Porous MOX-type fuel for fast neutron reactors

Abstract

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 preferably being between 3% and 30%, more than 50% or 80% of the porosity being an open intergranular porosity,

Description

Title of the invention: Porous MOX-type fuel for fast neutron reactors. Technical field

[0001] 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. EARLIER ART

[0002] A reactor's ability to operate at varying power levels is a key factor in the development of the nuclear power sector. The objective 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, while remaining ready to rapidly increase its output if necessary.

[0003] Power variations are necessary due to the increasing 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 requires a constant balance between electricity production and consumption to maintain stability. A flexible nuclear reactor, which can adjust its power output rapidly, helps maintain this balance when electricity production from renewable energy sources varies significantly.

[0004] 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 needle. A fuel needle is defined as a fuel element, namely a sealed steel cladding (the first safety barrier) containing primarily a stack of fuel pellets. The acronym MOX stands for Mixed Oxide, which designates a fuel comprising both UO2 and PuO2.

[0005] Under the effect of irradiation, the fuel is the site of fission reactions, which lead to the production of fission products, some of which are gaseous, mainly Xe and Kr.

[0006] The quantity of gas produced increases during irradiation, depending on the burnup rate, i.e., 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 strong 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, located at the ends of the fuel rods, and designed to collect fission gases and helium.

[0007] At low power (< 300 W / cm²), a significant proportion of gas is retained in the fuel, as the thermal gradient is 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, which respectively reduces the amount of gas or promotes the thermal release of this gas.

[0008] 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 a 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 outer layers of the core of a fast neutron reactor, so as to allow the transmutation of 241Am by neutron capture.

[0009] The Asakura publication “Developments in the fabrication technology of low density MOX pellets for fast breeder reactor fuel” describes the manufacture of non-annular cylindrical fuel pellets for fast neutron reactors. This publication describes the use of various porogens incorporated into a powder with particle sizes ranging from 125 µm to 850 µm. The fuel is intended for use at high burn-up rates, exceeding 100 GW day / tonne.

[0010] The publication Morimoto K. “Thermal conductivities of (U, Pu, Am)O2 solid solutions” describes the manufacture of MOX fuel with an Am content between 0.7% and 3%, with a closed porosity between 10% and 15%.

[0011] 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 a 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. Description of the invention

[0012] 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, 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.

[0013] The fuel may comprise a mass fraction of between 10% and 50% of at least one fissile isotope under the effect of irradiation by fast neutrons.

[0014] The volume fraction of porosity can be between 5% and 13%.

[0015] Preferably, the mass fraction of the isotope, fissile under the effect of irradiation by of fast neutrons, is less than 95% or 80%.

[0016] The average diameter of the grains is preferably less than 20 pm or 15 pm or 10 pm.

[0017] The cumulative mass fraction of metal, comprising U and / or Pu and / or Th, is preferably greater than 80% or greater than 85% or greater than 87%.

[0018] Each metal, chosen from U, Pu or Th, can be in the form of metal oxide, or nitride, or pure metal or metal alloy.

[0019] The fuel may comprise UyPui yOx, with y between 0.9 and 0.5 and x between 1.5 and 2.

[0020] 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 of Am or Np or Cm.

[0021] A second object of the invention is a method for manufacturing a fuel according to the first object of the invention, comprising: - a) preparation of a powder containing grains of U and / or Pu and / or Th, with the possible presence of other minor actinides; - b) mixing of the powder with an organic porogenous agent, with a mass fraction for example between 3% and 7%; - c) compression; - d) sintering, in particular at a temperature above 1500 °C.

[0022] The average diameter of the grains may be less than 20 pm or 15 pm, or less than 10 pm.

[0023] 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.

[0024] The invention will be better understood upon reading the description of the exemplary embodiments presented later in this description, in connection with the figures listed below. FIGURES

[0025] Fig. 1A schematically represents a 3D view of a representative volume of a fuel according to the invention.

[0026] [Fig.1B] is a 2D view extracted from [Fig.1A].

[0027] Fig. 2 schematically represents a 3D view of a representative volume of MOX fuel, intended for fast neutron reactors, according to the prior art.

[0028] Fig. 3 illustrates the main steps of a process for optimizing a fuel microstructure according to the invention.

[0029] Figure 4 illustrates a parameter of the microstructure of a fuel according to the invention.

[0030] Figures 5A and 5B represent microstructures respectively before and after optimization.

[0031] 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.

[0032] Fig. 7 represents the relative density of porous fuels (ordinate axis), according to the invention, as a function of a mass fraction of organic porous agent used during manufacturing.

