EP0173602A1 - Emergency heat exchanger for cooling the primary fluid of a nuclear reactor, and method of assembling this heat exchanger - Google Patents

Abstract

A liquid metal cooled pool type nuclear reactor incorporates an emergency heat exchanger having a novel geometry. The heat exchanger comprises circular and annular tube plates coaxially aligned. The tube bundle has a vertical straight part connected to the central tube plate, a bent horizontal circular portion extending over one-third of the heat exchanger's circumference for returning the bundle, and a vertical straight return part joining the peripheral tube plate.

Description

The invention relates to an emergency heat exchanger for cooling the primary fluid of a nuclear reactor, the core of which consists of fuel assemblies releasing heat is immersed in a primary fluid contained in a tank.

In the case of fast neutron nuclear reactors, the primary coolant for the reactor is generally constituted by liquid sodium filling a large stainless steel tank closed by a very thick horizontal slab.

when the reactor is stopped after a certain operating time, it is necessary to continue cooling the core assemblies since there remains a certain residual radioactivity generating heat in this core.

In large nuclear power reactors, the amount of heat to be removed is large and the main heat exchange circuit of the reactor is generally used to cool it after shutdown. This circuit includes, in the case of integrated type reactors, sodium-sodium intermediate exchangers and pumps to circulate the primary sodium. These pumps operate at low speed during cool down after shutdown.

However, if a technical incident causes the normal operation of the main cooling circuit to stop, the core can no longer be sufficiently cooled. Excessive overheating of the heart can lead to very serious accidents, so that an emergency cooling circuit completely separate from the main circuit is provided, which is very simple and reliable.

Such an emergency circuit includes a sodium-sodium heat exchanger partly immersed in the primary fluid of the reactor. This heat exchanger comprises a bundle of tubes inside which circulates secondary sodium which heats up in contact with the primary sodium contained in the reactor vessel. The secondary sodium circulated in the bundle is itself cooled outside the reactor vessel, in a sodium-air exchanger.

In the case of high power fast neutron reactors, for example 1500 or 1800 MWe, it is necessary to use several sodium-sodium emergency exchangers immersed in the reactor vessel. It is necessary to limit the number of these sodium-sodium emergency exchangers, for cost reasons and to reduce the number of passages in the reactor slab. The sodium-sodium emergency exchangers must therefore be relatively large. These heat exchangers also undergo very high thermal stresses, so that their design poses technical problems difficult to solve.

The sodium-sodium emergency exchangers are most of the time of the type of pin tube bundles dipping directly into the primary sodium. These tubes are placed inside an external ferrule open at its base and pierced over a large part of its lateral surface. The U-shaped tubes are connected at one end to a first tube plate and at their other end to a second tube plate offset from the first according to the height of the heat exchanger. These tubular plates make it possible to send the secondary sodium in the tubes to the central part of the exchanger and to recover it at its peripheral part. The cooled sodium descends into the branches of the tubes located at the central part of the exchanger and rises through the branches of the tubes situated at the periphery of the latter. As it travels through the tubes, the secondary liquid sodium heats up in thermal contact with the primary sodium through the wall of the tubes. This results in very significant temperature differences between the different parts of the exchanger. It can also experience significant temperature variations over time. This results in thermal stresses which can be very high in certain parts of the exchanger and it is necessary to design exchangers of a structure such that it makes it possible to reduce these thermal stresses to acceptable levels.

In addition, the tubes constituting the exchange bundle must be effectively braced, to avoid their relative displacement under the action of heat and under the action of vibrations. This results in problems which are difficult to solve for the mounting of the heat exchanger.

The object of the invention is therefore to propose an emergency heat exchanger for cooling the primary fluid of a nuclear reactor contained in a tank comprising a substantially horizontal closing plate and enclosing the core of the reactor immersed in the primary fluid, comprising a support flange resting on the closing plate, a bundle of exchange tubes folded in a pin and fixed on two tubular plates, a cylindrical ferrule with vertical axis enveloping the beam ceau which is immersed in the primary fluid and a circuit for supplying the tubes of the bundle with heat exchange fluid comprising a means for cooling the exchange fluid heated by the primary fluid disposed outside the tank, heat exchanger heat which makes it possible to limit the thermal stresses in its various constituent elements and which is of a simple structure and allowing easy assembly.

