WO2009083677A2

WO2009083677A2 - Fast breeder reactor

Info

Publication number: WO2009083677A2
Application number: PCT/FR2008/052240
Authority: WO
Inventors: Alain Cros
Original assignee: Areva Np
Priority date: 2007-12-28
Filing date: 2008-12-08
Publication date: 2009-07-09
Also published as: FR2925972B1; FR2925972A1; WO2009083677A3

Abstract

The present invention relates to a fast breeder reactor (FBR) of integrated type, cooled by a liquid metal such as sodium. This FBR comprises a main vessel (1) containing a core (4) made of fuel assemblies (9) able to be cooled by a coolant, an element for supporting the said core, known as a diagrid (5), an element for supporting the diagrid (5), known as a decking (6), and a pump (7) for circulating the coolant. The pump (7) comprises a vertical body (18) and a delivery capacity (25) communicating with the lower part of the vertical body (18). The delivery capacity (25) secured to the decking (6) is in communication with the external side wall (11) of the decking (6) via a substantially cylindrical spool piece (27) secured to the decking (6) and to the delivery capacity (25) such that the coolant flows from the delivery capacity (25) towards the inside of the decking (6), the said reactor comprising means adapted to the differential expansion of the two structures (35) to supply the diagrid (5) with coolant from inside the decking (6).

Description

Fast neutron nuclear reactor

The present invention relates to an integrated type fast neutron nuclear reactor (RNR) cooled by a liquid metal such as sodium.

In the framework of the Generation IV International Forum, fast neutron nuclear reactors are now enjoying a renewed interest. RNRs first derive their energy from plutonium. Their heart, consisting of plutonium and natural uranium, has a dual function:

- produce heat (fission of plutonium) converted into electricity; - transform natural or depleted uranium (little fissile) into plutonium

(fissile). RNRs are referred to as "regenerator" or "breeder" if they produce more plutonium than they consume.

In addition to this dual function, research is being conducted to use this type of reactor to "burn" long-lived radioactive waste, transuranium and actinide, and turn it into short-lived radioactive elements.

An RNR is cooled by a coolant consisting of a liquid metal such as sodium. Sodium, in the liquid state, is a remarkable cooling fluid, thanks in particular to its high boiling point.

In known manner, the primary circuit of a fast neutron nuclear reactor is formed by:

- a main tank containing the reactor core provided with fuel assemblies;

primary pumps for circulating the cooling fluid and;

intermediate heat exchangers for transmitting the heat of the cooling fluid of the heated reactor in contact with the core to a secondary fluid. Traditionally the support of the heart is provided by two separate structures to separate the support functions and sodium supply of the heart:

- a first welded structure in pressure (typically 6 bar) called box spring in which are positioned the feet of fuel assemblies and which is supplied with cold sodium (400 ¹ O) by primary pipes from po primary MEPs;

- A second mechanically welded structure called decking on which the bed frame comes to rest. The RNRs are distributed according to two different configurations, characterizing the mode of the primary circuit:

integrated RNRs for which the primary pumps and exchangers are entirely contained inside the main vessel containing the core and are immersed in the cooling fluid of said main vessel through the closure slab of this vessel;

the so-called "loop" RNRs for which the primary pumps and the intermediate heat exchangers are placed in dedicated tanks outside the main reactor vessel which now contains only the core and the internal structure, the main vessel; and the component tank being connected by primary pipes. The present invention relates only to integrated RNRs. In integrated RNRs, the primary pumps comprise a generally cylindrical body vertically arranged and supported at its upper part by the slab of the reactor closing the tank. The body of the pump is in communication, at its lower part, with a discharge capacity which is generally of spherical shape. This spherical capacity is connected to two discharge pipes which open at their opposite end to the end connected to the spherical capacity, in the bed base supporting the reactor core. The liquid metal that is sucked by the pump at the level of gills passing through the envelope of its vertical body is pushed back into the spherical capacity and then distributed in the delivery pipes to be reinjected under the assemblies of the heart. Side pipes of pressure are welded in the outer shell of the bed base: they are relatively long, of the order of 4m in the case of the project "EFR" (European Fast Reactor), generally bent and connected to the spherical capacity through taps welded on both sides of the radial plane of the spheres. The discharge sphere is also physically connected to the deck which must take the hydrodynamic thrusts applied to the sphere and coming from the sodium pump (of the order of 150 tons).

