Nuclear reactor core

Abstract

The present invention relates to the field of nuclear power engineering, namely the placement of unsheathed fuel assemblies in a nuclear reactor core, aiming to improve the heat dissipation of fuel elements in a nuclear reactor. The nuclear reactor core comprises a hexagonal unsheathed fuel assembly having fuel elements located on a triangular grid with a spacing greater than the diameter of the fuel element cladding. The fuel elements comprise spiral spacer ribs on the cladding and are rigidly fixed in a given angular direction so that the fuel elements are in rib-to-rib contact at a height position at which the fuel elements reach the maximum fuel element cladding temperature during reactor operation. The fuel assembly is positioned in such a way that the fuel elements located on the surface of the fuel assembly form a single triangular grid with the remaining fuel elements in the core, and ensure that all the fuel elements are in rib-to-rib contact at a height position at which the highest fuel element cladding temperature is reached during reactor operation. At the same time, incorrect placement of the fuel assembly in the core is eliminated.

Description

Technical Field

[0001] The invention relates to the field of nuclear power engineering, namely the placement of jacketless fuel assemblies in a nuclear reactor core, and aims to improve the heat dissipation of fuel elements in the nuclear reactor. Background Art

[0002] List of abbreviations used in this article:

[0003] FA: fuel assembly;

[0004] FE: fuel element;

[0005] NR:nuclear reactor.

[0006] The core of a modular fast neutron nuclear reactor with liquid metal coolant is known (Russian Patent 2699229), which is formed by vertically mounted jacket fuel assemblies with carrier elements and spacer strips, the latter located in the upper part, with a triangular grid for mounting cylindrical fuel elements (FEs).

[0007] The core of a water-moderated nuclear reactor is known (Russian Patent 2216056), which comprises a fuel assembly comprising a bundle of fuel elements of hexagonal cross section in a frame, the surfaces of which have specular reflection. The invention solves the problem of reducing the non-uniformity of energy release in the core and preventing deformation of the fuel assemblies due to their close packing to each other.

[0008] The disadvantages of the above solution include too high "idle" flow rate of coolant in the square cells of the fuel elements on the face of the fuel assembly, which reduces the average coolant temperature at the core outlet and deteriorates the power unit performance. The above disadvantages are due to the placement of the fuel assembly in the core and the specular reflection along the surface.

[0009] A known technical solution is described in Russian patent 261770, which proposes a core of a thermal emission converter reactor of a nuclear power plant, in which power generation channels are located throughout the core along the nodes of an equilateral triangular grid, including at the joints of power generation channel bundles.

[0010] In the case of a jacketless design, known technical solutions exclude "idle" coolant leaks at the beam face joints (fuel assembly faces).

[0011] However, the technical implementation of the known invention can ensure reliable operation of the core only when the power generation channels (fuel elements) are "closely" packed (when the distance between the fuel elements is almost equal to their diameter).

[0012] In the case where the fuel elements are densely packed in a triangular grid, the relative area of ​​the free cross-section of the coolant channel in the core (coolant volume fraction) ε is given by the following formula ε = 1-π / (s 2 ·2√3), where s is the relative fuel element spacing, which is equal to the ratio of the fuel element spacing a in the triangular grid to the fuel element diameter d (in this case s=1) is 9.25%. This applies to the extremely low power thermal emission reactor claimed in Russian Patent 261770. For power reactors, the coolant volume fraction ε is in the range of 30%-50%. This value of the cooling liquid volume fraction can be guaranteed when the relative fuel element spacing pitch is in the range of 1.1-1.3, which means that there is a gap between the fuel elements. In this case, in order to avoid fuel element deflection (fuel element deflection will cause local "constriction" of the coolant flow area, thereby causing overheating of the fuel elements), and to eliminate fuel element vibrations that damage the thin-walled cladding, it is necessary to ensure the fuel element spacing along the core height. Summary of the invention

[0013] The technical problem to be solved by the present invention is to increase the reactor power output while maintaining the total coolant flow and average coolant temperature at the core outlet, and to enhance the reliability of correctly placing the fuel assembly at the corresponding position in the core.

[0014] The technical effects achieved by using the present invention are to lower the maximum temperature of the fuel element cladding, reduce corrosion and wear, improve the long-term strength of the fuel element cladding that is sensitive to temperature increases, and eliminate incorrect placement of fuel assemblies in the core.

[0015] The above technical achievements are achieved as follows: in a nuclear reactor core, a hexagonal unsheathed fuel assembly is included, the fuel assembly has fuel elements located on a triangular grid, the spacing of which is greater than the diameter of the fuel element cladding, the fuel elements include spiral spacer ribs on the cladding, and are rigidly fixed at a given angular direction to ensure "rib-to-rib" fuel element contact at a height position where the maximum fuel element cladding temperature is reached during reactor operation, and the fuel assemblies are placed in a manner such that fuel elements located on the surfaces of adjacent fuel assemblies form a single triangular grid with other fuel elements in the core, and ensure "rib-to-rib" contact of all fuel elements in the core at a height position where the fuel element cladding reaches the highest temperature during reactor operation.

