FR3072308B1 - DEVICE AND METHOD FOR CRYOGENIC MILLING WITH CRYOGENIC GAS MILLING MEDIA SOLIDIFIED - Google Patents

Abstract

The main object of the invention is a crushing device (1) cryogenic powder (P), characterized in that it comprises: a grinding vessel (2) heat-insulated under pressure, comprising the powder charge (P) grinding media (Bo) in the form of solidified cryogenic gas and liquefied gas (GL); a drive shaft (30) for rotating the milling vessel (2); a laser particle size analyzer (3) associated with a backlighting device (4); a generation and feed system (5) in suspension of powders / liquefied gas; a data processing and control system (6) of the grinding device (1), the laser granulometric analysis device (3) being connected to said processing and control system (6), a device for producing media of grinding (8) from solidified cryogenic gas in the form of cryogenic gas, and a device for admitting liquefied gas and carbon dioxide.

Description

DEVICE AND METHOD FOR CRYOGENIC MILLING WITH SOLIDIFIED CRYOGENIC GAS CRUSHING MEDIA

DESCRIPTION

TECHNICAL AREA

The present invention relates to the field of cryogenic grinding of powders, in particular actinide powders comprising in particular uranium oxide powders (UO 2) and / or plutonium oxide powders (PuO 2), for obtaining controlled particle size powders and / or mixtures with controlled particle size and increased homogeneity compared to granular media obtained by grinding and / or dry mixing. The invention is preferably used for the manufacture of nuclear ceramic parts, namely the manufacture of nuclear fuel, in particular nuclear fuel pellets. The invention thus proposes a device for the cryogenic grinding of powders comprising grinding media in the form of a solidified cryogenic gas, as well as an associated grinding method.

STATE OF THE PRIOR ART

Grinding operations are relatively common in the industry in many areas. Depending on the applications, it is possible to use grinders that can be very different depending on the loads to be milled and their ability to fragment such as, for example, knife mills, flail mills, hammers, rollers, ball mills, jet mills. air, among others.

These different devices exploit four major mechanisms inducing the fragmentation of the charge at the origin of the size reduction of the particles constituting the charge to be ground, namely: impaction; shearing; the compression ; and attrition.

In the context of the manufacture of ceramic parts such as nuclear fuels, ball mills are conventionally and industrially used for co-grinding powders of uranium and plutonium oxides. Documents such as the "Process engineering of size reduction: lease milling" monograph, Austin LG et al., AIME, 1984 or GB 189626501A, US 6,473,336A, US 2,235,985A and US 2,041,287A describe several versions of these mills. balls that can be used for this type of application.

It should nevertheless be noted that these devices suffer from several drawbacks, and in particular: the pollution of the load by the abrasion induced by the balls and the grinding tank (to minimize this effect, it is potentially used uranium balls in the nuclear industry); difficulty in controlling the grinding process in a continuous manner to avoid phenomena such as over-grinding or reagglomeration; relatively long grinding kinetics inducing times of operation which are binding (several hours of grinding necessary for a few hundred grams to tens of kilograms to be treated in the case of nuclear fuel operations) otherwise the crushed load may not not be sufficiently homogeneous and / or crushed; and the difficulty of being able to work continuously without the risk of accumulation and clogging of the solid filler, especially when it is not very flowable, which is common in the case of uranium oxide and plutonium powders.

Evolutions have therefore been proposed according to the applications with, in particular, US Pat. No. 4,715,547 A implementing grinding in the liquid phase. It should be noted that this type of device does not answer either the problem posed because the liquids envisaged are aqueous or organic phases generating risks (including criticality and radiolysis) and very constraining effluents for certain loads to be milled (powders of fissile material, reacting with water or some organic, ...).

Knowing that to minimize the energy to be used during grinding, it is also possible to make the material fragile by cooling it to relatively low temperatures, that is to say classically below -50 ° C., certain known grinding devices implement a cryogenic cooling of the feedstock to be milled before treating it, as described in the application WO 2008/110517 A1. It should nevertheless be noted that this type of cryogenic mill does not make it possible to have a fast grinding as is attainable in liquid way. Other devices such as those described in US 3,734,412 A, EP 2,535,114 A1, GB 1,508,941 A1 or EP 1405,927 A1 implement suspensions consisting of a liquefied gas and a feedstock to be milled favoring efficiency. grinding and mixing. Nevertheless, these devices use grinding media that induce a risk of increased pollution or that do not include an element to optimize their control and ensure a true continuous driving. Finally, the quantities of liquid nitrogen used per unit of load to be milled appear important, which leads to increased cost and risk levels.

