FR3148491A1 - Nuclear fuel pellet incorporating thermally conductive inserts in the form of bars distributed at different azimuths and altitudes, associated nuclear fuel rod and assembly. - Google Patents

Abstract

Nuclear fuel pellet incorporating thermally conductive inserts in the form of bars distributed at different azimuths and altitudes, associated nuclear fuel rod and 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 the diameter (Ø) of the pellet; - a plurality of inserts (8; 80, 81, 82) made of thermally conductive material, the inserts being solid bars, distributed according to different azimuthal and altitude positions within the straight cylinder, while being distant from each other. Figure for the abstract: fig. 4

Description

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

The present invention relates to the field of fuel elements for nuclear reactors, in particular Pressurized Water Reactors (PWRs), also known by the English acronym PWR (Pressurized Water Reactor).

More specifically, it is in the field of ceramic fuels made from uranium oxide ( _UO2 ) or MOX, consisting of approximately 8.5% plutonium and 91.5% depleted uranium.

The invention essentially aims to improve the thermal properties of these fuels, both in nominal operation and in incidental or accidental conditions (for example in the event of a Primary Coolant Loss Accident (PCLA)).

By "nuclear reactors", throughout the application, is understood the usual meaning of the term to date, namely power plants for producing energy from nuclear fission reactions using fuel elements in which fissions occur which release the heat power, the latter being extracted from the elements by heat exchange with a heat transfer fluid which ensures their cooling.

By "nuclear fuel rod", throughout the application, is understood the official meaning defined for example, in the dictionary of Nuclear Sciences and Techniques, 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 pin" whose usage favors the name, is a nuclear fuel rod within the meaning of the present invention.

Although described with reference to the application to REP reactors, the invention relates to fuel elements which can be dedicated to all types of reactors for electrogenic or experimental purposes, such as Boiling Water Reactors (BWR), Fast Neutron Reactors (FNR), including those cooled by a liquid metal, in particular by liquid sodium (FNR-Na).

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

One sector is currently very largely dominant industrially in the world: Pressurized Water Reactors (PWR).

A PWR essentially consists of two main components: the primary circuit which constitutes the nuclear part of the reactor, and the secondary circuit which constitutes the non-nuclear part of it. For example, we can refer to a principle diagram under the link https://fr.wikipedia.org/wiki/R%C3%A9acteur_%C3%A0_eau_pressuris%C3%A9e.

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

The secondary circuit mainly includes non-nuclear equipment to produce electricity (piping, steam generators, turbines, etc.). 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, with the alternator, constitutes the electrical energy generation system.

To return to the primary circuit, this integrates the key part, called the reactor core. The core groups together all the assemblies, which number 157 to 241 in a PWR of the French sector. A schematic view of a PWR fuel assembly with its control cluster comprising neutron absorbing material rods is shown in under the link:

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

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).

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

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

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

This fuel is introduced into a cylinder of circular cross-section closed at each of its two ends by a plug. This cylinder is called a pencil and is described more precisely below. The pencil is sealed and constitutes the first containment barrier.

The rods are grouped in bundles and arranged in a square-pitch network. For the French industry's installation families called "bearings" of 900 MWe and 1300 MWe, the number is 264 rods to which are added 25 other tubes, which then make up an assembly that also incorporates numerous structural elements (lower end piece, retaining grids, instrumentation tube, 24 guide tubes where 24 control rods slide, upper end piece, retaining springs) as shown in the figure from the aforementioned .pdf document.

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

Pencil 1 consists of a sheath 2, conventionally made of Zircaloy-4 (Zr4), closed at each of its ends by a cap, respectively upper 3 and lower 4, which is welded onto it. This sealed pencil is filled with helium, typically at 25 bars when cold for standard fuels, to partly counterbalance the effect of the external pressure of 150 bars of the heat transfer fluid.

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

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

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

A helical compression spring 7, generally made of Inconel®, is housed in the expansion chamber 5 with its lower end resting against the upper face of the pellet 6, the highest in the stack of pellets, and its other end resting against the upper plug 3.

In addition to maintaining the stack of pellets 6 along the longitudinal axis XX' and "absorbing" the longitudinal swelling of the pellets 6 over time, the other function of this spring 7 is to prevent buckling of the section of the sheath in its ovalization mode. In other words, it must prevent extreme ovalization of the section of the sheath.

