FR3040234A1 - ASSEMBLY FOR RNR-NA TYPE NUCLEAR REACTOR, HAVING A BOX PROVIDED WITH ENHANCED STIFFNESS SPACER PLATES - Google Patents

Abstract

The invention relates to an assembly for a nuclear reactor, in particular for a sodium-cooled rapid neutron reactor RNR-Na, comprising a housing (10) of longitudinal axis (X), each main face of the housing comprising, in its central part, a spacer plate (2) with an adjacent assembly, including an outwardly projecting portion, the assembly further comprising a reinforcing sleeve (3) attached and held within the housing (10) and arranged next to the plates (2).

Description

ASSEMBLY FOR NUCLEAR REACTOR TYPE RNR-Na, A HOUSING PROVIDED WITH SPACER PLATES WITH IMPROVED STIFFNESS

Technical area

The present invention relates to a fuel assembly for a fast neutron nuclear reactor cooled with liquid metal, especially liquid sodium called RNR-Na or SFR (acronym for "Sodium Fast Reactor") and which is part of the family of so-called reactors. fourth generation. The aim of the invention is first of all to propose a fuel assembly that can be used in the short term in the fourth generation French technology demonstrator project called ASTRID.

The fuel assemblies targeted by the invention can be used both in a nuclear reactor of integrated type, that is to say for which the primary circuit of sodium with pumping means is totally contained in a vessel also containing and heat exchangers, only in a loop-type reactor, that is to say for which the intermediate heat exchangers and the primary sodium pumping means are located outside the tank.

By fuel assembly is meant an assembly comprising fuel elements and charged and / or discharged into / from a nuclear reactor.

By fuel assembly type RNR-Na or SFR is meant a fuel assembly adapted to be irradiated in a fast neutron nuclear reactor cooled with liquid sodium called RNR-Na or SFR.

Although described with reference to the main application aimed at, namely a fuel assembly for a nuclear reactor, the invention can be applied to any type of assembly for a nuclear reactor, such as a reflector, a lateral neutron protection (NLP). ), a control bar, an experimental assembly, a complementary safety device ...

State of the art

The fuel assemblies intended for use in fast reactors cooled with liquid sodium (RNR-Na), have a particular mechanical structure in particular to let liquid sodium pass through them.

FIG. 1 shows a fuel assembly 1 already used in a nuclear reactor RNR-Na known under the name "Phoenix".

Such an assembly 1 of elongate shape along a longitudinal axis X firstly comprises a tube or casing 10 with a hexagonal section, the upper portion 11 of which forms the gripping head of the assembly and houses an upper neutron protection device (PNS ), and whose central portion 12 envelops unrepresented fuel needles.

In other words, the portions 11, 12 form the same tubular envelope 10 or identical hexagonal section box over its entire height. The head 11 of the assembly has a central opening 110 opening therein. The assembly 1 finally comprises a lower portion 13 forming the foot of the assembly, in the extension of the housing 10. The foot 13 of the assembly has a distal end 15 cone-shaped or rounded to be inserted vertically in the candles of the bed base (support) of a reactor core. The foot 13 of the assembly has at its periphery openings 16 opening therein.

Thus, in the installed configuration of a fuel assembly, that is to say in a loaded position in a reactor core, the foot 13 of a male-shaped assembly 1 is inserted into an opening of the reactor base in thus maintaining the assembly 1 in the latter with its longitudinal axis X vertically.

The primary sodium can circulate inside the housing 10 of the assembly 1 and thus heat-transfer the heat generated by the fuel needles. The sodium is thus introduced through the openings 16 of the foot 13 and out through the central opening 110 of the head 11, after having passed through the bundle of combustible needles.

The central portion 12 of an assembly comprises a plurality of nuclear fuel needles. Each needle is in the form of a sealed cylindrical sheath tube inside which is stacked a column 14 of fissile fuel pellets in which occur nuclear reactions that give off heat. All columns 14 define what is usually called the fissile area which is approximately halfway up an assembly 1. It is shown schematically in the form of a black rectangle in FIG.

All the assemblies of the same reactor are arranged vertically on a bed base to form a core network with hexagonal mesh.

The assemblies in position on the bed base are spaced from each other at their base (foot), typically a few mm between the facing faces of two adjacent hexagonal section housings.

It is necessary that this spacing be kept substantially constant over the entire height of the assembly during operation of the reactor.

In fact, a combination of two adjacent fuel assemblies immediately leads to a reactivity insertion, ie a rapid increase in power, which can have heavy consequences, such as overheating, clogging, and lead to a meltdown accident. of the heart.

To overcome this, it is known in existing RNR-Na reactors, to add spacers in the upper part of the assembly housing, just above the area of fissile needles. Generally, these devices are arranged at a height approximately equal to 2/3 of the height of the assembly projecting above the bed base.

These spacers, usually called "platelets", consist essentially of bosses, that is to say, extra thickness, projecting outwardly of the assembly. Each face of the hexagonal section of the housing is provided with a boss (plate).

These plates therefore have the function of locally reducing the clearance between the adjacent assemblies and thus make it possible: to ensure the compactness of the assembly network, during the normal operation of the reactor and the handling operations (at a reduced temperature), to limit the compaction of the core, that is to say a movement of approximation of the assemblies, during an earthquake, or the recompaction of the core consecutive to the elastic return of the assemblies after stacking, that is to say a movement of separation of the assemblies generated by a release of energy internal to the core, such as a gas expansion.

The platelets already used on the assemblies of the RNR-Na reactors operated in France, known under the names "Phénix", "Superphénix" or "Rapsodie", are obtained by stamping with a punch on each of the six faces of the hexagonal case in order to to obtain the desired deformation towards the outside of the case.

