RU2252458C1 - Nuclear reactor fuel assembly - Google Patents

Abstract

FIELD: nuclear power engineering; rigid frame components for fastening fuel assemblies.

SUBSTANCE: proposed fuel assembly has top and bottom nozzles intercommunicating through guide channels disposed in subchannels of spacer grids. Guide channels are fixed to at least but-end spacer grids. Spacer grid height and subchannel wall thickness δ are chosen so that their quantitative characteristics meet design and experimentally evaluated conditions. Such mechanical design of fuel assembly provides for enhancing its resistance to transverse and longitudinal bending and angular stiffness in guide channel-subchannel pairs, reducing intrinsic flexure of fuel assembly in spans between spacer grids and free bending of fuel assembly in nonuniform neutron and temperature fields due to reduced liability of alloy E635 to radiation growth.

EFFECT: improved mechanical design of fuel assembly.

9 cl, 3 dwg

Description

FIELD OF THE INVENTION

The invention relates to nuclear engineering and relates to the design of fuel assemblies of water-cooled nuclear power reactors, especially of the type VVER-1000, in particular, the construction of elements of a rigid frame.

State of the art

Known fuel assembly containing a bunch of fuel elements located in the cells of the spacer grids (Kramerov A.Ya. Questions of designing nuclear reactors, M., Atomizdat, 1971, p.198, Fig. 7.2.2.). The rigidity and strength of this fuel assembly is provided by the presence of a hexagonal cover connecting the head and tail of the assembly. However, the presence of a cover introduces parasitic neutron capture in the core and increases the linear thermal load of the fuel elements due to the forced increase in the step between the fuel assemblies.

A known fuel assembly of a nuclear reactor containing a hexagonal beam of fuel elements located in cells located along the length of the assembly of spacer grids (A.Ya. Kramerov, Questions of designing nuclear reactors, M., Atomizdat, 1971, p.204, Fig. 7.1.11b). The head and tail parts are connected by guide channels, in which rods are moving, including neutron absorbing material. The cover in this fuel assembly is absent, which allows to reduce the gap between adjacent fuel assemblies. As a result, the unevenness of energy release and the linear loads of fuel elements are reduced.

The absence of a cover reduces spurious neutron capture and reduces material consumption. But the use of the assembly at power units with VVER-1000 reactors showed that after operation for three years there is a curvature of the guide channels due to mechanical stress from the side of the head part, which is preloaded to prevent the fuel assembly from floating under the action of the coolant flow. In addition, during the operation of the reactor, thermomechanical loading of the assembly structure as a whole occurs due to deformations of the fuel elements transmitted through spacer grids, which also undergo deformation. A fundamental role in the development of curvatures of fuel assemblies is played by the relaxation of elastic tension in distance grids created when heat elements and guide channels are placed in grids.

A significant decrease in the bending stiffness of the fuel assembly fundamentally changes the nature of its behavior in the active zone during long-term operation: bends of the fuel assembly of complex spatial shape arise with deviations of the assembly axes from the initial position by an amount that reaches the maximum allowed based on the geometric possibilities of packing the assemblies in the active zone, taking into account structural assembly clearances. At the same time, large gaps may appear between the peripheral fuel elements of adjacent curved fuel assemblies, which violates the heat engineering reliability of these fuel elements due to a surge in energy release. It should also be noted that for the hexagonal design of fuel assemblies, in particular, used on power units with VVER-1000 reactors, angular anisotropy of bending stiffness is observed (bending stiffness in the direction from “corner to corner” is not equal to bending stiffness in the direction “from face to face” ”), While the flexural rigidity of the assembly in the tangential direction (relative to the center of the core) is less than its flexural rigidity in the radial direction. Greater freedom of movement in the tangential direction causes a predominantly vortex twist of the active zones and, consequently, the maximum curvature of the fuel assemblies under operating conditions. There are experimental data on the curvature of fuel assemblies by a value of about 20 mm. Such significant distortions of the fuel assemblies violate the original geometry of the core, lead to changes in energy release and thermal properties of the core.

To eliminate the identified negative factors of the fuel assembly and ensure its stable behavior, which is expressed in the exclusion of excessive distortion of the assembly under operating conditions during operation for 4-5 years, it is necessary

- to ensure the guaranteed value of the bending stiffness of the fuel assemblies during longitudinal bending by introducing longitudinal elements that do not slip relative to the spacing grids during the entire period of operation;

- to ensure a guaranteed higher value of the bending stiffness of the fuel assemblies during transverse bending by reducing under individual working conditions of the rods or other longitudinal elements in the spans between the spacing grids while increasing the stiffness of rotation of the rods or longitudinal elements in the cells of the spacing grids.

