EP0006795B1 - Intermediate heat exchanger for a nuclear reactor with fast neutrons - Google Patents

Description

The present invention relates to reactors, in which the cooling of the core is ensured by continuous circulation of a liquid metal, usually sodium, contained inside a main tank in which the core is immersed, the calories acquired by the liquid metal passing through the latter in contact with the fuel assemblies being transferred to a secondary fluid, also sodium, internally traversing the tubes of an intermediate exchanger plunging into the tank, the liquid metal cooling the core playing the role of fluid primary circulating outside these tubes, in heat exchange relationship with the secondary fluid. In known manner, the secondary fluid which is thus heated, is then returned outside the reactor vessel to a generator capable of supplying pressurized steam directly under pressure in an electricity production installation.

The present invention relates more particularly to an arrangement of the internal structure of such an intermediate exchanger, comprising, in known manner, a cylindrical internal ferrule with vertical axis, an external ferrule coaxial with the internal ferrule, two plates of horizontal annular tubes disposed in the vicinity of the upper and lower extremities of said ferrules, a bundle of straight tubes extending between the tube plates in cylindrical plies coaxial with the ferrules, these plies being braced mutually by transverse belts formed by horizontal bands carrying elements of generally hollow bracing housed between the tubes of the plies and in contact with them, the intake and discharge manifolds of the secondary fluid circulating inside the tubes being respectively provided under the lower plate and above the upper plate, the internal ferrule forming a conduit for supplying the secondary fluid to the intake manifold while the outer shell is extended upwards by an outlet duct for the same secondary fluid collected in the evacuation manifold after passage through the tubes, and inlet and outlet windows distributed around the axis of the external shell for admission and evacuation, in the vicinity of the tube plates of a primary fluid exchanging calories with the secondary fluid through the wall of the tubes.

In a conventional structure of this kind, the primary fluid therefore penetrates transversely into the exchanger through the inlet windows, spreading in the space between the external and internal ferrules, in order to circulate after a first change of direction substantially. at 90 ° in contact with the beam tubes, the flow taking place over most of the length of these tubes against the current of the secondary fluid, the primary fluid undergoing a second change of direction again by 90 ° for s '' evacuate out of the exchanger through its outlet windows.

It is therefore easily understood that, as a result of this double change of direction, it is possible to note a significant difference in the temperatures of the secondary fluid at the outlet of the tubes of the bundle in the evacuation manifold, the cylindrical layers arranged as close as possible to the internal ferrule leaving a secondary fluid colder than the tubes of the layers close to the external ferrule, the heat exchange with the primary fluid having been in these latter internal layers less effective. This difference is in particular due to a lower supply of the tubes of the internal plies by the primary fluid, in particular at the level of the entry and exit windows. This results in significant differential expansion between the internal and external ferrules, which generates high mechanical stresses which are detrimental to the good performance of the exchanger.

The present invention aims to overcome this drawback, by ensuring a more homogeneous distribution of the hot primary fluid at the entrance into the exchanger through the tubes of the bundle, in particular between the layers of internal and external tubes.

To this end, the exchanger according to the invention is characterized in that the bundle of tubes comprises means capable of producing a greater supply in the region occupied by the tubes of the internal plies than in the region occupied by the tubes of the plies. external, said means creating, in the circulation of the primary fluid inside the exchanger, a variable pressure drop.

According to a first embodiment, the means for producing said supply consist of interlacing elements integral with the bracing belts of the tubes, these elements being hollow or solid and defining at least two concentric zones with different pressure losses.

In another embodiment, the tubes of the bundle are distributed in at least two zones with, in the internal plies, a radial and / or circumferential pitch more reduced than in the external plies.

• - la Fig. 1 est une vue schématique en coupe axiale d'un échangeur intermédiaire selon l'invention, conforme à un premier mode de réalisation,
• - la Fig. 2 est une vue de détail en perspective d'une des ceintures d'entretoisement des tubes de l'échangeur selon la Fig. 1,
• - la Fig. 3 est une vue en coupe de l'échangeur de la Fig. 1 selon la ligne III-III de cette dernière,
• - les Fig. 4 et 5 sont respectivement des demi-vues en coupe axiale, illustrant deux autres variantes de réalisation.
• - la Fig. 6 est une vue partielle en coupe horizontale d'un échangeur montrant un autre mode de réalisation des pertes de charge.

Other characteristics of an intermediate exchanger for a fast neutron nuclear reactor established in accordance with the invention will become more apparent from the following description of several exemplary embodiments, given by way of non-limiting example, with reference to the drawings appended to which:

• - Fig. 1 is a schematic view in axial section of an intermediate exchanger according to the invention, in accordance with a first embodiment,
• - Fig. 2 is a detail view in perspec tive of one of the spacer belts of the exchanger tubes according to FIG. 1,
• - Fig. 3 is a sectional view of the exchanger of FIG. 1 along line III-III thereof,
• - Figs. 4 and 5 are respectively half-views in axial section, illustrating two other alternative embodiments.
• - Fig. 6 is a partial view in horizontal section of an exchanger showing another embodiment of the pressure drops.

