Fuel element with spacer elements for a nuclear reactor

Abstract

A fuel element (1) for a nuclear reactor comprises a casing (2) containing a plurality of fuel rods (3) extending along respective axes (A) and provided with respective spacer elements (4); each spacer element (4) comprises an internal fastener element (9), fitted on a containment tube (5) of the fuel rod (3); a contact element (10), joined to the fastener element (9) and located radially outside the fastener element (9) and cooperating in use with contact elements (10) of other spacer elements (4) located on adjacent fuel rods (3); at least some spacer elements (4) are provided with axial restraining members (22) engaging respective adjacent spacer elements (4) to prevent relative axial sliding between the spacer elements (4).

Description

[0001] "FUEL ELEMENT WITH SPACER ELEMENTS FOR A NUCLEAR REACTOR"

[0002] Cross-Reference to Related Applications

[0003] This Patent Appl ication claims priority from Italian Patent Application No . 102023000018663 filed on September 12 , 2023 , the entire disclosure of which is incorporated herein by reference .

[0004] Technical Field

[0005] The present invention relates to a fuel element for a nuclear reactor, in particular a fuel element provided with spacer elements .

[0006] Background

[0007] As is known, a nuclear reactor has a core composed of a plurality of fuel elements . Each fuel element is in turn composed of a plurality of fuel rods more or less spaced apart , between which the cooling f luid circulates . The fuel elements generally have a square or hexagonal section . The fuel rods generally have a cylindrical shape closed at the ends and are slender ( thin) , having a length that can reach even a few hundred times the diameter . Given the geometry, in order to avoid the risk of vibrations and deformations with respect to the theoretical position of the axi s , it is necessary to restrain the rods with respect to one another by means of spacer elements .

[0008] It is known to use grids as spacer elements of the fuel rods inside a fuel element , said grids being positioned at di f ferent heights along the rods , as described for example in EP0084947A2 .

[0009] Another known spacer element is a wire helically wrapped around the rod, as described in WO2013162957A2 . The grids are especially utili zed when the cooling fluid is water, whereas helical wire spacer elements are especially utili zed in the sodium-cooled reactors .

[0010] However, the known solutions have some drawbacks .

[0011] The grids are complicated and dif f icult to manufacture ; furthermore , during the operation of the reactor, they have a slight di f ferential axial sliding with respect to the rods with possible local erosion of the rods and do not allow an optimal uni formity of the temperature of the cooling fluid along the entire circumferential development of the rods . One of the maj or dif ficulties concerning the grids is their mounting because , i f they are pre-assembled, the rods must be mounted through them with axial sliding and relative rubbing; whereas , i f they are assembled together with the rods , this makes the design and the assembl ing thereof di f ficult .

[0012] The helical wire is suitable only for applications in which the distance between the rods is very small ; otherwise , its diameter would be excessive with consequent excessive axial sti f fness with respect to the rod and excessive neutron absorption .

[0013] EP0534254A1 describes a further solution wherein the spacer element is sleeve-shaped and is placed around the fuel rod and connected to the rod, for example by means of welding on a contact line . The drawback of this solution is that it is necessary to make welds or another type of connection which alter the surface of the protective covering ( cladding) which covers the surface of the fuel rod and thus results to be not acceptable .

[0014] For some applications , such as the lead-cooled reactors with protective covering ( cladding ) of the surface of the fuel rod, none of the proposed solutions are satis factory and for years there has been debate and no consensus among persons skilled in the art as to which is the preferable solution, without a satis factory solution having been found to date . In particular, any solution involving a sliding between spacer element and rod, both in the mounting step and in the operating step, can ruin the thin protective covering of the rod . Any connection weld between rod and spacer element ruins the protective covering .

[0015] Summary

[0016] An obj ect of the present invention is to provide a fuel element for a nuclear reactor which overcomes the highlighted drawbacks of the known solutions and has further advantages from a constructive point of view and in terms of ef ficiency, reliability and safety .

[0017] The present invention thus relates to a fuel element for a nuclear reactor as defined in the appended claim 1 and, with regard to its ancillary features , in the dependent claims .

