RU2524172C1 - Production of spacer grate - Google Patents

Abstract

FIELD: power engineering.

SUBSTANCE: production of spacer grate of fuel assembly comprises making of elements with inner ledges, securing them between themselves and to shell and annealing. At final step, definite residual strains are created in grate elements which allow a gradual decrease in internal inscribed diameter of elements at operation of fuel assembly in nuclear reactor.

EFFECT: tighter contact with fuel element shells.

2 dwg

Description

Technical field

The invention relates to nuclear energy, and more specifically to fuel assemblies (FAs) for nuclear reactors.

State of the art

The distance gratings (DR) of modern caseless fuel assemblies have a number of features, among which there is a high creep rate of the material of the 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. Given the decrease in the diameter of the fuel cladding, which is characteristic of the initial and average period of operation of fuel assemblies, between the DR cells and the fuel cladding, a gap usually appears, 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 can not contact the DR cells three beetles, as is usually the case at the very beginning of operation, and two or one beetles. Thus, the reliability of the fixation of fuel claddings is deteriorating, and in the contact pairs of the “fuel cell DR” when pulsating loads from the side of the coolant flow occur, mutual displacements can occur, which, in principle, can lead to problems associated with fretting wear of fuel claddings .

A known method of manufacturing spacer grids (RF Patent No. 2322710 "Method for manufacturing spacer grids for a fuel assembly of a nuclear reactor", claimed 02.05.2006, IPC G21C 3/344), including the mechanical molding of individual cells, set them in the field with the introduction into the holes for the guides channels and a central pipe of technological inserts, the size of which takes into account the deformation of the spacer grid during welding and heat treatment. This manufacturing method has the disadvantage that it does not take into account the conditions for the interaction of fuel cladding with the cells of the spacer grids both in the process of beam assembly and in subsequent operation.

A known method of manufacturing a spacer grid (RF Patent No. 2360306 "Method of spacing grids of a fuel assembly of a nuclear reactor and a device for its implementation", claimed April 19, 2007, IPC G21C 3/34, G21C 2/00), including the placement of cells in the rim and welding them between themselves and the rim. After annealing, the cells are controlled with the help of balls-calibers and, according to the results of the control, an annealing or crimping of the cells is performed to obtain the desired inscribed diameter of the cell hole for the fuel rod installation. The disadvantage of this solution is also the neglect of the subsequent conditions for the interaction of fuel cladding with the cells of the spacer grids both in the process of beam assembly and in subsequent operation.

A known method of manufacturing a spacer grid (RF Patent No. 2256960 "Spacer grid and a method of manufacturing a spacer grid", claimed 10/22/2003 IPC G21C 3/34, G21C 21/00), in which mounting surfaces are made on the mating surfaces of the figured cells with their calibration to a predetermined size of the outer surface by plastic deformation of the workpieces and subsequent heat setting. This method of manufacturing spacer grids will reduce the "turnkey" size deviations of the spacer grids. But it also does not solve issues related to the optimization of the conditions for the interaction of fuel claddings with DR cells in reactor conditions.

The closest in technical essence and the achieved effect is a method of manufacturing spacer grids / 2 /, which is taken as a prototype.

Disclosure of invention

The aim of the invention is to improve the thermomechanical behavior of the fuel Assembly during operation.

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, reducing the likelihood of gaps between the fuel cladding and the cells of the spacer grids during operation.

The technical result of the invention is the creation of a technology for manufacturing cells of spacer grids, which helps to reduce the gaps between fuel claddings and cells during operation.

This goal is achieved by the fact that the proposed method of manufacturing cells of the spacer grid (1) of the fuel assembly, including the manufacture of cells (2) with internal protrusions (3), fixed between themselves and the rim (4) with subsequent annealing. What is new is that at the final stage of manufacturing the spacer grid in cells (2), certain residual stresses are created, due to which, when the FA is operated in reactor conditions, the inscribed internal diameter of the cells gradually decreases, providing more tight contact with the fuel claddings.

With this manufacturing method, the effect of a partial return of the shape of the cells of the spacer grids after their plastic deformation is used due to relaxation of residual stresses in the material of the cells under conditions under which creep of structural materials is manifested, including in reactor conditions. The return of the shape of the DR cells in reactor conditions should lead to a gradual decrease in the inscribed diameter of the cells due to the relaxation of residual stresses in the cell material.

This solution allows for a longer operation of the contact pair “fuel cell DR” in the elastic region and more reliable fixation of fuel cladding during operation. This solution can also be used in order to reduce the initial elastic interference in the contact pairs of the "fuel cell DR", which will lead to a decrease in the maximum forces in the elements of the fuel assemblies arising during operation with excessive elongation of the fuel rods relative to the guide channels.

Brief Description of the Drawings

The invention is illustrated by drawings, on which:

figure 1 - spacer grid;

figure 2 - cell with internal protrusions.

Embodiments of the invention

A method of manufacturing the cells of the spacer grid (1) of the fuel assembly includes the manufacture of cells (2) with internal protrusions (3), fixing them between themselves and the rim (4), followed by annealing. At the final stage of the manufacture of the spacer grid, certain residual stresses are created in its cells, due to which, during operation of the fuel assemblies in reactor conditions, the inscribed internal diameter of the cells gradually decreases, providing more tight contact with the fuel claddings.

In order to create the necessary residual stresses in the material of the DR cells, under which, in the reactor conditions, a gradual decrease in the inscribed cell diameter should occur due to relaxation processes due to radiation creep of the cell material, it is proposed to use the equipment setting at the cell stamping stage in which the inscribed cell diameter is used obtained after their stamping and subsequent annealing would be obviously less than the design value (for the cells of the DR fuel assemblies for VVER - by Well, the order of several tenths of a millimeter). The temperature and duration of annealing can be standard, at which relaxation of residual stresses in the material of the DR cells is ensured.

At the last stage of the manufacture of the spacer grid, or when assembling the fuel rod bundle, it is necessary to plastically deform the cells again to increase their inscribed inner diameter in order to create internal residual stresses in the cells, which will subsequently relax due to creep of the structural material of the cells, reducing their internal inscribed diameter, partially returning the cells to their original (before plastic deformation) form. The magnitude of repeated plastic deformation of the DR cells should be such that the inscribed inner diameter of the cells increases by approximately an order of magnitude larger than the elastic deformation of the cells (for DR fuel cells for WWER, by an order of several tenths of a millimeter).

To perform this operation, when assembling a bundle of fuel rods, it is proposed to use a technological mandrel, which causes an increase in the inscribed diameter of the DR cells due to plastic deformation of the cell material. In this case, the design value of the inscribed diameter of the DR cells after their disassociation should be chosen taking into account its subsequent decrease during operation due to the partial return of the shape of the DR cells during relaxation of residual stresses in the cell material under reactor conditions.

After plastic deformation of DR cells is performed in order to increase their inscribed inner diameter, the cells should not be exposed to impacts where creep or plastic deformation of their structural material is possible until the operation of the fuel assemblies.

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 bundle 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 forces in the fuel assembly elements during operation.

Claims (1)

A method of manufacturing the cells of the spacer grid (1) of the fuel assembly, including the manufacture of cells (2) with internal protrusions (3), fixing them together and the rim (4), subsequent annealing, characterized in that at the final stage of manufacturing the spacer grid in its cells create residual stresses, due to which during operation of a fuel assembly in reactor conditions, the inscribed internal diameter of the cells gradually decreases, as a maximum by the value of the possible gap (Δ) between the internal protrusions and the cladding of the fuel element, ensuring a tighter contact with the fuel elements.

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