RU2344502C2 - Method of producing tableted fuel for nuclear reactor heat-emitting elements and production line to this effect - Google Patents

Abstract

FIELD: physics, nuclear physics.

SUBSTANCE: mixing the uranium oxide initial powders is carried out in two stages, i.e. preliminary mixing in a planetary-auger mixer in quantities and subsequent mixing in the complex processing plant built around a hammer-type mill-crusher or a circular milling-scattering device. The propose production line is additionally furnished with a mixing plant and a complex processing plant to be consecutively installed ahead of the plant designed to mix uranium oxide powder up with a dry plastifier.

EFFECT: expanded range of uranium oxide initial powders, higher process efficiency, higher yield of fuel tablets, reduced dust formation and better ecology.

6 cl, 5 dwg, 2 tbl

Description

The invention relates to the nuclear industry and can be used in enterprises for the manufacture of pelletized fuel from uranium dioxide for fuel elements (fuel elements) assembled in fuel assemblies (FAs) and used in commercial nuclear reactors (for example, VVER type).

A known method of producing powders of uranium oxides with a given value of the conditional mass fraction (UMD) of uranium-235, which consists in the fact that uranium-containing powders with high and low UMD of uranium-235 are pre-mixed to the required UMD of uranium-235 (see Mayorov A.A. ., Braverman IB Technology for the production of powders of ceramic uranium dioxide. M: Energoatomizdat, 1985, chapters 4 and 7). This method involves dissolving the entire initial mixture of powders in nitric acid, extraction purification followed by precipitation of the obtained ammonium polyuranate and obtaining the finished uranium dioxide powder by hydrogen reduction of calcined ammonium polyuranate. The disadvantages of this method are the high complexity, losses, high consumption of chemicals, a large amount of waste.

There is also a method of producing powders of uranium oxides with a given value of the conditional mass fraction (UMD) of uranium-235, in which only part of the mixture of powders with different values of the UMD of uranium-235 is subjected to dissolution in nitric acid (see patent RU 2186031 IPC ⁷ C01G 43/025 dated 05/04/2000, published July 27, 2002), and the entire volume of the powder with high UDM of uranium-235 and part of the powder with low AMD of uranium-235 are subjected to mixing, providing a certain content of uranium-235 in the mixture after preliminary mixing. The resulting mixture was dissolved in nitric acid and subjected to extraction purification (if necessary). Ammonium polyuranates are precipitated with ammonia, then they are dried and calcined. The obtained powder of uranium oxide-uranium is crushed in a known manner, for example, using disk, impact disk or ball mills, to particle sizes less than 100 microns. Next, the product thus treated is mixed with the remainder of the powder with low UDM of uranium-235. The mixing process is carried out, in particular, in a planetary screw mixer. At the final stage, hydrogen reduction of the resulting mixture is carried out to obtain uranium dioxide powder. It is also possible to recover only the remaining part of the powder with low UDM of uranium-235, followed by mixing with the processed product.

The disadvantages of this method include:

- the inability to use at the stage of the final mixing of powders only uranium dioxide;

- the inability to use at the stage of the final mixing of powders with a particle size of more than 100 microns, which generally leads to the production of a non-flowing mixed powder;

- the method does not disclose the possibility of using the obtained powder for direct compression with a dry plasticizer, followed by the manufacture of tablet fuel without using the granulation stage;

- high complexity of the process.

A known method of producing powders of uranium oxides with a given value of the conditional mass fraction (UMD) of uranium-235, namely, that uranium-containing starting powders with high and low UMD of uranium-235 are mixed to the required degree of uniformity and conditional mass fraction of uranium-235, with this is directed to the mixing of powders whose particle size satisfies certain ratios (see RF patent 2200130 C1 IPC ⁷ C01G 43/025 of 03/03/2001, publ. 10.03.2003). According to this method, preliminary sieving of samples of low- and highly enriched powders is carried out by dry method through a set of sieves or direct measurement of particle sizes, for example, using a laser analyzer. If the particle sizes do not meet the specified values, the powders are sent for grinding.