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

To date, a fuel pellet as usually used in a PWR reactor consists of uranium oxide _UO2 enriched in ^U235 to approximately 5%, the remainder being fertile ^U238 .

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

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.

• un fort gradient thermique entre le centre et la périphérie de la pastille induit par la faible conductivité thermique de l’UO₂;
• un gradient thermique radial entre pastille et gaine du crayon. En effet, le joint entre pastille et gaine, entièrement gazeux (de l’hélium) en début d’irradiation devient totalement comblé au cours du second cycle. Toutefois, la rugosité de la pastille permet la présence discontinue de gaz qui n’est plus simplement de l’hélium mais comprend également des GdF : ainsi, le contact entre pastille et gaine n’est jamais parfait et crée donc, par sa résistance thermique, un gradient thermique radial;
• la conduction thermique radiale à travers la gaine;
• la résistance due à l’échange convectif radial entre la face externe de la gaine et le fluide caloporteur.

This power is evacuated towards the cold source of the primary circuit (the primary cooling water) by encountering a certain 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 UO ₂ ;
• 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 start of irradiation, becomes completely filled during the second cycle. However, the roughness of the pellet allows the discontinuous presence of gas which is no longer simply helium but also includes GdF: thus, the contact between the pellet and the cladding is never perfect and therefore creates, by its thermal resistance, a radial thermal gradient;
• radial heat conduction through the sheath;
• the resistance due to radial convective exchange between the external face of the sheath and the heat transfer fluid.

Heat evacuation in nominal operating mode and in incident 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 convective exchanges by a heat exchange coefficient between the sheath and the heat transfer fluid.

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.

Numerical simulations of a Loss of Primary Coolant (APRP) and Reactivity Insertion Accident (RIA) type accident have demonstrated the benefit, 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.

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

Generally speaking, 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 situations (IPG acronym for Pellet-Clad Interactions) and accident situations (APRP, RIA, etc.), improve the flexibility of controlling a nuclear reactor, and increase the maximum combustion rate (also called burn-up or irradiation rate) of the latter.

There , from publication [1] gives the order of magnitude of the temperatures in a fuel pellet in nominal operating mode.

There are various ways to improve the power evacuation function of a UO ₂ fuel pellet or pellet stack. Since some characteristics can hardly be modified, such as the primary fluid, the cladding material, the most open innovation path is that of improving the thermal conductivity of UO ₂ .

This path can be divided into two categories which have been explored.

The first category concerns the improvement of thermal conductivity without adding another phase within the _UO2 fuel.

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

- a modification of the shape of the pencil, in order to increase the exchange surface between the hot source and the cold source. This modification is located by definition at the level of each pencil of a fuel assembly. This solution has the major drawbacks of requiring a new design of the assembly and of having a possible impact on the manufacturability of the pencil and the performance of the assembly;

- a change in the nature of the fuel by switching from a combustible oxide to a better heat-conducting fuel, for example a metallic fuel. This modification is localized by definition at the level of each fuel pellet. This solution has the major drawbacks of inducing different behavior in the reactor, requiring research and development to develop/qualify this new fuel, developing new manufacturing processes and having to redefine the downstream fuel cycle;

- a combination of the two previous subcategories which is a concept designated under the name of LIGHTBRIDGE: see in particular [2]. This concept consists of a generally helical shaped rod whose sheath contains a metallic fuel, UZr. In addition to the disadvantages explained for the two previous subcategories, this solution has the major disadvantages of significantly reducing the mass of fissile material, typically by around 25% and of requiring over-enrichment of the pellet with U5 fuel.

The second category concerns the improvement of thermal conductivity with the addition of another phase within the UO ₂ fuel. The addition is localized by definition at the level of each fuel pellet.