The functional part of these wafers, that is to say their flat contact surface 20 as illustrated in FIG. 3C, is generally a rectangle of 20 to 50 mm side with a deep drawing depth of the wafers, ie the altitude boss 20 on the face, a few millimeters maximum.

The inventors have analyzed that the concept of platelets retained for example for fuel assemblies for the earlier reactors, as illustrated in FIGS. 2 to 3C, can not be retained for fuel assemblies for ASTRID, because the existing plates are not enough. rigid vis-à-vis the objective of safety sought and presented below, or in other words their stiffness is not high enough. The stiffness of a wafer, noted K, characterizes the resistance of a wafer to crushing by an external force. It is defined in the elastic range as being equal to the ratio between the force applied to the wafer and the displacement of the face of the wafer with respect to the axis of the hexagonal housing.

In fact, the stiffness K of the known platelets is insufficient in the context of the ASTRID reactor as explained hereinafter.

Firstly, the limitation of compaction of the core in accident situations (earthquake, release of internal energy to the core, etc.) is clearly a safety objective to which the ASTRID reactor must respond with a view to a reactor reactor stream. fourth generation.

For the heart called "Low Draining Heart (or CFV)" of ASTRID, which has the particularity of presenting a negative sodium drain coefficient, studies have shown that the criterion of increase of reactivity following the compaction of the heart , set at a maximum of $ 1 ($), is respected by implementing pads whose stiffness is increased by a factor of 5 compared to known stamped pads while remaining compatible with the other specifications of a fuel assembly intended to be used in the ASTRID reactor.

These specifications are numerous and only those having an influence in the platelet design process are detailed below.

The CFV core negative drain coefficient is the keystone of the safety demonstration for the ASTRID reactor. In essence, the negative emptying, characterized by a natural decrease of reactivity of the heart in case of emptying of the sodium, is reached in particular by minimizing the quantity of steel in the zone, called "plenum", situated just above the combustible needles . Indeed, steel is a reflective material of neutrons.

However, in a configuration where the sodium of the plenum would be drained, for example in an accidental situation generating a boiling of sodium, a large amount of steel in the plenum would result in a reflection of the neutrons leak to the fuel, leading to an increase reactivity, which is precisely the opposite of the desired effect for the CFV core.

As a reminder, a fuel assembly of a CFV core therefore comprises, starting from the top to the bottom of the core: an upper absorbent zone, consisting of a neutron-absorbing material, a liquid metal plenum zone, an upper zone; of fissile material, - an intermediate zone of fertile material, - a lower zone of fissile material.

In addition, the horizontal median plane of the intermediate zone of fertile material is located below the horizontal median plane of the assembly formed by the upper zone of fissile material, the intermediate zone of fertile material and the lower zone of fissile material, and the ratio of the height of the intermediate zone of fertile material to the height of the assembly formed by the upper zone of fissile material, the intermediate zone of fertile material and the lower zone of fissile material is in the range of 0, 25 to 0.40.

It should be noted that, moreover, the plates are positioned on the hexagonal case just above the upper end of the fuel needles in order to guarantee an optimum spacing between the fissile zones of two adjacent assemblies, and thus to limit the compaction. In other words, the plates are positioned in the lower part of the plenum.

The guarantee of maintaining the negative drain coefficient of the core therefore leads to minimizing the amount of embedded steel at the platelets. In practical terms, only neutron calculations are able to estimate the impact of platelet geometry on the drain coefficient.

Moreover, as in any nuclear reactor, it is necessary to minimize the forces during the handling of the assemblies in the ASTRID reactor.

When they are embedded in the bed base, the assemblies are in contact or near contact at the platelets. The compactness of the heart results in a negative or no play at the level of the platelet plane. This compactness is sought in nominal operation, corresponding to average sodium temperatures of about 550 ° C at platelets and 400 ° C at the bed base, to ensure the static mechanical equilibrium of the heart.

However, if the heart is compact in nominal operation, it is not necessarily cold during tank assembly handling operations, the entire sodium of the tank then being lowered to 200 ° C. Indeed, the level of compactness of the cold core depends on the differential thermal expansion between the steel constituting the bed base and the steel constituting the platelets.

The bed base is usually made of austenitic steel with a high coefficient of thermal expansion, which is AISI 316 LN stainless steel.

The inserts can be made either of austenitic steel type AISI 316, whose expansion coefficient is identical to that of the bed base, or of martensitic steel type EM10 (9% Cr and 1% Mo) or ferritic steel of which the coefficients of expansion are lower than that of 316 steels.

The following two situations can be distinguished: the platelets and the bed base are both made of austenitic steel: the compactness at platelets in nominal operation, ie when hot, then implies a positive play on the platelets in the handling configuration, ie when cold . This positive handling game is favorable for the insertion and extraction of network assemblies by minimizing the friction between assemblies and therefore handling efforts. It also avoids any risk of blocking the assembly in the network; the plates are made of ferritic or martensitic steel whereas the base is made of austenitic steel: the compactness of the pads in nominal operation also implies a compactness in the pads in handling with a clearance slightly less than or equal to zero. This negative clearance is unfavorable because it induces significant extraction efforts necessary, at the risk of exceeding the traction capacity of the handling machine, or damage the outer surface of the pads (friction, scratches ...). The extraction force of the assembly depends on the force applied on the platelets and therefore on their stiffness with imposed displacement, the contact surface between platelets, and the coefficient of friction. In other words, having very rigid pads and / or with a large contact surface is unfavorable vis-à-vis the objective of minimizing the forces during handling assemblies.