This type of pump-bed connection has some disadvantages.

Thus, the pump Sleep bond is a relatively flexible connection and highly stressed during transients, particularly when the sodium of the cold collector (typically 400o) devi ent hot (hot puff 500 ¹ C leading to stratification due to the flow rate fallback ). Such a particular thermal regime generates differential displacements between the three structures (Sphere, box spring and decking) which strongly urge the delivery pipes. Piping of several meters can cause differential movements of several tens of millimeters.

These cyclic solicitations (about 200 expected over 60 years) affect more particularly the piping / sphere and piping / box spring piping areas which generally comprise welds at these connections.

This constructive arrangement is therefore likely to cause a breakage of the pump-bed connection.

In the event of a common mode fault, a break in two pump-to-bed connections could lead to under-power of the heart and a risk of accident.

In this context, the present invention aims to provide an integrated type fast neutron nuclear reactor to significantly reduce the risk of rupture of the pump-bed connection.

To this end, the invention proposes an integrated type fast neutron nuclear reactor comprising a main vessel containing:

a core composed of fuel assemblies able to be cooled by a coolant,

a support element of said heart called a bed base, a support element of said bed base called decking,

a pump for circulating said coolant comprising: a vertical body, a delivery capacity communicating with the lower part of said vertical body, said reactor being characterized in that said delivery capacity is in communication with the side wall; external said deck by means of a substantially cylindrical sleeve integral with said deck and said discharge capacity so that said coolant flows from said discharge capacity to the interior of said decking, said reactor comprising means for supplying said coolant liquid bed from the interior of said decking. .

Thanks to the invention, it eliminates the connecting pipes between the bed base and the discharge sphere: the decking here serves as a feed box of the cold sodium bed base. The constructive arrangement according to the invention makes it possible to route the cold sodium from the pumps to the bed base via the decking. It is thus possible to use a short sleeve (typically having a length of the order of 500 mm for a 1500 MWe reactor and preferably equal to 440 mm), integral with the sphere and the decking so that the differential displacements are extremely small. .

In addition, the sleeve may be a sleeve machined from a monoblock forged blank so that it does not have solder in the transition zones with the sphere and the decking which are the seat of stress concentrations. In other words, it is possible to move away the welds allowing the decking-sleeve connection and the cuff-sphere connection of the zones that are usually highly stressed.

Furthermore, according to the invention, the mattress no longer has side and radial taps on its sides, allowing a simplification of its structure and improving its dimensional stability. This dimensional stability is favored in particular by the reduction of its diameter due to the feeding of the sodium bed base from below, thus avoiding a glue peripheral power supply device required in the solutions of the prior art to minimize the loss of load and vibration hazards due to the impact of the sodium jet on the candles.

In addition, the arrangement according to the invention makes it possible to have the choice not to weld the lower part or redan of the inner tank (delimiting an area of the main tank, called hot collector, receiving the hot sodium leaving the heart and a zone, called cold collector, receiving the cooled sodium) on the bed base as is the case on the EFR reactor. This choice was not conceivable in the provisions of the prior art except to use deformable watertight bellows bellows in the inner vessel (case Superphénix reactor), the use of these bellows complicate the constructive layout of the area and require embodiments of bellows by processes requiring heavy R & D work to be acceptable in the sense of the building code. In a reactor according to the invention, the lower part of the inner vessel can be welded directly to the platform without bellows.

This frees the bed frame from any functional constraints other than its own function of supplying the heart.

