Work floor for the maintenance of a nuclear reactor steam generator and assembly comprising such a floor

Abstract

The invention relates to a steam generator (1) comprising: - U-shaped tubes (5); - a dome (18) covering the curved branches (9) of the U-shaped tubes; - a plurality of steam ducts (23), each connected to a steam port (21) of the dome and arranged in a circle about the central axis (C); the work floor (29) comprising at least one floor sector (31) intended to be arranged circumferentially between two steam ducts (23), the or each floor sector (31) comprising: - a support (33) having a shape designed to be arranged radially between a central zone of the dome (18) and the circle of steam ducts (23) and circumferentially between the two neighbouring steam ducts (23), the support (33) being placed on the dome (18) and having a bearing point (37) on each of the two neighbouring steam ducts (23) and at least one bearing point (39) on the dome (18); - at least one extension plate (35) connected to the support (33) and extending from the support (33) in a longitudinal direction between the two steam ducts (23).

Description

[0001] TITLE: Work platform for the maintenance of a nuclear reactor steam generator and assembly comprising such a platform

[0002] The invention relates generally to the maintenance of nuclear reactor steam generators.

[0003] More specifically, the invention relates to a work platform designed for the maintenance of such a steam generator.

[0004] The steam generator has a vertical central axis and comprises:

[0005] - an outer casing;

[0006] - a tubular plate arranged inside the outer casing, in a lower area of ​​the outer casing;

[0007] - U-shaped tubes, each having two straight branches with ends fixed to the tube sheet and a curved branch connecting the two straight branches to each other;

[0008] - a dome, housed inside the outer envelope and covering the curved branches, the dome being convex towards an upper end of the outer envelope having steam vents for the passage of steam;

[0009] - a plurality of steam conduits for the circulation of steam, each connected to a steam outlet and extending towards the upper end of the outer casing, the steam conduits being arranged in at least one circle around the central axis.

[0010] When maintenance work needs to be carried out in the upper part of the steam generator, above the dome, it is possible to set up a work platform to facilitate the work of the operators.

[0011] The existing work floors are not satisfactory.

[0012] Their installation is lengthy and tedious. This drawback is particularly problematic, given that intervention time inside the steam generator must be minimized. Operators working inside the steam generator are exposed to exceptionally harsh working conditions. The dose rate inside the steam generator is significant, even though the lower section is filled with water to create a biological barrier. The temperature is high. Furthermore, operators must wear protective suits, making the work particularly arduous. In addition, the existing floors are not very rigid, which complicates interventions and slows down work inside the steam generator.

[0013] The existing floors also consist of a large number of parts, including small elements that could fall to the bottom of the steam generator. These small elements could then become migrating bodies, potentially affecting the safe operation of the steam generator.

[0014] In this context, the invention aims to provide a work floor that does not have the above defects.

[0015] To this end, the invention relates to a work floor for the maintenance of a nuclear reactor generator of the type described above, the work floor comprising at least one floor sector intended to be arranged circumferentially between two steam pipes adjacent to at least one circle, the floor sector(s) comprising:

[0016] - a support of suitable shape to be arranged radially between a central area of ​​the dome and at least one circle of steam ducts and circumferentially between said two neighboring steam ducts, the support being placed on the dome and having a support on each of the two neighboring steam ducts and at least one support on the dome;

[0017] - at least one extension plate connected to the support and extending from the support in a longitudinal direction between the two adjacent vapor ducts.

[0018] Thus, each floor section is of a particularly simple design. It comprises a small number of parts: a support, and a small number of extension plates.

[0019] Typically, it includes one or two extension plates.

[0020] The support is easy to assemble, as it simply rests on the dome and supports the steam ducts. The extension plate is then attached to the support. It allows operators to access the space between the steam ducts.

[0021] Each floor section can be assembled without small parts that could come loose and fall to the bottom of the steam generator.

[0022] The supports ensure its stability. They also ensure the stability of each extension plate attached to the support.

[0023] The connection of at least one extension plate to the support is designed so that the floor sector offers good rigidity.

