Method of manufacturing cantilever truss for core meltdown catcher device

Abstract

The present invention relates to a technology for manufacturing a system for ensuring the safety of a nuclear power plant. A method for manufacturing a cantilever truss for a core meltdown catch device, in which two symmetrical parts of the cantilever truss are formed, each part consisting of an outer, middle and inner half shell, which are connected by radial and parallel bearing ribs, upper and lower semicircular force plates by welding to form parallel and radial sectors. The two symmetrical parts are then connected to each other by welding at two parallel sector parts located on one orthogonal axis, such that an upper horizontal joint connects each upper semicircular force plate, a lower horizontal weld joint connects each lower semicircular force plate, and outer, inner and middle vertical weld joints connect each outer, inner and middle half shell. In this way, the reliability of the core meltdown catch device and the core cooling equipment is improved.

Description

[Technical field]

[0001] The present invention relates to a manufacturing technique for a core meltdown catch device (hereinafter referred to as CMLD), and in particular to a manufacturing method for a cantilever truss for a CMLD. [Background technology]

[0002] The greatest radiological hazard is a core meltdown accident that could occur in the event of multiple failures in the core cooling equipment.

[0003] In such an accident, molten core material (corium) melts the reactor internals and reactor vessel, and then flows beyond its critical limits. The residual heat remaining within the reactor can potentially compromise the integrity of the nuclear power plant's containment shell, which is the last barrier to the release of radioactive material into the environment.

[0004] To prevent this, it is necessary to capture the molten core (corium) flowing out of the reactor vessel and ensure its continuous cooling until it completely crystallizes. This function is performed by the core melt catcher, which prevents damage to the nuclear power plant's sealed shell and protects the population and the environment from radiation exposure that would occur in the event of a severe reactor accident.

[0005] The cantilever truss protects the CMLD housing and internal structure from destruction by the core melt and supports the guide plate that transmits static and dynamic shocks to the cantilever truss fixed to the reactor well, while ensuring operability in the event of destruction of the guide plate.

[0006] A method of manufacturing a cantilever truss for CMLD is known, which consists of forming annular shells of different diameters and welding them to each other with a number of ribs (Patent of the Russian Federation No. 2576517, IPC G21C 9 / 016, priority dated 16.12.2014; Patent of the Russian Federation No. 2576516, IPC G21C 9 / 016, priority dated 16.12.2014; Patent of the Russian Federation No. 2575878, IPC G21C 9 / 016, priority dated 16.12.2014; Core meltdown catcher for nuclear power plants using Russian pressurized water reactors VVER-1200, 7th International Scientific and Technical Conference "Ensuring the safety of nuclear power plants using VVER", IA Sidorov, Development and Design Bureau "GIDROPRESS", Russian Federation, 17-20 May 2011.). Ring plates are welded to the top and bottom of the welded structure.

[0007] The disadvantage of this method of manufacturing the cantilever truss for CMLD is that after welding, the strength of the cantilever truss is equal in all radial directions, so that if the cantilever truss is subjected to a shock load greater than the design load from the side of the guide plate, which is simultaneously subjected to the thermo-mechanical and thermo-chemical shock from the molten core and the shock from the destroyed bottom of the reactor vessel, the cantilever truss will not break in a specific area, but in a random area determined by the manufacturing process and the nature of the heat treatment of the welded joints.

[0008] If high intensity, such an impact could have a destructive effect on the load-bearing elements of the CMLD cantilever truss and on the supporting elements of the CMLD cantilever truss in the reactor well, exceeding the designed residual pressures and causing higher than designed pressures within the reactor vessel.

[0009] It therefore needs to be established as a response to the management of a severe accident beyond the design basis in the event of a bottom rupture of the reactor vessel. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Russian Patent No. 2576517 [Patent Document 2] Russian Patent No. 2576516 [Patent Document 3] Russian Patent No. 2575878 Summary of the Invention [Problem to be solved by the invention]

[0011] The technical result of the present invention is to increase the reliability of the core meltdown catch system.

[0012] The object of the present invention is to create a method for manufacturing a cantilever truss for CMLD, which makes the failure of the load-bearing elements predictable in the event of an accident beyond the design basis, vibration effects on the reactor well, or impacts on the cantilever truss from the side of the guide plate. This task is solved by the method for manufacturing a cantilever truss for a core meltdown catch device according to the present invention.

