COMPOSITION FILLED WITH ACTINIDE POWDER AND AROMATIC POLYMER AND/OR PMMA

Abstract

A composition filled with actinide powder, comprising an organic matrix and an actinide powder or a mixture of actinide powders, comprises at least: a plasticizer comprising an alkane whose longest radical chain comprises at least a few tens of carbon atoms and is in a volume content of between 20% and 70% of the total volume of the organic compounds alone; a binder comprising at least one aromatic polymer and/or polymethyl methacrylate and which is in a volume content of between 20% and 50% of the total volume of the organic compounds alone; a dispersant comprising a carboxylic acid or salts thereof, the volume content of which is less than 10% of the total volume of the organic compounds alone; said actinide powder or said mixture of actinide powders represent between 40% and 65% of the volume of the filled matrix.

Description

The field of the invention is that of compositions based on actinide powder, and which have the advantage of being injectable since they allow a rheology that is compatible with injection systems. One of the main fields of application may concern (but not exclusively) the manufacture of nuclear fuels (or more generally of actinide-based components/materials).

More generally, the present invention relates to the production of components with more or less complex shapes containing actinides, whether in metallic, oxide, carbide or nitride form. The standard and industrial production of fuel currently and mainly proceeds via exploitation of powder metallurgy (based on the pressing of the constituent powders of components/fuels to be formed and the sintering of the compacts obtained after pressing).

However, the use of powder metallurgy induces a certain number of drawbacks and obstacles when it is desired to make components of complex shapes or when it is desired to have very good control of the size of the components (and all the more so when these components are of complex shapes) to be produced, without the need for a rectification step.

Currently, the manufacture of nuclear fuels (actinide compounds) is typically performed via standard processes based on the metallurgy of powders. Two major steps are exploited to do so:

• • the forming of the constituent powders of the fuel (pressing with a potential prior preparation of the powders);
  • the sintering of the compact obtained after the powder pressing step.

This type of process is proven and industrial, but induces at least four types of drawbacks:

• • the difficulty in controlling the shape of the components obtained from the sintering, which is itself conditioned by the control of the granular stack in the pressing molds (linked to the homogeneity of distribution of the material). Now, since actinide powders are, for some of them, relatively cohesive, this control is not trivial and usually requires preparation of the powders prior to their forming. For certain uses, the geometrical specifications impose rectification of the combustible objects obtained by metallurgy of the powders;
  • this preparation of the powders often induces powder dissemination, which leads to an increase in retention in the confinement chambers of the manufacturing process. The result of this is an increased radiological risk;
  • the impossibility of obtaining components/fuels whose shape is complex (i.e. any shape) and/or not axisymmetric since the forming is performed industrially by uniaxial pressing;
  • the need to render the confinement chambers containing the actinide powders inert so as to limit the risks of pyrophoricity (when the actinides are in metallic or carbide form notably).

To act on all of these drawbacks, the Applicant proposes filled compositions that make it possible to use a process known as powder injection molding (PIM).

However, in order for this type of process to be operative for the use of actinide powders, it is necessary to have available a fluid organic matrix consisting of organic components, generally based on polymers that allow good (in the sense of homogeneous distribution) incorporation of the powder into said organic matrix. This organic matrix must satisfy all of the objective functions and constraints imposed by this type of process in the light of the specificities of the nuclear materials to be used and of the specifications of the targeted fuels.

At the present time, no formulation of fluid organic matrix for preparing actinide components is mentioned in the technical and scientific literature. This may notably be explained by the number of constraints/criteria weighing on a filled organic matrix. These are to be taken into account for the use of actinide powders which have specific properties, and under satisfactory conditions (i.e. conditions making it possible to obtain components whose characteristics are at least equivalent to those obtained by powder metallurgy).

Thus, to satisfy this general problem of manufacturing actinide fuels/components via the PIM process in a satisfactory manner (i.e. in a manner making it possible to obtain components whose characteristics are at least equivalent to those obtained by powder metallurgy), it is necessary for the envisioned filled matrix to concomitantly satisfy the following criteria:

• • an actinide powder filler content in the filled matrix that is sufficient to obtain after debinding granular stack densities of greater than 40%. (It is recalled that the debinding operation consists in removing the constituent carbon-based compounds of the composite filler. This debinding may be performed conventionally via thermal action to volatilize the filler.)