[0024] The work floor may also have one or more of the following characteristics, considered individually or in all technically possible combinations: - at least 90% of the forces borne by at least one extension plate are taken up by the support, the supports of the support being configured to prevent the support from tilting in the longitudinal direction relative to the dome;

[0025] - at least one extension plate is separated from the neighboring vapor ducts and the outer casing by a gap;

[0026] - at least one extension plate has a flexible border closing the gap between said at least one extension plate and the outer casing;

[0027] - the support comprises a support plate substantially perpendicular to the central axis, at least one extension plate has a proximal longitudinal end bearing on an upper surface of the support plate, and hooks cooperating with the support to prevent the extension plate from tilting longitudinally relative to the support;

[0028] - the support plate has hooking holes into which the hooks are engaged, each hook cooperating with a lower surface of the support plate to prevent the extension plate from tilting longitudinally relative to the support;

[0029] - each hook cooperates with an edge of the corresponding hooking hole to lock at least one extension plate in position relative to the support in a plane perpendicular to the central axis;

[0030] - each floor sector comprises two extension plates arranged side by side circumferentially;

[0031] - each floor sector includes a fixing of the two extension plates to each other;

[0032] - the fixing includes at least one finger attached to one of the two extension plates, and a hole made in the other of the two extension plates, the finger carrying a pin engaged in the hole;

[0033] - the working floor comprises several floor sectors, each arranged circumferentially between two adjacent vapor ducts, said floor sectors being mechanically entirely independent of each other;

[0034] - the working floor comprises several floor sectors each arranged circumferentially between two adjacent vapor ducts, and at least one end plate arranged radially between one of the vapor ducts and the outer casing, at least one end plate being supported on the extension plates of the two floor sectors arranged on either side of said vapor duct;

[0035] - a bar is fixed to a distal longitudinal end of the extension plate(s). According to a second aspect, the invention relates to an assembly comprising a nuclear reactor steam generator and a work platform for the maintenance of said steam generator, the steam generator being of the type described above, the work platform having the characteristics described above.

[0036] Other features and advantages of the invention will become apparent from the detailed description given below, by way of example and not limitation, with reference to the attached figures, including:

[0037] - [Fig.1] Figure 1 is a simplified schematic representation, in axial section, of a nuclear reactor steam generator;

[0038] - [Fig.2] Figure 2 is a schematic top view taken from above the dome of a steam generator, with a working floor in place inside this steam generator;

[0039] - [Fig.3] Figure 3 is an enlarged perspective view of an area of ​​the steam generator in Figure 2;

[0040] - [Fig.4] Figure 4 is a schematic representation, in radial section, of a sector of the work floor of figures 2 and 3;

[0041] - [Fig. 5] Figure 5 is a schematic top view representation of the two extension plates of a floor sector, the extension plates being shown separated from each other to reveal the means of fixing the two extension plates to each other; and

[0042] - [Fig.6] Figure 6 is a schematic top view representation of an area of ​​the steam generator, illustrating variants of the working floor design.

[0043] The work floor is intended for the maintenance of a steam generator 1 of a nuclear reactor of the type shown in Figure 1.

[0044] The steam generator 1 has a vertical central axis C.

[0045] The circumferential and radial directions in this description are taken relative to the central axis C of the steam generator. Up and down, the superior and inferior directions, are understood relative to the vertical direction defined by the central axis C.

[0046] Steam generator 1 comprises:

[0047] - an outer envelope 3;

[0048] - U-shaped tubes 5;

[0049] - a tubular plate 7 arranged inside the outer casing 3, in a lower area of ​​the outer casing 3. The outer casing 3 is pressure-resistant. It has an upper area of ​​relatively larger diameter, the lower area being of relatively smaller diameter.

[0050] The steam generator 1 comprises a large number of U-shaped tubes 5, forming a tube bundle. Each U-shaped tube 5 has two straight branches 8 with ends fixed to the tube sheet 7, and a curved branch 9 connecting the two straight branches to each other.

[0051] Each tube thus has an inverted U shape.

[0052] The U-shaped tubes 5 carry pressurized water from the primary circuit of the nuclear reactor. This primary water is introduced into a first compartment of the steam generator 1, under the tube sheet 7, through an inlet port.

[0053] 10. The primary water flows through the tubes 5 of the tube bundle, then returns under the tube sheet 7 into a second compartment separate from the first, and exits through the outlet port.

[0054] 11.

