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 pencil technical field

[0001] The present invention relates to the field of nuclear reactor fuel rods. Its application is particularly advantageous 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 side by side in the reactor vessel. The assemblies themselves typically consist of a bundle of fuel rods. Each fuel rod typically comprises a plurality of pellets of fissile nuclear fuel material stacked one on top of the other inside a cladding. A manufacturing clearance is conventionally provided between the stack of pellets and the surrounding cladding. This makes it easier to insert the pellets into the cladding for stacking.

[0003] When the reactor is operating, the fission reactions occurring within the fuel generate gaseous and solid fission products, which in turn cause the pellets to swell throughout the irradiation. This swelling can cause the side wall of the pellets to contact the cladding, locally closing the manufacturing gap. It can even continue until the manufacturing gap is closed around the entire perimeter of the pellet. This expansion of the pellet against the inner wall of the cladding causes significant mechanical stresses within the cladding. This phenomenon can be further accentuated by the reduction in the cladding diameter due to the pressure of the coolant flowing around it.

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

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

[0006] These constraints are also detrimental to the reactor's control flexibility. Nuclear reactors must, however, be flexible in order to adapt Electricity consumption varies depending on the time of day. This need for flexibility is increasingly important due to the growing reliance on intermittent renewable energy sources. Solar and wind power, for example, whose production is inherently irregular, need to be coupled with a highly flexible energy source, such as nuclear power. This translates into significant technical constraints on reactor control, reactor components, and especially the cladding that houses the fuel rods. This cladding must be able to accommodate the rods under both low and high production conditions.

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

[0008] Another known solution is to limit power cycling. However, this solution is not satisfactory in terms of power and flexibility provided to the grid, since the reactor must be able to compensate for a sharp drop in the renewable energy component of 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 extending mainly along a longitudinal axis is provided, comprising a plurality of nuclear fuel pellets stacked one on top of the other along the longitudinal axis, the plurality of pellets having an outer wall, and a cladding surrounding the plurality of pellets, the cladding having an inner wall, wherein, at any point p of the inner wall of the cladding located by an angle 0, with 0<0<2ir, around the longitudinal axis, the radial distance measured radially from the longitudinal axis between the outer wall of the plurality of pellets and the point p defines a pellet-cladding clearance j(0), characterized in that the inner wall has at least three first zones, called tightening 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 spreading zones,each presenting a pellet-sheath set j greater than or equal to a second set, called j2, with 0 <jl / j2<0,7, et de préférence 0<jl / j2<0,5. ,

[0011] The fact that the sheath has such constriction and spacing zones limits the extent of the contact areas between the pellets and the sheath. This also results in the sheath being subjected to more bending than tension compared to current pencils.

[0012] One of the advantages of the sheath according to the invention is that it is fully compatible with standard, unmodified pellets, and in particular cylindrical pellets. This has the benefit of not incurring additional production costs and of facilitating the supply of pellets and the industrialization of the 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 in use. Here again, the industrialization of the fuel rod is facilitated and production costs are limited.

[0014] A second object of the invention relates to an assembly for a nuclear reactor, comprising rods according to the first aspect of the invention and 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, features and advantages of the invention will become clearer from the detailed description of an embodiment thereof, which is illustrated by the following accompanying drawings in which:

[0017] [Fig.1] Fig.1 represents schematically, an example of a combustible pencil according to the invention.

[0018] [Fig.2A] Fig.2A represents, in cross-section and schematically, a combustible pencil according to an example of the invention, having a quadrifoliated sheath.

[0019] [Fig.2B] Fig.2B represents, in cross-section and schematically, a combustible pencil according to another example of the invention, having a trifoliate sheath.

[0020] [Fig. 3] Fig. 3 represents a cross-section of a portion of an assembly not square receiving a combustible pencil with a quadrifoliated sheath.

[0021] [Fig.4] Fig.4 represents a cross-section of a combustible pencil the state of the art.

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

[0023] Before proceeding with a detailed review of embodiments of the invention, optional features that may be used in combination or alternatively are listed below:

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

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

[0026] By way of example, the first pad-sheath clearance j1 is strictly less than the so-called conventional clearance jconv that would have been measured if the pad were surrounded by a conventional cylindrical sheath with a circular base. This value, well known to those skilled in the art, is typically approximately 0.085 mm for PWRs. Thus, for PWRs, we typically have, within the scope of the present invention, j1 < 0.085 mm.