Currently, the mounting method selected for an assembly for the ASTRID reactor provides for the insertion of the fuel needle bundle through the top of the hexagonal housing equipped with the welded foot, followed by mounting in the upper part of the assembly consisting of the PNS. and the head to close the assembly.

The bundle of combustible needles occupies the entire interior space of the hexagonal case. The plates being positioned above the needle beam, the feasibility of inserting the latter in the housing therefore depends on the geometry of the wafers.

Thus, when the wafers have a geometry that does not reduce the internal gap of the housing, that is to say the distance separating two facing faces, they do not prevent the insertion of the beam. These platelets of which say non-intrusive. The pads can then be mounted or manufactured directly on the original hexagonal case. This type of pads is the simplest and is therefore compatible with assembly assembly.

On the other hand, when the plates have a geometry that reduces the internal gap, such as rigid plates whose thickness is increased towards the inside of the case, they are said to be intrusive and prevent the insertion of the beam from the top of the case. . These pads must then be reported on the housing only once the beam set up.

However, this operation is critical. On the one hand, the proximity of the pads with the top of the combustible needles, of the order of 8 to 10 cm, prohibits a welded connection between the pads and the housing due to the local heating at very high temperature of this area when welding, or even during a thermal annealing stabilization, which would be likely to damage the needles. On the other hand, this welding is a delicate operation to achieve and control. A defective weld made with the fuel bundle in place would result in complete loss of assembly.

Finally, spacing plates must be compatible with the thermal hydraulics of the assemblies and the reactor core.

As already mentioned, the pads are oversize provided on the outer faces of the housing, which reduce or locally eliminate the clearance between adjacent assemblies. The contact surface, or more precisely the width of the pads should not be too large at the risk of plugging the space between adjacent assemblies and thereby prevent the flow of sodium between the housings of the assemblies.

The sodium flow between the assemblies is very low and does not participate in the cooling of the assemblies in normal operation. On the other hand, the circulation of sodium between the housings of the assemblies which is installed by natural convection in certain accidental situations, such as the loss of primary flow, becomes necessary for the evacuation of the residual power of the assemblies.

In addition, the thermohydraulics internal to an assembly is essential for all phases of operation.

In the case of non-intrusive plates, that is to say which do not reduce the internal gap of the hexagonal case, the flow of sodium inside an assembly is not disturbed.

In the opposite case of intrusive wafers, the reduction of internal gap locally represents an obstacle increasing the pressure losses in the assembly and can disrupt the flow of sodium at the assembly outlet at the level of the monitoring instrumentation (temperature and sodium flow) located above the heart. It is therefore imperative to adapt the geometry of the intrusive platelets to have a satisfactory sodium flow.

The inventors then sought to identify, among known solutions of spacing devices between adjacent fuel assemblies in a nuclear reactor vessel, those that could be suitable for guaranteeing a rigid spacing between fuel assemblies for a fourth generation RNR-Na reactor of the ASTRID type.

US Pat. No. 4,149,234 discloses a fuel assembly for a nuclear reactor, in particular for an RNR-Na reactor, the hexagonal case of which comprises inserts attached to each face, each plate consisting of two half-plates arranged one beside the other. . The two half-wafers have the same dimensions, but are made of different materials of which one is chosen to have a low coefficient of friction, for example Stellite, and the other is a steel. The arrangement of the half-wafers is such that when two half-wafers of an assembly are in contact with the two half-wafers of the adjacent assembly, each half-wafer is in contact with a half wafer of 'a different material. An assembly according to this US4142934 patent is incompatible with the functional specifications of the fourth-generation RNR-Na reactor chips of the ASTRID type, since the platelets each divided into two half-platelets do not make it possible to obtain an increase in their stiffness.

The patent FR2509896 also discloses a fuel assembly for a nuclear reactor, in particular for a reactor RNR-Na, in which the plates are in the form of stampings in each of the corners of the hexagonal tube of the housing. If the corner pads according to this patent FR 2509896 would allow a priori to increase the stiffness of platelets in the desired proportions, the inventors believe that they can not be retained as a separation solution for RNR-Na reactor of fourth generation of ASTRID type, because of the lack of tolerance that they induce vis-à-vis a defect of angular orientation of the assemblies. Indeed, a slight angular defect orientation of an assembly according to this patent FR 2509896 would result in a greater displacement at the corners, which would lead to contact pressures and therefore to friction between the platelets between the larger assemblies. This does not go in the direction of the function sought to minimize the efforts of handling assemblies.

The patent FR2403626 discloses a fuel assembly for a nuclear reactor, in particular for a reactor RNR-Na, comprising a housing tube with a dodecagonal section with a flat or convex side located in each corner of the tube, in place of a tube with usual hexagonal section for an assembly box. This tube geometry with a dodecagonal section makes it possible to limit the expansion of the assembly box under irradiation. This thus guarantees the geometry and the neutron performances of the core since the vibrations are reduced because of the maintenance of the clearances between the fuel needles and the casing, and thus the instabilities of reactivity are limited. Assuming that a dodecagonal section is adopted for the assembly box as disclosed in this patent FR 2 403626 with wafers still positioned individually in the middle of each main face, the increase in stiffness of the wafers would certainly be effective, because of the limitation of the bending of the main faces, but probably insufficient vis-à-vis the objective set in the context of a RNR-Na reactor of fourth generation type ASTRID, because the thickness of steel at the platelet level would not be however, increased.

US Pat. No. 4,543,233 also discloses a fuel assembly for a nuclear reactor RNR-Na, with platelets of circular section housed in the outer face of the hexagonal case and fastened to it by means of a spring washer.