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

- a homogeneous dispersion of a material, such as diamond, graphene, etc., with a thermal conductivity higher than UO ₂ ceramic at the nanometer scale to the micron scale. This solution does not seem relevant to date from the point of view of its thermal behavior. The inventors believe that it is however difficult to decide on this solution due to a lack of information and knowledge. This solution has the major drawbacks of being only at the laboratory scale and therefore requiring complete research and development to be done for the fuel, an analysis of manufacturability, and finally of not having any modeling tools;

- a homogeneous dispersion of a second metallic phase on a microscopic scale with the aim of reducing the thermal conductivity of the UO ₂ pellet: see in particular [1]. This solution has the major drawbacks of inducing a manufacturing difficulty and requiring the finalization and then validation of existing modeling tools;

- a heterogeneous dispersion of a second metallic phase on a macroscopic scale, i.e. on a scale of a tenth of a millimeter or less, with the aim of promoting the heat flow from the UO ₂ pellet to the cold source. This second metallic phase is characterized by a metallic insert within the fissile material of the pellet.

Such a metal insert is described in two distinct designs in the publication [3].

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

However, the analysis carried out by the applicant's experts on this design shows that if, indeed, the choice of the structural characteristics of the fins makes it possible to achieve a significant reduction in the core temperature of the pellet, on the other hand this choice potentially generates several drawbacks which could have very inconvenient consequences on the behavior of the pellet, or even completely call into question the relevance of the solution.

• une garantie hypothétique de la fonction thermique sous irradiation et de la tenue mécanique sous irradiation des ailettes compte tenu de leur faible épaisseur;
• une tenue mécanique de la gaine non garantie, avec des gradients de température élevés et locaux sur celle-ci, typiquement de 100 à 300 °C/mm, qui sont dus aux contacts de chaque ailette avec la gaine ;
• l’absence de certitude de ne pas aggraver le phénomène d’Interaction Pastille Gaine avec une Corrosion Sous Contrainte (IPG-CSC) ;
• un niveau de corrosion susceptible d’être présent sur la gaine en cours d’irradiation ;
• une fabricabilité des ailettes métalliques dans un combustible céramique non démontrée avec notamment la possibilité du montage des ailettes.

The main potentially negative consequences are:

• a hypothetical guarantee of the thermal function under irradiation and of the mechanical resistance under irradiation of the fins given their low thickness;
• mechanical resistance 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 Interaction Pellet Sheath with Stress Corrosion (IPG-CSC);
• a level of corrosion likely to be present on the sheath during irradiation;
• a manufacturability of metal fins in a ceramic fuel not demonstrated, in particular with the possibility of mounting the fins.

To overcome the aforementioned drawbacks of the design shown in Figure 1a of publication [3], the applicant proposed in patent applications WO2022/223387, WO2022/223504, WO2022/223510, different forms of metal insert in substance respectively, with a four-arm cross-section, and with solid disc(s) and solid rod.

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.

In other words, there is a need to improve the thermal design of nuclear fuel pellets, particularly those based on UO ₂ oxides, in order to reduce their core temperature as much as possible, in nominal operating mode for a Pressurized Water Reactor (PWR) core while respecting the following constraints:

- minimization of thermal gradients within a pellet;

- minimization of the quantity of second phase provided in a pellet;

- absence of degradation or at least limited degradation of neutron behavior;

- easiest possible manufacturability;

- assembly of a fuel column as simple as possible;

- absence of the aforementioned drawbacks of the design shown in Figure 1a of this publication [3].

The aim of the invention is to meet at least part of this need.

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

• une pluralité d’inserts en matériau conducteur thermique, les inserts étant des barreaux pleins, répartis selon différentes positions azimutales et d’altitude au sein du cylindre droit, en étant distants les uns des autres.

- a straight cylinder of fissile material with a central axis (X) whose length and diameter respectively define 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.

The bars can be straight. The inserts can advantageously be metallic or made of metal alloy or ceramic.

They can be square, rectangular, circular or elliptical in cross section.

The bars are preferably distributed evenly throughout the volume of the straight cylinder.

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 a plane oriented at an angle offset by 90° relative to those of an adjacent plane.

According to this mode, each bar advantageously extends in a radial direction intersecting with the central axis (X).

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

According to another advantageous variant embodiment, the pitch measured on 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.

Preferably, the section of each bar is less than 1 mm², preferably less than 0.1 mm².

More preferably, the length of each bar is less than or equal to the diameter of the pellet, i.e. a few mm.

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

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

With n=3, each plane has 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 has 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).

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

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

More preferably, the ratio between section and length of each bar is less than 0.5, preferably less than 0.1.

Advantageously, the material of the bars is chosen from a zirconium alloy, in particular Zircaloy-4 (Zr ₄ ), chromium (Cr), molybdenum (Mo).

