Nuclear fuel pellet incorporating thermally conductive inserts in the form of rods distributed at different azimuths and altitudes, associated fuel rod and nuclear fuel assembly.

Abstract

Nuclear fuel pellet incorporating thermally conductive inserts in the form of rods distributed at different azimuths and altitudes, fuel rod and associated nuclear fuel assembly. The invention consists of a nuclear fuel pellet (6), comprising: - a straight cylinder (60) of fissile material with a central axis (X) whose length and diameter respectively define the length (H) and diameter (Ø) of the pellet; - a plurality of inserts (8; 80, 81, 82) of thermally conductive material, the inserts being solid rods, distributed at different azimuth and altitude positions within the straight cylinder, and spaced apart from each other. Figure for the abstract: Fig. 4

Description

Title of the invention: Nuclear fuel pellet incorporating thermally conductive inserts in the form of bars distributed at different azimuths and altitudes, associated fuel rod and nuclear fuel assembly. technical field

[0001] The present invention relates to the field of fuel elements for nuclear reactors, in particular Pressurized Water Reactors (PWR).

[0002] More specifically, it is in the field of ceramic-type fuels made of uranium oxide (UO2) or MOX consisting of approximately 8.5% plutonium and 91.5% depleted uranium.

[0003] The invention essentially aims to improve the thermal properties of these fuels, both in nominal operation and in incident or accident regime (for example in a Loss of Coolant Accident (LOA) situation).

[0004] For the purposes of this application, "nuclear reactors" means the current common meaning of the term, namely power plants that produce energy from nuclear fission reactions using fuel elements in which fissions occur that release heat power, the latter being extracted from the elements by heat exchange with a heat transfer fluid that ensures their cooling.

[0005] The term “nuclear fuel rod,” as used throughout this application, is understood to mean the official definition, for example, found in the Dictionary of Nuclear Science and Technology, namely a narrow tube of small diameter, closed at both ends, constituting the core of a nuclear reactor and containing fissile material. Thus, a “nuclear fuel needle,” the term most commonly used, is a nuclear fuel rod within the meaning of the present invention.

[0006] Although described with reference to the application to PWR reactors, the invention relates to fuel elements which can be dedicated to all types of reactors for power generation or experimental purposes, such as Boiling Water Reactors (BWRs), Fast Neutron Reactors (FNRs), including those cooled by a liquid metal, in particular by liquid sodium (Na-FNR). Previous technique

[0007] Nuclear reactors that use fission energy to produce heat can be classified into several different categories according to their characteristics Risks: the final form of energy produced (electricity, heat, etc.), the type of neutron flux (fast neutrons or thermalized neutrons), the cooling fluid used (liquid metal, water, etc.), the physical state of the cooling fluid (solid, liquid, or gaseous), the pressure level of the cooling fluid (e.g., atmospheric for Boiling Water Reactors and 150 bar for PWRs, etc.)...

[0008] One sector is currently very largely dominant industrially in the world, that of Pressurized Water Reactors (PWR).

[0009] A PWR essentially comprises two main components: the primary circuit, which constitutes the nuclear part of the reactor, and the secondary circuit, which constitutes its non-nuclear part. Reference can be made, for example, to a schematic diagram at the following link: https: / / fr.wikipedia.org / wiki / R%C3%A9acteur_%C3%A0_eau_pressuris%C3%A9e.

[0010] The primary circuit essentially comprises a tank assembly with internal structures and fuel assemblies.

[0011] The secondary circuit essentially comprises non-nuclear equipment for producing electricity (piping, steam generators, turbine, etc.). The steam generators are heat exchangers that transfer heat from the core to the secondary fluid, i.e., water in a PWR, which vaporizes and then expands in a turbine which, together with the alternator, constitutes the electrical energy generation system.

[0012] Returning to the primary circuit, this incorporates the key part, called the reactor core. The core comprises all the fuel assemblies, which number from 157 to 241 in a French PWR. A schematic view of a PWR fuel assembly with its control rod assembly, including neutron-absorbing fuel rods, is shown in [Fig. 4] under the following link:

[0013] http: / / www.cea.fr / Documents / monographies / combustibles-nucl%C3%A9aires_r%C3%A9acteurs-eau.pdf

[0014] The core itself is placed in a tank which constitutes the second containment barrier for the fuel and its fission waste (called Fission Products or FP).

[0015] The core is cooled by a heat transfer fluid, water at a pressure of 150 bars, which also acts as a moderator (slowing down neutrons), in order to promote the fission reaction.

[0016] This core also includes internal structures ensuring specific functions, such as maintaining assemblies, channeling the heat transfer fluid, etc., which are not detailed here.

[0017] Fuel assemblies make it possible to produce energy by taking advantage of the nuclear fission reaction of a fuel composed of a small part of nuclei heavy fissile elements - the 235 isotope of uranium - as well as the 238 isotope of uranium which is only fertile (it will produce plutonium 239 by neutron capture).

[0018] This fuel is introduced into a cylinder with a circular cross-section, closed at each end by a plug. This cylinder is called a fuel rod and is described in more detail below. The fuel rod is sealed and constitutes the first containment barrier.

