A method of manufacturing pellets of nuclear fuel from mixed oxide (u, pu)o2from stagnant powder uo2

 

The invention relates to a method for preparing a mixed oxide (U, Pu)O2from stagnant powders UO2. The method includes the dosage and the first mixing (1) powders PuO2and UO2and/or defective products fuel production, the micronization (2) and forced sieving (3) this first mixture, a new dosing and second mixing (4) the thus treated first mixture of powders UO2and, if necessary, rejected products, the introduction of additives (5), tableting (6) the second mixture and sintering (7) formed so tablets. In addition, at least for part of powders UO2undertaking the selection of stagnant UO2and its machining (29) granulating to make him yield before the introduction of granulated thus UO2at least the second mixture. Effect: increase the mechanical strength and fluidity of the powder UO2and, therefore, expansion of the range of powders UO2that can be used for the production of MOX fuel. 10 C. p. F.-ly, 3 ill.

The invention concerns a method of preparation of a mixed oxide (U, Pu)O2from stagnant powders is aemula fuel MOSS, has been subject to various developments, driven by the desire to reuse plutonium extracted in the processing of spent fuel.

Production and irradiation of MOX fuel in light-water reactors are currently being considered as a possible solution to enhance the resistance to the spread of plutonium that exists in a separate fission products form, regardless of civilian or military origin plutonium.

Over the last two decades, we developed several methods of cooking fuel MOSS, and to thin the mixture in some cases used the full grinding powders UO2and PuO2and others were limited to grinding only some parts of these powders.

The way MIMAS (an acronym for MIcronization and MASter blend = micronizing original mix) developed by the applicant of the present invention (see Fig. 1) includes the micronization grinding only part of the final mixture and uses two sequential mixing to ensure isotopic homogenization and take advantage of the fluid products UO2input (especially to ensure proper filling of the matrices presses used for the preformed the first mixture simplifies manufacture (for example, due to the abolition of operations pre-compaction/granulation or spheroidizing mixture mixed oxide) and simplified at the beginning of its industrial development defining quality fuel MOSS users and permits from the Authorities Nuclear Supervision (due to the similarity in the behavior of this MOX fuel and fuel UO2).

Applied in a variety of ways ways MIMAS under the names sometimes differ from MIMAS, but all of them are characterized by two sequential mixing, while in the second mixing use fluid UO2.

UO2that is a raw material for cooking fuel with enriched uranium and in most cases for making MOX fuel, is produced by conversion of uranium hexafluoride. There are industrial methods of transformation, under which receive fluid powder UO2. This occurs, in particular, in two industrial wet methods of transformation, known in this technical field under the appropriate heading "AUC" intermediate product (AmmoniumUranyl Carbonate), and "TU2" unit for converting uranium, in which the transformation. One of the disadvantages of these wet methods of transformation is proizvodnja, some of which do not receive fluid UO2were gradually replaced by dry methods, which allow recycling of waste gas, but which usually give stagnant powder UO2.

Thus, in order to diversify sources of powder UO2for the production of MOX fuel by means of type MIMAS should have the ability to use non-leaking powder UO2.

Known methods of conditioning non-leaking powder UO2to convert the fluent granular material UO2having properties suitable for loading them press tablet press. Have been developed and are used in industrial scale plants UO2various mechanical methods of granulation, such as pre-seal - granulating or agglomerating - spheroidizing.

Experience has shown that such methods of granulation give granules with insufficient mechanical strength for the proper implementation of the second mixing, which distinguishes the ways MIMAS and the like. Under optimum operating conditions of the second mixer granules are damaged and the fluidity of the second mixture is disturbed resulting fuel tabledit in the differential volume reduction, and so on). Alternatively, if the operation mode of the second mixer, or if used for the second mixing apparatus suitable for mild stirring to be mixed powders, the uniform distribution of plutonium in the fuel may be violated, and thus obtained tablets MOSS does not always meet the criteria for a maximum variability of the plutonium content.