[0033] Figure 8 shows micrographs of a fuel according to the invention with two different scales.

[0034] The [Fig.9] is a scanning electron microscope image of a fuel according to the invention. PRESENTATION OF SPECIFIC IMPLEMENTATION METHODS

[0035] Figure 1A 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.

[0036] 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 (excluding oxygen), selected from isotopes of uranium (in particular 238U) and / or plutonium (in particular Pu) and / or thorium (in particular Th). Preferably, it comprises a mass fraction of at least 10% of isotopes that become fissile under the effect of fast neutrons, for example 239Pu, so that the energy density of the fuel is high.

[0037] The fuel may, during its manufacture, comprise 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%.

[0038] Preferably, the mass fraction of the fissile isotope under irradiation by fast neutrons (such as 239Pu) is greater than 10%. This allows the fuel to be used as feed fuel, which can be exposed to intense irradiation, in a central part of the core of a fast neutron reactor. The mass fraction of the fissile isotope under irradiation by fast neutrons is, for example, between 10% and 80% or between 10% and 95%.

[0039] The fuel material is porous. The pores allow the gases formed during fission (He, Kr, Xe) to escape, even at low temperatures. This porosity is open and interconnected, with at least 50%, or even 60%, 70%, or 80% of the pores opening onto another pore which itself opens outside the pellet (percentages of porosity given by volume). The interconnected porosity allows the gases to be released through the interconnected pores, which form a network of microchannels within the fuel matrix.

[0040] Fig. 1B is a two-dimensional detail of Fig. 1A: it represents an interconnected porosity. Fig. 2 illustrates a microstructure of a standard MOX fuel, in which the porosity is low (typically less than 5% by volume), being essentially closed.

[0041] 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.

[0042] Thus, the fuel must satisfy two opposing conditions: - the highest possible porosity, in order to promote the diffusion of fission gases at low temperature; - the highest possible thermal conductivity, bearing in mind that high porosity tends to decrease thermal conductivity. Indeed, the conductivity thermal conductivity, in each pore, is less than the thermal conductivity of the fuel matrix.

[0043] The inventors believe that a predominantly open porosity of between 3% and 30% (by volume) is achievable. The 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.

[0044] Beyond 28% or 30%, the porosity is too high and this results in a degradation of the mechanical strength of the fuel.

[0045] This refers to the porosity of the fresh fuel, that is, before irradiation in the reactor.

[0046] The fuel matrix is ​​formed of grains, or agglomerates of grains, whose average size is preferably less than 100 µm, and preferably less than 80 µm, 50 µm, or 30 µm. A grain size as small as possible promotes the formation of intragranular gas diffusion. By size, we mean the average or largest diameter of the grains. The grains, or agglomerates of grains, are surrounded by interconnected pores. This contrasts with a standard MOX fuel microstructure, in which the porosity is predominantly closed. It is considered that in MOX fuel, the volume fractions of open and closed porosity are less than 1% and less than 4%, respectively.

[0047] The grains may be formed from oxides of fissile or fertile material, for example UO2 and / or PuO2 and / or (U,Pu)O2, also denoted UyPui yOx, where x and y are positive real numbers. They may also be: - of nitride: Uranium nitride UN, Plutonium and Uranium nitride (U,Pu)N, Thorium nitride ThN); - or of metal: Metallic Uranium, Uranium and Plutonium alloys, Uranium-Molybdenum alloys.

[0048] Nitride-based fuels or metallic fuels exhibit high thermal conductivity, as well as high power density.

[0049] The inventors have designed a (U,Pu)O2 type fuel that provides a high gas release rate and thermal conductivity suitable for use at high power and high temperature. 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.

[0050] 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.

[0051] 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.

[0052] At the microscopic level, the gas diffusion was calculated using the TMFFT (Thermo-Mechanical Fast Fourier Transform) numerical solver - a fast Fourier transform solver for simulating thermo-mechanical properties.

[0053] 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 corresponds to the total number of iterations.