For this purpose, the two tubular plates are placed coaxially horizontally and at the same level, one of these tubular plates of annular shape located peripherally relative to the second central plate of circular shape being fixed to a ferrule with axis vertical located above the tubular plates and connecting them to the support flange and a second ferrule coaxial with the first, located below the tubular plates and carrying a connecting piece of a third ferrule connected to the plate central tubular,

and each of the tubes of the bundle comprises a straight vertical part connected to the central tube plate, a bent part for the inversion of the tube, a straight vertical return part, a horizontal circular portion around about a third of circumference and a vertical part connecting to the peripheral tubular plate.

The invention also relates to a method for mounting the heat exchanger.

• La figure 1 est une vue en élévation de l'échangeur de chaleur de secours en position dans la cuve d'un réacteur nucléaire à neutrons rapides.
• La figure 2a est une vue en coupe par un plan vertical de symétrie de la partie supérieure de l'échangeur de chaleur de secours représenté à la figure 1.
• La figure 2b est une vue en coupe par un plan vertical de symétrie de la partie inférieure de l'échangeur de chaleur représenté sur la figure 1.
• La figure 3a est une coupe suivant AA de la figure 2b.
• La figure 3b est une coupe suivant BB de la figure 2b.
• La figure 3c est une coupe suivant CC de la figure 2b.
• La figure 4 est une vue de dessus d'une portion d'une entretoise de maintien des tubes de l'échangeur de chaleur représenté sur la figure 1.
• Les figures 5, 6 et 7 sont relatives à un second mode de réalisation d'une entretoise de maintien des tubes.
• Les figures 5 et 5a sont des vues de dessus d'une partie de cette entretoise, à des échelles différentes.
• La figure 6 est une vue suivant F de la figure 5a.
• La figure 7 est une vue partielle en élévation dumoyen de maintien des éléments de l'entretoise.
• La figure 8 est une vue en coupe des éléments permettant le montage des tubes du faisceau dans les entretoises.

In order to clearly understand the invention, there will now be described, by way of nonlimiting example, an emergency heat exchanger of a fast neutron nuclear reactor cooled by liquid sodium.

• Figure 1 is an elevational view of the standby heat exchanger in position in the tank of a fast neutron nuclear reactor.
• FIG. 2a is a sectional view through a vertical plane of symmetry of the upper part of the emergency heat exchanger shown in FIG. 1.
• FIG. 2b is a sectional view through a vertical plane of symmetry of the lower part of the heat exchanger shown in FIG. 1.
• Figure 3a is a section along AA of Figure 2b.
• Figure 3b is a section along BB of Figure 2b.
• Figure 3c is a section along CC of Figure 2b.
• FIG. 4 is a top view of a portion of a spacer for holding the tubes of the heat exchanger shown in FIG. 1.
• Figures 5, 6 and 7 relate to a second embodiment of a tube holding spacer.
• Figures 5 and 5a are top views of part of this spacer, at different scales.
• Figure 6 is a view along F of Figure 5a.
• FIG. 7 is a partial view in elevation of the means for holding the elements of the spacer.
• Figure 8 is a sectional view of the elements for mounting the bundle tubes in the spacers.

In Figure 1, we see the heat exchanger generally designated by the reference 1 in position inside the tank of a fast neutron nuclear reactor filled with liquid sodium up to level 2. The exchanger 1 passes through the slab 3 via a bushing 4 and rests by means of a flange 5 on a support flange 6 carried by the ferrule of the bushing 4 of the slab 3.

On the upper part of the exchanger 1 are fixed insulated conduits 7 and 8 respectively ensuring the return of the cooled secondary sodium in the heat exchanger and the removal of the heated secondary sodium which is sent, for cooling, in an exchanger sodium-air heat disposed outside the tank, not shown, and located on the secondary sodium circuit.