The listed conditions are satisfied by the closest in technical essence and achieved result to the described fuel assembly of a nuclear reactor containing the head and tail parts connected by guide channels placed in the cells of the spacer grids, which are located at a distance from each other along the length of the assembly (RU 2093906, G 21 C 3/30, 10.20.97). Additional stiffening elements are introduced into the design of the known fuel assembly - longitudinal angles extending from the lower support grid to the upper spacer grid and welded six corners from the outside to each spacer grid. A rigid connection of the corners with the spacer grids provides a sharp increase in the stiffness of the fuel assembly for longitudinal bending, regardless of the relaxation of elastic interference in the core system of the fuel assembly. The spatial shape of the corners has a very high value of the intrinsic moments of inertia and thereby provides sufficient structural rigidity during transverse bending of the fuel assembly. In addition, the inability to rotate the corners relative to the axis of the spacer grids in places of their mutual fastening additionally increases the rigidity of the fuel assembly.

At the same time, the known fuel assembly has the following disadvantages:

- increasing the metal structure, which leads to a decrease in the neutron-physical characteristics of the active zone;

- deterioration of heat dissipation from angular and peripheral fuel elements;

- the complexity of the manufacturing technology of the fuel assembly due to the introduction of additional corner elements, an increase in the volume of welding and control;

- reduction of visual control capabilities at the manufacturing stages and during the inspection during operation.

SUMMARY OF THE INVENTION

The objective of the present invention is to develop and create a fuel assembly of a nuclear reactor with increased stability during operation in fuel cycles of increased duration without the accumulation of unacceptable bends while reducing metal consumption.

As a result of solving this problem, it is possible to obtain new technical results, namely, that the stiffness of the fuel assembly increases in transverse and longitudinal bends, the angular stiffness in the pairs “the guide channel - the cell of the spacer grid” increases, the intrinsic bend of the fuel assembly in the spans between the spacer bars decreases and free bending of the fuel assembly in inhomogeneous neutron and temperature fields due to the reduced tendency of the E635 alloy to radiation growth.

These technical results are achieved by the fact that in the fuel assembly of a nuclear reactor containing a head and a tail, connected by guide channels placed in the cells of the spacer grids, which are located at a distance from each other along the length of the assembly, the guide channels are rigidly connected, at least with end spacing grids, and the height h of the spacing grids and the thickness δ of the walls of the cells of the spacing grid are selected in such a way that their numerical values satisfy the condition ju

Where

h is the height of the spacer grid, mm;

δ is the thickness of the elements forming the cells of the spacer lattice, mm, and for 8 lattices, C ₀ = 39.17, A ₀ = 5.563, A ₁ = -3.482, A ₂ = 1.322, B ₀ = 2.245, B ₁ = - 4,500, B ₂ = 6,072, B ₃ = -3,128, B ₄ = -0,620, for 9 gratings C ₀ = 22,74, A ₀ = 4,988, A ₁ = -2,985, A ₂ = 1,119, B ₀ = 2.225, B ₁ = -4.005, B ₂ = 5.145, B ₃ = -2.595, B ₄ = -1.113, for 10 gratings C ₀ = 13.06, A ₀ = 4.481, A ₁ = -2.568, A ₂ = 0.932, B ₀ = 2.203, B ₁ = -3.568, B ₂ = 4.324, B ₃ = -2.127, B ₄ = -1.510, for 11 gratings C ₀ = 8.84, A ₀ = 4.138, A ₁ = -2.281, A ₂ = 0.811, B ₀ = 2.170, B ₁ = -3.250, B ₂ = 3.752, B ₃ = -1.814, B ₄ = -1.675, for 12 gratings C ₀ = 6.90, A ₀ = 3,895, A ₁ = -2,088, A ₂ = 0,732, B ₀ = 2,126, B ₁ = -3,042, B ₂ = 3,400, B ₃ = -1,623, B ₄ = -1,695, for 13 lattices C ₀ = 5.73, A ₀ = 3.697, A ₁ = -1.937, A ₂ = 0.667, B ₀ = 2.068, B ₁ = -2.910, B ₂ = 3.199, B ₃ = -1.505, B ₄ = -1.651, for 14 gratings C ₀ = 4.70, A ₀ = 3.526, A ₁ = -1.813, A ₂ = 0.614, B ₀ = 2.003, B ₁ = -2.815, B ₂ = 3.062, B ₃ = -1.422, B ₄ = -1.575, for 15 lattices C ₀ = 3.78, A ₀ = 3.356, A ₁ = -1.684, A ₂ = 0.560, B ₀ = 1.940, B ₁ = -2.722, B ₂ = 2.928, B ₃ = -1.336, B ₄ = -1.490.

