EP0190075A1 - Steam generator with liquid metal as the heat transport medium and with leak detection by removing such a liquid metal - Google Patents

Abstract

The end of the steam generator through which the heat-transfer liquid metal enters comprises a tubular plate (2), a thermal protection plate (10) and a hollow flow threshold (22) constituting the end part of the envelope. beam (4). Tubes (20) for protecting the tubes 3) of the bundle are placed around each of the tubes (3) between the protection plate (10) and the tube plate (2). The space (17) between these two plates is closed at its periphery by a ferrule (15). The hollow threshold (22) communicates with the space (17) by tubes (27) and with the interior volume or the bundle envelope (4). A liquid metal sampling device (31, 32) is placed in the hollow threshold (22). The invention applies, in particular to the steam generators of fast neutron nuclear reactors cooled by liquid sodium.

Description

The invention relates to a steam generator, the heat transfer fluid is a liquid metal, to de- _t ection leakage by sampling the liquid metal.

The steam generators of fast neutron nuclear reactors very often use a liquid metal such as sodium as heat transfer fluid, for heating and vaporizing feed water, from the heat taken from the reactor core.

These steam generators are very often formed by an outer casing of generally cylindrical shape arranged with its vertical axis and connected at each of its ends to a very thick tubular plate. The exchange part of the steam generator is constituted by a bundle of tubes placed in the outer casing inside a bundle casing coaxial with this outer casing. Each of the tubes of the bundle is connected by welding at one of its ends, to the lower tube plate and at its other end, to the upper tube plate, in the extension of corresponding bore passing respectively through the lower tube plate and the plate upper tubular. The lower tubular plate separates the internal volume of the envelope of the steam generator from a water box allowing to distribute the food water in the tubes of the bundle. The upper tube plate separates the internal volume of the envelope of the steam generator from a vapor collector collecting the vapor formed in each of the tubes of the bundle.

The heat transfer liquid metal enters the internal volume of the envelope of the steam generator, in its upper part, that is to say a little below the upper tube plate. The envelope of beam has its upper end situated at a certain distance below the upper tube plate, so that the molten metal penetrates into the beam envelope, above the upper end of this beam envelope. To obtain the most homogeneous possible distribution of sodium along the bundle tubes, the upper part of the bundle envelope constitutes a flow threshold of rounded and generally hollow shape.

To avoid direct contact of the underside of the upper tube plate with the hot liquid metal entering the envelope of the steam generator, very often, parallel to the tube plate and a little below it, a thermal protection plate thinner than the tube plate. The bundle tubes pass through the thermal protection plate in openings machined in this protection plate, to be welded at their upper end to end pieces machined in the tube plate.

Between the thermal protection plate and the upper tubular plate, the bundle tubes are surrounded over part of their length by protective tubes having an internal diameter such that a radial clearance remains between the bundle tube and the protective tube. corresponding. The radial space between the bundle tube and the protection tube opens at one of its ends into the interior space of the steam generator below the thermal protection plate and at its other end in the space between the thermal protection plate and the upper tube plate.

It is extremely important to quickly detect any possible leakage of drinking water or vapor in the heat transfer liquid metal. Indeed, in the case of sodium for example, a leak of water or vapor whose pressure is higher than that of sodium can result in a very violent chemical reaction accompanied by a shock wave in the internal volume of the generator. steam. The disastrous effects of such a leak can be reduced or eliminated if its detection is fast enough to allow the necessary safety measures to be taken before an appreciable quantity of water or vapor has entered the liquid metal.

Such leaks are much more likely to occur in the welded areas of the tubes on the tube plates at their ends than in other areas of the tube. More specifically, leaks often occur at the weld between the upper end of the tube and the upper tube plate, this end of the tube being in contact with liquid sodium much hotter than the lower end.

Liquid sodium sampling devices have therefore been proposed, for the purpose of detecting and dosing hydrogen in the sampled sodium arranged in the vicinity of the upper end of the steam generator. However, these samples making it possible to detect the presence of hydrogen indicative of a water leak in the liquid sodium, are carried out in an area where the hot sodium entering the steam generator arrives, so that they do not concern actually a flow of sodium having circulated in contact with the weld zone of the tubes and consequently. highly charged with hydrogen in the event of a leak on one of the tubes.

The detection is therefore random and depends on the speed with which the hydrogen diffuses into the liquid sodium circulating in its upper part.