Identical reference figures have been used in these figures to designate bodies which are similar from one to the other.

In Fig. 1, the reference 1 designates as a whole, an intermediate exchanger according to the invention, intended to be mounted inside a tank (not shown) of a fast neutron nuclear reactor, comprising in particular a transverse step 2 crossed by the body of the exchanger. This step delimits inside the tank, two regions respectively 3 and 4, region 3 receiving the coolant metal leaving the reactor core, therefore having acquired calories in contact with the fuel assemblies, this sodium after crossing the 'intermediate exchanger 1 being collected under the step 2 in region 4, at a significantly lower temperature, as a result of the heat exchange carried out in the exchanger with a secondary fluid. Preferably, the primary and secondary fluids consist of liquid sodium.

The exchanger 1 mainly consists of a cylindrical internal ferrule 5, with a vertical axis, and an also cylindrical external ferrule 6, coaxial with the first, these two ferrules being joined, respectively in the vicinity of their upper and lower ends, by two horizontal tube plates 7 and 8. Between these plates 7 and 8, extend the tubes 9 of a bundle of straight tubes, inside which circulates the secondary fluid exchanging calories with the primary fluid, itself even flowing in the exchanger outside the tubes 9. In the bundle, the tubes 9 are suitably braced between them to form cylindrical sheets, coaxial with the ferrules and in which the tubes are maintained at a spacing determined by l 'Intermediate transverse belts 10, the detail of which appears more clearly in the partial view of FIG. 2. At its lower end, the exchanger comprises a bottom 11 which delimits with the tubular plate 8 an intake manifold 12 for the secondary fluid, brought into the exchanger by the interior of the internal ferrule 5, the latter comprising an open lower end 13. The secondary fluid thus admitted inside the manifold 12 flows into the tubes 9 of the bundle and it is finally collected in an upper manifold 14 delimited between the internal ferrule 5 and an extension 15 of the ferrule external 6.

The hot primary fluid in region 3 enters the interior of the intermediate exchanger through inlet windows 17, regularly distributed around the axis of the exchanger, in the outer shell 6. This primary fluid flows then from top to bottom in contact with the tubes 9 of the bundle over most of the length of the latter, against the current of the secondary fluid which passes inside these same tubes from bottom to top. At the outlet of the exchanger, the primary fluid flows out of the outer shell 6 through outlet windows 18, also regularly distributed around the axis of this shell.

Fig. 2 illustrates in more detail the practical embodiment of the bracing belts 10, making it possible to maintain the tubes 9 of the bundle with a determined spacing, at the same time as ensuring protection of these tubes from vibrations due to circulation of the primary fluid. To this end, the belts 10 are in particular constituted by means of horizontal bands 19 against which are welded elements of hollow tubes, such as 20 and 21.

According to the invention and in a first variant embodiment, precisely illustrated in FIGS. 1 to 3, bracing elements which are constituted here by the tube elements 20 and 21 of the bracing belts 10 provided between the ferrules 5 and 6, are determined so that they have different lengths, in particular such that the hollow elements 20, of greater length, are arranged in the external plies of the bundle, while the elements 21 of shorter length are provided in the internal plies.

Thanks to these provisions, there is in fact a supercharging of primary fluid of the internal plies in the intermediate part of the exchanger, in particular in the region where this primary fluid, after having penetrated transversely through the inlet windows 17 and having undergone a first change of direction at 90 °, flows parallel to the direction of the tubes against the current of the secondary fluid inside the latter. This supercharging thus created between the tubes 9 appreciably increases the heat exchange for the tubes concerned in their central part and achieves a better overall balance in the heat exchange between the different zones or regions of the tubes, in particular by making it possible to restore to the outlet of these in the collector 14, a roughly uniform temperature.

Of course, the above solution can be improved, by making a greater number of zones, for example three or four comprising in each, hollow elements of different lengths, varying gradually from the external layers to the internal layers.

According to the variant embodiment shown in FIG. 6 the different concentric zones are produced by giving the cross section horizontal bracing elements bearing the general reference 30 of the shapes or contours suitable for creating different pressure drops the pressure drop decreasing when going from the periphery to the center. There is a first zone with bracing elements 30a which are constituted by portions of tubes of thickness e ₁ . There is then a second zone in which the bracing elements 30b are still portions of tubes of thickness e ₂ greater than e _1, the tubes having the same outside diameter as the tubes 30a. In the third zone, the bracing elements 30c consist of solid rods whose outside diameter is equal to that of the tubes 30a and 30b.

Finally, the bracing elements 30d are formed by the association of a solid rod 30'd similar to the rod 30c and an extension 30 "d inserted between the exchange tubes 9.