[0018] Substantially, the fuel element of the invention is provided with a restraining and spacing system of the fuel rods forming the fuel element , said restraining and spacing system ensures , in a simple , ef f icient and fully reliable manner, the correct positioning and spacing of the fuel rods , resulting to be particularly simple to manufacture and mount .

[0019] Brief Description of the Drawings

[0020] The invention i s described in the following nonlimiting example embodiments , with reference to the figures of the accompanying drawings , wherein : Figure 1 is a schematic overall longitudinal section view of a fuel element for a nuclear reactor in accordance with a first embodiment of the invention, comprising fuel rods provided with, at predefined heights, spacer elements;

[0021] Figure 2 is a cross-section view of line II-II and on an enlarged scale of the fuel element of Figure 1;

[0022] Figure 3 is an axonometric view of a spacer element included in the fuel element of Figure 1;

[0023] Figures 4A and 4B are a longitudinal section view (of line IVa-IVa) and a cross-section view (of line IVb- IVb) , respectively, of the spacer element of Figure 3;

[0024] Figures 5 and 6 are a longitudinal section view (of line V-V) and a cross-section view (of line VI-VI) , respectively, of a detail of the fuel element of Figure 1, in particular of two adjacent portions of respective fuel rods provided with spacer elements;

[0025] Figure 7 is a cutaway in axonometric view of a further detail of the fuel element of Figure 1, in particular a portion of fuel rod provided with spacer element;

[0026] Figures 8A and 8B are axonometric views on an enlarged scale of the spacer element of Figure 3;

[0027] Figure 9 is an axonometric view of a variation of the spacer element;

[0028] Figures 10A-10C illustrate a further variation of the spacer element and are a cross-section view of only the spacer element, a cross-section view of a plurality of adjacent fuel rods, each of which is provided with the spacer element, and a plane development view of one single spacer element, respectively;

[0029] - Figures 10D-10F illustrate a further variation of the spacer element and are a cross-section view of only the spacer element , a cross-section view of a plurality of adj acent fuel rods , each of which is provided with the spacer element , and a plane development view of one single spacer element , respectively;

[0030] Figures 11A- 11D illustrate a further variation of the spacer element applicable to the upper end o f the fuel rod, and are , in order, a longitudinal section view of the spacer element located on an upper plug of the fuel rod, a cross-section view thereof , a side elevation view and a top plan view;

[0031] Figures 12A- 12C illustrate a further variation of the spacer element and are an axonometric view and a top plan view of only the spacer element , and a top plan view, with parts removed for clarity, of a plurality of adj acent fuel rods , each o f which is provided with the spacer element , respectively .

[0032] Description of Embodiments

[0033] With reference to Figures 1-2 , a fuel element 1 of a nuclear reactor comprises a hollow casing 2 having, for example , a hexagonal cross-section (but other shapes are possible , in particular the one with a square section) and which supports and contains , on the inside , a plurality of fuel rods 3 side by side one another and arranged, for example , with equilateral triangular step (but other arrangements are j ust as possible , for example with square step ) and spaced apart from one another by spacer elements 4 .

[0034] Each fuel rod 3 extends along a longitudinal axis A and comprises a containment tube 5 ( for example with a circular cross-section) containing nuclear fuel and closed at an upper end by a first plug 6 and at a lower end by a second plug 7 also having the function of fastening the fuel rod 3 to the casing 2 by means of pins 8 which proj ect from a lower end of the casing 2 and engage the plug 7 .

[0035] According to the invention, a number of spacer elements 4 are integrally applied to each fuel rod 3 , said spacer elements 4 being inserted on the tube 5 and axially spaced apart from one another along the fuel rod 3 ; the spacer elements 4 of adj acent fuel rods 3 , entering into contact with one another , maintain the fuel rods 3 at a preset distance .

[0036] Each spacer element 4 extends along and around an axis coinciding in use with the longitudinal axis A of the fuel rod 3 on which the spacer element 4 is mounted, and comprises an internal fastener element 9 , fitted around the fuel rod 3 on an outer lateral surface of the tube 5 , a contact element 10 , cooperating in use with the contact element 10 of another spacer element 4 located on an adj acent fuel rod 3 , and a plurality of structural connecting elements 11 which j oin the fastener element 9 to the contact element 10 .

