nuclear reactor pencil

Abstract

Title: Nuclear Reactor Fuel Rod The invention relates to a nuclear fuel rod (1) extending mainly along a longitudinal axis (ZZ'), comprising a plurality of nuclear fuel pellets (200) stacked along the longitudinal axis, the plurality of pellets (200) having an outer wall (210), and a cladding (100) surrounding the plurality of pellets, the cladding having an inner wall (110). At any point p on the inner wall of the cladding located by an angle θ, with 0≤θ≤2π, around the longitudinal axis, the radial distance measured radially from the longitudinal axis between the outer wall and the point p defines a pellet-cladding clearance j(θ). The inner wall has at least three constriction zones (111), each having a pad-sheath clearance less than a first clearance, called j1, and at least three separation zones (112), each having a pad-sheath clearance greater than a second clearance, called j2, with 0

Description

Title of the invention: Nuclear reactor rod Technical field

[0001] The present invention relates to the field of fuel rods for nuclear reactors. It finds a particularly advantageous application in the field of fuel rods intended for nuclear power reactors such as pressurized water reactors (PWRs), boiling water reactors (BWRs) or fast neutron reactors (FNRs). Another advantageous application concerns fuel rods intended for experimental reactors. STATE OF THE ART

[0002] The core of a reactor is conventionally divided into fuel assemblies arranged next to each other in the reactor vessel. The assemblies themselves are typically made up of a bundle of fuel rods. Each fuel rod typically comprises a plurality of pellets of fissile nuclear fuel materials stacked on top of each other inside a cladding. A manufacturing clearance is conventionally provided between the stack of pellets and the cladding surrounding them. In this way, it is easier to introduce the pellets inside the cladding in order to stack them.

[0003] When the reactor is in operation, the fission reactions taking place within the fuel generate gaseous and solid fission products which themselves generate swelling of the pellets throughout the irradiation. This swelling can cause the side wall of the pellets to come into contact with the cladding, locally closing the manufacturing gap. It can even last until the manufacturing gap is closed around the entire periphery of the pellet. This expansion of the pellet against the internal wall of the cladding causes high mechanical stresses within the cladding. This phenomenon can also be accentuated due to the reduction in the diameter of the cladding by pressure of the heat transfer fluid flowing around the latter.

[0004] Excessive stresses within the cladding can lead to its rupture, disseminating fuel particles into the primary circuit. In addition to contamination of the primary fluid, fission and therefore fission products can then occur directly in the primary circuit. Cladding rupture must therefore naturally be avoided.

[0005] Thus, the constraints within the sheath currently limit the safety of reactors, particularly in incident or accident situations.

[0006] These constraints are also detrimental to the flexibility of reactor control. Nuclear reactors must, however, be flexible in order to adapt to electricity consumption, which varies depending on the time of day. This need for flexibility is increasingly strong due to the ever-increasing use of intermittent renewable energies. Solar and wind energy, for example, whose production is inherently irregular, need to be coupled with an energy source, such as nuclear, which offers great flexibility. This results in significant technical constraints on reactor control, reactor components, and in particular the cladding housing the fuel rods. The latter must be able to accommodate the rods in both low and high production situations.

[0007] A known solution consists of increasing the clearance between the pellets and the cladding in order to avoid any contact between these two elements. It has been observed that such an increase in the manufacturing clearance is, however, accompanied by a negative impact on the fuel temperature.

[0008] Another known solution consists of limiting power cycling. However, this solution is not satisfactory in terms of power and flexibility provided to the network since the reactor must be able to compensate for a sharp drop in the renewable energy component within the energy mix.

[0009] An object of the present invention is therefore to propose a solution improving the safety and performance of a nuclear reactor. SUMMARY

[0010] To achieve this objective, according to one embodiment, a nuclear fuel rod is provided extending mainly along a longitudinal axis, comprising a plurality of nuclear fuel pellets stacked on top of each other along the longitudinal axis, the plurality of pellets having an external wall, and a cladding surrounding the plurality of pellets, the cladding having an internal wall, in which, at any point p of the internal wall of the cladding identified by an angle 0, with 0<0<2ir, around the longitudinal axis, the radial distance measured radially from the longitudinal axis between the external wall of the plurality of pellets and the point p defines a pellet-cladding clearance j(0), characterized in that the internal wall has at least three first zones, called constriction zones, each having a pellet-cladding clearance j less than or equal to a first clearance, called jl, and at least three second zones, called separation zones,each presenting a pellet-sheath clearance j greater than or equal to a second clearance, called j2, with 0 <jl / j2<0,7, et de préférence 0<jl / j2<0,5. ,

[0011] The fact that the sheath has such areas of tightening and separation makes it possible to limit the extent of the contact areas between the pellets and the sheath. This also has the effect of making the sheath work more in bending than in traction compared to current rods.

