Nuclear reactor fuel assembly (embodiment)

Abstract

The present invention relates to the field of nuclear engineering, and more particularly to a fuel assembly for a VVER-type nuclear reactor. The fuel assembly of the present invention comprises: bundles of fuel elements arranged in a regular triangular pattern at a pitch of 12.2-12.3 mm in a spacer grid connected to a central tube; a hexagonal housing connected to the top nozzle and the bottom nozzle; and a debris filter. The outer diameter of the cladding of the fuel element is 0.720-0.733 times the spacing of the pattern, and the diameter of the fuel pellet is 7.57-7.60 mm. In a second embodiment, the fuel assembly has a regular triangular pattern with a pitch of 12.60-12.75 mm and a reinforcing structure of spacer grids connected by longitudinal tubes and corner elements, where the outer diameter of the cladding of the fuel elements is 0.695-0.710 times the pitch of the pattern and the diameter of the fuel pellets is 7.57-7.60 mm. The invention makes it possible to increase the water-uranium ratio of fuel assemblies, reduce the metal demand for such assemblies, and improve their neutron characteristics and energy production.

Description

Technical Field

[0001] The present invention relates to the field of nuclear engineering and more particularly to nuclear reactor fuel assemblies of the type such as VVER-440, VVER-1000, VVER-1200, etc. Background Art

[0002] Nuclear reactor fuel assemblies VVER-440, VVER-1000 are well known (cf. Kirillov PL et al., Handbook for Thermohydraulic Calculation (Nuclear Reactors, Heat Exchangers, Steam Generators). Energoatomizdat, Moscow, 1990, fig. P.8.1, P.8.3 and P.8.5, pp.317-319).

[0003] In the nuclear reactor VVER-440, the fuel assemblies of the first and second generations have a hexagonal shell connecting the top and bottom nozzles, where the fuel elements are arranged in a regular triangular pattern using a spacer grid connected to the central tube.

[0004] The fuel elements consist of fuel pellets sealed in cladding with plugs and tips using butt resistance welding, the fuel pellets are secured against longitudinal movement with 2-stage snap locks.

[0005] The spring step with the larger diameter is a fixed part placed on the inner surface of the cladding with a certain tension, and the thin step is a compensating part of the lock located on the upper fuel pellet, compressing the fuel stack. The advantage of the lock is that it does not load the welded joint of the cladding and the plug during operation.

[0006] The outer diameter of the fuel element cladding is 9.1 mm, and the outer diameter of the fuel pellet is 7.6 -0.03 mm.

[0007] The grid spacing of the fuel elements in the shell assembly of the first generation VVER-440 reactor is 12.2 mm, and for the second generation, it is 12.3 mm.

[0008] A disadvantage of the known case fuel assembly for the VVER-440 nuclear reactor is the low water-to-uranium ratio.

[0009] The water-uranium ratio n of the regular triangular lattice of fuel elements is calculated using the following formula (see Skvortsov S.A., Pressurized water reactors (VVER) in the Soviet Union. Atomic Energy. Vol. 5, Edition 3, 1958, p. 247)

[0010] , (1)

[0011] Where: s- fuel element grid spacing,

[0012] d- outer diameter of the fuel element cladding,

[0013] d t - External diameter of the fuel pellets.

[0014] Formula (1) shows that for a known fuel assembly of a VVER-440 nuclear reactor having a hexagonal shell and a fuel element grid spacing of 12.2 mm-12.3 mm, the water-uranium ratio is 1.41-1.47.

[0015] For pressurized water VVER reactors (including the VVER-440 reactor), the optimum amount of water in the unit from a thermal engineering point of view is the same as the optimum amount of water from a neutronics point of view. As shown in neutronics calculations, the corresponding value of the water-uranium ratio is n opt =1.9.

[0016] Due to the low water-to-uranium ratio of the shell fuel assembly, a design of fuel assembly without shells was developed, making it possible to increase the grid spacing of the fuel elements in existing VVER-440, VVER-1000 and VVER-1200 nuclear reactors.