The lower part 1b of the exchanger consists of a ferrule with a vertical axis comprising holes and enveloping the exchange bundle.

We will now describe in more detail the structure of the heat exchanger 1 with reference to Figures 2a and 2b.

It can be seen that the upper part 1a of the heat exchanger comprises an external envelope on which are fixed the sodium conduits 7 and 8 in communication inside the envelope of the heat exchanger with a chamber 10 of arrival of the secondary liquid sodium and with a chamber 11 for the return of the heated secondary sodium, respectively.

The secondary liquid sodium inlet and return chambers 10 and 11 are coaxial and have the vertical axis ZZ 'of the heat exchanger as a common axis.

The secondary sodium inlet chamber 10 is arranged at the central part and has a double wall. The chamber 11 for the return of the heated secondary sodium, of annular shape, is arranged at the periphery of the chamber 10. The chambers 10 and 11 are constituted by ferrules cy lindrics and frustoconical ferrules welded end to end.

The volume between the two walls of the chamber 10 is filled with an inert gas.

The volume between the external wall of the cylindrical part of the chamber 11 and the external envelope of the heat exchanger is filled with heat-insulating blocks 12. Heat-insulating is also arranged in the extension of these blocks 12 around the conduits 7 and 8. A metal block 13, arranged around the frustoconical part of the chambers 10 and 11, makes it possible to achieve biological protection of the crossing 4.

Under the flange 5 are fixed two coaxial ferrules 14 leaving between them a space of very small width and a height sufficient to surround the envelope of the heat exchanger 1 over the entire height of the bushing 4. These ferrules 14 provide in known manner thermal protection of the crossing.

Under the flange 5 is also fixed a ferrule 15 of greater thickness constituting the external wall of the heat exchanger connecting the flange 5 to the lower part 1b of the exchanger. In this part 1b of the exchanger is the bundle 17 consisting of a set of tubes bent into a pin which each has one end connected to an outer tubular plate of annular shape 18 and one end connected to a central annular plate of circular shape 19 .

The two tubular plates 18 and 19 both have the axis ZZ ′ of the heat exchanger as an axis and are placed opposite one another in this heat exchanger, the annular plate 18 surrounding the circular plate 19.

The tubular plate 18 is connected on its periphery to the shell 15 of the heat exchanger which thus ensures the connection between the tubular plate 18 and the flange 5. On its internal edge, the annular plate 18 is connected to one of the walls of the secondary sodium inlet chamber 10. Finally, on its periphery and at its upper part, the central plate 19 is connected to the second envelope of the chamber 10.

The envelope of the chamber 11 is connected to the shell 15 at its lower part by means of a Y-piece 21.

The ferrule 15 is extended below the tubular plates 18 and 19 by the ferrule lb enveloping the bundle of tubes of the heat exchanger. This ferrule 1b is fixed by welding along the outer edge of the annular plate 18, below this plate.

Along the internal edge of the plate 18, on its underside, a ferrule 20 of short length is fixed. The lower part of this ferrule 20 is connected to an annular junction piece with Y-shaped section 22 which makes it possible to connect the ferrule 20 to a ferrule 23 of a substantially identical length coaxial with the ferrule 20 and fixed along the edge of the central plate 19 on its underside.

At the lower part of the annular part 22 with a Y section are fixed tie rods 25 whose circumferential distribution is visible in FIGS. 3a and 3b. These tie rods 25 maintain, by means of short sleeves 26, a set of spacer grids 27 ensuring the transverse maintenance of the tubes 28 of the bundle 17.

Each of the tubes 28 of the bundle comprises a straight descending part 28a fixed at its upper end in the tubular plate 19, a bent part for turning the tube 28b, a straight return part 28c, a horizontal circular portion 28e visible in FIG. 3a and finally a terminal straight part 28f fixed inside the tubular plate 18. In this way, for each of the tubes 28, the inlet end communicates with the sodium inlet chamber 10 and the outlet end with the return chamber 11 of the liquid sodium. The lower part of the heat exchanger up to the level of liquid sodium 2 being immersed in the primary sodium to be cooled, the secondary sodium circulating in the tubes 28 heats up before leaving in the chamber 11. The different parts of the tubes and the heat exchanger are therefore at different temperatures. The primary sodium is in contact with the bundle over its entire submerged length and perforations 30 are provided for the passage of the primary sodium in the envelope 1b.