[0019] The pencils are grouped into bundles and arranged according to a square-pitch network. For the French-spec installation families called "levels" of 900 MWe and 1300 MWe, the number is 264 pencils to which are added 25 other tubes, which then make up an assembly which also incorporates numerous structural elements (lower end cap, retaining grids, instrumentation tube, 24 guide tubes in which 24 control pencils slide, upper end cap, retaining springs) as shown in [Fig.4] of the aforementioned .pdf document.

[0020] In [Fig.1], a nuclear fuel rod 1 is shown according to the state of the art in its configuration for use in a PWR nuclear reactor, i.e. in a vertical position with the pellets 6 towards the lower part as specified below.

[0021] The fuel rod 1 consists of a sheath 2 conventionally made of Zircaloy-4 (Zr4) closed at each end by a cap 3 at the top and 4 at the bottom, respectively, which is welded to it. This sealed fuel rod is filled with helium, typically at 25 bar cold for common fuels, to partially counteract the effect of the external pressure of 150 bar of the heat transfer fluid.

[0022] The interior of the cladding is essentially divided into two compartments, one of which 5 in the upper part, between the top of the fissile column and the upper cap 3, constitutes a gas expansion chamber and the other houses the fissile column formed by the stacking of nuclear fuel pellets 6 which each extend along the longitudinal direction XX' of the rod 1.

[0023] The expansion chamber is a free volume intended to receive the Fission Products in gaseous form, commonly called Fission Gas (FG).

[0024] In the stack shown, each pellet 6 has substantially the same length or height H.

[0025] A helical compression spring 7, generally made of Inconel®, is housed in the expansion chamber 5 with its lower end bearing against the upper face of the pad 6, the highest in the stack of pads, and its other end bearing against the upper plug 3.

[0026] In addition to maintaining the stacking of the pellets 6 along the longitudinal axis XX' and "absorbing" over time the longitudinal swelling of the pellets 6, the other function of this spring 7 is to prevent buckling of the sheath cross-section on its mode of ovalization. In other words, it must prevent extreme ovalization of the sheath's cross-section.

[0027] The primary function of a fuel rod is to produce, and then transmit, the heat produced by the fission reactions within the fuel.

[0028] To date, a fuel pellet as it is usually implemented in a PWR reactor, consists of uranium oxide UO2 enriched in U235 to about 5%, the complement being fertile U238.

[0029] Each pellet releases energy in the form of heat by nuclear fission, which varies over time depending on fuel wear but also on the variation in the altitude of the control rods and the temperature of the primary circuit.

[0030] The power thus dissipated is also a function of the position of the pellet in the pencil, the position of the pencil in the assembly, and the position of the assembly in the core.

[0031] This power is dissipated towards the cold source of the primary circuit (the primary cooling water) by encountering a number of thermal obstacles which can be summarized as follows: - a strong thermal gradient between the center and the periphery of the pellet induced by the low thermal conductivity of UO2; - a radial thermal gradient between the pellet and the pencil cladding. Indeed, the joint between the pellet and the cladding, entirely gaseous (helium) at the beginning of irradiation, becomes completely filled during the second cycle. However, the roughness of the pellet allows for the discontinuous presence of gas which is no longer simply helium but also includes gases: thus, the contact between the pellet and the cladding is never perfect and therefore creates, through its thermal resistance, a radial thermal gradient; - radial thermal conduction through the sheath; - the resistance due to radial convective exchange between the outer face of the duct and the heat transfer fluid.

[0032] The heat evacuation in nominal operating mode and in incidental and accidental mode is mainly governed by the heat conduction equation (or Fourier equation) with energy dissipation for the fuel pellet to which is added the Newton equation which models the convective exchanges by a heat exchange coefficient between the cladding and the heat transfer fluid.

[0033] These equations apply in incidental and accidental situations, as long as the geometry of the pencil (including pellets) remains intact and the heat transfer fluid has not vaporized.

[0034] Numerical simulations of a Loss of Primary Coolant (LPRC) type accident and of a Reactivity Insertion (RIA) type accident Accident”), demonstrated the advantage, from the point of view of the temporal thermal consequences of this accident, of having, at the nominal therefore initial regime of this accident, a fuel that is as cold as possible at the core.

[0035] The main objective is therefore, in terms of safety, to lower the thermal temperature of the pencil and more particularly of the fuel pellet.

[0036] In general, reducing the operating temperature of a fuel pellet makes it possible to increase the power density, reduce the release of fission gases, improve the safety of reactors in incident (IPG acronym for Pellet-Cladding Interactions) and accident (APRP, RIA...) situations, improve the flexibility of controlling a nuclear reactor, and increase the maximum burn-up rate (also called bum-up or irradiation rate) of the latter.

[0037] Figure 2, taken from publication 1, gives the order of magnitude of the temperatures in a fuel pellet under nominal operating conditions.

[0038] There are various ways to improve the heat dissipation function of a fuel pellet or stack of UO2 pellets. Since some characteristics can hardly be modified, such as the primary fluid or the cladding material, the most promising avenue for innovation is improving the thermal conductivity of UO2.

[0039] This route can be divided into two categories which have been explored.

[0040] The first category relates to the improvement of thermal conductivity without the addition of another phase within the UO2 fuel.