[0017] In the fuel assemblies of VVER-1000 reactors and third-generation fuel assemblies of VVER-440 reactors, the shells are not included, but in order to ensure their initial transverse rigidity, a reinforcement structure consisting of longitudinal tubes, spacer grids and guide channels in the form of a central tube connecting the top and bottom nozzles is formed. In addition, in TVS-2M (fuel assemblies) of VVER-1000 and VVER-1200 reactors, the spacer grids are rigidly attached to the guide channels using spot welding, and in TVSA of VVER-1000 reactors and third-generation assemblies of VVER-440 reactors, there are additional longitudinal corner elements, to which the spacer grids are also rigidly attached using spot welding.

[0018] According to a first embodiment, the fuel assembly closest to the proposed one in terms of technical essence and achieved results is the second generation fuel assembly of the VVER-440 nuclear reactor, in which the outer diameter of the fuel elements is 9.1 mm, the fuel elements are arranged in a hexagonal shell in a regular triangular pattern, and the spacing between the fuel elements is 12.3 mm (see Bondar Yu.N. et al., Improving the WWER-440 Fuel Design and Technology. 8th International Conference on WWERFuel Performance, Modelling and Experimental Support. Sofia, Bulgaria, Institute for Nuclear Research and Nuclear Energy, 2010, pp.186-190).

[0019] According to the second embodiment, the fuel assembly closest to the proposed one in terms of technical essence and achieved results is the third-generation fuel assembly for the VVER-440 reactor (RU2728894, dated 03.08.2020, G21C3 / 30), having a reinforcing structure consisting of a central tube and 6 longitudinal corner elements, to which the spacer grid is fixed using spot welding, wherein the outer diameter of the fuel elements is 9.1 mm, and the fuel elements are arranged in a regular triangular pattern with a pitch of 12.6 mm.

[0020] The selection of 9.1 mm as a single value for the outer diameter of the fuel element cladding without specifying a range of required values ​​of the outer diameter of the fuel element, a corresponding range of grid spacings of the spacer grids and their relationship (which means a combination of specific values ​​included therein) does not allow for an increase in the water-uranium ratio or a reduction in metal consumption of fuel assemblies for nuclear reactors VVER-440, VVER-1000. Known fuel assemblies have a water-uranium ratio of 1.45-1.61, which is significantly less than the optimal value. Summary of the invention

[0021] The object of the invention is to increase the energy yield of fuel assemblies of VVER-440, VVER-1000 and VVER-1200 nuclear reactors.

[0022] The technical effect of the present invention is to increase the water-uranium ratio of the fuel assembly of the VVER-440 and VVER-1000 nuclear reactors by reducing the outer diameter of the fuel element cladding, reduce metal consumption, and ultimately improve the neutron characteristics and energy yield thereof.

[0023] Technical Solution

[0024] The technical effect is achieved through the following means:

[0025] - Nuclear reactor fuel assembly, comprising: a bundle of fuel elements, the bundle of fuel elements comprising fuel pellets, compressed in the axial direction with a 2-stage spring lock placed on the fuel element cladding, sealed with plugs and tips using butt resistance welding, arranged in a regular triangular pattern with a pitch of 12.2 mm-12.3 mm in a spacer grid connected to a central tube; a hexagonal shell connected to the top nozzle and the bottom nozzle; a debris filter, the outer diameter of the fuel element cladding is 0.720 times-0.733 times the fuel element pitch of the pattern, wherein the diameter of the fuel pellets is 7.57 mm-7.60 mm.

[0026] - Nuclear reactor fuel assembly, comprising: a bundle of fuel elements, said bundle of fuel elements containing fuel pellets, said fuel pellets compressed in the axial direction with a 2-stage spring lock placed on the fuel element cladding, sealed with plugs and tips using butt resistance welding, arranged in a regular triangular pattern with a pitch of 12.60 mm-12.75 mm in a spacer grid; a load-bearing frame, in which tubes and corner elements are connected to the top nozzle and the bottom nozzle; a debris filter, characterized in that the outer diameter of the fuel element cladding is 0.695 times-0.710 times the fuel element pitch of the pattern, wherein the diameter of the fuel pellets is 7.57 mm-7.60 mm.

[0027] - The spacer grid is made of zirconium alloy E110 and the units are connected to each other using spot welding.