The nuclear reactor according to the invention may also have one or more of the following characteristics, considered individually or in any technically possible combination:

said sleeve is a machined monobloc forged part;

said sleeve is welded to said side wall of said decking and to said delivery capacity, said sleeve comprising a central part and two ends on either side of said central part projecting outwards and welded respectively to an opening of said outer side wall of said decking and an opening of said discharge capacity;

said two ends are machined to present the respective shape of said outer side wall of said decking and of said delivery capacity so that they are respectively integrated in the continuity of said external lateral wall and of said discharge capacity; said decking comprises a sealed annular collector adapted to receive said heat transfer fluid;

said annular collector is formed by a plurality of concentric ferrules; said means for supplying said bed base with heat transfer liquid are distributed around said annular collector;

said decking comprises an upper plate and said base comprises a lower plate, said upper and lower plates being provided with orifices facing each other, said means for supplying said bed base with heat-transfer liquid being formed by cuffs, each said sleeves being welded to said lower and upper plates via said orifices;

each of said sleeves comprises a central part for the passage of the heat transfer fluid of substantially cylindrical shape with a vertical axis formed by an upper ferrule and a welded lower ferrule;

each of said cuffs comprises: a lower part forming substantially one half of a torus; an outer portion formed by a virion of diameter greater than the diameter of said central portion of the sleeve and concentric with it, said lower part providing continuity between said central portion and said outer portion;

- The upper end of said outer portion is welded to said upper plate of said decking and the upper end of said central portion is welded to said lower plate of said bed base;

said sleeve is formed by two pieces welded together: a first piece comprising:

¹ said lower part forming substantially a half torus; "said outer portion;

• the lower part of said central passage portion of the coolant; a second part comprising the upper part of said central portion for the passage of the coolant;

said first and second parts are welded end to end;

said decking comprises an upper plate and said bed base comprises a lower plate, said upper and lower plates being provided with orifices facing each other, said means for feeding said bed base with heat transfer liquid being formed by labyrinth systems, each said systems comprising: a substantially cylindrical female portion of vertical axis fixed to said decking and engaged within one of said orifices of said upper plate; a substantially cylindrical portion of a vertical axis engaged in the female part so that the interface between the inner wall of said female part and the outer wall of said male part forms a labyrinth interface;

a vertical clearance is provided so that said maie portion is able to slide vertically in the female part;

said maie portion comprises an upper end placed on said bed base via a support piece, the interface between said support piece and said upper end being of the ball joint type and forming a labyrinth interface;

the reactor according to the invention comprises an inner vessel delimiting a zone of the main vessel receiving hot liquid leaving the core and a zone receiving cooled liquid, said inner vessel comprising a lower part directly fixed to said decking;

- Said discharge capacity is welded at the bottom to said decking via stiffeners integrated in said decking.

Other features and advantages of the invention will emerge clearly from the description which is given below, by way of indication and in no way limiting, with reference to the appended figures, among which:

- Figure 1 shows a simplified sectional view through a vertical plane of the lower part of the main vessel of a nuclear reactor fast integrated type of neutron according to the invention; FIG. 2 is an enlarged view of the connecting sleeve between the discharge sphere and the decking as represented in FIG.

1;

- Figure 3 illustrates a first embodiment of the means for supplying the sodium bed base from the decking in a reactor according to the invention;

- Figure 4 shows the two parts forming the supply means as illustrated in Figure 3.

- Figure 5 illustrates a second embodiment of the means for supplying the sodium bed base from the decking in a reactor according to the invention. FIG. 1 represents a simplified sectional view through a vertical plane of the lower part of a main tank 1 of an integrated type fast neutron nuclear reactor according to the invention. The main tank 1 has a cylindrical lateral wall 2 axis of symmetry X and a curved bottom 3.

The main vessel 1 comprises:

a core 4 cooled by a liquid-metal heat transfer fluid such as sodium; a bed base 5 comprising a lower plate 43;

a decking 6;

a primary pump 7 (only the lower part is represented);

an internal tank 60 (only the lower part is shown);

- A bearing shell 16. The bed frame 4 is a mechanically welded structure comprising candles 8.

