RU2770610C2 - METHOD FOR PRODUCING POWDER CONTAINING URANIUM OXIDE UO2, IF NECESSARY, PLUTONIUM OXIDE PuO2 AND, IF NECESSARY, AMERICIUM OXIDE AmO2 AND/OR OXIDE OF ANOTHER MINOR ACTINOID - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to the technology of production of powder containing uranium oxide UO₂, if necessary, plutonium oxide PuO₂ and, if necessary, americium oxide AmO₂ and/or oxide of another minor actinoid MO₂, where M is neptunium or curium. The method includes a) the stage of preparation of an aqueous suspension by contacting water, powder of uranium oxide UO₂, if necessary, powder of plutonium oxide PuO₂ and, if necessary, powder of americium oxide AmO₂ and/or powder of oxide of another minor actinoid MO₂, where M is neptunium or curium, at least one additive selected from anticoagulants, organic binders or mixtures thereof, wherein the additive or additives are introduced in such an amount that the dynamic viscosity of the aqueous suspension does not exceed 1000 MPa·s; b) the stage of cryogenic granulation of the suspension prepared at the stage a); c) the stage of sublimation drying of granules obtained at the stage b), by which powder is obtained, containing uranium oxide UO₂, plutonium oxide PuO₂ and, if necessary, americium oxide AmO₂ and/or oxide of another minor actinoid MO₂, where M is neptunium or curium.

EFFECT: resulting powder consists of spherical particles that provide good flowability and uniform distribution of granules in size ranging from 150 to 350 mcm, has a high compaction capability and excellent reactivity during sintering.

9 cl, 2 ex

Description

The field of technology to which the invention belongs

The invention relates to a method for producing a powder containing uranium oxide UO ₂ , optionally plutonium oxide PuO ₂ and optionally americium oxide AmO ₂ and/or oxide of another minor actinide.

For further discussion, it should be clarified that minor actinides are understood to mean actinide elements other than uranium, plutonium and thorium, formed in reactors by successive captures of neutrons by nuclei of standard fuel, while minor actinides are americium, curium and neptunium.

More specifically, the invention relates to a process for the production of a free-flowing powder, the particle size distribution of which has a narrow monomodal orientation, intended for pressing without prior mixing and capable of having, in particular, the following specific physico-chemical properties:

- good ability to spontaneous flowability,

- distribution of granules by size with a monomodal orientation by a value from 150 to 350 microns,

- good uniformity of the elements inside the powder particles,

- the minimum proportion of fine particles in the powder to prevent their dispersion inside the equipment and in glove boxes,

- good sealing ability,

- excellent reactivity during natural sintering.

Due to the physical and chemical properties listed above, the powder obtained by the method according to the invention can be used for the manufacture of the following materials:

- mixed fuel from oxides of uranium and plutonium (U,Pu)O ₂ , the so-called MOX fuel, currently used in pressurized water reactors,

- coatings saturated with actinide(s), such as transmutation targets based on actinide(s) for conducting experiments on nuclear transformation in fast neutron reactors, in particular, for a better understanding of the mechanism of transmutation of said minor actinides, these targets may consist of from a material of the MOX type containing from 1 to 5 wt.% minor actinide (actinides) (this material can be denoted by the formula (U, Pu, Am, Np, Cm)O ₂ ) or from a material containing a matrix of uranium oxide with a content of from 10 to 20% by weight of minor actinide(s) (this material may be denoted by the formula (U, Am, Np, Cm)O ₂ .

Prior Art

The production of mixed fuel from uranium and plutonium oxides (U, Pu)O ₂ , the so-called MOX fuel, has been the subject of various developments aimed at reusing reprocessed plutonium in spent fuel regeneration. The reuse of plutonium in the manufacture and irradiation of MOX fuel is now considered a means to limit the spread of plutonium.

Over the past two decades, several methods have been developed for the manufacture of MOX fuel, some of them require complete grinding of powders from UO ₂ and PuO ₂ to obtain a homogeneous mixture, while others are limited to grinding only one fraction of these powders.

At present, the (U, Pu)O ₂ oxide mixture is prepared by mechanical mixing of UO ₂ and PuO ₂ oxides. After pressing, sintering, and refining, the resulting mixture makes it possible to manufacture MOX fuel pellets that meet modern requirements. The most proven industrial process, which is known as MIMAS, includes two main steps in the preparation of powders, this is the joint grinding of powders of uranium and plutonium oxides to obtain an initial mixture, called the parent mixture, differing in the content of plutonium from 25 to 30%, and dry dilution mother mixture with uranium oxide to obtain the required final content of plutonium.

