EP0647815A1 - Steam generator with superposed blocking elements in the tube lane - Google Patents

Abstract

The invention relates to steam generators with a bank of curved tubes connected to a horizontal tube plate, the tubes which are closest together delimiting a central transverse region (60) known as the tube lane, which it is appropriate to obstruct at least in part by solid elements which limit the circulation of the secondary fluid around the tubes on the exterior of these. According to the invention, these elements are constituted by a stack (62) of blocks (66, 68, 70) accommodated in the central region, these blocks being removable and capable of being extracted or put in place in the enclosure of the generator through an access hole (92) made in its external wall (10). <IMAGE>

Description

The present invention relates to a steam generator, in particular for a nuclear power plant, comprising an outer casing with a vertical axis, a horizontal tube plate, fixed in a sealed manner inside and at the lower end of the casing, a bundle of inverted U-shaped tubes, comprising vertical branches, respectively hot and cold, joined by a curved part at their upper end and traversed by a primary fluid yielding its calories, inside the exchanger, to a secondary fluid circulating in the outer casing, the branches of the tubes having their ends connected to the tube plate and opening out under the latter, respectively in an intake manifold for the hot primary fluid and an evacuation manifold for this cooled fluid, an inner envelope covering the bundle of tubes, the lower edge of which is spaced from the tube plate and delimits with the outer envelope there is an annular space traversed by the secondary fluid introduced inside this envelope into this space before vaporizing on contact with the tubes traversed by the primary fluid, means for extracting the vapor, disposed above the bundle of tubes in the outer casing, and means for deflecting and distributing the flow of the secondary fluid, provided at the lower part of the inner casing, these deflection means delimiting with the tube plate on the one hand and the hot branches and cold of the closest tubes on the other hand, an elongated central region, extending transversely through the outer envelope, this region being occupied by means of blocking or partial occupation of the passage offered to the secondary fluid in this region .

The secondary fluid, usually water, contains particles of matter, mainly in the form of iron oxides or copper compounds, or traces of other metals, which tend to deposit on the tube plate, in particular in the central region thereof, between the branches closest to the U-shaped tubes where the flow speed of the secondary fluid from one end to the other of the plate is insufficient to prevent the deposition of mud or other residues, harmful, because creating concentrations of corrosion agents along the external walls of these tubes.

The blocking or partial occupation means arranged in this central region, generally called "water street", by reference to the classic Anglo-Saxon expression in the technique of "tube lane blocking device", the term blocking signifying rather obstructs this region, therefore aims to minimize the flow of secondary fluid in the corresponding passage between the closest tubes, in order to increase the lateral flow through the bundle, reducing the deposition mud in this passage. These means are generally constituted by metal blocks, parallelepiped, mounted in a fixed position in the central region between the tubes and resting on the plate according to the diameter of the latter in this region.

Note that, depending on the type of steam generator considered, of the type known as "axial economizer" or "boiler", the street of water constituting the aforementioned central region, is or is not separated in the median plane of the generator by a vertical deflector, fixed to the tube plate and extending in the middle of the passage delimited between the branches of the tubes in this region, the blocking means being in one piece and housed in the latter in the first case, or formed by two symmetrical parts, disposed respectively on either side of the vertical deflector in the other, between this deflector and the branches of the tubes opposite.

However, with such blocking means, immobilized on the tube plate, it is sometimes difficult to carry out the necessary maintenance operations in line with this plate, in particular in its surface directed towards the inside of the steam generator, to clean the tube plate or to control the tubes and in particular their connection with the plate. In addition, in certain solutions where the tube plate is equipped with a continuous purge device, comprising in particular a pipe parallel to the plate and pierced with holes distributed along its length to operate a permanent water withdrawal through this plate, the presence of these means can be troublesome to obtain optimal operation.

As an indication, to carry out such a purging system, one can implement the provisions described in French patent FR-A-92 07903 in the name of the Applicant Company, filed on June 26, 1992, in which these means of purging include at least one passage through the tube plate and through which an opening opens on the upper face of this plate in the central region, this opening communicating with drainage means located outside the envelope of the generator.