STATEMENT OF THE INVENTION

Accordingly, there is thus a need to provide a new type of device and method cryogenic crushing powders, especially actinide powders. In particular, there is a need to have a grinding device containing a liquefied gas phase allowing suspension, or at least increased stirring (compared to what is achievable in the dry process) or even a fluidization, of the granular filler for grinding, or grinding and mixing.

Beyond the reduction in the particle size of the charge, there is more precisely a need to be able to concomitantly: - to minimize the grinding energy to be applied for this operation, - to improve the homogeneity of the powder mixtures to be ground, - to minimize the quantities of liquefied gas to be used, - limit the pollution of the powders to be ground, - limit the heating of the powders, - avoid the generation of liquid effluents (aqueous or organic), - limit the risks of criticality when placing use of fissile material, - increase the crushing kinetics, - ensure a continuous and controlled operation of the grinding of the load. The object of the invention is to at least partially remedy the needs mentioned above and the drawbacks relating to the embodiments of the prior art, in particular those mentioned above. According to one of its aspects, the subject of the invention is a device for the cryogenic grinding of powders, in particular actinide powders, characterized in that it comprises: a grinding vessel insulated under pressure, in particular between 1 and 40 bar absolute, comprising the charge of powders to be milled, grinding media in the form of solidified cryogenic gas and liquefied gas, - a drive shaft for rotating the milling vessel, - a particle size analyzer laser associated with a backlighting device, a system for generating and supplying powder suspensions / liquefied gas, a data processing system and controlling the grinding device, the laser particle size analysis device being connected to said processing and control system, - a device for producing grinding media from solidified cryogenic gas, - a device for the intake of liquefied gas and carbon dioxide.

Advantageously, the present invention can rely on several elements of the grinding device, namely: a heat-insulated grinding vessel and pressurized; a device for limiting the rise in pressure; grinding media (balls, balls, rollers, etc.) optionally consisting of dry ice containing or not a portion of the load to be milled to avoid any pollution of the load to be milled; liquefied gas dosers and the solid charge to be introduced into the tank; a level probe measurement of the liquefied / solid gas suspension; a crusher control system and feeding and emptying devices in connection with the crusher; a pelletizing nozzle of the grinding vessel connected to a feed of carbon dioxide (CO2) under pressure; a device for generating pellets loaded with powders for grinding.

Among the technical effects advantageously exploited in the present invention, mention may be made of grinding without pollution of the powders by using a grinding configuration with grinding media in the form of beads, balls or related forms, consisting of solidified cryogenic gas, whether or not the powders must themselves be crushed. The use of solidified gas, such as dry ice, does not induce any detrimental pollution because the subsequent sublimation of the solidified gas, for example CO 2, is rapid and complete under normal conditions of temperature and pressure. The incorporation of powders for grinding in this dry ice is mainly used, optionally, to increase if necessary the density of the grinding media, which gives more efficient grinding if necessary.

The solid phase of the liquefied gas constituting the grinding media is chosen so that it is denser than the liquid phase constituting the suspension. This is the case, for example, of CO 2, the solid phase of which is significantly denser than liquid nitrogen, about a factor of two, the liquid nitrogen having a density of about 0.8 and the solid CO 2 a density of about 1.5. The incorporation of the load in dry ice media also makes it possible to increase this differential, as shown in FIG. 9 described below.

It should be noted that, in the usual way, a cryogenic fluid here designates a liquefied gas kept in the liquid state at low temperature. This liquefied gas is chemically inert under the conditions of implementation of the invention for the powders to be mixed / milled.

The grinding device according to the invention may further comprise one or more of the following characteristics taken separately or in any possible technical combinations.

The grinding tank can be oriented vertically relative to the horizontal plane on which it is arranged, or even in an inclined manner, for example by means of a rocker, depending on the energy to be imposed on the medium to be crushed. .

The grinding tank may advantageously consist of a sieve.

The grinding media may consist of solidified cryogenic gases and the charge to be milled.

The grinding media may in particular comprise balls, balls and / or rollers, among others.