[0055] The tube bundle is surrounded, up to the vicinity of its lower part, by a secondary envelope 13 internally delimiting a vaporization chamber 15 inside which the secondary water is in contact with the U-shaped tubes 5 through which the pressurized water, at high temperature, flows from the core of the nuclear reactor.

[0056] The upper part of the steam generator 1 constitutes a tank 17 collecting the steam obtained by evaporation of the secondary water.

[0057] The steam generator includes a dome 18, housed inside the outer casing 1 and covering the curved branches 9 of the U-tubes.

[0058] Dome 18 is located at a distance above and opposite the curved branches 9.

[0059] The balloon 17 is separated from the vaporization chamber 15 by the dome 18, which forms the roof of the vaporization chamber 15.

[0060] Dome 18 is curved towards an upper end of the outer envelope 3.

[0061] It is centered on the central axis C.

[0062] A secondary water inlet 19 opens into the annular volume 20 separating the secondary envelope 13 from the outer envelope 3. This annular volume 20 communicates fluidly with a lower end of the vaporization chamber 15, at the level of the tube plate 7.

[0063] The dome 18 has vapor vents 21 for the passage of steam.

[0064] The steam generator 1 further comprises a plurality of steam conduits 23, each connected to a steam port 21 and extending towards the upper end of the outer casing 3. The steam conduits 23 are arranged in at least one circle around the central axis C.

[0065] In the example shown in Figure 2, the steam pipes 23 form two concentric circles around the central axis C. In the example shown in Figure 1, the steam pipes form a large number of concentric circles around the central axis C.

[0066] The steam generator 1 comprises a plurality of centrifugal separators forming a first stage 24 for steam drying. The centrifugal separators are arranged in the steam ducts 23, or as extensions thereof.

[0067] Thus, the steam ducts 23 channel the steam from the vaporization chamber 15 to the internal volume of the flask 17, through the centrifugal separators.

[0068] The steam generator 1 also includes a second steam drying stage 25, located downstream of the first stage. Downstream in this description refers to the normal direction of steam flow.

[0069] A steam vent 27 is provided at the upper end of the outer casing 3, downstream of the second drying stage 25.

[0070] The operation of the steam generator is as follows.

[0071] Secondary water enters the steam generator 1 through the inlet 19 and passes through the annular space 20 between the outer shell 3 and the secondary shell 13, up to the lower end of the secondary shell 13.

[0072] Secondary water passes between the tube sheet 7 and the lower end of the secondary jacket 13, and enters the vaporization chamber 15. The secondary water gradually vaporizes as it rises inside the vaporization chamber 15. The steam can only leave the vaporization chamber 15 through the steam ducts 23, which carry the wet steam to the centrifugal separators of the first drying stage 24. The steam abandons most of the liquid water carried with it in the centrifugal separators of the first stage 24. It then passes into the second drying stage 25, and exits the steam generator through the steam outlet 27.

[0073] The work floor 29 illustrated in Figure 2 is intended to be installed in the steam generator 1, to facilitate maintenance operations inside the outer casing 3 of this steam generator.

[0074] It is designed to be mounted inside the balloon 17, on the dome 18. It is intended to support operators while they perform maintenance operations. It therefore provides a substantially flat and horizontal surface, that is, perpendicular to the central axis C, on which the operators can stand. Maintenance operations performed using the work platform 29 are of all types: work on the cyclones of the first steam drying stage, work in the feedwater torus, etc.

[0075] As can be seen in Figure 2, the working floor 29 includes at least one floor sector 31, intended to be arranged circumferentially between two vapor conduits 23 adjacent to one of the vapor conduit circles.

[0076] Typically, the working floor 29 comprises a plurality of floor sectors 31 each intended to be arranged circumferentially between two adjacent vapor conduits 23 of said circle.

[0077] Each floor sector 31 is designed to be arranged between two vapor conduits 23 of the same circle.

[0078] For example, it includes as many floor sectors 31 as there are intervals between the vapor conduits 23 of the circle.

[0079] Alternatively, the working floor 29 has only one floor sector 31, or two floor sectors 31, or a number of floor sectors 31 less than the number of intervals between the vapor conduits of the circle considered.

[0080] In the example shown, the floor sectors 31 are designed to be housed between the vapor conduits 23 located on the outermost radially circle.

[0081] Alternatively, they are designed to be inserted between the vapor conduits 23 of the innermost circle, or any other circle.