[0027] According to one example, the first pellet-sheath 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-sheath clearance j1 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 sheath with a circular base. Thus, for PWRs, we typically have, within the framework of the present invention, 0.0255 <jl mm et / ou jl <0,0595 mm.

[0029] By way of example, the second pad-sheath clearance j2 is strictly greater than the so-called conventional clearance jconv that would have been measured if the pad were surrounded by a conventional cylindrical sheath with a circular base. This value, well known to those skilled in the art, is typically approximately 0.085 mm for PWRs. Thus, for PWRs, we typically have, within the scope of the present invention, j2 > 0.085 mm. By way of example, the second pad-sheath clearance j2 is greater than 0.095 mm

[0030] These latter characteristics (j1 less than 0.075 mm and j2 greater than 0.095 mm) allow for significant expansion of the pads. Indeed, the more the sheath exhibits variations in play with the pads, the more it will be able to 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 us to maintain a useful manufacturing clearance when inserting the pellets into the sheath.

[0032] According to one example, the second pad-sheath 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 pad were surrounded by a conventional cylindrical sheath with a circular base. This value, well known to those skilled in the art, is typically approximately 0.085 mm for PWRs. Thus, for PWRs, we typically have, within the scope 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 in the plurality of pellets has a shape circular of radius called radius of disc Rp.

[0035] According to one example, the constriction zones and the spacing 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 areas with high stress levels.

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

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

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

[0039] According to one example, the areas of separation and the areas of tightening exhibit 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 inner wall of the sheath has three constriction zones and three separation zones.

[0042] According to one embodiment, the inner wall of the sheath has four constriction zones and four spacing zones.

[0043] According to one embodiment, the spacing zones are regularly distributed around the longitudinal axis. Preferably, the same is true for the narrowing zones. In combination with an alternation between these two types of zones, This arrangement allows for a symmetrical distribution of stresses within the duct. This helps maintain a uniform and relatively low stress level within the duct.

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

[0045] According to an example, e(0) is substantially equal to eO*cos(30) within manufacturing tolerances, with e0=jmoy-jl.

[0046] According to an example, e(0) is substantially equal to eO*cos(40) 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 inner wall, in which, at any point p of the inner wall located at an angle θ, with θ < θ < 2π, around the longitudinal axis, the distance between the longitudinal axis and point p defines an internal radius R(θ). This sheath is characterized in that the inner wall has at least three first zones, called constriction zones, each having an internal radius R(θ) corresponding to a minimum, called Rmin, and at least three second zones, called expansion zones, each having an internal radius R(θ) corresponding to a maximum, called Rmax, with Rmax > 1.01 * Rmin. Typically, Rmin is approximately equal to 4.5 mm.

[0048] The terms "approximately", "about", "on the order of" mean "to within 10%, preferably to within 5%" or, when referring to an angular orientation, "to within 10°". Thus, a direction substantially normal to a plane means a direction having an angle of 90+10° with respect to the plane.

[0049] “Internal” means the elements or faces turned towards the inside of the pellets combustibles, for example facing towards the longitudinal axis, and "external" refers to the elements or faces facing outwards from the combustible pellets, for example by being opposite to the longitudinal axis.

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

[0051] A cylinder is defined as a solid bounded 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 may not be circular. Note that the term "cylinder" includes a substantially cylindrical shape, that is to say of imperfect cylindrical shape, for example in connection with manufacturing tolerances.

[0052] The bending and tensile stresses referred to below are measured in the duct cross-section on a plane perpendicular to the circumferential direction. These bending and tensile stresses can be calculated, in particular, by numerical simulation, especially 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 following 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, for the purposes of the invention, a pencil does not necessarily include combustible pellets.

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

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

[0058] Fig. 1 illustrates a non-limiting example of a combustible pencil.

[0059] The fuel rod 1 typically comprises a plurality of fuel pellets 200 stacked one on top of the other along the longitudinal axis ZZ', an axis extending along the Z direction of the XYZ coordinate system. The fuel rod 1 also conventionally comprises a sheath 100 surrounding the plurality of pellets 200 in the transverse plane XY. The sheath 100 is cylindrical in shape and extends primarily along the Z direction. It is closed at each of its ends along the longitudinal axis ZZ' by plugs, referred to as the upper plug 11a and the lower plug 11b. The sheath 100 may contain one or more free expansion chambers for fission gases 12. A chamber 12 may accommodate a spring 13 and / or a spacer to ensure that the pellets 200 are held within the sheath 100.

