EA031829B1

EA031829B1 - Fibrous fuel rod manufacturing method

Info

Publication number: EA031829B1
Application number: EA201700560A
Authority: EA
Inventors: Евгений Константинович Дьяков; Рафаэль Хайбуллович Жалилов; Павел Александрович Зайцев; Александр Антонович Колганов; Анатолий Васильевич Мартыненко; Владимир Николаевич Нужин; Дмитрий Михайлович Солдаткин; Владимир Анатольевич Солнцев; Шамиль Талибулович Тухватулин
Priority date: 2017-11-16
Filing date: 2017-11-16
Publication date: 2019-02-28
Also published as: EA201700560A1

Abstract

The invention is related to nuclear power industry, in particular, to dispersion fuel rods based on fissionable fuel materials for research power reactors where fuel in the form of separate elements (fibres) is distributed uniformly in a matrix of non-fissionable material. A fibrous fuel rod manufacturing method comprises formation of cells in a primary blank, and placement of metal uranium with a preset fuel phase volume fraction Vƒ in said cells. Extrusion of the blank is performed, and the bar obtained is divided into parts. Then cells are formed in the subsequent blank, and parts of the bar obtained from the primary blank are placed in the cells. The steps of dividing the bar obtained from each previous blank and placing its parts into pre-formed cells in subsequent blanks are performed until obtaining the required efficient diameter of fuel fibres, fuel rod cross-section area, distance between fibres and number of fibres. The invention makes it possible to increase fuel rod strength and thermal conductivity due to reduction of interface surfaces between fuel rod elements, provision of uniform and warranted cohesion of elements over interface surfaces, and elimination of a boundary between a fuel rod cladding and a core.

Description

The invention relates to nuclear energy, in particular to dispersive fuel elements (fuel rods) based on fissile materials for research, power reactors, in which the fuel in the form of individual elements (fibers) is evenly distributed in a matrix of non-dividing material. A method of manufacturing a fiber fuel element is that cells are formed in the initial billet in which metal uranium is placed with a given volume fraction of the fuel phase V /. Carry out the extrusion of the workpiece, after which the resulting bar is divided into parts. Then, in the subsequent billet, cells are formed into which the bar parts obtained from the initial billet are placed. The operations of dividing the rod obtained from each previous billet and placing its parts in preformed cells in subsequent billets are carried out until the required effective diameter of the fuel fibers, the cross-sectional area of the fuel rod, the distance between the fibers and the number of fibers is obtained. The invention allows to increase the strength and thermal conductivity of a fuel element by reducing the interfaces between the elements of the fuel element, ensuring uniform and guaranteed adhesion of elements along the interfaces, as well as eliminating the boundary between the shell and the core.

The technical field to which the invention relates.

The invention relates to nuclear energy, in particular to dispersive fuel elements (fuel rods) based on fissile materials for research, power reactors, in which the fuel in the form of individual elements (fibers) is evenly distributed in a matrix of non-dividing material.

Prior art

Dispersion fuel rods for research reactors have a significant variety of geometric shapes - plates (straight and curved), pipes of various profiles, rods having 2, 3 or 4 beam sections in cross section and more complex configurations. Structurally, the main elements of the dispersive fuel rods are the core and the shell, which contains the core. The core is a composition of particles of fissile materials that are distributed in a matrix of non-fissionable materials (metals, alloys, ceramics). Accordingly, there is a wide variety of methods for manufacturing dispersion fuel rods based on smelting, casting, powder metallurgy, rolling, and combinations thereof.

The constructive and technological schemes used must meet the constantly increasing requirements for dispersive fuel elements, one of the main ones being durability and thermal conductivity.

A known method of manufacturing a fuel rod (RU No. 2374705, IPC G21C 21/02, publ. 27.11.2009), according to which an appropriate proportion of the matrix is placed in the casing, heated to the melt and the remaining fraction of fissile material in the form of grains is fed into the melt with constant vibration. Formed dispersion composition.

The disadvantage of this method can be attributed to the uneven distribution of fuel particles with the possibility of the formation of clots (sticking together) and pores between them. In addition, the matrix material has a relatively low strength due to the fact that it is in the cast state, in which the strength of metals is much lower than in the deformed state. Thus, the tensile strength of melted from alloy E-110 (Zr-1% Nb) is 2 times lower than that of cold-deformed by 80%. It is also obvious that any kind of defects arising during the hardening of the matrix material (shrink shells, nestsity, gas inclusions, segregation, cracks) adversely affect the strength and thermal conductivity of the core and fuel rod as a whole.