A distinctive feature of the present invention is the rigid connection of the guide channels, at least with the end, but preferably with all, spacing grids, which eliminates the slipping of the guide channels relative to the cells of these grids. In this case, the total bending stiffness of the fuel assembly during irradiation increases, since a constant component of the bending stiffness of the assembly appears, equal to the stiffness of the associated frame of the guide channels. In this case, the fuel elements can cease to play the role of a connected multi-rod system and, when they slip in the cells of the spacer grids, they will contribute to the total bending stiffness of the cartridge not as a connected multi-rod bundle, but as a set of independent rods, which is much smaller in magnitude. It is possible to increase the rigidity of the fuel assembly as a whole and to ensure the stability of the fuel assembly, in which the gaps formed between the assemblies do not exceed the value that is extremely possible from the point of view of ensuring the permissible energy stress of the fuel elements and the heat engineering reliability of the core, only if the condition

In addition, the above calculated experimental expression relates the number of spacing grids to their characteristic parameters, at which the stresses arising in the spacing grids and caused by axial forces arising during bending at the points of attachment of the guide channels do not exceed the permissible value.

It is also advisable to choose the wall thickness of the spacer grids from 0.25 to 0.35 mm, and its height from 20 to 50 mm, and make guide channels from the E635 alloy, and the spacer grids from the E110 alloy or preferably from the E635 alloy.

The guide channels can be rigidly connected to at least the end spacer grids directly or by means of intermediate bushings located in the respective cells of the spacer grids. The connection of the guide channels with the spacer grids is preferably performed symmetrically with respect to the longitudinal axis of the guide channel from the side of both ends of the spacer grid at a distance of not more than 0.15 h from the end of the spacer grid.

A rigid connection of the guide channels, at least with the end spacer grids, can be made in the form of spot welding.

It is most rational to use 18 guide channels, the diameter of which is from 12 to 14 mm.

List of drawings

Figure 1 shows a General view of the fuel assembly, figure 2 shows a variant of the node A in figure 1 in section, figure 3 shows a second variant of the node A in figure 1 in section.

Information confirming the possibility of carrying out the invention

The fuel assembly of a nuclear reactor contains a head part 1 and a tail part 2 connected by guide channels 3. The guide channels 3 are located in cells 4 of the spacer grids 5, which are located at a distance from each other along the length of the assembly. The guide channels, as a rule, in the amount of 18 pieces pass through cells symmetrically located around the central channel 6. In the remaining cells of the spacer grids 5, fuel elements 7 are placed. The spacer grids 5 together with the guide channels 3 form a rigid frame of the fuel assembly due to the fact that the guide channels 3 are rigidly connected to the end spacer grids or, preferably, to all spacer grids 5. The height h of the spacer grids and the thickness δ the walls of the cells of the spacer grid are chosen in such a way that their numerical values satisfy the condition

Where

h is the height of the spacer grid, mm;

δ is the thickness of the elements forming the cells of the spacer lattice, mm, and for 8 lattices, C ₀ = 39.17, A ₀ = 5.563, A ₁ = -3.482, A ₂ = 1.322, B ₀ = 2.245, B ₁ = - 4,500, B ₂ = 6,072, B ₃ = -3,128, B ₄ = -0,620, for 9 gratings C ₀ = 22,74, A ₀ = 4,988, A ₁ = -2,985, A ₂ = 1,119, B ₀ = 2.225, B ₁ = -4.005, B ₂ = 5.145, B ₃ = -2.595, B ₄ = -1.113, for 10 gratings C ₀ = 13.06, A ₀ = 4.481, A ₁ = -2.568, A ₂ = 0.932, B ₀ = 2.203, B ₁ = -3.568, B ₂ = 4.324, B ₃ = -2.127, B ₄ = -1.510, for 11 gratings C ₀ = 8.84, A ₀ = 4.138, A ₁ = -2.281, A ₂ = 0.811, B ₀ = 2.170, B ₁ = -3.250, B ₂ = 3.752, B ₃ = -1.814, B ₄ = -1.675, for 12 gratings C ₀ = 6.90, A ₀ = 3,895, A ₁ = -2,088, A ₂ = 0,732, B ₀ = 2,126, B ₁ = -3,042, B ₂ = 3,400, B ₃ = -1,623, B ₄ = -1,695, for 13 lattices C ₀ = 5.73, A ₀ = 3.697, A ₁ = -1.937, A ₂ = 0.667, B ₀ = 2.068, B ₁ = -2.910, B ₂ = 3.199, B ₃ = -1.505, B ₄ = -1.651, for 14 gratings C ₀ = 4.70, A ₀ = 3.526, A ₁ = -1.813, A ₂ = 0.614, B ₀ = 2.003, B ₁ = -2.815, B ₂ = 3.062, B ₃ = -1.422, B ₄ = -1.575, for 15 lattices C ₀ = 3.78, A ₀ = 3.356, A ₁ = -1.684, A ₂ = 0.560, B ₀ = 1.940, B ₁ = -2.722, B ₂ = 2.928, B ₃ = -1.336, B ₄ = -1.490.