[0082] The floor sector 31 is designed to be simply placed in the steam generator, without rigid attachment to the structures of this steam generator by means of, for example, straps, suspensions, screws, etc.

[0083] As can be seen in Figure 2, each floor sector 31 comprises:

[0084] - a support 33 of suitable shape to be arranged radially between a central area of ​​the dome 18 and the circle of vapor conduits 23, and circumferentially between said two neighboring vapor conduits 23;

[0085] - at least one extension plate 35 linked to the support 33 and extending from the support 33 in a longitudinal direction between the two adjacent vapor conduits 23.

[0086] The support 33 and the extension plate(s) 35 are sized to be inserted into the steam generator through the upper manhole. Furthermore, these components are of suitable mass for easy handling by an operator inside the steam generator. Preferably, each component has a mass of less than 15 kg.

[0087] The support 33 and the extension plate(s) 35 are preferably made of a metallic material, for example stainless steel. In the embodiment described below, the support 33 and the extension plate(s) 35 are independent elements designed to be assembled together by an operator inside the steam generator without tools or additional fasteners such as screws or straps.

[0088] After assembly, the support 33 and the extension plate(s) 35 form a highly rigid structure.

[0089] The support 33 is designed to be arranged radially inside the circle of vapor ducts.

[0090] The support 33 is placed on the dome 18. It is not rigidly fixed to the dome 18 or to the vapor ducts 23. It simply rests on the dome 18 and the vapor ducts 23.

[0091] More specifically, the support 33 has a bearing 37 on each of the two adjacent steam conduits 23, and at least one bearing 39 on the dome 18.

[0092] The supports 37, 39 are configured so that the vertical forces applied to the support 33 are transmitted by the supports 37, 39 to the dome 18 and to the steam ducts 23.

[0093] These forces are directly transmitted by supports 37, 39 to dome 18 and to steam conduits 23.

[0094] The support 33, as can be seen more particularly in figure 3, comprises a support plate 41 substantially perpendicular to the central axis C. The support plate 41 has towards the upper end of the outer envelope 3 a substantially flat upper face 43.

[0095] The support plate 41 has a circumferentially elongated shape, i.e. a circumferentially larger dimension than radially.

[0096] The support plate 41 is delimited radially outwards, i.e. towards the outer envelope 3, by an outer edge 45. It is delimited radially inwards, i.e. towards the central axis C, by an inner edge 47. It is also delimited by two lateral edges 49 opposed to each other.

[0097] The lateral edges 49 are turned circumferentially towards the floor sectors 31 framing the support 33.

[0098] The supports 37 correspond to arched sectors formed in the edge of the support plate 41, interposed between the two lateral edges 49 and the outer edge 45.

[0099] These arched sectors are concave towards the outside of the plate, i.e. towards the steam ducts 23. Each arched sector has a shape complementary to that of the surface of the steam duct 23 against which it rests.

[0100] For example, each support 37 is an arc of a circle, with a radius equal to the radius of the steam duct against which it rests. In the example shown in Figure 3, the support 33 has two supports 39 on the dome 18. The support 33 has spacer plates 51 attached to the lower face 53 of the support plate 41. The lower edges of the spacer plates 51 rest on the dome 18 and constitute the supports 39. These edges have a profile corresponding to the shape of the dome 18.

[0101] For example, the edges of the spacer plates 51 defining the supports 39 extend substantially in radial planes relative to the central axis C. They follow the curvature of the dome 18 in said planes.

[0102] As mentioned above, the extension plate or each extension plate 35 extends from the support 33 along the longitudinal direction between the two adjacent vapor conduits 23.

[0103] The longitudinal direction corresponds approximately to a radial direction taken from the central axis C of the steam generator.

[0104] The extension plate or plates 35 therefore extend cantilevered from the support 33. It extends practically to the outer casing.

[0105] At least one extension plate 35 is separated from the neighboring vapor conduits 23 and the outer casing 3 by a gap.

[0106] This gap is necessary to accommodate the geometric tolerances on the position of the vapor conduits 23 and the outer casing 3.

[0107] The gap is continuous. In other words, at least one extension plate 35 is not supported by the vapor conduits 23 or by the outer casing 3.

[0108] At least one extension plate 35 is also not supported on the dome 18.