The object of the invention is therefore to propose a steam generator in which the heat transfer fluid is a liquid metal, with leak detection by sampling this liquid metal, comprising an outer casing of generally cylindrical shape connected at each of its ends to a very thick tubular plate, a bundle of tubes arranged inside a bundle of coaxial bundle with the external envelope and placed inside this envelope, having one of its ends or inlet end of the heat transfer fluid located at a certain distance from the corresponding tube plate, at least one inlet opening for the hot liquid metal passing through the external envelope in the vicinity of the entry end of the bundle envelope which constitutes a threshold of flow of rounded and hollow shape over which the liquid metal passes to enter the bundle envelope, each of the bundle tubes being welded at one of its ends to one of the tubular plates and at its other end on the other tube plate, at a bore passing through the tube plate. a thermal protection plate placed parallel to the tube plate corresponding to the inlet end of the heat transfer fluid and between this tube plate and the end of the bundle envelope, protective tubes each surrounding a bundle tube with a certain radial clearance between the thermal protection plate and the corresponding tubular plate, so that the radial space formed between the two tubes opens at one of its ends on the face of the protective plate diri towards the inside of the steam generator and at its other end in the space between the tube plate and the corresponding thermal protection plate, steam generator which makes it possible to quickly and reliably detect a leak of water or steam at one end of the bundle tubes, without requesting any significant modification to the structure of the steam generator.

• - l'espace situé entre la plaque tubulaire et la plaque de protection thermique est fermé à sa périphérie par une virole coaxiale à l'enveloppe externe et à l'enveloppe de faisceau,
• - le volume interne du seuil d'écoulement creux est en communication, à l'une de ses extrémités, avec le volume interne de l'enveloppe de faisceau et à son autre extrémité, avec l'espace situé entre la plaque tubulaire et la plaque de protection thermique, par l'intermédiaire de tubes sensiblement verticaux, de manière que le métal liquide circule depuis l'intérieur du générateur de vapeur, d'abord vers l'espace entre la plaque de protection thermique et la plaque tubulaire, dans les tubes de protection, puis vers le seuil d'écoulement dans les tubes verticaux,
• - et un moyen de prélèvement de métal liquide est disposé dans le volume du seuil d'écoulement.

For this purpose, on the side of the inlet end of the steam generator:

• - the space between the tubular plate and the thermal protection plate is closed at its periphery by a ferrule coaxial with the external envelope and with the bundle envelope,
• - the internal volume of the hollow flow threshold is in communication, at one of its ends, with the internal volume of the bundle envelope and at its other end, with the space between the tubular plate and the plate thermal protection, via substantially vertical tubes, so that the liquid metal flows from inside the steam generator, first to the space between the thermal protection plate and the tube plate, in the tubes protection, then towards the flow threshold in the vertical tubes,
• - And a liquid metal sampling means is arranged in the volume of the flow threshold.

In order to clearly understand the invention, we will now describe, by way of nonlimiting examples, with reference to the attached figures, two embodiments of a steam generator whose heat transfer fluid is liquid sodium, allowing the detection of a leak by taking liquid sodium in the steam generator.

• - la Fig. 1 est une demi-vue en coupe par un plan vertical de la partie supérieure d'un générateur de vapeur suivant l'invention ;
• - la Fig. 1a est une vue agrandie du détail A de la Fig. 1 ;
• - la Fig. 2 est une vue suivant BB de la Fig. 1 ;
• - la Fig. 3 est une vue en coupe par un plan vertical de la partie supérieure d'un générateur de vapeur suivant l'invention et suivant une variante de réalisation ;
• - la Fig. 4 est une vue suivant BB de la Fig. 3.

In these figures:

• - Fig. 1 is a half-view in section through a vertical plane of the upper part of a steam generator according to the invention;
• - Fig. 1a is an enlarged view of detail A of FIG. 1;
• - Fig. 2 is a view along BB of FIG. 1;
• - Fig. 3 is a sectional view through a vertical plane of the upper part of a steam generator according to the invention and according to an alternative embodiment;
• - Fig. 4 is a view along BB of FIG. 3.