[0037] Each spacer element 4 is composed of an integral monolithic piece formed by the fastener element 9 , the contact element 10 and the structural connecting elements 11 .

[0038] In the embodiment illustrated more speci fically in Figures 3-7 , the fastener element 9 is defined by a tubular, for example cylindrical , body 9b fitted on the tube 5 of the fuel rod 3 , and the contact element 10 is defined by an annular body 10b placed around and radially outward with respect to the fastener element 9 .

[0039] In the illustrated example , the annular body 10b has a cylindrical shape with circular cross-section, but it could also have a prismatic shape with, for example , a square or hexagonal cross-section .

[0040] The contact element 10 is placed concentric around the fastener element 9 and radially spaced apart from it ; the fastener element 9 and the contact element 10 are radially spaced apart from each other and separated by an annular interspace and are j oined by the structural connecting elements 11 which extend radially through the interspace .

[0041] The structural connecting elements 11 are composed of ribs which proj ect from the outer lateral surface of the fastener element 9 and are j oined to it by a rectilinear root edge ; preferably, the structural connecting elements 11 are inclined with respect to the fastener element 9 both on a plane perpendicular to the axis A, where they are oriented at a pre-established angle a with respect to an axis orthogonal to the axis A and passing through the root edge ( Figure 4B ) ; and along the axis A, being inclined at an angle p with respect to the axis A ( Figure 3 ) .

[0042] As is illustrated in Figures 8A, 8B, some or the totality of the structural connecting elements 11 protrude axially beyond a lower ( Figure 8A) or upper ( Figure 8B ) axial end of the spacer element 4 and in particular beyond an ( a lower or upper ) axial end edge 12 of the spacer element 4 .

[0043] In particular, at least some connecting elements 11 have respective lower axial extens ions 14 or respective upper axial extensions 15 arranged to extend the respective structural connecting elements 11 ; the extensions 14 , 15 are provided with respective radial teeth 16 , 17 which proj ect perpendicular to the extensions 14 , 15 and to the structural connecting elements 11 and protrude radially beyond an outer lateral surface 20 of the contact element 10 so as to engage respective axial end edges 12 of contact elements 10 of adj acent spacer elements 4 . The extensions 14 , 15 and the teeth 16 , 17 are formed integral to the structural connecting elements 11 and to the spacer element 4 as a whole .

[0044] In the embodiment of Figure 9 , the contact elements 10 are provided with radial teeth 18 which proj ect radially from the outer lateral surface 20 of the contact element 10 , and with recesses 19 formed in the surface 20 ; the teeth 18 and the recesses 19 are arranged aligned and circumferentially spaced apart on each contact element 10 , so that , after mounting the fuel rod 3 inside the fuel element 1 , the teeth 18 of a contact element 10 engage corresponding recesses 19 of contact elements 10 of adj acent fuel rods 3 .

[0045] In the embodiment of the spacer elements 4 represented in Figures 10A, 10B, 10C, the contact element 10 is totally similar to what already described with reference to Figures 3- 9 , whereas the fastener element 9 is composed of a plurality of internal fastening structures 21 which proj ect inside the annular body 10b and are structurally connected to the annular body 10b of the contact element 10 and angularly spaced apart from one another . The fastening structures 21 are defined by respective cavities formed in the side wall of the annular body 10b, which thus has a series of corresponding notches on its outer lateral surface .

[0046] In the illustrated example , the fastening structures 21 extend axially along the annular body 10b on an inner lateral surface of the annular body 10b and have a cross-section shaped as an arc , with a convex surface facing the axis A and the fuel rod 3 . Preferably, the fastening structures 21 are inclined at an angle p with respect to the axis A of the fuel rod 3 . Also in this case , the contact element 10 is provided with radial teeth 18 which proj ect radially from the outer lateral surface 20 of the contact element 10 , and with recesses 19 formed in the surface 20 ; the teeth 18 and the recesses 19 are arranged aligned and circumferentially spaced apart on each contact element 10 so that the teeth 18 of a contact element 10 engage corresponding recesses 19 of contact elements 10 of adj acent fuel rods 3 . Optionally, the recesses 19 have dimensions such that the teeth 18 are inserted into them with an axial clearance ( i . e . with a limited possibility of excursion in a direction parallel to the axis A) .