[0109] Thus, at least 90% of the forces experienced by at least one extension plate 35 are taken up by the support 33. Typically, all of the forces experienced by at least one extension plate 35 are taken up by the support 33.

[0110] The forces exerted on a given extension plate 35 are taken up directly by the support 33 and / or indirectly through another extension plate 35 belonging to the same sector 31 and connected to the same support 33.

[0111] Advantageously, the supports 37, 39 of the support 33 are configured to prevent the support 33 from tilting in the longitudinal direction relative to the dome 18.

[0112] Longitudinal tilting means a tilting of the support 33 relative to the dome 18 in such a way that the outer edge 45 of the support plate 41 pivots downwards around the supports 39, and the inner edge 47 of the support plate 41 upwards.

[0113] In other words, the supports 37, 39 are designed and positioned so that the support 33 is stable despite the forces transmitted to the support 33 by the at least one extension plate 35. The forces transmitted by the at least one extension plate 35 to the support 33 include in particular the weight of the at least one extension plate 35, the weight of a worker standing on the at least one extension plate 35, the weight of the tools used by this worker, etc.

[0114] Typically, the supports 37, 39 of the support 33 are configured to prevent the support 33 from tipping over when a downward vertical force equivalent to the weight of an operator with their equipment is exerted on the geometric center of at least one extension plate 35. The supports 37, 39 of the support 33 are, for example, configured to prevent the support 33 from tipping over when a load of 200 kg is placed on the geometric center of at least one extension plate 35.

[0115] The stability of the support under the effect of the torque transmitted by at least one extension plate 35 is obtained in particular by the correspondence of shape between the supports 37 and the vapor conduits 23.

[0116] Furthermore, due to the shape of the dome 18, each steam orifice 21 has a normal axis N forming an acute, non-zero angle α with the central axis C of the steam generator.

[0117] The normal axis N is the axis perpendicular to the plane of the vapor orifice 21 and passing through its center. It is shown in Figure 3.

[0118] The steam conduit 23 connected to this orifice has a central portion 55 that is substantially vertical, i.e. parallel to the central axis C. It also has a curved end 57, connecting the central portion 55 to the steam orifice 21.

[0119] The support 37 rests on a surface 59 of the curved end 57 which is turned towards the central axis C of the steam generator and upwards, i.e. towards the upper end of the outer casing 3.

[0120] The orientation of the surface 59 on which the support 37 rests helps to prevent the support 33 from tipping over, and to ensure its stability.

[0121] Each extension plate 35 has a proximal longitudinal end 61 bearing against the upper face 43 of the support plate. As can be seen in particular in Figure 4, the extension plate 35 has hooks 63 cooperating with the support 33 to prevent the extension plate 35 from tilting longitudinally relative to the support 33.

[0122] Longitudinal tilting refers to a tilting of the extension plate 35 relative to the support 33 in such a way that the distal longitudinal end 67 of the extension plate 35 pivots downwards, and the proximal longitudinal end 61 upwards. The distal longitudinal end 67 is located longitudinally opposite the proximal longitudinal end 61. To achieve this, the support plate 41 has latching holes 69 into which the hooks 63 are engaged.

[0123] Each extension plate 35 has, for example, two hooks 63. The support plate 41 typically has a hooking hole 69 for each hook 63.

[0124] Each hook 63 cooperates with the lower face 53 of the support plate 41 to prevent the extension plate 35 from tilting relative to the support 33.

[0125] In the example shown, each bracket 63 has an L-shape. Thus, each bracket 63 comprises an end section 71 with a longitudinal orientation, and a connecting section 73 connecting the end section to the extension plate 35. The connecting section 73 is rigidly fixed under the extension plate 35, and is typically perpendicular to this plate 35. The end section 71 extends longitudinally from the connecting section 73 towards the proximal longitudinal end 61.

[0126] The hooking hole 69 is sized so that it is possible to slide the end section 71 inside this hole.

[0127] Advantageously, each hook 63 cooperates with the edge of the corresponding hooking hole 69 to lock the extension plate 35 in position relative to the support 33 in a plane perpendicular to the central axis C.

[0128] The extension plate 35 is thus locked relative to the support in all directions, radially, circumferentially, and in all horizontal directions inclined at the same time with respect to the radial direction or the circumferential direction.

