaces  29 , and the lower portion  35  of the door is guided by the lower guide surfaces  27 . 
     Once the door  17 A,  17 B is in its holding position, the screws  63  are screwed into the threaded orifices  61 . The serpentine form, the surface state and the manufacturing tolerances of the guide surfaces  27  and  29  and of the shoulder  31 , on the one hand, and of the doors  17 A,  17 B, on the other hand, are such that the housings  15 A,  15 B are well sealed and that the nuclear materials are confined in the housings  15 A,  15 B in the event of a serious accident which would have caused cladding bursts in the assemblies. 
     The assembly  10  is subsequently swung as far as the horizontal position and then comes to rest on the cradle  8  where it is secured by bolts. 
     After placing the upper half-shell back on the lower half-shell of the casing  2  and securing the two half-shells by screws and nuts, it is possible to handle and transport the container, for example, by lifting the container by means of the lifting feet  75  and  75 ′ secured to the upper half-shell of the external casing, as is shown in  FIG. 1 . 
     The procedure for unloading the nuclear fuel assemblies is the reverse of the procedure for loading these assemblies into the container. It will not be explained in detail here. 
     The transport container described above can be used for fresh or irradiated nuclear fuel assemblies, regardless of the nuclear fuel UO 2 ,PuO 2  . . . . It can also be used to transport equipment having a space requirement similar to that of a nuclear fuel assembly, for example rod boxes, quiver-like supports, or skeletons of nuclear fuel assemblies. 
     The container described above has multiple advantages. 
     It is possible in the same container, with the same internal structure, to transport nuclear fuel assemblies of different sizes. This result is achieved owing to the fact that it is possible to adjust the spacing between the first and second bearing surfaces  21  and  39  by displacing the doors  17 A,  17 B along the screws  47 . 
     The adjustment described above is also effected using simple and economical means: the screws  47  mounted in the bearings  53 , and the nuts  49  provided with shafts  51  engaged in the blind holes  59  of the door. 
     It is possible in the same container, with the same internal structure, to transport nuclear fuel assemblies having different grid positions. This result is achieved owing to the fact that the bearing surfaces  21  and  39  are smooth and the doors  17 A,  17 B do not have movable runners which are to rest on the grids of a transported nuclear fuel assembly. This feature is also advantageous with regard to the cleaning and decontamination of the receiving housings  15 A and  15 B and the doors  17 A and  17 B of the container, the surfaces  21  and  39  of which are smooth and may be free from retention zones. 
     This feature is also advantageous with respect to the increase in mass associated with the absence of runners in that it enables more assemblies to be transported for the same external space requirement. 
     The operation of the container is particularly simple owing to the fact that it comprises only a small number of screws  47  enabling the position of the door to be adjusted, and a small number of securing screws  63 . 
     The container described above may have multiple variants. 
     Thus, the means for displacing the doors  17 A and  17 B on the support  13  may have structures other than that described above. By way of example, they may be in the form of connecting rods arranged to form an arm of the pantograph type. Such arms are known from the prior art and therefore will not be described in detail here. They make it possible to obtain a movement of the door, first of all of rotation and then of translation, to pass from its release position to its holding position, like the screw and nut displacement means described above. 
     More generally, these displacement means do not necessarily ensure a movement of translation and then of rotation. 
     It is thus possible to provide that each door will pass from its holding position to its release position by a simple translational movement along the screws  47  perpendicularly to the upper face  19  of the support  13 , without a 180° rotation as in the embodiment described above. In that case, the release position corresponds substantially to the position of the door illustrated on the left in  FIG. 5 . The introduction of the nuclear fuel assemblies into the housings is then effected by a longitudinal movement, by means of a crane. The withdrawal of the assemblies from the housings is effected in the same manner. 
     In a variant, it is also possible to provide that the door is dismountable, in which case the screws  47  can be replaced by securing screws of the same type as the screws  63 . In order to load and unload the assemblies into and from the housings, all of the securing screws are then unscrewed and the doors  17 A,  17 B are subsequently removed completely, for example by means of a travelling crane. 
     Protective means may be located around the nuclear fuel assemblies, inside the support  13  and/or the doors  17 A,  17 B. These protective means may be of different types. They may be of the mechanical type in order to stiffen the internal equipment of the container and to protect the fuel assemblies in the event of the container falling or in the event of impact. These protective means may also be of the neutron type and may absorb the neutrons emitted by the nuclear fuel assemblies. The protective means may also be of the thermal type in order to prevent the heat generated by the fuel assembly from being conducted through the support or the door. The protective means may also be of the biological type and may absorb the ionizing radiation emitted by the nuclear fuel assemblies, for example gamma radiation. It is even possible that these protective means may be sufficient to transport a nuclear fuel assembly without an external casing  2  being necessary. 
     The container described above is suitable for transporting nuclear fuel assemblies for a BWR reactor (boiling water reactor) or a PWR reactor (pressurized water reactor). These assemblies may be of the type 17×17, 10×10, 18×18, or of any other type. It will be recalled that these numbers characterize the square network in accordance with which the fuel rods are arranged. Thus, a 17×17 assembly has a network of seventeen rows of seventeen rods or accessories. 