Specifically, when the PIM process is applied to actinide powders whose purpose is to result in objects whose characteristics are similar to those obtained by powder metallurgy, it is necessary after the step of debinding of the formed polymers to result in granular stacks that need to be cohesive, i.e. to keep their shape, and whose density is equivalent to that obtained by uniaxial powder pressing (powder metallurgy). A powder may be considered as cohesive if it notably satisfies the definition of Geldard (class C) or has a Hausner coefficient of greater than 1.4, “Techniques de l'ingenieur mise en forme des poudres, J 3 380-1”. To achieve this minimum filler content value, it is necessary for the powder, especially if it is cohesive, as is conventionally the case for actinide powders (and notably the oxides thereof), to be deagglomerated during the blending/preparation of the filler. This prerequisite is not trivial per se for the following reasons:

• • the injectability of the filler: despite the filler content criterion mentioned above, it is necessary to be able to use the filled matrix in a mold (or through a die if extrusion is performed), which imposes a shear viscosity range of between 50 and 10 000 Pa·s during injection with a preferential range of less than 1000 Pa·s for a rate gradient of 100 s⁻¹;
  • the shear-thinning behavior and robustness of the rheological behavior with temperature, or more generally the blending conditions. The rheological behavior of the filler may prove to be prohibitive. Moreover, since actinide powders can be relatively dense, cohesive and polymodal, it is notably necessary to limit any risk of segregation/sedimentation in the filled matrix in the event of poor formulation or mixing condition during the blending;
  • the stability of the properties of the filled matrix, which means the following criteria:
    • physicochemical compatibility, notably immiscibility of the polymers under the working conditions of the PIM process;
    • chemical stability (i.e. absence of notable chemical interaction between the polymers and between the polymers and the actinide powders used). Notably, this criterion demands that the mixture of the constituent polymers of the matrix be stable at least down to the lowest decomposition temperature of the constituents of the matrix of organic compounds.

Given that actinides are moreover compounds that are reputed to promote the decomposition of the constituent carbon-based compounds of the filled matrix (cf. “The activity and mechanism of uranium oxide catalysts for the oxidative destruction of volatile organic compounds”, S. H. Taylor, C. S. Heneghana, G. J. Hutchingsa et al., Catalysis Today, 59:249-259, 2000; A study of uranium oxide based catalysts for the oxidative destruction of short chain alkanes, Applied Catalysis B: environmental, 25:137-149, 2000, S. H. Taylor et al.), this stability criterion of the properties is not trivial to achieve with, notably, either a risk of modification of the degree of oxidation of the actinides in contact with the constituent compounds of the matrix, or a risk of formation of non-debindable carbon-based residues (which may thus be disadvantageous at the end of the manufacture depending on the residual content) during the implementation of the PIM process;

• • a debindable filled matrix without the need to use an aqueous solution and not containing any water. Specifically, the use of actinide powders induces an increased risk of criticality during the use of water and this use moreover induces a generation of liquid effluents that are always difficult to process in a nuclear environment.

Moreover, many actinides intrinsically induce radiolysis phenomena. This induces potential degradation of the constituent organic compounds of the fluid organic matrix which are liable to be prohibitive for the intended use of the product (loss of mechanical strength, swelling, increase of the carbon content, evolution of hydrogen or flammable gas in unacceptable amounts, etc.). Thus, the constituent organic compounds of the organic matrix must be sufficiently resistant to these radiolysis phenomena so as to preserve the acceptability of said organic matrix with respect to the other criteria mentioned previously.

This is why the Applicant proposes compositions filled with actinide powder that are capable of withstanding these radiolysis phenomena and that are compatible with the properties necessary for good behavior in the process for forming actinide powders via the standard PIM process.

Notably, polymers whose monomer comprises an aromatic nucleus are relatively resistant to radiolysis and impart to the formed objects substantial maintenance of their shape. By identifying filled compositions that are also compatible with the abovementioned injection problems, it becomes possible to define filled matrices that are resistant to radiolysis.

One difficulty may remain, concerning the carbon-based residue, which must remain low after the debinding operation due to potential coking of the aromatic nuclei. However, the compositions of the present invention make it possible to avoid this problem on account precisely of the possibility notably of using aromatic polymers which provide this radiolysis protection.

