JP7258853B2 - Process for producing powders containing uranium oxide UO2, optionally plutonium oxide PuO2, and optionally americium oxide AmO2 and/or oxides of other minor actinides - Google Patents

Description

The present invention relates to a method for producing powders comprising uranium oxide _UO2 , optionally plutonium oxide _PuO2 , and optionally americium oxide _AmO2 and/or other oxides of minor actinides.

However, in the remainder of this disclosure, the term minor actinides refers to actinide elements other than uranium, plutonium and thorium. They are formed in nuclear reactors by the continuous capture of neutrons by standard fuel cores. Minor actinides are americium, curium and neptunium.

More specifically, the present invention provides a flowable, narrow unimodal trending particle size distribution, intended to be compressed without prior mixing, more specifically the following specific It relates to a method for producing powders that can have physico-chemical properties:
- suitability for naturally occurring flows;
- particle size distribution with a unimodal trend centered on values between 150 and 350 μm;
- good homogeneity of the elements within the powder particles;
- a minimum proportion of fine particles in the powder to prevent diffusion of the powder within the equipment and glovebox;
- suitability for compression; and
- Excellent reactivity to natural sintering.

As a result of the above physico-chemical properties, the powder obtained by the method of the invention may be suitable for the production of the following materials:
- Mixed oxide fuel of uranium and plutonium (U,Pu) _O2 , called MOX fuel, currently used in light water reactors;
- A blanket with minor actinides, such as a transmutation target with minor actinides, aimed at conducting transmutation experiments in fast neutron reactors, in particular to better understand the transmutation mechanisms of these minor actinide elements. (the target is a MOX type material containing 1-5 wt% minor actinides (the material can be represented by the _formula (U, Pu, Am, Np, Cm) O A material comprising a uranium oxide matrix containing minor actinides (the material can be represented by the formula (U, Am, Np, Cm) O ₂ ).

The production of mixed oxides of uranium and plutonium (U,Pu) _O2 , called MOX fuel, has been the subject of various developments related to the desire to recycle plutonium recovered during spent fuel reprocessing. . Recycling of plutonium by manufacturing MOX fuel and irradiation is today considered a means of limiting plutonium proliferation.

Several methods have been developed to produce MOX fuel over the past two decades. Some utilize complete grinding of UO ₂ and PuO ₂ powders to provide a homogeneous mixture, while others are limited to grinding only a portion of these powders.

Currently, the production of mixed oxide (U,Pu) _O2 is carried out by mechanical mixing of _UO2 and _PuO2 oxides. The resulting mixture makes it possible, after pressing, sintering and rectification, to produce pellets of MOX fuel that meet current specifications. The most reliable and proven industrial method, known as the MIMAS process, involves two major steps in the production of powders: co-grinding uranium oxide and plutonium powders to produce plutonium content of 25-25 An initial mixture, called a masterbatch, characterized by 30%, is produced and then dry-diluted with uranium oxide until the desired final plutonium content is obtained.

Fuel production requires that the powders used meet precise properties. In particular good fluidity suitability, good compressibility and suitability for densification by sintering are required. An important quality criterion in the final properties of the sintered material is the homogeneity of the plutonium distribution. Good homogeneity in each sintered pellet is, on the one hand, wholly favorable for the behavior of MOX in the reactor, especially in terms of increasing the burn rate and smoothing the way, and on the other hand, irradiation during reprocessing operations. Completely dissolves spent fuel.

As for transmutation targets, they are the subject of detailed studies to enable the recycling of minor actinides resulting from the processing of spent fuel from pressurized water reactors, in addition to the above end states.

This type of recycling is carried out through two separate channels known as:
- heterogeneous recycling; and
- Homogeneous recycling.

In the case of heterogeneous recycling, minor actinides are separated from uranium and plutonium during spent fuel processing and then incorporated into the fuel components at high contents (approximately 10-20% atomic). It contains a non-fissionable matrix (eg, depleted UO ₂ ) separate from the standard fuel components of nuclear reactors. A fuel component containing minor actinides may, for example, consist of a blanket of elements arranged around the core of a nuclear reactor. In particular, the recycle channel prevents the degradation of standard fuel properties due to contamination with minor actinides by concentrating the problems caused by these actinides in reducing the flow rate of the material.

