Porous mox fuel for fast-neutron reactors

Abstract

The invention relates to a fuel intended to be irradiated in a fast-neutron nuclear reactor, comprising grains of at least one metal selected from U, Pu or Th, the grains being compressed in a volume, the fuel being porous, the fuel being characterised in that it comprises open intergranular pores forming an interconnected porosity, the porosity volume fraction preferably being between 3% and 30%, and more than 50% or 80% of the porosity being open intergranular porosity.

Description

[0001] Description

[0002] Title: Porous MOX-type fuel for fast neutron reactors TECHNICAL FIELD

[0003] The technical field of the invention relates to nuclear fuel for fast neutron reactors. The invention is a fuel compatible with use at varying thermal power levels, ranging from 50 W / cm² to 500 W / cm².

[0004] EARLIER ART

[0005] A reactor's ability to operate at varying power levels is a key factor in the development of the nuclear power industry. The goal is to allow power output to be adjusted according to the demand on the electricity grid. When there is high electricity demand on the grid, the reactor must operate at full power to meet energy needs. Outside of peak periods, the reactor can operate at reduced power, remaining ready to rapidly increase its output if necessary. These power variations are necessary due to the growing share of renewable energy sources, such as solar and wind power, in the energy mix. However, these are intermittent energy sources. A nuclear reactor capable of modulating its power output can compensate for fluctuations in renewable energy production.Indeed, the electrical grid needs a constant balance between electricity production and consumption to maintain stability. A flexible nuclear reactor, which can adjust its power output quickly, helps maintain this balance when electricity production from renewable energy sources varies significantly.

[0006] Currently, MOX (U,Pu)O2 fuels for fast neutron reactors are suitable for high-power operation, with thermal power levels typically between 300 and 500 W / cm². The unit W / cm² designates power per unit length, or linear power, along a fuel rod. A fuel rod is defined as a fuel element, namely a sealed steel cladding (l ère safety barrier) mainly containing a stack of combustible pellets. The acronym MOX stands for Mixed Oxide, which refers to a fuel containing both UO2 and PuO2.

[0007] Under irradiation, the fuel undergoes fission reactions, leading to the production of fission products, some of which are gaseous, primarily Xe and Kr. The amount of gas produced increases during irradiation, depending on the burnup rate, which is the integral of the initial fissile atoms present in the fuel that have fissioned (at%), or the energy released by a nuclear fuel relative to its initial mass of fissile heavy metal (GW day / tonne). In a high-power fast neutron reactor, the fuel is subjected to high temperatures, typically between 1000 and 2400 K, as well as a steep temperature gradient, for example, on the order of 4000 K / cm, radially within the fuel pellet. The gases produced escape by diffusion.They can be collected in a plenum (free volume), located at the ends of the fuel needles, and intended to collect fission gases and helium.

[0008] At low power (< 300 W / cm²), a significant proportion of gas is retained within the fuel, as the temperature and thermal gradient are insufficient to allow release by thermal diffusion. This gas retention leads to gas swelling, which can cause interaction between the fuel pellet and the cladding (the first safety barrier) during nominal reactor operation, but especially during power transients. Since the fuel element for fast neutron reactors is designed to prevent pellet-cladding interaction (a safety criterion for the cladding), the risk of cladding damage necessitates either a reduction in the burnup rate or operation at a power level above 300 W / cm², respectively reducing the amount of gas or promoting its thermal release.

[0009] Patent EP2474000 describes a process for manufacturing, without a pore-forming agent, fuel intended for irradiation in a fast neutron reactor. This fuel is heavily loaded with minor actinides, the objective being to enable their transmutation. The fuel has partially open porosity, with a total porosity between 14% and 16%. This type of fuel is intended for irradiation at low power, typically 30 to 150 W / cm². This patent gives an example of the manufacture of a Uo·9Amo·1O₂ pellet with an open porosity of 10%. This type of fuel is intended to be placed in the peripheral parts of the core of a fast neutron reactor, so as to allow for the transmutation of 241 Am by neutron capture.

