Patent Number: 
Section: description

FIGS. 1 and 2 show a transport container for new fuel assemblies of a pressurized water nuclear reactor denoted overall by the reference number 1. The transport container 1, which is designed to transport two fuel assemblies in a horizontal position, has an external envelope 2 formed by a lower shell 2a and an upper shell 2b, both semi-cylindrical in shape and connected one on top of the other along a joining plane of the envelope 2 passing through the longitudinal axis of the cylindrical-shaped envelope. Each of the shells 2a and 2b is made from steel sheet and has semicircular reinforcing ribs 3a, 3b respectively, distributed over the length of the half-shell. Sections 4 and 4xe2x80x2 are also fixed to the lower part of the lower half-shell 2a, said sections forming support feet for the container. Furthermore, adjustable support elements 5 and 5xe2x80x2, which have screw jacks and which are secured to a longitudinal end part 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 of the container respectively. By using the adjustable feet 5 and 5xe2x80x2 of the container, it is possible to place the container, on its transport support, in a perfectly horizontal position, i.e. in a position in which the longitudinal axis of the container is perfectly horizontal. The two half-shells 2a and 2b are brought together one on top of the other via rectangular peripheral flanges forming an upper planar support part of the lower half-shell 2a and a lower planar support part of the upper half-shell 2b of the container. In the closed position of the container, shown in FIGS. 1 and 2, the flanges of the two half-shells 2a and 2b are brought together and fixed one on top of the other by screws and nuts, forming an assembly flange 6. FIGS. 3A and 3B show a part of the container in the open state, i.e. with the upper half-shell of the container envelope separated from the lower half-shell and removed. FIGS. 3A and 3B show the internal structure of the container, denoted overall by the reference number 7, which has in particular a cradle 8 resting on supports 9 formed by shock-absorber pads, in the lower half-shell 2a of the external envelope 2 of the container. A second part of the internal structure of the container is formed by a unit 10 for receiving and supporting two fuel assemblies in the horizontal position placed side by side. The unit 10, which rests on the cradle 8, defines two completely closed housings for two fuel assemblies, as will be explained hereinafter. The cradle 8 has two side rails 8a, 8b formed by angle brackets fixed on the support pads 9 and which are held in parallel positions, with a separation corresponding to the width of the unit 10 for receiving the container, by crossmembers. At one of its ends, the cradle has a pivoting stiffening and mounting unit comprising two plates 11a and 11b which are parallel to one another and two crossmembers formed by hollow sections fixed to the side rails of the cradle and to the plates 11a and 11b.  The mounting of the cradle on the lower shelf of the container such that it can pivot about a horizontal axis of transverse direction, is ensured via the pivoting stiffening and mounting unit comprising the plates 11a and 11b.  Furthermore, as will be explained hereinafter, a retaining plate for fuel assemblies is also mounted between the plates 11a and 11b.  As can be seen in FIG. 3B, a shock absorber 43 is inserted between the longitudinal end of the internal structure 7 and the internal circular end wall of the external envelope 2, in such a way as to limit the effect of a shock to the fuel assemblies, for example the effect of dropping a container. The shock-absorber 43, in the shape of a disc whose cross section is identical to the internal cross section of the container envelope, is made up of a balsa disc surrounded by an envelope made from stainless steel sheet. Of course, an identical shock absorber is positioned at the second longitudinal end of the container, between the second longitudinal end of the internal structure and the second end of the external envelope. As can be seen in FIG. 4, the fuel assembly support and reception unit 10 has a frame 12 having a Tshaped cross section and two doors 14a and 14b mounted is such that they pivot on the sides of the frame 12, as will be explained hereinafter. In the closed position of the doors, as shown in FIG. 4, the door 14a together with the right part of the frame 12 defines a housing 13a for one fuel assembly and the door 14b together with the left part of the frame 12 defines a second housing 13b. The housings have a square cross section which has the dimensions of the cross section of a spacer-grid of a pressurized water nuclear reactor fuel assembly for which the container 1 ensures transport. To load the container, the cradle 8 is made to tilt about the transverse axis located at one of the ends of the cradle into a position which is substantially vertical. In its tilted position, the fuel assembly reception and support unit 10, is in a vertical position. The doors 14a and 14b are tilted towards the outside, in such a way as to give access to the housings 13a and 13b.  