Another method involves vibrating the mixture of pellets of fuel and the matrix into the zirconium cladding of a fuel element and capillary impregnation, short-term (1-5 min) annealing at a temperature of 840-900 ° C (A. Savchenko, A. Vatulin, I. et. Al. Dispersion type zirconium matrix fuels fabricated by capillary impregnation method, J. Nucl. Mater., 362 (2007), pp. 356-363). At this temperature, the zirconium alloy is completely melted and, under the action of capillary forces, moves and fills the joints (cavities) between the fuel granules, as well as the fuel and the cladding, forming a metallurgical bond, which ensures high thermal conductivity of the fuel core.

However, this method has the same drawbacks as the above, besides, there is always the likelihood of incomplete filling of pores in the process of capillary impregnation, which at the boundary between the core and the shell will lead to the formation of local overheating of the latter.

Both methods are quite complicated and time consuming.

The closest in technical essence and achieved when using the technical result of the prototype of the claimed invention is a method of manufacturing a fuel rod according to the Eurasian patent No. 027036, the date publ. 06/30/2017, IPC G21C 3/02, G21C 21/02, according to which at the first stage of manufacturing a fuel rod form a bimetallic rod with a given proportion of fuel volume phase V ^. Then, the specified bar is divided into parts that are combined into an assembly (for example, seven pieces each), placed into a sealed casing and pushed through a die, reducing it by 3-5 times in diameter. A multi-fiber rod is obtained, which is also divided into parts that are combined into an assembly (for example, seven pieces), placed in a sealed casing, pushed through a matrix (die), obtaining a rod with an increased amount of fuel fibers. The operation is repeated to obtain the required amount of fuel fibers N with a certain diameter d and distance Δ between the fuel fibers. Then, the resulting rod is brought to the desired diameter by drawing, profiled, divided into fuel rods of the desired length and sealed ends. The method ensures uniform distribution of fuel in the core of a fuel element.

However, this method is not free from drawbacks:

between the cladding core and the core, as well as between adjacent shells of the fuel fibers along their lateral surfaces, splitting and uneven adhesion are possible, leading to inequality and deterioration of the thermal conductivity of the fuel rod and due to the presence of cavities between the parts of the assembly rods along their lateral surfaces. These cavities can be formed when stacking rods in a sealed envelope before the extrusion operation and significantly reduce the adhesion of their side surfaces during the process of compression during flow through the die plate;

the boundaries between the shell and the core, as well as between the shells of adjacent fibers, are

- 1 031829 stress concentrators, and reduce the strength of the fuel rod, and reduce thermal conductivity in the radial direction due to thermal contact resistance.

DISCLOSURE OF INVENTION

The task was to increase the strength and thermal conductivity of a fuel element, due to the fact that, with the same parameters as the prototype (for example, the number of fibers), the number of elements forming the fuel elements and, accordingly, the number of interfaces between the elements, including the boundary between the core, as well as to form the conditions for uniform and guaranteed adhesion of elements along the interfaces.

To solve the task and achieve when using the invention, the technical result in the dispersive (fiber) fuel element in its manufacture, including placing in the billet fuel rod of uranium metal with a given volume fraction of the fuel phase Vf, extrusion of the workpiece, followed by dividing it into several parts, according to the invention in the initial billet, cells are formed in which metal uranium is placed, and after extrusion, the resulting bar is divided into parts, then in the subsequent billet to form tons of cells in which parts of the rod are obtained from the original billet, while dividing the rod obtained from each previous billet and placing its parts in the pre-formed cells in subsequent billets is carried out until the required effective diameter of the fuel fibers, cross-sectional area of the fuel element, distance between the fibers and the number of fibers.

In particular cases of carrying out the invention, in the last workpiece of the cell, in longitudinal and / or cross sections, they perform different profiles and / or sizes and are placed at different distances from each other to provide a variable volume fraction of the fuel phase in the axial and radial directions, respectively;

the distance between the cells is performed from the condition of uniformity of the thermal field over the cross section of a fuel element;

one or more cells are left free;

one or more cells filled with burnable absorber or propellant.

Also, in any or all of the blanks, the central cell / cells are performed to a lesser depth than the peripheral cells.

For the manufacture of fuel rods according to the proposed method, as well as in the prototype, it is advisable to use uranium metal hardened from the β-phase, which due to the fine-grained structure has a high ductility, which allows to obtain thin fuel fibers. In addition, the effect of high pressure and temperature in the manufacturing process during the reduction of uranium quenched from the β-phase, decreases its radiation swelling during operation.

The invention allows to obtain a fuel element with good strength and thermal conductivity due to the absence of cavities between the shells of the fibers, the core and the sheath of the fuel rod, i.e.

a shell-free, symmetric, equal-strength fuel rod structure is formed, in which the presence of matrix bridges between the fuel fibers ensures, during compression, the same plastic deformation of the matrix and the fuel and their reliable adhesion along the adjacent surfaces;

the matrix and the sheath of the fuel rod form a single body, which improves the heat transfer from the matrix to the coolant;

the possibility of making a fuel rod with variable V _f both in radial and axial directions, it allows to align the temperature of the fuel element along the axis and in the radial direction, to increase the fuel burnup;

simplifies the process of manufacturing a fuel rod by eliminating the need (in relation to the prototype) of manufacturing the original bimetallic uranium-zirconium rod.