[0010] The Asakura publication "Developments in the fabrication technology of low-density MOX pellets for fast breeder reactor fuel" describes the fabrication of non-annular cylindrical fuel pellets for fast neutron reactors. This publication describes the use of various pore-forming agents incorporated into a powder with particle sizes ranging from 125 µm to 850 µm. The fuel is intended for use at high burnup rates, exceeding 100 GW day / tonne. The objective of this article is to obtain a MOX fuel with fully closed, unconnected intragranular porosity to retain gases (the fuel for the MONJU reactor).

[0011] The publication Morimoto K. “Thermal conductivities of (U, Pu, Am)O2 solid solutions” describes the impact of the manufacturing process of the previous publication (Asakura) on the thermal properties of MOX fuel with an Am content between 0.7% and 3%, with a closed porosity between 10% and 15%.

[0012] US3320179 describes a process for the formation of porous microspheres by sol-gel method.

[0013] FR3072822 describes the formation of a fuel rich in at least one minor actinide. The publication Manaud J. et al., "Direct sintering of UO2," is also relevant. 2+X oxides prepared under hydrothermal conditions", Journal of the European ceramic society; vol. 41, no. 13, June 6, 2021, describes a sintering process for powders prepared by hydrothermal conversion of oxalates, at temperatures of 1500°C or 1600°C.

[0014] US3755513 describes a process for obtaining a closed-porosity fuel.

[0015] The inventors propose a fuel whose structure is compatible with both low and high power levels, while limiting the risk of swelling at low power, and exhibiting thermal conductivity compatible with high-power operation, typically exceeding 300 W / cm² or 350 W / cm². Such a fuel is compatible with a fast neutron reactor operating in a hybrid mode, combining high power (> 300 W / cm²) and low power (between 50 W / cm² and 300 W / cm²), without impacting safety.

[0016] DESCRIPTION OF THE INVENTION

[0017] A first object of the invention is a fuel, intended to be irradiated in a fast neutron nuclear reactor, comprising grains of at least one metal chosen from U, Pu or Th, the grains being compressed in a volume, preferably cylindrical, the fuel being porous, the fuel being characterized in that it comprises open intergranular pores, forming an interconnected porosity, the volume fraction of porosity being between 3% and 30%, of which more than 50% or 80% of the porosity is an open intergranular porosity.

[0018] The fuel may contain a mass fraction of at least 10% of at least one fissile isotope under the effect of fast neutron irradiation. Preferably, more than 10%, or even more than 50%, of the open porosity volume fraction is formed of pores having a diameter or largest diagonal greater than 10 pm or 20 pm, and preferably 10 pm and 30 pm.

[0019] The volume fraction of porosity can be between 5% and 13%.

[0020] Preferably, the mass fraction of the isotope, fissile under the effect of irradiation by fast neutrons, is less than 95% or 80%.

[0021] The average grain diameter is preferably less than 20 pm, 15 pm, or 10 pm. The cumulative mass fraction of metallic elements, including U and / or Pu and / or Th, is preferably greater than 80%, greater than 85%, or greater than 87%.

[0022] Each metal, chosen from U, Pu or Th, can be in the form of a metal oxide, or nitride, or pure metal or metal alloy.

[0023] The fuel may contain U 1-y Could y O x , with y between 0.1 and 0.5 and x between 1.5 and 2.0.

[0024] Preferably, the mass fraction of minor actinide is less than 10% or less than 8% or less than 5%, the term minor actinide denoting an isotope or mixture of isotopes of Am or Np or Cm.

[0025] A second object of the invention is a method for manufacturing a fuel according to the first object of the invention, comprising:

[0026] - a) preparation of a powder containing grains of U and / or Pu and / or Th, with a possible presence of other minor actinides;

[0027] - b) mixing of the powder with an organic porogenous agent, with a mass fraction for example between 3% and 7%;

[0028] - c) compression;

[0029] - d) sintering, in particular at a temperature above 1500 °C.