Advantageously, the fissile material of the right 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.

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

The invention also relates to a nuclear fuel rod extending in a longitudinal direction (XX') comprising:

- a plurality of nuclear pellets as described above, stacked on top of each other;

- a sheath made of neutron-transparent material surrounding the stack of pellets.

Advantageously, the sheath is made of zirconium alloy, in particular Zircaloy-4 (Zr ₄ ), or M5 ^® alloy (ZrNbO).

The invention also relates to a nuclear fuel assembly comprising a plurality of fuel rods as above and arranged between them in a network.

The invention also relates to the use of a nuclear fuel pellet as described above or of 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 (FNR-Na).

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.

The microstructure-scale bars have a geometry and distribution that minimize the volume of the added metallic phase required relative to the volume of the pellet, for the given objective of reducing and homogenizing the combustible temperature.

The bars thus inserted into the fuel cylinder create thermal bridges which reduce the thermal gradients in the pellet, preferably by being oriented in the direction of the main heat flow, which for a pellet is very largely 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.

The thermal conductive bars are thus preferentially oriented in the radial direction of the right fuel cylinder.

Thus, the directions of the inserts in the form of bars to be favored for standard cylindrical pellets are overwhelmingly radial, then azimuthal and, to a lesser extent, axial, that is to say according to the height of the right cylinder of the pellet.

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 in the right fuel cylinder, while minimizing their percentage in added volume and therefore maximizing the fissile volume of the pellet. This makes it possible to minimize the over-enrichment induced to compensate for the loss of fissile mass.

Thermally conductive bars can be made from molybdenum, chromium or SiC (silicon carbide) ceramic, 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).

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

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

Due to the radial then lateral direction which is mainly preferred for a standard cylindrical format pellet, a greater bar width is preferable to a greater height, depending on the height of the fuel cylinder.

Depending on the desired temperature reduction and distribution, the number and positioning of the conductive bars, i.e. their volume density in the pellet, will need to be adjusted.

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.

Advantageously, the characteristic axial pitch, that is to say according to the height of the straight cylinder, between the bars is of the same order as the characteristic horizontal pitch between the bars on the periphery of the pellet, so as to homogenize the temperatures within the pellet as much as possible. Thus, the bars will be preferentially oriented with a phase shift angle, from one plane to the other axially adjacent for a more homogeneous mesh of the pellet so as to homogenize the temperatures within the pellet as much as possible.

Therefore, thanks to the plurality of inserts in the form of bars, the operating temperature of the fuel pellet is significantly reduced.

• une diminution du relâchement des gaz de fission et de la fragmentation du combustible, ce qui est bénéfique pour la pression interne du crayon,
• une diminution du gonflement des bulles de gaz et donc du combustible,
• une diminution de l’interaction mécanique entre la pastille et la gaine, ce qui est bénéfique pour la déformation diamétrale de la gaine et la densité d’énergie de déformation de la gaine,
• un gain de marges sur les différents critères de sûreté précités, en particulier sur la limite à fusion du combustible, ce qui augmente ainsi les marges de fonctionnement d’un réacteur nucléaire notamment afin d’être plus flexible en pilotage, et d’obtenir un gain en puissance maximale voire en burn-up maximal, en particulier afin d’améliorer la capacité de pilotage à haut burn-up.

A ceramic fuel pellet operating at a lower temperature has many advantages, including:

• a reduction in the release of fission gases and fuel fragmentation, which is beneficial for the internal pressure of the rod,
• a reduction in the swelling of gas bubbles and therefore of the fuel,
• a decrease in the mechanical interaction between the pellet and the cladding, which is beneficial for the cladding diametrical deformation and the cladding strain energy density,
• a gain in margins on the various safety criteria mentioned above, in particular on the fuel fusion limit, which thus 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 burn-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 (REP, BWR, RNR), as well as certain experimental reactors, with the manufacture and stacking of fuels with improved safety and performance.

For the safety aspect, a pellet according to the invention can be classified in the so-called E-ATF fuel category (acronym for “ Enhanced Accident Tolerant Fuel ”).