The list of figures drawings and other materials

The invention is illustrated by figures of graphic images.

FIG. 1 shows the appearance of the original seven-cell blank;

in fig. 2 shows the appearance of the subsequent (second) billet with parts (segments) of the rod obtained in its cells, obtained after the extrusion of the initial billet;

in fig. 3 shows the appearance of the workpiece with 343 fuel fibers;

in fig. 4 shows a longitudinal section of the last preform in which cells are made of different diameters in the axial direction;

in fig. 5 shows the cross section of the last workpiece in which the cells are made of different diameters in the radial direction;

in fig. 6 - section of a fuel rod in which the distance between the cells is selected from the condition of uniformity of the thermal field;

in fig. 7 shows a workpiece in which the central cell or cells are made to a lesser depth than the peripheral cells;

in fig. 8 shows the sectional diagrams of the proposed fiber fuel element (a) and fuel rod adopted as prototype (b), showing the difference in the structure of their boundaries with the same number of fuel fibers

- 2 031829

133.

The implementation of the invention

For the manufacture of a cylindrical fuel rod with a circular cross section (for example, S = 4 mm ² with a diameter of D = 2.3 mm) with a predetermined volume fraction of the fuel phase V / = 0.1, the initial (first) cylindrical billet is first made of E110 zirconium alloy with seven cells in the form of holes (Fig. 1), which provide the most dense packaging fuel fibers in a cylindrical billet. The dimensions of the billet are determined on the basis of V / and the degree of compression of the billet, in which metallic uranium is placed in the form of rods. The distance Δ between the fuel fibers with a diameter d of the fuel fibers is determined from the ratio

when the fuel fibers are located on an equilateral triangle.

Next, the sealed initial cylindrical billet of alloy E110 with fuel rods in the holes is enclosed in a copper sheath and subjected to extrusion (compression) by extrusion through a die. The diameter of the workpiece is reduced by 3-5 times. Then the copper shell is removed. The resulting preform in the form of a rod with seven fibers (see Fig. 1) is calibrated to give it a cylindrical shape, and then divided into parts.

Next, make the next (second) cylindrical billet of alloy E110 also with seven holes (Fig. 2), in which are placed the rod parts, obtained after compression and division of the original (first) blank. Seal, enclose it in a copper shell and push through a die plate. The diameter of the workpiece is also reduced by 3-5 times. The copper shell is removed. Dragging the resulting rod to the required size of the fuel rod cross section (S = 4 mm ² and diameter D ~ 2.3 mm). The bar already contains 49 fuel fibers with an effective diameter of d ~ 0.25 mm and a distance between fuel fibers Δ ~ 0.5 mm. The bar is profiled by drawing through a die or on rolls of the required profile, twisted with a pitch (20-50) mm, divided into fuel rods of the desired length with subsequent sealing of the ends.

To obtain fuel rods with the number of fuel fibers N = 343 and a smaller diameter of the fuel fibers, the next (third) cylindrical billet is made from E110 alloy also with seven cells, into which sections of the rod obtained after crimping the previous (second) billet are laid (see Fig. 2 ). The workpiece is enclosed in a copper sheath and is also forced through a die plate with the subsequent removal of the copper sheath. In this case, with seven cells in the workpiece, we get a bar in which there will be 343 fuel fibers (see Fig. 3) with an effective diameter d ~ 0.05 mm and a distance between the fuel fibers Δ ~ 0.1 mm. Next, the rod is adjusted by drawing to obtain the desired cross-section of the fuel rod D ~ 2.2 mm, profiled on the appropriate rolls (or by drawing) to give the desired shape of the cross-section of the fuel rod. After twisting, cut into fuel rods of the desired length and seal the ends.

For the manufacture of square fuel rods with a square cross-section of the cell in the workpieces are placed in the nodes of a square grid and the geometry of the original workpiece is determined from the ratio

The initial billet of alloy E110 may be square (or some other shape), in which cells for uranium metal rods are made in compliance with the specified V /. Further, the manufacture of fuel rods is made similarly to the previous version of the manufacture of a round fuel rod.

Other options.

1) FIG. 4 shows a longitudinal section in the last workpiece, where cells are made of different profiles (sizes) to provide a variable volume fraction of the fuel phase in the axial directions. For the manufacture of a fuel element with a variable content of uranium metal along the length of a fuel element at the last stage of reduction, the resulting 49-fiber rods are calibrated to different diameters, then the segments with different diameters are placed in the cells provided for them. Such a performance of a fuel rod also allows you to equalize the temperature along its length.