The subject of the present invention is a steam generator of the aforementioned kind, in which the means for blocking the water street are arranged so as not to hinder the maintenance of the tube plate, in particular allowing access to that -this by tools introduced into the outer casing through suitable orifices, and also eliminating dead zones which are hardly or not accessible during the operations of cleaning the tube plate or servicing the tubes.

To this end, the generator considered, in which the blocking means are in the form of a plurality of blocks, preferably of generally parallelepipedal shape, removable, these blocks being capable of being individually extracted or placed in the central region, in the inner envelope of the generator, through an access passage formed laterally therein, is characterized in that the blocks are superimposed in the central region to form a stack resting on the tube plate, these blocks being immobilized at their ends close to the axis of the envelope by means of a bridge stop, integral with the tube plate, extending perpendicularly to the latter and to the direction of the blocks stacked in the central region, an anti-theft blocking member being provided at the opposite ends of the blocks to prevent them from being lifted from the tube plate.

Preferably, the bridge immobilization stop for the blocks comprises an opening for receiving the end of the latter, comprising a face having a recessed slope formed opposite a homologous profile provided on the block disposed at the part top of the stack. Advantageously, the superimposed blocks each comprise at least one orifice for circulation of the secondary fluid; preferably also, the stop trigger guard has an inverted U-shaped arch profile, the central part of which has the embedding slope.

According to a particular characteristic, the blocking blocking device for blocking the stack of blocks comprises a support stop, arranged under the lower end of the inner envelope and in contact with the block at the upper part of the stacking, this stop being immobilized against this block by means of a shutter engaged in the access passage through which the blocks are put in place and extracted from the enclosure. Preferably, the shutter has one end with a substantially conical profile, cooperating with an inclined face which is homologous to the support stop.

Preferably, the bearing stop comprises a flat face on which a piston is applied, slidably mounted in the shutter under the effect of a thrust spring housed between the shutter and a tight closing cover of the passage. access.

According to another characteristic also, the support stop comprises at least one lug penetrating into a housing formed in the block disposed at the upper part of the stack.

Advantageously, the tube plate comprises at least one pin for centering the block arranged at the lower part of the stack, so that the latter is thus maintained in the upper part and in the lower part, opposite from the tube plate.

According to another characteristic, each block has in its upper face a centering stud capable of cooperating with a hole (blind or opening) of the block superimposed in the stack. Alternatively, each block has a rib forming a slide for the positioning of successive blocks superimposed in the stack.

According to yet another particular characteristic, the blocks superimposed in the stack, with the exception of the block located at the upper part thereof, are separated from one block to the next by a sliding sole, having a flat face, engaged between two successive blocks and an external vertical rim, for holding the block located under the sole.

According to yet another characteristic, the stack of superimposed blocks is surrounded, along its lateral sides, by parallel guide rails, mounted in line with the access passage formed in the outer envelope.

Advantageously and to perfect the maintenance of the blocks in the stack, preferably each of these blocks, possibly the block disposed at the top of said stack, comprises a curved metal blade, forming a spring, mounted between the end of the block opposite to the trigger guard and the inner wall of the outer casing.

Finally and to facilitate handling of the blocks, each of them usefully comprises, opposite the access passage in the outer envelope, an ear provided with a gripping hole.

The provisions of the invention apply equally, whether the steam generator considered is of the boiler type or of the economizer type, the central region being in the latter case separated in its median plane by a vertical partition secured to the tube plate and extending perpendicular thereto. In the latter case, the parallelepipedic blocks superimposed in the stack are each formed at by means of two symmetrical elements, mounted on either side of the partition.