[0030] The process may include, prior to step c), for example between steps b) and c), sieving the powder to remove grains or agglomerates of grains whose diameter is greater than a threshold, between 100 pm and 500 pm.

[0031] The average grain diameter may be less than 20 µm, 15 µm, or 10 µm. The U and / or Pu and / or Th content may be as described in relation to the first object of the invention. The same applies to the minor actinide content.

[0032] Advantageously, the open porosity forms a porosity network. The fuel comprises matrix zones, formed by grains or agglomerates of grains during sintering, each matrix zone being delimited by the open porosity network, each matrix zone having an average diameter of less than 250 µm, and preferably less than 80 µm, 50 µm, or 30 µm. Thus, the porosity network delimits said matrix zones.

[0033] The invention will be better understood by reading the explanation of the examples of embodiment presented, in the continuation of the description, in connection with the figures listed below.

[0034] FIGURES

[0035] Figure IA schematically represents a 3D view of a representative volume of a fuel according to the invention.

[0036] Figure IB is a 2D view extracted from figure IA.

[0037] Figure 2 schematically represents a 3D view of a representative volume of MOX fuel, intended for fast neutron reactors, according to the prior art.

[0038] Figure 3 illustrates the main steps of a process for optimizing a fuel microstructure according to the invention.

[0039] Figure 4 illustrates a parameter of the microstructure of a fuel according to the invention.

[0040] Figures 5A and 5B represent microstructures before and after optimization respectively.

[0041] Figure 6 shows a model of a spatial temperature distribution of a reference fuel and two fuels according to the invention, exhibiting respectively different levels of porosity.

[0042] Figure 7 represents the relative density of porous fuels (ordinate axis), according to the invention, as a function of a mass fraction of organic porogenous agent used during manufacturing.

[0043] Figure 8 shows micrographs of a fuel according to the invention with two different scales.

[0044] Figure 9 is a scanning electron microscope image of a fuel according to the invention.

[0045] PRESENTATION OF SPECIFIC IMPLEMENTATION METHODS

[0046] Figure IA schematically illustrates an example of the microstructure of a fuel according to the invention. The fuel is said to have open and interconnected intergranular porosity.

[0047] The fuel is intended to power a fast neutron reactor. It comprises a fissile or fertile isotope matrix of uranium and / or plutonium and / or thorium. During manufacturing, it consists of at least 80%, or even at least 85% or 87%, by mass of heavy metallic isotopes, selected from isotopes of uranium and / or plutonium and / or thorium. Preferably, it contains a mass fraction of at least 10% of isotopes that become fissile under the effect of fast neutrons, for example 239 Pu or 241 Pu or 233 U or 235 U, so that the energy density of the fuel is high.

[0048] The fuel may, during its manufacture, contain one or more minor actinides, for example, isotopes of americium, neptunium, or curium. The mass fraction of minor actinide(s) is preferably less than 10%, and preferably less than 8%, 5%, or 3%. Preferably, the mass fraction of fissile isotope under irradiation by fast neutrons (such as 239 Pu and / or 241 Pu and / or 233 U and / or 235 U) is greater than 10%. This allows the fuel to be used as a feed fuel (heat source), potentially exposed to intense irradiation, in a central part of the core of a fast neutron reactor. The mass fraction of the fissile isotope under fast neutron irradiation is, for example, between 10% and 80% or between 10% and 95%.

[0049] The fuel material is porous. The pores allow the gases formed during fission or alpha reactions (Kr, Xe, He) to escape, even at low temperatures. This porosity is open and interconnected, with at least 50%, and possibly 60%, 70%, or even 80% of the pores opening into another pore, which in turn opens outside the pellet (percentages of porosity given by volume). This interconnected porosity allows the gases to escape through the interconnected pores, which form a network of microchannels within the fuel matrix.

