EP0117191A1 - Steam generator for a liquid metal-cooled nuclear reactor - Google Patents

Abstract

The invention relates to a steam generator for a nuclear reactor cooled by liquid metal. The steam generator comprises a bundle consisting of double-walled tubes (20), placed inside an envelope in which the liquid metal circulates. The tubes (20) substantially straight and directed in the axial direction of the envelope of the steam generator are each formed of at least two successive sections (20a and 20b) in the axial direction. Each section (20a or 20b) comprises an external tube (27a or 27b) welded at one of its ends at least to a leakage collecting chamber (25) disposed entirely inside the envelope. The adjacent sections of external tubes (27a, 27b) are not interconnected. Each section also includes an internal tube (28a or 28b) welded to the adjacent internal tube, inside the chamber (25). The leakage space between the tubes (27 and 28) is in communication only with the interior space of at least one collecting chamber (25). The invention applies in particular to fast neutron nuclear reactors cooled by liquid sodium.

Description

The invention relates to a steam generator for a nuclear reactor cooled by liquid metal, such as a fast neutron nuclear reactor cooled by sodium.

Such a reactor comprises a tank containing the core of the reactor constituted by fuel assemblies immersed in liquid sodium filling the tank called primary sodium.

The heat taken from the reactor core by the primary sodium which comes into direct contact with the fuel assemblies is used to vaporize drinking water, the vapor then being sent to the turbine of the nuclear power plant. To avoid any contact between the radioactive primary sodium and the water, the heat transfer between this sodium and the drinking water can be done using an intermediate exchange fluid which is often liquid sodium called secondary sodium.

The primary sodium coming into contact with the reactor core raises the temperature of the secondary sodium in sodium-sodium heat exchangers called intermediate exchangers.

Secondary sodium is then used to vaporize drinking water inside steam generators.

The nuclear reactor therefore generally comprises a secondary circuit comprising at least one intermediate exchanger, a pump, a steam generator for the pipes and various control devices.

This secondary circuit is therefore complex and costly, whether it is partially integrated into the reactor vessel or whether it is entirely disposed outside of this vessel.

The intermediate circuit avoids direct heat exchange between the primary sodium contaminated by radioactive products and drinking water. In the event of a leak in the wall of the heat exchange device, sodium and water can come into contact, which causes the appearance of a violent chemical reaction which can eventually lead to a leak in the sodium outwards.

_' When using clean secondary sodium, radioactive sodium release is avoided.

To simplify the heat exchange devices associated with reactors cooled by liquid metal, and to protect against any reaction between active sodium and water, it is necessary to re-establish a double separation between the two fluids. For this, we proposed to use steam generators of a special type comprising double wall heat exchange tubes, with the interposition of an exchange fluid which may be liquid sodium between the two walls of the tube.

It has also been proposed, in the case of double-walled tubes constituted by an internal tube and an external tube threaded one on the other, to fill the small space remaining between the tubes with a neutral gas such as l helium under pressure.

The spaces between internal and external tubes constituting the double-walled tubes are placed in communication with a leakage control space making it possible to detect, for example by pressure measurement, a possible leak in the wall of one of the tubes, during the operation of the steam generator.

The steam generator consists of a bundle of tubes contained in an envelope, each of the internal tubes of the double-walled tubes being placed in communication at one of its ends with a water distribution system and at its other end with a vapor collection system.

The liquid sodium heated up in contact with the reactor core is brought into the envelope of the steam generator, at its upper part, and flows in this envelope from top to bottom in contact with the external tubes of the double-walled tubes constituting the beam.

There is a pressure difference between the primary sodium flowing in contact with the outer surface of the tubes and the water or vapor circulating inside the bundle tubes. The helium filling the leakage control spaces is at an intermediate pressure between these two pressures.

Any leakage through the inner tube or through the outer tube constituting the double-walled tube results in a change in pressure in the leakage control space.