It was experimentally established that when mixing uranium-containing powders with various enrichment, the determining factor that significantly affects the quality of the resulting nuclear fuel is the particle size of the used initial uranium-containing powders. The following relationship between the particle sizes of low- and highly enriched powders and enrichment was revealed:

Dn≤530-14 * Rv;

Db 0 0.85 * Dn * (Rb-Rn) ^-0.4 * (R ₃ ) ^0.6 ;

Dн / Dв≤3;

where Dн and Дв are the particle sizes of the initial powders with low and high values of the conditional mass fraction (UMD) of uranium-235, respectively, mcm;

Rz - the specified value of UMD of uranium-235, wt.%;

Rн and Rв - UMD of uranium-235,%, in the initial powders with low and high enrichment, respectively, with (Rв-Rн) ≤22.

Grinding of "relatively large" powders is carried out using impact disk or ball mills. The grinding regimes are determined based on the particle size of the powders. Then crushed powders with various enrichment are sent to the mixing, which is carried out in a mixer type "drunk barrel" or a planetary screw mixer. The spread of the conditional mass fraction (UMD) of uranium-235 in point samples is determined. The result must comply with the acceptance requirements, namely - "the specified value of the UMD of uranium-235 ± 0.05%." If necessary, the resulting mixture can be re-ground to improve the quality of mixing.

The required degree of homogeneity of the mixed product is calculated based on the operating conditions of nuclear fuel in the reactor. For tablets of nuclear ceramic fuel enriched up to 5%, the existing requirement for uniformity in the conditional mass fraction (UMD) of uranium-235 is as follows: the UMD of uranium-235 determined in three point samples weighing 0.3 g, taken in one piece from one tablet, must be in the range of "the specified value of the UMD of uranium-235 ± 0.05%." For ceramic grade uranium oxide powders, the homogeneity requirement is formulated in the same way: URM-235 UMD values determined in at least three point samples of 0.3 g mass taken from the studied volume of powder should be in the range “preset URM-235 ± 0.05 ± 0 , 05%. "

As the initial uranium-containing powders can be used as uranium oxide U ₃ O ₈ which before or after mixing is reduced to UO ₂ and directly uranium dioxide UO _2. Also, uranium dioxide UO ₂ can be used as a powder with a low content of uranium-235, and uranium oxide U ₃ O ₈ as a powder with a high content of uranium-235.

The method allows to increase the uniformity of the mixed powder and to reduce the amount of product directed to dissolution and extraction, due to the introduction of the grinding stage. The method provides the possibility of direct mixing of UO ₂ powders that satisfy the specified requirements for the maximum particle size, without additional grinding.

The disadvantages of this method:

- there is no possibility of mixing the initial powders of ceramic grade uranium dioxide with different values of the conditional mass fraction (UMD) of uranium-235 (in the range up to 5.0%), the particles of which have a size greater than the calculated maximum size (460-490 microns for a lower mixed powder enrichment and 300-1000 microns for a mixed powder of higher enrichment, depending on the combination of the UMD of uranium-235 in the starting and powders and the final mixture);

- the method does not disclose the possibility of obtaining a fluid mixed powder. As is well known from experience, grinding UO ₂ powder to a particle size of less than 460-490 μm results in a powder that does not have flowability (the ability to freely fill molds of a pressing unit having an inner diameter of not more than 10 mm) and the need to use granulation or agglomeration stages together with a grinding stage. Since each stage is dusty, an increase in the number of stages in a line leads, in addition to increasing the overall complexity, to serious difficulties in the practical implementation of known methods from the point of view of compliance with environmental standards;

- not disclosed the possibility of implementing the method in a single technological line for the production of pelletized fuel for fuel elements, which allows to obtain tablets of uranium dioxide UO ₂ from the obtained powder with a given content of uranium-235. It is known that uranium dioxide powders obtained by different technologies UO ₂ noticeably differ from each other both in technological properties and in the stability of these properties. Therefore, in each case, the development and optimization of the entire production line is required, taking into account the features of the used uranium dioxide powders UO ₂ .