[0050] Figure IB is a two-dimensional detail of Figure IA: it represents an interconnected porosity. Figure 2 illustrates a microstructure of a standard MOX fuel, in which the porosity is low (typically less than 5% by volume), being essentially closed.

[0051] Operation under high thermal power requires good thermal conductivity, in order to avoid the formation of excessively high temperature gradients and overheating of the fuel.

[0052] Thus, the fuel must satisfy two opposing conditions:

[0053] the highest possible porosity, so as to promote the diffusion of fission gases at low temperature;

[0054] the highest possible thermal conductivity, bearing in mind that high porosity tends to decrease thermal conductivity. Indeed, the thermal conductivity in each pore is lower than the thermal conductivity of the fuel matrix, and the pore network constitutes thermal barriers.

[0055] The inventors believe that a predominantly open porosity of between 3% and 30% (by volume) is achievable. A porosity range of between 5% and 12% or 13% is considered optimal, as it ensures a good balance between the ability to release gases at low temperatures and the ability to conduct heat.

[0056] Beyond 28% or 30%, the porosity is too high and this also results in a degradation of the mechanical strength of the fuel.

[0057] This refers to the porosity of the fresh fuel, that is, before irradiation in the reactor. However, this pore network must remain stable under irradiation. The manufacturing process presented ensures the stability of the microstructure (open porosity network) during the temperature increase above the sintering temperature (which is 1700°C).

[0058] The fuel is composed of combustible matrix zones delimited by pores forming an open porosity network. These matrix zones are formed from grains or grain agglomerates during sintering. The average size of the matrix zones is preferably less than 250 µm, and more preferably less than 80 µm, 50 µm, or 30 µm. Keeping the matrix zone size as small as possible promotes gas diffusion within the matrix. Size refers to the average or largest diameter of the matrix zones. The matrix zones are surrounded by interconnected open pores, forming the open porosity network. This contrasts with the microstructure of a standard MOX fuel, in which the porosity is predominantly closed. In MOX fuel, the volume fractions of open and closed porosity are considered to be less than 1% and 4%, respectively.

[0059] The grains can be formed from oxides of fissile or fertile material, for example UO2 and / or PuÛ2 and / or (U, Pu)Û2, also noted Ui. y PuyO x , where x and y are positive real numbers. It could also be:

[0060] of nitride: Uranium nitride UN, Plutonium and Uranium nitride (U,Pu)N, Thorium nitride ThN);

[0061] or metallic: Metallic uranium, uranium-plutonium alloys, uranium-molybdenum alloys. Nitride-based fuels or metallic fuels exhibit high thermal conductivity and high power density.

[0062] The inventors designed a (U,Pu)O2 type fuel that achieves a higher gas release rate than conventional fuel, as well as thermal conductivity suitable for high-power, high-temperature operation. The design was performed using the Merope code. The Merope code is a microstructure generation tool for nuclear fuels, for all types of reactors. This microstructure generator then allows the thermomechanical behavior of the fuel to be studied. The Merope code is described in the publication by Marc Josien, "Merope: a microstructure generator for simulation of heterogeneous materials," *Journal of Computational Science* (2024): 102359.

[0063] Simulations

[0064]

[0065] related to the invention.

[0066] The Merope code was used to simulate the behavior of a representative elementary volume (REV), which is an approximation of a homogeneous and infinite medium. The fuel consists of MOX (U,Pu)O2 grains and porous inclusions forming an open pore network. It should be noted that the study was conducted at a scale smaller than that of the experimental microstructures that form the basis of the claims in this document; while the thermal conclusions are immediately transferable to the scale of fabricated microstructures, the quantitative conclusions regarding gas release must be validated by further studies.

[0067] At the microscopic level, the thermal conductivity of the VER was estimated using the AMITEX-FFTP numerical solver, developed by the CEA (French Alternative Energies and Atomic Energy Commission, applicant for this patent). AMITEX-FFTP is based on Fast Fourier Transforms (FFTs) to solve full-field homogenization problems on volumes representative of complex microstructures, such as composites or alloys. It allows for the modeling of the thermo-mechanical properties of heterogeneous materials.