The speed of rotation of the motor shaft can oscillate between a few tens of revolutions per minute to a few hundred revolutions per minute.

The data processing and control system of the grinding device can process data derived in particular from: the pressure measurement prevailing within the grinding vessel; the measurement of the quantities of materials introduced by the solid and liquefied gas dosers; the particle size of the load suspended and milled during operation.

The data processing and control system of the grinding device can in particular ensure by control-command action: the constancy of the ratio between quantity of liquid gas and solid charge; the adaptation of the grinding parameters, such as rotational speed and grinding time, or even inclination, according to the instructions and measures mentioned above.

The grinding media in the form of a cryogenic gas are advantageously dry ice balls, whether or not loaded with materials to be milled in order to possibly increase the density of the balls without introducing pollution at the level of the powders.

The grinding media production device can thus comprise a carbon dioxide snow generator by expansion of carbon dioxide (CO2), liquid or gaseous, and compaction thereof within a compaction system giving the desired shape of the media. grinding.

The compaction system can in particular be constituted by a roller-type device capable of applying the compaction pressure. In addition, in order to incorporate a portion of solid filler within the balls, the powders to be grinded can be suspended with liquid carbon dioxide (CO 2) before being expanded.

The grinding tank may include a carbon dioxide (CO 2) inlet for forming a dry ice layer on the inner walls in the tank, as shown in Figure 4B described below. This can be liquid and / or gaseous, which can be introduced at different temperatures, either to generate dry ice, or to sublimate it at the end of the operation. Note that the grinding tank can be equipped with a sieve basket, as shown in Figure 2 described below, to allow increased wetting of the liquid carbon dioxide C02 can be introduced before grinding and charging to form after cooling, by introducing liquefied gas colder such as for example liquid nitrogen, a layer of dry ice adhering to the wall and thus constituting a protective layer of the tank to prevent pollution of the load to grind by abrasion of the and also to prevent the passage of fine powder particles through the mesh of the sieve. After sublimation / evaporation of the solid phase of cryogenic gas closing the mesh of the sieve basket, a drain can be operated of the particle size of interest. Subsequently another cycle can be operated.

The grinding device may further comprise a relief valve to allow to limit the pressure within the grinding vessel.

In addition, the system for generating and supplying powder suspensions / liquefied gas, may comprise: - a doser of powders and liquefied gas, - a recirculation loop suspensions after introduction into the grinding tank through a recirculation valve and a pump for reinjecting the suspensions into the grinding chamber.

The grinding device may also include a device for electrostatically charging the powders to be grinded.

The grinding device may further comprise a piezoelectric device and / or a sonotrode for the application of mechanical and / or acoustic vibrations when necessary.

In addition, another subject of the invention is, according to another of its aspects, a process for the cryogenic grinding of powders, characterized in that it is implemented by means of a device as defined above.

The method may implement a volume amount of liquefied gas between the porosity of the bed of powder to be ground, in particular of the order of 40 to 50% of the apparent volume of the powder, and the apparent volume of the bed of powder.

The process can be applied to actinide powders.

The process may comprise the following steps: - optionally, introduction of pressurized or liquefied carbon dioxide into the grinding vessel to form a layer of dry ice, in order to limit any pollution of the feed to be grinded by abrasion of the vessel ( this is a similar phase to a culottage), - introduction of grinding media in the form of dry ice pellets into the grinding tank, - dosing of the feed of powders to grind, - introduction of the feedstock. grinding powders dosed in the grinding tank, - introduction of a specified amount of liquefied gas into the grinding tank, in order to guarantee an increased mixing (compared with grinding in the dry phase) or even a fluidization of the solid charge to grind - rotating the grinding tank and optionally inclining the grinding vessel in predetermined cycles, - monitoring the parameters of the grinding device, in particular rotational speed, degree of inclination, adjustment of the quantity of liquid and solids, particle size of the suspended charge, and control of these parameters by means of a control-command module, - stop of the grinding on crossing a threshold of one or more target parameters, in particular particle size of the suspended solid filler, - ambient temperature of the grinding vessel for evaporating the liquefied gas phase, - separation, for example by sieving, or sublimation of the grinding balls and the crushed load.