[0016] The specific fuel assembly layout is ensured by using mechanical "encoders" in the fuel assembly support grid and in the core grid, where the fuel assemblies manufactured according to the "key and lock" principle are installed.

[0017] The main technical problem solved by the mechanical "encoder" design is that it is technically impossible to install the fuel assembly in the wrong position and / or in the wrong orientation with respect to its angular position, even taking into account potential erroneous behavior of the personnel assembling the core.

[0018] One of the options for realizing the encoder is to use a hollow pin, one end of which is fixed in the lower core support grid in any known manner. The other end of the pin should protrude in the direction of the lower fuel assembly grid. In this case, there should be a hole in the lower fuel assembly grid, exactly opposite to the pin. When the correct position of the pin matches the corresponding hole, the pin is inserted into the hole and the fuel assembly can be installed in the lower core support grid. If the pin does not enter the hole, it rests on the lower fuel assembly grid, not allowing the fuel assembly to be lowered to the lower operating position so that it can be reliably fixed.

[0019] The geometrical arrangement of the pins and their corresponding holes must be unique for each group of fuel assemblies and / or for each individual fuel assembly and this must be ensured during the design of the core and production. If necessary, multiple pins and correspondingly multiple corresponding holes in the support plate can be used. At the same time, the position of at least one pin must be different in different groups of fuel assemblies and / or in individual fuel assemblies.

[0020] An alternative solution providing the same technical result is to use hollow pins fixed in the lower fuel assembly grid and corresponding holes in the lower core support grid. Pins of different cross-sections can also be used (e.g., round, square, triangular, with or without coolant channels). The main requirement that must be met is that it is physically impossible to install the fuel assembly in the wrong position and / or in the wrong angular orientation.

[0021] The pins can be used, for example, for fuel assembly groups with different uranium enrichments (mixed uranium-plutonium fuel with different plutonium contents).

[0022] In the proposed solution, the fuel elements located on the faces of the fuel assemblies form a single fuel element triangular grid in the core, which eliminates excessive coolant flow rates in the fuel element units on the faces of the fuel assemblies, while the distances between the fuel elements within a fuel assembly are the same as the fuel elements located on the surfaces of adjacent fuel assemblies, which is ensured by the presence of spaced spiral ribs on the fuel element cladding and the arrangement of the fuel assemblies, so that all fuel elements in the core are located at a height that reaches the maximum fuel element cladding temperature during reactor operation to ensure "rib-to-rib" contact.

[0023] The positions of the fuel elements of adjacent fuel assemblies at the boundaries of these fuel assemblies are geometrically identical to the positions of the fuel elements within the fuel assembly, thus creating the same flow resistance in the area near all fuel elements. By arranging the fuel elements in a triangular grid, any fuel element is surrounded by 6 fuel elements at the same distance from it. There are 6 identical conventional channels around each fuel element for coolant flow, limited by the lines connecting the centers of adjacent fuel elements and the fuel element cladding. Any deviation of the fuel element placement from the above arrangement results in the formation of conditional channels with increased flow area. The flow resistance to longitudinal flow in such channels is reduced, resulting in the formation of so-called "idle leaks". Due to the increase in coolant flow rate through these channels, the heating of the coolant is reduced, and in other channels with a "contracted" cross-section, the flow rate is reduced, the coolant heating is increased, and the temperature of the fuel element cladding is increased.

[0024] The fuel elements are rigidly fixed to the outer surface of the cladding in the fuel element grid by means of spiral ribs and the fuel assembly is accordingly fixed in the core in a predetermined manner, while ensuring that the "rib-to-rib" fuel elements are in contact at a height position where the maximum fuel element cladding temperature is reached during reactor operation, making it possible to create identical regular channels around each fuel element.

[0025] When the fuel elements in the fuel assembly are placed on a triangular grid, in the case of a jacketless design, the "idle" coolant leaks at the fuel assembly face joints are eliminated, resulting in an increase in the coolant flow rate in the regular triangular units, thereby reducing the maximum temperature of the fuel element cladding, reducing corrosion wear, and improving the long-term strength of the fuel element cladding that is sensitive to temperature increases. This makes it possible to increase the reactor power output while maintaining the total coolant flow and average coolant temperature at the core outlet. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 The layout of a plurality of fuel assemblies 1 in a given direction in the core is shown, wherein a plurality of fuel elements 2 form a single triangular grid. A plurality of holes 3 are formed in the lower tail pieces of the fuel elements 2, into which cables are inserted to orient the fuel elements 2 in the fuel assembly 1. Figure 1 Also shown are pins 4 for orienting the fuel assembly 1 in the core support grid in which the fuel assembly 1 is mounted, for placing the fuel assembly 1 in a given direction in the core, pins 5 for mounting the fuel assembly by enrichment if the fuel assembly 1 contains fuel elements 2 with different uranium enrichments (mixed uranium-plutonium fuel with different plutonium contents), and holes 6 of the mounting (orienting) support grid 7, into which the corresponding pins are inserted when assembling the fuel assembly 1 in the core.

