WO2018234694A1 - PROCESS FOR PREPARING PELLETS OF MIXED DENSE FUEL BASED ON URANIUM, PLUTONIUM AND POSSIBLY MINOR ACTINIDE (S) - Google Patents

Abstract

The invention relates to a process for preparing pellets of a dense mixed fuel comprising uranium dioxide UO₂, plutonium oxide PuO₂ and optionally at least one oxide of minor actinide(s) or of a fuel comprising a solid solution of uranium dioxide, plutonium and optionally at least one minor actinide successively comprising the following steps: a) a step of co-micronization of a mixture of powders comprising a powder of uranium oxide(s), of which all or some of this powder consists of triuranium octoxide U₃O₈, a powder of plutonium oxide PuO₂ and optionally a powder of oxide(s) of minor actinide(s); b) a step of compacting the mixture of powders in the form of pellets; c) a step of sintering the pellets obtained in b).

Description

 PROCESS FOR PREPARING PELLETS OF A MIXED DENSE FUEL BASED ON URANIUM, PLUTONIUM AND, POSSIBLY, ACTINIDE (S)

 MINOR (S)

DESCRIPTION TECHNICAL FIELD

The invention relates to a process for the preparation of a nuclear fuel based on mixed uranium and plutonium oxide and, more specifically, to a process for preparing dense pellets based on mixed uranium oxide and plutonium from a mixture of powders, part of which is constituted by triuranium octaoxide (U3O8), this type of fuel can be qualified as MOX fuel (this acronym corresponding to the terminology "Mixed Oxide"). Because of their high energy value, this type of fuel can be upgraded in power reactors, such as fast neutron reactors (also known under the abbreviation RN R) or light water reactors (known, also , under the abbreviation REL).

 This process can therefore be part of the field of plutonium recycling, which is derived from the irradiation of conventional fuels in a nuclear reactor. STATE OF THE PRIOR ART

In general, processes for the fabrication of MOX type nuclear fuels are based on powder metallurgy processes, involving a step of preparing a mixture of powders followed by a step of shaping the mixture. (For example, by pressing) followed by a densification step of the shaped mixture (for example, by sintering). Among these processes, two main ways are distinguished for the manufacture of MOX fuels:

a first route which may be described as "direct co-grinding", in which a powder of UO ₂ and a powder of PuO ₂ are mixed and co-crushed immediately in the proportions desired to obtain the specified plutonium content, that is to say the plutonium content that the fuel has at the end of manufacture, namely after shaping and sintering of the mixture of powders;

 a second route which combines both grinding and dilution stages, in which a primary mixture of "overconcentrated" plutonium powders is initially constituted with respect to the specified plutonium content (this mixture may also be qualified as a mixture mother), which primary mixture is then diluted by addition of uranium dioxide.

 These two routes are, in particular, illustrated by two reference methods, which are the COCA process (for Cadarache crushing) and the MIMAS process (for Mlcronized MASterblend in English). The COCA process consists, before forming in the form of pellets, cobrying in a single step a mixture of powders mainly comprising a UO2 powder and comprising PuO2, while the MIMAS process consists of initially establishing a mixture of UO2 and PuO2 with a high proportion of PuO2, which mixture is then milled and then diluted by adding UO2 to achieve the target content, before shaping into pellets.

 Whether for one or the other of these processes, the raw material used is mainly a UO2 powder.

 In the field of fuel fabrication of the type

MOX, other processes implement the use, in addition to a powder of UO2, of another uranium input, known as U3O8 triuranium octaoxide sought for its ability to perform the function of pore-forming agent, as described in particular in patent FR 2949598. This patent more particularly describes a method of manufacturing a nuclear fuel comprising uranium, possibly plutonium and at least one minor actinide comprising, in a first step, a compaction step in the form of pellets of a mixture of powders. (uranium oxide powders, optionally plutonium oxide and minor actinide oxide (s) powder (s)), a part of the uranium oxide powder being a powder of U3O8 and comprising, in a second step, a step of reducing triuranium octaoxide U3O8 to uranium dioxide U0 ₂ .