The Applicant has observed that it is also possible to introduce a decoy which may, on the other hand, be relatively sensitive to radiolysis. Such a decoy (it may be a polymer of polymethyl methacrylate type) absorbs the energy induced by the radiation emitted by the actinide powders protecting the other constituent molecules of the organic matrix. However, in order for there not to be, for example, any risk of swelling of the component during the intended radiolysis resistance time (of the order of two days), an excessive decoy content should not be exceeded or a decoy that is nonetheless too sensitive should not be used (i.e. a decoy that has an excessive radiolytic degradation yield with respect to the actinide powder to be incorporated into the organic matrix), this condition may be respected by means of ranges of percentages selected in the present invention.

The above characteristics are to be respected concomitantly with those of the targeted actinide fuels/components which must have characteristics at least equivalent to those that may be achieved by powder metallurgy, i.e., notably:

• • a density equivalent to at least 95% of the theoretical density of the target actinide compound after sintering of the debonded components;
  • homogeneity of the microstructure, i.e. a uniform distribution of grain size and porosity;
  • control of the size, i.e. a variation of the dimensions of the fuel relative to the expected mean dimensions of less than 0.2%, i.e. a value of ±0.012 mm;
  • a residual carbon mass content of less than 0.05% (for the cases of powders other than carbides).

In synthesis, it should thus be noted that all the criteria weighing directly on the filled matrix and those expected on the object that may be achieved by PIM of this same matrix define a specific nontrivial problem that the present invention proposes to solve, given, moreover, that these criteria must be complied with over a sufficient time (which may typically be up to two days at least after the manufacture of the filled matrix) corresponding to the time of possible submission of said matrix to radiolysis before complete debinding in a fuel manufacturing plant.

More specifically, one subject of the present invention is a composition filled with actinide powder comprising an organic matrix and an actinide powder or a mixture of actinide powders, characterized in that it comprises at least:

• • a plasticizer comprising an alkane whose longest radical chain comprises at least a few tens of carbon atoms and which is in a volume content of between 20% and 70% of the total volume of the organic compounds alone;
  • a binder comprising at least one aromatic polymer and/or polymethyl methacrylate and which is in a volume content of between 20% and 50% of the total volume of the organic compounds alone;
  • a dispersant comprising a carboxylic acid or salts thereof, the volume content of which is less than 10% of the total volume of the organic compounds alone;
  • said actinide powder or said mixture of actinide powders representing between 40% and 65% of the total volume of the filled matrix.

These compositions make it possible to achieve the specifications defined in the specific problem mentioned previously, namely limitation of the effects of radiolysis on the rheology of the filled pastes obtained and the mechanical strength of the injected objects before debinding.

According to one variant of the invention, the binder comprises polystyrene.

According to one variant of the invention, the binder comprises polystyrene and a polyolefin.

According to one variant of the invention, the binder comprises polymethyl methacrylate and a polyolefin which may be polyethylene.

According to one variant of the invention, the plasticizer comprises paraffin.

According to one variant of the invention, the plasticizer comprises polypropylene.

According to one variant of the invention, the specific surface area of the grains of said actinide powder(s) is between about 1 m²/g and 15 m²/g.

According to one variant of the invention, the tapped density of said actinide powder is between about 10% and 70% of the theoretical density of the powder compound(s).

According to one variant of the invention, the theoretical density of the constituent materials of the powder is between 2 and 20.

According to one variant of the invention, the theoretical density of the constituent materials of the powder is between 7 and 19.

According to one variant of the invention, the polyolefinic polymer has a mean molar mass of at least 10 000 g/mol.

According to one variant of the invention, the carboxylic acid or salts thereof have a molar mass at least equal to 100 g/mol.

According to one variant of the invention, the mass proportion of said carboxylic acid or salts thereof relative to the mass of actinide powders is between about 0.01% and 1% by mass.