In the devices of the prior art, described for example in French patents 2,371,655 and 2,379,881, leakage control spaces are provided at each end of the steam generator, near the water collectors and steam. This requires the provision of double tubular plates which require numerous welds in a highly stressed area, in particular by thermal stresses.

On the other hand, for large power steam generators, it is necessary to use double-walled tubes of very long length, possibly coiled to form bundles of ac length. ceptable.

It is generally not possible to produce double-walled tubes of sufficient length and it is therefore necessary to connect end to end several sections of double-walled tubes. This end-to-end connection can of course only be made for the internal tube, in an area where the latter is not protected by the peripheral tube. These portions of single tube inserted between two portions of double-walled tube must be placed in the steam generator so as not to come into contact with the liquid sodium.

For this, provision is made for annular chambers arranged outside the envelope of the steam generator, through which pass the portions of internal tubes allowing the connection of the double-walled tubes, by sealed crossings in the envelope of the steam generator. .

Such a device is complex and requires particular manufacturing operations which are difficult to implement.

The object of the invention is therefore to provide a steam generator for a nuclear reactor cooled by liquid metal comprising a generally cylindrical envelope disposed with its vertical axis, enclosing a bundle of double-walled tubes each consisting of two coaxial tubes of which an internal tube communicating at one of its ends with a water distribution device or collector and at its other end with a vapor collector and an external tube in contact by its external wall with the liquid metal circulating from bottom to up in the casing of the steam generator, said steam generator having no pla _- a double tubular connecting chamber or tubes outside of the shell and enabling a highly sensitive and reliable detection of leaks of double-walled tubes, while being of a design allowing a simplified and relatively inexpensive construction.

• - un tube externe soudé à l'une de ses extrémités au moins à la paroi d'une chambre collectrice de fuite disposée entièrement à l'intérieur de l'enveloppe du générateur de vapeur dans une zone éloignée de ses extrémités, traversant cette paroi et n'étant pas relié au tube externe du tronçon adjacent soudé à la même chambre collectrice,
• - et un tube interne soudé au tube interne du tronçon adjacent, la soudure de raccord étant placée à l'intérieur de la chambre collectrice,
• - l'espace de fuite de très faible largeur entre les parois en vis-à-vis des tubes interne et externe d'un tronçon de tube quelconque étant uniquement en communication avec l'espace intérieur d'au moins une chambre collectrice.

For this purpose, the bundle is constituted by substantially straight tubes directed in the axial direction of the envelope of the steam generator each formed from at least two successive sections in the axial direction each comprising:

• an external tube welded at one of its ends at least to the wall of a leakage collecting chamber disposed entirely inside the envelope of the steam generator in a zone remote from its ends, passing through this wall and not being connected to the external tube of the adjacent section welded to the same collecting chamber,
• - and an inner tube welded to the inner tube of the adjacent section, the weld of connection being placed inside the collecting chamber,
• - The very small leakage space between the walls facing the internal and external tubes of any tube section being only in communication with the interior space of at least one collecting chamber.

In order to clearly understand the invention, a description will now be given, by way of nonlimiting example, of an embodiment of a steam generator according to the invention associated with a _r a-pid neutron nuclear reactor cooled by liquid sodium.

• Figure 1 shows, in a sectional view through a vertical plane, a steam generator according to the invention.
• FIG. 2 represents a very enlarged view of the zones of connection of the tubes of the bundle with the tubular plates and with the leak collecting chamber of the generator represented in FIG. 1.
• FIG. 3 represents a first alternative embodiment of a collector of a generator according to the invention.
• FIG. 4 represents, in a very enlarged view, the detail A of FIG. 3.
• FIG. 5 represents a second alternative embodiment of a manifold of a steam generator according to the invention.
• Figure 6 shows detail B of Figure 5, greatly enlarged.

In FIG. 1, we see the envelope of a steam generator 1 constituted by a cylindrical bundle envelope over the greatest height of the steam generator and by two enlarged zones, also of cylindrical shape, 2 and 3 each. of the ends of the beam. The envelope 1 also comprises an enlarged central zone 4 of toroidal shape.