[0068] At the microscopic level, gas diffusion was calculated using the TMFFT (Thermo-Mechanical Fast Fourier Transform) numerical solver. Figure 3 summarizes the process of determining an optimal MOX fuel microstructure. Steps 100 to 140, shown schematically in Figure 3, are iterative. Each iteration is assigned an iteration rank k, where k is an integer incremented at each new iteration, with 1 < k < K. K represents the total number of iterations.

[0069] 100 generation of a VER (representative elementary volume) model. This model is generated with 6 parameters k .

[0070] Parameter 6 k are renewed at each iteration. 6 k can be considered as a parameter vector, whose terms are:

[0071] Ptarget '■ desired porosity, i.e., the volume fraction of pores (%) that we wish to obtain. p ta rget is a constraint that the optimization algorithm must respect (constrained optimization).

[0072] where Rg and R p correspond to the respective average radii of the matrix zones and pores, the latter being assumed to be spherical;

[0073] Rg where o R 9 corresponds to the standard deviation on the radius of the matrix areas following a Gaussian distribution;

[0074] I y.., IIII

[0075] cr R corresponds to the standard deviation on the pore radius, the next pore radius

[0076]

[0077] p

[0078] a Gaussian distribution;

[0079] i is the volume fraction of pores in the intergranular space, that is, between the different matrix zones. For this purpose, two adjacent matrix zones were considered to be separated by a thickness corresponding to the pore diameter. Thus, if i = 100%, the matrix zones are completely surrounded by a layer of gas. i = 0% corresponds to an absence of pores between the zones;

[0080]

[0081] P: factor quantifying the average interconnectivity of the pores, f> = - - where d represents R p

[0082] an average distance between the respective centers of two adjacent pores, and R p corresponds to the average radius of two pores. Figure 4 schematically represents the quantities d and R p .

[0083] Among the modeling assumptions, the matrix zones and pores were considered to be spherical. The intragranular pores were left fixed and considered homogeneous: they affect the thermal conductivity of the fuel. The thermal conductivity of the pores was assumed to be zero, which is a commonly accepted assumption. 110 Calculation of the thermal conductivity λ eq (θ k ) of the VER by the AMITEX-FFTP solver. During this step, the steady-state heat diffusion equation on the VER is solved, with different thermal conductivities for the two phases (matrix and pores) and a temperature gradient, between 500K and 1500K. The model output is an equivalent thermal conductivity.

[0084] Sensitivity analyses were performed on the equivalent thermal conductivity to study the separate effects of the different geometric parameters defined in step 100. The most influential of these parameters, apart from the total porosity p, is the dispersion

[0085]

[0086] on the R g size of matrix areas and the ratio - between the average size of matrix areas and the R p

[0087] pores.

[0088] calculation of a characteristic duration 6 k The release of gases from the VER was calculated using the TM FFT solver. The gas transport model solves the gas mixture diffusion equation, assuming a combustion rate of 14 at% (14 atomic percentage), meaning that 14 out of every 100 heavy atoms were fissioned. Dirichlet boundary conditions were applied, assuming a zero gas concentration at the boundaries.

[0089] The characteristic duration T(0 k ) release corresponds to a ratio between the total quantity of fission gas atoms contained in the ERV and the fission gas source term, i.e., the quantity of fission gas produced in the ERV per unit time.

[0090]

[0091] Or:

[0092] C(r) is the concentration of fission gas at a point in the VER;

[0093] S v is the quantity of fission gas produced per unit volume and time: unit atoms, pim' 3 .s -1 ;

[0094] V VER is the volume of the worm;

[0095] C t is the concentration of fission gas (atoms / pm -3 ) in a voxel i, after discretization of the VER into I voxels, I denoting the number of voxels.