This reduction step generates a porosity within the fuel. Indeed, the allotropic transformation of U3O8 triuranium octoxide into uranium dioxide U0 ₂ generates a reduction in volume of the space occupied by triuranium octoxide by approximately 30%, the space left vacant constituting pores in the fuel. For example, with a proportion of 40% by mass of U3O8 for a content (U / U + Am) = 0.1, a fuel having a porosity of about 14% is obtained (ie other words, a density equal to about 86% of the theoretical density). This large porosity will thus allow the evacuation of fission gases and decay helium without physical degradation of the fuel.

 The use of triuranium octoxide U3O8 as a blowing agent is also described in WO 03/015105 using powder mixtures with U3O8 contents of up to 25%.

 Moreover, besides the possibility for a U3O8 powder to fulfill the function of porogenic agent, other advantages can be put forward such as:

the stable nature of this powder, the oxygen content evolving very little compared to a UO ₂ uranium dioxide powder whatever its storage mode (for example, under air or in an inert atmosphere), which constitutes a robustness element for its use in an industrial ceramization process; the non-pyrophoric character of this powder with respect to a UO ₂ uranium dioxide powder;

-the possibility of developing other material flows than U0 ₂ thus allowing a diversification of inputs to manufacture uranium-based nuclear fuels.

 Also, in summary, in the state of the art illustrating the preparation of mixed fuels and using U3O8 triuranium octaoxide, it appears that the purpose of use of this additive is mainly to obtain porous fuels.

 On the basis of this observation, the authors have turned towards an antagonistic objective which is that of plutonium-based MOX-type dense fuels while using, as a whole or part of the uranium source, U3O8 triuranium octoxide. advantages associated with the use of this source (such as those mentioned above, such as the stable and non-pyrophoric character of triuranium octoxide powders).

 To achieve this antagonistic objective, the authors have developed a method involving a very specific step of preparation of a mixture of powders comprising U3O8 triuranium octaoxide and, for example, an amount of U3O8 may exceed 25% by mass. .

STATEMENT OF THE INVENTION

Thus, the invention relates to a process for preparing pellets of a dense mixed fuel comprising uranium dioxide U0 _2, the Pu0 plutonium oxide ₂ and optionally at least one oxide of actinide (s) minor (S) or comprising a solid solution of uranium dioxide, plutonium and optionally at least one minor actinide comprising successively the following steps:

a) a step of co-micronization of a mixture of powders comprising a powder of uranium oxide (s), all or part of which powder consists of U3O8 triuranium octaoxide, a PuO ₂ plutonium oxide powder and optionally a minor actinide oxide (s) powder (s);

 b) a step of compacting the mixture of powders in the form of pellets;

 c) a sintering step of the pellets obtained in b).

 As already suggested above, the authors of the present invention have gone against a technical a priori to propose the use of U3O8 triuranium octoxide as a partial or total source of uranium for the manufacture of dense fuel pellets. mixtures based on uranium, plutonium and possibly minor actinide (s).

Indeed, to substitute all or part of the uranium source conventionally used U0 ₂ to manufacture dense fuels does not present an obviousness in the light of the state of the art. Indeed, using U3O8, the embodiments put forward a shrinkage phenomenon during the sintering step in a hydrogenated medium, this shrinkage being due to the allotropic transformation of U3O8 into U0 ₂ , which conventionally generates a reduction in volume of 30% with creation of a concomitant porosity.

Before step a) of co-micronisation, the process of the invention may comprise a preliminary step of preparing the powder mixture by contacting a uranium oxide powder (s), all or part of which this powder consists of U3O8 triuranium octaoxide, a PuO ₂ plutonium oxide powder and optionally a minor actinide oxide (s) powder (s).

Whether for the preceding step or step a), the uranium oxide powder (s) may be composed exclusively of a U3O8 triuranium octoxide powder or may consist of a mixture of octahydroxide oxide. U3O8 triuranium and uranium dioxide U0 ₂ . In the case of a mixture, U3O8 triuranium octoxide may be in a majority proportion, that is to say it represents more than 50% by weight of the mixture comprising the UO ₂ powder and the U 3 O 8 powder. For example, the triuranium octoxide U3O8 may be present in a proportion ranging from 10 to 60% by weight of the mixture comprising the UO ₂ powder and the U 3 O 8 powder.

