Nuclear reactor core fuel assemblies

Abstract

The present invention relates to nuclear power engineering, more particularly to nuclear reactor fuel assemblies, and can be used in nuclear reactors, preferably with liquid metal coolants, in particular with heavy liquid metal coolants. A square or hexagonal nuclear reactor core fuel assembly comprises a fuel rod, whose cladding is made of a ferrite-martensitic grade steel resistant to corrosion in heavy liquid metal coolants, and has spiral ribs arranged equidistantly along the periphery of its outer surface, a lower grid and an upper grid, and a locking wire passing through an opening in an end plug of the fuel rod and a slot of one of the grids of the fuel assembly, the axis of the opening being oriented parallel to one of these edges of the fuel assembly, wherein the fuel rod is rigidly fastened in one of these grids of the fuel assembly in such a way that for each fuel rod in the fuel assembly, the angle between the axis of the fuel rod end plug opening through which the locking wire passes and the middle of the base of one of the ribs on the end of the fuel rod cladding corresponds to the angle at which the fuel rod can contact the rib to rib at the height at which the fuel rod cladding reaches its maximum temperature when the reactor is in operation, wherein the end plug with the opening is fastened to the end of the fuel rod cladding.

Description

Technical Field

[0001] The present disclosure relates to nuclear power engineering, namely to fuel assemblies of nuclear reactors (NR), and can be used in NR, preferably with liquid metal, in particular with heavy liquid metal coolant (HLMC). Background Art

[0002] A fuel assembly (FA) for a nuclear reactor core is known (Russian patent 2298848), which comprises a fuel element (FE), a lower grid and an upper grid, a lanyard and a hexagonal tube with a tail piece. The cylindrical part of the tail piece comprises an external thread and a clamping nut. The lower grid is equipped with a perforated retaining ring and is made vertically of two parts fastened together. The lower part of the grid comprises a groove milled along the center line of the hole for the lanyard to fasten the FE tail piece.

[0003] A disadvantage of this solution is that there are no spacings of FE along the length of the FA between the supporting grids, which may lead to their bending and thus to an increase or decrease in the cross section of the cooling channels of certain rods and the formation of hot spots, which in turn reduces the operational reliability of the FA.

[0004] FE cladding, FE and FA are known (Russian Patent 2551432). The cladding for fuel elements of reactors with heavy liquid metal coolants is a solid-rolled tubular element with helical twisted ribs on the outer surface of the element, and the element is made of ferrite-martensite chromium-silicon steel and includes four helical twisted ribs located equidistant from each other. FE is the cladding sealed at the ends using plugs and the nuclear fuel placed in the cladding.

[0005] To obtain a fuel assembly, the known invention proposes to mount the assembled FEs in a carrying frame with a "rib to rib" distance between them and to fasten them in upper, middle and lower grids mounted on the carrying frame.

[0006] However, the claims and specification of the patent do not disclose how to ensure the "rib-to-rib" spacing of the FE.

[0007] It should also be mentioned that the technical solution does not require a certain identical angular orientation of the ribs of each FE in the FA to ensure the established target of "rib-to-rib" spacing. In the absence of such a requirement, the angular orientation of the ribs of each FE in the FA will be random. At the same time, assuming that there are thousands or tens of thousands of FEs in a nuclear reactor core (NRC), regulations may arise when the distance between the planes where the ribs of the FEs contact each other may be large enough. As a result, the FEs may bend toward each other, the flow area shrinks, local consumption decreases, and the FEs overheat. In addition, the gaps generated will contribute to the vibration wear of the FEs because the spacing of the FEs in such a cell will be larger with a larger span. The spacing issue of the FEs located on the FA face remains open, and the FEs do not have support points on the outside of the FA where these FEs can protrude. Therefore, the disadvantages of the FA include insufficient operational reliability. Summary of the invention

[0008] The problem to be solved by the present disclosure is to increase the operational reliability of the FA by ensuring the most rigid spacing of the FE along the entire length by taking into account the tolerances of the torque of the FE cladding, in particular the tolerances of the torque of the upper hottest part of the FE cladding.

[0009] When the present disclosure is implemented, the following technical results are obtained: the possibility of deflection and vibration of FE and the uneven coolant flow distribution in the cell formed by the nearby FEs adjacent to the FE along its periphery with the highest blanket temperature are reduced. This increases the operational reliability and serviceability of the FE.

[0010] The task is solved due to the fact that a hexagonal or square fuel assembly comprising rods FE on the outer surface of a cladding made of HLMC corrosion-resistant ferritic-martensitic grade steel comprises spiral ribs uniformly positioned along the perimeter, a lower grid and an upper grid, lanyards passing through holes in the FE tail piece and through grooves of the FA grid, a centerline oriented parallel to one FA face, and said FE is rigidly fixed in one FA grid so that for each FE in the FA the angle between the centerline of the hole in the FE tail piece through which the lanyard passes and the middle of the base of one rib on the end of the FE cladding corresponds to the angle ensuring "rib-to-rib" contact of the FE at the height at which the maximum temperature of the FE cladding is reached during reactor operation, the tail piece with the hole being fixed in the middle of the base.

[0011] The proposed solution takes into account that the tolerance of the torque of the ribs on the FE cladding cannot be equal to zero, which is caused by the elastic-plastic properties of the cladding metal, the heating temperature of the cladding during torsion and the torsion forces. The use of the proposed FA design makes it possible to avoid the selective shielding of the cladding by the rib twist angle, which leads to a reduction in product yield, and to reduce the requirements on the torque tolerance value.