[0013] That is, a method for manufacturing a cantilever truss for a core meltdown catcher device, comprising forming two symmetrical sections of the cantilever truss, each section consisting of outer, middle and inner half shells, which are connected by welding with radial and parallel support ribs, upper and lower semicircular force plates to form a plurality of parallel and radial sectors, and then the two symmetrical sections are connected to each other by welding at two parallel sector sections located on one orthogonal axis, such that an upper horizontal joint connects each of the upper semicircular force plates, a lower horizontal weld joint connects each of the lower semicircular force plates, and outer, inner and middle vertical weld joints connect each of the outer, inner and middle half shells.

[0014] The manufacturing method of the cantilever truss for CMLD allows to ensure structural inhomogeneity between the load-bearing elements, so that during thermal expansion the occurrence of maximum deformations of the cantilever truss is localized between two parallel bearing ribs along the radial axis, where the welded joint connecting the two symmetrical parts of the cantilever truss is located.

[0015] In addition, in carrying out the method of manufacturing a cantilever truss for a core meltdown catcher according to the present invention, the thicknesses of the top and bottom horizontal weld joints and the outer, inner and middle vertical weld joints are made thinner than the thicknesses of the base metal and other weld joints, thereby allowing for a greater strength imbalance between the load-bearing elements of the cantilever truss and maximizing stress concentration in the areas where the thinner weld joints are located.

[0016] Furthermore, when carrying out the manufacturing method of the cantilever truss for a core meltdown catcher device of the present invention, the outer, inner and middle vertical weld joints are partially fabricated, and cover plates are attached to the outer and inner vertical weld joints and the top and bottom horizontal weld joints.

[0017] The invention is illustrated by the drawings. [Brief description of the drawings]

[0018] [Figure 1] FIG. 1 shows a cantilever truss core meltdown catch device. [Diagram 2] FIG. 1 illustrates a cantilever truss for a core meltdown catch device. [Diagram 3] FIG. 1 shows one of two symmetrical sections of a cantilever truss for a core meltdown catch device. [Figure 4] FIG. 1 illustrates a cantilever truss for a core meltdown catcher (showing two symmetrical sections connected). [Diagram 5] FIG. 1 illustrates a cantilever truss for a core meltdown catcher, showing the cover plate placed at the welded joint of the two connected symmetrical parts. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] As shown in Figure 1, the core meltdown catcher is installed under the reactor vessel (2) and includes a guide plate (1) mounted on a cantilever truss (3). A housing (4) equipped with a filler (7) for collecting and dispersing the molten core is installed under the cantilever truss (3).

[0020] The top of the housing (4) has a flange (5) with a desuperheater (6). The filler (7) consists of several cassettes (8) mounted on top of each other, each containing a central hole and several peripheral holes (9).

[0021] The method of manufacturing a cantilever truss for a core meltdown catcher is as follows.

[0022] As shown in Figure 2, one of the two symmetrical parts (28) of the cantilever truss for CMLD is manufactured by welding. Since the other part has the same structure, only one of the two symmetrical parts of the cantilever truss for CMLD is shown in Figure 2.

[0023] Each symmetrical section (28) of the CMLD cantilever truss is composed of the following load-bearing elements: upper semicircular force plate (19), lower semicircular force plate (20), outer, inner and middle half shells (16), (17), (18) and bearing ribs (15). These load-bearing elements form the force frame (13) of the CMLD cantilever truss.

[0024] The two symmetrical sections formed in the cantilever truss for CMLD are connected by welding: the upper horizontal weld joint (26) connects the two upper semicircular force plates (19), the lower horizontal weld joint (27) connects the two lower semicircular force plates (20), and the outer, inner and middle vertical weld joints (12), (22), (24) connect the two outer, inner and middle half shells (16), (17), (18), respectively.

[0025] In this case, the horizontal weld joints (26), (27) located on one cross section and the vertical weld joints (12), (22), (24) also located on one cross section ensure the structural uniformity between the other elements of the cantilever truss for CMLD, thereby making it possible to localize the location of maximum deformations of the load-bearing elements of the cantilever truss for CMLD during thermal expansion to the sector between the parallel bearing ribs (15) in which the weld joints are located.

[0026] In an embodiment of the invention, the thickness (B) of the horizontal weld joints (26), (27) and the vertical weld joints (12), (22), (24) connecting the two symmetrical sections (28) of a cantilever truss for a CMLD is made thinner than the thickness (A) of the base metal or other weld joints used in the manufacture of each of the two symmetrical sections (28).