[0054] Step 100: generation of a VER (representative elementary volume) model. This model is generated with @k- parameters

[0055] The parameters 6k are renewed at each iteration. 0k can be considered as a parameter vector, whose terms are: - Ptarget: desired porosity, i.e., the volume fraction of pores (%) we wish to obtain. Ptarga is a constraint that the optimization algorithm must respect (constrained optimization). - kg, where Rg and Rp correspond to the respective average radii of the grains and R? pores, the latter being assumed to be spherical; - 2L where aR* corresponds to the standard deviation on the radius of the grains, the radius of the grains following a Gaussian distribution; - where corresponds to the standard deviation on the pore radius, the radius of Rp pores following a Gaussian distribution; - V: volume fraction of pores in the intergranular space, that is, between the different grains. For this, we considered that two adjacent grains are separated by a thickness corresponding to the diameter of the pores. Thus, if ip = 100%, the grains are completely surrounded by a layer of air, ip = 0% corresponds to an absence of pores between the grains; - fi: factor quantifying the average interconnectivity of the pores. _ A °ù represents the average distance between the respective centers of two adjacent pores, and Rp corresponds to the average radius of two pores. Figure 4 schematically represents the quantities d and Rp.

[0056] Among the modeling assumptions, the grains 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.

[0057] Step 110: Calculation of the thermal conductivity of the VER using the AMITEX-FFTP solver. During this step, the steady-state heat diffusion equation for the VER is solved, with different thermal conductivities for the two phases (matrix and pores) and a temperature gradient between 500 and 1500 K. The model output is an equivalent thermal conductivity.

[0058] 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 — on the grain size and the ratio between the average grain size RP and the pores.

[0059] Step 120: Calculation of a characteristic release time of the VER gases using the TMFFT solver. The gas transport model solves the gas mixture diffusion equation taking into account a combustion rate of 14 at% (14 atomic percent), which means that 14 out of 100 heavy atoms have been fissioned. Dirichlet boundary conditions were taken into account, assuming a zero gas concentration at the boundaries.

[0060] The characteristic release time corresponds to a ratio between the total quantity of fission gas atoms contained in the VER and the fission gas source term, i.e. the quantity of fission gas produced in the VER per unit time.

[0061] JC(rWr3 (1) T W = ~s7

[0062] where:

[0063] C(r) is the concentration of fission gas at a point in the VER;

[0064]

[0065]

[0066]

[0067]

[0068]

[0069]

[0070]

[0071]

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[0073]

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[0075]

[0076]

[0077]

[0078]

[0079]

[0080] Sv is the quantity of fission gas produced per unit volume and time: unit atoms, pm 3.s 1; VVER is the volume of the VER; C; 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. The characteristic release time is a duration during which the fission gases produced within the VER are retained within the latter. Step 130: Calculating a cost function During this step, a cost function is calculated. 'h'ErLJw T o / 1 AW) + ^td denotes the thermal conductivity at the theoretical density, i.e. without pores. P(Gk) is the porosity obtained by taking into account the parameters 0*. Ttarget cs( a previously established setpoint value for the characteristic release time. For example, T <arget= q g mois. il s’agit d’une autre contrainte assignée à l’algorithme d’optimisation. is the characteristic release time, as described in (1), taking into account the parameters 0^. [x]+ denotes the positive part of x The cost function combines the thermal conductivity properties resulting from step 110 and the gas diffusion properties resulting from step 120. Factors 103 and 100 are pre-established weighting factors, which tend to minimize the cost function when porosity and gas diffusion constraints are respected. The cost function is a decreasing function of and of ^us 'a interconnected porosity is important, the more it decreases, and the more it increases. Step 140: Minimization. During this step, a set of parameters 0^+i is determined using a gradient descent algorithm to decrease the value of the cost function in the next iteration. Steps 100 to 140 are repeated until a predetermined number of iterations is reached, or until the algorithm converges, i.e., until a minimum value of J(dk) is obtained. The algorithm described in connection with steps 100 to 140 was implemented, modeling a Uv yPyO2x fuel, with 2-x = 1.98 and y = 0.3 in order to determine an optimal microstructure in terms of thermal conductivity and gas diffusion ability.

[0081] Table 1 sets out the optimized parameters for an interconnected porosity MOX.

[0082] [Tab 1] ï H •L $35k, ss 8. sa*

[0083] A fuel whose microstructure is such 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 of 14 at%, without significant degradation of thermal conductivity compared to a dense, non-porous fuel: = 0.813, which is a loss of thermal conductivity of approximately 19%.

[0084] Generally, a MOX fuel exhibits a certain porosity, essentially closed, generally <5%, of which less than 1% corresponds to open porosity. Compared to such a fuel, the microstructure described in relation to Table 1 induces a loss of thermal conductivity of 12%.

[0085] Figures 5A and 5B represent an interconnected porosity microstructure without optimization ([Fig.5A]) and with optimization ([Fig.5B]), the average grain diameter being 20 pm.

[0086] Table 2 presents the main microstructure characteristics of an interconnected porosity MOX fuel.