[0060] The pellets 200 have an outer wall 210. They are characterized geometrically, in particular, by the distance between the longitudinal axis ZZ' and the outer wall 210, this distance being able to vary, in the case of non-circular pellets, depending on the location on the outer 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 cross-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 elOO, preferably constant. The sheath thickness is measured in the transverse plane XY and at any point of the sheath along a direction perpendicular to the direction tangential to the profile of the sheath 100.

[0063] The inner wall 110 of the sheath 100 has constriction zones 111 and spacing zones 112. In other words, the constriction zones 111 and spacing zones 112 are distributed along the inner wall 110 of the sheath 100. Thus, the cross-section of the sheath, at least of its inner wall 110, taken in the transverse plane XY, is not circular. The constriction zones 111 and spacing zones 112 have a distance from the pad 200 that is respectively smaller and larger than any other zone of the inner wall 110. There are at least three constriction zones 111 and spacing zones 112. Preferably, these constriction zones 111 and spacing zones 112 are located in the same transverse plane XY. These zones typically extend along the longitudinal axis ZZ' along the entire height of the stack of pellets 200 and the sheath 100.According to a preferred embodiment, the sheath 100 comprises as many constriction zones 111 as separation zones 112.

[0064] During the inflation of the pellet 200, contact between the pellet 200 and the sheath 100 occurs at the constriction zones 111. Therefore, only a small area of ​​the inner wall 110 of the sheath is in contact with the pellet. In this configuration, the sheath is subjected to more bending and less tension compared to a prior art sheath, which reduces stress in the sheath 100.

[0065] Reducing the mechanical interaction between the cladding 100 and the pellet 200 also limits the diametral deformation of the cladding, the cladding strain energy density, and the cladding failure time, and improves the cladding's durability. This improves the margins on these various safety criteria commonly used to characterize a fuel element. Consequently, the reactor's operating margins are improved, resulting in increased flexibility and / or maximum power.

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

[0067] The sheath 100 is configured such that, under the effect of the expansion of the pad 100 against the constriction zones 111, the latter move away from the longitudinal axis ZZ' while the separation zones 112 move towards it. The displacement of the separation zones compensates for that of the constriction zones, directly induced by the swelling of the pad 200. This prevents the sheath from being subjected to excessive stress. Thanks to its geometry, the sheath 100 essentially acts like a spring, allowing it to flexibly accommodate the expansion of the pad 200. The sheath 100 is typically made of a metallic alloy.

[0068] Due to the particular shape of its inner wall, the sheath 100 has a plurality of hollows and peaks comprising, respectively, the spacing zones 112 and the constriction zones 111. The sheath 100 and the disc 200 define, on the one hand, cavities 150, delimited by the disc 200 and the hollows of the sheath 100, and on the other hand, grooves 160, delimited by the disc 200 and the peaks of the sheath 100. The cavities 150 and the grooves 160 thus constitute the empty space between the disc 200 and the sheath 100. The grooves 160 are intended to be at least partially closed during the expansion of the disc 200 until it contacts the constriction zones 111.

[0069] The constriction zones 111 and the spacing zones 112 are preferably placed alternately around the pad 200. According to a preferred example, if N denotes the number of constriction zones 111, these are placed symmetrically around the longitudinal axis ZZ', with a rotational symmetry of an angle of 2ir / N. The same is preferably true for the spacing zones 111. Such a configuration ensures a symmetrical distribution of stresses in the sheath 100, which avoids a concentration of stresses at a single point or in a small area of ​​the sheath 100, which would have the disadvantage of weakening it. 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 angles within such a profile facilitates the deformation of the sheath 100 to accommodate the expansion of the pellet. Furthermore, angles constitute areas of weakness in the sheath 100 during its deformation due to the expansion of the pellet 200. Eliminating them further reduces the stress levels within the sheath 100 and improves its mechanical strength.

[0071] According to a preferred alternative, the inner wall 110 has a convex profile around its entire perimeter. The radius of curvature of the constriction zones 111 is then greater than the radius of curvature of the separation zones 112. A wall having a convex profile around its entire perimeter helps to limit the areas of weakness in the sheath 100.

[0072] According to another alternative, the inner wall 110 may, for example, have alternating concave and convex regions around the pad 200. The convex regions extend around the constriction zones 111. Such a convex shape makes it possible to further reduce the tensile stress between the sleeve 100 and the pad 200. This allows the stresses in the sleeve 100 to be further reduced. The radius of curvature of the convex regions can be dimensioned so as to favor bending interaction as much as possible over tensile interaction.