The core 2 is formed by fuel assemblies 9 whose lower part or foot is engaged in the candles 8 of the bed base 4.

The decking 6 is a mechanically welded structure which comprises in particular:

a plurality of concentric shells, of which: an outer lateral wall formed by a ferrule; o a second ferrule 12 inside the outer side wall 11; a third shell 13 inside the second shell 12;

an upper plate 10, called upper sole;

- A lower plate 14, called lower sole;

- Radial webs 15 forming stiffening spacers of the decking structure to limit the deflection of the latter and fixing the lower and upper flanges 10 and 14; The decking 6 rests on the bottom 3 of the vessel 1 via the bearing shell 16 (on which the deck 6 is welded) coming into contact with a reinforced portion 17 of the bottom 3. The bed base 5 is in support on the deck 6 via the end of the outer shell of the bed base 5 which rests on the upper flange 10.

The deck 6 further comprises a recuperator 20 suspended from the outer shell 11. This recuperator 20 is intended to receive the debris of the molten core following a serious accident which caused a sufficient rise in temperature.

The spherical central portion of the vessel bottom 3 has an X-axis hole in which a forging member 21, referred to as the receiving part of the pivot 22, is fitted and welded. This machined part 21 comprises a cylindrical bore.

The deck 6 comprises in its low central portion, a pivot 22 formed in the form of a cylindrical shell which is housed in the bore 21. The lower flange 14 is machined at its center and the pivot 22 is welded to the bottom flange 14 via a reinforced collar 23. The pivot 22 forms a positioning pivot plate 6 and earthquake recovery forces. Note that there is a clearance of a few millimeters (typically between 1 and 20 mm) between the bottom of the receiving part 21 and the lower end of the central pivot 22 when the deck 6 rests on the bearing shell 16 of so that the pivot 22 remains slightly above the bottom of the receiving part 21. In the event of collapse of the decking 6, the lower end of the pivot 22 would thus bear against the bottom of the receiving part 21. constructive arrangement thus provides emergency support from the central part. The pump 7 comprises a vertical body 18 of globally cylindrical shape, placed inside a chimney 19, enclosing the active elements of the pump 7 and having unrepresented openings for suction of the liquid sodium into the chimney 19. the type of fast neutron reactor whose construction is planned in the future, the tank 1 contains three identical pumps 7 placed in an annular zone at the periphery of the tank 1.

The upper part of the body 18 of the pump 7 not shown in Figure 1 passes through the horizontal slab of the reactor closing the vessel 1 and is fixed to this slab which ensures the suspension of the pump.

The lower or redan part of the inner vessel 60 is welded directly to the decking 6: the inner vessel defines a zone 24, called cold collector, in which the cooled liquid sodium is collected after passing through intermediate heat exchangers not shown - Sinks plunging into the tank and ensuring the heating of sodium secondary liquid using the heat taken by the primary liquid sodium in the core 4 of the reactor. Note that in a reactor according to the invention, the notch of the inner vessel 60 can be welded directly to the deck 6 without using bellows (deformable leaktight seal device placed in the inner vessel).

The stack 19 of the pump 7 which communicates with the cold collector 24 is also filled with cooled liquid sodium, the pump 7 ensures the reinjection into the core 4 according to a constructive arrangement that will be described in what follows. The pump 7 comprises a spherical discharge capacity 25, or discharge sphere, into which the lower part of the body 18 of the pump 7 opens.

The decking 6 comprises two webs or stiffeners 26 (only one is shown in Figure 1) parallel (typically spaced about 1000mm) located in line with the spheres and disposed substantially radially relative to the ferrules of the decking 6. The sphere 25 is welded in the lower part to the decking 6 via the two webs 26. The sphere 25 is completely integral with the decking 6. The decking 6 further comprises a sleeve 27 (typically of the order of 1200mm diameter) connecting the plate 6 with the sphere 25. This sleeve 27 is shown in enlarged view in Figure 2. The sleeve 27 is a short sleeve (ie having a length in the cutting plane of the order of 500 mm, preferably equal to 440 mm): it comprises a cylindrical central portion 28 and two ends