The UO ₂ and U ₃ O 8 powders may be derived from all types of processes and, in particular, from dry or wet conversion processes.

The PuO ₂ powder may be present in a content greater than or equal to the specified content desired for the plutonium in the final fuel. In the case where this content is greater than the specified content, the method of the invention may comprise a dilution step which is defined more precisely below.

When present, the minor actinide oxide (s) powder (s) may be an americium oxide (s) powder (such as AmO ₂ , AlT ₂ O 3), a powder of curium oxide (s) and / or a powder of neptunium oxide (s).

 The constituent powders of the mixture intended to be co-micronized may consist of different types of particles, for example, substantially spherical, such as:

 crystallites, which are the smallest constituents constituting a powder and having a size generally less than 1 micrometer, for example, of the order of 0.1 μιη;

 aggregates, which consist of the intimate and solid association of crystallites and having a size generally ranging from 1 to 10 μιη;

 agglomerates, which consist of a set of aggregates linked by weak bonds (for example, Van der Waals type bonds, electrostatic bonds) and having a size generally ranging from 10 to 100 μιη.

In addition, the powders may have a specific surface area ranging from 1 to 15 m ² / g As mentioned above, the powder mixture is subjected to a co-micronization step, that is to say a step of mixing and grinding, concomitantly, the powder grains constituting the mixture, until to obtain an intimate mixture of powders, the powders constituting the mixture thus micronized may consist of different types of particles, for example, substantially spherical, such as:

 crystallites, which are the smallest constituent entities of a powder, these crystallites having a size generally less than 1 micrometer, for example, of the order of 0.1 μιη;

 aggregates, which consist of the intimate and solid association of crystallites and having a size generally ranging from 1 to 5 μιη;

 agglomerates, which consist of a set of aggregates linked by weak bonds (for example, Van der Waals type bonds, electrostatic bonds) and having a size generally ranging from 10 to 200 μιη ( micronization may contribute to disaggregate and then reagglomerate the particles),

 the proportions between crystallites, aggregates and agglomerates may be different from those of the mixture before the co-micronization step.

In addition, the powders may have a specific surface area ranging from 1 to 15 m ² / g

By this step, it is possible to access a homogeneous granular mixture at a scale of a few micrometers, which has the effect of not only homogeneously distributing U3O8 triuranium octaoxide within the mixture but also of reducing the size of the mixture. U3O8 triuranium octoxide, so that its allotropic conversion to U0 ₂ during sintering generates pores of sufficiently small size that they do not affect the density of the fuel.

From a practical point of view, this co-micronization step can be carried out by subjecting the powder mixture to grinding in a ball mill for a time sufficient to obtain the desired micronized mixture.

 After this co-micronization step, the method of the invention may comprise a sieving step, and more specifically, a sieving-forcing step, in particular to eliminate larger agglomerates.

After the co-micronization step and the possible sieving step, the process may comprise a step of adding to the micronized and optionally sieved mixture of a UO ₂ uranium dioxide powder, this step being capable of being described as dilution step, in that it contributes to reducing the content of other elements than uranium in the fuel and also the U3O8 content in the mixture resulting from the adding step. This step is particularly necessary when the content of PuO ₂ in the powder mixture in step a) is greater than the specified content of plutonium that is desired in the final fuel.

 In the case of the implementation of this step, the micronized mixture obtained at the end of step a) can be qualified as "mother mixture", which mother mixture has, as mentioned above, a source of uranium which may consist, in whole or in part, of triuranium octoxide U3O8.

Then, the mixture of powders (that obtained at the end of step a) or that obtained at the end of the dilution step) is subjected to a shaping step in the form of pellets, by example, by uniaxial pressing. More specifically, this shaping step may consist of compacting the mixture, which amounts to placing the mixture in one or more molds of suitable shape to form pellets and, secondly, to subject this mixture to a pressing for example, uniaxial, this uniaxial pressing being obtainable by means of a piston applying a pressure on the placed mixture in the mold or molds, this pressure may range from 300 to 500 MPa for a period ranging from 1 second to 1 minute.