• Les Figures 1 et 2 sont des vues schématiques en coupe verticale partielle de générateurs de vapeur d'un type général connu dans la technique, la Figure 1 représentant un générateur de type bouilleur et la Figure 2 un générateur similaire mais de type économiseur axial.
• Les Figures 3 et 4 sont des vues à plus grande échelle, respectivement en élévation et de dessus, d'une partie d'un générateur de vapeur selon l'une quelconque des Figures 1 ou 2, représentant la structure en elle-même connue de blocs ou organes d'obstruction ou de blocage, logés à poste fixe dans la région centrale ou rue d'eau d'un tel générateur.
• La Figure 5 est une vue en coupe transversale d'une partie de la rue d'eau d'un générateur de vapeur, du type illustré sur la Figure 1, muni d'un empilement de blocs superposés réalisés conformément à l'invention.
• La Figure 6 est une vue schématique en bout de l'empilement de blocs illustré sur la Figure 5.
• La Figure 7 est une vue de détail à plus grande échelle d'une disposition mise en oeuvre sur le bloc supérieur de l'empilement de la Figure 5.
• Les Figures 8 et 9 sont respectivement des vues de dessus et en bout du bloc supérieur représenté sur les Figures 5 et 6.
• La Figure 10 est une vue en bout schématique de la rue d'eau dans le cas d'un générateur de vapeur du type représenté sur la Figure 2.
• La Figure 11 est une vue de dessus de l'empilement des blocs, mis en oeuvre dans le cas du générateur de vapeur selon les Figures 2 et 10.
• Les Figures 12 et 13 sont des vues en coupe partielle, analogues à la Figure 5, mais illustrant deux autres variantes de réalisation.

Other characteristics of a steam generator comprising means for blocking or partially obstructing the water street, consisting of superimposed removable blocks established in accordance with the invention, will become more apparent from the following description of several examples of realization, given as an indication and not limiting, with reference to the appended drawings, on which:

• Figures 1 and 2 are schematic views in partial vertical section of steam generators of a general type known in the art, Figure 1 representing a boiler type generator and Figure 2 a similar generator but axial economiser type.
• Figures 3 and 4 are enlarged views, respectively in elevation and from above, of a part of a steam generator according to any one of Figures 1 or 2, showing the structure in itself known from blocks or obstructions or blocking devices, housed at a fixed post in the central region or water street of such a generator.
• Figure 5 is a cross-sectional view of part of the water street of a steam generator, of the type illustrated in Figure 1, provided with a stack of superimposed blocks produced in accordance with the invention.
• FIG. 6 is a diagrammatic view at the end of the stack of blocks illustrated in FIG. 5.
• FIG. 7 is a detail view on a larger scale of an arrangement implemented on the upper block of the stack of FIG. 5.
• Figures 8 and 9 are respectively top and end views of the upper block shown in Figures 5 and 6.
• Figure 10 is a schematic end view of the water street in the case of a steam generator of the type shown in Figure 2.
• FIG. 11 is a top view of the stack of blocks, used in the case of the steam generator according to FIGS. 2 and 10.
• Figures 12 and 13 are views in partial section, similar to Figure 5, but illustrating two other alternative embodiments.

In Figure 1, there is shown schematically a steam generator of the boiler type illustrating a first possible embodiment of the invention. This steam generator is designed to transfer heat between the primary water circuit and the secondary water / steam circuit of a pressurized water nuclear reactor.

In this same Figure 1, the reference 10 designates the outer envelope of revolution, of vertical axis, of the steam generator. This envelope 10 delimits an enclosed interior space which is separated into a primary lower zone and a secondary upper zone by a horizontal tube plate 12 tightly connected to the envelope 10.

A vertical partition 14 divides the primary lower zone, usually called a water box, into an intake manifold 16 and an evacuation manifold 18 of the water circulating in the primary circuit of the reactor. Tubing 20 and 22, welded or forged or molded on the outer casing 10 of the generator steam connect collectors 16 and 18 respectively to this primary circuit.

An inverted U-shaped tube bundle 24 is tightly connected to the tube plate 12, in the secondary upper zone delimited by the latter, so that the two ends of each of the tubes open respectively into the intake manifold 16 and in the discharge manifold 18. The vertical branches 24a of the tubes 24 which open into the intake manifold 16 are called hot branches and the vertical branches 24b of the tubes 24 which open into the discharge manifold 18 are called cold branches .

The bundle of tubes 24 is surrounded and capped by an inner casing 26, arranged coaxially in the outer casing 10. The upper horizontal wall of this inner casing 26 opens into water / vapor separators 28 surmounted by dryers 29 which communicate the space 27 formed inside the envelope 26 with a steam outlet pipe 30 located at the top of the outer envelope 10. The lower edge of the inner envelope 26 is placed at a determined distance above the tube plate 12, so as to form a passage between an annular recirculation space 32 delimited between the envelopes 10 and 26 and the space 27 formed in the inner envelope 26.