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

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

[0075] Similarly, the internal radius R(0) of the sheath 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-sheath clearance javg, an average internal radius Ravg is defined as 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 j1, but it is understood that it can take lower values ​​at least in some of the tightening zones 111. j1 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 fuel rod. Furthermore, jl is preferably less than 0.085 mm for a standard PWR fuel 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 disc-sheath set is identical in each of the constriction zones 111, as in figures 2A and 2B. The pad-sheath clearance j(0) admits in this case as many minima as there are constriction zones.

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

[0078] The values ​​j1 and j2 are target values ​​for the pad-sheath clearance in the constriction and separation zones. As mentioned above, it is advantageously expected that the pad-sheath clearance will actually be equal to j1 in the constriction zones 111 and to j2 in the separation zones 112, as illustrated in [Fig. 2A]. However, it is possible that j<jl au niveau de certaines au moins des zones de resserrement 111 et j> j2 at the level of at least some of the constriction zones 111. Furthermore, when describing the preferred embodiment in which the pad-sheath clearance is equal to j1 at the level of the constriction zones 111 and to j2 at the level of the separation zones 112, it is understood that these equalities are understood to the manufacturing errors of the sheath 100 and the pad 200.

[0079] Advantageously, javg is substantially equal to the pad-sheath clearance that would have been measured if the pad 200 had been surrounded by a conventional cylindrical sheath with a circular base, referred to as the conventional clearance and denoted jconv. jconv is typically substantially equal to 0.085 mm for PWRs. Thus, for PWRs, we typically have, within the scope of the present invention, javg = jconv = 0.085 mm. j1 is then advantageously between 0 and 0.085 mm, and j2 is advantageously between 0.085 mm and 0.170 mm. j1 can therefore be substantially equal to 0 mm and j2 substantially equal to 0.170 mm.

[0080] The values ​​of j1 and j2 are outside the manufacturing tolerance range of the sheath 100. This condition is defined further with reference to the sheath deflection.

[0081] The average internal radius Rmoy is substantially equal to that of a prior art sheath. This preserves, compared to existing systems, the free volume of the rod and the surface area for the heat transfer fluid within the assembly. Furthermore, the hydraulic diameter of the assembly is only very slightly modified compared to prior art rods.

[0082] Figure 2A illustrates the fact that the inner wall 110 of the sheath 100 is located alternately on either side of a theoretical path (in dotted lines) distant from the gap The average radius of the duct is javg of the duct 200. If the duct is circular, this path is also circular and coincides with a circle of internal radius Ravg around the longitudinal axis ZZ'. The deflection e(0) is defined at point p as the distance measured radially around the longitudinal axis ZZ' between this theoretical path and this point p on the inner wall 110 of the duct 100. If the inner wall 110 is located between the theoretical path and the duct 200, the deflection e(0) is negative. If, on the other hand, it is located beyond the theoretical path, the deflection e(0) is positive.

[0083] The deflection is less than a first deflection e1 at the constriction zones 111 and greater than a second deflection e2 at the separation zones 112. Advantageously, in absolute values, these minimum and maximum values ​​are equal: e2 = -e1 = e0. Preferably, the deflection is identical in each of the constriction zones 111. The deflection e(0) has as many minima as there are separation zones in this case. Similarly, preferably, the deflection is identical in each of the separation zones 112. The deflection e(0) has as many maxima as there are separation zones in this case.

[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 constriction zones 111 and the separation zones 112 can be point-like in the transverse plane XY or, if the clearance and the deflection admit maxima / minima over a continuous range of values ​​of 0, correspond to a non-point-like portion of this internal wall.

[0086] The pad-sheath clearance and deflection functions advantageously follow sine or cosine laws. By following these laws, the inner wall 110 of the sheath exhibits continuity and particularly advantageous radii of curvature within the scope 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 be extrapolated to those of the sheath 100 as a whole.

[0088] Figure 2A illustrates more particularly an embodiment in which the sheath 100 comprises four constriction zones 111 and four spacing zones 112. This embodiment can be described as quadrifoliated. According to a particular example of this embodiment, the constriction zones 111 are arranged symmetrically around the longitudinal axis ZZ', with a rotational symmetry of 90°. Preferably, the same is true for the spacing zones 112. A constriction zone In this embodiment, 111 and an adjacent separation zone 112 are preferably separated by an angular distance of 45° with respect to the longitudinal axis ZZ' (example illustrated in [Fig. 2A]). An example of a pad-sheath function adapted to the quadrifoliated 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^=jmoy+e0CO^e)

[0090] It is understood that this equality is understood as "substantially equal", and that manufacturing tolerances are to be taken into account.