[0129] The one or each extension plate 35 is substantially flat, and extends in a plane perpendicular to the central axis C.

[0130] It has on its underside a stiffening rib 74, visible in figure 3.

[0131] In the example shown, the floor sector or sectors 31 comprise two extension plates 35, arranged side by side circumferentially.

[0132] In this case, each of the two extension plates 35 is delimited towards the other extension plate 35 by an inner edge 75, and is delimited towards one of the two steam conduits 23 framing the extension plates by an outer edge 77.

[0133] The inner edges 75 of the two extension plates extend against each other, without any gap between the two edges 75. They are parallel to each other. In the example shown, they are straight.

[0134] On the contrary, in the example shown in Figure 2, the outer edges 77 are curved around the corresponding vapor conduit 23, at the level of the proximal longitudinal end 61. Advantageously, the floor sector 31 then includes a fixing 79 of the two extension plates 35 to each other.

[0135] As illustrated in Figure 5, the fastener 79 has at least one finger 81 attached to one of the two extension plates 35, and a hole 83 provided in the other of the two extension plates 35. The finger 81 carries a pin 85, engaged in the hole 83.

[0136] The finger 81 is placed under the corresponding extension plate 35, and protrudes from the inner edge 75 of said plate, towards the other extension plate 35. It extends under the lower surface of the other extension plate 35.

[0137] The hole 83 passes through the extension plate 35 through its entire thickness and is open at the lower surface of this plate. It has an internal cross-section slightly larger than the external cross-section of the pin 85.

[0138] In the example shown in Figure 5, the fastener 79 also includes a second finger 81, attached to the other extension plate 35. In other words, each of the two extension plates has a finger 81 carrying a pin 85, and each of the two plates has a hole 83 provided to receive the pin 85 of the finger 81 carried by the other plate.

[0139] In the embodiment shown in Figure 2, the work floor 29 comprises several floor sectors 31 arranged circumferentially between two adjacent vapor conduits 23, the floor sectors 31 being mechanically completely independent of each other.

[0140] In the embodiment shown in Figure 6, the floor sectors 31 are not mechanically completely independent of each other.

[0141] On the contrary, the working floor 29 includes at least one end plate 87 arranged radially between one of the vapor conduits 23 and the outer casing 3. The end plate 87 extends between the extension plates 35 of the two floor sectors 31 arranged on either side of the vapor conduit 23.

[0142] This end plate 87 is circumferentially elongated. It has two opposite ends 89 along the circumferential direction which rest on the extension plates 35 of the floor sectors arranged on either side of the steam duct 23.

[0143] According to another embodiment shown in Figure 6, which may or may not be combined with the previous embodiment, at least one extension plate 35 has a flexible rim 91 closing the gap between said at least one extension plate 35 and the outer casing 3. The flexible rim 91 is rigidly fixed to the distal longitudinal end 67 of the or each extension plate 35. It extends to the outer casing 3, and preferably touches the outer casing 3.

[0144] A flexible border 91 can also be placed on the outer plate 87, and close the gap between the outer plate 87 and the outer envelope 3.

[0145] According to an advantageous variant shown in figures 4 and 5, there is a bar 93 at the distal longitudinal end 67 of the or each extension plate 35.

[0146] The 93 bar is made of the same material as the floor, and more specifically as the extension plate 35. It is fixed mechanically or by welding to the extension plate 35.

[0147] The bar 93 is a metal profile, with a substantially circumferential orientation. It is placed on the large upper face of the extension plate 35, typically along the edge of the extension plate 35.

[0148] This bar 93 serves, in particular, as a gripping or support element for the operator when working near the enclosure 3, for example, when lowering tools under the floor using ropes. This bar 93, located at the distal longitudinal end 67, also allows the operator, when lying on the floor and working in the lower space (under the floor between the ferrule and the bundle enclosure), to brace their forearm against the bar to prevent slipping (while their other hand is manipulating a tool, for example).

[0149] A bar 93 may also be provided on the end plate 87, along the edge of the end plate 87 closest to the outer envelope 3.

[0150] Advantageously, the flexible border 91 is fixed to the bar 93, as illustrated in figure 6.

[0151] The flooring described above has multiple advantages.