For homogeneous recycling, the minor actinides are mixed at low content (less than 5% atomic) and distributed substantially uniformly in all reactor standard fuel components. To do this, uranium, plutonium and minor actinides are processed together to form oxides during spent fuel processing. It is then used to make the fuel.

In recent years, the manufacturing methods used, whether for fuels or transmutation targets, have tended to move towards methods that limit the diffusion of fines and improve the homogeneity of the elements in the pellet. be. This produces homogeneous globules of mixed oxide (U,Am) _O2 without going through a granulation step, in contrast to conventional powder metallurgical methods that carry out the steps of granulation such as grinding, screening, and mixing. This is the case for the WAR method, which can obtain , which severely limits the diffusion of fine particles.

Yet another method involving a spray-drying step is described in US Pat. The method does not use grinding, mixing or screening steps. However, it still produces a non-negligible proportion of fine particles during spraying.

Therefore, in light of what already exists and the shortcomings of the methods of the prior art, the authors of the present invention have developed uranium oxide UO ₂ , plutonium oxide PuO ₂ and optionally americium oxide AmO ₂ and/or oxides of other minor actinides. The goal was set to propose a new method for producing powders containing This provides for obtaining spherical particles of the powder, thus making it possible to access good fluidity and having good homogeneity of the distribution of the elements that make up the powder. Finally, the method of the present invention makes it possible to obtain powders that can be used directly in the design of compacted materials (i.e. without the need for the addition of additives for compaction, preventing the formation of dispersed fine particles). (no milling or mixing steps via a dry route are required).

WO 00/30978

Accordingly, the present invention provides for the manufacture of powders comprising uranium oxide _UO2 , optionally plutonium oxide _PuO2 and optionally americium oxide _AmO2 and/or oxides _MO2 of another minor actinide, where M is neptunium or curium. Regarding the method, the manufacturing method includes the following steps:

a) A process of preparing an aqueous suspension comprising contacting with water; uranium oxide _UO2 powder, optionally plutonium oxide _PuO2 powder and optionally americium oxide _AmO2 powder and/or another minor actinide oxide MO2 _. powder (M is neptunium or curium); at least one additive selected from anticaking agents, organic binders and mixtures thereof, said additive having a kinematic viscosity of the aqueous suspension exceeding 1000 mPa.s; not, preferably added in an amount not exceeding 300 mPa.s);

b) a cold granulation step of the suspension prepared in a);
c) a freeze-drying step of the granules obtained in b), whereby uranium oxide UO ₂ , optionally plutonium oxide PuO 2 and optionally americium oxide AmO ₂ and _/ or another minor actinide oxide MO ₂ (M is neptunium or curium) is obtained.

In addition to the objectives already mentioned in the technical field part and achieved by the practice of the method of the invention, the method of the invention also has the following advantages:

- The use of water as dispersion medium of particular interest. because it limits the use of organic products and thereby limits impurities in the final powder obtained;

- simple, rapid and reproducible implementation during step a), which results in a suspension that can be transported without difficulty by simple pumping into the injection nozzle of the cryogenic granulator;

- A combination of suspension, cold granulation and freeze-drying that can yield powders containing particles with controlled porosity and good flowability. The particles are solids with a homogeneous distribution of the elements (U, optionally Pu and optionally Am and/or another minor actinide) and are highly spherical;

- Possibility to obtain a very high percentage of dry matter in suspension. This can result in powder particles that are dense, solid and highly spherical; and

- Criticality, ie the feasibility of carrying out the process in production units of industrial installations, taking into account the structure of the equipment.

In the method, an aqueous suspension is prepared by first contacting with water; uranium oxide _UO2 powder, optionally plutonium oxide _PuO2 powder and optionally americium oxide _AmO2 powder and/or another minor actinide. oxide _MO2 (M is neptunium or curium) powder; at least one additive selected from anti-agglomeration agents, organic binders and mixtures thereof (said suspension does not exceed 1000 mPa.s, preferably 300 mPa.s) suitable for cold granulation operations with kinematic viscosities not exceeding .s).