[0091] It is conceivable that the sheath 100 may have a different number of constriction zones and / or separation zones. Figure 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 arranged symmetrically around the longitudinal axis ZZ', with a rotational symmetry of 120°. Preferably, the same is true for the separation zones 112. In this embodiment, an adjacent constriction zone 111 and a separation zone 112 are preferably separated by an angular distance of 60° with respect to the longitudinal axis ZZ'. An example of a pad-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 implementation.

[0092] [Math.2] j($) = jmoy + e o COs(30)

[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 spacing zones and six tightening zones (hexafoliated embodiment).

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

[0096] PWR reactor assemblies typically include a grid 400 having a square element pattern 410 (square pitch assembly). Fuel rods 1 are inserted through the grid 400, each in a elementary motif 410. The sheath 100 surrounding one of these pencils 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 motif 410. As illustrated in [Fig.3], it is advantageous to make these holds at the level of the constriction areas 111.

[0097] Combining a quadri-layered sheath with a square-pitch assembly and matching the retaining points to the tightening zones 111 of the sheath 100 allows for better retention of the pencil 1 and better distribution of stresses within the sheath 100.

[0098] Other combinations are also possible. The trifoliate and hexafoliate embodiments, for example, are very well suited, due to the shape of the sheaths, to a triangular-pitch assembly. An assembly of this type has pencils 1 arranged in a triangular pitch and typically separated by spacer wires.

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

[0100] In view of the preceding description, it is clear that the invention proposes a fuel rod that improves the performance in terms of flexibility and power of a reactor during normal operation, while also limiting damage during incidental or accidental operation. It also appears that the described solution is fully compatible with industry standards (shape of the pellets, spring-pin attachment, assembly, etc.).

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

Claims

Demands

1. A nuclear fuel rod (1) extending mainly along a longitudinal axis (ZZ'), comprising a plurality of nuclear fuel pellets (200) stacked one on top of the 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 θ, with θ < θ < 2π, 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 initial zones, called constriction zones (111), each having a The game's disc-sheath size is less than or equal to that of a first game (jl), and there are at least three second zones.said separation zones (112), each having a pad-sheath set greater than or equal to a second set j2, with 0 <jl / j2<0,7, dans lequel les zones de resserrement (111) et les zones d’éloignement (112) sont disposées en alternance autour de l’axe longitudinal (ZZ’), et dans lequel le deuxième jeu pastille-gaine j2 est strictement supérieur à 0,085 mm et de préférence supérieur ou égal à 0,1105 mm.,

2. Pencil (1) according to the preceding claim in which 0 <jl / j2<0,5.

3. Pencil (1) according to any one of the preceding claims wherein the sheath (100) is configured to deform so as to accommodate a radial expansion of the stack of pellets (200), the constriction zones (111) and the separation zones (112) being configured to move away and towards the longitudinal axis (ZZ') respectively under the action of the expansion.

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

5. Pencil (1) according to any one of the preceding claims wherein 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.

6. Pencil (1) according to any one of the preceding claims wherein the spacing zones (112) and the tightening zones (111) have a sinusoidal profile around the longitudinal axis (ZZ').

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

8. Pencil (1) according to any one of claims 1 to 6 in which the inner wall (110) of the sheath (100) has four constriction zones (111) and four separation zones (112).

9. Pencil (1) according to any one of the preceding claims wherein the spacing zones (112) are distributed regularly around the longitudinal axis (ZZ').

10. Pencil (1) according to any one of the preceding claims in which, at any point p of the inner wall (110) of the sheath (100), j(0) = jmoy + e(0), jmoy being the average of the pad-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 pad-sheath clearance jmoy of the outer wall (210) and the point p and e(0) being named sheath deflection, with 0.075 mm < jmoy < 0.095 mm.

11. Pencil (1) according to the preceding claim in combination with claim 7 wherein e(0) is substantially equal to eO*cos(30), with e0=jmoy-jl.

12. Pencil (1) according to claim 10 in combination with claim 8 wherein e(0) is substantially equal to eO*cos(40), with e0=jmoy-jl.

13. Nuclear reactor assembly, comprising fuel rods (1) according to any one of the preceding claims and a body within which the fuel rods are arranged in the form of a bundle.

14. Nuclear reactor comprising a core within which are arranged assemblies according to the preceding claim.

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