At the end of the compacting, the pellets thus obtained are subjected to a sintering step, this sintering step being advantageously carried out under conditions that are effective for obtaining the desired density and the reduction of U3O8 in U0 ₂ or even for fixing. the desired stoichiometry (ie, the molar ratio between the amount of oxygen and the amount of U, Pu and / or minor actinide (s) elements). Those skilled in the art, depending on the density and the stoichiometry that he wishes to obtain, will determine the operating conditions necessary to obtain these characteristics. To do this, it will be able to practice routine tests to test different sets of operating conditions and verify by simple analysis techniques the density and the stoichiometry obtained, until the set of operating conditions necessary to obtain the desired density and stoichiometry desired. Dilatometry measurements may also be performed to determine the optimal sintering conditions.

 This set of operating conditions can be the atmosphere in which the sintering step is carried out, the heating temperature and the duration of application of this temperature and the rate of rise in temperature until obtaining the maximum temperature of bearing heating.

By way of example, the sintering step may thus consist in subjecting the pre-obtained pellets or pellets to heating, for example, in a controlled reducing atmosphere (for example, this atmosphere possibly consisting of a mixture comprising a reducing gas optionally, oxygen and / or water vapor) at a temperature and a time required to obtain the desired relative density, this temperature being able to range from 1000 ° C. to 2000 ° C. for a period of time. from 1 hour to 48 hours. The constitution of the atmosphere may be chosen so as to obtain, in the end, a pellet having a final relative density at a defined value and / or a pellet having a ratio O / M (M corresponding to the actinide elements) final predefined. Regarding the latter criterion, the important factor in the constitution of the atmosphere is its moisture content.

 The sintering temperature can be reached according to different modes, such as:

 a dynamic mode where the heating temperature is reached according to a linear rise in temperature or not, this temperature can then be maintained up to a duration of 48 hours;

 a stepped temperature rise mode, until reaching a plateau corresponding to the heating temperature, this temperature step being able to be maintained up to a duration of 48 hours.

 By way of example, the sintering temperature can be obtained by a linear rise in temperature until reaching a temperature plateau of 1700 ° C., which can be maintained for a period of 4 hours.

 As regards the atmosphere, this may be of a different nature, in particular depending on the proportion of plutonium desired in the fuel.

Thus, for fuels whose proportion of plutonium is of the order of 15% or of the order of 30% (such fuels being suitable for fast neutron reactors), the atmosphere may consist of a mixture comprising argon and hydrogen (4% by volume of hydrogen) and respectively a moisture content of the order of a few tens of vpm, for example, from 1 to 50 vpm of water for a mixture Ar / lh (4% by volume of hydrogen) for a plutonium content of the order of 15% and a moisture content of the order of a few hundred vpm, for example from 100 to 500 vpm of water for a mixture Ar / lh (4% by volume of hydrogen) for a plutonium content of the order of 30%.

 For fuels with a plutonium content of less than 12% (such fuels being suitable for pressurized water reactors), the atmosphere may consist of a mixture comprising argon and hydrogen (4% in volume of hydrogen) and a humidity level of more than 1000 vpm.

 At the end of the process of the invention, fuel pellets having a high density, in particular a relative density greater than 94%, are obtained, this relative density corresponding to the ratio of the apparent density to the theoretical density. (ie, in other words, less than 6% porosity) and, more specifically, a relative density greater than or equal to 96%.

 By way of example, the theoretical density of a MOX fuel pellet in the form of a solid solution of uranium dioxide and plutonium is deduced from the mesh parameter by the following equation:

Theoretical density = 1.6598 * 10 ⁶ * [(4 * M (U, Pu) + 64 * (2-x)) / a ³ ] in which:

 -M (U, Pu) corresponds to the molar mass of the elements U and Pu

(in g mol ^_1 );

 -x corresponds to the stoichiometry difference of the oxygen of the solid solution;

 -a corresponds to the mesh parameter (in pm).

The mesh parameter of a sintered MOX chip based on a solid solution (Ui _y Pu _y ) 02-x between 0 and 30% Pu is given by the following equation: a (pm) = 547,0-7,4y + (30 + II, 0y) * x

 in which :

 -a corresponds to the mesh parameter;

 y corresponds to the plutonium content of the solid solution; and -x corresponds to the stoichiometry difference of the oxygen of the solid solution.