In the production of fuels, the powders used must exactly meet the requirements. In particular, they must have good flowability, good compressibility and sintering properties. An essential criterion for the quality of a sintered material with final properties is a uniform distribution of plutonium. Good distribution uniformity in each sintered pellet is on the one hand favorable for the behavior of MOX in the reactor, in particular from the perspective of increasing the amount of fuel, and on the other hand facilitates the complete dissolution of the spent fuel during regeneration operations.

As for the targets for transmutation, they were the object of extensive research aimed, in addition to the final goals indicated above, to ensure the reuse of minor actinides formed during the processing of spent fuel in a pressurized water reactor.

This type of reuse of recovered fuel is carried out in two different ways, known as:

- heterogeneous reuse of recovered fuel,

- uniform reuse of recovered fuel.

In the case of inhomogeneous reuse of regenerated fuel, minor actinides are separated during spent fuel processing from uranium and plutonium and then introduced at an increased content (about 10 - 20 atomic %) into fuel elements containing a non-fissile matrix (for example, depleted UO ₂ ) and different from elements of standard fuel for the reactor. Fuel elements containing minor actinides may consist, for example, of coating elements located along the periphery of the reactor core. This type of reuse of the recovered fuel makes it possible, in particular, to avoid the deterioration of the properties of the standard fuel by introducing minor actinides while concentrating the problems created by these actinides in a reduced material flow.

In the case of homogeneous reuse of reclaimed fuel, the minor actinides are mixed at a low content (less than 5 atomic %) and with an almost uniform distribution throughout the mass of standard reactor fuel elements. At the same time, during the processing of spent fuel, uranium, plutonium and minor actinides are processed together to form oxides used later in the production of these fuels.

Regardless of the type of fuel or transmutation target, existing manufacturing methods gravitate toward methods that limit dispersion of the fine powder and increase the uniformity of the elements in the tablet. This refers to the WAR method, which makes it possible to obtain homogeneous spherical particles from mixed oxides (U, Am)O ₂ without the need for a granulation step, which significantly limits the dispersion of fine particles, in contrast to traditional powder metallurgy methods, which use steps such as grinding, sieving and mixing.

However, WO 00/30978 discloses another method comprising a spray-drying phase. This process, which does not involve grinding, mixing and screening, is nevertheless characterized by a significant content of fines when sprayed.

Therefore, taking into account the already existing state of the art and the shortcomings of these methods known from the prior art, the authors of the present invention set themselves the goal of creating a new method for obtaining a powder containing uranium oxide UO ₂ , plutonium oxide PuO ₂ and, if necessary, americium oxide AmO ₂ and/or oxide of another minor actinide, which makes it possible to obtain spherical powder particles, providing good flowability, and creating a good uniformity in the distribution of powder elements. Finally, the method according to the invention should provide powders that can be used directly in the production of densified materials, i.e. without the need for additives for compaction, grinding or dry blending, thus avoiding the formation of dispersed fine particles.

Statement of the Invention

The invention thus relates to a process for producing a powder containing uranium oxide UO ₂ , optionally plutonium oxide PuO ₂ and optionally americium oxide AmO ₂ and/or oxide of another minor actinide MO ₂ , where M is neptunium or curium, comprising the following steps :

a) preparation of an aqueous suspension containing bringing into contact with water a powder of uranium oxide UO₂,_,if necessary plutonium oxide powder PuO₂ and at the need for americium oxide powder AmO₂ and/or oxide powder of another minor actinide MO₂, where M means neptunium or curium, and at least one additive selected from anticoagulants, organic binders and mixtures thereof, and the additive or additives are introduced in such an amount that the dynamic viscosity of the aqueous suspension does not exceed 1000 mPa.s, preferably does not exceed 300 mPa.s;

b) cryogenic granulation prepared in step a) suspension;

c) freeze-drying the granules obtained in step b), by means of which a powder is obtained containing uranium oxide UO ₂ , optionally plutonium oxide PuO ₂ and optionally americium oxide AmO ₂ and/or oxide of another minor actinide MO ₂ , where M means neptunium or curium.