A known line for the manufacture of pelletized fuel for fuel elements, including:

- a unit for mixing powders of uranium dioxide UO ₂ , uranium oxide U ₃ O ₈ and dry plasticizer in biconical containers with a volume of 250 l;

- a unit for pressing tablets from a mixture of powders of uranium dioxide UO ₂ , uranium oxide U ₃ O ₈ and dry plasticizer;

- a tablet sintering unit, which is a tunnel-type furnace, having several temperature zones in which heating, sintering in a reducing medium and tablet cooling are carried out, with the advance of boats with tablets through the furnace zones;

- unit wet grinding the surface of the tablets;

- drying unit (see “Development, production and operation of fuel elements of power reactors” under the editorship of F.G. Reshetnikov, M., Energoatomizdat, 1995, book 1, pp. 93-95, 98-99, 101- 102).

The disadvantage of this line: when mixing powders of uranium dioxide UO ₂ , uranium oxide U ₃ O ₈ with different UMD values of uranium-235 in biconical containers with a volume of 250 l in two-plane mixing machines with the subsequent introduction of a dry plasticizer, the required degree of homogeneity in UMD of uranium-235 is not achieved in fuel pellets made in this way.

The closest in technical essence and the achieved result to the proposed one is a method for the manufacture of pelletized fuel for fuel elements, including the preparation of initial uranium oxide powders with high and low uranium-235 content and particle sizes satisfying certain ratios, mixing the prepared powder with uranium oxide powder U ₃ O ₈ and dry plasticizer, pressing tablets from a mixture of powders, sintering tablets, grinding, drying and rejecting tablets (see patent RU 2158030 C2, IPC ⁷ G 3/62, 21/10 of 11/18/1998, published on 10/20/2000) - a prototype, the disadvantage of which is the inability to achieve the required degree of uniformity in the conditional mass fraction of uranium-235 in fuel pellets.

The closest in technical essence and the achieved effect (prototype) is the line for the production of pelletized fuel for fuel elements, including:

- a unit for mixing powders of uranium dioxide UO ₂ , uranium oxide U ₃ O ₈ and dry plasticizer;

- a unit for pressing tablets from a mixture of powders of uranium dioxide UO ₂ , uranium oxide U ₃ O ₈ and dry plasticizer;

- a unit for sintering tablets, which is a tunnel-type furnace with several temperature zones, in which heating, sintering in a reducing medium and cooling of tablets are carried out, with the promotion of boats with tablets through the zone of the furnace;

- unit wet grinding the surface of the tablets;

- tablet drying unit;

- unit for processing defective tablets;

- vehicles wiring from the unit to the unit of powder and tablets (see patent RU 2158030 C2, IPC ⁷ G 3/62, 21/10 of 11/18/1998, publ. 10/20/2000).

In the prototype device, the units are combined into a production line, which is equipped with a vehicle for transporting powders and tablets from the unit to the unit.

The disadvantage of this device is the inability to manufacture pelletized fuel with a given value of the conditional mass fraction (UMD) of uranium-235 from uranium dioxide powders with different UMD values of uranium-235.

A single technical objective of the invention is to provide a method for the manufacture of pelletized fuel with a given value of the conditional mass fraction of uranium-235 (not exceeding 5%) from uranium dioxide powders with different values of the conditional mass fraction of uranium-235 using aggregates combined in a line and providing a powder and uranium dioxide tablets with a given value of the conditional mass fraction of uranium-235 with optimal technological properties, as a result of which it is possible to expand the range of initial powders of wild uranium sulfide, to increase the productivity of the process, increase the yield of suitable tablets and their quality, reduce dust formation and reduce the environmental hazard of production.

