RU2532261C1 - Apparatus for spacing fuel elements - Google Patents

Abstract

FIELD: physics, atomic power.

SUBSTANCE: invention relates to nuclear reactor fuel assemblies. The invention employs hydraulic forces acting on cells of spacer grids on the side of coolant flow to further press pins of the cells to the fuel element envelopes. The spacer grid of the fuel assembly comprises shaped cells on which pins are made such that the outer surface of the cells in the area of the pins is directed opposite the flow of the coolant. The pins are placed in the lower part of the cells, moving the region of contact of the cells with the fuel element envelopes closer to the ends of the cells and thereby reducing radial stiffness of the cells. The lead-in surface of the pins is in the form of a bend of the end of the cell.

EFFECT: pressing pins to fuel element envelopes with incident flow of the coolant during operation.

3 cl, 5 dwg

Description

Technical field

The invention relates to nuclear energy, and more specifically to a spacer grid of a fuel assembly (FA) for nuclear reactors.

State of the art

The distance gratings (DR) of modern caseless fuel assemblies have a number of features, among which are high radial stiffness and high creep rate of the material of DR cells in reactor conditions.

In reactor conditions, due to the radiation creep of the material of the DR cells, which are usually made of zirconium alloys and the contact interaction of the cells with the fuel claddings, an increase in the internal inscribed diameter of the cells occurs. Taking into account the decrease in the diameter of the fuel cladding, which is characteristic of the initial and average period of operation of fuel assemblies, a gap usually appears between the DR cells and the fuel cladding, and the conditions for the contact interaction of the cladding of the fuel rods with the DR qualitatively change, since in this case, the fuel rods may not contact the DR cells three beetles, as is usually the case at the very beginning of operation, and two or one beetles each. Thus, the reliability of the fixation of fuel claddings is deteriorating, and mutual displacements may occur in the contact pairs of the DR fuel cell under pulsating loads from the coolant flow, which, in principle, can lead to problems associated with fretting wear of fuel claddings.

The high radial stiffness of the DR cells can lead to significant contact forces between the claddings of the fuel rods and the DR, which, in turn, can cause high stresses in the elements of the fuel assemblies with excessive elongation of the fuel cladding relative to the guide channels during operation. The large radial stiffness of the DR cells can also be the reason for the appearance of large forces arising during the assembly of the fuel rod bundle, which complicates the assembly technology, for example, due to the use of varnish.

Known spacer grid (VVER active zones for nuclear power plants. V.D.Shmelev, Yu.G. Dragunov and others, Moscow, 2004, p. 107, 108, Fig. 6.11) having a rim, cells with them beetles. The disadvantages of this spacer grid are increased forces when the fuel rods pass through the cells of the spacer grids during assembly of the fuel rod bundle, as well as the occurrence of gaps between the cell bullets and the claddings of the fuel rods during operation.

This spacer grid is the closest in technical essence and the achieved effect, which is taken as a prototype.

Disclosure of invention

The objective of the invention is to optimize the interaction of the cladding of the fuel rods with the cells of the spacer grids during operation by optimizing the geometry of the DR cells, in which the radial stiffness of the cells is reduced, and in the reactor conditions there are efforts from the side of the coolant flow, gradually reducing the internal inscribed cell diameter due to creep of the cell material .

The technical result of the invention is to reduce the likelihood of gaps between the fuel cladding and the DR cells and to reduce the maximum stresses in the fuel assembly elements during operation, as well as to reduce the fueling forces of the fuel rods during beam assembly.

This goal is achieved by the fact that a method is proposed for spacing fuel rods in a fuel assembly having spacer grids (1) containing curly cells (2) with beadlets (3), in which it is new that the coolant flow acts on the cells (2) in this way that hydraulic forces arise near the pupples, gradually decreasing the inscribed inner diameter of the cells (2) by the pupples (3) due to creep of the cell material under reactor conditions, by a maximum of the possible gap (D) between the pupple (3) and olochkoy fuel element (7).

The technical result is achieved by the fact that in the spacer grid (1) containing curly cells (2) with puklevki (3), it is new that the puklevki (3) are made in such a way that most of the outer surface (4) of the cells (2) in the area of the beetles, it faces the coolant flow.