[0152] Because at least 90% of the forces exerted on the at least one extension plate are absorbed by the support, and because the support's bearings are configured to prevent the support from tilting along its length relative to the dome, a rigid attachment of the support to the dome is not necessary. The existing bearing arrangement is sufficient to ensure the stability of both the support and the at least one extension plate.

[0153] When at least one extension plate is separated from the adjacent steam ducts and the outer casing by a gap, it is possible to disregard the positional tolerances of the steam ducts and the outer casing. In other words, even if the steam ducts and the outer casing are not precisely in the positions shown on the drawings, for example, due to deformation of the steam ducts or the outer casing, it is still possible to install the floor section. When at least one extension plate has a flexible edge closing the gap between said extension plate and the outer casing, it is possible to reduce the risk of a small part falling to the bottom of the steam generator.

[0154] The use of hooks cooperating with the support to prevent the extension plate from tilting longitudinally relative to the support makes it easy to guarantee the stability of the extension plate or each extension plate relative to the support.

[0155] When the support plate has hooking holes into which the hooks are engaged, each hook cooperating with a lower surface of the support plate to prevent the extension plate from tilting longitudinally relative to the support, it is possible to put the extension plate(s) on the support plate very easily and very quickly.

[0156] It is then possible to use the hooks and mounting holes to easily lock the extension plate in position relative to the support in a plane perpendicular to the central axis. This is achieved without the need for any additional means.

[0157] Providing that each floor sector has two extension plates arranged side by side circumferentially allows the floor sector to be adapted to geometric configurations where the spacing between vapor ducts is significant.

[0158] Securing the two extension plates to each other ensures the stability and rigidity of each floor section.

[0159] When the fastener has at least one finger attached to one of the two extension plates and a hole in the other extension plate, with the finger carrying a pin engaged in the hole, the two extension plates can be conveniently fastened to each other without tools. This fastening is easily performed by the operator when assembling the work platform inside the steam generator.

[0160] When floor sectors are mechanically independent of each other, it is possible to use only one floor sector, or several floor sectors not adjacent to each other.

[0161] Providing an end plate arranged radially between a steam duct and the outer casing, and supported by extension plates of the floor sections arranged on either side of said steam duct, allows for interventions requiring the operator to stand in the space between the steam duct and the outer casing. The work floor described above can have multiple variations.

[0162] The supports for the steam pipes could have a different shape than described above. For example, the support plate could incorporate an intermediate piece to increase the bearing surface on the steam pipes.

[0163] Similarly, the support on the dome could be achieved differently from what has been described above. For example, the spacer plates could be replaced by the ends of the support plate, bent perpendicularly to the central part of said support plate.

[0164] In the description above, each floor section is divided into several independent parts: the support, and one or more extension plates. These different parts are assembled by an operator inside the steam generator. According to a non-preferred variant, the floor section could consist of only two independent parts: one part comprising half of the support and one extension plate, and another half comprising the other half of the support and one extension plate. After assembly of these two parts inside the steam generator, they form the support described above, and the extension plates described above. The assembly means are of any suitable type, for example, hooks.

[0165] The means of attaching the extension plate(s) to the support could differ from the hooks described above. The hooks could be replaced, for example, by hinges or removable pivots allowing for easy assembly.

[0166] The method of attaching the two extension plates to each other could differ from the solution proposed above. It could be achieved, for example, using hinges, toggle latches, or lever closures.

Claims

DEMANDS 1. Work platform (29) for the maintenance of a nuclear reactor steam generator, the steam generator (1) having a vertical central axis (C) and comprising: - an outer envelope (3); - a tubular plate (7) arranged inside the outer casing (3), in a lower area of ​​the outer casing (3); - U-shaped tubes (5), each having two straight branches (8) with ends fixed to the tube plate (7) and a curved branch (9) connecting the two straight branches (8) to each other; - a dome (18), housed inside the outer envelope (3) and covering the curved branches (9), the dome (18) being convex towards an upper end of the outer envelope (3) and having vapor orifices (21) for the passage of vapor; - a plurality of steam ducts (23) for the circulation of steam, each connected to a steam outlet (21) and extending towards the upper end of the outer casing (3), the steam ducts (23) being arranged in at least one circle around the central axis (C); the working floor (29) comprising at least one floor sector (31) intended to be arranged circumferentially between two steam ducts (23) adjacent to the at least one circle, the floor sector(s) (31) comprising: - a support (33) of a shape adapted to be arranged radially between a central area of ​​the dome (18) and at least one circle of vapor conduits (23) and circumferentially between said two neighboring vapor conduits (23), the support (33) being placed on the dome (18) and having a support (37) on each of the two neighboring vapor conduits (23) and at least one support (39) on the dome (18), the supports (37, 39) being preferably configured so that the vertical forces applied to the support (33) are transmitted by the supports (37, 39) to the dome (18) and to the vapor conduits (23); - at least one extension plate (35) linked to the support (33) and extending from the support (33) in a longitudinal direction between the two adjacent vapor conduits (23).