[0012] One of the advantages of the sheath according to the invention is that it is fully compatible with unmodified standard pellets, and in particular cylindrical pellets. This has the advantage of not inducing additional production costs and of facilitating the supply of pellets and the industrialization of pencils.

[0013] The cladding is also compatible with the standard grids used in current fuel assemblies: no modification is necessary to assemble them to the supports currently used. Here again, the industrialization of the rod is facilitated and production costs are limited.

[0014] A second subject of the invention relates to an assembly for a nuclear reactor, comprising rods according to the first aspect of the invention as well as a body inside which the rods are arranged in the form of a bundle.

[0015] A third object of the invention relates to a nuclear reactor comprising a core inside which are arranged assemblies according to the second object of the invention. BRIEF DESCRIPTION OF THE FIGURES

[0016] The aims, objects, as well as the characteristics and advantages of the invention will emerge more clearly from the detailed description of an embodiment thereof which is illustrated by the following accompanying drawings in which:

[0017] [Fig.l] [Fig.l] schematically represents an example of a fuel rod according to the invention.

[0018] [Fig.2A] [Fig.2A] represents, in section and schematically, a fuel rod according to an example of the invention, having a quadrifoliate cladding.

[0019] [Fig.2B] [Fig.2B] represents, in section and schematically, a fuel rod according to another example of the invention, having a trifoliate cladding.

[0020] [Fig.3] [Fig.3] represents a cross-section of a portion of an assembly at not square receiving a fuel rod with quadrifoliate sheath.

[0021] [Fig.4] [Fig.4] represents a cross-section of a fuel rod of the state of the art.

[0022] The drawings are given as examples and are not limiting of the invention. They constitute schematic representations of principle intended to facilitate the understanding of the invention and are not necessarily to the scale of practical applications. In particular, the dimensions are not representative of reality. For the sake of clarity, the curvatures of the sheath have been exaggerated relative to the other dimensions. DETAILED DESCRIPTION

[0023] Before beginning a detailed review of embodiments of the invention, optional features which may possibly be used in combination or alternatively are set out below:

[0024] According to an advantageous embodiment, the sheath is configured to deform so as to accommodate a radial expansion of the stack of pellets, the tightening zones and the separation zones being configured to respectively move away from and towards the longitudinal axis under the action of the expansion. Such a movement of the tightening and separation zones during the radial expansion of the pellets makes it possible to maintain the integrity of the sheath and therefore to avoid any breakage.

[0025] According to an advantageous embodiment, under the effect of a radial expansion of the pellets, the tightening zones and the separation zones are configured to deform in tension between 0.1 and 10 times the deformation in bending, preferably so as to deform in tension between 0.5 and 5 times the deformation in bending. Bending work in addition to deformation in tension allows better accommodation of the pellets by the sheath. It reduces the level of stress in the sheath, which is therefore less stressed. The integrity of the sheath is therefore improved.

[0026] According to one example, the first pellet-cladding clearance j 1 is strictly less than the so-called conventional clearance jconv that would have been measured if the pellet were surrounded by a conventional cylindrical cladding with a circular base. This value, well known to those skilled in the art, is typically substantially equal to 0.085 mm for PWRs. Thus, for PWRs, in the context of the present invention, jl <0.085 mm is typically the case.

[0027] According to one example, the first pellet-cladding clearance j 1 is less than 0.075 mm. Here again, the impact on the fuel temperature is limited.

[0028] According to one example, the first pellet-cladding clearance j 1 is greater than or equal to 0.3 times and / or less than or equal to 0.7 times the so-called conventional clearance jconv that would have been measured if the pellet were surrounded by a conventional cylindrical cladding with a circular base. Thus, for the REPs, we typically have, in the context of the present invention, 0.0255 <jl mm et / ou jl <0,0595 mm.

[0029] According to one example, the second pellet-cladding clearance j2 is strictly greater than the so-called conventional clearance jconv that would have been measured if the pellet were surrounded by a conventional cylindrical cladding with a circular base. This value, well known to those skilled in the art, is typically substantially equal to 0.085 mm for PWRs. Thus, for PWRs, in the context of the present invention, j2>0.085 mm is typically the case. According to one example, the second pellet-cladding clearance j2 is greater than 0.095 mm.