The upper part of the portions 28a of the tubes, the circular portions 28e in their entirety and the portions 28f are arranged above the level 2 of the primary liquid sodium.

The differential expansion of the portions of the tube subjected to fluids at different temperatures is largely compensated for by the circular portion 28e of the tube disposed above the level of liquid sodium. These portions are therefore subjected to flexions which they can however absorb without too much difficulty given their length corresponding to an arc of circumference having an angle at the center of the order of 140 ° and in any case greater than 120 ° C. that is to say a third of the circumference. On the other hand, these circular portions 28e are not subject to too unfavorable conditions of use since they are located above the level of the primary liquid sodium.

7

The two tubular plates 18 and 19 comprise "môyh§- junction between them as well as with the support flange 5 of the exchanger which make it possible to absorb any deformation of the bundle and of the exchanger ferrules. At the same time , these joining means allow effective maintenance of the tubular plates and of the external ferrule lb of the bundle. In addition, the transverse holding of the bundle against vibrations is ensured by the spacers 27 fixed to the lower part of the annular joining piece 22 .

The base of the bundle formed by the bent portions 28b visible in FIGS. 2b and 3c is constituted by a simple juxtaposition of hairpin tubes having good resistance to deformation in the transverse directions.

In Figure 4, we see an embodiment of a spacer grid 27 for fixing the tubes 28. This spacer grid 27 is constituted by a set of circular and concentric hoops 34 all having as axis the axis ZZ 'of the heat exchanger, between which are arranged metal strips 32 with sinusoidal folding fixed on each side on the corresponding ribs. The hoops 34 are constituted by successive portions connected by welded junction pieces 35. The bands 32 folded in the form of sinusoids provide the junction between the different hoops and constitute with them three external retaining rings 36a, 36b and 36c and six rings internal 37.

Between the set of internal rings 37 and the external rings 36 is provided a space in which the sleeves 26 for holding the tie rods 25 are fixed by means of parts also allowing the junction between the internal part of the spacer grid and the external part.

The tie rods 25 ensure the suspension of the spacer grid 27 under the tubular plates.

The method of mounting the tubes 28 in the spacer grids 27 will be explained later.

In FIGS. 5, 6 and 7, a second embodiment of a spacer grid is seen, the latter comprising a set of concentric rings 40 all having as axis the axis ZZ 'of the heat exchanger as previously. These hoops 40 have rectangular cutouts 41 as visible in FIG. 6 and the grid frame is made up, in addition to the hoops 40, by radial elements 42 and a strip 43 folded so as to provide a housing for the tubes 28 between the strip 43 and the corresponding hoop 40. Between two portions folded into a cylinder to come in contact with the tubes 28, the strip 43 is folded at right angles to form a .part of dimensions corresponding to the cutouts 41 of the hoop 40. These portions 44 bent at right angles are introduced into the cutouts 41 of the hoop 40 and held in place place by pieces 45 playing the role of stirrup. These parts 45 have the form of portions of rings cut into slots as visible in FIG. 7 or of combs.

The concentric rings 40, the radial elements 42 and the strip 43 constituting the framework of the grid 27 are connected together to ensure the cohesion of the structure. The radial elements 42 also carry the sleeves 26 for fixing the tie rods 25 for hanging the grid 27.

As well in the case of the grid represented in FIG. 4 as in the case of the grid represented in FIGS. 5, 6 and 7, the mounting and the fixing of the various elements constituting them are carried out for all of the six internal rings, before mounting the harness.

For mounting the harness, the interior parts of the spacer grids suspended by the tie rods 25 are placed under the tubular plates 18 and 19, then the branches 28a of the tubes 28 are introduced into the spacer grids one by one so as to constitute a first complete outer layer. The ends of the tubes 28 are then connected to the tubular plates 18 and 19 respectively and the external hoops are put in place so as to constitute a first external ring for fixing the tubes 28. The fixing elements such as 32 (FIG. 4) or such that 43 and 45 (Figure 5) are connected to the or the hoops set up in the outer part of the grid.