A fuel assembly may be placed in each of the housings 13a and 13b, using a fuel assembly lifting tool, for example the hoist of an overhead crane. The fuel assemblies come to rest, via their bottom nozzles, on the fuel assembly support plate fixed between the two plates 11a and 11b of the cradle 8. The doors of the fuel-assembly reception and support unit 10 are closed and the unit 10 is tilted into the horizontal position, coming to rest on the cradle 8. After having placed the upper half-shell back on the lower half-shell of the envelope 2 and fixed the two half-shells by screws and nuts, the container can be handled and transported, for example by lifting the container using lifting lugs 15 and 15xe2x80x2 fixed on the upper half-shell of the external envelope, as shown in FIG. 1. FIG. 5 shows an exploded view in perspective of the cradle 8 and the various elements forming the fuel assembly reception and support unit 10. The frame 12, which has a T-shaped transverse section, has a parallelepipedal base 12a and a wall 12b perpendicular to the base 12a, separating the housings 13a and 13b for two fuel assemblies 16a and 16b, the spacer-grids 17a and 17b, the bottom nozzles 18a and 18b and the top nozzles 18xe2x80x2a and 18xe2x80x2b of which are shown. The housings 13a and 13b of the fuel assemblies 16a and 16b are defined at one of the ends of the frame 12, by a support plate 20 intended to be fixed such that it pivots, via stub shafts, between the plates 11a and 11b of the cradle 8 and a second end plate 21 mounted such that it pivots at the second end of the frame 12, about a transverse pivot axis. The fuel assemblies rest, via their top nozzles 18xe2x80x2a and 18xe2x80x2b, on the plate 20. The transverse holding plate 21 has adjustable supporting end-stops on the bottom nozzles 18a and 18b of the fuel assemblies. The plate 21 could also have adjustable means for holding the fuel assemblies in the longitudinal direction. When the end plates 20 and 21 are pulled down into their closed position, the fuel assemblies are held in the longitudinal direction by being clamped between the support devices 22 and the plate 20. The pivoting lateral doors 14a and 14b of the unit 10 holding and supporting the fuel assemblies 16a and 16b have an inverted L-shaped cross section and have, along their lower edge, at the end of one of the branches of the L, articulating parts 23 in the form of hinges spaced out over the length of the doors 14a and 14b.  The doors shown in FIG. 5 have six hinges. 23 spaced apart over the length of a first lower edge of the doors 14a or 14b.  Along its opposite second edge, at the end of the second branch of the L, each of the doors 14a and 14b has fixing lugs 24 having a part pierced by an opening and projecting slightly towards the outside with respect to the edge of the door. The hinge shaped articulating parts 23 have all openings aligned in a direction parallel to the edge of the door and each one engages on an articulation axis 25, fixed so that it projects from a lateral edge of the base 12a of the frame 12 of the fuel-assembly support. Similarly, the openings in the parts projecting from the lugs 24 located along the second edge of the doors are aligned in a direction parallel to the edge of the door. The median wall 12b of the frame 12 has on its upper edge guide parts 26 and 26xe2x80x2 having openings which are all aligned in a direction parallel to the upper edge of the median wall 12b of the frame 12. When the doors, which are mounted articulated on the articulation axes 25 via hinges 23, are pulled down to the closed position, the second edges of the doors 14a and 14b along which the lugs 24 are located, are pulled down onto the upper edge of the median wall 12b of the frame 12, each of the lugs 24 coming to a position inserted between two successive guide posts 26 and 26xe2x80x2 fixed on the upper edge of the median wall 12b of the frame 12. The doors 14a and 14b, when in the closed position, can be locked by introducing a rod into the aligned openings of the parts 26 and 26xe2x80x2 and of the lugs 24. Furthermore, the doors 14a and 14b have pegs 27a, 27xe2x80x2a and 27b, 27xe2x80x2b respectively at their longitudinal ends projecting towards the outside in the longitudinal direction. The end plates 20 and 21 of the frame 12 each have, along their upper and lateral edges, slots 28 and 28xe2x80x2, each one intended to receive one of the pegs 27a or 27b or one of the pegs 27xe2x80x2a and 27xe2x80x2b respectively, in the closed position of the doors, after the end walls 20 and 21 have been pulled down. Furthermore the walls 20 and 21 have openings passing through them, facing each of the nozzles of the fuel assemblies, in their transport position inside the housings 13a and 13b.  