[0041] This first category can itself be subdivided into three subcategories as follows:

[0042] - a modification of the shape of the pencil, in order to increase the exchange surface between hot source and cold source. This modification is, by definition, localized at the level of each fuel rod in a fuel assembly. The major drawbacks of this solution are that it requires a new design of the assembly and may have an impact on the manufacturability of the fuel rod and the performance of the assembly;

[0043] - a change in the nature of the fuel by switching from a combustible oxide to a A better heat conductor fuel, for example a metallic fuel. This modification is by definition localized at the level of each fuel pellet. The major drawbacks of this solution are that it induces different behavior in the reactor, requires research and development to develop / qualify this new fuel, necessitates the development of new manufacturing processes, and requires redefining the downstream fuel cycle;

[0044] - a combination of the two preceding subcategories which is a designated concept under the name LIGHTBRIDGE: see in particular [2]. This concept consists of a generally helical-shaped pencil whose cladding contains a fuel tallique, of UZr. In addition to the disadvantages explained for the two previous subcategories, this solution has the major disadvantages of greatly reducing the mass of fissile material, typically by about 25% and of requiring the pellet to be super-enriched in U5 fuel.

[0045] The second category concerns the improvement of thermal conductivity with the addition of another phase within the UO2 fuel. The addition is localized by definition at the level of each fuel pellet.

[0046] This second category can itself be subdivided into three subcategories as follows:

[0047] - a homogeneous dispersion of a material, such as diamond, graphene... Thermal conductivity superior to UO2 ceramic at the nanometer to micron scale. This solution does not appear relevant at present from the point of view of its thermal behavior. The inventors believe, however, that it is difficult to make a definitive judgment on this solution due to a lack of information and knowledge. The major drawbacks of this solution are that it is only at the laboratory scale and therefore requires complete research and development for the fuel, a manufacturability analysis, and finally, the lack of any modeling tools.

[0048] - a homogeneous dispersion of a second metallic phase on the micro scale scopic with the objective of reducing the thermal conductivity of the UO2 pellet: see in particular [1]. This solution has the major drawbacks of inducing manufacturing difficulties and requiring the finalization and validation of existing modeling tools;

[0049] - a heterogeneous dispersion of a second metallic phase on a macro scale scopic, that is, at the scale of a tenth of a millimeter or less, with the objective of promoting the heat flow from the UO2 pellet towards the cold source. This second metallic phase is characterized by a metallic insert within the fissile material of the pellet.

[0050] Such a metallic insert is described under two distinct designs in publication [3].

[0051] One such design, shown in Figure 1a of this publication [3], consists of a set of six thin fins, uniformly distributed at an angular angle like the spokes of a bicycle wheel. This design effectively improves the thermal conductivity of the fuel pellet at the center.

[0052] However, the analysis carried out by the applicant's experts on this design shows that while the choice of structural characteristics of the fins does indeed allow for a significant reduction in the core temperature of the pellet, this choice potentially generates several drawbacks that could have very troublesome consequences on the behavior of the pellet, or even to completely call into question the relevance of the solution.

[0053] The main potentially negative consequences are as follows: - a hypothetical guarantee of the thermal function under irradiation and of the mechanical strength under irradiation of the fins given their small thickness; - mechanical strength of the sheath not guaranteed, with high and local temperature gradients on it, typically from 100 to 300 °C / mm, which are due to the contacts of each fin with the sheath; - the lack of certainty of not aggravating the phenomenon of Pellet Sheath Interaction with Stress Corrosion (IPG-CSC); - a level of corrosion that may be present on the cladding during irradiation; - the manufacturability of metallic fins in a ceramic fuel has not been demonstrated, including the possibility of mounting the fins.

[0054] To overcome the aforementioned drawbacks of the design shown in figure la of publication [3], the applicant proposed in patent applications WO2022 / 223387, WO2022 / 223504, WO2022 / 223510, different forms of metallic insert in substance respectively, with cross section in four branches, and with solid disc(s) and solid rod.

[0055] If these solutions appear satisfactory, the inventor of the present invention has set himself the objective of further reducing the thermal gradients in a fuel pellet in operation.

[0056] In other words, there is a need to improve the thermal design of nuclear fuel pellets, particularly those based on UO2 oxides, in order to reduce their core temperature as much as possible under nominal operating conditions for a Pressurized Water Reactor (PWR) core, while respecting the following constraints:

[0057] - minimization of thermal gradients within a pellet;

[0058] - minimization of the quantity of second phase supplied in a pellet;

[0059] - absence of degradation or at least limited degradation of the behavior of the new tronics;

[0060] - easiest possible manufacturability;

[0061] - assembly of a fuel column in the simplest possible way;

[0062] - absence of the aforementioned drawbacks of the design shown in figure la of this publication [3].

[0063] The aim of the invention is to meet at least part of this need. Description of the invention

[0064] To this end, the invention relates, in one of its aspects, to a nuclear fuel pellet, comprising:

[0065] - a right cylinder of fissile material with central axis (X) whose length and diameter define respectively the length and diameter of the pellet; - a plurality of inserts made of thermally conductive material, the inserts being solid bars, distributed according to different azimuthal and altitude positions within the right cylinder, being distant from each other.

[0066] The bars can be straight. The inserts can advantageously be metallic or made of metallic alloy or ceramic.

[0067] They can have a square, rectangular, circular or elliptical cross-section.

[0068] The bars are preferably distributed homogeneously in the volume of the straight cylinder.

[0069] According to an advantageous embodiment, the bars are distributed equally in several planes parallel to each other and perpendicular to the central axis (X), with the coplanar bars of one plane oriented at an angle offset by 90° with respect to those of an adjacent plane.