The stated technical problem is solved in that in the method for producing powders of uranium dioxide of a nuclear ceramic grade obtained by any of the methods, namely, by wet ADU or AUK process, by the dry method - gas-flame, dry conversion, or another method, which consists in that uranium-containing starting powders with high and low uranium-235 content are mixed to the required degree of homogeneity, according to the invention, the starting powders are mixed in the calculated ratio in an inert gas medium, processed in Sulfur gas in the unit, combining the functions of grinding, sieving and mixing.

Mixing is carried out in two stages: preliminary mixing in a planetary screw mixer and grinding-sieving-mixing in the unit performing complex powder processing (the options for such units are a hammer mill - a rotor-type mill and an annular grinding-scattering device) with continuous loading and unloading in biconical containers.

Mixing is carried out with alternate loading of portions (weighing no more than 700 kg) of the initial UO ₂ powders with different values of the conditional mass fraction of uranium-235, with stirring in four to eight cycles with a rotation speed of the mixing screw of 0.5-1.0 rpm, with a duration of each cycle of 15-20 minutes with stops between stages of 10-15 minutes, when 250-350 kg of powder is unloaded after four cycles and loaded into the upper part of the mixer, nitrogen is supplied to the mixer with a flow rate of 1-3 m ³ / h , with a working volume of the mixer of 0.5-3.5 m ³ , with a volumetric load factor of cm At least 0.35 inhabitants.

The problem is also solved by the fact that complex processing in a hammer mill of rotary type is carried out at a speed of rotation of hammers of 100-2000 rpm, with a mesh size of 500-800 microns.

The problem is also solved by the fact that complex processing in an annular grinding and dispersing device is carried out with a mesh size in the range of 500-630 microns.

The stated technical problem is solved by the fact that the line for implementing the method of manufacturing pelletized fuel for fuel elements of a nuclear reactor, comprising a unit for mixing powders of uranium dioxide UO ₂ , uranium oxide U ₃ O ₈ and dry plasticizer; a unit for compressing tablets from a mixture of powders of uranium dioxide UO ₂ , uranium oxide U ₃ O ₈ and dry plasticizer; the tablet sintering unit, which is a tunnel type furnace with several temperature zones, in which heating, sintering in a reducing medium and tablet cooling are carried out, with the promotion of boat tablets through the furnace zones; unit wet grinding the surface of the tablets; tablet drying unit; processing unit for defective tablets; vehicles wiring from the unit to the unit of powder and tablets, according to the invention, the line is additionally equipped with an unit for mixing uranium dioxide powders with different values of the conditional mass fraction of uranium-235 in the form of a planetary screw type conical mixer and a complex powder processing unit UO ₂ in the form of a hammer mill rotary type, equipped with six U-shaped hammers and mesh with cells made of stainless steel, or in the form of an annular grinding and scattering device containing a housing in the form of an annular container placed on vibration mounts, in the annular space of which crushing mechanisms are placed at equal distance from each other in the form of mechanical plates pivotally mounted on one side with corrugated lower surfaces interacting with metal plates located under each of them, and removable grids with cells a given size, located on the same level with metal plates in the rest of the annular space of the tank.

The application of the two-stage mixing method in a line equipped with the described mixing units, due to the intensity of the impact of the grinding body on the powder, is maintained at a level sufficient to eliminate the largest powder particles, but not leading to a significant increase in the proportion of small particles (less than 71 microns) that impair fluidity. In addition, due to the presence of a grid with cells of a given size, complex processing units also provide screening of small particles without excessive grinding, which eliminates the risk of a non-flowing powder. Subsequent filling of the press powder into the matrix of rotary press machines does not cause difficulties due to the optimum technological properties of the powder, primarily fluidity.