 The invention will now be described with reference to the particular embodiment explained below, this embodiment made in the laboratory being provided for illustrative and non-limiting purposes.

DETAILED DESCRIPTION OF A PARTICULAR EMBODIMENT

 EXAMPLE

 This example illustrates the preparation of fuel pellets obtained according to the process of the invention according to two variants:

a first variant that does not involve a step of diluting the micronized mixture with a UO ₂ powder;

a second variant involving a step of diluting the micronized mixture with a UO ₂ powder.

 For comparison, it is also proceeded to the preparation of pellets of fuels obtained by processes not using U3O8 triuranium octaoxide also according to a first variant not involving a step of diluting the micronized mixture and a second variant involving a step of diluting the micronized mixture. a) First variant according to the invention

In this variant, a mixture of powders (20 g) is prepared to obtain a mixture corresponding to a composition comprising 35.5 at%. of U ₃ 0 ₈ (ie, 7.2 g), 35.5% at. U0 ₂ (7.1 g) and 29 at%. of Pu0 ₂ (ie 5.7 g). The powder of U3O8, the U0 ₂ powder and powder Pu0 ₂ respectively have a surface area of 2.2 m ² / g, 3.7 m ² / g and 5.7 m ² / g.

 The mixture is then subjected to dry co-micronisation using a ball mill consisting of a 0.5 liter cylindrical vessel filled with grinding bodies (which are orthocylindrical rollers), the tank being subjected to a speed of rotation of 60 rpm and inclination of 10 ° for a period of 4 hours (8 cycles of 30 minutes with unclogging of the powder mixture between each cycle).

 This step is intended to homogenize the mixture of powders at the micrometer scale, the mechanisms involved being mechanisms of fragmentation and deagglomeration-agglomeration and also leads to an increase in the specific surface of the powder mixture.

The resulting powder mixture has a specific surface area of 4.2 m ² / g.

 The mixture is then screened so as to separate agglomerates of more than 200 μιη from the mixture of powders (which corresponds to 10% by weight of the mixture). The agglomerates thus isolated are then ground in an agate mortar and then ironed with a sieve. The purpose of this sieving is thus to eliminate the large agglomerates formed in the previous step. Indeed, the presence of large agglomerates could generate problems of filling press dies as well as sintering problems during sintering, which could cause the creation of inter-agglomerated porosity.

The sieved mixture is shaped by compacting and more specifically by uniaxial Osterwalder floating matrix pressing under a pressure of 350 MPa. An accompanying pressure of 35 MPa is applied during the ejection. The matrix is lubricated before each compacting cycle with an organometallic powder of zinc stearate. Finally, 10 tablets of 1.5 g are obtained having a geometric density of 6 g. cm ³ .

 A first batch of pellets is subjected to a sintering cycle in an atmosphere comprising a mixture of argon and hydrogen (4% by volume of hydrogen) and a moisture content of 1200 vpm in which a temperature step 1700 ° C is applied for 4 hours.

 A second batch of pellets is subjected to the same sintering cycle.

 b) Second variant according to the invention

In this variant, a mixture of powders (20 g) is prepared to obtain a mixture corresponding to a composition comprising 35.5 at%. of U ₃ 0 ₈ (ie, 7.2 g), 35.5% at. U0 ₂ (7.1 g) and 29 at%. of Pu0 ₂ (ie 5.7 g).

The powder of U3O8, the U0 ₂ powder and powder Pu0 ₂ respectively have a surface area of 2.2 m ² / g, 3.7 m ² / g and 5.7 m ² / g.

 The mixture is then subjected to dry co-micronisation using a ball mill consisting of a 0.5 liter cylindrical vessel filled with grinding bodies (which are orthocylindrical rollers), the tank being subjected to a speed of rotation of 60 rpm and inclination of 10 ° for a period of 4 hours (8 cycles of 30 minutes with unclogging of the powder mixture between each cycle).

The resulting powder mixture has a specific surface area of 4.2 m ² / g.