[0070] According to this mode, each bar extends advantageously along a radial direction secant with the central axis (X).

[0071] According to this mode and an advantageous configuration variant, the pellet comprises, per plane, a bar which extends according to the diameter of the pellet.

[0072] According to another advantageous embodiment, the pitch measured at the periphery of the distribution pad between two adjacent bars within the same plane is equal to or greater than the axial distance between two adjacent planes.

[0073] Preferably, the cross-section of each bar is less than 1 mm2, preferably less than 0.1 mm2.

[0074] Preferably, the length of each bar is less than or equal to the diameter of the pellet, i.e. a few mm.

[0075] Preferably, the length of each bar is a fraction of the radius of the pellet. Each plane comprises a through bar of length 2*R, and 4 bars of each length L defined by i=(l :1 : n-1) (L=i*R / n), n being an integer greater than 1.

[0076] With n=2, each plane has one bar of length 2*R and 4 bars of length 1R / 2 (5 bars per plane).

[0077] With n=3, each plane comprises one bar of length 2*R and 4 bars of length 2R / 3 and 4 bars of length 1R / 3 (9 bars per plane, Figures 4 and 4A). With n=4, each plane comprises one bar of length 2*R and 4 bars of length 3R / 4, 4 bars of length 2R / 4 and 4 bars of length 1R / 4 (13 bars per plane, figures 5 and 5A).

[0078] With n=5, each plane has 17 bars per plane (figures 6, 6A and 7).

[0079] The bars on the same plane are distributed homogeneously in azimuth, one of their ends being on the periphery of the disc, the different lengths of bars being alternated as shown in figures 4 to 7.

[0080] Preferably, the ratio between section and length of each bar is less than 0.5, preferably less than 0.1.

[0081] Advantageously, the material of the bars is chosen from a zirconium alloy, in particular Zircaloy-4 (Zr4), chromium (Cr), Molybdenum (Mo).

[0082] Advantageously still, the fissile material of the straight cylinder is chosen from uranium(IV) oxide (UO2), mixed oxide (U,Pu)O2 or a mixed mixture based on uranium oxide and reprocessed plutonium oxides.

[0083] Preferably, the volume percentage of the bars being between 1 and 10%.

[0084] The invention also relates to a nuclear fuel rod extending along a longitudinal direction (XX1) comprising:

[0085] - a plurality of nuclear pellets as described above, stacked one on top of the other;

[0086] - a sheath of neutron-transparent material surrounding the stack of pellets.

[0087] Advantageously, the sheath is made of zirconium alloy, in particular Zircaloy-4 (Zr4), or of M5® alloy (ZrNbO).

[0088] The invention also relates to a nuclear fuel assembly comprising a plurality of fuel rods as above and arranged among themselves in a network.

[0089] The invention also relates to the use of a nuclear fuel pellet as described above or a fuel rod as described above in a pressurized water reactor (PWR), a boiling water reactor (BWR), a fast neutron reactor (FNR), in particular cooled by liquid metal, such as liquid sodium (Na-FNR).

[0090] Thus, the invention essentially consists of a nuclear fuel pellet which integrates, within a straight cylinder of fissile material, a plurality of thermally conductive inserts, in particular metallic, in the form of solid bars, advantageously distributed homogeneously in a straight cylinder of combustible material.

[0091] The bars at the micro-structure scale have a geometry and distribution that minimize the volume of added metallic phase required relative to the volume of the pellet, with a given objective of reducing and homogenizing fuel temperature.

[0092] The bars thus inserted into the fuel cylinder create bridges thermals that reduce thermal gradients in the pellet, preferably by being oriented in the direction of the main heat flow, which for a pellet is very predominantly radial (except for the end pellet of a low and high fissile column), between the core of the pellet and its periphery and the cladding.

[0093] The thermally conductive bars are thus preferentially oriented in the radial direction of the straight fuel cylinder.

[0094] Thus, the directions of the bar-shaped inserts to be preferred for standard cylindrical pellets are predominantly radial, then azimuthal and minorityally axial, i.e. according to the height of the right cylinder of the pellet.

[0095] The metal rods make it possible to reduce the maximum fuel temperature by several hundred degrees Celsius, depending on their dimensions, their constituent material, and their distribution within the straight fuel cylinder, while minimizing their added volume percentage and thus maximizing the fissile volume of the pellet. This minimizes the over-enrichment required to compensate for the loss of fissile mass.

[0096] Thermally conductive bars, can be based on molybdenum, chromium or SiC ceramic (silicon carbide), and each have a characteristic length of a few mm, a characteristic width of the order of 1 mm or less (a few microns), a characteristic height of the order of 1 mm or less (a few microns).

[0097] The bars are introduced into the pellet at different altitudes and different azimuths.

[0098] In general, the more the width and height of the bars can be reduced while preserving their radial continuity, the more homogeneous and efficient their distribution in the pellet can be with a maximized gain at a given percentage of conductive phase.

[0099] Because of the radial then lateral direction which is predominantly preferred for a standard cylindrical format pellet, a larger bar width is preferable to a larger height, depending on the height of the fuel cylinder.

[0100] Depending on the desired temperature decrease and distribution, the number and positioning of the conductive bars, i.e. their volumetric density in the pellet, will need to be adjusted.

[0101] In particular, depending on the radial area of ​​the pellet to be cooled, the distribution of the bars in the pellet will need to be adjusted.