In addition to the objects already mentioned in the section relating to the field of technology and objectives, and achieved by using the method according to the invention, the method according to the invention also has the following advantages:

- the use of water as an extremely effective dispersion medium, since it allows you to limit the use of organic products and, therefore, limit impurities in the resulting target powder,

- a simple, fast and reproducible implementation, leading in step a) to obtain a suspension, which can be easily pumped to the injector nozzle of the cryogenic granulation apparatus,

- the joint use of suspension, cryogenic granulation and freeze drying, which allows to obtain a powder containing particles with controlled porosity, solid and perfectly spherical with good uniformity in the distribution of elements (U, optionally Pu and optionally Am and/or other minor actinide), and with good flowability;

- the possibility of obtaining very high values of the dry matter content in the suspension, which can lead to dense, continuous and perfectly spherical powder particles,

- the possibility of using this method at a production facility with industrial productivity, taking into account the criticality and, consequently, the geometry of the equipment.

According to the method, first of all, an aqueous suspension is prepared by contacting with water a powder of uranium oxide UO ₂ , if necessary, a powder of plutonium oxide PuO ₂ and, if necessary, a powder of americium oxide AmO ₂ and / or another minor actinide MO ₂ (where M means neptunium or curium), at least one additive selected from anticoagulants, organic binders and mixtures thereof, the suspension having a dynamic viscosity not exceeding 1000 mPa.s, preferably not exceeding 300 mPa.s, in order to be consistent with the cryogenic granulation operation.

Traditionally, dynamic viscosity is measured with a rheometer at a shear rate of at least 10 ³ s ^-1 (e.g. equal to 1500 s ^-1 ) using a conical roll configuration system at ambient temperature and pressure (i.e. without the use of external heating and pressurization, but only at ambient temperature and pressure, while the ambient temperature can be 20°C, the pressure is atmospheric). Preferably, the dynamic viscosity does not exceed 300 mPa.s, which corresponds to a very liquid fluid capable of circulating freely through the supply pipes and through the spray nozzle of the cryogenic granulation device.

The content of uranium oxide powder UO ₂ , optionally plutonium oxide powder PuO ₂ and optionally americium oxide powder AmO ₂ and/or oxide powder of another minor actinide MO ₂ (where M is neptunium or curium) is preferably 10 to 50% by volume. on the volume of water in suspension.

When preparing the suspension, at least one additive selected from anticoagulants (also called dispersants), organic binders and mixtures thereof is used, preferably a mixture consisting of at least one anticoagulant and at least one organic binder.

The anticoagulant is designed to thin the suspension. It may consist of an organic, easily removable product, such as ammonium polymethacrylate, such as a commercial product from Polyplastic S.A. under the name DARVAN C, which is an aqueous solution of ammonium polymethacrylate with a concentration of 25 wt.%. The anticoagulant may also be a polycarboxylate ether, such as the commercial product called MasterGlenium 27 from BASF.

The mass amount of the anticoagulant used is usually from 0.02 to 1 wt. % by weight of the dry matter of the suspension, i.e., based on the total weight of the oxide(s) UO ₂ , optionally PuO ₂ , optionally AmO ₂ , optionally MO ₂ (where M means neptunium or curium).

Organic binders are used in suspension to accelerate powder agglomeration during cryogenic granulation. The choice falls mainly on organic binders, which can be easily removed. By way of example, mention may be made of polyvinyl alcohol (PVA), polyethylene glycol (PEG), poly(vinyl butyral) (PVB), acrylic latex or mixtures thereof.

The mass amount of the organic(s) binder(s) used can be from 0.1 to 3 wt.% based on the weight of the dry matter of the suspension, i.e. based on the total weight of the oxide(s) UO ₂ , optionally PuO ₂ , optionally AmO ₂ , optionally MO ₂ (where M is neptunium or curium).

UO ₂ or a mixture of UO ₂ and/or PuO ₂ and/or AmO ₂ and/or oxide of another minor actinide MO ₂ (where M means neptunium or curium) in the crude state can be used to prepare the suspension, which are added to an aqueous mixture containing additive or additives (anticoagulant and/or organic binder). The whole can then be mixed by mechanical mixing, preferably using a roller mixer and grinding balls (made, for example, of yttrium-containing zirconium dioxide or alumina) for several hours. Also, the slurry can be prepared by ball milling or attrition.

The next cryogenic granulation step can be carried out in a commercial granulator or a specially made in the laboratory device for carrying out the mentioned step. This device may consist of a peristaltic pump for supplying the slurry to a nozzle for granulating the slurry. The microdroplets formed and sprayed by the nozzle enter the Dewar vessel with liquid nitrogen and are directly frozen in a spherical form. At the end of the granulation, for the drying step, the frozen granules can be placed in a lyophilizer to sublimate the frozen water and maintain the shape of the granules (in particular, their sphericity) and their characteristics.

