WO2014037901A1

WO2014037901A1 - Nuclear control device for an fnr liquid metal-cooled reactor

Info

Publication number: WO2014037901A1
Application number: PCT/IB2013/058319
Authority: WO
Inventors: Pierre Allegre; Frédéric VARAINE
Original assignee: Commissariat A L'energie Atomique Et Aux Energies Alternatives
Priority date: 2012-09-07
Filing date: 2013-09-05
Publication date: 2014-03-13
Also published as: FR2995437A1; FR2995437B1

Abstract

The invention relates to a nuclear control device (17), intended for being inserted in a dedicated location among a plurality of fuel assemblies (6) which in turn are inside a core (4) of a liquid metal-cooled reactor, said assemblies (6) having a shape that is elongated along a longitudinal axis X, each comprising a cladding (6A) having a portion (60) surrounding the fissile area (6') and a foot portion (62) forming an extension of the cladding, the section thereof that is transverse to the axis X thereof being smaller than that of the cladding (6A), the foot (62) being inserted into an opening of the diagrid (40) of the core (4), supporting the assembly (6) therein, the axis X thereof being vertical. According to the invention, the device (17) includes a support stand female element (19) in particular which, by design, prevents a fuel assembly from descending to the opening (400) of the diagrid (40). The invention enables attempts to mistakenly insert a fuel assembly (6) instead of a nuclear control device (17) into the dedicated location of the latter to be prevented.

Description

Nuclear control device for liquid metal cooled reactor type RNR Technical field

The present invention relates to a fast neutron nuclear reactor cooled with liquid sodium called RNR-Na or SFR (acronym for "Sodium Fast Reactor") or any other reactor cooled with a liquid metal and which may be part of the family of reactors so-called fourth generation.

In a SFR nuclear reactor, the permanent control of nuclear chain reactions is ensured by means of nuclear control devices each consisting of at least nuclear control rods.

The non-transparent nature of liquid sodium makes it impossible to visualize nuclear fuel assembly handling operations and nuclear control devices (control or shutdown) by a camera system. One of the situations that could be penalizing is the one according to which during a handling, is inserted in place of a nuclear control device a fuel assembly.

The invention therefore relates to a new nuclear control device to avoid this situation.

Although described with reference to 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 tank also containing and heat exchangers, the invention is also applies to 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 formed of fuel elements (containing fissile material) and loaded and / or discharged in one piece in / 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 metal, in particular liquid sodium said RNR-Na or SFR (acronym for "sodium Fast Reactor ").

By nuclear control device is meant a device comprising at least neutron absorber elements for controlling the nuclear chain reactions in the fuel assemblies within a reactor core. State of the art

FIG. 1 shows a sodium-cooled nuclear reactor (SFR) with an integrated type of architecture, as already retained in France in the "Superphénix" reactor or as planned in France under the name EFR: see publication [1].

Such a reactor 1 comprises a reactor vessel 2 filled with liquid sodium 3, called primary sodium, and inside which is present the heart 4. A plug 5, said cap heart cover, is arranged at the heart of the heart 4. Within heart 4 are implanted a plurality of fuel assemblies 6, only one of which is shown in FIG. 1. More exactly, each fuel assembly 6 is inserted into a support, usually called bed base 40, below the core 4. being held vertically.

In such a reactor 1, the extraction of the heat produced during the nuclear reactions within the core 4 is carried out by circulating the primary sodium 3 by means of pumping not shown, arranged in the reactor vessel 2, to Intermediate exchangers 7. Thus, the primary sodium enters an intermediate exchanger 7 through its inlet window 70 passes through the exchanger tubes, gives up its heat to the secondary sodium before emerging through the outlet window 71 of the exchanger. Thus, the extraction of heat is carried out by the secondary sodium reaching cold through its supply duct 8 to an intermediate exchanger 7 before coming out hot through its outlet duct 9. The heat extracted is then used to produce heat. the steam in steam generators not shown, the steam produced being fed into one or more turbines also not shown. The turbine (s) transform (s) the mechanical energy of the steam into electrical energy.