[0087] [Tab 2] 5: - 38:^ To £ - 5.2 % W ■«A Table 2

[0088] The thermal behavior of the fuel, as described in relation to Table 1, was studied, taking into account two porosities: 9% and 11% (volume fractions). A power level of 500 W / cm² was considered. 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 ref, 9%, and 11% curves respectively designate the reference fuel, whose porosity is estimated at 5% (essentially closed, dispersed porosity), and the fuel of open and interconnected porosity of 9% as well as the fuel with open and interconnected porosity of 11%.

[0089] An increase in the 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% 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, so as to improve heat dissipation.

[0090] It has also been determined that all fission gases are vented. Manufacturing process

[0091] The fuel as previously described can be produced by introducing an organic porogen into a MOX powder, then performing compression and sintering.

[0092] The organic porogen agent may be azodicarbonamide (AZB).

[0093] A MOX mixture with a Pu / (U + Pu) ratio of 28.4% was used. Different fuels were formed, using a mass fraction of AZB ranging from 3 to 7%, to obtain different levels of porosity. The AZB was mixed and then discs with a diameter of 5 mm and a height between 1.3 mm and 1.5 mm were formed by pressing 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 for all batches.

[0094] Figure 7 represents the relative density (ordinate axis %) as a function of the mass fraction of porogen (abscissa 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.

[0095] Table 3 shows the main results obtained. The first column corresponds to the mass fraction of porogen added. The table shows, for each mass fraction of porogen added, the average values ​​of apparent density relative, 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.

[0096] [Tab 3] %AZ B Apparent relative density Porosity or green Closed porosity Theoretical density after sintering 0.0% 97.23 0.5 2.3 11.160 3.6% 86.03 10.6 3.4 11.173 4.4% 82.70 15.1 2.2 11.173 5.0% 82.18 16.1 1.8 11.133 5.6% 79.02 19.1 1.9 11.120 6.2% 71.64 25.0 2.9 11.120

[0097] Table 3

[0098] Depending on the mass fraction of added porogenic agent, the relative density varies between 95% and approximately 70%, corresponding to a porosity between 5% (0% added porogenic agent) and 30% (6.2% added porogenic agent). It has been observed that beyond a mass fraction of 7%, the resulting discs lose their mechanical strength and crumble into powder after sintering.

[0099] Optical micrographs (see [Fig. 8]) and scanning electron micrographs (see [Fig. 9]) were performed on a fuel sample with a porosity of 18% (relative density of 82%). The porosity is observed to be essentially open and interconnected.

[0100] The organic porogen may be selected from: Avicel (microcrystalline cellulose), zinc stearate, or porogens 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. Advantages

[0101] 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 / cm, compared to 300 to 500 W / cm for current fuels intended for fast neutron reactors, for example the fuel intended for the Superphénix reactor, composed of MOX (Uo.sPuoJCL). Compared to the latter, the fuel according to the invention has a thermal conductivity of 1.76 W / mK at 2400 K (compared to 2.0 W / .K for the Superphénix fuel), i.e. a reduction of a little over 10%.

[0102] A significant advantage is the release of 100% of the fission gases, regardless of the power level. The Superphénix fuel allows for the release of 80% of the fission gases at high power, and 20% at low power (150 W / cm²).

[0103] Due to its ability to vent all fission gases, the fuel according to the invention avoids the risk of swelling. In the case of Superphénix fuel, each pellet has an annular shape, extending around a central hole. Swelling of the fuel can cause the central hole to close, which increases the risk of melting, as the heat dissipation capacity is reduced. Fuel swelling can also damage the cladding, leading to a risk of direct contact between the fuel and the coolant (liquid sodium).

[0104] 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 an open intergranular porosity, the fuel comprising a mass fraction greater than 10% of at least one fissile isotope under the effect of irradiation by fast neutrons.

2. Fuel according to claim 1, wherein the porosity volume fraction is between 5% and 13%.

3. 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%.

4. Fuel according to any one of the preceding claims, wherein the average grain diameter is less than 20 pm or 15 pm or 10 pm.

5. 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%.

6. Fuel according to claim 5, wherein each metal, selected from U, Pu or Th, is in the form of metal oxide, or nitride, or pure metal or metal alloy.

7. Fuel according to claim 6, comprising UyPui yOx, with y between 0.9 and 0.5 and x between 1.5 and 2.

8. 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.

9. A method for manufacturing a fuel according to any one of the preceding claims, comprising: - a) preparation of a powder comprising grains of U and / or Pu and / or Th; 15 - 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.

10. A method according to claim 9, wherein the average grain diameter is less than 20 pm or 15 pm, or less than 10 pm.

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