[0028] - The fuel elements are attached to the support grid and welded to the bottom nozzle by means of clip joints.

[0029] - The cladding, plug and tip of the fuel element are made of zirconium alloy E110.

[0030] - The spring locks of the pellets in the fuel elements are made of chrome-nickel alloy or stainless steel.

[0031] The entire set of essential characteristics ensures the solution of the objects of the invention and the achievement of the above-mentioned new technical effects. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 A fuel assembly according to a first embodiment (including a casing) is shown.

[0033] Figure 2 A fuel assembly according to a second embodiment (excluding the casing) is shown.

[0034] Figure 3 Fuel elements of the proposed fuel assembly are shown.

[0035] Figure 4 A cell spacer grid is shown. DETAILED DESCRIPTION

[0036] A nuclear reactor fuel assembly according to a first embodiment (including a shell) comprises: a bundle of fuel elements 2 fixed in a lower grid 5, the bundle of fuel elements 2 comprising fuel pellets 12 compressed in the axial direction by a two-stage spring lock placed on the cladding 13 of the fuel elements 2, sealed with a plug 10 and a tip 14 using butt resistance welding, arranged in a regular triangular pattern 15 with a pitch of 12.2 mm to 12.3 mm in a spacer grid 4 connected to a central tube 3; a hexagonal shell 6 connected to a top nozzle 1 and a bottom nozzle 8; a debris filter 7, wherein the outer diameter of the fuel element cladding is 0.720 to 0.733 times the spacing 15 of the grid in which the fuel elements 2 are arranged, wherein the fuel pellets 12 have a diameter of 7.57 mm to 7.60 mm.

[0037] A nuclear reactor fuel assembly according to a second embodiment (excluding the shell) comprises: a bundle of fuel elements 2 fixed in a lower grid 5, the bundle of fuel elements 2 comprising fuel pellets 12, said fuel pellets 12 being compressed in the axial direction by a two-stage spring lock 11 placed on the cladding 13 of the fuel elements 2 and sealed by a plug 10 and a tip 14 using butt resistance welding, arranged in a regular triangular pattern at a pitch of 12.60 mm to 12.75 mm in the spacer grid 4; a reinforcement structure of the spacer grid 4, said reinforcement structure being connected to the longitudinal tubes 17 or to the longitudinal tubes 17 and the corner elements 9, and the reinforcement structure being connected to the top nozzle 1 and the bottom nozzle 8; a debris filter 7, which is installed in the bottom nozzle 8, wherein the outer diameter of the fuel element cladding is 0.695 times to 0.710 times the grid pitch 15 in which the fuel elements 2 are arranged, wherein the fuel pellets 12 have a diameter of 7.57 mm to 7.60 mm.

[0038] The cladding 13, the plug 10, the tip 14 and the spacer grid 4 are made of zirconium alloy E110, and the cells 16 of the spacer grid 4 are connected using spot welding.

[0039] The spring lock 11 is made in the form of a 2-stage spring, which is a chrome-nickel alloy EK173-ID type or a stainless steel 12X18N10T type.

[0040] The fuel element is connected to the lower support mesh 5 with the tip 14 via a clamp joint.

[0041] As an example of the design of a fuel assembly according to the first embodiment, for a VVER-type nuclear reactor fuel assembly with a fuel element spacing of 12.2 mm-12.3 mm, it is proposed to reduce the outer diameter of the fuel element cladding to 8.784 mm-9.0159 mm, preferably 8.9 mm, and to make the outer diameter of the uranium dioxide pellets the same, 7.6 -0.03 mm. The outer diameter of the fuel element cladding ranges from 8.784 mm to 9.016 mm, which makes it possible to practically implement the proposed fuel assembly for the VVER-440 nuclear reactor as well as the VVER-1000 and VVER-1200 nuclear reactors. The research analysis shows that, compared with the known fuel assemblies, the fuel assembly proposed according to the first embodiment, in which the outer diameter of the fuel element cladding is 8.784 mm to 9.016 mm, preferably 8.9 mm, has a significantly higher water-uranium ratio of up to 1.54, reduces metal consumption, and ultimately improves its neutron characteristics and energy yield.