[0027] To maintain the impact strength of the force frame (13), a factor (U) equal to the ratio of the total thickness of the weld joint (B) to the thickness of the base metal being welded (A) must be:

[0028] 0.8<(U=B / A)<1.0 At a value of U=1.0, the localization effect of thermal expansion of the force frame (13) is enhanced, but at U<0.8, the impact strength of the welded joints of the force frame (13) is not guaranteed.

[0029] In another embodiment of the invention, the force frame (13) consisting of two symmetrical parts (28) is welded such that the upper horizontal welding joint (26) connects the upper semi-circular force plate (19), the lower horizontal welding joint (27) connects the lower semi-circular force plate (20), and the outer, inner, and middle half-shells (16), (17), (18) are partially connected by vertical welding joints.

[0030] In this case, to maintain the impact strength, the value of the coefficient (W), which is the ratio of the total area (F) of the actual welding joints including the actual upper and lower horizontal welding joints (26), (27) and the actual vertical welding joints (12), (22), (24), to the theoretical total area (Q) of the potential welding joints that connect all of the upper and lower semi-circular force plates (19), (20) and the outer, inner, and middle half-shells (16), (17), (18) and are made on a closed loop, needs to satisfy the following.

[0031] 0.75 < (W = F / Q) < 1.0 When the value of W = 1.0, the localization effect of the thermal expansion of the force frame (13) is enhanced, but when W < 0.75, the impact strength of the welding joints of the force frame (13) is not guaranteed. To improve the impact strength and satisfy the criterion of 0.75 < W < 1.0, the cover plate (11) is attached from above to the welding joints (12), (22), (26), (27) to increase the area of the welding joints between the two symmetrical parts (28) of the force frame (13).

[0032] As shown in FIGS. 3 to 5, each of the two symmetrical parts of the force frame (13) of the cantilever truss for the core melting catcher device includes radial support ribs (14), parallel support ribs (15), an outer half-shell (16), an inner half-shell (17), a middle half-shell (18), an upper semi-circular force plate (19), and a lower semi-circular force plate (20).

[0033] The middle half-shell (18) and the upper semi-circular force plate (19) may include steam vents (23). Attached to the outside of the outer half-shell (16) are support legs (21) with anchor ribs (10).

[0034] The cantilever truss for a core meltdown catch device manufactured by the method of the present invention functions as follows.

[0035] When thermal exposure and mechanical shock exceeding the design criteria are applied from both the melt surface and the guide plate (1), the force frame (13) may be destroyed uncontrollably. To eliminate this phenomenon, the cantilever truss force frame (13) is welded to two symmetrical parts (28) at the top and bottom horizontal weld joints (26), (27) and the outside, inside and middle vertical weld joints (12), (22), (24), ensuring that the strength of these joints is lower than the strength of the base metal and weld joints of each symmetrical part (28) of the force frame (13).

[0036] When a cantilever truss is fabricated with two symmetrical sections (28), both symmetrical sections (28) of the force frame (13) are independently secured and any movement is restricted in order to hold both sections (28) in place and to prevent the equipment and materials they hold from collapsing into the CMLD housing (4). For this purpose, both symmetrical sections (28) of the force frame (13) are secured separately using support legs (21).

[0037] In the event of high impact strength, the use of the support legs (21) creates the necessary interaction area between the concrete of the reactor well (25) subjected to wave loads and the support legs (21), thereby ensuring uniform distribution of forces from the sides of the force frame (13) to the reactor well (25).

[0038] When the support legs (21) are used, the wave loads transmitted from the outer half-shell (16) to the support legs (21) are distributed over the volume of concrete in the reactor well (25), and the support legs (21) themselves are resistant to local wave loads and will not be destroyed even when subjected to strong impact loads, without resulting in a decrease in the strength of the concrete in the reactor well (25).

[0039] When the reactor vessel (2) is breached, the molten core begins to flow under the influence of hydrostatic and residual pressure onto the surface of the guide plate (1) held by the cantilever truss (3). The molten core flowing through the guide plate (1) heats it up and partially melts it.