The patent JP2006145506 discloses a fuel assembly for RNR-Na nuclear reactor, with inserts reported on the outside of the housing of the assembly, similar to those of patent US4543233. The plates also of circular section are each housed in a hole made on each side of the tube of the housing and attached to it by screwing or welding. The inventors believe that the inserts reported on the outside of the faces of the casing according to the patents US4543233 and JP2006145506 do not lead to a significant increase in stiffness, at least not in the proportions sought for a fourth-generation RNR-Na reactor of the type ASTRID.

The patent FR2921509 discloses a fuel assembly for a nuclear reactor RNR, in particular of the RNR-Na type, comprising inside the hexagonal case, a six-pointed star-shaped structure, arranged above the bundle of combustible needles. . The structure may have longer or shorter branches, and may optionally incorporate lateral bars between branches to stiffen the structure. The free end of each of the branches is extended by a spacing plate which passes through an opening in the middle of each face of the housing. An assembly according to this patent FR2921509 is not compatible with at least some of the functional specificities of a fourth generation RNR-Na reactor fuel assembly of the ASTRID type. In particular, the star-shaped structure reported inside the casing constitutes a large mass of steel added in the plenum zone, which necessarily leads to degrade the drainage coefficient to the point of not being able to guarantee the CFV effect of the core. . Furthermore, the star structure is inherently an obstacle to the flow of sodium in the assembly, with a consequent increase in pressure losses and a disruption of the output flow assembly.

US4306938 discloses a fuel assembly for RNR-Na nuclear reactor, comprising platelets stamped in the form of a bead or continuous strip over the entire periphery of the hexagonal housing. The stamped pads are stiffened by the introduction of a sleeve disposed inside the housing and which is stamped at the same time as the wafers and is housed once deformed in the deformation depression of the wafers. The gap behind the pads is then identical to that of the hexagonal case. An assembly according to this US4306938 patent can not meet many of the functional specificities of a fuel assembly for RNR-Na reactor of fourth generation of ASTRID type. First, the sleeve being housed exactly in the hollow formed by the stamping platelets, its thickness is in fact limited to the drawing depth which is equal to half the distance between two adjacent assemblies, or typically of the order of 1.5 to 3 mm. This thickness is small and insufficient so that the equivalent stiffness of the assembly consisting of the pads and the sleeve, can increase the intrinsic stiffness of the wafers by a factor of 5. Then, the wafers being stamped continuously over the entire outer periphery of the housing they completely clog the space between assemblies. This prevents a circulation of sodium or disrupts the installation of a natural convection between the assemblies. Finally, because a wafer is stamped across the entire width of a face of the housing, the contact surface at the wafers of two adjacent assemblies is important. In addition, the pads are stamped in a ferritic steel housing, so the play at the platelets at the handling temperature is close to zero. These two aspects are in the sense of increasing the handling efforts during the extraction / insertion of the assembly in the network.

The patent application US2014 / 185734 discloses a fuel assembly for RNR-Na nuclear reactor, comprising a double-wall structure housing consisting of an inner tube housed in an outer tube of hexagonal section. The inner tube deforms under the effect of internal loads until coming into contact with the outer tube. The outer tube cash forces transmitted by the inner tube. The good distribution of the deformations / stresses between the two tubes makes it possible to limit the deformations on the outer tube. Additional stiffeners may be arranged between the two tubes to limit deformation of the outer tube. In this patent application, the problem concerns the transmission of forces, under the effect of the coolant pressure, from the inside of the structure of the internal assembly to the outside. This problem is the opposite of that which arises for a fourth-generation RNR-Na reactor fuel assembly of the ASTRID type, for which it is a question of limiting the forces coming from the outside of the structure inwards under the effect of the thrust forces of adjacent assemblies.

There is therefore a need to improve the spacing devices between adjacent fuel assemblies of a compact network within a nuclear reactor core, in particular in order to meet the specifications of the spacers of a fuel assembly for a reactor RNR-Na of fourth generation of ASTRID type.

The object of the invention is to respond at least in part to this need.

Presentation of the invention

To do this, the subject of the invention is an assembly for a nuclear reactor, in particular for a sodium-cooled fast neutron reactor (RNR-Na), comprising a longitudinal axis housing (X), each main face of the housing comprising, in its central part, a spacing plate with an adjacent assembly, comprising an outwardly projecting portion, the assembly further comprising a reinforcing sleeve attached and held inside the housing and arranged facing the pads .

According to one embodiment, the housing is of hexagonal section.

According to an advantageous embodiment, each wafer is a stamped wafer, the sleeve being arranged opposite stamping recesses of the wafers.

Preferably, each wafer has an outer surface of contact with the assembly, which has a rectangular shape.

According to an advantageous embodiment, the reinforcing sleeve further comprises an inclined straight edge forming a convergent connecting the inner periphery of the underside of the sleeve to the outer periphery of the underside of the sleeve and an inclined straight edge forming a divergent connecting the inner periphery of the sleeve. above the sleeve at the outer periphery of the top of the sleeve. The pads reinforced by means of a sleeve according to the invention constitute intrusive plates insofar as the reported sleeve is an obstacle to the flow of sodium in the assembly. Thus, by arranging a convergent upstream and a divergent downstream of the thick part of the sleeve, on the one hand minimizes the pressure losses and thus contributes to the realization of the internal thermohydraulic assembly, and other The contribution is made by reducing the amount of steel to maintain the negative drain coefficient (CFV) sought in the context of the ASTRID reactor.

Advantageously, the reinforcing sleeve has a height less than that of the plates, the heights being measured along the longitudinal axis (X).