Such a solution of the problem posed allows the coolant flow incident on the DR cells to create a pressure difference on different sides of the bellows - from the outside, the coolant pressure exceeds the coolant pressure from the inside of the bellows. Thus, an additional effort is created to compress the sticks to the fuel claddings during the operation of the fuel assemblies. Taking into account the radiation creep of zirconium alloys, such a design of DR cells will reduce the likelihood of gaps in the contact elements of the DR fuel cell due to a gradual decrease in the inscribed diameter of the cells when exposed to a coolant flow in reactor conditions under which the creep of material of DR cells .

According to the variant, the pupples (3) are located in the lower part of the cells (2).

This solution, when the pupples are close to the end edge of the cells and have an insignificant distance from the contact points with the fuel claddings to the end of the cells, provides reduced radial stiffness compared to standard DR cells, in which the pupples are in the central region of the cell height. Thus, the proposed design of the DR should provide a reduction in the forces arising from the assembly of the fuel rod bundle and the stresses in the fuel assembly elements during operation.

According to the variant, the pupples (3) have a lead-in surface (the inner surface of the DR cell near its end), made in the form of a bend (5) of the end (6) of the cell (2).

This solution will eliminate the bite of the fuel rod when it moves relative to the cells in the longitudinal direction.

Brief Description of the Drawings

The invention is illustrated by drawings, on which:

figure 1 is a change in the inscribed diameter of the cell on the bullets when exposed to a coolant flow;

figure 2 - spacer grid for arranging the beam on a triangular grid;

figure 3 - spacer grid for arranging the beam on a square grid;

4 is a General view of the proposed cell with a fragment of the fuel clad with puklevkmi in the lower part;

5 is a cross-section of a cell with a bead having a bend in the end part of the cell.

Embodiments of the invention

The distance grid (1) contains curly cells (2) with beetles (3). Puklevki (3) of cells (2) are made in such a way that most of the outer surface (4) of cells (2) in the region of puklevki is facing the heat carrier flow.

Puklevki (3) can be located in the lower part of the cells (2).

Puklevki (3) may have a lead-in surface made in the form of a bend (5) of the end face (6) of the cell (2).

The work of the spacer grid is as follows.

The coolant flow enters the fuel rod bundle of the fuel assembly, which is fixed in the cells of the spacer grids. Since the cells are inclined with respect to the direction of flow of the surface, a hydrodynamic force is created, which tends to compress the cells along the bullets. This ensures tight contact of the cells with the cladding of the fuel rods (7), ensuring the absence of fretting wear of both the shells and the cells themselves.

When assembling a bundle of fuel rods due to the fact that the inscribed diameter along the bullets is less than the outer diameter of the cladding of a fuel rod, the fuel rod is installed with a certain force. Placing the pupples closer to the lower end of the cell allows one to reduce the radial stiffness of the DR cell and, as a result, reduce the contact force of the interaction of the fuel cladding and the cell. This ultimately leads to a decrease in the forces arising during the assembly of the fuel rod bundle, and increases the manufacturability of the entire fuel assembly. In this case, the presence of a bend in the end of the cell in the region of the bite to the side of the fuel rod cladding ensures that the shell does not bite during longitudinal movement of the fuel rods relative to the DR.

Industrial applicability

The invention has industrial applicability for the active zones of domestic reactors of WWER type and the active zones of foreign PWRs. The invention can also be implemented in any industrial heat exchange equipment containing heating elements formed into a beam along a triangular or square grid, and spacer grids.

The invention extends to any type of honeycomb spacer grids used in reactor cores.

The present invention will improve the manufacturing conditions of the fuel assembly, reduce the likelihood of fretting wear of the fuel elements and reduce the maximum voltage in the elements of the fuel assembly during operation.

Claims (3)

1. A device for spacing fuel elements, consisting of a spacer grid (1) containing curly cells (2) with beetles (3), characterized in that the beetles (3) of the cells (2) are made in such a way that most of the outer surface (4 ) cells (2) in the area of the beetles facing the heat carrier flow. 2. The device for spacing fuel elements according to claim 1, characterized in that the beetles (3) are located in the lower part of the cells (2). 3. A device for spacing fuel elements according to claim 1, characterized in that the beetles (3) have a lead-in surface made in the form of a bend (5) of the end (6) of the cell (2).

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