In order to avoid the above disadvantages of the method of production of MOX fuel from non-leaking powder UO2which is the subject of the present invention includes a mechanical graining treatment stagnant powder UO2that does not change the chemical (such as stoichiometry) and morphological (such as grain size) properties of powder UO2but which allows nevertheless to give a powder of the required mechanical strength and fluidity for the effective conduct of operations respectively of the second mixing and tableting.

Thus freed from the necessity to use methods type MIMAS flowing powders UO2as raw materials.

At a preferential variant of the invention taken stagnant UO2one of which is supplied in this state is.

In the embodiment of the invention, as the invention non-limiting example, the processing granulation can also be applied to the fraction of non-leaking UO2supplied to the first mixing.

To eliminate the above drawback associated with insufficient mechanical strength UO2, granulated by means of one of the most common methods of air conditioning, mechanical process according to the invention are carried out either by forcing non-leaking powder UO2through a sieve or mesh, or by pressing of the powder into tablets at a high pressure, so what you want to achieve appropriate nerassypchatyh, followed by crushing these pills. If necessary in advance can be added to the powder UO2one or more binder(s) and/or lubricant(s) substance (substances).

Other details and features of the invention will be understood from the claims and detailed description of the drawings attached to the present description and illustrated as non-limiting examples of the production method according to the invention.

In Fig.1 illustrates schematically the preparation stage see the value of fuel from mixed oxide method according to the invention.

In Fig.3 schematically depicts embodiments of the method according to the invention.

In the various figures to denote identical or similar elements use the same legend.

In the basis of the method according to the invention to use non-leaking powder UO2is the method of fabrication of fuel pellets from mixed oxide (U, Pu)O2i.e. in General (Fig.2): - dosing and the first mixing stage (stage 1) powders Rio2and powders UO2and/or defective products of the fuel production - micronization (stage 2) this first mixture, in particular by grinding and compulsory screening (stage 3) product, for example, through a sieve with a mesh size of 250 μm, a new dosing and the second mixing stage (stage 4) the thus treated mixture UO2and, if necessary, defective products, production of fuel - adding and mixing the second mixture lubricating substances and/or substances for regulating the porosity (stage 5), and this last stage may wholly or partly be combined with the stage 4, the pressing (stage 6) of the second mixture in tablet presses (tableting) and sintering (hundred/p> This method of fabrication of fuel pellets of the mixed oxide may also for the thus obtained tablets include stage: - dry grinding (stage 8), - sort by appearance (stage 9), - stacking tablets (stage 10),
- conclusions tablets in the shell and its seal with the formation of fuel rods (stage 11, Fig.1),
- sealing rods
- non-destructive control rods (stage 12) and
Assembly rods (stage 13).

This method according to the invention comprises moreover (Fig.2) mechanical processing by pre-granulation of the entire amount or part of the non-leaking UO2(stage 29). This processing may include, for example:
any (Fig.3) the stage of pressing into tablets (stage 30) stagnant UO2and crushing tablets (stage 31) and, if necessary, screening (stage 32) of crushed material with the formation of flowing granules with the appropriate properties for introduction as a major component of the second mixture (stage 4) or as an option in both mixtures (stages 1 and 4), while maintaining the original chemical composition and grain size of the original UO2,
or agglomerating stage/pre-compaction/granulares the additives in(wok), the mesh or grid and applied to the powder pressure is regulated to form granules, which are relevant to the above properties.

Some non-limiting, method of manufacturing tablets below as an example:
- work rather private parties than continuous,
the plutonium content of the first mixture: 20 - 40% (stage 1),
- grinding (stage 4) in the amount of 60 kg, at least during the effective period of 5 hours,
- use non-leaking powders UO2obtained by wet method (for example: powder ex-ADU or ammonium diuranate) or obtained by the dry method (which is known to experts in the art),
add 0.2-0.5% zinc stearate, and 0-1% of the pore-forming AZB, well-known specialists in the field of technology
- pressing to form tablets (stage 6) at a pressure of from 400 to 700 MPa,
- sintering (stage 7) at least for 4 hours at a temperature of from 1600 to 1760oWith in argon with 5% hydrogen and the ratio of H2/H2O from 10 to 30,
dry centerless grinding (stage 8).