The reactor vessel 2 is separated into two distinct zones 3A, 3B by a separation device consisting of at least one tank 10 arranged inside the reactor vessel 2. This separation device is also known by the name of redan. In general, as shown in Figure 1, the separating device consists of a single inner vessel 10 whose shape is frustoconical in its lower part and cylindrical in its upper part. As described and claimed in the application WO 2010/057720A1, the separation device may also consist of two walls of homothetic shape and through each of which the heat exchangers are arranged with clearance. As illustrated in FIG. 1, the zone 3 A of primary sodium delimited internally by the inner vessel 10 collects the sodium leaving the core 4 and feeds the intermediate exchangers 7: it constitutes the zone in which the sodium is the hottest and so is commonly called hot zone 3A or hot collector. The zone 3B of primary sodium delimited between the inner vessel 10 and the reactor vessel 2 collects the primary sodium which has been cooled in the intermediate exchangers and feeds the pumping means: it constitutes the zone in which the sodium is the coldest and so is commonly called cold zone 3B or cold collector. As illustrated in FIG. 1, a seal is produced by the inner vessel 10 and around all the components passing through it, such as the intermediate exchangers 7: this forces the primary sodium to circulate in a necessarily closed loop respectively from the core 4 to the hot collector 3A, then to the intermediate exchangers 7, the cold collector 3B, pumping means not shown and again through the heart.

As illustrated in FIG. 1, the reactor vessel 2 is closed by a closure slab 11 supporting the various components, such as the intermediate exchangers 7, the core cover cap 5 and the pumping means. The space 12 between the closure slab 11 and the free levels 30A, 30B of sodium, commonly called stack sky, is filled with a neutral gas vis-a-vis sodium, typically Argon.

When a new fuel assembly 6 is loaded into the reactor core 4 or a spent fuel assembly is discharged, i.e. an assembly 6 in which there is insufficient fissile material to maintain nuclear reactions in the reactor, a handling device is used. Such a handling device is for example described in patent FR 2235462. In a schematic manner, such a handling device comprises two rotating plugs 13, 14 which can rotate in the closure slab 11 being eccentric with respect to the other and a recovery arm 15 passing through the closure slab 11. The position, the eccentricity and the relative diameters of each of the two rotating plugs 13, 14 allow the recovery arm to be positioned vertically to any point of the heart. 4 and in particular at the vertical of any of the fuel assemblies 6 to be implanted or already implanted in the core 4. The recovery arm 15 comprises at its lower end a gripping grapple not shown adapted to catch an assembly 6 by its head 61 which forms a gripping head. A fuel assembly 6 is thus caught by the grapple, and thanks to a vertical translation movement of the grapple carried by the arm of recovery and appropriate rotations of the two plugs 13, 14, the fuel assembly 6 is transferred.

FIG. 2 shows a fuel assembly 6 already used in an SFR nuclear reactor known under the name "Phoenix". Such an assembly 6 of elongate shape along a longitudinal axis X firstly comprises, as a central portion, a first tube 60 of hexagonal section which forms a housing and which envelops unrepresented fuel rods. The assembly comprises, as an upper portion forming the head of the assembly, a second tube 61 in the extension of the first tube 60 of the same hexagonal section as this tube 60, and which usually envelops neutron absorbent pellets not shown . In other words, the tubes 60, 61 form the same tubular envelope 6A of identical hexagonal section over its entire height. The head 61 of the assembly has a central opening 610 opening therein. The assembly 6 finally comprises a lower portion 62 forming the foot of the assembly, a third tube 62 in the extension of the envelope 6A, and more precisely in the extension of the first tube 60. The foot 62 of the present assembly a distal end 620 cone-shaped or rounded to be inserted into the bed base 40 of the heart 4. The foot 62 of the assembly has at its periphery openings 621 opening therein.

Thus, in the installed configuration of a fuel assembly, that is to say in the loaded position in the core 4 (FIG. 1), the foot 62 of an assembly 6, of male shape, is inserted into an opening of the bed base 40 of the reactor and thus maintaining the assembly 6 in the latter with its longitudinal axis X vertically. The primary sodium can circulate inside the assembly 6 and thus take the heat released by the fuel rods by thermal conduction. The sodium is thus introduced through the openings 621 of the foot 62 and out through the central opening 610 of the head 61.