The invention is illustrated by drawings. The drawings show:

- figure 1 is a block diagram of a line for implementing a method of manufacturing a tablet fuel;

- figure 2 - aggregate mixing powder;

- figure 3 - aggregate complex processing of powder: rotary hammer mill;

- figure 4 - hammers,

- figure 5 - aggregate complex processing of the powder: ring grinding and dispersing device.

The line for implementing the method of manufacturing pelletized fuel for fuel elements of a nuclear reactor includes a unit 1 for mixing uranium dioxide powders UO ₂ with different values of the conditional mass fraction of uranium-235, complex processing unit 2 (with simultaneous sieving and intensive mixing) of a mixture of uranium dioxide powders with a given value UMD uranium-235, mixing unit 3 uranium dioxide powder with a predetermined conditional value of mass fraction of the uranium-235, uranium oxide powder of U ₃ O ₈ and a dry plasticizer (stearate ching a, DISED or the like.), the unit 4 pressing the tablets from a mixture of uranium dioxide powder UO _2, uranium oxide U ₃ O ₈ and a dry plasticizer unit 5 as a furnace tunnel type sintering pellets in a reducing hydrogen atmosphere, the wet grinding machine 6 side tablet surfaces, unit 7 for drying tablets in a nitrogen medium, unit 8 for sorting tablets in appearance, unit 9 for processing defective tablets.

Units 1, 2, 3, 4, 5, 6, 7, 8 are combined into a production line, which is equipped with vehicles 10 for transferring powders from one unit to another. The complex processing unit 2 may precede the mixing unit 1.

The powder mixing unit 1 (FIG. 2) in the form of a planetary screw type conical mixer with a working volume of 0.5 m ³ to 3.5 m ³ contains a mixing vessel 11, a pivot arm 12, a mixing screw 13, an outlet 14, a support part 15 of the bridge structure, mounting loop 16, connecting flange 17, mixing auger motor 18, rotary arm gear motor 19.

The complex powder processing unit 2 (Figs. 3, 4) in the form of a mill (crusher) of a rotor type with U-shaped hammers includes a loading device 20, a mesh 21 with cells made of stainless steel, six U-shaped hammers 22 located in the rotor 23 mills; powder discharge device 24.

The complex powder processing unit 2 (Fig. 5) in the form of an annular grinding and scattering device contains a housing 25 made in the form of an annular container, in the annular space 26 of which crushing mechanisms 27 are arranged in the form of metal joints hinged on one side plates 28 with corrugated lower surfaces 29 interacting with metal plates 30 under each of them, at the same level with which the rest of the annular space 26 of the annular container 25 is blocked by removable grids 31 annymi sizes of holes in them. The annular tank 25 is placed on the vibration mounts 32 with the possibility of reciprocating rotation around its axis and reciprocating movement up and down, and the source 34 with the imbalances 35 of the transmission of the reciprocating movement of the annular tank 25 is placed in the inner cavity 33 of the annular tank 25, for adjustment grinding intensity is designed adjusting bolt 36 in the metal plate 28.

The method is carried out, and the line works as follows. The required quantities of mixed starting powders are calculated, for which uranium oxide U ₃ O ₈ can be used, which, before or after mixing, is reduced to UO ₂ , directly uranium dioxide UO ₂ , uranium dioxide can be used as a powder with a low content of uranium-235 UO ₂ , and as a powder with a high content of uranium-235 - uranium oxide U ₃ O ₈ , uranium oxides.

The calculation is carried out by solving the system of equations of material balance:

where the "SM" index refers to a mixed batch of UO ₂ powder, the "i" index refers to the i-th batch of the initial UO ₂ powder, M is the mass of the UO ₂ powder, N is the mass of the mixture of uranium isotopes (in a dry product), A is the mass the proportion of a mixture of uranium isotopes (in a dry product), H is the mass fraction of moisture, Q is the mass of uranium-235, C is the conditional mass fraction of uranium-235.