In Fig. 1, we see the upper part of a steam generator comprising an outer casing 1 of generally cylindrical shape connected to an end portion in the form of a spherical sector itself welded to a very thick tubular plate 2. A bundle casing 4 of cylindrical shape is arranged inside the outer casing 1 and coaxially with it. The bundle consists of a large number of straight tubes 3, the upper part of which is connected by welding to the tube plate 2 is visible in FIG. 1a. In this figure, we see the upper end of the tube 3 connected by a weld 6 to a nozzle 5 machined on the underside of the tube plate 2. The tube plate 2 has a bore 7 in the extension of the internal bore of the tube 3 which is thus in communication with the vapor collector 9 situated above the tube plate 2. A single tube has been shown in FIG. 1a but it is obvious that the tube plate has a very large number end pieces such as 5 for connecting all the tubes of the straight tube bundle of the steam generator. In Fig. 1, there is shown the axes 3 'of a set of tubes 3 of the bundle.

Each tube 3 is connected at its part in- _f érieure not shown in Figs. 1 and 1a, to the lower tubular plate of the steam generator secured to the lower part of the outer casing 1. The ends of the tube are connected to the lower tubular plate and to the upper tubular plate by welding on a connector provided on the upper face of the lower tube plate and on the lower face of the upper tube plate respectively.

In Fig. 1, we see a tube 12 fixed on the outer casing 1 of the steam generator allowing the introduction of hot liquid sodium inside the steam generator. A thermal protection plate 10 parallel to the tubular plate 2 and of a thickness much thinner than that of the plate 2 is arranged below the latter at a certain distance. This plate 10 prevents the hot liquid sodium entering through the tubing 12 and circulating according to the arrows 13, from coming into direct contact with the tubular plate 2 of great thickness and very sensitive to thermal shocks. The thermal protection plate 10 is fixed under the tubular plate 2 by means of a frustoconical ferrule 15 completely closing on its periphery, the space 17 between the plates 2 and 10. A deflection ferrule 16 of the sodium liquid is also fixed on the lower external edge of the thermal protection plate 10.

The thermal protection plate 10 is pierced with a very large number of bores 18 with a diameter substantially greater than the diameter of the tubes 3, along the axes 3 ′ of these tubes of the bundle.

Around each of the tubes 3, a protective sheath 20 is placed, the internal diameter of which is such that a relatively large radial clearance remains between the tube 3 and the sheath 20. Each of the sheaths 20 is fixed inside a bore 18 and welded at its lower portion on the protective plate _t ion 10. the sheath 20 has a sufficient length such that the weld zone 6 of the tube 3 of the tube plate 2 is located within the sheath.

The upper part of the bundle envelope 4 constitutes a hollow threshold generally designated by the reference 22. This hollow threshold consists of a cylindrical-conical ferrule 23 welded to the upper part of the cylindrical bundle envelope 4, a cylindrical internal ferrule 24 and an upper annular and rounded closure part 25 visible in FIGS. 1 and 2. A space of small thickness is provided between the ferrules 23 and 24 at their lower part in order to put the interior volume of the threshold 22 into communication with the interior space of the bundle envelope 4.

The upper part of the bundle envelope 4 constituted by the flow threshold 22 makes it possible to guide the flow of hot sodium circulating according to the arrows 13 and to distribute it homogeneously in the bundle, at the moment when it flows at the above the rounded part 25 of the flow threshold. This part 25 has openings 26 distributed with a constant angular spacing over its entire circumference. In each of the openings 26 is fixed the lower end of a vertical tube 27 whose end upper part is engaged in an opening 28 passing through the thermal protection plate 10, at its periphery.

In Fig. Ja. we see that the end of the tube 27 is machined to form a spherical bearing 27a allowing its centering inside the opening 28 of the protective plate.

On the internal cylindrical shell 24 of the hollow threshold 22 are fixed devices 30 for supporting and bracing the tubes 3 of the bundle, regularly distributed in the axial direction of the steam generator. These retaining and bracing devices 30 introduce a pressure drop on the downward circulation of the liquid sodium (arrow 13 "). The pressure is therefore lower at the bottom of the hollow threshold 22 than under the tube plate. a sodium circulation in the direction indicated by the arrows 13 ′ in FIGS. 1 and 1a. Part of the sodium passing above the flow threshold 22 is directed upwards inside the radial space arranged between the tube 3 and its sheath 20, this radial space opening at its lower part below the thermal protection plate 10 and at its upper part in the space limited by the thermal protection plate 10 and the upper tubular plate 2 The upward circulation of liquid sodium therefore comes to lick the area of the weld 6 between the tube 3 and the tube plate 2 before descending into the space between the thermal protection plate 10 and the tube plate 2, towards the end. é superior of the tubes 27. The sodium then descends by the tubes 27 into the interior volume of the hollow threshold 22 from which the sodium is sucked by the pressure difference, towards the interior space of the beam where it mixes with the main flow 13 "descending in the beam.