As a non-limiting example of the present invention, the pressing stage 30 moorosi. These pressure values are therefore higher than the values pelletizing pressure (4-10 MPa), typically used in fabrication plants, nuclear fuel UO2. Before pressing in non-leaking powder UO2you can enter a binding and/or lubricating substance, well known to specialists in this field of technology. As a non-limiting example of the present invention, the pressing can be carried out at a pressure of 40-100 MPa.

Also, as a non-limiting example of the present invention, the above-mentioned crushing tablets may be conducted in one or more jaw or roller crushers with a gap 200-250 microns. This fragmentation can be performed screening, if the crusher is missing or may miss granules with a particle size above 250 microns. The detail that can be obtained by crushing, may be usefully introduced as source material in the first mixture (stage 1).

As another non-limiting example of the present invention, punching through a sieve (stage 29) can be carried out in the plant of the type used in the methods of the type MIMAS (stage 3), for conditioning the first mixture (after micronisation stage 2) and before the second CME is the regulation of the maximum particle size of the granules by passing through the same sieve, can directly give granules with the desired characteristics.

Experience has shown that non-leaking powder, prepared according to the method which is the subject of the present invention can be used in existing installations for the production of MOX by regulating the parameters of the second blending operation (stage 4), tabletting (stage 6) and sintering (stage 7) within the range normally used for optimization of the production process depending on the characteristics of different flowable powders UO2used for the production of MOX fuel.

The method according to the invention allows thus to expand the range of powders UO2that can be used for the production of MOX fuel without loss of benefits caused by the similarity between the fuel MOSS, obtained according to the invention, and fuel UO2cooked in an industrial environment using the currently known methods of the same stagnant powder UO2.

Naturally, the present invention is in no way limited to the above-described forms of exercise and it can be made many changes, not going beyond the below fo coming from another type of transformation, other than the conversion of uranium hexafluoride in UO2.


Claims

1. Method for the production of pellets of nuclear fuel from mixed oxide (U, Pu) O2including dosing and the first mixing (1) powders PuO2and UO2and/or defective products fuel production, the micronization (2) and forced sieving (3) this first mixture, a new dosing and second mixing (4) first thus treated mixture, UO2and, if necessary, rejected products, adding and mixing a lubricating substances and/or substances for regulating the porosity (5), separately or in combination with the second stage of mixing (4), tableting (6) the second mixture and sintering (7) thus obtained tablets, characterized in that, at least for part of powders UO2undertaking the selection of stagnant UO2and its machining (29) granulating to make him yield before using granulated thus UO2at least in the second mixing.

2. The method according to p. 1, characterized in that it contains for the specified processing granulation sub>2fragmentation (31) of the obtained tablets before formation fluid crushed material and using at least part of this crushed fluid material to the second mixing (4).

3. The method according to p. 2, characterized in that the pressing (30) is carried out at a pressure of from 40 to 200 MPa.

4. The method according to p. 2, characterized in that for crushing (31) used jaw or roll crusher.

5. The method according to any of paragraphs. 1-4, characterized in that it includes, in addition, the selection of the particle size by sieving (32) granulated UO2before using it.

6. The method according to p. 5, characterized in that by sifting (32) share granulated UO2on at least two fractions of different grain size, while the fine fraction is introduced into the first mixture (1) while the other is introduced into the second mixture (4).

7. The method according to p. 1, characterized in that it includes to perform granulation stagnant UO2his burst through the mesh or sieve, and the amount of additive(s), the mesh or grid and applied to the powder pressure to regulate the formation of granules having the desired properties, such as fluidity and compressibility tableting coast is the, when granulating stagnant UO2it adds grease.

9. The method according to any of paragraphs. 1-8, characterized in that when the granulation stagnant UO2it adds a binder.

10. The method according to any of paragraphs. 1-9, characterized in that sintering (7) of fuel pellets is carried out in argon and hydrogen or nitrogen and hydrogen, at a temperature of from 1600 to 1760oC.

11. The method according to any of paragraphs. 1-10, characterized in that during sintering (7) regulate the partial pressure of oxygen, preferably by regulating the relationship of H2/N2O in the gas to purge for more effective vzaimodeistvie oxides RHO2and UO2.

 

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