[0012] The rigid fixing of the FEs to the spiral ribs on the outer surface of the blanket located in the FA grid makes it possible to exclude the situation where in any cell between the FEs in the area of ​​maximum FE blanket temperature the FEs with the smallest and largest deviations from the nominal torque of the ribs will be positioned, which would enable the FEs in this cell to deflect towards each other, contracting the flow area and increasing the temperature in the most stressed parts of the FEs, wherein the spiral ribs are not arranged randomly, but in order to ensure "rib-to-rib" contact of the FEs at the height where the maximum temperature of the FE blanket is reached during reactor operation.

[0013] If the claimed FA is used in a reactor operating with partial refueling, or if the ribs of such FE contact the ribs of the FE of an adjacent FA or the formed shell of an NRC basket in a reactor operating without partial refueling, then protrusions of the FE on the FA face are ensured to be excluded by using a FA jacket (shell). BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The accompanying drawings illustrate the essence of the present disclosure. Figure 1 An FE with a fixing hole in the tail piece is shown; Figure 2 A section AA is shown showing a fixed angle of FE in the FA grid; Figure 3 shows a cross section of a square unjacketed FA at a height where the maximum temperature of the FE is reached and precise "rib-to-rib" contact is ensured; Figure 4 A cross section of a hexagonal non-jacketed FA is shown at the height where the maximum temperature of the FE is reached and precise "rib-to-rib" contact is ensured. DETAILED DESCRIPTION

[0015] A description of possible, but not exclusive, options for the claimed invention is given below.

[0016] FE( Figure 1 ) includes a hollow tube (cladding) 1 made of ferrite-martensitic grade steel, which is corrosion-resistant in HLMC, in which the fuel is placed; a neutron reflector; a gas space; springs and other necessary parts (not shown in the figure). On the outer surface of the cladding 1, spiral ribs 2 (preferably four ribs) are evenly spaced apart with torque to ensure the spacing of FEs with adjacent FEs when their ribs are fixed in contact with each other at appropriate angles of FE in FA along the height of the core in a given number of planes. For a given number of spacing planes, a reduction in the number of ribs on the cladding will result in a reduction in the torque of the ribs, an increase in the hydraulic resistance of the NRC, and a complication of the cladding manufacturing technology. An increase in the number of ribs on the cladding will in turn lead to an undue increase in the amount of steel used in the NRC, which will deteriorate its physical properties.

[0017] The FE blanket is sealed by two tails 3 (upper and lower), one of which has a prefabricated structural element 4 (hole) to fix the FE in the FA grid, thereby preventing axial and angular displacement of the FE. The tail piece 3 with hole 4 is sealed in the end part of the FE blanket 1 to ensure the "rib to rib" contact of the FE in the plane along the height of the NRC, at which the maximum temperature of the FE blanket is reached. To ensure this, the height of the NRC at which the maximum temperature of the FE blanket is reached is first determined by thermal-hydraulic calculations taking into account the radial and axial inhomogeneities of the energy release field, according to the method used when designing the reactor. Next, the lower / upper tail piece 3 with prefabricated structural element 4 (hole) is positioned at a certain angle to be fixed in the corresponding FA support grid. The angular positioning of the grid of holes 4 in the tailpiece 3 relative to the position of one of the ribs 2 on the cladding 1 is ensured using a device located in a plane perpendicular to the centerline of the FE (e.g. a clamp with slots for the ribs), which is placed at a given, previously determined distance from one of the ribs at the end of the FE cladding 1, where the maximum temperature of the cladding 1 is reached. Then, using lanyards, the FE tailpiece 3 is fixed in one FA grid, preventing its angular and axial displacements and ensuring a free sliding axial displacement of the FE tailpiece 3 in the other FA grid. In this case, the FA can be of jacketed or unjacketed design and can include at least two support grids (upper and lower support grids).

[0018] The present disclosure makes it possible to space the FE from adjacent FE ("rib to rib") between the upper and lower supporting (for FE) spacing FA grids, and taking into account the tolerance of the rib torque, to achieve the most precise "rib to rib" spacing in the plane along the height of the NRC, where the FE cladding has the highest temperature along the height of the NRC, which ensures long-term operation of the FA in the HLMC medium (lead, eutectic lead-bismuth alloy).

[0019] Industrial Applicability

[0020] The technical solution according to the invention can be used in power plants using reactors with heavy liquid metal coolant (HLMC) based on lead or on lead-bismuth alloy.The proposed FA design makes it possible to increase the design life of the FA in HLMC media.

Claims

1. A square or hexagonal fuel assembly for a nuclear reactor core, comprising a rod FE, the outer surface of whose cladding is made of HLMC corrosion-resistant ferrite-martensitic grade steel, comprising spiral ribs uniformly positioned along the perimeter, a lower grid and an upper grid, a lanyard passing through a hole in the FE tail piece and through a groove of the FA grid, a centerline oriented parallel to one FA face, the cross section of the cladding of the rod FE being circular, wherein the FE is rigidly fixed in one of the FA grids, so that for each FE in the FA, the angle between the centerline of the hole in the FE tail piece through which the lanyard passes and the middle of the base of one rib on the end of the FE cladding corresponds to the angle ensuring "rib to rib" contact of the FE at the height at which the maximum temperature of the FE cladding is reached during reactor operation, the tail piece with the hole being fixed inside the end of the FE cladding.

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