The two following layers are formed successively, in the same way.

When the whole of the bundle and the spacer grids have thus been assembled, the outer ferrule lb enveloping the bundle is put in place, then this ferrule lb is fixed by welding to the tubular plate 18.

In the case of the grid shown in FIGS. 5, 6 and 7, the tubes 28 are first put in place against the hoops 40, then the fixing pieces 43 are introduced into the cutouts 41 of the hoops 40. Finally, the whole is immobilized by the parts 45 in the form of a comb.

For the assembly of the pre-assembled internal parts of the spacer grids and the assembly of the tubes in these pre-assembled parts, use is made of the means represented in FIG. 8 showing the end 28a of a tube 28 being assembled in a grid. 27. A 50-piece ogival nozzle the end of the tube 28a to be introduced into the grid 27 which has sleeves 51 having the same internal and external diameters as the tubes 28 which were put in place when the elements of the grid were assembled in the location which must occupy the tubes, to maintain a spacing of these elements corresponding exactly to the dimension of the tubes 28. The sleeves 51 are held by the radial forces exerted by the elastic elements constituting the grid 27.

When the end of the tube 28a comprising the ogival endpiece 50 is introduced into the endpiece 51, the latter is pushed out, while the tube 28 comes to occupy its place in the spacer grid 27. The tube 28a is thus maintained perfectly in the spacer grid.

We see that the main advantages of the heat exchanger according to the invention are to allow expansion of the different parts of this exchanger, during its operation and in particular of the bundle tubes, without creating excessive stresses in these elements.

All of the exchanger elements are also perfectly maintained against vibrations, in particular in the transverse direction.

The mounting of the tubular plates at the same level in the exchanger, while allowing them relative displacements under the effect of expansions, makes it possible to optimize the structure of the heat exchanger.

Finally, the heat exchanger according to the invention can be produced by simple and perfectly defined assembly operations.

The invention is not limited to the embodiment which has just been described; on the contrary, it includes all variants.

où R est le rayon de la virole externe lb de l'échangeur de chaleur et t l'épaisseur des viroles de jonction.Thus, the length of the ferrules 20 and 23 for connecting the tubular plates and for suspending the spacer grids can vary between certain limits. In practice, this length L of the ferrules 20 and 23, in the case of a heat exchanger as used in a fast neutron nuclear reactor could be such that it verifies the following inequalities:

where R is the radius of the outer shell lb of the heat exchanger and t is the thickness of the joining ferrules.

In the case of heat exchangers for fast neutron nuclear reactors, this thickness is generally between 6 and 10 mm.

In such heat exchangers, the temperature difference T between the hottest parts and the coldest parts is generally around 200 ° C.

One can also imagine making spacer grids in a different way from those which have been described.

Finally, the bending of the bundle tubes may be slightly different from that which has been described and shown.

The invention applies in all cases where an emergency exchanger is used for cooling the primary fluid of a nuclear reactor, this heat exchanger plunging into a tank containing the primary fluid.

Claims (10)