Kinematic viscosity is typically measured at ambient temperature using a cylinder-cone configuration system (i.e., without external heating or pressure other than the temperature and pressure of the ambient atmosphere, where the ambient temperature can be a temperature of 20° C., Ambient pressure is atmospheric pressure), measured using a rheometer with a shear rate of at least 10 ³ s ^-1 (eg, 1,500 s ^-1 ). More preferably, the kinematic viscosity is 300 mPa.s. not exceed s. This corresponds to a very fluid suspension that can be easily circulated through the feed pipes and spray nozzles of the cryogenic granulator.

Uranium oxide UO ₂ powder, optionally plutonium oxide PuO ₂ powder and optionally americium oxide AmO ₂ powder and/or another minor actinide oxide MO ₂ (M is neptunium or curium) powder are advantageously in suspension It is present in a content ranging from 10 to 50% by volume relative to the amount of water in the.

For the preparation of the suspension, at least one additive selected from anti-flocculating agents (also called dispersants), organic binders and mixtures thereof, preferably at least one anti-flocculating agent and at least A mixture of one organic binder is used.

Anti-flocculants are used to fluidize the suspension. This is an easy-to-remove organic product, such as Polyplastic S.T. A. Ammonium polymethacrylate (aqueous solution of 25% by weight of ammonium polymethacrylate) such as the product marketed under the name DARBAN C by the company. The anti-agglomeration agent may be a polycarboxylic acid ether such as the product marketed under the name MasterGlenium 27 by BASF.

The amount of anti-flocculating agent used is usually the sum of the _weight of dry matter of the suspension (i.e. oxide _UO2 , optionally PuO2, optionally _AmO2 , optionally _MO2 (M is neptunium or curium)). weight) is 0.02 to 1% by weight.

Organic binders are used in the suspension to favor cohesion of the powder during cold granulation. The choice is primarily made in easily removable organic binders. Examples include polyvinyl alcohol (PVA), polyethylene glycol (PEG), poly(vinyl butyral) (known by the abbreviation PVB), acrylic latex or mixtures thereof.

The amount of _organic binder used is the weight of the dry matter of the suspension (i.e. the total weight of the oxide _UO2 , optionally PuO2, optionally _AmO2 , optionally _MO2 (M is neptunium or curium)). 0.1 to 3% by weight of the.

For the preparation of suspensions, raw UO ₂ or raw UO 2 and/or PuO ₂ and/or AmO ₂ and/or oxides of other minor actinides _{MO 2} (M is neptunium or curium) to a mixture of water containing additives (anti-agglomeration agents and/or organic binders). The whole can then be mixed by mechanical stirring, preferably using a roller stirrer and grinding beads (eg made of yttrium zirconia or alumina) for several hours. Suspensions can also be prepared using a bar mill or by attrition.

The next step of cold granulation can be carried out on commercial granulators or specially prepared laboratory equipment for carrying out the steps. It can be formed with a peristaltic pump that allows the suspension to be conveyed to a nozzle to allow granulation of the suspension. These microdroplets formed and sprayed by the nozzle fall into a Dewar bottle of liquid nitrogen where they are frozen into a spherical shape. In order to carry out a freeze-drying step at the end of granulation, the frozen granules can be placed in a freeze-dryer to allow sublimation of the frozen water, preserving the shape of the granules (especially their spherical shape) and their individuality. can.

At the end of freeze-drying, the residual moisture content of the granules is very low, preventing drying of the powder prior to use.

After this final step, a powder is obtained which can in particular have the following properties:

- Particle size distribution with a unimodal trend centered on values between 150 and 350 μm.
- sufficient cohesiveness of the granules to withstand the operation of making pellets;
- excellent liquidity;
- suitability for compression;
- excellent suitability for natural sintering; and
- Good homogeneity of the elemental distribution within the powder.

The virtually perfect sphericity of these granules provides very good flowability in the press mold, resulting in a pellet that is subsequently sintered.