[0030] These latter characteristics (j 1 less than 0.075 mm and j2 greater than 0.095 mm) make it possible to accommodate a significant expansion of the pellets. Indeed, the more the sheath has variations in clearance with the pellets, the more it will be able to deform to accommodate large radial expansions of the pellets, like a spring.

[0031] The first pellet-sheath clearance j 1 is greater than or equal to 0, preferably strictly greater than 0. This allows for a useful manufacturing clearance to be maintained when inserting pellets in the sheath.

[0032] According to one example, the second pellet-cladding clearance j2 is greater than or equal to 1.3 times and / or less than or equal to 1.7 times the so-called conventional clearance jconv that would have been measured if the pellet were surrounded by a conventional cylindrical cladding with a circular base. This value, well known to those skilled in the art, is typically substantially equal to 0.085 mm for PWRs. Thus, for PWRs, we typically have, in the context of the present invention, 0.1105 <j2 et / ou j2<0,1445 mm.

[0033] According to one example, the longitudinal axis is an axis of symmetry for the sheath.

[0034] According to one example, each pellet of the plurality of pellets has a shape circular radius called pellet radius Rp.

[0035] According to one example, the constriction zones and the separation zones are arranged alternately around the longitudinal axis. Such alternation improves the symmetry of the stress distribution in the sheath. This makes it possible to limit the zones with a high level of stress.

[0036] According to an advantageous embodiment, the sheath has a continuous profile along the longitudinal axis.

[0037] According to an advantageous embodiment, the sheath has a height called sheath height along the longitudinal axis and has a constant profile along the longitudinal axis over at least 50% of its sheath height, preferably over at least 80% of its sheath height. A constant profile means a constant section, the section being taken in a plane perpendicular to the longitudinal axis.

[0038] This avoids angles, which can constitute weak points in the sheath in that they concentrate the stresses during the mechanical interaction between the sheath and the pellet.

[0039] According to one example, the separation zones and the contraction zones have a sinusoidal profile around the longitudinal axis.

[0040] According to one example, the sheath has a constant thickness elOO.

[0041] According to one embodiment, the internal wall of the sheath has three narrowing zones and three separation zones.

[0042] According to one embodiment, the internal wall of the sheath has four tightening zones and four separation zones.

[0043] According to one embodiment, the separation zones are distributed regularly around the longitudinal axis. The same is preferably true for the tightening zones. In combination with an alternation between these two types of zones, such an arrangement allows a symmetrical distribution of the stresses in the sheath. This makes it possible to maintain the stress level in the sheath at a uniform and relatively low level.

[0044] According to one embodiment, at any point p of the internal wall of the sheath, j(0) = jmoy + e(0), jmoy being called the average pellet-cladding clearance, e(0) being the distance measured radially from the longitudinal axis between a point distant from the average pellet-cladding clearance jmoy of the external wall and the point p and e(0) being called the cladding deflection, with 0.075 mm < jmoy < 0.095 mm.

[0045] According to one example, e(0) is substantially equal to eO*cos(30) to within manufacturing tolerances, with eO=jmoy-jl.

[0046] According to one example, e(0) is substantially equal to eO*cos(40) to within manufacturing tolerances, with e0=jmoy-jl.

[0047] Another aspect of the invention relates to a sheath extending along a longitudinal axis and having an internal wall, in which, at any point p of the internal wall identified by an angle 0, with 0<0<2ir, around the longitudinal axis, the distance between the longitudinal axis and the point p defines an internal radius R(0). This sheath is characterized in that the internal wall has at least three first zones, called narrowing zones, each having an internal radius R(0) corresponding to a minimum, called Rmin, and at least three second zones, called distance zones, each having an internal radius R(0) corresponding to a maximum, called Rmax, with Rmax>1.01*Rmin. Typically, Rmin is substantially equal to 4.5 mm.

[0048] The terms “substantially”, “approximately”, “of the order of” mean “to within 10%, preferably to within 5%” or, when it is an angular orientation, “to within 10°”. Thus, a direction substantially normal to a plane means a direction having an angle of 90+10° relative to the plane.

[0049] “Internal” means the elements or faces facing the interior of the pellets fuels, for example facing the longitudinal axis, and "external" means the outwardly facing elements or faces of the fuel pellets, for example facing away from the longitudinal axis.

[0050] In the context of the present invention, and unless otherwise stated, the thicknesses of an element or part are taken in a direction normal to the tangent to the surface of the element or part.