A laboratory analysis of the starting powders is carried out to determine the values of the above parameters, as well as the values of the mass fractions of fractions of less than 71 microns and more than 500 microns (and possibly other fractions).

Powders of uranium oxides are mixed with various values of the conditional mass fraction of uranium-235 to the required degree of homogeneity, defined as follows: the values of the conditional mass fraction of uranium-235 of three or more samples of a mixed powder weighing 0.3 g should deviate from the specified value of the conditional mass fraction of uranium -235 no more than ± 0.05%.

The quantities of powders calculated in this way are loaded in a nitrogen medium into a powder mixing unit 1, where uranium dioxide powders UO ₂ with different values of the conditional mass fraction of uranium-235 in a conical mixer 11 of planetary-screw type with a working volume of from 0.5 m ³ to 3, 5 m ^{3 are} mixed in a nitrogen medium in two stages at a rotation speed of the mixing screw of 18 0.5-1.0 rpm The loading is carried out in layers, that is, portions of the powder with a higher conditional mass fraction of uranium-235 alternate with portions of the powder with a lower conventional mass fraction of uranium-235, choosing a portion weight of not more than 700 kg.

The first stage consists of four mixing cycles lasting 15-20 minutes with breaks of 10-15 minutes. Then, 250-350 kg of powder is unloaded from the lower conical part of the mixer 11 through the outlet 4 and loaded into the upper part of the planetary screw mixer 11. The second stage consists of four mixing cycles lasting 15-20 minutes with breaks of 10-15 minutes. Breaks of 10-15 minutes are provided to reduce the temperature of the powder, which is heated during the mixing process. After mixing, the powder is discharged into 250 L biconical containers (not shown).

Determine the uniformity of the mixed powder. The experimental results showed that if the maximum particle size of all the initial UO ₂ powders does not exceed 500 μm, that is, the mass fraction of a fraction of more than 500 μm is zero, the powder mixed in a planetary screw mixer under the above conditions will have the necessary uniformity in conditional mass share of uranium-235. In this case, after mixing, homogenization quality control is carried out according to the conditional mass fraction of uranium-235, for which three samples weighing 0.3 g are taken from each biconical container with the unloaded powder to determine the conditional mass fraction of uranium-235 by the spectrometric method. Mixed powder, satisfying in all samples the requirement “set value of URM uranium-235 ± 0.05%”, is sent to the unit 3 for mixing the powder with a dry plasticizer and subsequent units of the line.

Experimental results showed that if the maximum particle size of one or more of the starting powders exceeds 500 μm, that is, the mass fraction of the fraction of more than 500 μm is greater than zero, the uniformity of the powder mixed in a planetary screw mixer, in most cases, falls outside the range of the “specified value conditional mass fraction of uranium-235 ± 0.05% ”, and the deviation from the permissible range is the greater, the higher the difference in the values of the conditional mass fraction of uranium-235 in the initial powders of uranium dioxide and the higher the mass fraction fractions were more than 500 microns in the starting powders. In this case, after the mixing operation in a planetary screw mixer of the mixing unit 1, this powder is sent for complex processing to the unit 2, combining grinding, sieving and intensive mixing, which can be a horizontal rotor type hammer mill with six U-shaped hammers 22, interchangeable mesh 21 with cells of a given size and adjustable speed of rotation of the hammers (190-2900 rpm), where the uranium dioxide powder is processed at a speed of rotation of the hammers 22 190-2900 rpm with continuous loading and unloading powder. The optimal mesh size of the mill 21 is 500-800 microns. It is also possible to use a mill without a net. The speed of rotation of the hammers 22 and, consequently, the intensity of grinding and mixing is chosen as the minimum allowable, at which the uniformity of the powder of uranium dioxide is achieved with respect to the conditional mass fraction of uranium-235. Excessive grinding of the uranium dioxide powder leads to a loss of its fluidity and difficulties at the pressing stage (usually when the speed of rotation of the hammers is more than 1500 rpm). To determine the optimal speed of the hammers 22 should be guided by the experimentally obtained values presented in table 1.