When a need to increase the density of the grinding balls exists, the step of introducing grinding media in the form of dry ice balls into the grinding tank can comprise the following steps: - dosage of a specified portion the charge to be ground, - mixing the metered charge with a given volume of liquefied carbon dioxide, - introducing the charge / carbon dioxide suspension into an expansion tank to form a loaded dry ice at the specified solids content, - Pressing the carbon dioxide snow thus loaded on a cavity, or mold, spherical by means of a roller press, - introduction in the crushing tank of charged carbon dioxide pellets thus obtained.

The device and the grinding method according to the invention may comprise any of the features set forth in the description, taken alone or in any technically possible combination with other characteristics.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be better understood on reading the detailed description which follows, non-limiting examples of implementation thereof, as well as the examination of the figures, schematic and partial, of the accompanying drawing, in which: - Figure 1 schematically illustrates an example of grinding device according to the invention, - Figure 2 schematically illustrates an example of grinding tank for a grinding device according to the invention such as that FIG. 3 illustrates the variation of the parameter U, ie the ratio between the apparent volume of the feed to be ground and that of the grinding media, for U <1, U = 1 and U> 1 in the tank. 4A and 4B illustrate, respectively in the case of a sieve tank and in the case of a full tank, two types of dry ice pelletization obtainable by the process according to the invention. - Figures 5 and 6 illustrate different configurations of grinding devices according to the orientation of the axis of rotation of the grinding chamber for a device and a method according to the invention; FIG. 7 illustrates the evolution of the energy as a function of the size / granulometry of the powders to be ground, - figure 8 represents the centrifugal force and the force of gravity applied to a ball during the grinding operation, - figure 9 represents the evolution of the density of the dry ice balls. as a function of the volume fraction of solid (case of plutonium oxide) in the balls, - FIG. 10 represents the typical evolution of the particle size distribution of powders as a function of the milling time in a ball mill, specifically evolution of the percentage by volume with respect to the size of the powder particles to be considered, FIGS. 11A and 11B illustrate two images taken by scanning electron microscope (SEM) on two scales In the examples of the present invention, FIGS. 12A and 12B illustrate two images taken at the SEM according to two viewing scales of the second PuO 2 powder used in one of the FIGS. Embodiments of the present invention, - Figures 13 and 14 illustrate the microstructure of UPu02 fuel produced by a manufacturing method according to the invention for minimal grinding energy (almost simple mixing) and after completion of a phase thermal densification (cf. FIG. 15); FIG. 15 represents the sintering cycle applied to the UPuO 2 pellet made by a process according to the invention; FIG. 16 allows the comparison of the grinding paths (simple mixing reference path (to turbulence), ball mill method in the dry phase and mill N2L route according to the invention, and - Figure 17 illustrates the impact of the milling of the present invention, compared to other reference channels as mentioned above, on the specific surface of the load to be milled and its granulometry.

In all of these figures, identical references may designate identical or similar elements.

In addition, the different parts shown in the figures are not necessarily in a uniform scale, to make the figures more readable.

DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS

It is noted that in the embodiments described hereinafter, the powders under consideration are actinide powders for the manufacture of nuclear fuel pellets. In addition, the cryogenic fluid considered here is liquefied nitrogen. However, the invention is not limited to these choices.

Referring to Figure 1, there is shown an example of grinding device 1 according to the invention, and with reference to Figure 2, an enlarged view of the grinding chamber 2 of the device 1.

The grinding device 1 thus comprises a grinding tank 2, consisting of a full tank 2 'as in FIG. 2 or optionally a sieve basket 2' as in FIG. 4A, heat-insulated under pressure, comprising the powder filler P to grind, Bo grinding media in the form of dry ice pellets and liquefied gas GL. It is thus formed a ball bed Bo / powder charge P with liquefied gas GL.

In addition, the device 1 comprises a drive shaft 30 for rotating the milling vessel 2 ', and a laser particle size analysis device 3 associated with a counter-lighting device 4. The motor shaft 30 is hollowed out for allow the passage of the backlight device 4, here in the form of an optical fiber connected to the laser particle size analysis device 3.

In addition, the grinding device 1 comprises a system for generating and supplying 5 suspensions of powder / liquefied gas, and a data processing and control system 6 of the grinding device 1, the laser particle size analyzer 3 being connected to this processing and control system 6.