[0051] A cylinder is understood to mean a solid delimited by a surface of substantially parallel generating lines and two substantially parallel planes, preferably perpendicular to the generating lines. The cross-section of a cylinder may be circular. Alternatively, the cross-section of the cylinder is not circular. Note that the term "cylinder" includes a substantially cylindrical shape, i.e. an imperfect cylindrical shape, for example in connection with manufacturing tolerances.

[0052] The bending and tensile stresses discussed above are measured in the section of the sheath on a plane perpendicular to the circumferential direction. The bending and tensile stresses can in particular be calculated by si- numerical simulation, particularly using the finite element method.

[0053] It should be noted that in the present application, unless otherwise stated, an object extending along an axis may also extend along any parallel axis.

[0054] In the remainder of the description, normal operation of the reactor or power plant is defined as a phase of operation in the absence of accidents or serious accidents.

[0055] It is understood that within the meaning of the invention, a pencil does not necessarily comprise combustible pellets.

[0056] A reference frame, preferably orthonormal, is shown in Figures 2A to 2B and 4.

[0057] Conventionally, a nuclear reactor core comprises assemblies such as fuels within which fission reactions occur. Each fuel assembly comprises a body housing a plurality of fuel rods arranged in the form of bundles. The primary fluid circulates in the reactor core between the fuel rods of the assembly. The rods are held within the assembly at their lower ends and / or at their upper ends. They may also be held by holding grids arranged between their upper and lower ends, or by a peripheral tube.

[0058] [Fig.l] illustrates a non-limiting example of a fuel rod.

[0059] The rod 1 typically comprises a plurality of fuel pellets 200 stacked on top of each other along the longitudinal axis ZZ', an axis extending along the Z direction of the XYZ reference frame. The rod 1 also conventionally comprises a cladding 100 surrounding the plurality of pellets 200 in the transverse plane XY. The cladding 100 is cylindrical in shape and extends mainly along the Z direction. It is closed at each of its ends along the longitudinal axis ZZ' by plugs, called upper 11a and lower 11b. The cladding 100 may contain one or more free volumes for expansion of the fission gases 12. A volume 12 may accommodate a spring 13 and / or a wedge making it possible to ensure the pellets 200 are held within the cladding 100.

[0060] The pellets 200 have an external wall 210. They are notably characterized geometrically by the distance between the longitudinal axis ZZ' and the external wall 210, this distance being able to vary, in the case of non-circular pellets, depending on the location of the external wall 210 where it is measured. The pellets 200 are typically all circular and of the same radius Rp.

[0061] Figures 2A and 2B illustrate sectional views of the pencils taken at the level of the pellets and allow the shape of the sheath to be described in more detail.

[0062] The sheath 100 has a sheath thickness el00, preferably constant. The sheath thickness is measured in the transverse plane XY and at any point of the sheath in a direction perpendicular to the direction tangential to the profile of the sheath 100.

[0063] The inner wall 110 of the sheath 100 has narrowing zones 111 and separation zones 112. In other words, the narrowing zones 111 and the separation zones 112 are distributed along the inner wall 110 of the sheath 100. Thus, the section of the sheath, at least of its inner wall 110, taken in the transverse plane XY, is not circular. The narrowing zones 111 and the separation zones 112 have a distance from the pellet 200 which is respectively smaller and larger than any other zone of the inner wall 110. The narrowing zones 111, like the separation zones 112, are at least three in number. These narrowing zones 111 and separation zones 112 are preferably found in the same transverse plane XY. These zones typically extend along the longitudinal axis ZZ' over the entire height of the stack of pellets 200 and the cladding 100.According to a preferred embodiment, the sheath 100 comprises as many tightening zones 111 as separation zones 112.

[0064] When the pellet 200 swells, the contact between the pellet 200 and the sheath 100 occurs at the tightening zones 111. Only a small surface area of ​​the internal wall 110 of the sheath is therefore in contact with the pellet. In such a configuration, the sheath works more in flexion and less in traction compared to a sheath of the prior art, which is a factor in reducing the stresses in the sheath 100.

[0065] Reducing the mechanical interaction between the cladding 100 and the pellet 200 also makes it possible to limit the diametrical deformation of the cladding, the deformation energy density of the cladding and the time to rupture of the cladding, and to improve the strength of the cladding. The margins on these various safety criteria commonly used to characterize a fuel element are therefore improved. The operating margins of the reactor are therefore improved, which makes it possible to gain flexibility and / or maximum power.