In the case of stamped platelets, the reinforcing sleeve may, on the other hand, have a height greater than that of the stamped platelets, the heights being measured along the longitudinal axis (X). In this case, the sleeve comprises means for allowing the filling and emptying of liquid and the non-accumulation of gas inside each cavity formed between the sleeve and a wafer. These means may advantageously consist of at least two holes passing through the sleeve, which each open into each cavity formed between the sleeve and a plate, these holes being located respectively at the lower part and the upper part of the cavity.

According to a first embodiment, the reinforcing sleeve comprises a hollow cylinder of height greater than or equal to that of the wafers and of outer diameter substantially equal to the largest dimension of the internal cross section of the housing.

According to a second alternative embodiment, the reinforcing sleeve comprises a part whose outer periphery comprises hexagonal portions matching the hexagonal inner section of the housing. According to this variant, the inner periphery may be circular in cross section.

According to a third embodiment, the reinforcing sleeve comprises a part whose outer periphery is of hexagonal cross-section matching the hexagonal inner section of the housing. According to this variant, the inner periphery may also be of hexagonal cross-section, the height of the inner periphery of the part being less than that of its outer periphery.

According to an advantageous embodiment, the reinforcing sleeve is made of a material whose coefficient of thermal expansion and irradiation swelling is greater than that of the housing material comprising the wafers of the assembly.

In this mode, the reinforcing sleeve is preferably made of austenitic steel and the housing is preferably made of ferritic or martensitic steel.

According to an advantageous embodiment, in order to best fix the sleeve inside the housing, the reinforcing sleeve is attached to one or more structures of the assembly by attachment means disposed at the periphery of the internal section of the housing. housing. The assembly which has just been described advantageously constitutes a fuel assembly, the housing being intended to be inserted vertically into the support of the reactor core, the housing comprising an upper part forming the head of the assembly housing a device for superior neutron protection (SNP) consisting of neutron absorbents and a central portion housing nuclear fuel needles, the spacing plates being disposed in a plane above the fuel needles.

In this mode, the coupling means of the sleeve are rods connected to the lower structures of the PNS. The fuel assembly as defined makes it possible to meet the specifications of the spacers of a fuel assembly for RNR-Na reactor of fourth generation of ASTRID type. The assembly described may also constitute any other type of non-combustible assembly that can be inserted in a reactor core cooled with sodium RNR-Na, such as for example a reflector assembly, a Neutron Side Protection (NLP) assembly, a control, an experimental assembly, a complementary safety device, a breeder assembly or a transmutation assembly. The invention also relates to a method for producing an assembly described above, comprising the following steps: - production of a reinforcement sleeve with external dimensions adapted to the actual dimensions of the inner section of the assembly box in order to guarantee a reduced editing game; preheating the housing to increase the mounting clearance; - Insertion of the sleeve provided with its attachment means inside the preheated housing to its position opposite the wafers; - hooking of the sleeve to the internal structures of the assembly; cooling the housing. The invention also relates to the use of a fuel assembly described above in a fast neutron nuclear reactor RNR, such as a reactor cooled with gas or liquid metal, the liquid metal being selected from sodium, lead or lead-bismuth.

DETAILED DESCRIPTION Other advantages and features of the invention will emerge more clearly from a reading of the detailed description of the invention given by way of illustration and without limitation with reference to the following figures, in which: FIG. 1 is an external perspective view of a fuel assembly according to the state of the art, already used in a sodium-cooled nuclear reactor RNR-Na; FIG. 2 is a perspective view of a fuel assembly according to the prior art, which has already been used in the "Phoenix" nuclear reactor, the figure showing devices in the form of spacing plates with a fuel assembly adjacent in the reactor core; Figure 2A is a longitudinal sectional view of a fuel assembly envisaged for ASTRID at the beginning of the project, showing more precisely the positioning of the plates relative to the other elements of the assembly; FIGS. 3A and 3B are respectively perspective and cross-sectional views of a portion of a hexagonal fuel assembly cross-section housing according to FIGS. 2 and 2A showing the stampings forming the plates; Figure 3C is a front detail view of a wafer according to Figures 3A and 3B; Figure 4 is identical to Figure 3B and shows the deformation of a face of the housing when it is subjected to a force applied to a wafer while the three opposite faces of the housing are maintained; FIG. 5 is a cross-sectional view of a portion of a hexagonal cross-section housing fuel assembly showing a platelet spacing device according to FIG. 2A and an attached sleeve according to the invention, as envisaged for the ASTRID nuclear reactor; Figure 6 is a perspective view of a portion of a box hexagonal section with stamped plates showing a first variant of the sleeve according to the invention; Figure 7 is a perspective view of a portion of a hexagonal section box with stamped plates showing a second variant of the sleeve according to the invention; Figure 8 is a perspective view of a portion of a hexagonal section box with stamped plates showing a third variant of the sleeve according to the invention; Figure 9 is a perspective view of a portion of a hexagonal section housing with stamped plates showing the third variant of the sleeve according to the invention with its fixing means inside the assembly housing; Figure 10 is a detailed sectional view of an advantageous variant of a reinforcing sleeve according to the invention, for improving the thermohydraulic within the fuel assembly which is provided with; FIGS. 11A and 11B are schematic sectional views of another advantageous variant of a reinforcing sleeve according to the invention, making it possible to improve the thermohydraulics within the fuel assembly which is provided with them, FIG. a schematic sectional view of a reinforcing sleeve according to the invention with a dodecagonal section fixed in a hexagonal section assembly box.

For the sake of clarity, the same references designating the same fuel assembly elements and platelet spacers according to the prior art and according to the invention are used for all of Figures 1 to 12.

Throughout the present application, the terms "vertical", "lower", "upper", "lower", "high", "below" and "above" are to be understood by reference with respect to a fuel assembly such as that it is in a vertical configuration in a nuclear reactor.