More specifically, the system for generating and supplying powder suspensions / liquefied gas comprises: a proportioner 20 of powders P and of liquefied gas GL, and a recirculation loop 24 of the suspensions after introduction into grinding tank 2 by through a recirculation valve 11 and a pump 26, or circulator, for reinjecting the suspensions in the grinding tank 2.

In addition, in Fig. 1, the reference CP denotes a pressure sensor and the reference numeral 10 designates a relief valve.

Furthermore, the device 1 comprises a device for producing grinding media 8 in the form of solidified cryogenic gas. This grinding media production device 8 comprises a carbon dioxide snow generator 31 by expansion of carbon dioxide, preferentially liquid, and compaction of the dry ice within a compaction system 32 giving the desired shape of the grinding media Bo . This is done by means of a powder mixer P and CO 2 L liquid carbon dioxide, then passing through a dispensing nozzle 34 and into the compaction system 32, preferably consisting of a roller press device. Bo balls are then introduced into the tank 2 through an admission of IBO balls.

In addition, the grinding tank 2 comprises a carbon dioxide inlet AD for forming a dry ice layer to the right of the inner wall of the grinding tank 2, if necessary.

Furthermore, it should be noted that, in FIG. 2, the reference 40 designates bearings, the reference 41 designates the heat insulation, and the reference 42 designates a recirculation pipe connected to the valve 11.

In addition, the liquefied gas GL present in the grinding tank 2 may advantageously be present in a quantity limited to the volume allowing just exceeding the outcrop of the bed of the load to be ground, by integrating the volume of the grinding media Bo. The ratio between the apparent volume of the feedstock to be milled and that of the milling media can then be given by the parameter U. FIG. 3 illustrates the variation of the parameter U, ie the ratio between the apparent volume of the feed to be milled and that grinding media, for U <1, U = 1 and U> 1 in the grinding vessel. In optimized operation, the value of this parameter U can oscillate between 0.6 and 1.2.

Figures 4A and 4B illustrate, respectively in the case of a sieve tank and in the case of a full tank, two types of dry ice pelletization obtainable by the process according to the invention.

Thus, in FIG. 4A, at the level of the wall of the tank 2, visible on the enlarged portion of the left part, a liquid carbon dioxide film FLCo 2 appears between portions of Tarn sieve, which is transformed at the end of an integer number of cycles (alternating admission of CO 2 and liquefied gas) into a layer of carbon dioxide C 2 O 2

FIG. 4B also shows the layer of carbon dioxide C 2 formed on the wall of the tank 2, after pelleting.

Figures 5 and 6 show two types of grinding devices 1 according to the invention. Thus, the grinding device 1 may be in the form of a ball mill, as shown in FIG. 5 with a vertical plane of rotation, or in the form of a planetary mill, as illustrated in FIG. 6 with a plane of horizontal rotation, they are nevertheless designed so that they can contain a liquefied gas.

To do this, they are advantageously insulated and potentially equipped with a relief valve 10 to adjust the operating pressure. In addition, optionally, in order to limit the pollution of powders P to grind, Bo grinding media consist of GS solidified gas, for example carbon dioxide CO2, whose density is greater than the density of liquefied gas GL used in the grinder.

In FIGS. 5 and 6, R represents the direction of rotation, Bo the dry ice balls, which may contain part of the powder P to grind / agglomerate, GS represents the solidified gas, GL represents the liquefied gas, GL + P represents the cryogenic suspension or slip containing the liquefied gas bath GL and the powder bed P, and gP represents the powder grains.

EXAMPLE OF IMPLEMENTATION In general, in order to limit the evaporation of the liquefied gas GL, the devices, for example the grinding device 1, will be designed with a concern for thermal insulation (dewar tank, specific heat insulation, etc.). ) and powders P to grind can advantageously and before contacting with the liquid nitrogen be cooled. In addition, this can also be done to avoid calefaction phenomena. To do this, ideally the temperature of the powders P would be decreased below the Leidenfrost temperature of the liquefied gas used, ie of the order of -73 ° C for liquid nitrogen, as described in "Inertial drops: from the staggering with spreading ", AL HIMBERT BIANCE, Thesis of doctorate of Paris VI, 2004.

Moreover, in order to size the volume of the grinding chamber 2, it can be advantageously sought an equidistribution between the volume representing the grinding media Bo, the volume of the powder P to be grinded and the porosity of the P powder bed. the following parameters: roller filling rate (J), powder filling rate (fc), and powder-ball (U) filling factor, the following ranges can be retained: J is between 10% and 60% , fc is between 10% and 50%, and U is between 0.6 and 1.4.