The supply of secondary steam to the steam generator is carried out by a toroidal feed distributor 38 located immediately above the annular recirculation space 32. A feed pipe 34 passes tightly through the outer casing 10 of the steam generator and opens into the feed distributor 38. The latter can in particular be connected to the annular recirculation space 32 by inverted J-shaped tubes 36.

Horizontal flow distribution plates 40, regularly spaced, are mounted inside the inner casing 26. These plates have perforations making it possible to support the tubes 24 of the bundle over their entire height and to achieve distribution radial as homogeneous as possible of the secondary water flow which rises in the casing 26.

The lower flow distribution plate, designated by the reference 40 in Figure 1, is located slightly above the lower edge of the inner casing 26. It is distinguished from other plates 41 by the fact that it has an opening central 42 of relatively large section. This lower plate 40 thus makes it possible to prevent the secondary water descending through the annular recirculation space 32 from rising immediately as soon as it has crossed the lower edge of the inner envelope 26, which would in particular have the consequence of creating above the central part of the tube plate 12 a static area.

In a manner known in the art, in order to avoid as much as possible that is established between the hot and cold branches of the tubes 24 in the central region of the generator, called water street, an area where the secondary fluid is practically stagnant, there are in this region blocks 46 suitable for occupying and partially closing it. However, these blocks 46 are generally put in place and then immobilized relative to the tube plate 12 and therefore have certain disadvantages to allow maintenance of the tubes at their connection with the plate.

Figure 2 illustrates a steam generator practically identical to that shown in Figure 1, but this times of the so-called axial economizer type, the central region or street of water between the closest branches in the exchanger, being separated by a vertical partition 44, integral with the tube plate 12 and extending vertically from this one. In this variant, the blocks occupying the water street are formed by means of separate elements, respectively 47 and 48, arranged on either side of the vertical partition 44.

Figures 3 and 4 illustrate in more detail the structure of the shutter blocks 46 housed in the central region of the bundle of tubes in the case of a generator of the boiler type according to Figure 1, similar arrangements being of course implemented. works with the generator in Figure 2.

In these Figures, the blocks 46 have vertical support uprights 54 and are advantageously provided with orifices 56, intended not to unduly prevent the circulation of the secondary fluid. These blocks rest side by side on the tube plate 12 extending transversely in the waterway and being associated, in the vicinity of this plate, with a longitudinal tube 49 for collecting sludge and other residues accumulating on where appropriate on the plate, by being joined at one end substantially in the center of the plate to a manifold 50, provided with sampling orifices 53, of the kind more particularly described in patent application 92 07903 already cited, and extending at its opposite end so as to exit laterally from the plate under the lower end of the outer casing 10 to be connected by a conduit 52, associated with a hollow end piece 55 to an installation for removing these sludges or residues.

In the assembly thus envisaged, the blocks 46 partially occupying the water street above the tube plate 12 are immobilized with respect to the latter, which leads to the drawbacks already mentioned.

To avoid these, it is therefore planned, in accordance with the invention, to arrange the occupation blocks of the central region between the tubes of the bundle so as to be able to move them easily inside this region and in particular the extract or place them at will in the latter, in order to clear the plate and more easily ensure maintenance of the lower end of the primary fluid circulation tubes which are connected thereto.

As seen in Figure 5, there is in the central region 60 constituting the water street between the branches closest to the tubes 24, in particular between the tube plate 12 and the plate 40 for deflection and distribution of the flow of secondary fluid extending parallel above the tube plate, a stack 62 formed of a set of juxtaposed parallelepipedal blocks, these blocks being removable in the manner described below.

In the embodiment illustrated in this Figure, the stack consists, from a support sole 64 on the tube plate 12, of three individual blocks, respectively 66, 68 and 70, each of these blocks, in the form general of elongated parallelepiped, extending in the longitudinal direction of region 60, consisting of several adjacent elements, integral with one another according to the length of the block considered, such as 66a, 66b and 66c for the block lower 66 for example, the upper block itself being formed of three homologous elements 70a, 70b and 70c.

The elements constituting the blocks of the stack, with the exception of those located most to the right in each block, that is to say closest to the outer wall 10 of the generator, comprise transverse orifices 72 for the circulation of the secondary fluid.