29 and 30 on either side of the central portion projecting outwardly of the cylinder. The fact of using a short sleeve 27 with the discharge sphere 25 completely integral with the deck 6 makes it possible to considerably reduce the differential displacements. The sleeve 27 is obtained from a machined machined axisymmetric monobloc foraminous blank. The ends 29 and

30 are machined so that the sleeve 27 is perfectly integrated with the outer side shell 11 of the deck via the end 29 and the sphere 25 via the end 30. In other words, the end 30 has a conical or spherical primer that will fit perfectly (ie without visible asperity after welding) in the sphere 25 in which an opening is provided for this purpose. The end 30 is welded to the sphere 25. Similarly, the end 29 has a "horse saddle" primer which will fit perfectly into the outer lateral shell 11 of the decking 6 in which an opening is provided for this purpose. The respective welds of the ends 29 and 30 to the ferrule 11 and to the sphere 25 are located outside the transition zone seat stress concentrations, the most stressed parts being at the level of the zones 31 and 32 located exactly between the part central 28 and the ends 29 and 30. In the systems according to the prior art, the delivery pipes stitched on the spheres and on the bed base had a weld directly located in the highest stress zones. Each of the three spheres arranged in the tank will therefore have a decking feed cuff (ie three cords in the decking). The absence of significant welds and the reduction of the differential displacements leads to a considerable reduction of the cyclic stresses in the zones of strong discontinuities and thus of the risk of rupture of the connection. The annular zone 33 formed by the three concentric rings 11, 12 and 13 and closed on the top by the upper flange 10 and from below by the lower flange 14 constitutes a box or collector tight and sized to support, in addition to loads conventional mechanical (weight, hydrodynamic pushes of the pumps, earthquake forces), a pressure of the order of 5-6 bars corresponding to the sodium under pressure from the pump 7. The intermediate shell 12 is provided with an opening 34. This box 33 receives the three sleeves from the three discharge spheres and will allow the supply of the bed: the liquid liquid is pumped by the pump 7 to the collector 33 via the sphere 25 and is transferred to the bed frame 5 via means 35 for supplying the sodium bed base 5 from the interior of the collector 33: we will detail these means 35 in what follows.

Figure 3 shows an enlarged view of the means 35 for supplying the sodium bed base from the decking.

The upper flange 10 of the decking and the lower plate 43 of the bed base are provided with orifices facing each other able to receive the means 35.

The means 35 are formed by a flexible sleeve 35 comprising a central ferrule 36 (having a diameter of the order of 500 mm) of passage of the coolant (represented by an arrow), of substantially cylindrical shape, formed by an upper ferrule 37 and a lower ferrule 38 welded, for example by an orbital welding process.

The sleeve 35 further comprises:

a lower part 39 forming substantially one half of a torus; an outer part 40 formed by a ferrule of diameter greater than the central ferrule 36 and concentric with it, the lower toric portion 39 ensuring continuity between the central ferrule 36 and the outer portion 40.

Practically, as shown in Figure 4 illustrating the embodiment of the sleeve 35 before welding, the sleeve comprises two parts: - a first part 41 comprising: o the lower ring portion 39; o the outer shell 40; o the lower ferrule 38; a second part 42 comprising the upper ferrule 37.

These two pieces 41 and 42 are docked and then welded end to end by orbital welding.

The upper end of the outer shell 40 is welded via a counter-flange 44 on the upper flange 10 of the decking in the hole provided for this purpose.

The upper end of the upper shell 37 is welded via a counter-flange 45 to the bottom plate 43 of the bed in the hole provided for this purpose. The use of a structure "glove finger" formed by the lower ring portion 39 and the outer ring 40 gives the sleeve 35 a certain flexibility to accommodate differential displacements between the frame 5 and the deck 6 ( typically of the order of a few mm in case of severe transient regimes). With reference to FIG. 1, the sleeve 35 is located between the second ferrule 12 and the third ferrule 13 of the decking 6.