[0042] As an example of the design of a fuel assembly according to the second embodiment, for a VVER-type nuclear reactor fuel assembly with a fuel element spacing of 12.6 mm-12.75 mm, it is proposed to reduce the outer diameter of the fuel element cladding to 8.757 mm-9.0525 mm, preferably 8.9 mm, and to make the outer diameter of the uranium dioxide pellets the same, 7.6 -0.03 mm. The outer diameter of the fuel element cladding ranges from 8,757 mm to 9,052 mm, which makes it possible to practically implement the proposed fuel assembly for the VVER-440 nuclear reactor, as well as the VVER-1000 and VVER-1200 nuclear reactors. The research analysis shows that, compared with the known fuel assemblies, the fuel assembly proposed according to the second embodiment, in which the outer diameter of the fuel element cladding is 8.757 mm to 9.052 mm, preferably 8.9 mm, has a significantly higher water-uranium ratio of up to 1.76, which is very close to the optimal value of 1.9, reduces metal consumption, and ultimately improves its neutron characteristics and energy yield.

[0043] In order to practically implement the proposed design, the technology was developed and cladding tubes 13 with an outer diameter of 8.9 mm were manufactured, the technology for manufacturing spacer grids 4, fuel elements 2 and fuel assemblies was developed, and a pilot batch of fuel assemblies with a fuel element spacing of 12.3 mm were manufactured and put into trial operation at the Paks NPP.

[0044] Compared to known fuel assemblies, the proposed fuel assembly has a significantly higher water to uranium ratio, reduces metal consumption and ultimately improves neutron characteristics and energy yield.

Claims

1. A nuclear reactor fuel assembly, comprising: A bundle of fuel elements, the bundle of fuel elements comprising fuel pellets compressed in the axial direction by a 2-stage spring lock placed on the cladding of the fuel elements, sealed with plugs and tips using butt resistance welding, arranged in a regular triangular pattern with a pitch of 12.2 mm to 12.3 mm in a spacer grid connected to a central tube; a hexagonal shell connected to a top nozzle and a bottom nozzle; a debris filter characterized in that the outer diameter of the cladding of the fuel elements is 0.720 to 0.733 times the arrangement pitch of the fuel element pattern, wherein the diameter of the fuel pellets is 7.57 mm to 7.60 mm.

2. The nuclear reactor fuel assembly according to claim 1, characterized in that: The spacer grids are made of zirconium alloy E110 and are connected to each other by butt resistance welding of the units.

3. The nuclear reactor fuel assembly according to claim 1, characterized in that: The fuel elements are connected by clip joints to a support grid welded to the end nozzle.

4. The nuclear reactor fuel assembly according to claim 1, characterized in that: The cladding, plug and tip of the fuel element are made of zirconium alloy E110.

5. The nuclear reactor fuel assembly according to claim 1, characterized in that: The spring locks of the pellets in the fuel element are made of chrome-nickel alloy or stainless steel.

6. A nuclear reactor fuel assembly comprising: A bundle of fuel elements, the bundle of fuel elements comprising fuel pellets compressed in the axial direction by a 2-stage spring lock placed on the cladding of the fuel elements, sealed with plugs and tips using butt resistance welding, arranged in a regular triangular pattern with a pitch of 12.60 mm to 12.75 mm in a spacer grid; a reinforcement structure connected to the top nozzle and the bottom nozzle; a debris filter, the assembly being characterized in that the outer diameter of the cladding of the fuel elements is 0.695 to 0.710 times the pitch of the pattern, and the diameter of the fuel pellets is 7.57 mm to 7.60 mm.

7. The nuclear reactor fuel assembly according to claim 6, characterized in that: The spacer grids are made of zirconium alloy E110 and are connected to each other by butt resistance welding of the units.

8. The nuclear reactor fuel assembly according to claim 6, characterized in that: The fuel elements are connected by clip joints to a support grid welded to the end nozzle.

9. The nuclear reactor fuel assembly according to claim 6, characterized in that: The cladding, plug and tip of the fuel element are made of zirconium alloy E110.

10. The nuclear reactor fuel assembly according to claim 6, characterized in that: The spring locks of the pellets in the fuel element are made of chrome-nickel alloy or stainless steel.

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