1.- Emergency heat exchanger for cooling the primary fluid of a nuclear reactor contained in a vessel comprising a substantially horizontal closing plate (3) containing the core of the reactor immersed in the primary fluid, comprising a support flange (5) resting on the closure plate (3), a bundle (17) of exchange tubes folded into a pin and fixed on two tubular plates (18, 19), a cylindrical ferrule (lb) with vertical axis enveloping the bundle (17) which is immersed in the primary fluid and a supply circuit (7, 8) of the tubes (28) of the bundle (17) in heat exchange fluid comprising a means for cooling the exchange fluid heated by the primary fluid, placed outside the tank,
characterized by the fact that the two tubular plates (18, 19) are placed coaxially horizontally and at the same level, one of these tubular plates (18) of annular shape located peripheral to the second plate (19 ) central, of circular shape, being fixed to a ferrule (15) with vertical axis located above the tubular plates (18, 19) and connecting these to the support flange (5) and to a second ferrule (20 ) coaxial with the first (15), located below the tubular plates (18, 19) and carrying a connecting piece (22) of a third ferrule (23) connected to the central tubular plate (19),
and that each of the tubes (28) of the bundle (17) comprises a straight vertical part (28a) connected to the central tubular plate (19), a bent part (28b) for turning the tube (28), a straight part vertical return (28c), a horizontal circular portion (28e) about a third of the circumference and a vertical portion (28f) connecting to the peripheral tubular plate (18). 2. A heat exchanger according to claim 1,
characterized in that the connecting piece (22) carries at its lower part a set of vertical tie rods (25) for the suspension of spacer grids (27) holding the tubes (28) of the bundle (17) in the radial directions. 3.- heat exchanger according to any one of claims 1 and 2,
characterized in that the second ferrule (20) and the third ferrule (23) have substantially equal lengths in the vertical direction, this length (L) being defined by the following inequalities:

where R is the radius of the ferrule enveloping the exchanger bundle and t the thickness common to the ferrules (lb, 20 and 23). 4.- Heat exchanger according to claim 3, characterized in that the thickness of the ferrules connecting the tubular plates (20, 23) is between six and ten millimeters. 5.- Heat exchanger according to any one of claims 1, 2, 3 and 4, characterized in that the horizontal circular portion (28e) of the tubes (28) allowing their expansion is arranged in a zone of the exchanger heat (1) located above the level (2) of the primary fluid in the nuclear reactor vessel. 6.- Heat exchanger according to claim 2, characterized in that the spacer grids (27) are constituted by a set of circular rings (34, 40) concentric and horizontal on which are fixed elastic elements (32, 43) holding tubes (28). 7.- Heat exchanger according to claim 6, characterized in that the elastic elements are metal strips (32) sinusoidal folding interposed between two successive hoops (34) providing spaces for housing the tubes (28). 8.- Heat exchanger according to claim 6, characterized in that the hoops (40) of circular section and of cylindrical shape have rectangular openings (41) inside which are engaged parts (44) bent at an angle right of a strip constituting the elastic means (43) for holding the tubes (28) against one of the faces of the hoop (40), immobilization combs (45) being introduced into the parts (44) bent at right angles elastic means (43), on the side of the face of the hoop (40) which is not in contact with the tube (28). 9. A method of mounting a heat exchanger according to any one of claims 6, 7 and 8 having at least one spacer grid (27) comprising a set of hoops (34, 40) delimiting successive retaining rings. tubes inside which the elastic elements (32, 43) come into contact with the tubes (28) and constitute two sets, one internal located towards the central part of the exchanger and receiving the parts (28a) of the tubes (28) connected to the central tube plate (19) and the other external located towards the external shell (lb) of this exchanger, characterized by the fact that the set of internal rings constituted by the hoops (34, 40) and the elastic elements (32, 43) is pre-assembled, that this internal set is fixed under the tubular plates (18, 19), that the inlet ends (28a) of the tubes (28) are introduced one by one into housings provided in the spacer grids (27), until constituting a complete external row corresponding to a crown of the external assembly, that the hoop (s) (34, 40) corresponding to this outer ring and the corresponding elastic elements (32, 43) are put in place and assembled, repeating the assembly operations of the tubes, outer ring by outer ring, until when the bundle is fully assembled, the ends of the tubes (28) are fixed in the tubular plates (18 and 19) and finally the external ferrule (1b) enveloping the bundle (17) is put in place and fixed ). 10.- A method of mounting a heat exchanger according to claim 9, characterized in that introduced at the time of their assembly, in the internal rings of the spacer grids (27) sleeves (51) of the same diameter as the tubes (28), in the positions which must be occupied subsequently by the tubes (28), that the end (28a) of the tubes (28) with fittings (50) and that at the time of introduction of the tubes (28) into the pre-assembled internal part of the spacer grids (27), by pushing through the end piece (50), the corresponding sleeve (51), including the tube (28), is pushed out takes its place in the spacer grid (27).

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