The reactor according to the invention comprises, by way of example, 18 flexible sleeves distributed around the annular zone located between the second ferrule 12 and the third ferrule 13 of the decking. The embodiment of the means 35 described above involves an on-site welding of parts 41 and 42 which may require in-service inspection.

FIG. 5 illustrates a second embodiment of means 35 'for feeding the sodium bed base from the decking into a reactor according to the invention and making it possible to overcome a solder site (with a possible gain in time when an inspection campaign in service).

As for the means 35 shown in Figures 1 and 3, the means 35 'are located between the second ferrule and the third ferrule of the decking. The upper flange 10 of the decking and the lower plate 43 of the bed base are provided with orifices vis-à-vis able to receive the means 35 '. These means 35 'for supplying the bed with heat transfer liquid are formed by a labyrinth system comprising: a female part 46 of substantially cylindrical shape engaged inside the orifice provided for this purpose in the upper soleplate (this female part 46 comprises a flange 48 bolted to the upper soleplate 10); - a maie part 47 of substantially cylindrical shape engaged in the female part 46.

A vertical clearance is provided so that the maie part 47 can slide vertically in the female part 46: there may indeed be displacements (of very small amplitude) between the bed base and the decking and this game allows the maie part 47 to follow these movements.

The interface 49 between the inner wall of the female portion 46 and the outer wall of the maie portion 47 forms a labyrinth interface, that is to say a succession of chambers and throttling for reducing the sodium pressure. flowing in this interface. It is therefore conceivable that the totality of the liquid sodium will not pass from the collector to the bed, the presence of this labyrinth causing sodium micro-leaks: these microfuites can be advantageously used to complete the cooling circuit of the main tank.

A counter-flange 50 is bolted to the upper flange 43 of the bed base. A support piece 51 bears on this counter-flange 50.

The maled portion 47 has an upper end 53 placed on the support member 51, the interface 54 between the support member 51 and the upper end 53 being of the ball joint type; this interface 54 also forms a labyrinth interface ensuring micro-leaks of sodium. During certain hot shocks, it may occur deformations of the type "banana" is the bed base or decking; the ball joint makes it possible to follow these deformations.

The upper end 53 is simply placed on the support piece

51, the background effect being always favorable to the plating of the piece maie 47. For more security, especially in case of earthquake, the upper end

53 is, however, maintained by an anti-foaming device 52, to avoid the risks of dislodgement of the parts 51 and 53. It will be noted that the interface between the support piece 51 and the counter-flange 50 also forms a labyrinth interface.

In known manner, labyrinths can be obtained by machining.

Of course, the invention is not limited to the embodiment just described.

Note that the dimensions given in the description are provided for guidance for a reactor with a power of 1500 MWe and are obviously likely to vary depending on the power of the reactor.

In addition, the delivery capacity may have a shape different from that of a sphere.

Claims

1. Integrated fast neutron nuclear reactor comprising a main tank (1) containing:

a core (4) composed of fuel assemblies (9) capable of being cooled by a coolant,

a support element of said core (4) called bed base (5),

a support element of said bed base (5) called a decking (6),

a pump (7) for circulating said coolant comprising: a vertical body (18), a delivery capacity (25) communicating with the lower part of said vertical body (18), said reactor being characterized in that that said delivery capacity (25) is in communication with the external lateral wall (11) of said decking (6) via a sleeve (27) of substantially cylindrical shape integral with said decking (6) and said discharge (25) so that said coolant flows from said discharge capacity (25) to the interior of said decking (6), said reactor comprising means (35) for supplying said bed (5) with heat transfer fluid from the interior of said decking (6).

2. Nuclear reactor according to claim 1 characterized in that said sleeve (27) is a machined monobloc forged part.

3. nuclear reactor according to one of the preceding claims characterized in that said sleeve (27) is welded to said side wall of said decking (11) and said discharge capacity (25), said sleeve (27) having a central portion (28) and two ends (29, 30) on either side of said central portion (28) protruding outwardly and welded respectively to an opening of said outer side wall (11) of said decking (6) and on an opening of said discharge capacity (25).