Figures 1 to 3C relating to the state of the art have already been described in detail in the preamble and are therefore not discussed below.

On the basis of the fact that the spacing plates 2 with an adjacent fuel assembly did not have sufficient stiffness K to meet the specificities of a 4th generation RNR-Na nuclear reactor fuel assembly, the inventors analyzed the mode of deformation of the platelets.

Thus, the low stiffness of a wafer 2 made by stamping is characterized, by crushing force given on this wafer, by a significant displacement thereof.

The inventors have been able to demonstrate that this displacement or significant erasure of the wafer subjected to a force is caused firstly by the bending deformation of the face of the hexagonal case which integrates the wafer in question. This phenomenon has been reproduced by finite element calculation and is shown in FIG.

The inventors then thought to reinforce the inside of the stampings 2 by a reinforcing sleeve 3 added and held inside the hexagonal-section box 10 and positioned opposite the plates 20, more precisely at the hollow 21 of the stampings of the platelets 2.

A fuel assembly 1 incorporating such a sleeve 3 according to the invention, as it is intended to be used in an RNR-Na nuclear reactor of the ASTRID type, is shown in FIG. 5. As for the fuel assemblies according to the state of the art intended for RNR reactors, the assembly 1 according to the invention is of elongate shape along a longitudinal axis X and comprises a housing 10 having a hexagonal cross section, the end of the upper portion 11 of which forms the head of the assembly, and which surrounds a neutron protection device called PNS comprising neutron absorbents 18. The central portion 12 of the assembly 1 envelops fuel needles 14 forming the fissile area of the assembly. The assembly 1 finally comprises a lower portion not shown forming the foot of the assembly, in the extension of the housing 10, as in the assembly according to the state of the art. The foot of the assembly has a distal end cone-shaped or rounded to be inserted vertically into the bed base of a reactor core. The foot of the assembly also has at its periphery openings opening therein for the entry of sodium into the assembly.

As shown in FIG. 5, the reinforcing sleeve 3 according to the invention is housed and held inside the casing 10 above the fuel needles 14 and positioned facing the platelet embossing recesses 2.

Each stamped wafer 2 always has an outer surface 20 of contact with the adjacent assembly, which has a rectangular shape and which is not impacted by the sleeve 3, each wafer 2 still having a pressed recess 21 without material to the inside the case.

It is thus possible to consider platelets in other forms, such as circular, or any other form feasible and satisfying the constraints of spacing and rigidity sought.

The reinforcing sleeve 3 according to the invention makes it possible to limit the deformation by bending of each of the six faces of the casing 10 and thus to increase the stiffness of the stamped plates 2. The overall stiffness equivalent is then the sum of the stiffness of the stamped platelets. 2 and the stiffness of the sleeve 3.

The reinforcing sleeve 3 according to the invention being an insert inside the housing 3, it can be given a multitude of geometries (shape of the section, thickness, height) according to the stiffness and the constraints of manufacture / assembly inherent to the assembly.

It is thus possible to envisage sleeves in the form of rings with a circular, hexagonal, dodecagonal cross section and a square, trapezoidal, "T" (rib), "U" -shaped radial cross-section. In general, the Reinforcing sleeve 3 according to the invention can be easily adapted in terms of geometries, dimensions and / or materials.

Three variants of different sleeve geometry are shown in FIGS. 6 to 8, which can therefore be: a hollow cylinder 3 of outside diameter substantially equal to the internal gap of the housing 10 (FIG. 6); a piece 3 'whose outer periphery 30 is of hexagonal cross-section matching the inside of the casing 10 and whose inner periphery 31 is of circular cross-section (FIG. 7); a part 3 "whose outer periphery 30 is of hexagonal cross-section matching that inside the case 10 and whose inner periphery 31 is also of hexagonal cross section, the height of the inner periphery of the part being less than that of its periphery exterior. The outer periphery 30 can be connected to the inner periphery 31 by a central rib 32 (FIG. 8). The radial section of the sleeve is then shaped "T".

The choice of the material of the sleeve 3 is free. Advantageously, the sleeve 3 is made of a material whose coefficient of thermal expansion and swelling under irradiation is greater than that of the housing material comprising the wafers of the assembly. Preferably, the sleeve 3 is austenitic steel type AISI 316, because it is the material that has the best compromise vis-à-vis the functions to be completed in the context of a fuel assembly 1 for nuclear reactor RNR-Na of type ASTRID, provided with a case of martensitic steel of type EM10.

FIG. 9 shows an advantageous variant of attachment of the sleeve 3 "of FIG. 8. According to this variant illustrated, the sleeve 3" is hooked (suspended) to the fixed lower structures of the PNS (not shown) by means of rods 4 are each arranged in an angle of the internal hexagonal section of the housing 10. The attachment rods 4 shown are circular section but any other section may be suitable.

With such hooking rods 4, the mounting method of the fuel assembly is compatible with the reference assembly range already advanced for the ASTRID reactor, only the connection of the sleeve 3 with the PNS being added as a preliminary step . The advantage of the mounting method according to the invention of the sleeve 3 in the housing 10 is that it does not require additional mechanical connection between the sleeve 3 with the housing 10, of the welding, screwing, crimping ...

The sleeve 3 is inserted with play inside the housing 10. It must then be ensured that the sleeve clearance 3 is correctly calibrated with respect to the inner wall of the housing 10.