[0047] In the further variation shown in Figures 10D- 10F, similar to what described with reference to Figures 10A- 10C, the fastener element 9 is composed of a plurality of internal fastening structures 21 which proj ect inside the annular body 10b and angularly spaced apart from one another .

[0048] With regard to the rest , the spacer element 4 is totally similar to what already described, having in particular radial teeth 18b and recesses 19b arranged so that the teeth 18b of a contact element 10 engage corresponding recesses 19b of contact elements 10 of adj acent fuel rods 3 , as is shown in Figure 10E .

[0049] In this case , the teeth 18b and the recesses 19b are obtained at an axial end of the spacer element 4 . In particular, the recesses 19b are defined by slits formed in a peripheral end edge of the annular body 10b ; and the teeth 18b are defined by respective tongues folded towards the outside of the spacer element 4 ; the teeth 18b can be for example obtained through simple folding of a portion of the lateral surface 20 of the spacer element 4 .

[0050] The extensions 14 , 15 with the respective teeth 16 , 17 ( Figures 8A, 8B ) , or the teeth 18 and the recesses 19 ( Figure 9 , Figures 10A- 10C ) , or the tongue-shaped teeth 18b and the respective recesses 19b formed at the end of the spacer element 4 ( Figures 10D- 10F) , define restraining members 22 of the spacer element 4 which by engaging respective adj acent spacer elements 4 prevent relative axial sliding between the spacer elements 4 .

[0051] Clearly, the restraining members 22 can be configured in a di f ferent manner from what described herein by way of example .

[0052] Preferably, the restraining members 22 , both in the case of the teeth 16 , 17 and in the case of the teeth 18 , 18b and respective recesses 19 , 19b ( as well as of further variations ) , are configured to allow small relative displacements in axial direction (parallel to the axis A) between the spacer elements 4 and thus between the fuel rods 3 . For example , the teeth 16 , 17 are arranged at one single end of the spacer element ; or are arranged at both opposite ends of the spacer element 4 with a preset axial clearance , i . e . the axial distance (measured parallel to the axis A) between the teeth 15 , 16 is slightly greater than the height (measured along the axis A) of the spacer elements 4 . The teeth 18 , 18b are for example inserted into the respective recesses 19 , 19b with an axial clearance . The teeth 16 , 17 , 18 , 18b and the respective recesses 19 , 19b prevent displacements in a direction perpendicular to the j oining of the axes of two adj acent fuel rods 3 .

[0053] In an embodiment not illustrated, the casing 2 is provided with axial strikers ( for example proj ections which proj ect radially towards the inside of the casing) configured to prevent axial translation of the spacer elements 4 even in case of loss of adhesion between all the tubes 5 of the fuel rods 3 and all the spacer elements 4 .

[0054] In all the embodiments described above , each fuel rod 3 is provided with a plurality of spacer elements 4 which are inserted on the tube 5 and axially spaced apart from one another ; and the fastener element 9 , directly or through the fastening structures 21 of the spacer element 4 , is force- fitted with radial interference on the tube 5 of the fuel rod 3 .

[0055] According to an aspect of the present invention, the fastener element 9 of the spacer element 4 has an inner diameter, understood as maximum diameter of a cylindrical element insertable inside the fastener element 9 , which is slightly smaller than the outer diameter of the tube 5 of the fuel rod 3 prior to inserting the spacer element 4 on the tube 5 and at the same temperature .

[0056] Therefore , both the tubular body 9b ( embodiments of Figures 3- 9 ) and the fastening structures 21 ( embodiments of Figures 10A- 10F) can contain, on the inside , a maximum cylindrical structure the inner diameter of which is slightly smaller than the outer diameter of the tube 5 of the fuel rod 3 prior to inserting the spacer element 4 on the tube 5 and at the same temperature .