As best illustrated in Figure 2, the foot section 62 of the assembly is smaller than the section of the central portion 60 of the assembly. The connection 600 between these two sections 60, 62 forms a more or less rounded or conical shoulder so as to be able to make a sphere / cone type connection with the bed 40 of the reactor core 4.

The central portion 60 of an assembly comprises a plurality of nuclear fuel rods. Each pencil is in the form of a sheath inside which is stacked a 6 'column of pellets of fissile material in which produce nuclear reactions that give off heat. All the columns 6 'define what is usually called the fissile zone and is approximately halfway up an assembly 6. It is shown schematically in the form of a black rectangle in FIG.

As in all nuclear reactors, in the SFR 1 reactor, permanent control of the nuclear chain reactions is ensured by means of nuclear control devices each constituted at least by control or stop bars, also called control rods, containing neutron absorbents which are, for example, boron-based. As illustrated in FIG. 1, these bars 16 are movable through the heart cover plug 5 in the core 4 of the reactor by a mechanism not shown: they can be raised or extracted depending on the number of neutrons to be absorbed and thus drive the reactor. More exactly, in an SFR reactor, a bar 16 is inserted in a dedicated location within a plurality of fuel assemblies 6 already implanted in the core 4.

One of the handling constraints of fuel assemblies and nuclear control devices in an SFR reactor is related first of all to the fact that in such a reactor, the intrinsic opacity of the coolant (liquid sodium) forces the handling of assemblies and devices control in the reactor core without having direct visualization by a camera visualization system that can be found in particular in pressurized water nuclear reactors.

Thus, there is the risk, during handling, accidental insertion of a fuel assembly instead of a nuclear control devices, which can lead to an incident at least penalizing its taking into account. the reactor's nuclear control system as a whole in order to avoid any risk of accidental divergence.

To avoid this risk, it has already been proposed in the "PHENIX" reactor put into service in France, to set up in the dedicated location, a guiding sleeve of a control bar, and to lock in position this sheath by the six fuel assemblies surrounding it contiguously. This solution has a major disadvantage of swelling under irradiation of this sheath, because it is found directly opposite the fissile areas of these six assemblies. Similarly, it has already been proposed in the "SUPERPHENIX" reactor, to set up in the dedicated location, a guiding sheath of outer section identical to the hexagonal one of the casing 6A of a fuel assembly 6 described below. above, with a nuclear control bar sliding vertically inside this sheath to control more or less nuclear reactions.

In the two PHENIX and SUPERPHENIX reactors, a pin adapted to separate the assemblies and / or the control rods according to the flow rate necessary for their cooling was implanted at a part called candle in the bed base of the heart. This solution has the major disadvantage of a risk of introduction of a fuel assembly, by mistake, up to a few centimeters from the normal position (risk of criticality proven) instead of a nuclear control device.

However, in the two PHENIX and SUPERPHENIX reactors, the reactor can not be started in case of error, that is to say in case of accidental insertion of a fuel assembly instead of one of the devices. nuclear control. Indeed, a prohibition lock which is arranged at the foot of the assembly prohibits the assembly to be completely inserted into the bed base. That being so, the insertion error is taken into account only after the foot of an assembly has already been introduced, that is to say that this assembly is already coupled with the other assemblies at the fissile zone level. . In other words, in the two aforementioned reactors, there still remained a risk of criticality, that is to say a risk of triggering an uncontrolled fission chain reaction.

The general object of the invention is to overcome all or part of the disadvantages of the prior art and therefore to propose a solution to increase the safety of liquid fast neutron reactors cooled with liquid sodium RNR-Na or SFR, more generally to metal liquid, equipped with nuclear control devices, in particular by prohibiting the possibility of insertion of a fuel assembly in place of such a device.

A particular object of the invention is to propose a solution to prohibit the possibility of insertion of a fuel assembly in place of a nuclear control device while minimizing the risk of swelling under irradiation of its elements. Another particular object of the invention is to prohibit the possibility of insertion of a fuel assembly instead of a nuclear control device while allowing the reconfiguration of the nuclear reactor core, that is to say say the possibility of arranging fuel assemblies and nuclear control devices with respect to each other in the reactor core.