4. Nuclear reactor according to the preceding claim characterized in that said two ends (29, 30) are machined to present the respective shape of said outer side wall (11) of said decking (6) and said delivery capacity (25) so that they are respectively integrated in the continuity of said outer side wall (11) and said discharge capacity (25).

5. Nuclear reactor according to one of the preceding claims characterized in that said decking (6) comprises a sealed annular collector (33) adapted to receive said heat transfer fluid.

6. Nuclear reactor according to the preceding claim characterized in that said annular collector (33) is formed by a plurality of concentric rings (11, 12, 13).

7. Nuclear reactor according to one of claims 5 or 6 characterized in that said means (35, 35 ') for supplying said bed base (5) heat transfer liquid are equidistributed around said annular collector (33).

8. Nuclear reactor according to one of the preceding claims characterized in that said decking (6) comprises an upper plate (10) and said bed base (5) comprises a lower plate (43), said upper and lower plates (10, 43). ) being provided with orifices facing each other, said means (35) for supplying said bed (5) with heat transfer liquid being formed by sleeves (35), each of said sleeves being welded to said lower and upper plates (10, 43) via said orifices.

9. Nuclear reactor according to the preceding claim characterized in that each of said sleeves (35) comprises a central portion (36) for passage of heat transfer fluid of substantially cylindrical shape of vertical axis formed by an upper ferrule (37) and a lower ferrule (38) welded.

10. Nuclear reactor according to the preceding claim characterized in that each of said cuffs (35) comprises:

a lower part (39) forming substantially one half of a torus;

an outer part (40) formed by a ferrule of diameter greater than the diameter of said central portion (36) of the sleeve (35) and concentric with it, said lower part (39) ensuring continuity between said portion central (36) and said outer portion (40).

11. Nuclear reactor according to the preceding claim characterized in that the upper end of said outer portion (40) is welded to said upper plate (10) of said decking and the upper end of said central portion (36) is welded to said lower plate (43) of said bed base.

12. Nuclear reactor according to one of claims 10 or 11 characterized in that said sleeve (35) is formed by two parts welded together:

a first part (41) comprising: said lower part (39) forming substantially one half of a torus; said outer portion (40); o the lower part (38) of said central portion (36) for passage of the coolant;

- A second part (42) comprising the upper part (37) of said central portion (36) for the passage of heat transfer fluid.

13. Nuclear reactor according to the preceding claim characterized in that said first (41) and second parts (42) are welded end to end.

14. Nuclear reactor according to one of claims 1 to 7 characterized in that said decking comprises an upper plate (10) and said bed base comprises a lower plate (43), said upper and lower plates being provided with orifices vis- opposite, said means (35 ') for feeding said heat transfer liquid bed being formed by labyrinth systems, each of said systems comprising:

a female part (46) of substantially cylindrical shape with a vertical axis fixed to said decking and engaged inside one of said orifices of said upper plate (10);

a male part (47) of substantially cylindrical shape with a vertical axis engaged in said female part (46) so that the interface (49) between the inner wall of said female part and the outer wall of the said part maie forms an interface to labyrinths.

15. nuclear reactor according to the preceding claim characterized in that a vertical play is provided so that said maie part (47) is able to slide vertically in the female part (46).

16. Nuclear reactor according to one of claims 14 or 15 characterized in that said maie portion (47) has an upper end (53) placed on said bed base via a support piece (51), the interface (54) between said support member and said upper end being of the ball joint type and forming a labyrinth interface.

17. Nuclear reactor according to one of the preceding claims characterized in that it comprises an inner vessel (60) defining a zone of the main vessel receiving hot liquid leaving the core and a zone receiving cooled liquid, said inner vessel ( 60) having a lower portion directly attached to said decking (6).

18. Nuclear reactor according to one of the preceding claims characterized in that said discharge capacity (25) is welded at the bottom to said piatelage via stiffeners (26) integrated in said decking.