[0102] Advantageously, the axial characteristic pitch, that is to say along the height of the straight cylinder, between the bars is of the same order as the horizontal characteristic pitch between the bars at the periphery of the disc, so as to homogenize at

[0103]

[0104]

[0105]

[0106]

[0107] To maximize the temperatures within the pellet, the bars will preferentially be oriented with a phase angle, from one axially adjacent plane to another, for a more homogeneous mesh of the pellet, in order to homogenize the temperatures within the pellet as much as possible. Therefore, thanks to the plurality of bar-shaped inserts, the operating temperature of the fuel pellet is significantly reduced. A ceramic fuel pellet operating at a lower temperature offers numerous advantages, including: - a reduction in the release of fission gases and fuel fragmentation, which is beneficial for the internal pressure of the fuel rod, - a decrease in the swelling of gas bubbles and therefore of the fuel, - a decrease in the mechanical interaction between the pellet and the sheath, which is beneficial for the diametral deformation of the sheath and the deformation energy density of the sheath, - an increase in margins on the various safety criteria mentioned above, in particular on the fuel meltdown limit, which increases the operating margins of a nuclear reactor in particular in order to be more flexible in control, and to obtain a gain in maximum power or even in maximum bum-up, in particular in order to improve the control capacity at high burn-up. The industrial applications targeted for the invention are all fuels for power-generating reactors (PWR, BWR, RNR), as well as certain experimental reactors, with the manufacture and stacking of fuels with improved safety and performance. From a safety perspective, a pellet according to the invention can be classified in the category known as E-ATF fuel (Anglo-Saxon acronym for "Enhanced Accident Tolerant Fuel"). The major developments in this E-ATF category brought about by the invention are as follows: - a lower fuel operating temperature and a lower internal pressure in the fuel element; - an introduction of inserts with thermal conductivity greater than UO2 in the direction of the main heat flow, to minimize the volume proportion of inserted bars; - a minimization of the volume of inserted rods introduced with respect to the volume of the ceramic phase of the fuel pellet by optimization of the geometry and distribution of the inserts to minimize the over-enrichment induced to compensate for the loss of fissile mass; - inserts in the form of bars on a scale of 1 mm or less (a few microns, depending on the resolution of the manufacturing process) for maximized efficiency, a more homogeneous temperature in the pellet and less internal mechanical stress; - a search for the smallest possible section / length ratios of inserts and the most homogeneous possible volumetric distribution in the areas of the pellet where we want to increase thermal conductivity.

[0108] Other advantages and features of the invention will become clearer from the detailed description of examples of implementation of the invention given by way of illustration and not limitation with reference to the following figures. Brief description of the drawings

[0109] [Fig-1] [Fig.1] is a schematic longitudinal cross-sectional view of a pencil nuclear fuel according to the state of the art, as implemented in a PWR nuclear reactor.

[0110] [Fig.2] [Fig.2] illustrates in the form of a curve the temperatures in a nuclear fuel pellet according to the state of the art, in nominal operating mode.

[0111] [Fig.3] [Fig.3] is a schematic perspective view of a basic motif M of a nuclear fuel rod whose cladding houses a fuel pellet with metallic inserts not shown according to the invention.

[0112] [Fig.4], [Fig.4A] Figures 4 and 4A are perspective views and in cross-section, resulting from a numerical simulation of a fuel pellet according to the invention, with metallic inserts in the form of rectangular bars distributed at eight altitudes, i.e., horizontal planes along the height of the straight fuel cylinder. C

[0113] [Fig.5], [Fig.5A] Figures 5 and 5A are respectively perspective and in cross-section, from a numerical simulation of a fuel pellet according to the invention, with metallic inserts in the form of rectangular section bars distributed according to a number of eleven altitudes, i.e. horizontal planes along the height of the straight fuel cylinder.

[0114] [Fig.6], [Fig.6A] Figures 6 and 6A are perspective views and in cross-section, from a numerical simulation of a fuel pellet according to the invention, with metallic inserts in the form of bars of circular section distributed according to a number of twelve altitudes, i.e. horizontal planes along the height of the straight fuel cylinder.

[0115] [Fig.7] [Fig.7] is a view from a numerical simulation of a pellet. combustible according to the invention, with metallic inserts in the form of bars of cross-section el liptic distributed according to a number of eighteen altitudes, that is to say horizontal planes along the height of the straight fuel cylinder.

[0116] [Fig-8] [Fig.8] illustrates in the form of a straight line, the optimal equivalent thermal conductivity as a function of the percentage of Mo insert phase relative to a UO2 fuel, for a thermal conductivity of Mo of 100 W / m / K and UO2 of 3 W / m / K.

[0117] [Fig.9] [Fig.9] illustrates in the form of curves, the maximum temperature gain in a fuel pellet as a function of the volume percentage of Mo metal phase inserted in a UO2 fuel matrix for a pellet of the type intended to operate in a PWR reactor, respectively at 373W / cm and 600W / cm. Detailed description

[0118] For the sake of clarity, the same element according to the state of the art and according to the invention is designated by the same numerical reference.

[0119] Figures 1 and 2 have already been discussed in the preamble. They will therefore not be detailed below.

[0120] In the examples below, the fuel pellets according to the invention are modeled by software marketed under the name "Scilab" which allows the pellets with inserts to be drawn.