Each of the fuel assembly housings 13a or 13b, which is defined on two lateral faces by two mutually perpendicular surfaces of the frame 12, on its opposite lateral faces by two internal perpendicular surfaces of a door 14a or 14b and at its ends by the plates 20 and 21, is completely closed and ensures effective containment of a fuel assembly. Should the container be subjected to a shock, leading to a partial destruction of the fuel assembly, pieces of fuel assemblies, for example pieces of fuel pellets or rods, cannot escape from the fuel assembly housing and be spread in the container. The doors 14a and 14b and the end walls 20 and 21 which are mounted such that they pivot, form a box having two housings for fuel assemblies, which can be opened to give access to the fuel assembly housings. Furthermore, as will be explained hereinafter, the base 12a and the median wall 12b of the frame and the walls of the doors 14a and 14b are constructed in the form of a double wall inside the thickness of which a neutron-absorbing resin i.e. a synthetic resin to which is added an element which strongly absorbs neutrons, is placed. FIG. 6 shows an exploded view in perspective of the elements forming the frame 12 of the fuel assembly reception and support unit. The frame 12 has a baseplate 30 reinforced by welded ribs 29 and by transverse sections 31 at the end of which are fixed articulation axes 25 for the doors 14a and 14b and lugs 32 for fixing the frame 12 to the lateral sides of the cradle 8, via screws and nuts (FIGS. 7 and 8). On either side of each of the sections 31, on top of the plate 29, in its median part, columns 33 are fixed perpendicular to the plate 29. To the upper part of the columns 33 are fixed elements 26xe2x80x2 for guiding the means of locking the doors of the fuel assembly reception and support unit. The second element forming the frame 12 is a profiled element in folded metal sheet 34 comprising two elements of metal sheet folded into an L-shape extended towards the bottom by two sills and connected at their upper part by elements which are folded and/or attached forming guide parts 26 for guiding the upper edge of the median wall 12b of the frame 12. On the folded-down lateral edges of the profiled sheet-metal element 34 passages are provided for the articulation axes of the doors and the pads for fixing the frame on the cradle which are fixed to the end of the reinforcing sections 31. Two T-shaped spacer parts 35a and 35b are fixed to the end of the plate 29. The frame 12 is produced by assembling the folded sheet-metal element 34 and the baseplate 29 having reinforcing elements and the columns 33. The end spacers 35a and 35b of the baseplate 30 are inserted into the internal profile of the folded sheet-metal element 34. Similarly the six columns 33 are inserted into the vertical part of the internal profile of the folded sheet-metal element 34, between the two vertical branches of the two L-shaped lateral sheet-metal elements. The guide parts 26xe2x80x2 fixed to the end of the columns are inserted between two successive guide parts 26 connecting the two L-shaped folded sheet-metal elements, in the form of the profiled element 34 with a T-shaped transverse cross section. In the assembled position of the frame 12, the horizontal parts of the sheet-metal elements folded into an L-shape come to rest on the spacers 35 and on the sections 31, in such a way that an empty space is kept between the horizontal parts of the sheet-metal element 34 and the baseplate 29. As can be seen in FIG. 8, this free space 36 is filled with a neutron-absorbing resin. The resin is a dense resin whose density is between 1.5 and 2. Similarly, an empty space 37 between the vertical parts of the sheet-metal element 24 is filled with a high density neutron-absorbing resin. The resin and the spacer elements ensure the mechanical integrity of the frame 12. By assembling the plate 30, its reinforcing elements and the columns 33 with the folded sheet-metal element 34, a double walled, stiff frame 12 is obtained. By filling the empty spaces 36 and 37 of the double wall with a neutron-absorbing resin, a frame whose baseplate 12a and the separating median wall 12b are capable of absorbing neutron flux produced by fuel assemblies placed in the housings 13a and 13b of the frame 12 is obtained. FIG. 9 shows the right hand door 14a of the fuel assembly reception and support unit. The door 14a (and likewise the second door 14b) is formed by sheet-metal elements folded into an L-shape which are connected to one another at the ends of the branches of the L by extensions of one of the branches, the articulating parts 23 and the locking lugs 24. Furthermore, between the two metal sheets forming the L-shaped door, spacers 38 are placed at a certain distance from one another over the length of the door 14a.  