After freeze drying, the residual moisture of the granules remains very low, which makes it possible to eliminate the drying of the powder before its use.

This final step results in a powder having the following specific properties:

- distribution of granules by size with a monomodal directivity with a center in the range of values from 150 to 350 microns,

- sufficient stickiness of granules for the manufacture of tablets,

- excellent flow properties,

- good sealing ability,

- excellent ability to natural sintering,

- good uniformity of distribution of elements in the powder.

The almost perfect sphericity of the granules makes it possible to obtain very good flowability in the molds when preparing tablets, which are then sintered.

With regard to the uniformity of the distribution of elements, it should be noted that it is extremely important for plutonium when it is present. Once the powder is compacted and sintered to make MOX fuel, the plutonium distribution uniformity is very favorable for the behavior of the fuel in the reactor, in particular from the perspective of increasing fuel content, and furthermore, complete dissolution of the spent fuel in future regeneration operations is facilitated.

The parameters affecting the diameter of the granules are the rheology of the slurry to be granulated, as well as the air and slurry flow rate during granulation.

The powder obtained by the method according to the invention can be used directly (i.e. without the need to add other ingredients) to obtain a densified material, for example in the form of fuel pellets.

Thus, the invention also relates to a method for manufacturing a nuclear fuel pellet(s), comprising the following successive steps:

d) obtaining a powder in the manner described above,

e) compaction of the powder obtained in step d) in the form of tablets,

f) sintering the tablets obtained in step e).

Compaction step e) can consist, on the one hand, in placing the powder in a mold for forming one or more tablets and, on the other hand, in uniaxially pressing the powder, for example by means of a piston acting on the powder in the mold during compression, which pressing can reach 250 - 1500 MPa within 1 second to 30 minutes.

The sintering step e) may consist in heating the pellets mentioned above, for example, to a temperature of 1000 to 1800° C. for 1 to 8 hours in an atmosphere of neutral gas such as argon, optionally in the presence of hydrogen and water or alternatively in a reducing atmosphere. a medium containing hydrogen and, if appropriate, a neutral gas such as argon, wherein hydrogen is present in the mixture in an amount of up to 5% by volume and, if appropriate, contains water in an amount of up to 20,000 ppm.

Other features and advantages of the invention will become apparent from the additional description below, which relates to an example of the preparation of a mixed powder and fuel pellets according to embodiments of the methods of the invention.

It goes without saying that this additional description is given only to illustrate the invention and does not limit it at all.

Detailed description of particular embodiments

Example 1

This example shows the application of the method according to the invention to obtain a mixed powder containing uranium oxide UO ₂ and plutonium oxide PuO ₂ at a ratio of (Pu/U+Pu) = 10 at. %, and the method was completely carried out in a glove box.

70 ml of demineralized water (representing 40 wt.% of the total weight of the final suspension) was introduced into a 250 ml plastic container containing about 250 g of zirconium dioxide grinding balls (with an average diameter of 3 mm). Were introduced dispersant and binder (respectively DARVAN C and polyethylene glycol 300) in the amount of respectively 0.5% and 2% by weight of dry oxide substance, namely the total weight of UO ₂ and PuO ₂ .

After rapid mixing of these ingredients, a powder mixture of UO ₂ and PuO ₂ was introduced in an amount of 60 wt.% of the final mass of the suspension, i.e. 105 g of the powder mixture, which contained 89 g of UO ₂ powder and 16 g of PuO ₂ powder. The resultant mixture was spun in a rock and roll mixer at 35 rpm to prevent sticking of the powders and provide better dispersion. Stirring was continued for at least 5 hours until a fluid suspension was obtained.

The viscosity of the suspension was checked with an ANTON PAAR RHEOLAB QC rheometer at 1500 s ^-1 and it was 100 mPa.s at a shear rate of 1500 s ^-1 , which corresponds to the preferred range of less than 300 mPa.s, which is especially preferable for carrying out cryogenic granulation of the suspension.

Cryogenic granulation was carried out in a device containing:

- a beaker for said suspension, connected to a peristaltic pump, with which the suspension was supplied to the nozzle of the atomizer, while the pump capacity was not more than 2 l/h at an air pressure of 0.15 bar,

- a Dewar-type reactor filled with liquid nitrogen, connected to the atomizer nozzle and providing instantaneous freezing of suspension droplets emerging from the atomizer nozzle.