It is thus necessary to check the following conditions: - the clearance must be strictly positive and of a sufficient value when mounting the sleeve 3 in the housing 10, that is to say in an environment of assembly workshop to ambient temperature of about 20 ° C; - The clearance must be zero or slightly negative (clamping) nominal reactor operation, that is to say at a temperature of 550 ° C average platelets 2. This condition ensures good contact between the sleeve 3 and the housing 10 in order to effectively have the increase in stiffness of platelets sought in the context of the invention.

The two conditions on the game, described above can be fulfilled with a sleeve of the following characteristics: the sleeve 3 is made of austenitic steel, which has a coefficient of thermal expansion and a swelling under higher flux than the ferritic steel casing or martensitic. As a result, the backlash in operation at 550 ° C between sleeve 3 and casing 10 is facilitated; - For mounting the sleeve 3 in the housing 10, the latter is preheated to a temperature of about 100 to 200 ° C while the sleeve 3 is maintained at about 20 ° C. The difference in expansion between the two constituent steels thus makes it possible to increase the mounting clearance; - The external dimensions of the sleeve 3, which define the contact surface with the housing 10 are machined at the last moment before mounting and adapted according to the actual measurement of the inner gap of the TH. This eliminates manufacturing tolerances of the housing 10 which can be relatively large.

Because it is reported, the sleeve 3 according to the invention can be chosen with a height independent of the height of the wafers 2, unlike the sleeve solution according to US4306938 which has de facto the same height as that of the wafers . By way of example, the inventors consider that a sleeve 3, as shown in FIG. 10, with a useful height H1 equal to 50 mm for stamped plates 2 having a drawing height h of 80 mm is entirely satisfactory. With this useful height, the thickness E of the sleeve 3 necessary to increase the stiffness by a factor of 5 is 8.6 mm.

In order to minimize the pressure drops induced by a sleeve according to the invention, an advantageous variant illustrated in FIG. 10, consists in producing the sleeve 3 with an inclined straight edge 33 forming a convergent connecting the inner periphery 31 of the underside of the sleeve to the outer periphery 30 of the underside of the sleeve and an inclined straight edge 34 forming a diverging connecting the inner periphery 31 of the top of the sleeve to the outer periphery 30 of the top of the sleeve 3.

This variant is all the more effective as the thickness of the sleeve is important.

In the variant where the total height H3 of the sleeve 3 including the convergent 33 and the divergent 34 exceeds that of the stamped plates, the volume between the stamped plate 2 and the sleeve 3 is delimited by the stamping cavity 21 and forms a closed cavity . It is then necessary to provide, in the lower part of the convergent 33, at least one opening hole for filling and emptying the liquid sodium of this cavity. At least one other hole opening in the upper part of the diverging part may also be necessary in order to avoid trapping gas when filling the sodium cavity.

Instead of convergent 33 and divergent 34, other means may be provided which limit the disruption of the sodium flow in the assembly.

To illustrate this last point, there is shown in Figures 11A and IB, vent openings 35, 36 respectively larger or smaller in the thickness of the sleeve and allow to pass through the sodium. At this stage, the inventors believe that this solution is less optimized than a convergent / divergent solution, and that it can not be applied to high throughput assemblies or those requiring high platelet stiffness.

With a sleeve 3 of low height in front of that of the plates, it is possible to guarantee a globally negative drain coefficient (CFV core) which is a key point of the ASTRID reactor. Indeed, studies carried out by the inventors have shown that the parameter minimizing the increase of reactivity in case of emptying is a low sleeve height 3, even if its thickness must then be increased to remain at equivalent stiffness.

In addition, it is also shown by calculation that the stiffness of a sleeve 3 stressed in compression varies linearly with its height, but varies with the cube of its thickness.

To maximize the stiffness of the sleeve 3 according to the invention, and therefore that of the stamped plates 2, it is then preferable to consider a sleeve 3 of low height and thick.

The inventors have finally shown, through the various studies, that the invention was able to meet the need for fuel assemblies for the ASTRID reactor, namely an increase in platelet stiffness by a factor of 5 with respect to state of the art while maintaining a negative drain coefficient, and being compatible with the handling, manufacture and thermohydraulics of the reactor.

A fuel assembly 1 according to the invention which has just been described thus makes it possible to meet all the functional specifications of spacing of a fuel assembly of a fourth-generation RNR nuclear reactor such as ASTRID. The invention has been described in the particular context of a fuel assembly for a fourth generation RNR nuclear reactor such as ASTRID, that is to say CFV core, which involves many constraints to be checked simultaneously.

In other RNR reactors or for non-combustible assemblies, some of these constraints can be relaxed or abandoned, as mainly: a non-CFV core architecture allows a quasi free choice of sleeve dimensions (unrestricted steel quantity); the absence or low flow of sodium makes it possible to dispense with a convergent 33 and a divergent 34 on the sleeve 3 as shown in FIG.

Also, in a general way, that is to say if one goes out of the particular context of the ASTRID reactor, the invention applies to any type of assembly: whatever the geometry, the cross-section, the radial section, the dimensions of the sleeve. In particular, a sleeve with a dodecagonal cross-section could be envisaged in a box with a hexagonal cross-section as shown in FIG. 12; whatever the type of wafers: it may be platelets stamped in the housing, but also any other type of non-stamped and non-intrusive wafers, such as inserts reported on the outside of the housing; whatever the geometry of the pads on the case: it can be a rectangular shape, circular, square, or other. Other variants and improvements may be provided without departing from the scope of the invention.

Thus, if the reinforcing sleeve described 3 is reported in a fourth-generation fast neutron nuclear fuel (RNR) fuel assembly of the ASTRID type, it can be envisaged to do so to any other type of RNR whose spacing between the assemblies must be ensured by the housing of the assemblies and must be guaranteed with a certain stiffness. It can be all gas-cooled, sodium, lead, lead-bismuth, etc.