[0066] It should also be noted that the shape of the internal wall of the sheath according to the invention deviates relatively little from a circular shape. A shape too far from a circular outline, for example an elliptical shape, would induce significant modifications and disturbances in terms of flow of the heat transfer fluid and cooling. In particular, the flow and therefore the cooling could be inhomogeneous in the assembly. This would have significant repercussions on the operating margins of the reactor. The shape proposed here therefore constitutes a good compromise between a substantially cylindrical sheath and a sheath having another shape.

[0067] The sheath 100 is configured so that, under the effect of the expansion of the pellet 100 against the tightening zones 111, the latter move away from the longitudinal axis ZZ' while the distancing zones 112 move closer to it. The movement of the distancing zones makes it possible to compensate for that of the tightening zones, directly induced by the swelling of the pellet 200. This makes it possible to avoid that the sheath is subjected to too great constraints. Thanks to its geometry, the sheath 100 is ultimately similar to a spring allowing it to flexibly accommodate the expansion of the pellet 200. The sheath 100 is also typically based on a metal alloy.

[0068] Due to the particular shape of its internal wall, the sheath 100 has a plurality of hollows and peaks comprising respectively the separation zones 112 and the tightening zones 111. The sheath 100 and the pellet 200 define on the one hand cavities 150, delimited by the pellet 200 and the hollows of the sheath 100, and on the other hand grooves 160, delimited by the pellet 200 and the peaks of the sheath 100. The cavities 150 and the grooves 160 thus constitute the empty space between the pellet 200 and the sheath 100. The grooves 160 are intended to be at least partially closed during the swelling of the pellet 200 until contact with the tightening zones 111.

[0069] The tightening zones 111 and the separation zones 112 are preferably placed alternately around the pellet 200. According to a preferred example, if N denotes the number of tightening zones 111, the latter are placed symmetrically around the longitudinal axis ZZ', according to a rotational symmetry of an angle of a value of 2ir / N. The same is preferably true for the separation zones 111. Such a configuration ensures a symmetrical distribution of the stresses in the sheath 100, which avoids a concentration of the stresses at a point or a reduced zone of the sheath 100, which would have the disadvantage of weakening the latter. The integrity of the sheath 100 is therefore improved.

[0070] The inner wall 110 of the sheath 100 advantageously has a continuous profile. The absence of an angle within such a profile promotes the deformation of the sheath 100 to accommodate the expansion of the pellet. The angles also constitute areas of weakness of the sheath 100 during its deformation due to the expansion of the pellet 200. Removing them makes it possible to further reduce the stress levels within the sheath 100 and to improve its mechanical strength.

[0071] According to a preferred alternative, the internal wall 110 has a convex profile around its entire periphery. The radius of curvature of the tightening zones 111 is then greater than the radius of curvature of the separation zones 112. A wall having a convex profile around its entire periphery makes it possible to limit the zones of fragility of the sheath 100.

[0072] According to another alternative, the internal wall 110 may for example have an alternation of concave regions and convex regions around the pellet 200. The convex regions extend around the tightening zones 111. Such a convex shape makes it possible to further reduce the tensile work between the sheath 100 and the pellet 200. This makes it possible to further reduce the stresses in the sheath 100. The radius of curvature of the convex regions may be dimensioned so as to favor as much as possible a bending interaction compared to a bending interaction. traction.

[0073] The profile of the inner wall 110 of the sheath 100 in the transverse plane XY can be described more precisely by means of quantities called pellet-sheath clearance and deflection. These two quantities are defined at any point p of the inner wall 110 of the sheath 100. As illustrated in [Fig.2A], each point p is identified around the longitudinal axis ZZ' by an angle 0 expressed in radians and between 0 and 2ir. This point p is associated with a pellet-sheath clearance at the point pj(0) and an deflection at the point pe(0). By convention, the angle 0 has a positive value when it is measured in the counterclockwise direction, with for example 0=0 at the level of the X direction.

[0074] The pad-cladding clearance is defined at point p as the distance measured radially around the longitudinal axis ZZ' between the external wall 210 of the pad 200 and point p. The pad-cladding clearance is positive over the entire range 0<0<2ir. A clearance, called the average pad-cladding clearance and designated jmoy, is defined as being the average of the pad-cladding clearance j(0) over the range 0<0<2ir.

[0075] Similarly, the internal radius R(0) of the cladding 100 at point p is defined as the distance between the longitudinal axis ZZ' and point p. In the case where the pellet is circular with radius Rp, we have R(0)-j(0)=Rp. Similar to the average pellet-cladding clearance jmoy, we define an average internal radius Rmoy, the average of the internal radius R(0) over the range 0<0<2ir.