The wall thickness of the spacer grids 5 is selected from 0.25 to 0.35 mm, and the height of the spacer grids is selected from 20 to 50 mm. The shape of the cells can be performed in any known manner. The distance grids 5 and the guide channels 3 are preferably made of E635 zirconium alloy; It is possible to combine alloys E635 and E110: NK from alloy E635 and DR from alloy E110.

The guide channels 3 are rigidly connected to the spacer grids by means of intermediate bushings 8 located in the respective cells of the spacer grids. A rigid connection 9 of the guide channels with the spacer grids is carried out by spot welding, for example by electrical contact. A rigid connection of the guide channels with the spacer grids is made symmetrically with respect to the longitudinal axis of the guide channel from the side of both ends of the spacer grid at a distance of not more than 0.15 h from the end of the spacer grid. In this case, the bending strength of the guide channels is significantly increased.

The described fuel assembly can be manufactured on any equipment designed for these purposes, and does not require the creation of a fundamentally new equipment.

Claims (9)

1. A fuel assembly of a nuclear reactor comprising a head and a tail connected by guiding channels located in the cells of the spacer grids, which are spaced apart from one another along the length of the assembly, characterized in that the guiding channels are rigidly connected to at least the end spacing lattices, and the height h of the spacing lattices and the thickness δ of the walls of the cells of the spacing lattice are selected in such a way that their numerical values satisfy the condition

Where

h is the height of the spacer grid, mm; δ is the thickness of the elements forming the cells of the spacer grid, mm, moreover, for 8 lattices, C ₀ = 39.17, A ₀ = 5.563, A ₁ = -3.482, A ₂ = 1.332, B ₀ = 2.245, B ₁ = -4.500, B ₂ = 6.072, B ₃ = -3.128, B ₄ = -0.620, for 9 lattices C ₀ = 22.74, A ₀ = 4.988, A ₁ = -2.985, A ₂ = 1.119, B ₀ = 2.225, B ₁ = -4.005, B ₂ = 5.145, B ₃ = -2.595, B ₄ = -1.113, for 10 lattices C ₀ = 13.06, A ₀ = 4.481, A ₁ = -2.568, A ₂ = 0.932, B ₀ = 2.203, B ₁ = -3.568, B ₂ = 4.324 , B ₃ = -2.127, B ₄ = -1.510, for 11 lattices C ₀ = 8.84, A ₀ = 4.138, A ₁ = -2.281, A ₂ = 0.811, B ₀ = 2.170, B ₁ = -3.250, B ₂ = 3.752, B ₃ = -1.814, B ₄ = -1.675, for 12 lattices C ₀ = 6.90, A ₀ = 3.895, A ₁ = -2.088, A ₂ = 0.732, B ₀ = 2.126, B ₁ = -3.042, B ₂ = 3.400, B ₃ = -1.623, B ₄ = -1.695, for 13 lattices C ₀ = 5.73, A ₀ = 3.677, A ₁ = -1.937, A ₂ = 0.667, B ₀ = 2.068, B ₁ = -2.910, B ₂ = 3.199, B ₃ = -1.505, B ₄ = -1.651, for 14 gratings C ₀ = 4.70, A ₀ = 3.526, A ₁ = -1.813, A ₂ = 0.614 , B ₀ = 2.003, B ₁ = -2.815, B ₂ = 3.062, B ₃ = -1 , 422, B ₄ = -1.575, for 15 lattices C ₀ = 3.78, A ₀ = 3.356, A ₁ = -1.684, A ₂ = 0.560, B ₀ = 1.940, B ₁ = -2.722, B ₂ = 2.928 , B ₃ = -1.336, B ₄ = -1.490. 2. The fuel assembly according to claim 1, characterized in that the guide channels are made of alloy E635. 3. The fuel assembly according to claim 1, characterized in that the spacer grids are made of alloy E635 or alloy E110. 4. The fuel assembly according to claim 1, characterized in that the guide channels are rigidly connected to at least the end spacer grids by means of intermediate bushings located in the respective spacer grid cells. 5. The fuel assembly according to claim 1, characterized in that the rigid connection of the guide channels with the spacer grids is located symmetrically with respect to the longitudinal axis of the guide channel from the side of both ends of the spacer grid. 6. The fuel assembly according to claim 1, characterized in that the rigid connection of the guide channels with the spacer grids is located from the end of the spacer grid at a distance of not more than 0.15 h. 7. The fuel assembly according to claim 1, characterized in that the rigid connection of the guide channels with the spacer grids is made in the form of spot welding. 8. The fuel assembly according to claim 1, characterized in that the number of guide channels is 18. 9. The fuel assembly according to claim 1, or 2, or 3, or 4, or 5, or 6, or 7, or 8, characterized in that the diameter of the guide channels is selected from 12 to 14 mm

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