We can see that the pressure drop increases as we pass from the central zone to the peripheral zone.

Of course the elements 30d could have a cross section having another outline. Similarly, the different zones could be defined only by portions of tubes (30a, 30b) of different thicknesses. Finally, to adapt the pressure drop in each zone, one can play both on the length of the bracing elements and on the outline of their horizontal cross section.

In another alternative embodiment illustrated in FIG. 4, the overfeeding of the regions occupied by the external plies on the one hand and the internal plies on the other hand, can be obtained by shifting, according to the length of these tubes, the bracing belts 10, so that the elements bracing 22 which are fixed thereon, arranged in contact with these tubes, create in these layers a funneling effect, with the formation of a horizontal velocity component in the flow of the primary fluid towards the center of the device, notably improving the supply of the internal layers. Preferably the elements 22 are hollow.

It is understood that each hollow element creates a localized pressure drop and that it is the progressive offset of the hollow elements in the axial direction which achieves the funneling effect and therefore the horizontal component of the fluid flow speed.

According to a second embodiment, illustrated in FIG. 5, the tubes of the bundle are arranged in two zones, respectively 9a and 9b, where they have different radial and circumferential pitches, the tubes 9a in the vicinity of the internal ferrule 5 having smaller pitches than the tubes 9b in the vicinity of the outer shroud 6. Advantageously, the two preceding zones are separated by an inner skirt 23, coaxial with the shrouds 5 and 6. It can therefore be seen that in this embodiment, the best distribution of the fluid is obtained by creating pressure drops different in the same horizontal plane.

In this mode of implementation, it facilitates the penetration of the primary fluid in the region of the tubes close to the internal shroud 5 and thus a more efficient heat exchange is obtained in said region.

Note that to balance the pressure drops between the two zones, anti-vibration belts can also be placed in the latter, creating, as in the examples in FIGS. 1 and 4, differentiated pressure drops, in order to counterbalance the lower linear pressure drop due to the larger pitch of the tubes 9b.

Claims (9)

1. An intermediate heat-exchanger (1) comprising an inner vertical-axis cylinder (5), an outer cylinder (6) coaxial of the inner cylinder (5), two horizontal annular tube panels (7, 8) disposed near the top and bottom ends of said cylinders (5, 6), a nest of straight tubes (19) extending between the tube panels (7, 8) in cylindrical layers coaxial of the cylinders (5, 6), said layers being mutually braced by transverse belts (10) formed by horizontal bands (19) bearing bracing elements (20, 21, 22, 30) housed between the tubes (9) of the layers and in contact therewith, inlet and outlet headers (12) and (14) respectively for a secondary fluid flowing inside the tubes (9) being provided respectively beneath the bottom panel (8) and above the top panel (7), the inner cylinder (5) forming a conduit for the supply of the secondary fluid to the inlet header (12) while the outer cylinder (6) is extended upwards by an outlet conduit (15) for the same secondary fluid collected in the outlet header (14) after passing through the tubes (9), and inlet and outlet apertures (17) and (18) respectively distributed about the axis of the outer cylinder (6) for the admission and discharge of a primary fluid near the tube panels (7, 8), said primary fluid exchanging heat with the secondary fluid through the wall of the tubes (9), characterised in that the nest of tubes (9), comprises means adapted to provide a greater supply in the region occupied by the tubes of the inner layers than in the region occupied by the tubes of the outer layers, said means producing a variable pressure drop in the flow of primary fluid inside the heat-exchanger.

2. A heat-exchanger according to claim 1, characterised in that the means for providing the said supply consist in the said bracing elements (20, 21, 22, 30) themselves.

3. A heat-exchanger according to claim 2, characterised in that the bracing elements (19, 20, 30) are disposed in a plurality of horizontal planes, the varying pressure drop being produced by the different geometry of the said bracing elements according to their radial position in a given horizontal plane, thus providing at least two concentric annular zones with different pressure drops.

4. A heat-exchanger according to claim 3, characterised in that the bracing elements (19, 20) are tubes and have different lengths.

5. A heat-exchanger according to claim 3, characterised in that the bracing elements (30) have different thicknesses.

6. A heat-exchanger according to claim 3, characterised in that at least some of the bracing elements (30) are solid cylinders (30c, 30d) with vertical generatrices, the cross- section of the solid bracing elements in a given level being of a form adapted to produce different pressure drops.

7. A heat-exchanger according to claim 2, characterised in that the bracing elements (22) are offset along the length of the tubes to give a funnel effect to facilitate the increase in the rate of flow of the primary fluid between the tubes of the inner layers, by producing a complementary horizontal velocity component in the flow of the primary fluid.

8. A heat-exchanger according to claim 7, characterised in that the said bracing elements (22) are tubes.

9. A heat-exchanger according to claim 1, characterised in that the tubes (9) of the nest are distributed over at least two zones with a smaller radial and/or circumferential pitch in the inner layers than in the outer layers.

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