The mixture is then screened so as to separate agglomerates of more than 200 μιη from the mixture of powders (which corresponds to 10% by weight of the mixture). The agglomerates thus isolated are then ground in an agate mortar and then ironed with a sieve. The purpose of this sieving is thus to eliminate the large agglomerates formed in the previous step. Indeed, the the presence of large agglomerates could generate problems of filling the press dies as well as sintering problems during sintering, which could cause the creation of inter-agglomerated porosity.

The sieved mixture (hereinafter masterbatch) is then subjected to a dilution step of adding a powder of UO2, whereby a mixture (20 g) comprising the masterbatch (at 34.6%) and a powder of U0 ₂ (65.4%).

More specifically, the dilution step consists of a stirring in a mixture of Turbula type. To do this, the powders are introduced into a plastic container 150 cm ³ provided with a wire, which container is then placed in the tank of a mixer. Prior studies of qualification of the apparatus made it possible to set the speed of rotation of the tank at 24 rpm as well as the duration of the mixing cycle at 10 minutes. In addition to the dilution of the masterbatch, this step aims to homogenize the distribution of the particles constituting the masterbatch in the UO ₂ powder.

The diluted mixture has a specific surface area of 4.2 m ² / g

The resulting mixture is shaped by compacting and more specifically by Osterwalder floating matrix uniaxial pressing under a pressure of 350 MPa. An accompanying pressure of 35 MPa is applied during the ejection. The matrix is lubricated before each compacting cycle with an organometallic powder of zinc stearate.

Finally, 10 tablets of 1.5 g are obtained having a geometric density of 5.7 g. cm ³ .

 A first batch of pellets is subjected to a sintering cycle in an atmosphere comprising a mixture of argon and hydrogen (4% by volume of hydrogen) and a moisture content of 1200 vpm in which a temperature step 1700 ° C is applied for 4 hours.

A second batch of pellets is subjected to the same sintering cycle. c) Comparative example (first variant)

In this variant, a mixture of powders (20 g) is prepared to obtain a mixture corresponding to a composition comprising 71 at%. U0 ₂ (14.3 g) and 29 at%. of Pu0 ₂ (ie 5.7 g), this mixture having a specific surface area of 5.4 m ² / g

The mixture is then subjected to dry co-micronisation using a ball mill consisting of a 0.5 liter cylindrical vessel filled with grinding bodies (which are orthocylindrical rollers), the tank being subjected to a speed of rotation of 60 rpm and inclination of 10 ° for a period of 4 hours (8 cycles of 30 minutes with unclogging of the powder mixture between each cycle).

 The mixture is then screened so as to separate agglomerates of more than 200 μιη from the mixture of powders (which corresponds to 10% by weight of the mixture). The agglomerates thus isolated are then ground in an agate mortar and then ironed with a sieve. The purpose of this sieving is thus to eliminate the large agglomerates formed in the previous step. Indeed, the presence of large agglomerates could generate problems of filling press dies as well as sintering problems during sintering, which could cause the creation of inter-agglomerated porosity.

 The resulting mixture is shaped by compacting and more specifically by Osterwalder floating matrix uniaxial pressing under a pressure of 350 MPa. An accompanying pressure of 35 MPa is applied during the ejection. The matrix is lubricated before each compacting cycle with an organometallic powder of zinc stearate.

Finally, 10 tablets of 1.5 g are obtained having a geometric density of 6.5 g. cm ³ .

A first batch of pellets is subjected to a sintering cycle in an atmosphere comprising a mixture of argon and hydrogen (4% by volume of hydrogen) and a moisture content of 1200 vpm during which a temperature plateau of 1700 ° C is applied for 4 hours.

 A second batch of pellets is subjected to the same sintering cycle.

 d) Comparative example (second variant)

In this variant, a mixture of powders (20 g) is prepared to obtain a mixture corresponding to a composition comprising 71 at%. U0 ₂ (14.3 g) and 29 at%. of PuO ₂ (ie 5.7 g), this mixture having a specific surface area of 5.4 m ² / g. The mixture is then subjected to dry co-micronisation using a ball mill consisting of a cylindrical vessel of 0.5 liter filled with grinding bodies (which are orthocylindrical rollers), the tank being subjected to a rotation speed of 60 revolutions / min and an inclination of 10 ° for a duration of 4 hours (8 cycles of 30 minutes with unclogging of the powder mixture between each cycle).