2) FIG. 5 shows the cross section of the last billet of a fuel element in which cells are made of different diameters with their increase from the center to the periphery to provide a variable volume fraction of the fuel phase in the radial direction and, accordingly, to ensure the condition of uniformity of the thermal field over the cross section of the fuel element.

3) In FIG. 6 shows that the distance between the cells in the last workpiece under metal uranium rods is made with an increase in the distance between them in the peripheral zone relative to the central one. With this design, V / in the radial direction will be variable from the center to the edges. In the last billet (see Fig. 5), one or more cells are left free or filled with or burnable absorber. This allows more efficient use of fuel material due to the alignment of the neutron field, respectively, to lengthen the campaign. As a displacer

- 3 031829 heat-resistant materials are used (molybdenum, niobium, tantalum, titanium and other materials). A displacer may be located in the central cell to reduce the temperature in the center of the fuel element. As burnable neutron absorbers used boric (boron carbide, B _four C) or gadolinium (gadolinium trioxide, G2O3) compounds.

4) FIG. 7 shows a cross section, for example of an initial blank, with cells made at different depths, in which fuel rods of the appropriate length are placed. The central cell (or cells) is made to a smaller depth than the neighboring peripheral cells. This makes it possible to significantly reduce the displacement of the ends of the fuel filaments relative to each other by 1.5-2 times, obtained after crimping the workpiece ((Yu.N. Sokursky, Ya.M. Sterlin, VA Fedorchenko. Uranium and its alloys. M. Atomizdat, 1971, p. 448, p. 426).

FIG. 8 shows diagrams of sections of fuel rods made according to the proposed method (Fig. 8a) and the prototype method (Fig. 8b), where the thin lines show the interfaces between the fuel fibers and the matrix. It is seen that the number of thermal contact resistances in the fuel element, made by the method adopted for the prototype, is much more (2-4 times) than by the proposed method.

The manufacture of a fuel rod according to the proposed technical solution has several advantages.

A shell-free, symmetrical structure of a fuel element is formed, in which the function of the shell is performed by the peripheral region of the matrix, which does not contain fuel fibers.

TVEL, in essence, is a zirconium rod, reinforced by groups of uranium fibers, which provides higher strength and thermal conductivity compared to the prototype due to the smaller number of stress concentrators and contact thermal resistances.

The proposed method reduces the fragmentation of the fuel element and, accordingly, the number of interfaces between its constituent elements. With the same geometry with the prototype, the length of the borders is reduced by at least 2 times (see Fig. 8).

Equal strength at the boundaries of adhesion is ensured by uniform (identical) plastic deformation of fuel elements during extrusion due to the absence of voids in the original billet.

The method has a high variability in material composition and geometry, since allows you to make fuel rods of various shapes, macrostructures, and uranium loads using the same initial billets.

The possibility of producing a fuel rod with variable V / both in the radial and axial directions makes it possible to equalize the temperature of the fuel rod along the axis and in the radial direction, to increase fuel burnout.

The possibility of manufacturing a fuel rod with internal cavities by making additional free cells to reduce gas swelling (swelling).

Simplified the process of manufacturing a fuel rod by eliminating the need (in relation to the prototype) of manufacturing the original bimetallic uranium-zirconium rod.

Due to the specified technical result, the fuel elements manufactured according to the proposed method have high reliability, a longer service life and provide enhanced technological capabilities for the modernization of existing research reactors when switching from high enrichment fuel rods to fuel elements enriched in U ²³⁵ less than 20%.

Claims

CLAIM

1. A method of manufacturing a fiber fuel element, comprising placing in the billet a fuel element of uranium metal with a given volume fraction of the fuel phase V /, extrusion of the billet with its subsequent division into several parts, characterized in that the cells in which the uranium metal is placed are formed in the initial billet, and after extrusion, the resulting bar is divided into parts, then in the subsequent billet cells are formed into which the bar parts obtained from the original billet are placed, with the operations of dividing the bar, half ennogo from each previous workpiece and placing it in pieces of preformed cells in the subsequent workpieces is performed to obtain the necessary effective diameter of the fuel fiber cross-sectional area of the fuel element, the distance between the fibers and number of fibers.

2. The method according to claim 1, characterized in that the cells in the last billet in the longitudinal and / or transverse sections perform different profile and / or size to provide a variable volume fraction of the fuel phase in the axial and radial directions, respectively.

3. The method according to claim 1 or 2, characterized in that the distance between the cells in the last billet is chosen from the condition of achieving uniformity of the thermal field over the cross section of the fuel rod.

4. The method according to p. 1, characterized in that in the last billet one or more cells are left free.

5. The method according to p. 1, characterized in that in the last billet one or more cells are filled with a burnable absorber or propellant.

6. The method according to claim 1, characterized in that in any or all of the blanks, the central cell is performed at a smaller depth than the peripheral ones.