[0121] A basic motif M of a nuclear fuel rod with a sheath 2 housing a fuel pellet 6 according to the invention is shown in [Fig.3].

[0122] A pellet 6 according to the invention with a central axis (X) comprises a straight cylinder 60 of combustible oxide material releasing thermal power and a plurality of metallic inserts in the form of bars distributed 8 homogeneously according to different azimuthal and altitude positions within the straight cylinder 60 while being spaced apart from each other.

[0123] More specifically, the bars 8 are straight and distributed equally in several planes parallel to each other and perpendicular to the central axis (X), with the coplanar bars of one plane oriented at an angle offset by 90° with respect to those of an adjacent plane.

[0124] Figures 4 to 7 illustrate four variants of embodiment of a pellet 6 according to the invention for a volumetric density of metal bars 8 forming the inserts distributed homogeneously at 4.8% (Figures 4 and 5) and at 2.5% (Figures 6 and 7) with respect to the total volume of the pellet 6.

[0125] The cross-sections of these bars 8 are respectively rectangular equal to 0.6x0.3mm ([Fig.4]), rectangular equal to 0.5x0.2mm ([Fig.5]), circular equal to 0.2x0.2mm ([Fig.6]), and elliptical 0.5x0.05mm ([Fig.7]).

[0126] Additive manufacturing processes can be implemented to produce this type of insert geometries.

[0127] Figure 4 relates to a variant with eight altitudes, or horizontal planes, of metal bars 8. In each horizontal plane, arranged at intervals from one another are a bar 80 extending over a diameter 0 of the right cylinder, bars 81 of intermediate length extending radially over a length of 2 / 3 of the radius R of the right cylinder, and bars 82 of shorter length extending radially over a length of 1 / 3 of the radius R of the right cylinder. In this illustrated example, the coplanar bars 80, 81, 82 are separated from one another by an angle of 360° / 10 = 36°. From one plane to the adjacent one, the bars 8 are out of phase by an angle of 90°, that is to say that the bar 80 of one plane forms an angle of 90° with the adjacent bar 80 in front view of the disc.

[0128] More specifically, the equation for the distribution of the bars according to figures 4 and 4A is as follows.

[0129] The length of the bars L is defined by:

[0130] L=i*R / n, that is, L takes the values ​​i*R / n

[0131] where i=(l : 1 : n-1), i taking the values ​​between 1 and n-1 with an increment of 1.

[0132] In the configuration of Figures 4 and 4A, n=3, which gives a bar of length 2*R, 4 bars of length 2R / 3 and 4 bars of length 1R / 3, i.e. 9 bars per plane.

[0133] The angle between the bars of the same plane of 360° divided by 1+9 bars is therefore equal to 36°.

[0134] The distribution of the bars in the clockwise direction in each hemisphere delimited by the bar of length 2R is: short (R / 3) - long (2R / 3) - long (2R / 3) - short (R / 3).

[0135] The pitch (distance) between the planes is equal to the height of the pellet divided by the number of planes, i.e., here 14 mm / 8 = 1.75 mm. The axial position of the lowest plane is 1.75 mm / 2 = 0.875 mm, and the axial position of the highest plane is 14 - 0.875 = 13.125 mm. Thus, when the pellets are stacked in the fuel rod ([Fig. 3]), the pitch (distance) between the planes is uniform along the entire height of the fuel column.

[0136] Figures 5 and 5A relate to a variant with eleven altitudes, or horizontal planes, of metal bars 8. In each horizontal plane, arranged at intervals from one another are a bar 80 extending over a diameter 0 of the right cylinder, bars 83 of intermediate length extending radially over a length of ¾ of the radius R of the right cylinder, bars 81 of shorter length extending radially over a length of 2 / 4 of the radius R of the right cylinder, and bars 82 of even shorter length extending radially over a length of % of the radius R of the right cylinder. In this illustrated example, the Coplanar bars 80, 81, 82, 83 are separated from each other by an angle of 360° / l+13 bars= 27.7°. From one plane to the adjacent one, the bars 8 are phase-shifted by an angle of 90°, that is to say that the bar 80 of one plane forms a 90° angle with the adjacent bar 80 in the front view of the pellet.

[0137] More specifically, the equation for the distribution of the bars according to [Fig.5] is as follows.

[0138] The length of the bars L is always defined with i=(l : 1 : n-1) (L=i*R / n), with n=4, which gives one bar of length 2*R and 4 bars of length 3R / 4 and 4 bars of length 2R / 4 and 4 bars of length R / 4, i.e. 13 bars per plane.

[0139] Angle between the bars of the same plane of 360° divided by 1+13 bars = 27.7°.

[0140] The distribution of the bars in the clockwise direction in each hemisphere delimited by the bar of length 2R is: short (R / 4) - long (3R / 4) - medium (2R / 4) - medium (2R / 4) - long (3R / 4) - short (R / 4).

[0141] The pitch (distance) between the planes is equal to the pellet height divided by the number of planes, i.e., here 14 mm / l = 1.27 mm. The axial position of the lowest plane is L = 27 mm / 2 = 0.64 mm, and the axial position of the highest plane is 14 - 0.64 = 13.36 mm. Thus, when the pellets are stacked in the rod ([Fig. 3]), the pitch (distance) between the planes is uniform along the entire height of the fuel column.