In order to size a ball mill, it is possible to retain the bond law describing the power to be applied to the load to be milled from an average diameter DI to an average diameter

where: W1 represents the work index (in kWh / t); DI represents the diameter before fractionation; D2 represents the diameter after fractionation.

Classically, W1 varies between 4 and 40 (depending on the material to be grinded which can be more or less friable / hard).

More generally, the law to be retained connecting the grinding energy and the size reduction of the material to be milled is a function of the nature of the materials but also of the area of the particles to be milled (average diameter at the beginning and end of grinding operation) .

FIG. 7 illustrates the evolution of the energy E in kWh / t as a function of the size of the particles to be milled in μιτι, each time associating a specific law whose

The general form is dE = - K.dx / xn, the exponent n taking the values 1, 3/2 and 2 respectively for the laws of Kick (III), Bond (II) and Von Rittinger (I).

For the adjustment of the operating parameters of the ball mill 1, the speed of rotation of the mill is conventionally set between 60 and 80% of the critical speed (Vc), and less than 90% of this speed. This speed Vc corresponds to the equilibrium of the centrifugal forces and of the gravity applied to the grinding balls, as illustrated by FIG. 8 which represents the centrifugal force Fc and the force of gravity applied to a ball B.

In fact, this speed is given by the following expression:

with:

Di: inner diameter of the mill 1.

In the case of a mill incorporating media with dry ice containing itself the solid to grind, to increase the relative density compared to the liquefied gas used as grinding medium, the density of the balls pB is directly related to the rate of incorporation of the solid ε:

FIG. 9 represents the evolution of the density pB of the balls B of dry ice as a function of the volume fraction of solid ε (case of plutonium oxide) in the balls. In general, the grinding in the liquid phase can be considered more effective than in the dry phase insofar as it can promote the disagglomeration of powders P and allows to keep the fines in suspension, which induces a targeted grinding of large particles. Furthermore, in the case of the present invention, the use of liquefied gas GL mechanically fragile (because of the strong cooling imposed) the materials to be milled, which makes the grinding operation even more efficient.

In fact, knowing moreover that a suspension generates turbulence and a mixing entropy higher than what can be obtained in the dry phase for a

the same energy transmitted to the mill, it is possible to estimate that the grinding time required in the liquid phase is less than that required to apply in the dry phase.

In the context of grinding by ball mill or planetary mill, the evolution curves of the particle size distribution can be given in FIG. 10 which represents the evolution of the particle size distribution of powders as a function of the grinding time in a ball mill. , or precisely the evolution of the volume percentage% vol with respect to the size Ta, the curves tl, t2 and t3 respectively representing a time t = 10.0xh, a time t = xh and a time t = 0. Note that In the liquid phase, the energy transmitted to the load to be ground is greater than that transmitted in the dry phase. Energy saving can reach nearly 30%.

Example 1

Taking all these considerations into account, the following grinding conditions, shown in Table 1, were retained for grinding U02 / CeO 2 powders:

Table 1

The characteristics of the initial powders and after grinding are given in FIG. 16, which respectively allows the comparison of the grinding paths (single-melting reference path, dry-phase ball milling path and N2L grinding mill).

according to the invention), and in FIG. 17 which gives the evolution of the specific surfaces and the particle size, illustrating the impact of the grinding of the present invention, compared with other reference channels, on the specific surface of the load. to grind and its granulometry.

The grinding was carried out for 30 minutes and this was compared to a grinding stage carried out in the solid phase or to a simple parturbulat mixing step. Example 2

Another embodiment was made with UO 2 and PuO 2 powders. For this application, the grinding energy applied is close to zero to estimate the effect of a quasi simple mixture.

FIGS. 13 and 14 illustrate the microstructure of an UPuO 2 fuel produced by a manufacturing method according to the invention, ie the microstructure of the final material produced from the process according to the invention, a cryogenic pathway in mild grinding for 1 min with a mass loading of 50%, for a minimum energy applied to the PuO 2 and UO 2 powder used to produce a nuclear fuel.