Regarding the homogeneity of the distribution of the elements, the relationship with the plutonium element, if present, is of particular importance. Once the powder is compacted and sintered to make MOX fuel, the homogeneity of the plutonium distribution is absolutely favorable to the behavior of the fuel in the reactor, especially in terms of increasing and promoting the burning rate. On the other hand, the irradiated fuel during subsequent reprocessing operations is completely dissolved.

Parameters affecting the granule diameter are the rheology of the suspension to be granulated, the air flow rate and the flow rate of the suspension during granulation.

The powders obtained by the method of the invention can be used directly (ie without the need for the addition of other ingredients) to form compacted materials, for example in the form of fuel pellets.

The invention therefore also relates to a method for the production of nuclear fuel pellets comprising, in succession, the following steps:

d) carrying out a method for producing a powder as defined above;
e) compressing the powder obtained in d) into the shape of pellets; and
f) sintering the pellets obtained in e).

The compaction step e) can consist, on the one hand, of placing the powder in a mold of a shape suitable for forming one or more pellets and, on the other hand, of subjecting the powder to uniaxial compaction. For example, using a piston that applies pressure to the powder placed in the mold, the pressure can range from 250 to 1,500 MPa for a duration that can range from 1 second to 30 minutes. can.

Sintering step f) is carried out under a neutral gas atmosphere such as argon, optionally in the presence of hydrogen and water, or in a reducing medium comprising hydrogen and optionally a neutral gas such as argon (up to 5 vol. optionally including water in a content that can range up to 20,000 ppm in the mixture, with a content that can range from 1,000 ppm to heating at a temperature in the range of 1,800° C. for a duration that can range from 1 to 8 hours.

Other features and advantages of the present invention will appear in the following supplementary description of examples for producing mixed powders and fuel pellets according to embodiments of the method of the present invention.

Naturally, the supplementary description is provided only for the purpose of illustrating the invention and does not form a limitation in any way.

(Example 1)
This example demonstrates the implementation of the method of the invention for the production of a mixed powder containing uranium oxide UO ₂ and plutonium oxide PuO ₂ in an atomic ratio (Pu/U+Pu)=10%. The production is made entirely in the glove box.

70 mL of demineralized water (corresponding to 40% by weight of the total weight of the final suspension) is placed in a 250 mL plastic container containing approximately 250 g of grinding beads made of zirconia (average diameter 3 mm). Dispersant and binder (which are DARVAN C and polyethylene glycol 300 (PEG 300), respectively) in proportions of 0.5% and 2%, respectively, of the weight of the dry oxide (i.e. the total weight of _UO2 and _PuO2 ). Add with .

After rapidly mixing these raw materials, a mixture of _UO2 and _PuO2 powders is added at a rate of 60% by weight relative to the weight of the final suspension, or 89 g _UO2 powder and 16 g _PuO2 powder. Add 105 g of a powder mixture containing The resulting mixture is rotated using a rock and roll type roller stirrer with a rotation speed of 35 rpm. This allows deagglomeration of the powder and better dispersion. Stirring is maintained for at least 5 hours until a liquid suspension is obtained.

The viscosity of the ^suspension , measured at 1500 s ^-1 using an ANTON PAAR RHEOLAB QC rheometer, is 100 mPa. s and 300 mPa.s. falls within the preferred range of less than s. This is particularly advantageous for triggering cold granulation of suspensions.

Cold granulation is carried out in an apparatus comprising the following parts.
- A beaker containing the above suspension. The beaker is connected to a peristaltic pump that allows the suspension to be conveyed to the spray nozzle. The flow rate of the pump is up to 2 L/h at an air pressure of 0.15 bar;
- Dewar type reactor filled with liquid nitrogen. Said reactor is connected to a spray nozzle, said reactor allowing flash freezing of droplets of suspension coming from the spray nozzle.

From a practical point of view, the previously obtained suspension was placed in the above-mentioned beaker, then withdrawn by a peristaltic pump at a flow rate of 33 mL/min and an air pressure of 0.15 bar, and passed through a spray nozzle into a Dewar-type reactor. bring it into The droplets formed are directly frozen by the liquid nitrogen contained in the reactor. For the volume of suspension obtained in the example, less than 5 minutes are required for cold granulation.