[0142] Figures 6 and 6A relate to a variant with twelve altitudes, or horizontal planes, of metal bars 8. In each horizontal plane, arranged at a distance from each other, are a bar 80 which extends over a diameter 0 of the right cylinder, bars 83 of intermediate length which extend radially over a length of 4 / 5 of the radius R of the right cylinder, bars 81 of shorter length which extend radially over a length of 3 / 5 of the radius R of the right cylinder, bars 82 of even shorter length which extend radially over a length of 2 / 5 of the radius R of the right cylinder and bars 84 of even shorter length which extend radially over a length of 1 / 5 of the radius R of the right cylinder. In this illustrated example, the coplanar bars 80, 81, 82, 83, 84 are separated from each other by an angle of 360 / (1+17)=20.0°.From one plane to the adjacent one, the bars 8 are phase-shifted by an angle of 90°, that is to say that the bar 80 of one plane forms a 90° angle with the adjacent bar 80 in the front view of the disc.

[0143] More specifically, the equation for the distribution of the bars according to [Fig.6] is as follows. The length of the bars L is always defined with i=(l : 1 : n-1) (L=i*R / n), with n=5, which gives one bar of length 2*R, 4 bars of length 4R / 5, 4 bars of length 3R / 5, 4 bars of length 2R / 5 and 4 bars of length R / 5, i.e. 17 bars per plane.

[0144] Angle between the bars of the same plane of 360° divided by 1+17 bars = 20.0°.

[0145] The distribution of the bars in the clockwise direction in each hemisphere delimited by the bar of length 2R is: short (R / 5) - long (4R / 5) - medium short (2R / 5) - medium long (3R / 5) - medium long (3R / 5) - medium short (2R / 5) - long (4R / 5) - short (R / 5).

[0146] The pitch (distance) between the planes is equal to the height of the pellet divided by the number of planes, i.e., 14 mm / 12 = 1.17 mm. The axial position of the lowest plane is 17 mm / 2 = 0.58 mm, and the axial position of the highest plane is 14 - 0.58 = 13.42 mm. Thus, when the pellets are stacked in the fuel rod ([Fig. 3]), the pitch (distance) between the planes is uniform along the entire height of the fuel column.

[0147] The [Fig.7] relates to a variant with eighteen altitudes, or planes ho radial bars, 8. In each horizontal plane, arranged at intervals from one another are a bar 80 extending over a diameter 0 of the right cylinder, bars 83 of intermediate length extending radially over a length of 4 / 5 of the radius R of the right cylinder, bars 81 of shorter length extending radially over a length of 3 / 5 of the radius R of the right cylinder, bars 82 of even shorter length extending radially over a length of 2 / 5 of the radius R of the right cylinder, and bars 84 of even shorter length extending radially over a length of 1 / 5 of the radius R of the right cylinder. In this illustrated example, the coplanar bars 80, 81, 82, 83, and 84 are spaced 20° apart. From one plane to the adjacent one, the bars 8 are out of phase by an angle of 90°, that is to say that the bar 80 of one plane forms an angle of 90° with the adjacent bar 80 in front view of the disc..

[0148] More specifically, the equation for the distribution of the bars according to [Fig.7] is as follows.

[0149] The length L of the bars is always defined by: i=(l :1 : n-1) (L=i*R / n), with n=5, which gives one bar of length 2*R, 4 bars of length 4R / 5, 4 bars of length 3R / 5, 4 bars of length 2R / 5 and 4 bars of length R / 5, i.e. 17 bars per plane.

[0150] Angle between the bars of the same plane of 360° divided by 1+17 bars = 20.0°.

[0151] The distribution of the bars in the clockwise direction in each hemisphere delimited by the bar of length 2R is: short (R / 5) - long (4R / 5) - medium short (2R / 5) - medium long (3R / 5) - medium long (3R / 5) - medium short (2R / 5) - long (4R / 5) - short (R / 5).

[0152] The pitch (distance) between the planes is equal to the height of the pellet divided by the number of planes, i.e., here 14 mm / 18 = 0.78 mm. The axial position of the lowest plane is 0.78 mm / 2 = 0.39 mm, and the axial position of the highest plane is 14 - 0.39 = 13.61 mm. Thus, when the pellets are stacked in the pencil ([Fig. 3]), the The spacing (distances) between planes is homogeneous over the entire height of the fuel column.

[0153] Fig. 8 illustrates in the form of a straight line the theoretical gain expected by the optimal equivalent thermal conductivity as a function of the percentage of molybdenum inserts 8, compared to a UO2 fuel 60, for a thermal conductivity of Mo of 100 W / m / K and UO2 of 3 W / m / K.

[0154] Thus, assuming that Mo bars 8 are perfectly distributed and radially homogeneous, and that the combustible phase is UO2, the calculations carried out of thermal resistances in parallel of the pellets according to the invention, as illustrated in figures 4 to 7, show that the equivalent thermal conductivity of the fuel in the radial direction of the pellet can be increased by a factor of ~2 on the equivalent thermal conductivity for 3% of metal bars 8, or by a factor of ~4 on the equivalent thermal conductivity for 10% of metal bars 8.

[0155] Thus, for a linear power of 373 W / cm in a PWR pellet, the maximum temperature of the fuel is approximately 1100°C for a standard cylindrical UO2 pellet.