[0096] The characteristic release time is a duration during which the fission gases produced within the VER are retained within the latter.

[0097] Step 130: Calculation of a cost function J(0 fe ) During this step, a cost function is calculated

[0098]

[0099] TD denotes the thermal conductivity at the theoretical density, that is, without pores. p(0 fe ) is the porosity obtained by taking into account the parameters 6 k .

[0100] τ target is a pre-established setpoint value for the characteristic release time. For example, τ target = 0.6 months. This is another constraint assigned to the optimization algorithm.

[0101] r(0 k ) is the characteristic release time, as described in (1), taking into account the parameters 6 k .

[0102] [x] +denotes the positive part of x

[0103] The cost function J(0 k ) combines the thermal conductivity properties resulting from step 110 and the gas diffusion properties resulting from step 120.

[0104] Factor 10 3 and 100 are pre-established weighting factors, which tend to minimize the cost function when the constraints in terms of porosity and gaseous diffusion are respected.

[0105] The cost function is a decreasing function of λ eq (θ k ) and T(0 fe The greater the interconnected porosity, the greater λ eq (θ k ) decreases, and more r(0 k ) increase.

[0106] 140 minimization. During this step, a set of parameters is determined using a gradient descent algorithm. k+1allowing the value of the cost function to decrease in the next iteration. Steps 100 to 140 are repeated, up to a predetermined number of iterations, or until the algorithm converges, i.e., until a minimum value of J(θ) is obtained. k ).

[0107] The algorithm described in connection with steps 100 to 140 was implemented, modeling a U fuel 1-y Could y O x , with x = 1.98 and y = 0.3 in order to determine an optimal microstructure in terms of thermal conductivity and gas diffusion ability.

[0108] Table 1 lists the optimized parameters for an interconnected porosity MOX.

[0109]

[0110] Table 1

[0111] A fuel whose microstructure is as described in relation to Table 1 allows a release of 100% of the gases produced during irradiation in a fast neutron reactor, with a burnup rate of 14 at%, without significant degradation of thermal conductivity compared to a dense, pore-free fuel:

[0112]

[0113] = 0.813, representing a loss of

[0114]

[0115] thermal conductivity of approximately 19%.

[0116] Generally, MOX fuel exhibits a certain degree of porosity, primarily closed, typically ≤ 5%, with less than 1% being open porosity. Compared to such a fuel, the microstructure described in relation to Table 1 induces a 12% loss in thermal conductivity.

[0117] Figures 5A

[0118] and 5B represent a microstructure with interconnected porosity without optimization (Figure 5A) and with optimization (Figure 5B), the average diameter of the grain agglomerates being 20 pm.

[0119] Table 2 presents the main microstructure characteristics of an interconnected porosity MOX fuel.

[0120]

[0121] Table 2. The thermal behavior of the fuel, as described in Table 1, was studied, taking into account two porosities: 9% and 11% (volume fractions). A power level of 500 W / cm² was assumed. Figure 6 plots the temperature evolution (ordinate axis - unit K) as a function of the distance from the center of the pellet (abscissa axis - unit mm). In Figure 6, the curves ref, 9%, and 11% represent, respectively, the reference fuel, whose porosity is estimated at 5% (essentially closed, dispersed porosity), the fuel with 9% open and interconnected porosity, and the fuel with 11% open and interconnected porosity.

[0122] An increase in maximum temperature is observed for fuels with 9% and 11% porosity, this increase being represented by two double arrows. The geometry of the modeled fuel is an annular pellet with a diameter of 7 mm extending around a central hole with a radius of 1 mm. The maximum temperature is 2457 K for the reference fuel, compared to 2542 K for the 9% open and interconnected porosity fuel and 2588 K for the 11% open and interconnected porosity fuel. The maximum temperature difference is 130 K: the difference between the 11% porosity fuel and the reference fuel in the central part of the fuel, adjacent to the central hole. This increase is considered acceptable. It can be compensated for by increasing the diameter of the central hole to improve heat dissipation.It was also determined that all fission gases are evacuated, within the framework of the numerical model used, and for the sizes in Table 1.