[0057] The spacer element 4 is made of a material subj ect to thermal expansion; before performing the insertion of the spacer element 4 on the tube 5 , the spacer element 4 is heated, and i f necessary the tube 5 is cooled, in order to make the inner diameter of the fastener element 9 ( i . e . the inner diameter of the tubular body 9b, or the distance between the fastening structures 21 ) of the spacer element 4 slightly larger than the outer diameter of the tube 5 , exploiting the thermal expansion of the material of which the spacer element is made for making the coupling feasible . Once the spacer element 4 and the tube 5 are brought to the same temperature , their surfaces will match so as to guarantee a given forcing and, thanks to the friction, their immovability . Likewise , it is possible to proceed for restraining the spacer element provided with fastening structures 21 .

[0058] With reference to Figures 11A- 11D, one of the spacer elements 4 , in particular the spacer element 4b to be positioned at the upper end of the fuel rod 3 , can be located, instead of on the tube 5 , on the plug 6 which closes the tube 5 at the top and in particular on a bulb 23 of the plug 6 which remains outside the tube 5 when the plug 6 closes the tube 5 .

[0059] In this case, the spacer element 4b can be advantageously obtained in one single piece with the plug 6 : the fastener element 9 of the spacer element 4b is replaced by the bulb 23 of the plug 6 and the structural connecting elements 11 are connected directly to the bulb 23 and are made in one single piece with it and j oin the bulb 23 to the contact element 10 .

[0060] In the further embodiment represented in Figures 12A, 12B, 12C, each spacer element 4 always comprises a contact element 10 , defined by an annular body 10b, and a fastener element 9 , composed of a plurality of internal fastening structures 21 which proj ect inside the annular body 10b and are structurally connected to the annular body 10b of the contact element 10 and angularly spaced apart from one another . As is described with reference to Figures 10A- 10F, also in this case the fastening structures 21 are defined by respective recesses formed in the side wall of the annular body 10b, which thus has a series of corresponding notches on its outer lateral surface ; the fastening structures 21 extend axially along the annular body 10b on an inner lateral surface of the annular body 10b and have a cross-section shaped as an arc , with a convex surface facing the axis A and the fuel rod 3 ; preferably, the fastening structures 21 are inclined at an angle p with respect to the axis A of the fuel rod 3 .

[0061] Also in this case , the contact element 10 is provided with radial teeth 18c proj ecting radially from the outer lateral surface 20 of the contact element 10 , and with recesses 19c formed in the surface 20 .

[0062] The teeth 18c are composed of respective portions of the side wall of the body 10b located at an end of the body 10b and folded outward; and the recesses 19c are composed of respective slits left open in the side wall of the annular body 10b by the folding of the teeth 18c .

[0063] Also in this case , the teeth 18c and the recesses 19c define restraining members 22 of the spacer element 4 which by engaging respective adj acent spacer elements 4 prevent relative axial sliding between the spacer elements 4 ; the teeth 18c and the recesses 19c are in fact arranged aligned and circumferentially spaced apart on each contact element 10 so that the teeth 18 of a contact element 10 engage corresponding recesses 19 of contact elements 10 of adj acent fuel rods 3 . Optionally, the recesses 19 have dimensions such that the teeth 18 are inserted into them with an axial clearance ( i . e . with a limited possibility of excursion in a direction parallel to the axis A) .

[0064] As in the embodiments described in the foregoing, each spacer element 4 is composed of an integral monolithic piece of which also the teeth 18c are part . Therefore , no further components for connecting the spacer elements 4 to one another are required .

[0065] Based on what set forth, the advantages of the present invention are evident :

[0066] - the spacer elements 4 create a series of restraints equally spaced apart between rods , minimi zing the quantity of neutron absorbing structural material ;

[0067] - in particular the solutions in Figures 10A- 10F and 12A- 12C minimi ze the quantity of necessary material and the spacer elements can be manufactured in a simple manner with di f ferent techniques starting from tube or from section bar ;

[0068] - once the pitch between the fuel rods 3 has been fixed, by varying the thickness of the various parts of the spacer element 4 and their number , it is possible to optimi ze the radial sti f fness of the restraining and spacing system of the fuel rods 3 ; the inclination a with respect to the perpendicular to the fuel rod 3 of the structural connecting elements 11 of the spacer elements 4 gives a given elasticity to the spacer element 4 for absorbing possible swellings of the fuel rods 3 ;