     The container can also be adapted to transport nuclear fuel assemblies, the cross-section of which is not square but, for example, rectangular or hexagonal. 
     In such cases, the adjustment of the position of the door relative to the support  13  is not necessarily effected by a vertical translation as described above and represented by the vertical arrow F 1  in  FIG. 5 . Thus, in case of rectangular assemblies, this translation will be effected in a direction passing via the vertices  25  and  43  of the longitudinal bearing surfaces  21  and  39 . 
     In the case of a hexagonal nuclear fuel assembly, it is provided, for example, that the faces  23  of the first bearing surface  21  form between them an angle of approximately 60°. Likewise, it is provided, for example, that the faces  41  of the second bearing surface  39  form between them an angle of 60°. The assembly is arranged in the housing in such a manner that a first side of the hexagon is in contact with one of the faces  23 , and a second side of the hexagon is in contact with the other face  23 . A third side of the hexagon, connecting the first and second sides, extends from one face  23  to the other, opposite the vertex  25 . This third side is not placed against the bearing surface  21 . In the same manner, two other sides of the hexagon rest against the faces  41  of the second bearing surface  39 , one side of the hexagon extending between these two faces  41 , opposite the vertex  43 . 
     In the same manner, the internal structure of the container can be adapted to transport nuclear fuel assemblies having an octagonal or triangular cross-section or any other polygonal cross-section. 
     In a variant, it is possible to provide that the two pairs of faces  23  and  41  of the first and second bearing surfaces  21  and  39  form a continuous square, of variable size depending on the cross-section of the assembly to be transported, as illustrated in  FIG. 7A . In that case, the means for adjusting the spacing between the first and second surfaces  21  and  39  may comprise means for displacing in a co-ordinated manner the four faces  23  and  41  along each other, in order to vary the size of the square. The faces  23  and  41  remain perpendicular to each other in the course of this movement. 
     According to another variant, illustrated in  FIG. 7B , each of the first and second bearing surfaces  21  and  39  comprises a large face  23 ,  41  and a small face  23 ′,  41 ′ which is narrower than in the large face in a transverse plane. Each of the first and second surfaces  21 ,  39  also comprises an undercut  80  bordering the small face and delimited in part by a guide surface  82 . The guide surfaces  82  extend substantially parallel with the diagonal passing via the vertices  25  and  43 , that is to say, vertically in  FIG. 7B . As shown in  FIG. 7B , the two large faces  23 ,  41  are parallel and opposite, and the two small faces  23 ′,  41 ′ are parallel and opposite. The two faces of the same bearing surface are typically perpendicular to each other. As can be seen on the right in  FIG. 7B , for a nuclear fuel assembly having a large cross-section, the two opposite sides of this assembly resting on the large faces  23  and  41  are completely covered by those faces. On the other hand, the two opposite sides resting on the small faces  23 ′ and  41 ′ are only partially covered by those faces. It can be seen on the left in  FIG. 7B  that, for nuclear fuel assemblies having a small cross-section, the four sides of these assemblies are completely covered by the faces  23 ,  23 ′,  41 ,  41 ′, the free edges of the large faces  23  and  41  engaging in the undercuts  80  of the opposite surface. 
     The surfaces  82  enable the relative displacement of the first and second bearing surfaces  21  and  39  to be guided. Furthermore, they form baffles enabling sealing to be improved. 
     Finally, the bearing surfaces  21  and  39  are delimited by two identical members which are fitted together head to tail, which enables the production costs to be reduced. 
     Preferably, the faces of the first surface  21  form between them an angle substantially equal to the angle which the faces of the second surface  39  form between them. This angle is from 60° to 135°, depending on the geometry of the nuclear fuel assemblies to be transported. 
     More generally, the container  1  according to the invention can receive a number of nuclear fuel assemblies other than two. Thus, it may be configured to receive a single nuclear fuel assembly, or in some variants a much higher number, for example six or eight. 
     The container  1  may also comprise, in addition to the first and second longitudinal bearing surfaces, a third longitudinal bearing surface. 
     Each of these longitudinal bearing surfaces may comprise, as in the examples described above, two longitudinal faces, but the number of faces may also be different, for example a single longitudinal face may be envisaged, and three longitudinal faces may be envisaged. 
     Thus, for nuclear fuel assemblies having a hexagonal cross-section, it is possible to provide three longitudinal bearing surfaces each comprising a single bearing face, these faces being inclined relative to each other by 120° when they rest against an assembly. 
     Still for the same type of assembly, it is possible in another variant to provide a first surface comprising three faces inclined relative to each other by 120° and intended to rest on consecutive faces of a nuclear fuel assembly. The second surface may then comprise a single bearing face. 
     When the same surface comprises several longitudinal faces, the latter do not necessarily intersect at a vertex as described above. 
     Likewise, in the examples described above, the longitudinal bearing surfaces rest directly on the nuclear fuel assemblies, without using movable holding runners. 
     However, it is also possible to use such runners or other means to ensure contact between the longitudinal bearing surfaces and a transported nuclear fuel assembly. 
     The invention described above can be readily implemented simply by modifying existing packages.