Once frozen, the resulting granules are rapidly placed in a freeze dryer with the aim of sublimating the water trapped within the frozen granules while maintaining the spherical shape of the granules. The freeze-drying procedure takes at least 3 hours to achieve a stable vacuum of 10 ⁻³ mbar and a temperature of about −100° C. at the end of the experiment.

Once all the water has been removed from the granules, this results in granules with a monomodal particle size distribution centered at a value of 200 μm which are ready for use to form pellets by pressing.

(Example 2)
This example demonstrates the production of UO ₂ /PuO ₂ MOX fuel pellets from the granular powder obtained in Example 1 above.

To do this, the powder is subjected to cold uniaxial pressing at 700 MPa with external lubrication by stearic acid. Pellets with a diameter of 9.5 mm and a height of 10 mm are thereby obtained. The pellets obtained are then subjected to sintering at 1,750° C. for 4 hours in an argon atmosphere containing 4% by volume of hydrogen. A temperature of 1,750° C. is reached with a temperature increase of 3° C./min.

Pellets sintered in this way have a relative density of about 94-98% with good homogeneity of the elements U and Pu within the pellet (resulting in good homogeneity of these elements in the powder). be). These elements are more homogeneously distributed than in MOX pellets produced by powder metallurgy.

Claims (9)

A method of producing a powder comprising uranium oxide _UO2 , optionally plutonium oxide _PuO2 , optionally americium oxide _AmO2 , optionally neptunium oxide NpO2 _and optionally curium oxide CmO2 _, the method comprising the steps of:
a) Aqueous suspension preparation step including contacting with water; uranium oxide _UO2 powder, optional plutonium oxide _PuO2 powder , optional americium oxide _AmO2 powder , optional neptunium oxide NpO2 _powder and optional oxide Curium CmO2 _powder ; at least one additive selected from anticaking agents, organic binders and mixtures thereof, said additive having a kinematic viscosity of the aqueous suspension of 1000 mPa.s; added in an amount not exceeding s;
b) a cold granulation step of the suspension prepared in a);
c) freeze-drying the granules obtained in b) to give a powder comprising uranium oxide _UO2 , optionally plutonium oxide _PuO2 , optionally americium oxide _AmO2 , optionally neptunium oxide NpO2 _and optionally curium oxide _CmO2 Freeze-drying process to obtain. Uranium oxide UO ₂ powder, optional plutonium oxide PuO ₂ powder , optional americium oxide AmO ₂ powder , optional neptunium oxide NpO ₂ powder and optional curium oxide CmO ₂ powder are 10-50 to the amount of water in the suspension. 2. A process according to claim 1, present in a content in the range of % by volume. 3. Process according to claim 1 or 2, wherein the additive is a mixture of at least one anti-agglomeration agent and at least one organic binder. The production method according to any one of claims 1 to 3, wherein the anti-aggregation agent is polyammonium methacrylate or polycarboxylic acid ether. 0.02 to 1 of the weight of the dry matter of the suspension (i.e. the total weight of oxide UO ₂ , optional PuO ₂ , optional AmO ₂ , optional NpO ₂ and optional CmO ₂ ). A manufacturing method according to any one of claims 1 to 4, present in the range of weight percent. The production method according to any one of claims 1 to 5, wherein the organic binder is polyvinyl alcohol, polyethylene glycol, poly(vinyl butyral), acrylic latex or a mixture thereof. 0.1 to 3 weight of organic binder by weight of dry matter of suspension (i.e. total weight of oxide UO ₂ , optional PuO ₂ , optional AmO ₂ , optional NpO ₂ and optional CmO ₂ ) % range. The kinematic viscosity of the suspension is 300 mPa.s. The manufacturing method according to any one of claims 1 to 7, wherein s does not exceed. A method of producing nuclear fuel pellets comprising, in succession, the following steps:
d) carrying out a method for producing a powder as defined in any one of claims 1-8;
e) compressing the powder obtained in d) into the shape of pellets; and
f) sintering the pellets obtained in e).