• une température de fonctionnement plus basse du combustible et une pression interne plus basse dans l’élément combustible ;
• une introduction d’inserts de conductivité thermique supérieure à l’UO₂dans la direction du flux thermique principal, pour minimiser la proportion volumique de barreaux insérés ;
• une minimisation du volume de barreaux insérés introduite vis-à-vis du volume de la phase céramique de la pastille combustible par optimisation de la géométrie et de la répartition des inserts pour minimiser le sur-enrichissement induit pour compenser la perte de masse fissile ;
• des inserts sous la forme de barreaux à l’échelle de 1 mm ou moins (quelques microns, dépend de la résolution du procédé de fabrication) pour une efficacité maximisée, une température plus homogène dans la pastille et moins de contraintes mécaniques internes ;
• une recherche des ratios section/longueur d’inserts les plus petits possibles et une répartition volumique la plus homogène possible dans les zones de la pastille où l’on souhaite augmenter la conductivité thermique.

The major developments of this E-ATF category brought by the invention are as follows:

• lower operating fuel temperature and lower internal pressure in the fuel element;
• an introduction of inserts with thermal conductivity higher than UO ₂ in the direction of the main heat flow, to minimize the volume proportion of inserted bars;
• a minimization of the volume of inserted bars 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 constraints;
• a search for the smallest possible insert section/length ratios and the most homogeneous possible volume distribution in the areas of the pellet where we want to increase thermal conductivity.

Other advantages and characteristics of the invention will become more apparent upon reading the detailed description of examples of implementation of the invention given for illustrative and non-limiting purposes with reference to the following figures.

there is a schematic view in longitudinal section of a state-of-the-art nuclear fuel rod, as implemented in a PWR nuclear reactor.

there 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.

there is a schematic perspective view of a basic M pattern of a nuclear fuel rod whose cladding houses a fuel pellet with metal inserts not shown according to the invention.

, Figures 4 and 4A are respectively perspective and cross-sectional views, resulting from a digital simulation of a fuel pellet according to the invention, with metal inserts in the form of bars of rectangular section distributed according to a number of eight altitudes, that is to say horizontal planes along the height of the straight fuel cylinder. C

, Figures 5 and 5A are respectively perspective and cross-sectional views, resulting from a digital simulation of a fuel pellet according to the invention, with metal inserts in the form of bars of rectangular section distributed according to a number of eleven altitudes, that is to say horizontal planes along the height of the straight fuel cylinder.

, Figures 6 and 6A are respectively perspective and cross-sectional views, resulting from a digital simulation of a fuel pellet according to the invention, with metal inserts in the form of bars of circular section distributed according to a number of twelve altitudes, that is to say horizontal planes along the height of the straight fuel cylinder.

there is a view resulting from a digital simulation of a fuel pellet according to the invention, with metal inserts in the form of bars of elliptical section distributed according to a number of eighteen altitudes, that is to say horizontal planes along the height of the straight fuel cylinder.

there illustrates in the form of a straight line, the optimal equivalent thermal conductivity as a function of the percentage of Mo insert phase compared to a UO ₂ fuel, for a Mo thermal conductivity of 100 W/m/K and UO ₂ of 3 W/m/K.

there 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 UO ₂ fuel matrix for a pellet of the type intended to operate in a REP reactor, respectively at 373W/cm and 600W/cm.

Detailed description

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.

Figures 1 and 2 have already been commented on in the preamble. They will therefore not be detailed below.

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.

It was represented in , a basic pattern M of a nuclear fuel rod with a cladding 2 housing a fuel pellet 6 according to the invention.

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 metal inserts in the form of bars distributed homogeneously according to different azimuthal and altitude positions within the straight cylinder 60 while being distant from each other.

More precisely, 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 a plane oriented at an angle offset by 90° relative to those of an adjacent plane.

Figures 4 to 7 illustrate four variants of embodiment of a pellet 6 according to the invention for a volume 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) relative to the total volume of the pellet 6.

The cross sections of these bars 8 are respectively rectangular equal to 0.6x0.3mm ( ), rectangular equal to 0.5x0.2mm ( ), circular equal to 0.2x0.2mm ( ), and elliptical 0.5x0.05mm ( ).

Additive manufacturing processes can be implemented to achieve this type of insert geometry.