The upper enlarged zone 3 communicates with the sodium inlet 5 in the steam generator and the lower enlarged zone 2 communicates with the sodium outlet 6. Between these two ends, the sodium circulates from top to bottom in contact with the tubes of the bundle 10 arranged inside the bundle envelope la-over the entire height of the envelope of the steam generator.

At its lower part, the casing 1 is connected to a tubular plate 11 secured to a water collector 12 supplied with water by a pipe 13.

At its upper part, the casing 1 is connected to a tubular plate 14 secured to a vapor collector 15 comprising an outlet pipe 16.

The water collectors 12 and the steam collectors 15 are hemispherical in shape.

The tubular plates 11 and 14 are crossed by the tubes of the bundle 10 inside which circulates the food water supplied by the tubing 13 of the water collector 12, this food water gradually vaporizes during its circulation from bottom to high inside the bundle tubes in thermal contact with the hot sodium arriving in the steam generator via the sodium inlet pipe 5.

The very high beam consists of substantially straight and parallel tubes, the spacing of which is maintained by spacer plates such as 18 and 19. At each of the ends of the beam inside the enlarged zones 2 and 3 are arranged heat shields 21 and 22 respectively protecting the tubular plates against the heat flow from the sodium.

At the central part of the bundle 10 is disposed a leak detection chamber 25, the structure of which will be described in more detail with reference to FIG. 2. The internal volume of this collecting and leak detection chamber is connected by a tube 26 passing through the casing 1 at the level of the toric widening 4 to a pressure measuring device.

Referring to FIG. 2, it can be seen that the tubes 20 of the bundle are double-walled and constituted by two successive sections 20a and _20b connected at the level of the leakage collecting chamber 25.

The upper section 20a of the tube 20 is itself constituted by an external tube 27a and an internal tube 28a coaxial and threaded one on the other with very little play, without metallurgical connection between the internal surface of the tube 27a and the external surface of the tube 28g. A space of very small width therefore exists between the tubes over the entire length of the section ²⁰ a.

The internal surface of the tube 27a is also machined to form therein longitudinal grooves allowing the pressurization of the space between the two tubes.

The lower section _20b of the tube 20 is also constituted by an outer tube 27 _b and an inner tube 28 _b identical to the tubes 27a and 28a respectively.

The upper section 20a of the tube crosses the tubular plate 14 at its upper part so that the inner tube 28a opens into the vapor collector 15. A weld 29 inside the collector 15, on the outlet face of the tubular plate 14, allows to close her metric the space between the tubes 27a and 28a.

In the same way, the lower section 20 _b of the tube crosses at its lower part the tubular plate 11, so that the inner tube 28 _b opens into the water collector 12. A weld 30 on the entry face of the tubular plate 11 makes it possible to hermetically seal the leakage space between the tubes _28b and _27b .

The outer tubes 20a and _20b are also fixed by welds on the other faces of the tubular plates 11 and 14.

The collecting chamber leak 25 is constituted by two circular plates 25a and _25b having a ring edge following which they are connected by a weld 32.

Each of the half-walls 25a and 25 _b comprises a large number of pipe penetrations such as 34a and 34 _b for the passage sections 20a and _20b of the tube 20 respectively.

It can be seen that the outer tube of the double-walled tube is fixed by welding to each of the bushings 34.

Inside the collection chamber 25 leaks, the two sections 28a and _28b of the inner tube are connected via a solder 35. In contrast, the two sections of the outer tube 27a and 27 _b are not connected and simply open into the internal volume of the leakage collector box 25, on either side of the connection weld 35 between the two sections of the internal tube 28.

All the spaces between the internal and external tubes of the double-walled tubes 20 are in communication with the internal volume of this leakage collecting chamber.