[0040] The nature and consequences of the interaction of the molten core with the guide plate (1) depend heavily on the pressure inside the reactor vessel (2), which can be divided into two categories: design reference pressure and pressures beyond the design reference pressure. The design reference pressure is the pressure of the steam and gas medium inside the reactor vessel (2) that does not exceed the residual pressure after measures to reduce pressure are taken in the event of a severe accident beyond the design reference pressure.

[0041] Pressures above the design basis are pressures that would occur in the reactor vessel (2) in excess of the residual pressure due to uncontrolled water inflow into the reactor vessel (2) in the event of a core destruction, or pressures that would occur in the reactor vessel (2) if the efforts to reduce pressure were incomplete.

[0042] At the design reference pressure in the reactor vessel (2), a longer two-stage flow of the molten core occurs: the first stage is the flow of mainly the metallic part of the molten core, and the second stage is the flow of its oxide part. The relatively long flow of the molten core onto the guide plate (1) does not lead to the sector destruction of the guide plate (1) and the cantilever truss (3).

[0043] In the reactor vessel (2), at pressures above the design basis, the molten core may escape by a rapid general jet head pressure blowout, accompanied by a rapid axial increase of the molten core flowing out of the molten core outlet hole in the reactor vessel (2). In this case, if the bottom of the reactor vessel (2) is not broken early, the parallel flow of molten metal and molten oxide may cut a 60° high sector in the horizontal plane of the guide plate (1) and directly interact with the cantilever truss (3). The force frame (13) ensures the strength and stability of the cantilever truss (3) in the event of a vertical penetration break of the 60° high sector of the cantilever truss (3) in the horizontal plane.

[0044] When the force frame (13) is subjected to enormous heat exposure from the melt surface, the largest thermal deformations occur in the inner force shell, which consists of two interconnected inner half shells (17), and the upper force plate, which consists of two interconnected semicircular force plates (19).

[0045] The alternating arrangement of the radial support ribs (14) and the parallel support ribs (15) ensures azimuthal asymmetry, making it possible for the force frame (13) to localize thermal deformations occurring in the inner half shell consisting of two interconnected inner half shells (17) and the upper force plate consisting of an interconnected semicircular force plate (19) within the sector between the parallel support ribs (15).

[0046] The elements of the force frame (13) of the cantilever truss (3), namely the parallel bearing ribs (15), the radial bearing ribs (14), the outer half shell (16), the inner half shell (17), the middle half shell (18), the upper semicircular force plate (19) and the lower semicircular force plate (20), may be made of grade 09G2S steel.

[0047] The thickness of the force plates (19) and (20) is 60 mm.

[0048] The total thickness of the horizontal weld joints on both sides of the two parts (28) of the cantilever truss is 50 mm, and the gap between the weld joints is 10 mm.

[0049] The thickness of the vertical weld joints (22) on both sides connecting the two parts (28) of the cantilever truss is 50 mm, and the gap between the weld joints is 10 mm.

[0050] The thickness of the vertical weld joints (12), (24) on one side of the cover plate (11) is 2x30mm, and the gap between the weld joints of the paired plates is 20-80mm.

[0051] The cover plate (11) has a thickness of 20 mm.

[0052] By implementing the manufacturing method for the cantilever truss for the core meltdown catcher device, it will be possible to predict the destruction of the load-bearing elements of the cantilever truss in the event of an accident exceeding the design basis, an earthquake impact on the reactor well, or an impact on the cantilever truss from the side of the guide plate, thereby improving the reliability of the CMLD.

Claims

1. 1. A method of manufacturing a cantilever truss for a core meltdown catch device, comprising: Two symmetrical sections of the cantilever truss are formed, Each section is composed of outer, middle and inner half shells, which are connected by radial and parallel bearing ribs, upper and lower semicircular force plates, by welding to form multiple parallel and radial sectors. The two symmetrical sections are then connected to each other by welding at two parallel sector sections located on one orthogonal axis such that an upper horizontal joint connects the upper semicircular force plates, a lower horizontal weld joint connects the lower semicircular force plates, and outer, inner and middle vertical weld joints connect the outer, inner and middle half shells.

2. 2. The method of claim 1, wherein the thickness of the top and bottom horizontal weld joints and the outer, inner and middle vertical weld joints is less than the thickness of the base metal and other weld joints.

3. 2. The method of claim 1, wherein the outer, inner and middle vertical weld joints are partially fabricated, and cover plates are attached to the outer and inner vertical weld joints and the top and bottom horizontal weld joints.

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