Each of the spacers 38 has, as can be seen in FIG. 10, two L-shaped plates spaced out from one another in the longitudinal direction of the door and fixed at their ends to an articulating part 25 and to a locking lug 24, respectively. A fuel assembly clamping device, placed in the housing defined by the door, is fixed to each of the branches of the L at each spacer 38 between the two L-shaped plates forming the spacer, ensuring the fuel assembly is held in a transverse direction. As can be seen in FIG. 10, each of the clamping devices 39 has a flat pad 40 which can be manoeuvred from the outside of the door by a screw 41, in order to move it in a direction perpendicular to the branch of the L of the door in which the locking device 39 is mounted. At each of the spacers 38, the door 14a has two clamping devices 39 intended to come into contact with two external faces of a spacer-grid of a fuel assembly positioned in the housing defined by the door 14a. In this way, the fuel assembly is clamped into its housing, on two mutually perpendicular sides. FIG. 11 shows a longitudinal end of the door 14a which is closed by an L-shaped plate 41 to which are fixed, projecting towards the outside, pegs 27 for fixing the door 14a to the end wall 21. As can be seen on the cutaway part of FIG. 11, a blocking rod 42 is mounted so that it slides in aligned openings in the upper horizontal wall of the door 14a and between the pegs 27a. Furthermore, the rod 42 is manoeuvrable from the outside of the door 14a.  When the door 14a is in the closed position and the end plate 20 (or 21) is pulled down to the closed position of the longitudinal ends of the housings of the fuel assembly reception and support unit, the rod 42 can be introduced into aligned openings passing through the external parts of the plate 20 (or 21) between the slots 28, in the transverse direction and the openings between the pegs 27a placed in alignment with the openings of the plates 20 (or 21). In this way the end closure plates 20 and 21 are locked on the end parts of the door 14a.  Of course, each of the ends of the door 14a having pegs 27a and 27xe2x80x2a can be locked in an identical fashion. The same locking rod 42 can lock the second door 14b by being introduced into the openings of the plate 20 (or 21) and the pegs 27b (or 27xe2x80x2b). The empty space between the two elements of the L-shaped wall of the doors 14a and 14b is filled with a neutron-absorbing resin, in order to absorb any neutron flux originating from a fuel assembly and directed towards the outside of the fuel assembly reception and support unit. The resin, which has high density (density from 1.5 to 2), and the spacers ensure the mechanical integrity of the doors. The internal structure of the container according to the invention defines two housings for two fuel assemblies which are completely closed and inside which the fuel assemblies are held laterally and in the axial or longitudinal direction. As the housings are completely closed, if any shock should cause partial destruction of a fuel assembly, parts of the fuel assembly are incapable of escaping from the internal structure which ensures the containment of the fuel assembly. The pieces of the fuel assembly are therefore incapable of spreading inside the external envelope of the container. Furthermore, the fuel assemblies are separated from each other inside the internal structure of the container, by a neutron-absorbing wall. The fuel assembly housings defined by the internal structure also have a neutron-absorbing wall closing the housings on the outside, i.e. towards the internal surface of the external envelope of the container. Improved mechanical protection of the fuel assemblies during their transport inside the container is therefore obtained at the same time as a reduction in the risks of achieving criticality during transport of more than one fuel assembly. The invention is not limited to the embodiment which has been described. In this way, the internal structure of the container may have a different shape to that which has been described and may have elements other than a T shaped frame and tilting doors. The shape of the housings in the internal structure of the container depends on the shape of the fuel assemblies being transported. In all cases, the internal structure has walls assembled to each other defining at least one completely closed fuel assembly reception and holding housing. The invention is applicable to the transport of any nuclear fuel assembly having a right prismatic shape. The container according to the invention can be used not only for the transport of new fuel assemblies but also for the transport of used fuel assemblies having low activity.