 The mixture is then screened so as to separate agglomerates of more than 200 μιη from the mixture of powders (which corresponds to 10% by weight of the mixture). The agglomerates thus isolated are then ground in an agate mortar and then ironed with a sieve. The purpose of this sieving is thus to eliminate the large agglomerates formed in the previous step. Indeed, the presence of large agglomerates could generate problems of filling press dies as well as sintering problems during sintering, which could cause the creation of inter-agglomerated porosity.

The sieved mixture (hereinafter masterbatch) is then subjected to a dilution step of adding a powder of UO ₂ to reach a final content of the mixture of 10 mol%.

More specifically, the dilution step consists of a stirring in a mixture of Turbula type. To do this, the powders are introduced into a 150 cm ³ plastic container equipped with a wire, which container is then placed in the tank of a mixer. Prior studies of qualification of the apparatus made it possible to set the speed of rotation of the tank at 24 rpm as well as the duration of the mixing cycle at 10 minutes. In addition to the dilution of the masterbatch, this step aims to homogenize the distribution of the particles constituting the masterbatch in the UO ₂ powder.

The diluted mixture has a specific surface area of 4.6 m ² / g

The resulting mixture is shaped by compacting and more specifically by Osterwalder floating matrix uniaxial pressing under a pressure of 350 M Pa. An accompanying pressure of 35 M Pa is applied during the ejection. The matrix is lubricated before each compaction cycle with an organometallic powder of zinc stearate.

Finally, 10 tablets of 1.5 g are obtained having a geometric density of 5.9 g. cm ³ .

 A first batch of pellets is subjected to a sintering cycle in an atmosphere comprising a mixture of argon and hydrogen (4% by volume of hydrogen) and a moisture content of 1200 vpm in which a temperature step 1700 ° C is applied for 4 hours.

 A second batch of pellets is subjected to the same sintering cycle.

 e) Results

 The pellets obtained according to the different variants mentioned in paragraphs a), b), c) and d) were analyzed to determine their density, the ratio O / M (M being the elements other than oxygen), the porosity rate. open and compliance with a thermal stability test.

The determination protocols for these parameters are as follows. * Determination of apparent density

 The determination of the apparent density of the cylindrical samples is carried out either from geometric measurements or by immersion in a liquid.

 In the first case, measurements of the mass, the height and the average diameter of the sample make it possible to determine the apparent density. The precision obtained by this technique is of the order of 1%. This density measurement is applied to compacted materials after pressing.

 The immersion technique, also called hydrostatic measurement, is more precise. It consists in impregnating the sample with a wetting liquid of known temperature and density. This impregnation is carried out under a primary vacuum to ensure complete plumping of the open porosity. The liquid used in our case is bromobenzene. The density of the ceramic is obtained using the formula:

P =

m ₂ - m ₃

 with:

 > mi: mass of the dry sample weighed in the air

 > rri2: mass of the totally impregnated sample weighed in the open air at 22 ° C

 > rri3: mass at 22 ° C of the impregnated sample totally immersed in bromobenzene pb: density of bromobenzene at 22 ° C. The accuracy of the measurement on the bulk density is 0.2%. This density measurement is applied preferentially for the pellets after sintering.

The theoretical density is an intrinsic parameter to a material evaluated here by its mesh parameter, its isotopy, its oxygen stoichiometry. * Determination of the ratio O / M

In order to determine the O / M ratio (M corresponding to the U and Pu elements), a suitable oxido-reduction thermochemical treatment is carried out. This treatment is carried out under hydrogenated argon at 5% H ₂ saturated with water at a temperature of the order of 950 ° C., which makes it possible to access a very small uncertainty on the value of the ratio O / M (typically less than to 0.003).

 * Thermal stability test

The stability cycle is defined by a 24 hour treatment at 1700 ° C under a neutral atmosphere (argon). At the furnace outlet, weighing and metrology of the pellets and / or density are carried out by hydrostatic weighing. There is conformity to the thermal stability test when the density variation is less than 1.9% in densification and without swelling after the test.

The results are shown in the following table.