There relates to a variant with a number of eight altitudes, or horizontal planes, of metal bars 8. In each horizontal plane, there are arranged at a distance from each other, a bar 80 which extends over an entire diameter Ø of the right cylinder, bars 81 of intermediate length which extend radially over a length of 2/3 of the radius R of the right cylinder, and bars 82 of shorter length which extend 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 distant from each other 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 pellet.

More precisely, the equation for the distribution of the bars according to figures 4 and 4A is as follows.

Length of bars L is defined by:

L=i*R/n, that is, L takes the values i*R/n

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

In the configuration of figures 4 and 4A, n=3, which gives one bar of length 2*R, 4 bars of length 2R/3 and 4 bars of length 1R/3, i.e. 9 bars per plane.

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

The distribution of the bars in a 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).

The pitch (distance) between the planes is equal to the height of the pellet divided by the number of planes, i.e. here 14mm/8=1.75mm. The axial position of the lowest plane is 1.75mm/2=0.875mm, the axial position of the highest plane is 14-0.875=13.125mm. So when the pellets are stacked in the pencil ( ), the pitch (distances) between planes is uniform over the entire height of the fuel column.

Figures 5 and 5A relate to a variant with a number of eleven altitudes, or horizontal planes, of metal bars 8. In each horizontal plane, there are arranged at a distance from each other, a bar 80 which extends over an entire diameter Ø of the right cylinder, bars 83 of intermediate length which extend radially over a length of ¾ of the radius R of the right cylinder, bars 81 of smaller length which extend radially over a length of 2/4 of the radius R of the right cylinder, and bars 82 of even smaller length which extend radially over a length of ¼ of the radius R of the right cylinder. In this illustrated example, the coplanar bars 80, 81, 82, 83 are spaced from each other by an angle of 360°/1+13 bars= 27.7°. 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 pellet.

More precisely, the equation for the distribution of bars according to the is as follows.

The length of the bars L is always defined with i=(1 : 1 : n-1) (L=i*R/n), with n=4, which gives a 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.

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

The distribution of the bars in a 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).

The pitch (distance) between the planes is equal to the pad height divided by the number of planes, i.e. here 14mm/11=1.27mm. The axial position of the lowest plane is 1.27mm/2=0.64mm, the axial position of the highest plane is 14-0.64=13.36mm. So when the pads are stacked in the pencil ( ), the pitch (distances) between planes is uniform over the entire height of the fuel column.

Figures 6 and 6A relate to a variant with a number of twelve altitudes, or horizontal planes, of metal bars 8. In each horizontal plane, there are arranged at a distance from each other, a bar 80 which extends over an entire diameter Ø 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 smaller length which extend radially over a length of 3/5 of the radius R of the right cylinder, bars 82 of even smaller length which extend radially over a length of 2/5 of the radius R of the right cylinder and bars 84 of even smaller 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 spaced apart 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 a plane forms an angle of 90° with the adjacent bar 80 in front view of the pellet.

More precisely, the equation for the distribution of bars according to the is as follows. The length of the bars L is always defined with i=(1 : 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.

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

The distribution of the bars in a 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).

The pitch (distance) between the planes is equal to the height of the pellet divided by the number of planes, i.e. here 14mm/12=1.17mm. The axial position of the lowest plane is 1.17mm/2=0.58mm, the axial position of the highest plane is 14-0.58=13.42mm. Thus when the pellets are stacked in the pencil ( ), the pitch (distances) between planes is uniform over the entire height of the fuel column.

There relates to a variant with a number of eighteen altitudes, or horizontal planes, of metal bars 8. In each horizontal plane, there are arranged at a distance from each other, a bar 80 which extends over an entire diameter Ø 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 smaller length which extend radially over a length of 3/5 of the radius R of the right cylinder, bars 82 of even smaller length which extend radially over a length of 2/5 of the radius R of the right cylinder and bars 84 of even smaller 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 spaced apart from each other by an angle of 20°. 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 a plane forms an angle of 90° with the adjacent bar 80 in front view of the pellet.

More precisely, the equation for the distribution of bars according to the is as follows.

The length L of the bars is always defined by: i=(1:1: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.

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

The distribution of the bars in a 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).

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

There 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 8 molybdenum inserts, compared to a 60 UO ₂ fuel, for a thermal conductivity of Mo of 100 W/m/K and UO ₂ of 3 W/m/K.