2. Work floor according to claim 1, wherein at least 90% of the forces suffered by at least one extension plate (35) are taken up by the support (33), the supports (37, 39) of the support being configured to prevent tilting of the support (33) in the longitudinal direction relative to the dome (18).

3. Work floor according to any one of claims 1 to 2, wherein at least one extension plate (35) is separated from the neighboring vapor conduits (23) and the outer casing (3) by a gap.

4. Work floor according to claim 3, wherein at least one extension plate (35) carries a flexible edge (91) closing the gap between said at least one extension plate (35) and the outer casing (3).

5. Work floor according to any one of claims 1 to 4, wherein the support (33) comprises a support plate (41) substantially perpendicular to the central axis (C), at least one extension plate (35) has a proximal longitudinal end (61) bearing on an upper surface (43) of the support plate (35), and hooks (63) cooperating with the support (33) to prevent tilting of the extension plate (35) longitudinally relative to the support (33).

6. Work floor according to claim 5, in which the support plate (33) has hooking holes (69) in which the hooks (63) are engaged, each hook (63) cooperating with a lower surface (53) of the support plate (33) to prevent the extension plate (35) from tilting longitudinally relative to the support (33).

7. Work floor according to claim 6, wherein each hook (63) cooperates with an edge of the corresponding hooking hole (69) to lock at least one extension plate (35) in position relative to the support (33) in a plane perpendicular to the central axis (C).

8. Work floor according to any one of claims 1 to 7, wherein the floor sector or each floor sector (31) comprises two extension plates (35) arranged side by side circumferentially.

9. Work floor according to claim 8, wherein the floor sector or each sector (35) has a fixing (79) of the two extension plates (35) to each other.

10. Work floor according to claim 9, in which the fixing (79) comprises at least one finger (81) integral with one of the two extension plates (35), and a hole (83) provided in the other of the two extension plates (35), the finger (81) carrying a pin (85) engaged in the hole (83).

11. A work floor according to any one of claims 1 to 10, wherein the work floor (29) comprises several floor sectors (31), each arranged circumferentially between two adjacent vapor conduits (23), said 18 floor sectors (31) being mechanically completely independent of each other.

12. Work floor according to any one of claims 1 to 10, wherein the work floor (29) comprises several floor sectors (31) each arranged circumferentially between two adjacent vapor conduits (23), and at least one end plate (87) arranged radially between one of the vapor conduits (23) and the outer casing (3), the at least one end plate (87) being supported on the extension plates (35) of the two floor sectors (31) arranged on either side of said vapor conduit (23).

13. Work floor according to any one of claims 1 to 12, wherein a bar (93) is fixed to a distal longitudinal end (67) of the or each extension plate (35).

14. Assembly comprising a nuclear reactor steam generator (1) and a work platform (29) for the maintenance of said steam generator, the steam generator (1) having a vertical central axis (C) and comprising: - an outer envelope (3); - a tubular plate (7) arranged inside the outer casing (3), in a lower area of ​​the outer casing (3); - U-shaped tubes (5), each having two straight branches (8) with ends fixed to the tube plate (7) and a curved branch (9) connecting the two straight branches (8) to each other; - a dome (18), housed inside the outer envelope (3) and covering the curved branches (9), the dome (18) being convex towards an upper end of the outer envelope (3) and having vapor orifices (21) for the passage of vapor; - a plurality of steam conduits (23) for the circulation of steam, each connected to a steam orifice (21) and extending towards the upper end of the outer casing (3), the steam conduits (23) being arranged in at least one circle around the central axis (C); the working floor (29) being according to any one of the preceding claims.

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