At their opposite ends, therefore located furthest to the left and away from the envelope, therefore closer to the central vertical axis of the generator, the blocks of the stack 62 cooperate with a bridge immobilization stop 74, having in the form of an inverted U-shaped arch overlapping the blocks, the lower ends of this stop being made integral during the construction of the generator of the support sole 64. Advantageously, the stop is arranged practically at the end of the stack blocks, and internally has an inclined embedding slope 75, under which engages the nose 76 of the element 70a of the upper block 70 of the stack, shaped so as to have the same profile as the embedding slope 75.

The various blocks 66, 68 and 70 of the stack are preferably provided with an external sliding sole 78. In addition, the sole 64 secured to the tube plate 12 on the one hand, preferably the upper face of the protective jacket of each of the blocks 66 and 68 (excluding the upper block 70) on the other hand, have pins 80 which engage mutually, from one block to the next, in a hole 81 made in the sole sliding block superimposed on the lower part of the latter. As a variant, the positioning of the various blocks in the stack could be achieved by means of guide grooves cooperating with ribs from one block to the other, or even by means of appropriate slides.

The stack 62 of the various blocks thus superimposed is also blocked against the soleplate 64 and the tube plate 12 by means of a blocking member 82, constituting an anti-flight element for the various blocks, in particular under the effect of the thrust exerted by the secondary fluid under pressure which circulates in the generator coming from the annular space 32 and passing through the bundle of tubes under the lower end of the inner casing 26.

For this purpose, the blocking member 82 mainly comprises a support stop 84 and a shutter 86.

The stop 84 is in the form of a shoe, applied against the upper face of the last block 70 in the stack 62, this stop advantageously comprising at least one and preferably two lugs 85 (see FIG. 6), each engaged at through a slot 87 formed in the protective jacket of this block, in line with its element 70. The stop also has an inclined face 88 against which applies a bearing of the same profile 89 of the shutter 86, itself resting on its opposite edge against a slide 90 fixed to the lower end of the inner casing 26.

The shutter 86 is permanently subjected to the action of a return spring 91 which rests on the opposite side on a cover (not shown) closing an access passage formed in the external envelope 10 of the generator, the diametrical dimensions of this passage being determined by construction so that they can allow the introduction or the withdrawal of each of the blocks 66, 68 or 70 in the enclosure of the generator.

Advantageously, the upper block 70 comprises, in its straight part directed towards the internal wall of the external envelope 10, an extension 93 provided, as seen in more detail in Figures 7 to 9, with flanges 94 and 95, allowing to imprison the ends and consequently to keep curved a leaf spring 96, able to come to bear against this internal wall, so as to immobilize the stack, thus held at its left end by the trigger guard 74 and his straight end opposite by the anti-takeoff member 82 with its support stop 84.
Finally, the sliding sole 78 of the upper block 70 has in its lower face supported on the block 68, at the end of the element 70c, a return 97 forming an angle iron, completing the immobilization of this block vis-à-vis of those located under it in stack 62.

Thanks to these provisions, it is easily understood that the blocks which constitute this stack can be on demand, and in particular outside the normal phases of use of the generator, in particular during periods requiring maintenance of the tube plate or of the bundle. , or even when it is necessary to intervene on the connection between these tubes and the plate, be easily installed or extracted from the generator enclosure. For this purpose, it suffices to remove the shutter 86, then the support stop 84 through the access passage 92, before each of the blocks is successively removed by means of a lifting member and appropriate traction (not shown), making it possible to slightly pivot the first block 70 relative to the trigger guard 74 to bring it into a slightly inclined position, as shown in phantom in Figure 5, the following blocks 68, then 66, being similarly extracted from the enclosure.

Conversely, when it comes to putting these blocks back in place, it suffices to first introduce, through passage 92, the lower block 66, placed on the soleplate 64 and positioned thanks to its lug 80, the second block 68 then being introduced according to the same process, then finally the block 70, with engagement of the nose 76 thereof under the embedding slope 75 of the trigger guard 74. When the blocks are thus superimposed, the leaf spring 96 suitably positioning the stack between the bridge stop 74 and the internal wall of the outer casing 10, it suffices to set up the support stop 84 and finally to immobilize it by the shutter 86, the enclosure of the generator can then be put back under the pressure of the secondary fluid.