A sampling means makes it possible to recover part of the sodium circulating in the direction indicated by the arrows 13 ′ to send it to a device for detecting and dosing hydrogen. This sampling means consists of a rectangular section toroidal envelope 31 having openings 33 in its upper wall arranged inside the hollow threshold 22 and by a sampling pipe 32 visible in FIGS. 1 and 2 taking this sodium taken from the exterior of the external envelope 1 of the steam generator, to the hydrogen detection and metering installation. The sodium flow circulating along the path represented by the arrows 13 'comes into contact with the weld zone 6 between the tube and the tube plate and in the case of a vapor leak in the vicinity of this zone, the sodium flow is very quickly and highly contaminated with hydrogen. This sodium sampled by the device 31, 32 is taken directly to the detection and metering device without undergoing mixing with the sodium entering the steam generator. Detection is therefore very sensitive and very rapid.

The temperature of the sodium circulating along the path represented by the arrows 13 ′ undergoes a different evolution from that of the main sodium flow following the descending axial path represented by the arrows 13 ". The sodium exiting at the bottom of the threshold 22 is generally more as cold as the sodium circulating in the beam. To limit or eliminate this temperature difference, holes 35 are provided in the lower part of the threshold, below the sampling torus 31. Hot sodium entering along the path 13 comes to mix with the sodium leaving at the lower part of the threshold passing through it through the holes 35. This reintroduction of hot sodium makes it possible to balance the temperature of the flow 13 'with respect to the temperature of the 13 "main flow. This reintroduction of sodium below the sampling torus therefore does not dilute the sodium sampled with incoming hot sodium.

In Figs. 3 and 4, an alternative embodiment of the upper part of the steam generator is seen, the corresponding elements in FIGS. 3 and 4 on the one hand and 1 and 2 on the other hand bearing the same references.

In the variant shown in Figs. 3 and 4, the space 17 between the protective plate 10 and the tubular plate 2 is separated into six distinct parts 37 by vertical and radial partitions 36 substantially sealed arranged angularly at 60 ^* with respect to each other. Each of the parts 37 of the space 17 is associated with a vertical tube 27 disposed inside an opening 28 passing through the tubular plate 10. The tube 27 makes it possible to bring the sodium flow 13 ′ having circulated in contact with the welding zone of the tube 3 in a sampling tube 39. The six tubes 39 each corresponding to a part 37 of the space 27 are independently connected to the hydrogen detection and metering device. This device makes it possible, in the event of a leak in the weld zone of one of the tubes 3, to distinguish the conduit 39 having brought the flow of sodium polluted by hydrogen, that is to say the part 37 in the form sector in which the leaking tube is located. Individual sampling by sector therefore makes it easier to find the leakage and repair of the steam generator.

The main advantages of the steam generator according to the invention, as far as leak detection is concerned, appear clearly in the description above; this detection is made on a relatively low sodium flow rate and in which the hydrogen concentration immediately becomes very high in the event of a leak at the soldering zone of a tube 3; this sensitive and rapid detection of a leak is obtained with a slight modification of the upper part of the steam generator, practically not modifying the circulation of the main sodium flow; the protection of the welding area of the tubes and of the upper tube plate is at least equivalent if not greater than the protection obtained in the steam generators according to the prior art; in fact, the weld zone 6 of the tube is located inside the protective sheath 20 and the space 17 between the protective plate 10 and the tubular plate 2 is completely closed at its periphery; the hot sodium entering the steam generator along the path 13 therefore never comes into direct contact with the weld zone of the tube and with the tube plate 2; the detection can allow a partial localization of the leak, by simply providing for a partitioning of the upper space 17 of the steam generator, above the protection plate 10.

The invention is not limited to the embodiments which have been described; this is how the threshold can have a different shape from that which has been described, that the sampling assembly can be produced in a different way and that the vertical tubes joining the interior volume of the threshold to the upper space of the steam generator, above the thermal protection plate can be rigidly fixed at one of their ends on this protection plate and mounted sliding at their other end in the threshold, instead of reverse mounting that has been described. Any partitioning of the upper space 17 of the steam generator can be adopted to refine the location of the leaks.