The invention will be understood more clearly and other advantages will emerge more clearly on reading the description that follows, which is given without any limitation, and by means of the attached figures, among which:

FIG. 1 illustrates all of the steps of a PIM process performed with the filled compositions of the present invention;

FIG. 2 illustrates an example of rate of instability of the flow pressure as a function of the shear rate for a typical case of poor formulation or blending condition;

FIG. 3 illustrates the shear viscosity as a function of the shear rate at 220° C. for various filled compositions according to the invention;

FIGS. 4 a, 4b and 4c illustrate the change of the incorporation torque as a function of time for three examples of compositions filled with powders obtained via a dry route, according to the invention;

FIG. 5 shows the blending torque for three examples of compositions filled to 50% by volume with powder according to the present invention;

FIGS. 6 a, 6b and 6c illustrate the experimental change in loss of mass of examples of compositions Fd, Fe and Ff according to the invention, during the debinding operation, and are compared with the theoretical curves;

FIG. 7 illustrates an example of a debinding operation thermal cycle under Ar/H₂ atmosphere to which are subjected examples of filled compositions of the invention;

FIGS. 8 a, 8b and 8c illustrate responses of thermogravimetric analysis (TGA) and differential thermal analysis (DTA) measurements performed on compositions of the present invention;

FIGS. 9 a, 9b and 9c illustrate XRD spectra of examples of filled compositions of the present invention.

In general, the filled compositions of the present invention are intended to provide actinide fillers that have satisfactory properties and that allow implementation according to the PIM process described below and illustrated by the steps summarized in FIG. 1.

In a first step 1, corresponding to the mixing and blending of the starting materials, all of the starting materials are mixed together, namely, in the present invention: the organic matrix M_org comprising the plasticizer, the binder, the dispersant, and the filler based on actinide powder P_i. As regards the procedure, the powder is generally added gradually to the mixture of the other heated starting materials using a blender, which may be equipped with paddles making it possible to obtain high shear rates, thus ensuring homogeneity of the whole.

In a second step 2, the step of injection molding may be performed as follows: the fluid filled matrix obtained previously is placed in an injection press. The injection cycle then proceeds in the following manner: the material placed in the injection press hopper arrives in the sheath which is heated to a suitable temperature and is then conveyed via an endless screw to the injection nozzle connected to the mold having the desired shape. Once the material has been metered out (volume linked to that of the component to be injected), the screw stops turning and the mold is filled under pressure (the screw acts as a piston). The mixture is then compacted in the print during the maintenance under pressure. The component is then ejected when the mixture has sufficiently cooled (sufficient rigidity). The main parameters that govern this step are: the temperature of the starting materials, the temperature of the mold, the injection pressure and the injection speed.

The third step 3 corresponds to the debinding operation. Debinding is a key operation of the process, which consists in removing the organic materials from the filled matrix, once the component has been injected. The quality of this operation is fundamental so as not to cause any physical damage (cracks) or chemical damage (carbidation) in the component. A very large proportion of the defects that appear after sintering is generated by inadequate debinding.

The fourth step 4 corresponds to the sintering operation. Once the debinding step has been completed, the component must be consolidated by a sintering step. Sintering is a thermal process which makes it possible, by heating compacted powders, generally below their melting point, to give them cohesion after cooling and to obtain the desired microstructure of the final material. The principle of sintering is based on atomic scattering: particles in contact weld via atomic transport phenomena via scattering if they are subjected to temperatures higher than half of their absolute melting point so as to obtain a finished object O_F.

Examples of Filled Compositions Used in the Present Invention:

In order to demonstrate the possibility of using the compositions of the present invention in a satisfactory manner, in the sense of the abovementioned problem, several filled compositions comprising a plasticizer, a binder and a dispersant as described in the present invention with an actinide powder reputed to be cohesive were prepared, with industrial uranium oxide powders.

Since one of the main difficulties induced by the use of actinide powders in the PIM process is linked to the cohesive nature of this type of powder, the example of powder used for the illustration of the present invention is representative of this characteristic. To do this, uranium oxide powder was used, the crystallites of which (constituent elemental objects of the powder) are grouped into aggregates, which are themselves lumped into agglomerates.

The main characteristics of the powder mainly used for the illustration of the present invention are given below:

• • a formed agglomerate diameter: D_agglomerate of between 10 and 200 μm;
  • a formed aggregate diameter: D_aggregate equal to 1 μm;
  • a formed crystallite diameter: D_crystallite equal to 0.3 μm;
  • a specific surface area: Ssa=2 m²/g.

FIG. 2 illustrates the rate of the flow pressure as a function of the shear rate (unit: s⁻¹) for a typical case of poor formulation or blending condition, which may typically be obtained from an organic matrix comprising a standard polymer. The pressure undergoes large instabilities for shear rates of about 2000 s⁻¹.