[0027] Figure 2The aa section of the core is shown, showing the fuel elements 2, the support grid 7 of the fuel assembly 1, the holes 3 in the fuel elements and the cables for orienting the fuel elements 2 in the fuel assembly 1, the pins 4 for guiding the fuel assembly in the core, the pins 5 for mounting the fuel assembly 1 by enrichment, and the holes 6 of the support grid for mounting (orienting) the core. DETAILED DESCRIPTION

[0028] In order to achieve Figure 1 The fuel assembly layout in the core shown uses a hexagonal unsheathed fuel assembly 1, in which fuel elements 2 with spaced spiral ribs on the cladding are installed in a grid of the fuel assembly 1 along a triangular grid. The fuel assemblies 1 are placed in the core in such a way that the fuel elements 2 form a triangular grid, and the distance between the center lines of adjacent fuel elements 2 located on the surface of different fuel assemblies 1 is equal to the distance between the center lines of adjacent fuel elements 2 located inside the fuel assembly 1.

[0029] In the fuel assembly 1, the fuel element 2 is fixed in the support grid 7 of the fuel assembly 1 to prevent angular and axial displacements and to ensure that the axial displacement of the tail piece of the fuel element 2 in another grid of the fuel assembly 1 is not affected by its sliding fit. The fuel element 2 is fixed in such a way that it ensures "rib-to-rib" contact of the fuel element 2 in a plane along the core height, at the position along the plane along the core height at which the fuel element 2 reaches the maximum cladding temperature. To this end, the core height at which the maximum fuel element cladding temperature is reached is first determined by thermohydraulic calculation methods used in the design of reactors, taking into account the axial inhomogeneity of the energy release field. Next, the lower tail piece of the fuel element 2 with the prefabricated hole 3 is positioned at a certain angle and fixed by metal wire in the hole of the support grid 7 of the fuel assembly 1.

[0030] Then, the tail piece of the fuel element 2 in the support grid 7 of the fuel assembly 1 is fixed using tie wires to prevent angular and axial displacement, and to ensure that the axial displacement of the tail piece of the fuel element 2 in another grid of the fuel assembly 1 is not affected by its sliding fit.

[0031] The specified layout of the fuel assembly 1 is ensured by using the pins 4 and the pins 5 (if necessary) in the support grid 7 of the fuel assembly 1 and the core grid on which the fuel assembly 1 is installed. The pins 4 and 5 are hollow pins.

[0032] The working principle of the core is as follows: the coolant enters the core and is evenly distributed through the channels formed between the fuel elements 2, inside the fuel assembly 1 and between the fuel assemblies 1.

[0033] Industrial Applicability: The declared core is for a nuclear reactor, the core of which comprises a hexagonal unsheathed fuel assembly, the fuel elements of which are arranged on a triangular grid with a spacing greater than the diameter of the fuel element cladding.

Claims

1. A nuclear reactor core comprising a plurality of hexagonal unsheathed fuel assemblies, each fuel assembly having a plurality of fuel elements mounted in a fuel assembly grid along a triangular grid, the spacing of which is greater than the diameter of the fuel element cladding, characterized in that: The fuel elements include spiral spacer ribs on the cladding and are rigidly fixed in a given angular direction, and the fuel assemblies are placed in a manner such that the fuel elements located at the boundaries between adjacent fuel assemblies and other fuel elements in the core can be arranged in a triangular grid with the surrounding fuel elements, and when the fuel element cladding reaches the maximum temperature during reactor operation, all fuel elements in the core are in rib-to-rib contact in the height direction of the core.

2. The nuclear reactor core according to claim 1, characterized in that: The fuel assembly grid and the core grid on which the fuel assemblies are mounted are equipped with at least one mechanical encoder, which is a pin and a hole formed according to the key and lock principle.

3. The nuclear reactor core according to claim 2, characterized in that: The pin is hollow.

4. The nuclear reactor core according to claim 2, characterized in that: One end of the pin is fixed in the core grid, and the other end of the pin protrudes toward the fuel assembly grid, wherein a hole is provided at a position opposite to the pin.

5. The nuclear reactor core according to claim 2, characterized in that: The geometric arrangement of the pins and corresponding holes may be unique to multiple fuel assemblies and / or a single fuel assembly.

6. The nuclear reactor core according to claim 2, characterized in that: The pins are fixed in the fuel assembly grid, and corresponding holes are provided in the core grid.

7. The nuclear reactor core according to claim 2, characterized in that: The pin is formed in a cylindrical shape.

8. The nuclear reactor core according to claim 2, characterized in that: The pin is in the shape of a quadrangular prism.

9. The nuclear reactor core according to claim 2, characterized in that: The pin is in the shape of a triangular prism.

10. The nuclear reactor core according to claim 2, characterized in that: The pin includes a passage for coolant flow.

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