[0076] The tightening zones 111 are each characterized by the fact that the pellet-cladding clearance there is less than or equal to a first clearance denoted jl. jl is the maximum value that the pellet-cladding clearance j can take at the tightening zones. Figures 2A and 2B illustrate the case where the pellet-cladding clearance at the tightening zones 111 is equal to jl, but it is understood that it can take lower values ​​at least in some of the tightening zones 111. j 1 is greater than or equal to 0 and preferably strictly greater than 0 in order to preserve a useful manufacturing clearance when inserting the pellets into the cladding for the assembly of the rod, jl is moreover preferably less than 0.085 mm for a standard REP rod in order to limit the empty space between the pellet 200 and the cladding 100 and thus limit the impact on the fuel temperature.Preferably, the pellet-sheath clearance is identical in each of the tightening zones 111, as in Figures 2A and 2B. The pellet-sheath clearance j(0) in this case admits as many minima as there are tightening zones.

[0077] Similarly, the separation zones 112 are characterized by a pad-cladding clearance greater than or equal to a second clearance denoted j2. j2 is the minimum value that the pad-cladding clearance j can take at the separation zones. Figures 2A and 2B illustrate the case where the pad-cladding clearance at the separation zones 112 is equal to j2, but it is understood that it can take higher values ​​at least in some of the separation zones 112. j2 is preferably less than 0.17 mm for a standard REP in order to again limit the empty volume between the pellet 200 and the sheath 100. Preferably, the pellet-sheath clearance is identical in each of the separation zones 112, as in figures 2A and 2B. The pellet-sheath clearance j(0) in this case admits as many maxima as there are separation zones.

[0078] The values ​​j 1 and j2 are target values ​​of pad-sheath clearance for the constriction and separation zones. As specified above, it is advantageous to provide that the pad-sheath clearance is effectively equal to j 1 at the constriction zones 111 and to j2 at the separation zones 112, as illustrated in [Fig.2A]. It is however possible that we have j<j 1 au niveau de certaines au moins des zones de resserrement 111 et j> j2 at at least some of the tightening zones 111. Furthermore, when describing the preferred embodiment in which the pellet-sheath clearance is equal to j 1 at the tightening zones 111 and to j2 at the separation zones 112, it is understood that these equalities apply to the manufacturing errors near the sheath 100 and the pellet 200.

[0079] Advantageously, jmoy is substantially equal to the pellet-cladding clearance that would have been measured if the pellet 200 had been surrounded by a conventional cylindrical cladding with a circular base, called conventional clearance and noted jconv. jconv is typically substantially equal to 0.085 mm for the REPs. Thus, for the REPs, we typically have, in the context of the present invention, jmoy=jconv=0.085 mm. jl is then advantageously between 0 and 0.085 mm, and j2 is advantageously between 0.085 mm and 0.170 mm. jl can thus be substantially equal to 0 mm and j2 substantially equal to 0.170 mm.

[0080] The values ​​of j 1 and j2 are outside the manufacturing tolerance range of the sheath 100. This condition is further defined with reference to the deflection of the sheath.

[0081] The average internal radius Rmoy is substantially equal to that of a prior art cladding. This makes it possible to preserve, compared to existing systems, the free volume of the rod and the surface area for the passage of the heat transfer fluid in the assembly. In addition, the hydraulic diameter of the assembly is only very slightly modified compared to prior art rods.

[0082] [Fig.2A] illustrates the fact that the inner wall 110 of the sheath 100 is located alternately on either side of a theoretical trace (in dotted lines) distant from the average pellet-sheath clearance jmoy of the pellet 200. If the pellet is circular, this trace is also circular and coincides with a circle of internal radius Rmoy around the longitudinal axis ZZ'. The arrow e(0) is defined at point p as the distance measured radially around the longitudinal axis ZZ' between this theoretical trace and this point p of the inner wall 110 of the sheath 100. If the inner wall 110 is located between the theoretical trace and the pellet 200, the arrow e(0) is negative. If, on the contrary, it is located beyond the theoretical trace, the arrow e(0) is positive.

[0083] The arrow is less than a first arrow el at the level of the tightening zones 111 and greater than a second arrow e2 at the distance zones 112. Advantageously, in absolute values, these minimum and maximum values ​​are equal: e2=-el=e0. Preferably, the arrow is identical in each of the tightening zones 111. The arrow e(0) in this case admits as many minima as there are distance zones. In the same way, preferably, the arrow is identical in each of the distance zones 112. The arrow e(0) in this case admits as many maxima as there are distance zones.