[0123] The pore and agglomerate sizes in Table 2 result from modeling at a smaller scale than experimentally observed. In practice, for experimental reasons, it has been found that at least 10%, or even 20%, or 30%, of the pores have a diameter of at least 10 µm, or at least 20 µm, and preferably between 10 µm and 30 µm. This helps maintain the fuel's porosity properties at high temperatures, for example, up to 2400 K. Therefore, to preserve good thermal properties, it is necessary to have fuel matrix regions with a characteristic size (diameter or diagonal) between 60 µm and 200 µm, delimited by open and interconnected pores of 10 µm to 30 µm. This magnification (isometry) helps maintain the thermal conductivity properties compared to the original study. The presence of interconnected pores throughout the irradiation allows for better gas evacuation.

[0124] Manufacturing process The fuel as previously described can be produced by introducing an organic porogen, in the form of particles, into a MOX powder, and then performing compression and sintering.

[0125] The organic pore-forming agent can be azodicarbonamide (AZB). The particle size of the pore-forming agent is preferably between 5 µm and 150 µm, preferably between 5 µm and 100 µm, or between 5 µm and 60 µm. The mass fraction and particle size are adjusted to achieve open porosity.

[0126] A MOX mixture with a Pu / (U + Pu) ratio of 28.4% was used. Different fuels were formed, using an AZB mass fraction ranging from 3 to 7%, to obtain varying levels of porosity. The AZB was mixed and then formed into 5 mm diameter discs with a height between 1.3 mm and 1.5 mm under pressure at 400 MPa. The diameter and height are given after sintering. The discs underwent sintering at 1700 °C for 4 hours under Ar / 4.3% vol. H2 + 350 vpm (maximum water vapor pressure) H2O, followed by a reducing cycle at 1500 °C for 4 hours under Ar / 4.3% vol. H2 + 150 vpm to obtain an oxygen / metal ratio of 1.98 across all batches.

[0127] The particle size distribution of AZB is as follows: 10% fractile: 7 pm - median: 16 pm - 90% fractile: 28 pm. This allows for the formation of pores that correspond approximately to the size of the added particles.

[0128] Figure 7 shows the relative density (y-axis %) as a function of the mass fraction of porogen (x-axis %). The relative density was determined by measuring the apparent density using the triple weighing method (dry mass, mass immersed in bromobenzene, and wet mass), which yields the open and closed porosity fractions. The apparent density is representative of the total porosity level.

[0129] Table 3 shows the main results obtained. The first column corresponds to the mass fraction of added porogen. The table shows, for each mass fraction of added porogen, the average values ​​of relative apparent density, open porosity (%), and closed porosity (%). The last column shows the theoretical density after sintering without taking porosity into account, i.e., based on the oxygen / metal ratio after sintering.

[0130]

[0131] Table 3

[0132] Depending on the mass fraction of added porogen, the relative density varies between 95% and approximately 70%, corresponding to a porosity between 5% (0% added porogen) and 30% (6.2% added porogen). It has been observed that beyond a mass fraction of 7%, the resulting discs lose their mechanical strength and crumble to powder after sintering.

[0133] Optical micrographs (see Figure 8) and scanning electron micrographs (see Figure 9) were performed on a fuel sample with a porosity of 18% (relative density of 82%). The porosity was observed to be essentially open and interconnected.

[0134] The organic porogen agent may be chosen from: Avicel (microcrystalline cellulose), zinc stearate, or porogen agents as described in the Asakura publication "Developments in the fabrication technology of low density MOX pellets for fast breeder reactor fuel", cited in the prior art.