[0156] As illustrated in [Fig.9], with 8 metal bars in Mo according to the invention inserted:

[0157] - in a proportion of 3%, the maximum temperatures in the fuel are the order of 850°C (gain of 350°C);

[0158] - in a proportion of 10%, the maximum temperatures in the fuel of the order of 550°C (gain of 550°C).

[0159] For a linear power of 600W / cm, close to the technological limit (melting limit) for pellets intended to operate in a PWR reactor, the temperature gradient in the pellet exceeds 1100°C for a UO2 pellet according to the state of the art.

[0160] With Mo bars 8 inserted according to the invention, the temperature gradient would be on the order of 600°C for a proportion of 3% Mo and 300°C for 10%, i.e. gains in maximum fuel temperature and therefore on the fuel melting margin greater than respectively 550°C and 850°C.

[0161] The gains indicated above, with reference to figures 8 and 9, are theoretical.

[0162] In practice, for pellets according to the invention as illustrated in Figures 4 to 7, the initial thermal simulations performed show lower gains, with an actual proportion of inserted bars of 3.5% corresponding to an equivalent thermal conductivity of the order of 6 to 8 W / mK

[0163] In general, to approach the theoretical limit ([Fig.8]), it is necessary to seek the smallest possible section / length ratios of inserts and a distribution as homogeneous volumetric distribution as possible.

[0164] The volumetric distribution can be adjusted according to the desired specification. For example, countering a RIA requires a higher concentration of bars 8 at the periphery of the pellet where the main energy deposition takes place.

[0165] The invention is not limited to the examples just described; in particular, features of the illustrated examples can be combined in unillustrated variants.

[0166] Other variants and improvements may be envisaged without departing from the scope of the invention. List of cited references

[0167] [1]: Kim DJ - Rhee YW & al - “Fabrication of Micro-Cell_UO2-Mo with enhanced thermal conductivity". JNM 462 (2015) 289-295.

[0168] [2]: Malone J. - Totemeier A. - Shapiro N. - Vaidyanathan S. - Lightbridge Corp - “ Advanced Metallic Fuel for LWRs”, Nuclear Technology, 181:3 437-442 Nov 2012.

[0169] [3]: Medvedev PG & Mariani RD - ''Conductive inserts to reduce nuclear fuel”, JNM 531 (2020) 151966.

[0170] [4]: ​​H. Bailly & al. - “Nuclear fuel for pressurized water reactors and "Fast neutron reactors" - Eyrolles 1996

Claims

Demands

1. Nuclear fuel pellet (6), comprising: - a straight cylinder (60) of fissile material with a central axis (X) whose length and diameter respectively define the length (H) and diameter (0) of the pellet; - a plurality of inserts (8; 80, 81, 82) of thermally conductive material, the inserts being solid rods, distributed according to different azimuthal positions and altitudes within the straight cylinder, being spaced apart from each other, the rods being distributed homogeneously in the volume of the straight cylinder and the rods being distributed equally in several planes parallel to each other and perpendicular to the central axis (X), with the coplanar rods of a plane oriented at an angle offset by 90° with those of an adjacent plane.

2.

3. A disc (6) according to claim 1, the bars being straight. A disc (6) according to claim 1 or 2, the bars having a square, rectangular, circular or elliptical cross-section.

4. A pellet (6) according to any one of the preceding claims, each bar extending in a radial direction secant with the central axis (X).

5. A pellet (6) according to any one of the preceding claims, comprising, by plane, a bar extending along the diameter (0) of the pellet.

6. Pad (6) according to any one of the preceding claims, the distribution pitch measured at the periphery of the pad between two adjacent bars within the same plane being equal to or greater than the axial distance between two adjacent planes.

7. A pellet (6) according to any one of the preceding claims, the cross-section of each bar being less than 1 mm2, preferably less than 0.1 mm2.

8. Pellet (6) according to any one of the preceding claims, the length of each bar being less than or equal to the diameter of the pellet.

9. A pellet (6) according to any one of the preceding claims, the ratio between section and length of each bar being less than 0.5, preferably less than 0.

1.

10. Pellet (6) according to any one of the preceding claims, the material of the bars being selected from a zirconium alloy, in particular Zircaloy-4 (Zr4), chromium (Cr), molybdenum (Mo).

11. Pellet (6) according to any one of the preceding claims, the fissile material of the straight cylinder being selected from uranium(IV) oxide (UO2), mixed oxide (U,Pu)O2 or a mixed mixture based on uranium oxide and reprocessed plutonium oxides (MOx).

12. Pellet (6) according to any one of the preceding claims, the volume percentage of the bars being between 1 and 10%.

13. Nuclear fuel rod (1) extending along a longitudinal direction (XX1) comprising: a plurality of nuclear pellets (6) according to any one of the preceding claims, stacked one on top of the other; a sheath (2) of neutron-transparent material surrounding the stack of pellets.

14. Pencil (1) according to claim 13, the sheath being made of zirconium alloy, in particular Zircaloy-4 (Zr4), or of M5® alloy (ZrNbO).

15. Nuclear fuel assembly comprising a plurality of fuel rods according to any one of claims 13 or 14 and arranged together in a network.

16. Use of a nuclear fuel pellet (6) according to any one of claims 1 to 12 or of a nuclear fuel rod (1) according to claim 13 or 14 in a pressurized water reactor (PWR), a boiling water reactor (BWR), a fast neutron reactor (FNR), in particular cooled by liquid metal, such as liquid sodium (Na-FNR).

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