Various filled composite compositions according to the invention collated in Table 1 below were studied:

TABLE 1
		Percentages	Filler content
		(excluding	by volume of
Formulation	Constituents	powder base)	the powder
Fd	Polystyrene/paraffin/SA	40/55/5	50%
Fe	LDPE/PS/paraffin/SA	31.6/20/43.4/5	50%
Ff	LDPE/paraffin/PMMA/SA	31.6/43.4/20/5	50%

with LDPE: low-density polyethylene, and SA: stearic acid

Table 2 below gives examples of operating conditions under which the compositions of the present invention were prepared.

TABLE 2
Process step                    Operating conditions
Blending (performed in a paddle T° = 175° C.
blender)                        Time = 60 minutes
                                Paddle spin speed = 30 rpm
Forming by injection            Injection pressure: 1500 bar
                                Maintenance pressure: 1200 bar
                                Temperature: 225° C.
                                Cooling time: 30 s
                                Mold closing force: 80 kN
                                Injection speed: 20 cm3/s
Debinding (thermal)             Thermal cycle: various temperature
                                rises ranging from room temperature to
                                570° C. with different stages, under
                                Ar/5% H2 atmosphere
Sintering                       Thermal cycle: temperature rise at
                                300° C./h, then steady stage for 4 hours
                                at 1700° C. and temperature decrease
                                ramp at 600° C./h

The present description gives below the elements for illustrating the achievement of the numerous acceptability criteria for the filled compositions described, notably with regard to the problem of the present invention.

Injectability and Filler Content in Filled Compositions According to the Invention:

FIG. 3 gives an illustration of the injectability of the abovementioned compositions Fd, Fe and Ff and is representative of the shear viscosity as a function of the shear rate (unit: s⁻¹) at 220° C., with a blending temperature of 175° C. and a filler content of 50% by volume. The curves C_3Fd, C_3Fe and C_3Ff are, respectively, relative to the compositions Fd, Fe and Ff.

In the light of the shear viscosity values for these formulations, it is possible to indicate that these filled compositions are indeed acceptable with respect to the rheology criterion, despite a relatively large filler content, since it is between 50 and 10 000 Pa·s.

FIGS. 4 a, 4b and 4c illustrate the change of the blending torques as a function of time for compositions Fd, Fe and Ff, (in these figures, the right-hand y-axis corresponds to the blending temperature).

FIG. 5 illustrates the blending torque values for the formulations Fd, Fe and Ff for a degree of incorporation of UO₂ powder of 50% by volume, a temperature T_blending=145° C. for the filled compositions Fe and Ff and a temperature T_blending=175° C. for the filled composition Fd.

Stability of the Properties of the Filled Compositions According to the Invention:

The preceding three filled compositions were moreover evaluated during a debinding operation and these results were compared with theoretical results. FIGS. 6a, 6b and 6c are, respectively, relative to the filled compositions Fd, Fe and Ff and illustrate the virtual absence of interaction of the organic constituents of the matrix, the overall debinding behavior of which may be likened to a linear combination of the individual behaviors of the latter. More specifically, the curves C_6d1, C_6e1 and C_6f1 relate to the theoretical curves, and curves C_6d2, C_6e2 and C_6f2 relate to the experimental curves.

An example of a thermal cycle that may be used under an atmosphere of argon and hydrogen in the debinding process is illustrated in FIG. 7, and is applied to the three filled compositions: Fd, Fe and Ff, this short thermal cycle being performed to allow rapid evaluations of the compositions obtained. In general, long debinding cycles (typically a few hours) will be preferred during industrial treatments for the manufacture of formed powders to make it possible to conserve the integrity of the component.

FIGS. 8 a, 8b and 8c illustrate the debinding operations as regards the thermal behavior of the filled compositions Fd, Fe and Ff. More specifically, curves C_8d1, C_8e1 and C_8f1 relate to TGA measurement results and curves C_8d2, C_8e2 and C_8f2 relate to DTA measurement results. These are thermogravimetric analysis (TGA) and differential thermal analysis (DTA) measurements.