If the reinforcing sleeve described 3 is reported in a fuel assembly, it can also be added to reinforce any other type of assembly present in a core of RNR, such as a reflector assembly, a Neutral Neutron Protection (NLP) assembly. , a control bar, an experimental assembly, a complementary safety device, a breeder assembly, a transmutation assembly, etc. Other means of attachment of the reinforcing sleeve 3 that the attachment rods 4 to the PNS can be considered in the context of the invention. The inventors believe that any connection based on patella fasteners is preferable in particular to overcome hyperstatisms that are potential sources of mounting lock or effort in operation. Reference cited [1]: Manual "Sodium-cooled fast neutron reactors" - The techniques of the Engineer B 3 171

Claims (23)

CLAIMS 1 Assembly for a nuclear reactor, particularly for a sodium-cooled rapid neutron reactor RNR-Na, comprising a housing (10) of longitudinal axis (X), each main face of the housing comprising, in its central part, a plate ( 2) spacing with an adjacent assembly, comprising an outwardly projecting portion, the assembly further comprising a reinforcing sleeve (3) attached and held within the housing (10) and arranged facing the platelets (2). 2. Assembly according to claim 1, the housing (10) being of hexagonal section. 3. Assembly according to claim 1 or 2, each plate being a stamped plate (20), the sleeve being arranged opposite stamping recesses (21) of the plates (2). 4. Assembly according to one of the preceding claims, each plate having an outer surface (20) of contact with an adjacent assembly, which has a rectangular shape. 5. Assembly according to one of the preceding claims, the reinforcing sleeve (3) further comprising a tapered straight edge forming a convergent (33) connecting the inner periphery (31) of the underside of the sleeve to the outer periphery (30) of the below the sleeve and an inclined straight edge forming a diverging portion (34) connecting the inner periphery (31) of the top of the sleeve to the outer periphery (30) of the top of the sleeve. 6. Assembly according to one of claims 3 to 5, the reinforcing sleeve (3) having a height less than that of the pressed recess (21) of the stamped plates (2), the heights being measured along the longitudinal axis. (X). 7. Assembly according to one of claims 3 to 5, the reinforcing sleeve (3) having a height greater than or equal to that of the stamping recess (21) of the plates (2), the heights being measured along the axis longitudinal (X), the sleeve comprising means for allowing the filling and emptying of liquid and the non-accumulation of gas within each cavity formed between the sleeve and a wafer. 8. The assembly of claim 7, the means consisting of at least two holes passing through the sleeve, each of which opens into each cavity formed between the sleeve and a plate, these holes being located respectively at the lower part and the upper part. of the cavity. 9. Assembly according to one of the preceding claims, the reinforcing sleeve comprising a hollow cylinder (3) of height greater than or equal to that of the plates (2), and of outer diameter substantially equal to the largest dimension of the cross section. internal case. 10. Assembly according to one of claims 2 to 9, the reinforcing sleeve comprising a part (3 ') whose outer periphery (30) comprises hexagonal portions conforming to the hexagonal inner section of the housing. 11. The assembly of claim 10, the piece (3 ') having an inner periphery (31) circular section. 12. Assembly according to one of claims 2 to 9, the reinforcing sleeve comprising a part (3 ") whose outer periphery (30) is of hexagonal cross-section matching the hexagonal inner section of the housing. 13. The assembly of claim 12, the piece (3 ") having an inner periphery also hexagonal cross section, the height of the inner periphery of the workpiece being less than that of its outer periphery. 14. Assembly according to one of the preceding claims, the reinforcing sleeve being made of a material whose coefficient of thermal expansion and swelling under irradiation is greater than that of the housing material comprising the wafers of the assembly. 15. The assembly of claim 14, the reinforcing sleeve (3) being made of austenitic steel. 16. Assembly according to claims 14 or 15, the housing (10) being made of ferritic steel or martensitic. 17. Assembly according to one of the preceding claims, the reinforcing sleeve (3) being attached to one or more structures to the assembly by attachment means (4) disposed at the periphery of the internal section of the housing. 18. Assembly (1) in particular for a sodium cooled reactor RNR-Na, according to any one of the preceding claims, constituting a fuel assembly, the housing being intended to be inserted vertically in the support of the reactor core, the housing comprising an upper portion forming the head of the assembly (11) housing an upper neutron protection device (2) (PNS) and a central portion (12) housing nuclear fuel needles (14), the spacing pads (2) being arranged above the fuel needles. 19. fuel assembly (1) according to claim 18 in combination with claim 17, the coupling means (4) of the sleeve being rods connected to the lower structures of the PNS. 20. Assembly (1) in particular for a sodium cooled reactor RNR-Na, according to any one of claims 1 to 17, constituting a reflector assembly, a lateral Neutron Protection (NLP) assembly, a control rod, an assembly. experimental, a complementary safety device, a breeder assembly or a transmutation assembly. 21. A method of producing an assembly according to any one of the preceding claims, comprising the following steps: - realization of a reinforcing sleeve (3) with external dimensions adapted to the actual dimensions of the inner section of the housing (10). assembly to guarantee a reduced mounting clearance; - preheating the housing (10); - Insertion of the sleeve (3) provided with its attachment means (4) inside the housing (10) preheated to its position opposite the wafers; - hooking of the sleeve to the internal structures of the assembly; - cooling of the housing (10). 22. Use of a fuel assembly (1) according to one of claims 18 or 19 or any other non-combustible assembly according to claim 20, in a fast neutron nuclear reactor. 23. Use according to claim 22, the reactor being cooled with gas or liquid metal, the liquid metal being selected from sodium, lead or lead-bismuth.