[0084] The values ​​of el and e2 are, like jl and j2, outside the manufacturing tolerance range of the sheath 100. The manufacturing tolerance of the sheath is for example set at 1% of the internal radius / diameter of a sheath with a circular profile having an average internal radius equal to that of the sheath 100. el and e2 are therefore for example both greater in absolute values ​​than 1% of the internal radius / diameter of such a circular sheath.

[0085] The narrowing zones 111 and the separation zones 112 may be punctual in the transverse plane XY or, if the clearance and the deflection admit maxima / minima over a continuous range of values ​​from 0, correspond to a non-punctual portion of this internal wall.

[0086] The pellet-sheath clearance and arrow functions advantageously follow sine or cosine laws. By following these laws, the internal wall 110 of the sheath has continuity as well as radii of curvature which are particularly advantageous in the context of the invention.

[0087] It is understood that, in the case where the thickness elOO of the sheath 100 is constant, the characteristics of the geometry of the internal wall 110 of the sheath can quite easily be extrapolated to those of the sheath 100 as a whole.

[0088] [Fig.2A] more particularly illustrates an embodiment in which the sheath 100 comprises four constriction zones 111 and four separation zones 112. This embodiment can be described as quadrifoliate. According to a particular example of this embodiment, the constriction zones 111 are placed symmetrically around the longitudinal axis ZZ', according to a rotational symmetry of 90°. The same is preferably true for the separation zones 112. An adjacent constriction zone 111 and separation zone 112 are, in this embodiment, preferably separated by an angular distance of 45° relative to the longitudinal axis ZZ' (example illustrated in [Fig.2A]). An example of a pellet-sheath clearance function adapted to the quadrifoliate embodiment is given below. It is understood, however, that any sine and / or cosine function, and more broadly any continuous function, is very well suited to this embodiment.

[0089] [Math.l] j( e )=À„, + «oœs(4»)

[0090] It is understood that this equality is understood as “substantially equal”, and that manufacturing tolerances must be taken into account.

[0091] It is conceivable that the sheath 100 has another number of constriction zones and / or separation zones. [Fig.2B] for example illustrates an embodiment in which the sheath comprises three constriction zones 111 and three separation zones 112. This embodiment can be described as trifoliate. According to a particular example of this embodiment, the constriction zones 111 are placed symmetrically around the longitudinal axis ZZ', according to a rotational symmetry of 120°. The same is preferably true for the separation zones 112. An adjacent constriction zone 111 and separation zone 112 are, in this embodiment, preferably separated by an angular distance of 60° relative to the longitudinal axis ZZ'. An example of a pellet-sheath clearance function adapted to the trifoliate embodiment is given below.Here again, it is understood that any sine and / or cosine function, and more broadly any continuous function, is very well suited to this embodiment.

[0092] [Math.2] =J moy + ^^)

[0093] Here too, it is understood that this equality is understood as “substantially equal”, and that manufacturing tolerances must be taken into account.

[0094] According to another advantageous embodiment, the sheath 100 has six separation zones and six tightening zones (hexafoliate embodiment).

[0095] The sheath 100 according to the “quadrifolium” embodiment of the invention is particularly well suited to the spring-dimple supports usually used in PWR type fuel assemblies, as can be seen in [Fig.3].

[0096] PWR type reactor assemblies in fact typically comprise a grid 400 having a square elementary pattern 410 (square pitch assembly). Fuel rods 1 are inserted through the grid 400, each in an elementary pattern 410. The sheath 100 surrounding one of these rods 1 is typically held at different heights by springs and / or bosses 450 at each of the sides 411 of the profile of the elementary pattern 410. As illustrated in [Fig. 3], it is advantageous to carry out these holds at the level of the tightening zones 111.

[0097] Associating a quadrifoliate sheath with a square pitch assembly and matching the holding points to the tightening zones 111 of the sheath 100 allows better holding of the pencil 1 and better distribution of the stresses within the sheath 100.

[0098] Other associations are also conceivable. The trifoliate and hexafoliate embodiments are, for example, very well adapted, due to the shape of the sheaths, to an ace triangular pitch assembly. An assembly of this type has rods 1 arranged in a triangular pitch and typically separated by spacer wires.

[0099] The rod described above is intended for all types of nuclear reactors, whether electric or experimental, pressurized water reactors (PWRs), boiling water reactors (BWRs) or fast neutron reactors (FNRs). For some of these reactors, particularly FNRs, the rods may also be referred to as needles.