[0069] - the inclination p with respect to the parallel to the fuel rod 3 of the structural connecting elements 11 of the spacer elements 4 , or of the fastening structures 21 , by imparting a helical motion on the primary cooling fluid, contributes to its mixing and to the uni formi zation of its temperature circumferentially to the fuel rod 3 ;

[0070] - the restraining members 22 ( teeth 16 , 17 radial with respect to the contact element 10 ; teeth 18 , 18b, 18c radial with respect to the contact element 10 and corresponding recesses 19 , 19b, 19c ) prevent relative axial translation displacements between the spacer elements 4 ( except for the provided clearances ) and also displacements between adj acent fuel rods 3 in a direction perpendicular to the j oining of the axes of the fuel rods 3 ;

[0071] - the restraining members 22 ( teeth 16 , 17 radial with respect to the contact element 10 ; teeth 18 , 18b, 18c radial with respect to the contact element 10 and corresponding recesses 19 , 19b , 19c ) prevent axial translation of the spacer element 4 even in case of loss of adhesion between the tube 5 of the fuel rod 3 and the fastener element 9 of the spacer element 4 , and simultaneously the fuel rods 3 maintain the possibility of small relative axial sliding ;

[0072] - the presence of teeth 16 , 17 , 18 radial with respect to the contact element 10 and the presence of some corresponding strikers on the casing 2 prevent axial translation of the spacer elements 4 even in case of loss of adhesion between all the tubes 5 of the fuel rods 3 and all the fastening elements 9 or loss of adhesion of all the fastening structures 21 of the spacer elements 4 located at the same level ;

[0073] - the spacer elements 4 are simple to manufacture ;

[0074] - the spacer elements 4 are simple to mount with their simple heating with respect to the tubes 5 of the fuel rods 3 or with possible aid of expanders ;

[0075] - absence of rubbing between the fuel rods 3 and also between the fuel rods 3 and the respective supports due to vibrations or di f ferential expansions ;

[0076] - absence of damage of the fuel rods 3 and of their possible protection during the mounting of the spacer elements 4 due to the larger inner diameter of the spacer elements 4 with respect to the fuel rods 3 in the inserting step in conditions of thermal expansion of the spacer elements 4 ;

[0077] - easy loading o f the fuel rods 3 inside the cas ing 2 with simple approaching to one another at the moment of the insertion in the pins 8 without vertical sliding as it occurs in the case of introduction in pre-assembled grids ;

[0078] - the assembling of the spacer elements 4 with the tubes 5 is performed be fore the introduction of the fuel and thus with tubes easy to handle for an ef fortless control of the coupling between tube 5 and spacer element 4 .

[0079] Finally, it is understood that modi fications and variations can be made to the fuel element described and illustrated herein, which do not depart from the scope of the appended claims .

Claims

CLAIMS1. A fuel element (1) for a nuclear reactor, comprising a casing (2) containing a plurality of fuel rods (3) , which extend along respective longitudinal axes (A) parallel to one another and are provided with respective spacer elements (4) attached integral to the respective fuel rods (3) to make a restraining and spacing system between the fuel rods (3) and between the fuel rods (3) and the casing (2) in directions perpendicular to the axes (A) of the fuel rods (3) ; wherein each spacer element (4) comprises an internal fastener element (9) , fitted on a containment tube (5) of the fuel rod (3) ; and a contact element (10) , joined to the fastener element (9) and located radially outside the fastener element (9) and cooperating in use with contact elements (10) of other spacer elements (4) located on adjacent fuel rods (3) ; and wherein at least some spacer elements (4) are provided with restraining members (22) engaging respective adjacent spacer elements (4) to prevent relative axial sliding between the spacer elements (4) ; characterized in that the restraining members (22) of each spacer element (4) comprise radial teeth (16, 17, 18, 18b, 18c) which are integral with the spacer element (4) and project radially from the spacer element (4) and protrude radially beyond an outer lateral surface (20) of the contact element (10) to engage respective axial end edges (12) of contact elements (10) of adjacent spacer elements (4) , or to engage respective recesses (19, 19b, 19c) formed in respective outer lateral surfaces (20) of contact elements (10) of adjacent spacer elements (4) , so as to prevent relative axial sliding between the spacer elements (4) .