Table 1.
Optimum hammer mill parameters for various starting powders The difference in the UMD of uranium-235 in the initial powders,% The minimum rotation speed of the mill hammers, rpm The residual amount of a fraction of more than 500 microns,% Powder Flowability No more than 0.4% 190-570 8-15% there is 0.4-0.6% 190-1000 No more than 10% there is 0.7-1.6% 570-1900 No more than 5% No in some cases 1.7-2.0% 1500-2900 No more than 1% No in some cases

Experience has shown that due to the difference in the properties of the initial uranium dioxide powders obtained by various methods and under different modes, the minimum speed of hammers varies from batch to batch, therefore it is recommended to adjust it according to the results of trial processing and analysis of a small amount of mixed powder (100-200 kg).

The complex processing unit 2 can also be made in the form of an annular grinding and scattering device (KPRU).

The powder is loaded into the annular capacity of the housing 5 of the complex processing unit 2. The cell size of the KPRU mesh should be in the range of 500-630 microns, depending on the difference in the values of the conditional mass fraction of uranium-235. The minimum required cell size is chosen, guided by the experimentally obtained values presented in table 2.

Table 2.
Optimal KPRU parameters for various initial powders The difference in the UMD of uranium-235 in the initial powders,% The mesh size, microns The residual amount of a fraction of more than 500 microns,% Powder Flowability No more than 1,6% 630 No more than 5% there is 0.7-2.0% 500 0% No in some cases

The intensity of the impact of the crushing mechanisms 27, the crushing area, the grid area 31, the volume of the working chamber 25 and other parameters of the integrated processing units are selected based on the maximum productivity of the complex processing of uranium dioxide powder without excessive grinding.

Any of the described aggregates can also be used for preliminary reduction of the particle size of one or more of the initial uranium dioxide powders intended for subsequent mixing in the planetary-screw mixer of unit 1.

To determine the homogeneity of the crushed powder according to the conditional mass fraction of uranium-235, three samples weighing 0.3 g are taken from each biconical container with crushed powder to determine the conditional mass fraction of uranium-235 by spectrometric method. A powder that satisfies the requirement “preset URM value of uranium-235 ± 0.05%” in all samples is sent to the subsequent aggregates of the line — mixing 3 with a dry plasticizer, pressing 4, sintering 5, grinding 6, drying and sorting 7.

If there is insufficient homogeneity of the obtained powder with respect to the conventional mass fraction of uranium-235, the mixing operations in the planetary-screw mixer of the mixing unit 1, the complex processing in the complex processing unit 2 is repeated.

To improve the technological properties of the crushed powder, it can be subjected to additional processing (granulation, etc.).

A possible embodiment of the invention is the order of the units in the line in which the complex processing unit 2 is located in front of the mixing unit 1 (planetary-screw mixer), immediately behind the units for the recovery and stabilization of UO ₂ powder with the initial URM value of uranium-235. In this case, the parameters of the complex processing unit 2 are selected so as to obtain powders having a maximum particle size of not more than 500-630 microns. In most cases, at the subsequent mixing stage in a planetary screw mixer, this makes it possible to obtain a fluid mixed powder homogeneous in the relative mass fraction of uranium-235, suitable for further direct compression with a dry plasticizer. If necessary, to achieve uniformity in the conditional mass fraction of uranium-235, the mixed powder is sent to the complex processing unit.

All minimum and maximum parameters are optimal, tested in practice and any deviation to a smaller or greater side will lead to a decrease in the quality of pelletized fuel and to increase the complexity of the process. The proposed method and line are implemented in existing production, a positive result of their application is confirmed by tests. The distribution of the values of the conditional mass fraction of uranium-235 in the range of “the specified value of the UMD of uranium-235 ± 0.05%” achieved in a mixture of uranium dioxide powders provides the distribution of the values of the conditional mass fraction of uranium-235 in pelletized ceramic fuel (also called the variability of the conditional mass fraction of uranium-235) in the range of “preset UMP-uranium-235 ± 0.05%” value, which corresponds to modern requirements for the fuel of commercial nuclear reactors, for example, VVER type.