Presentation of the invention

To do this, the subject of the invention is a nuclear control device, intended to be inserted into a dedicated location within a plurality of fuel assemblies themselves within a reactor core cooled with the liquid metal, said assemblies of elongate shape along a longitudinal axis X each having an envelope with a portion enveloping the fissile zone and a foot portion in the extension of the envelope and whose cross section on its axis X is smaller than that of the envelope, the foot being inserted into an opening of the bed base of the heart while maintaining the assembly in the latter with its axis X vertically,

the nuclear control device comprising:

at least one nuclear control bar containing neutron absorbers defining an absorbing zone;

an element called insertion element, of elongated shape along a longitudinal axis XI comprising:

a casing forming a guiding sheath of the nuclear control bar between an extreme withdrawal position in which the absorbent zone is not housed in the sheath and an end position in which the absorbing zone is completely opposite a zone of an adjacent fuel assembly when the nuclear control device is inserted into the plurality of fuel assemblies,

A foot portion intended to be inserted into an opening of the bed base of the heart while maintaining the insertion element in the latter with its axis XI parallel to the axis X of the fuel assemblies,

• a foot extension portion between the guide sleeve and the foot; the extension being of cross section to the axis XI greater than that of the foot; the extension of the foot and the foot being of male type, a socket-like female element of elongate shape along a longitudinal axis X2, dimensioned to allow the insertion of the extension and the foot of the insertion element with projection of the latter beyond the base, and if necessary, the engagement of a foot of a fuel assembly but without projecting beyond the base, the base being intended to be at least in abutment against the opening of the bed base at the dedicated location so on the one hand to allow insertion of the foot of the insertion element in this opening and on the other hand to prevent the insertion of the foot of a fuel assembly into this opening.

In other words, the nuclear control device consists essentially of three distinct parts, namely respectively from bottom to top in configuration installed in the reactor core:

a lower fixed part with an anti-insertion function in the bed base of a fuel assembly,

a mobile part formed by at least one control bar containing the neutron absorbents and enabling the control of the nuclear reactor,

an upper fixed part ensuring the height guidance of the mobile part to more or less control the nuclear reactions.

Thanks to the invention, the attempt to insert by mistake a fuel assembly in place of a nuclear control device in its dedicated location is not possible because the base prohibited by design the descent of said fuel assembly until 'at the opening of the bed base.

Thus, the base according to the invention which prohibits during handling operations the establishment of a fuel assembly other than a control device at the dedicated location, which eliminates any risk of unwanted reactivity at the heart.

Furthermore, since the foot of the insertion element may be identical to a foot of a standard fuel assembly, ie designed to be inserted into a standard opening of a reactor bed, and the base may be placed above any standard opening, the device according to the invention can be implanted at any location on the bed base. The base according to the invention can be replaced or moved according to the operating life of the reactor. In other words, the solution according to the invention allows some flexibility for heart power management, since a nuclear control device can be inserted into any opening of the core bed.

Compared to the solutions according to the state of the art used up to now, the solution according to the invention considerably improves the safety of an SFR nuclear reactor, since it only allows the insertion of a nuclear control device in a site reserved for it, by preventing the untimely insertion of a fuel assembly at this location.

Advantageously, the length of the base is further dimensioned to block, in the event of inadvertent insertion of a fuel assembly into the opening at the dedicated location, said assembly in a position such that its fissile zone is not opposite. from that of another adjacent fuel assembly already inserted into an opening of the bed base. In other words, the geometry of the base of the nuclear control device makes it possible to guarantee that the fissile zone of a fuel assembly introduced by mistake into the location dedicated to the device will not be at the level, that is, at the same level. altitude, the fissile area of the assemblies already inserted into the reactor core.

Preferably, the length of the base is further dimensioned so as not to be opposite a fissile zone of an adjacent fuel assembly already inserted in an opening of the bed base. This allows the base to not be subjected to the intense neutron flux of the fissile zones of the fuel assemblies inserted in the heart and which surrounds it contiguously, since it is located below them. This avoids, at the very least reduces, the risk of swelling under irradiation of the female part.