[0135] Benefits

[0136] The advantages of the fuel according to the invention are its compatibility with a fast neutron reactor, enabling it to provide a linear power output of between 50 and 500 W / cm², compared to 300 to 500 W / cm² for current fuels intended for fast neutron reactors, for example, the fuel intended for the Superphénix reactor, composed of MOX (U 0.8 Could 0.2)O2. Compared to the latter, the fuel according to the invention exhibits a thermal conductivity of 1.76 W / m·K at 2400 K (compared to 2.0 W / m·K for the Superphénix fuel), representing a reduction of slightly more than 10%. A significant advantage is improved fission gas release, regardless of the power level. The Superphénix fuel allows for an 80% release of fission gases at high power and 20% at low power (150 W / cm).

[0137] Due to its ability to better evacuate all fission gases, the fuel according to the invention limits the risk of swelling. In the case of Superphénix fuel, each pellet has an annular shape, extending around a central hole. Fuel swelling can cause the central hole to close, increasing the risk of meltdown due to reduced heat dissipation capacity. Fuel swelling can also damage the cladding, leading to a risk of direct contact between the fuel and the coolant (liquid sodium).

[0138] It follows from the above that the fuel according to the invention allows adaptation to various power levels, typically from 50 W / cm to 500 W / cm, while significantly improving operational safety.

Claims

DEMANDS 1. Fuel, intended to be irradiated in a fast neutron nuclear reactor, comprising grains of at least one metal selected from U, Pu, or Th, the grains being compressed in a volume, the fuel being porous, the fuel being characterized in that it comprises open intergranular pores, forming an interconnected porosity, the volume fraction of porosity being between 3% and 30%, more than 50% or 80% of the porosity being open porosity, the fuel comprising a mass fraction greater than 10% of at least one isotope, fissile under the effect of irradiation by fast neutrons, selected from 239 Could, 241 Could, 235 U, 233 U, the fuel being such that more than 10% of the open porosity volume fraction is formed from pores having a diameter or greater diagonal greater than 10 pm or 20 pm, and preferably between 10 pm and 30 pm.

2. Fuel according to claim 1, wherein at least 50% of the open porosity volume fraction is formed from pores having a diameter or greater diagonal greater than 10 pm or 20 pm, and preferably between 10 pm and 30 pm.

3. Fuel according to any one of the preceding claims, wherein the volume fraction of porosity is between 5% and 13%.

4. Fuel according to any one of the preceding claims, wherein the mass fraction of fissile isotope under the effect of irradiation by fast neutrons is less than 95%.

5. Fuel according to any one of the preceding claims, wherein the open porosity forms a network, the fuel comprising matrix zones, formed by grains or agglomerates of grains, each matrix zone being delimited by the open porosity network, each matrix zone having an average diameter of less than 250 pm, and preferably less than 80 pm or 50 pm or 30 pm.

6. Fuel according to any one of the preceding claims, wherein the cumulative mass fraction of metal, comprising U and / or Pu and / or Th, is greater than 80% or greater than 85% or greater than 87%.

7. Fuel according to claim 6, wherein each metal, selected from U, Pu or Th, is in the form of metal oxide, or nitride, or pure metal or metal alloy.

8. Fuel according to claim 7, comprising U 1-y Could y O x, with y between 0.1 and 0.5 and x between 1.5 and 2.

9. Fuel according to any one of the preceding claims, wherein the mass fraction of minor actinide is less than 10% or less than 8% or less than 5%, the term minor actinide denoting an isotope of Am or Np or Cm.

10. A method for manufacturing a fuel according to any one of the preceding claims, comprising: - a) preparation of a powder containing grains of U and / or Pu and / or Th; - b) mixing of the powder with an organic porogenous agent, with a mass fraction between 3% and 7%; - c) compression; - d) sintering at a temperature above 1500 °C.

11. A method according to claim 10, wherein the average grain diameter is less than 20 pm or 15 pm, or less than 10 pm.

12. A method according to claim 10 or claim 11, comprising, prior to step c), sieving the powder to remove grains or agglomerates of grains whose diameter is greater than a threshold, between 100 pm and 500 pm.

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