[0100] In view of the above description, it clearly appears that the invention proposes a fuel rod making it possible to improve the performance in terms of flexibility and power of a reactor in normal operation, but also to limit damage in incidental or accidental operation. It also appears that the solution described is perfectly compatible with industry standards (shape of the pellets, spring-dimple attachment, assembly, etc.).

[0101] The invention is not limited to the embodiments previously described and extends to all embodiments covered by the invention.

Claims

Claims

1. Nuclear fuel rod (1) extending mainly along a longitudinal axis (ZZ'), comprising a plurality of nuclear fuel pellets (200) stacked on top of each other along the longitudinal axis (ZZ'), the plurality of pellets (200) having an outer wall (210), and a cladding (100) surrounding the plurality of pellets (200), the cladding having an inner wall (110), wherein, at any point p of the inner wall (110) of the cladding (100) identified by an angle 0, with 0<0<2ir, around the longitudinal axis (ZZ'), the radial distance measured radially from the longitudinal axis (ZZ') between the outer wall (210) of the plurality of pellets (200) and the point p defines a pellet-cladding clearance j(0), characterized in that the inner wall (110) has at least three first zones, called constriction zones (111), each having a pellet-sheath clearance less than or equal to a first clearance j 1, and at least three second zones,called distance zones (112), each having a pellet-sheath clearance greater than or equal to a second clearance j2, with 0 <jl / j2<0,7.,

2.

3. Pencil (1) according to the preceding claim in which 0 <jl / j2<0,5. Crayon (1) selon l’une quelconque des revendications précédentes dans lequel la gaine (100) est configurée pour se déformer de sorte à accommoder une expansion radiale de l’empilement de pastilles (200), les zones de resserrement (111) et les zones d’éloignement (112) étant configurées pour respectivement s’éloigner et se rapprocher de l’axe longitudinal (ZZ’) sous l’action de l’expansion.

4. Pencil (1) according to any one of the preceding claims wherein, under the effect of a radial expansion of the pellets (200), the tightening zones (111) and the distancing zones (112) are configured to deform in tension between 0.1 and 10 times the deformation in bending, preferably so as to deform in tension between 0.5 and 5 times the deformation in bending.

5. Pencil (1) according to any one of the preceding claims in which the first pellet-sheath clearance j 1 is strictly less than 0.085 mm and preferably less than or equal to 0.0595 mm.

6. Pencil (1) according to any one of the preceding claims in which the second pellet-sheath clearance j2 is strictly greater than 0.085 mm and preferably greater than or equal to 0.1105 mm.

7. Pencil (1) according to any one of the preceding claims in in which the narrowing zones (111) and the distancing zones (112) are arranged alternately around the longitudinal axis (ZZ').

8. Pencil (1) according to any one of the preceding claims in which the sheath (100) has a height called sheath height along the longitudinal axis (ZZ') and has a constant profile along the longitudinal axis (ZZ') over at least 50% of its sheath height, preferably over at least 80% of its sheath height.

9. Pencil (1) according to any one of the preceding claims in which the separation zones (112) and the narrowing zones (111) have a sinusoidal profile around the longitudinal axis (ZZ').

10. Pencil (1) according to any one of the preceding claims in which the internal wall (110) of the sheath (100) has three narrowing zones (111) and three separation zones (112).

11. Pencil (1) according to any one of claims 1 to 9 in which the internal wall (110) of the sheath (100) has four narrowing zones (111) and four separation zones (112).

12. Pencil (1) according to any one of the preceding claims in which the separation zones (112) are distributed regularly around the longitudinal axis (ZZ').

13. Pencil (1) according to any one of the preceding claims in which, at any point p of the internal wall (110) of the sheath (100), j(0) = jmoy + e(0), jmoy being the average of the pellet-sheath clearance j(0) over the range 0<0<2ir, e(0) being the distance measured radially from the longitudinal axis (ZZ') between a point distant from the average pellet-sheath clearance jmoy of the external wall (210) and the point p and e(0) being called the deflection of the sheath, with 0.075 mm < jmoy < 0.095 mm.

14. Pencil (1) according to the preceding claim in combination with claim 10 in which e(0) is substantially equal to eO*cos(30), with e0=jmoy-jl.

15. Pencil (1) according to claim 13 in combination with claim 11 in which e(0) is substantially equal to eO*cos(40), with e0=jmoy-jl.

16. An assembly for a nuclear reactor, comprising rods (1) according to any one of the preceding claims and a body inside which the rods are arranged in the form of a bundle.

17. Nuclear reactor comprising a core inside which are arranged assemblies according to the preceding claim.

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