It is also obvious that the steam generator can include a sampling assembly as described in its lower part, in the case where the lower end of the steam generator constitutes the inlet end of the heat-transfer fluid. In this case, the lower end of the bundle envelope constitutes its entry end 'produced in the form of a hollow and rounded threshold. A thermal protection plate is arranged above the lower tube plate at a certain distance from this plate. This avoids direct contact with the lower tube plate of hot sodium which, in this embodiment, is brought by a tube in the vicinity of the lower part of the steam generator. The arrangement of the elements allowing the sampling of sodium having circulated in the vicinity of the zone of welding of the tubes of the bundle on the lower tubular plate is moreover practically identical to the arrangement which has been described above. One can also conceive the application of the invention to steam generators whose axis is not vertical.

The steam generators according to the invention can use a heat transfer fluid different from liquid sodium and constituted for example by the eutectic sodium-potassium mixture, or by lithium.

The generator can be with straight tubes, or with helical tubes, the ends of these tubes being arranged parallel and vertical to be welded to the tube plate.

Claims (5)

1.- Steam generator, the heat transfer fluid of which is a liquid metal and the leakage detection of which is carried out by removal of this liquid metal, comprising an outer casing (1) of generally cylindrical shape with vertical axis connected at each of its ends to a very thick tubular plate (2), a bundle of tubes (3) arranged inside a bundle envelope (4) coaxial with the external envelope (1) and placed inside this envelope, having one of its ends or inlet end of the heat-transfer fluid situated at a certain distance from the corresponding tubular plate (2), at least one opening (12) for the arrival of hot liquid metal passing through the external envelope (1 ) in the vicinity of the entry end of the bundle envelope (4) which constitutes a flow threshold (22) of rounded and hollow shape over which the liquid metal passes to penetrate into the bundle envelope (4 ), each of the bundle tubes (3) being welded to one of its ends on one of the tubular plates (2) and at its other end on the other tubular plate (2), at a bore (7) passing through the tubular plate (2), a thermal protection plate ( 10) placed parallel to the tubular plate (2) corresponding to the inlet end of the heat transfer fluid and between this tubular plate (2) and the bundle envelope (4), protective tubes (20) each surrounding a bundle tube (3) with a certain radial clearance, arranged between the thermal protection plate (10) on which they are fixed and the corresponding tube plate (2), so that the radial space formed between the two tubes (3 , 20) leads to one of its ends on the face of the protection plate (10) directed towards the inside of the steam generator and at its other end in the space (17) between the tube plate (2) and the corresponding thermal protection plate (10) , characterized in that, on the side of the inlet end of the heat transfer fluid into the steam generator: - the space (17) located between the tubular plate (2) and the thermal protection plate (10) is closed at its periphery by a ferrule (15) coaxial with the external envelope (1) and the envelope beam (4), - the internal volume of the hollow flow threshold (22) is in communication at one of its ends with the internal volume of the bundle envelope (4) and at its other end, with the space (17) located between the tube plate (2) and the thermal protection plate (10) by means of substantially vertical tubes (27), so that the liquid metal flows from the interior of the steam generator, first towards the space (17) between the thermal protection plate (10) and the tube plate (2). in the protection tubes (20), then towards the flow threshold (22), in the vertical tubes (27), - And by the fact that a withdrawal means (31, 32, 33, 39) of liquid metal is arranged in the internal volume of the flow threshold (22). 2.- Steam generator according to claim 1, characterized in that the vertical tubes (27) are rigidly fixed at one of their ends to the flow threshold (22) or the thermal protection plate ( 10) and are slidably mounted at their other end in an opening provided in the protective plate (10) or the flow threshold (22) respectively. 3.- Steam generator according to any one of claims 1 and 2, characterized in that the protective tubes (20) fixed at one of their ends, to the protective plate (10) have a length such that they surround, at their other end, the weld zone (6) between the tube (3) of the bundle and the tube plate (2). 4.- Steam generator according to any one of claims 1, 2 and 3, characterized in that the upper space (17) of the steam generator between the thermal protection plate (10) and the tubular plate (2 ) is divided, by substantially watertight partitions (36) into several parts (37) in each of which the thermal protection plate (10) is pierced with an opening (28) in which is mounted a substantially vertical tube (27) connecting the corresponding part (37) of the space (17) to the interior space of the threshold (22) in which a sampling tube (39) is mounted in the extension of the vertical tube (27), the sampling tubes (39 ) being connected independently of each other to a hydrogen detection and metering installation. 5.- Steam generator according to any one of claims 1, 2, 3 and 4, characterized in that the hollow threshold (22) comprises, in its wall (23) directed towards the inlet (12) of metal hot liquid in the steam generator, orifices (35), below the sampling device (31.32).

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