Claims (6)

1. A method of manufacturing pelletized fuel for fuel elements of a nuclear reactor, comprising preparing the initial uranium oxide powders with high and low uranium-235 content, mixing the prepared powder with uranium oxide powder U ₃ O ₈ and dry plasticizer, compressing tablets from a mixture of powders, sintering tablets , grinding, drying and rejecting tablets, characterized in that the preparation of the initial powders of uranium oxides with a high and low content of uranium-235 is carried out by two-stage mixing in an inert medium in the first stage, the mixing of the powders is carried out in a planetary screw mixer in a nitrogen medium when it is fed into the mixer with a flow rate of 1-3 m ³ / h with a working volume of the mixer of 0.5-3.5 m ³ , with a volumetric loading factor a mixer of at least 0.35 and with variable loading of portions of powder with a mass of each portion of not more than 700 kg in four to eight cycles of 15-20 minutes each, with stops between cycles of 10-15 minutes, after four cycles, 250-350 kg of powder are unloaded again loaded into the upper part of the mixer, and in the second stage produce a mixture ue powder in an inert gas atmosphere to a powder processing unit is integrated, both the mixing step is performed at a continuous loading and unloading of containers in biconical then prepared powders fed to subsequent manufacturing operations pelletized fuel. 2. The method according to claim 1, characterized in that the mixing of the initial powders of uranium oxides in a planetary screw mixer is carried out at a rotation speed of the mixing screw of 0.5-1.0 rpm 3. The method according to claim 1, characterized in that the mixing of the initial powders of uranium oxides in the complex powder processing unit, made in the form of a rotary mill, is carried out at a hammer rotation speed of 100-2000 rpm and mesh sizes of 500-800 microns. 4. The method according to claim 1, characterized in that the mixing of the initial powders of uranium oxides in the complex powder processing unit, made in the form of an annular grinding and scattering device, is carried out at a mesh cell size of 500-630 microns. 5. A production line of pelletized fuel for fuel elements of a nuclear reactor, including a unit for mixing powders of uranium dioxide UO ₂ , uranium oxide U ₃ O ₈ and dry plasticizer, a unit for compressing tablets from a mixture of powders of uranium dioxide UO ₂ , uranium oxide U ₃ O ₈ and dry plasticizer, a unit for sintering tablets, which is a tunnel-type furnace with several temperature zones in which heating, sintering in a reducing environment and cooling of tablets are carried out, with the promotion of boats with tablets through the furnace zones, the unit for wet grinding of the surface of the tablets, the unit for drying the tablets, the unit for processing defective tablets, the vehicles for wiring from the unit to the unit for powder and tablets, characterized in that the line is additionally equipped with UO ₂ , uranium oxide powders arranged in series in front of the unit U ₃ O ₈ and dry plasticizer by a unit for mixing powders of uranium oxides with different values of the conditional mass fraction of uranium-235 in the form of a planetary screw type conical mixer and a complex powder processing unit, made in the form of a rotor-type hammer mill, equipped with six U-shaped hammers and a mesh with cells made of stainless steel, or in the form of an annular grinding and scattering device containing a housing in the form of an annular container placed on vibration supports, in in the annular space of which crushing mechanisms are placed at equal distances from each other in the form of mechanical plates pivotally mounted on one side with corrugated lower surfaces interacting with metal plates located under each of them, and removable grids with cells of a given size, located on the same level with metal plates in the rest of the annular space of the tank. 6. The line according to claim 5, characterized in that the complex processing unit of the powder of uranium oxides is located in front of the mixing unit.

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