Differential thermal analysis (DTA) is a method used for determining the temperatures corresponding to changes in the material as a function of the thermal treatment. It consists in measuring the temperature difference between a sample (Te) and a reference (Tr) (thermally inert material) as a function of time or temperature, when they are subjected to a programmed temperature variation, under a controlled atmosphere.

In general, the phase transitions and the evaporation of solvents are reflected by endothermic peaks. On the other hand, crystallization, oxidation and certain decomposition reactions are characterized by exothermic peaks. DTA is generally associated with a thermogravimetric analysis (TGA) which makes it possible to measure the variation of a mass of a sample as a function of the thermal treatment temperature. This mass variation may be a loss of mass such as the emission of vapors or a gain of mass during the fixing of a gas, for example. The curves of these figures do not show any exothermicity peaks or any notable event other than phenomena of melting and of degradation/volatilization of the feedstock constituents, which confirms the stability of these formulations.

These measurements are reinforced in their conclusion by the XRD measurements, which were taken at the end of the process for producing the powders and thus after the sintering operation. FIGS. 9a, 9b and 9c illustrate, to this end, the XRD spectra of the filled compositions Fd, Fe and Ff and do not reveal any change in the UO₂ phase of the fuel, which argues in favor of no significant interaction of the actinide powder with the forming polymers, which is targeted with the present filled compositions Fd, Fe and Ff.

Debinding Capacity of the Filled Compositions According to the Invention:

As regards the debinding capacity criterion, it is necessary for the debinding operation to be able to be performed while conserving the integrity of the component once the forming polymers have been debonded and without an excessive proportion of carbon-based residues that would not be removable during sintering and that might moreover modify the microstructure of the sintered actinide material.

To demonstrate the acceptability of the examples of filled compositions Fd, Fe and Ff with respect to this criterion, Table 4 below gives the percentages of carbon-based residues in the final components obtained from the sintering operation.

TABLE 4
            Residual carbon
Formulation content after sintering
Fd          0.0113% wt
Fe          0.0112% wt
Ff          0.0129% wt

Claims

1. A composition filled with actinide powder comprising an organic matrix and an actinide powder or a mixture of actinide powders, comprising: a plasticizer comprising an alkane whose longest radical chain comprises at least a few tens of carbon atoms and is in a volume content of between 20% and 70% of the total volume of the organic compounds alone;a binder comprising at least one aromatic polymer and/or polymethyl methacrylate capable of limiting the radiolysis effects and which is in a volume content of between 20% and 50% of the total volume of the organic compounds alone; anda dispersant comprising a carboxylic acid or salts thereof, the volume content of which is less than 10% of the total volume of the organic compounds alone;wherein said actinide powder or said mixture of actinide powders represent between 40% and 65% of the volume of the filled matrix.

2. The composition filled with actinide powder as claimed in claim 1, wherein the binder comprises polystyrene.

3. The composition filled with actinide powder as claimed in claim 1, wherein the binder comprises polystyrene and a polyolefin.

4. The composition filled with actinide powder as claimed in claim 1, wherein the binder comprises polymethyl methacrylate and a polyolefin which may be polyethylene.

5. The composition filled with actinide powder as claimed in claim 1, wherein the plasticizer comprises paraffin.

6. The composition filled with actinide powder as claimed in claim 1, wherein the plasticizer comprises polypropylene.

7. The composition filled with actinide powder as claimed in claim 1, wherein the specific surface area of the grains of said actinide powder is between about 1 m2/g and 15 m2/g.

8. The composition filled with actinide powder as claimed in claim 1, wherein the tapped density of said actinide powder is between about 10% and 70% of the theoretical density of the compound(s) of the powder(s).

9. The composition filled with actinide powder as claimed in claim 1, wherein the theoretical density of the constituent materials of the powder is between 2 and 20.

10. The composition filled with actinide powder as claimed in claim 9, wherein the theoretical density of the constituent materials of the powder is between 7 and 19.

11. The composition filled with actinide powder as claimed in claim 1, wherein the polyolefinic polymer has a mean molar mass of at least 10 000 g/mol.

12. The composition filled with actinide powder as claimed in claim 1, wherein the carboxylic acid or salts thereof have a molar mass at least equal to 100 g/mol.

13. The composition filled with actinide powder as claimed in claim 1, wherein the mass proportion of said carboxylic acid or salts thereof relative to the mass of actinide powders is between about 0.01% and 1% by mass.