Thus, assuming that the Mo bars 8 are perfectly distributed and homogeneous radially, and that the fuel phase is UO ₂ , the calculations carried out of thermal resistances in parallel with the pellets in accordance with the invention, as illustrated in FIGS. 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.

Thus, for a linear power of 373 W/cm in a REP pellet, the maximum fuel temperature is approximately 1100°C for a standard cylindrical format UO ₂ pellet.

As illustrated in the , with 8 Mo metal bars according to the invention inserted:

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

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

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.

With 8 Mo bars inserted according to the invention, the temperature gradient would be of 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 in the fuel melting margin of greater than 550°C and 850°C respectively.

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

In practice, for pellets according to the invention as illustrated in Figures 4 to 7, the first thermal simulations carried out show lower gains, a real proportion of inserted bars of 3.5% corresponding to an equivalent thermal conductivity of the order of 6 to 8 W/m.K.

Generally speaking, to tend towards the theoretical limit ( ), it is necessary to seek the smallest possible section/length ratios of inserts and the most homogeneous volume distribution possible.

The volume distribution can be adjusted according to the desired specification. For example, countering an RIA requires a higher concentration of 8 bars on the periphery of the pellet where the main energy deposition takes place.

The invention is not limited to the examples which have just been described; it is possible in particular to combine characteristics of the examples illustrated within non-illustrated variants.

Other variations and improvements may be envisaged without departing from the scope of the invention.

List of cited references

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

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

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

[4]: H. Bailly & al. – “ Nuclear fuel for pressurized water reactors and fast neutron reactors ” – Eyrolles 1996

Claims (18)

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 the diameter (Ø) of the pellet;

• a plurality of inserts (8; 80, 81, 82) made of thermally conductive material, the inserts being solid bars, distributed according to different azimuthal and altitude positions within the straight cylinder, being distant from each other.

Pastille (6) according to claim 1, the bars being rectilinear. Pellet (6) according to one of claims 1 or 2, the bars being of square, rectangular, circular or elliptical cross section. Pastille (6) according to one of the preceding claims, the bars being distributed homogeneously in the volume of the straight cylinder. Pastille (6) according to claim 4, the bars being distributed equally in several planes parallel to each other and perpendicular to the central axis (X), with the coplanar bars of a plane oriented at an angle offset by 90° relative to those of an adjacent plane. Pellet (6) according to claim 5, each bar extending in a radial direction intersecting with the central axis (X). Pellet (6) according to claim 5 or 6, comprising, per plane, a bar which extends along the diameter (Ø) of the pellet. Pellet (6) according to one of claims 5 to 7, the distribution pitch measured on the periphery of the pellet between two adjacent bars within the same plane being equal to or greater than the axial distance between two adjacent planes. Pellet (6) according to one of the preceding claims, the section of each bar being less than 1 mm², preferably less than 0.1 mm² . Pellet (6) according to one of the preceding claims, the length of each bar being less than or equal to the diameter of the pellet. Pellet (6) according to one of the preceding claims, the ratio between section and length of each bar being less than 0.5, preferably less than 0.1. Pellet (6) according to one of the preceding claims, the material of the bars being chosen from a zirconium alloy, in particular Zircaloy-4 (Zr ₄ ), chromium (Cr), molybdenum (Mo). Pellet (6) according to one of the preceding claims, the fissile material of the right cylinder being chosen from uranium(IV) oxide (UO ₂ ), mixed oxide (U, Pu)O ₂ or a mixed mixture based on uranium oxide and reprocessed plutonium oxides (MOx). Pastille (6) according to one of the preceding claims, the volume percentage of the bars being between 1 and 10%. Nuclear fuel rod (1) extending in a longitudinal direction (XX') comprising:
a plurality of nuclear pellets (6) according to one of the preceding claims, stacked on top of each other;
a sheath (2) made of neutron-transparent material surrounding the stack of pellets. Pencil (1) according to claim 15, the sheath being made of zirconium alloy, in particular Zircaloy-4 (Zr ₄ ), or M5® alloy (ZrNbO). Nuclear fuel assembly comprising a plurality of fuel rods according to any one of claims 15 or 16 and arranged between them in an array. Use of a nuclear fuel pellet (6) according to one of claims 1 to 14 or of a nuclear fuel rod (1) according to claim 15 or 16 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 (FNR-Na).