Figure 10 illustrates schematically, but on a larger scale, along a plane perpendicular to the median plane of the central region 60 or water street, the corresponding part of the latter with in particular the vertical partition 44, implemented when the generator is of the economiser type, unlike the solution more particularly envisaged with the example of FIGS. 5 to 9, concerning a generator of the boiler type.

In this second case and as seen diagrammatically in FIG. 11, the stack 62 of the blocks is split, each of the blocks 66, 68 and 70 being separated between two adjacent blocks arranged symmetrically on either side of the partition 44 In this figure, the upper block 70 thus consists of two symmetrical blocks 70/1 and 70/2, each of these blocks being associated with a shutter 86/1 and 86/2 housed side by side in the access passage. 92.

In this variant, as well as in the previous one, it is advantageously possible to provide the lower end of the inner casing 26 with slides 98, capable of coming to frame the stack 62 of the various blocks on each side of that -ci, by guiding these blocks between these slides and the central vertical partition 44 during the phases of installation or removal already explained.

FIGS. 12 and 13 illustrate other alternative embodiments of the stack 62 of the various blocks 66, 68 and 70, with different structural modifications, both for these blocks and for the holding members, immobilization and guidance with which they are associated.
In FIG. 12, the support stop 84 is directly secured to the shutter 86 mounted in the passage 92 of the outer casing 10, in the manner of a piston subjected to the effect of the support spring 91, the front end of this stop comprising a groove 100 cooperating with a key 102 provided in the upper face of the block 70.
In this same Figure 12, each of the blocks of the stack 62 is advantageously provided with a gripping lug 104, provided with a hole 106 for the passage of a hook or the like (not shown), making it possible to grasp or deposit each of the blocks successively.

In the variant illustrated in FIG. 13, the stop 84 and the shutter 86 are again distinct, this stop being immobilized in the blocking position of the stack 62 by a bearing surface 108 engaged under the end of the plate deflection 40. In this same variant, the two upper blocks 68 and 70 are capable of being linked to each other, once put in place by a screw-nut assembly 110.

The invention thus makes it possible, whatever the variant chosen, to ensure the installation inside the water street of the steam generator, of effective "blocking" means, achieving a suitable obstruction of the region. corresponding during operation of the generator, these means being able to be removed in a particularly simple manner to release this region and allow access to the plate and to the bundle of tubes in the vicinity thereof, by any desired maintenance or repair tool.

The removable blocks offered and the various arrangements provided for immobilizing them give the stack great security, in particular to with regard to the thrust of the secondary fluid, the association of a trigger guard at one end, of an anti-flight device to the other, making it possible to keep the blocks in abutment on the tube plate, whatever generator operating conditions.

Of course, it goes without saying that the invention is not limited to the schematic examples more particularly envisaged with reference to the appended drawings; on the contrary, it embraces all its variants.

Claims (16)