More preferably, the length of the base is further dimensioned to block, in case of inadvertent insertion of a fuel assembly into the opening at the dedicated location, said assembly in a position such that it can be detected by a control system for handling fuel assemblies in the reactor vessel. The blocking can thus be achieved with an inadvertently inserted assembly which projects at least one meter above the assemblies already inserted into the core. Obviously, the height difference in the heart between the normal position of a fuel assembly inserted at a location dedicated to it and the wrong one, that is to say with an assembly in abutment against the base and not inserted in the bed base, is a function of the position of the fissile zone in the assemblies and consequently of the absorbent zone in a control device according to the invention. This avoids any risk of criticality locally at the assemblies in question or fortuitous divergence.

According to an advantageous characteristic, the length of an insertion element considered between the end of the guide sleeve and the foot is equal to the length of a fuel assembly. It is thus possible to carry out the handling of a nuclear control device with the same means and in the same manner as those used for the handling of fuel assemblies in the reactor vessel.

According to another advantageous characteristic, the insertion element comprising a shoulder in the form of a truncated cone or spherical between its extension foot and its foot, Γ shoulder allowing a sphere / cone support against the inner wall of the opening of the bed base .

According to an advantageous embodiment, the base has at least one locking lug, preferably three lugs angularly distributed at 120 ° from one another, which extends (ent) transversely to its axis X2 to lock the positioning of the base against the bed base by wedging (s) the pin (s) by an assembly (s) adjacent fuel (s). The lug, advantageously the three lugs 120 ° from each other both to ensure that during the extraction of a nuclear control device according to the invention of the reactor core, the base remains in place, that is to say in support against the bed base of the reactor, and detect any seizure of the insertion element in the female part. On this last point, in fact, in the event of galling during the upward extraction of the reactor between the pedestal and the insertion element according to the invention, the difference of effort at the level of a handling grapple is important because instead of extracting only an insertion element, it supports the weight of the base with one or three fuel assemblies. This variation of effort to be supported by the grapple is easily measurable during extraction.

The base preferably has a hexagonal section transversely to its axis X2, identical to the hexagonal section of the casing of a fuel assembly. Guidance and positioning of the base, which are difficult to ensure in a liquid metal reactor, particularly liquid sodium, because of the lack of visibility due to the opacity of the liquid metal, and are advantageously achieved by the fuel assemblies which surround it contiguously. In case of reconfiguration of the heart, when the base comprises three locking lugs, removing the three adjacent fuel assemblies which wedge the base by its lugs, holding the other three assemblies in place, then proceeds to the extraction of the base, its guidance then being provided advantageously by the other three assemblies contiguous, remained in place.

Advantageously, the guide sleeve has a hexagonal section transversely to its axis X2, identical to the hexagonal section of the casing of a fuel assembly. Thus easy guidance of the insertion element by six fuel assemblies which surround it contiguously.

Preferably, the material constituting the base has the same coefficient of thermal expansion as that of the extension and the foot of the insertion element. This reduces the risk of seizure by thermal expansion between the base and the insertion element during operation of the reactor.

The subject of the invention is also a process for implementing the measuring device which has just been described, the tank being the reactor vessel of a fast neutron nuclear reactor cooled with liquid metal, such as liquid sodium ( SFR), a method in which the foot of the insertion element is pressed against the bed base around an opening of the reactor at a dedicated location, the insertion element is inserted into the opening through the foot. then the nuclear control rod is moved to accommodate at least part of it in the guide sleeve.

Finally, the subject of the invention is a liquid-cooled sodium fast neutron nuclear reactor (SFR), comprising at least one nuclear control device which has just been described, inserted into the bed base of the core while being contiguously surrounded by six SFR fuel assemblies.