The sintering was carried out with the following conditions in connection with FIG. 15 representing the sintering cycle applied to the UPuO 2 pellet made by the process according to the invention, T denoting the temperature in ° C, and t the time in hours.

The applied pressure was provided by a uniaxial press rather than casting / die casting. The pressure applied was between 200 and 300 MPa. The mass proportion of liquid nitrogen compared to the powder was about 50% by weight.

The powders used for this manufacture were as follows:

Uranium source: U02 powders from a dry synthesis process and 8% by weight of crude U308 (1.8 mV1 specific surface) · Figures 11A and 11B illustrate two images taken under a scanning electron microscope (SEM) ) according to two scales of visualization of this powder.

Plutonium source: plutonium oxide powders; characteristics: specific surface area of 5.7 m2 / g. FIGS. 12A and 12B illustrate two images taken at the SEM according to two viewing scales of this powder.

Of course, the invention is not limited to the embodiments which have just been described. Various modifications may be made by the skilled person.

Claims (13)

1. Crushing device (1) cryogenic powder (P), characterized in that it comprises: - a grinding vessel (2) insulated under pressure, comprising the powder load (P) to grind, grinding media (Bo) in the form of solidified cryogenic gas and liquefied gas (GL), - a driving shaft (30) for rotating the milling vessel (2), - a laser particle size analyzer (3) associated a backlighting device (4), - a generation and feed system (5) in powder / liquefied gas suspension, - a data processing and control system (6) of the grinding device (1), the laser granulometric analysis device (3) being connected to said processing and control system (6), - a device for producing grinding media (8) from solidified cryogenic gas, - a gas admission device liquefied and carbon dioxide. 2. Device according to claim 1, characterized in that the grinding tank (2) consists of a sieve. 3. Device according to claim 1, characterized in that the grinding media (Bo) consist of solidified cryogenic gas and the load to grind. 4. Device according to one of the preceding claims, characterized in that it further comprises a discharge valve (10). 5. Device according to any one of the preceding claims, characterized in that the generation system and feed (5) in suspensions powders / liquefied gas, comprises: - a doser (20) of powders (P) and gas liquefied (GL), - a recirculation loop (24) suspensions after introduction into the grinding tank (2) through a recirculation valve (11) and a pump (26) for reinjecting the suspensions in the grinding vessel (2). 6. Device according to any one of the preceding claims, characterized in that the grinding media (Bo) in the form of cryogenic gas are dry ice balls. 7. Device according to claim 6, characterized in that the device for producing grinding media (8) comprises a carbon dioxide snow generator (31) by expansion of carbon dioxide, liquid or gaseous, and compaction of the dry ice at within a compaction system (32) giving the desired shape of the grinding media (Bo). 8. Device according to claim 7, characterized in that the compaction system (32) is constituted by a roller press type device. 9. Device according to any one of the preceding claims, characterized in that the grinding tank (2) comprises a carbon dioxide inlet (AD) for forming a layer of dry ice on the inner walls in the grinding vessel. (2). 10. Process for the cryogenic grinding of powders (P), characterized in that it is implemented by means of a device according to any one of the preceding claims. 11. Grinding process according to claim 9, characterized in that it implements a volume quantity of liquefied gas (GL) between the porosity of the bed of powder to be ground, in particular of the order of 40 to 50% of the apparent volume of the powder, and the apparent volume of the powder bed. 12. Grinding process according to claim 9, characterized in that it is applied to actinide powders. 13. Grinding method according to claim 9, characterized in that it is implemented by means of a device according to any one of claims 5 to 8, and in that it comprises the following steps: - optionally introduction of pressurized or liquefied carbon dioxide into the grinding chamber (2) to form a dry ice layer; - introduction of grinding media (Bo) in the form of dry ice pellets into the grinding chamber (2) ), - dosing of the powder charge (P) to be ground, - introduction of the powder charge (P) to be milled in the grinding tank (2), - introduction of a specified quantity of liquefied gas (GL) in the grinding tank (2), - rotating the grinding tank (2) and optionally inclining the grinding tank (2) according to predetermined cycles, - monitoring the parameters of the crushing device (1) and controlling of these parameters by means of a control-command module, - stop of grinding on crossing a threshold of one or more target parameters, - bringing the grinding chamber (2) to ambient temperature to evaporate the liquefied gas phase (GL), - separation or sublimation of the grinding balls (Bo ) and the crushed load.