The leakage collecting chamber 25 is disposed entirely inside the casing 1 - of the steam generator, at the level of the toric enlargement 4. This collecting chamber 25 is not fixed inside the generator. steam only by the double-walled tubes 20 on which it is welded.

The internal volume of the chamber 25 and all of the spaces between the two walls of the tubes 20 are filled with helium at a pressure intermediate between the pressure of the sodium circulating in the steam generator in contact with the external wall of the tubes 20 and the water or steam circulating inside the tubes 28.

In the case of a leak caused by a cracking of one of the tubes 27 or 28 constituting the double-walled tubes 20, there is therefore a rise in pressure or a drop in pressure in the internal volume of the manifold 25 which can be recorded by the pressure gauge connected to the tube 26, outside of the envelope 1. Any leakage through one of the tubes 27 or 28 of the double-walled tubes 20 is therefore thus detected .

In FIGS. 3 and 4, an alternative embodiment of the water or vapor collector 15 shown in FIG. 1 is shown.

This vapor collector consists of a tubular plate 44, the internal face of which has a concave spherical shape and an envelope in the form of a portion of a sphere 45 which is connected by a weld 46 to the tubular plate 44.

At its upper part, the spherical casing 45 is connected to a steam outlet pipe 47. The tubular plate 44 is connected to the enlarged zone 43 of the generator casing corresponding to the enlarged part 2 or 3 shown in the figure 1.

In FIG. 4, it can be seen that the outer tube 27 of a double-walled tube 20 is welded to the sodium side face 48 and inside the passage hole 49 passing through the tubular plate 44, while the inner tube 28 is welded to the inside of the outer tube 27, which secures it and closes the leakage space between the two tubes 27 and 28.

In FIGS. 5 and 6, a second alternative embodiment of the water or steam collectors is seen, the collector being constituted, in this alternative, by a very thick spherical envelope 50 completely separated from the envelope 1 of the generator. steam. The envelope 1 is welded at its upper or lower part to a tubular plate 52 allowing the passage through the double-walled tubes 20 for their passage outside the steam generator.

On the. Figure 6, we see that the outer tube 27 is fixed by welding to the input face 'of the tube plate 52 and that the outer 27 and inner tubes 28 are fixed to each other by a weld 53 which allows _'also the closure of the leakage space between the tubes 27 and 28, the weld 53 being outside of the envelope of the steam generator.

Only the internal tubes 28 are connected to the manifold 50.

It can be seen that the main advantages of the device according to the invention are to allow leakage detection of double-walled tubes by means of a simple device, distinct and distant from the tube plates of the water and steam collectors of the steam generator. There is therefore no presence of a significant weld concentration in the region of the tubular plates.

On the other hand, the leak detection chambers allow the connection of successive sections of the tube bundle, inside the envelope of the steam generator but in an area isolated from liquid sodium.

The overall structure of the steam generator is extremely simple since it only has straight tubes formed by successive sections whose connection is made inside the leak detection chamber. This leak detection chamber is moreover practically part of the beam structure and has no connection with the envelope of the steam generator.

It is possible to choose for the internal and external tubes of the double-walled tubes of different steel grades, one of which has good resistance to liquid sodium and the other an increased resistance to water at high temperature or steam. For example, choose an austenitic stainless steel to form the outer tube and a ferritic stainless steel to form the inner tube. These two grades of steel must of course have expansion coefficients not too far apart for the operating temperature zone of the steam generator.

It may also choose different steel grades to form the sections 28a and 28 _b of the inner tube, selecting a shade adapted to steam to the upper portion 28a and a shade adapted to water for the part 28b.

However, the invention is not limited to the embodiments which have been described; on the contrary, it includes all variants.

This is how the bundle can be made up of more than two successive sections. For example, in the case of a bundle consisting of three successive sections, the tubes of the central section will be connected at each of their ends to a leakage collecting chamber.

In general, in the case of n successive sections, n - 1 leakage collecting chambers arranged inside the envelope of the steam generator will be used.