1 - Steam generator, in particular for nuclear power plants, comprising an outer casing (10) with a vertical axis, a horizontal tube plate (12), fixed in a sealed manner inside and at the lower end of the casing exterior, a bundle of tubes (24) in inverted U, comprising vertical branches, respectively hot (24a) and cold (24b), joined by a curved part at their upper end and traversed by a primary fluid yielding its calories, at l inside the exchanger, to a secondary fluid circulating in the outer casing, the branches of the tubes having their ends connected to the tube plate and opening under it, respectively in an intake manifold (16) of the hot primary fluid and an outlet manifold (18) for this cooled fluid, an inner casing (26) covering the tube bundle, the lower edge of which is spaced from the tube plate and delimits with the outer envelope an annular space (32) traversed by the secondary fluid introduced inside this envelope into this space before vaporizing on contact with the tubes traversed by the primary fluid, means for extracting the vapor, arranged at- above the bundle of tubes in the outer casing, and means (40) for deflection and distribution of the flow rate of the secondary fluid, provided at the lower part of the inner casing, these deflection means delimiting with the tube plate d on the one hand and the hot and cold branches of the closest tubes on the other hand, an elongated central region (60), extending transversely through the outer casing, this region being occupied by means of blocking or partial occupation of the passage offered to the secondary fluid in this region, generator in which the blocking means are in the form of a plurality of blocks (66, 68, 70), preferably e of form general parallelepiped, removable, these blocks being capable of being individually extracted or placed in the central region, in the interior envelope of the generator, through an access passage (92) formed laterally therein, characterized in that the blocks are superimposed in the central region to constitute a stack (62) resting on the tube plate (12), these blocks being immobilized at their ends close to the axis of the envelope by means of a bridge stop (74), integral with the tube plate, extending perpendicularly to the latter and to the direction of the blocks stacked in the central region, an anti-flight blocking member (82) being provided at the opposite ends of the blocks to avoid the lifting of these vis-à-vis the tube plate. 2 - Steam generator according to claim 1, characterized in that the trigger guard (74) for immobilizing the blocks (66, 68, 70) comprises an opening for receiving the end of the latter, comprising a face having an embedding slope (75) formed opposite a homologous profile (76), provided on the block (70) disposed at the upper part of the stack (62). 3 - Steam generator according to one of claims 1 or 2, characterized in that the superimposed blocks (66, 68, 70) each comprise at least one orifice (72) for circulation of the secondary fluid. 4 - Steam generator according to any one of claims 1 to 3, characterized in that the bridge stop (74) has an inverted U-shaped hoop profile, the central part of which includes the embedding slope (75). 5 - Steam generator according to any one of claims 1 to 4, characterized in that the anti-take-off blocking (82) of the stack (62) of the blocks comprises a support stop (84), disposed under the lower end of the inner casing (26) and in contact with the block (70) at the upper part of the stack, this stop being immobilized against this block by means of a shutter (86) engaged in the access passage (92). 6 - Steam generator according to claim 5, characterized in that the shutter (86) has one end with a substantially conical profile (89), cooperating with a homologous inclined face (88) of the support stop (84). 7 - Steam generator according to one of claims 5 or 6, characterized in that the support stop (84) has a flat face on which a piston is applied, slidably mounted in the shutter under the effect a thrust spring (91), housed between the shutter (86) and a sealed cover for the access passage (92). 8 - Steam generator according to any one of claims 5 to 7, characterized in that the support stop (84) comprises at least one lug (85) penetrating into a housing (87) formed in the block (70) disposed at the top of the stack (62). 9 - Steam generator according to any one of claims 1 to 8, characterized in that the tube plate (12) comprises at least one centering pin (80) of the block (66) disposed at the bottom of the stack (62). 10 - Steam generator according to any one of claims 1 to 9, characterized in that each block (66, 68) in the stack, with the exception of the block (70) disposed at the top of the stack (62), has in its upper face a centering stud (80) able to cooperate with a hole (blind or through) (81) of the superimposed block. 11 - Steam generator according to claim 10, characterized in that each block (66, 68) has a rib forming a slide for positioning the successive blocks superimposed in the stack (62). 12 - Steam generator according to any one of claims 1 to 11, characterized in that the blocks (66, 68) superimposed in the stack (62), with the exception of the block (70) located at the top from it, are separated from one block to the next by a sliding sole (78), having a flat face, engaged between two successive blocks and an external vertical rim (97), for holding the block located under the sole. 13 - Steam generator according to any one of claims 1 to 12, characterized in that the stack (62) of the superimposed blocks is surrounded, along its lateral sides, by parallel guide rails (98), mounted at right access passage (92) formed in the outer casing (10). 14 - Steam generator according to any one of claims 1 to 13, characterized in that at least the block (70) disposed at the upper part of the stack (62), comprises a curved metal blade (96), forming a spring, mounted between the end of the block opposite the trigger guard (74) and the internal wall of the outer casing (10). 15 - Steam generator according to any one of claims 1 to 14, characterized in that each block (66, 68, 70) comprises, opposite the access passage (92) in the outer envelope (10), an ear (104) provided with a gripping hole (106). 16 - Steam generator according to any one of claims 1 to 15, of the economiser type, the central region of which is separated in its median plane by a vertical partition (44) integral with the tube plate (12) and extending perpendicular thereto, characterized in that the superimposed blocks (66, 68, 70) in the stack (62) are each constituted by means of two symmetrical elements, mounted on either side of the partition.

Images