2. The fuel element according to claim 1, wherein the contact element (10) comprises an annular body (10b) placed around and radially outward with respect to the fastener element (9) and having a cross-section with a circular, square, or hexagonal shape.

3. The fuel element according to claim 1 or 2, wherein the fastener element (9) comprises a substantially cylindrical tubular body (9b) fitted on the tube (5) of the fuel rod (3) and joined to the contact element (10) by a plurality of structural connecting elements (11) .

4. The fuel element according to claim 3, wherein the structural connecting elements (11) are inclined at a first predetermined angle (a) relative to an axis perpendicular to said longitudinal axis (A) ; and / or at a second predetermined angle (p) relative to said longitudinal axis (A) .

5. The fuel element according to claim 1 or 2, wherein the fastener element (9) comprises a plurality of internal fastening structures (21) projecting inside the contact element (10) and structurally connected to the contact element (10) and angularly spaced apart from one another; and wherein the fastening structures (21) extend axially along the annular body (10b) on an inner lateral surface of the annular body (10b) , preferably inclined at an angle (p) with respect to the axis (A) , and have a cross-section shaped as an arc, with a convex surface facing the axis (A) .

6. The fuel element according to any one of the preceding claims, wherein the fastener element (9) is forced- fitted with radial interference on the tube (5) of the fuel rod ( 3 ) .

7. The fuel element according to claim 6, wherein thefastener element (9) has, prior to mounting the fastener element (9) on said tube (5) and at the same temperature, an inner diameter slightly smaller than the outer diameter of said tube (5) ; and is made of a material subject to thermal expansion in such a way that, by heating the spacer element (4) before and / or during the mounting of the spacer element (4) on the tube (5) of the fuel rod (3) , and if necessary by cooling the tube (5) of the fuel rod (3) , the inner diameter of the fastener element (9) of the spacer element (4) becomes slightly larger than the outer diameter of the tube (5) of the fuel rod (3) , so as to allow their coupling.

8. The fuel element according to claim 3 or 4, wherein the restraining members (22) comprise axial extensions (14, 15) arranged to extend at least some structural connecting elements (11) and protruding axially beyond an upper axial end or a lower axial end of the spacer element (4) ; and wherein said radial teeth (16, 17) project perpendicular from respective extensions (14, 15) and protrude radially beyond said outer lateral surface (20) of the contact element (10) to engage respectively above or below the contact element (10) of an adjacent spacer element (4) .

9. The fuel element according to claim 8, wherein the teeth (16, 17) have an axial clearance relative to the contact element (10) of an adjacent spacer element (4) to allow small differential axial displacements between fuel rods ( 3 ) .

10. The fuel element according to any one of the preceding claims, wherein said radial teeth (18, 18b, 18c) project radially from said outer lateral surface (20) of the contact element (10) , and said recesses (19, 19b, 19c) areformed in said outer lateral surface (20) ; the teeth (18, 18b, 18c) and the recesses (19, 19b, 19c) being arranged aligned and circumferentially spaced apart on each contact element (10) , so that, after mounting the fuel rod (3) inside the fuel element (1) , the teeth (18, 18b) of a contact element (10) engage corresponding recesses (19, 19b) of contact elements (10) of adjacent fuel rods (3) .

11. The fuel element according to claim 10, wherein the teeth (18, 18b) are inserted into the respective recesses (19, 19b) with an axial clearance to allow small differential axial displacements between the fuel rods (3) .

12. The fuel element according to claim 10 or 11, wherein the teeth (18, 18b) are inserted into the respective recesses (19, 19b) so as to prevent relative displacements between adjacent pairs of fuel rods (3) in a direction perpendicular to the line of contact between the respective contact elements (10) .

13. The fuel element according to any one of the preceding claims, wherein the casing (2) is provided with axial strikers configured to prevent axial translation of the spacer elements (4) even in case of loss of adhesion between all the tubes (5) of the fuel rods (3) and all the spacer elements (4) .

14. The fuel element according to any one of the preceding claims, wherein the fuel rods (3) have at an upper end a spacer element (4b) which is an integral monolithic piece with a plug (6) closing the tube (5) of the respective fuel rod ( 3 ) .

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