Each of these leakage collecting chambers will be associated with a manometric device allowing the monitoring of the corresponding tube sections. It will thus be possible to have redundant data for certain sections connected to two successive chambers by each of their ends.

In the case of several successive sections, as described above, the steel grades chosen for the constitution of the tubes may be different.

The shape and arrangement of the leakage collecting chambers may be different from what has been described. In the exemplary embodiment, an enlarged toric part of the envelope is provided to allow the circulation of sodium around the leakage collecting chamber. The circulation of sodium at the level of the upper face of this leakage collecting chamber becomes radial, which is favorable since the sweeping of the upper face of the leakage detection chamber avoids any deposit of impurities.

On the other hand, the enlarged toric portion 4 of the envelope allows longitudinal deformations of the latter making it possible to absorb differential thermal expansions. This is particularly important in the case of very high structures necessary for the constitution of generators of high power.

However, the leakage collecting chamber or chambers and the envelope may have different shapes from those which have been described.

The water and steam collectors can have a shape not only spherical but also toric or cylindrical, when they are completely independent of the envelope (1) of the steam generator and arranged as shown in FIG. 5.

Finally, the steam generator according to the invention can be associated with all fast neutron nuclear reactors cooled by a liquid metal whether they are of the semi-integrated or loop type.

Claims (7)

1.- Steam generator for a nuclear reactor cooled by liquid metal comprising an envelope (1) of generally cylindrical shape, arranged with its vertical axis, enclosing a bundle of double-walled tubes (10) each consisting of two coaxial tubes ( 27, 28) including an internal tube (28), communicating at one of its ends with a water distribution device (12) or collector and at its other end with a vapor collector (15) and an external tube (27) in contact by its outer wall with the liquid metal flowing from top to bottom in the envelope (1) of the steam generator, characterized in that the bundle (10) consists of substantially straight tubes (20) directed in the axial direction of the envelope (1) of the steam generator each formed from at least two successive sections (20a and 20 _b ) in the axial direction each comprising: - an external tube (27a or 27 _b ) welded at one of its ends at least to the wall of a leakage collecting chamber (25) disposed entirely inside the envelope (1) of the steam generator in a zone remote from its ends, passing through this wall and not being connected to the external tube of the adjacent section (27a, or 27 _b ) welded to the same collecting chamber (25), - and an internal tube (28a or 28 _b ) welded to the internal tube of the adjacent section (28a or 28 _b ), the connection weld (35) being placed inside the collecting chamber (25), the leakage space of small width between the walls facing the inner and outer tubes (27 and 28) of any length of tube (20a or _20b ) being only in communication with the interior space d '' at least one collecting chamber (25). 2.- Steam generator according to claim (1), characterized in that the leakage collecting chamber (25) is connected only to the tubes (20) of the bundle (10) and has no point of contact with the envelope (1) of the steam generator. 3.- Steam generator according to any one of claims 1 and 2, characterized in that the envelope (1) comprises in at least one zone remote from its ends, at least one widening (4) inside which is arranged a manifold (25). 4.- Steam generator according to any one of claims 1, 2 and 3, characterized by the fact that the internal tubes (28) on the one hand and the external tubes (27) on the other hand constituting the double-walled tubes are made of different steel grades. 5.- Steam generator according to any one of claims 1, 2 and 3, characterized in that the successive sections (28a, 28 _b ) constituting the internal tube (28) of the double-walled tubes (20) are formed by different steel grades. 6.- Steam generator according to any one of claims 1, 2, 3, 4 and 5, characterized in that at least one of the water and steam collectors (45) is constituted by a tubular plate (44) closing one of the ends of the steam generator and by a complementary wall (45). 7.- Steam generator according to any one of claims 1, 2, 3, 4 and 5, characterized in that at least one of the water and steam collectors (50) is completely independent of the casing (1) of the steam generator and that only the internal tubes (28) of the double-walled tubes are connected to this collector (50).

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