The pre-prepared suspension should be placed in said beaker, then withdrawn by means of a peristaltic pump at a flow rate of 33 ml/min. and an air pressure of 0.15 bar and direct to the Dewar vessel type reactor through the spray nozzle. The resulting droplets freeze directly under the action of liquid nitrogen in the reactor. The volume of suspension obtained in this example takes less than 5 minutes to carry out cryogenic granulation.

After freezing, the resulting granules are quickly placed in a lyophilizer to sublimate the water frozen inside the granules while maintaining their spherical shape. Such a freeze-drying operation takes at least 3 hours in order to reach a stable vacuum of 10 ^-3 and a temperature of about -100°C at the end of the run.

After complete removal of water from the granules, the latter are characterized by a particle size distribution with a monomodal focus on a size of 200 μm and they are suitable for use in this form for molding tablets by compression.

Example 2

This example shows the preparation of MOX UO ₂ /PuO ₂ fuel pellets from the granular powder obtained in Example 1 above.

Meanwhile, the powder was subjected to uniaxial cold pressing at 700 MPa while externally lubricated with stearic acid, whereby tablets of 9.5 mm in diameter and 10 mm in thickness were obtained. Then the resulting tablets were subjected to sintering at 1750°C for 4 hours in an argon atmosphere with a content of 4 vol. % hydrogen, while the temperature of 1750°C was reached at a growth rate of 3°C/min.

The sintered pellets had a relative density of about 94-98% with good uniformity of the U and Pu elements in the pellets (due to the good uniformity of these elements in the powder). These elements were distributed more evenly than in powder metallurgy-made MOX pellets.

Claims (15)

1. A method for producing a powder containing uranium oxide UO ₂ , if necessary, plutonium oxide PuO ₂ and, if necessary, americium oxide AmO ₂ and / or oxide of another minor actinide MO ₂ , where M means neptunium or curium, including the following stages: a) the stage of preparing an aqueous suspension by contacting water, uranium oxide powder UO₂, if necessary plutonium oxide powder PuO₂ and, if necessary, americium oxide powder AmO₂ and/or oxide powder of another minor actinide MO₂, where M means neptunium or curium, at least one additive selected from anticoagulants, organic binders and mixtures thereof, and the additive or additives are introduced in such an amount that the dynamic viscosity of the aqueous suspension does not exceed 1000 mPa⋅s; b) the stage of cryogenic granulation of the suspension prepared in stage a); c) a freeze-drying step for the granules obtained in step b), by which a powder is obtained containing uranium oxide UO ₂ , plutonium oxide PuO ₂ and, if necessary, americium oxide AmO ₂ and/or oxide of another minor actinide MO ₂ , where M means neptunium or curium. 2. The method according to claim 1, wherein the content of UO ₂ oxide powder, optionally PuO _{2 plutonium oxide powder and optionally AmO 2 americium} oxide powder and/or oxide powder of another minor actinide MO ₂ , where M means neptunium or curium, is from 10 to 15 vol. % of the volume of water in suspension. 3. The method according to claim 1 or 2, wherein the additive is a mixture of at least one anticoagulant and an organic binder. 4. The method according to any one of paragraphs. 1-3, in which the anticoagulant is an ammonium polymethacrylate or a polycarboxylate ether. 5. The method according to any one of paragraphs. 1-4, in which the content of the anticoagulant is from 0.02 to 1 wt.% of the weight of the dry matter of the suspension, i.e. based on the total weight of the oxide(s) UO ₂ , optionally PuO ₂ , optionally AmO ₂ , optionally MO ₂ , where M is neptunium or curium. 6. The method according to any one of paragraphs. 1-5, in which the organic binder is polyvinyl alcohol, polyethylene glycol, poly(vinyl butyral), acrylic latex, or a mixture thereof. 7. The method according to any one of paragraphs. 1-6, in which the content of the organic binder is from 0.1 to 3 wt.% of the weight of the dry matter of the suspension, i.e., of the total weight of the oxide(s) UO ₂ , optionally PuO ₂ , optionally AmO ₂ , with the need for MO ₂ where M means neptunium or curium. 8. The method according to any one of paragraphs. 1-7, in which the dynamic viscosity of the suspension does not exceed 300 mPa⋅s. 9. A method for manufacturing a nuclear fuel tablet (tablets), including the following successive stages: d) obtaining a powder by a method according to any one of paragraphs. 1-8, e) compaction of the powder obtained in step d) in the form of tablets, f) sintering the tablets obtained in step e).