Pellets Density Ratio O / M Porosity Test Rate Open Thermal Stability

Paragraph a) 10.65 g.cnr ³ , 2,000 <1% Complies with either

 density

 relative of 96

 % (the density

 theoretical

 being 11.1)

Paragraph b) 10.60 g.cnr ³ , 2,000 <1% Complies with either

 density

 relative of

 96.4% (the

 density

 theoretical

 being 11.0)

Paragraph (c) 10.73 g.cnr ³ , 2,000 <1% Complies with either

 density

 relative of

 96.7% (the

 density

 theoretical

 being 11.1)

Paragraph d) 10.65 g.cnr ³ , 2,000 <1% Compliant be one

 density

 relative of

 96.8% (the

 density

 theoretical

 being 11.0)

The theoretical densities indicated in the table are determined according to the method of the descriptive part.

Thus, for the pellets prepared according to the modalities of paragraph a) with a plutonium content of 29 at%, a molar mass M (U, Pu) of 238.5 g. mol ¹ and a stoichiometric deviation of 0, the theoretical density is calculated according to the following equation:

Theoretical Density = 1.6598 * 10 ⁶ * [((4 * M (U, Pu) + 64 * (2-x))) / (547.0-7.4y + (30, 1 + 11, 0y) * x) ³ ],

 that is 11.1.

For pellets prepared according to the modalities of paragraph b) with a plutonium content of 10 at%, a molar mass M (U, Pu) of 238.2 g. mol ¹ and a stoichiometric deviation of 0, the theoretical density is calculated according to the following equation:

Theoretical Density = 1.6598 * 10 ⁶ * [((4 * M (U, Pu) + 64 * (2-x))) / (547.0-7.4y + (30, 1 + 11, 0y) * x) ³ ],

 that is 11.0.

 It emerges, in particular, that all the pellets have a relative density greater than or equal to 96%.

It can thus be concluded that the use of U3O8 as a source of uranium does not in any way affect the density of the pellets obtained. density values obtained with U3O8 being equivalent to those obtained for pellets made solely with UO2, as a source of uranium.

 Thus, thanks to the method of the invention, it is possible to access the advantages arising from the use of U3O8 powder while accessing dense pellets of fuel using this powder.

Claims

A process for preparing pellets of a mixed dense fuel comprising uranium dioxide UO _2, plutonium oxide PuO ₂ and optionally at least one minor actinide oxide (s) or comprising a solid solution of uranium dioxide, plutonium and possibly at least one minor actinide comprising successively the following steps: a) a step of co-micronization of a mixture of powders comprising a powder of uranium oxide (s), all or part of this powder consists of U3O8 triuranium octaoxide, a plutonium oxide powder; PuO ₂ and optionally a minor oxide (s) oxide (s) powder (s);  b) a step of compacting the mixture of powders in the form of pellets;  c) a sintering step of the pellets obtained in b). 2. Method according to claim 1, wherein the uranium oxide powder (s) is composed exclusively of a U3O8 triuranium octaoxide powder or a mixture of U3O8 triuranium octoxide and carbon dioxide. uranium U0 ₂ . 3. A process according to any one of the preceding claims, wherein the co-micronization step comprises subjecting the mixture obtained in a) to ball milling for a time sufficient to obtain the micronized mixture. 4. Method according to any one of the preceding claims, furthermore comprising, after the co-micronisation step, a sieving step and more specifically, a sieving-forcing step, 5. Method according to any one of the preceding claims, further comprising, after the step of co-micromisation and the possible sieving step, a step of adding to the micronized mixture and optionally sieved a powder of dioxide uranium U0 ₂ . The method of any of the preceding claims, wherein the pelletizing step comprises subjecting the powder mixture to unixaxial pressing. 7. A process according to any one of the preceding claims, wherein the sintering step is carried out in an atmosphere consisting of a mixture comprising a reducing gas, optionally oxygen and / or water vapor. 8. Process according to any one of the preceding claims, comprising, before step a), a step of preparing the powder mixture by contacting a powder of uranium oxide (s), all or part of this powder consists of triuranium octaoxide U3O8, a plutonium oxide powder Pu0 ₂ and optionally a powder (s) actinide (s) minor (s).