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 among which:

- Figure 1 is a schematic sectional view of the tank of a SFR sodium-cooled nuclear reactor according to the state of the art, showing different handling positions of a fuel assembly and those of the nuclear control rods; - Figure 2 is an external perspective view of a nuclear fuel assembly according to the invention and already used in a SFR sodium-cooled nuclear reactor;

FIG. 3 is a side view in section of a part of a nuclear reactor core SFR made at the level of the upper plate of the bed base respectively showing an authorized insertion position of a fuel assembly, a position of insertion of a nuclear control device according to the invention at a dedicated location and finally an unauthorized position of a fuel assembly at a location dedicated to a control device according to the invention;

FIG. 4 is an enlarged view of the lower parts of the fuel assemblies and of a nuclear control device according to FIG. 3;

FIG. 4A is a detailed perspective view of the lower part of a fuel assembly and two nuclear control devices according to FIG. 3;

- Figure 5 is a perspective view of an embodiment of the base of the nuclear control device according to the invention.

FIG. 1 relates to an integrated type SFR nuclear reactor according to the state of the art and showing different handling positions of a fuel assembly to be respectively inserted in the heart and above the heart, as well as the positions of the nuclear control rods 16. FIG. 2 relates to a nuclear fuel assembly according to the invention and already used in an SFR-cooled nuclear reactor. These figures 1 and 2 have already been commented on in the preamble and are therefore not more so hereinafter.

For the sake of clarity, the same references designating the same reactor elements, fuel assembly and nuclear control device according to the invention are used for all of Figures 1 to 5.

Throughout the present application, the terms "vertical", "lower", "upper", "lower", "high", "below" and "above" are to be understood by reference to a tank filled with liquid sodium, a control device according to the invention and respectively of a fuel assembly such that they are in vertical configuration of operation. Thus, in an operating configuration installed in the heart, a fuel assembly 6 vertically has its foot down. Similarly, in an operating configuration installed in the heart, the control device 17 vertically has its foot down.

Throughout the present application, the term "next to" is to be understood to mean adjacent and at the same altitude, i.e. at the same depth in the reactor vessel. Thus, when the absorbent zone 16 'of a nuclear control device 17 is not facing the fissile zone 6' of a fuel assembly 6, it means that it is not at the same depth in the reactor vessel as the fissile zone 6 'even though the nuclear control rod 16 containing it may be adjacent to said assembly 6.

As illustrated in FIG. 3, the nuclear control device 17 is in its operating configuration, that is to say inserted into a dedicated location within a plurality of fuel assemblies 6 themselves within a reactor core 4 cooled to the liquid metal, with each foot assembly 62 inserted in an opening the upper plate 40A of the bed 40 maintaining the assembly with its axis X vertically.

The nuclear control device according to the invention comprises first of all as a moving part a nuclear control bar 16 containing neutron absorbers defining an absorbing zone 16 '.

It also comprises as upper fixed part, an element 18 said insertion element, of elongate shape along a longitudinal axis XI.

This insertion element 18, in the form of a single piece in the example shown, comprises a guide sleeve 180 of the nuclear control bar 16 between an extreme position of withdrawal in which the absorbent zone 16 'is not housed in the sleeve and an engaged end position in which the absorbent zone 16 'is completely opposite a fissile zone of an adjacent fuel assembly when the nuclear control device is inserted into the plurality of fuel assemblies 6. It also comprises a foot 181 inserted in an opening 400 of the upper plate 40A of the bed 40 maintaining the insertion element 18 in the latter with its axis XI parallel to the axis X of the fuel assemblies 6. An extension of the foot 182 is provided between the guide sleeve 180 and the foot 181. The extension 182 is of cross section to the axis XI greater than that of the foot 181. The device 17 also comprises, as a lower fixed part, a pedestal 19, of elongate shape along a longitudinal axis X 2 which is at least in abutment with the opening 400 of the bed 40 at a dedicated location in the installed configuration of the device 17 (FIG. and 4). This base 19 is of female type and is dimensioned to allow the nesting of the extension 182 and the foot 181 with projection thereof beyond the base 19 (right part in Figures 3 and 4) and the interlocking a foot 62 of a fuel assembly 6 but without projecting the latter beyond the base 19 (left part of Figures 3 and 4). Thus, the base 19 on the one hand allows the insertion of the foot 181 in the opening 400 (right part in Figures 3 and 4) and on the other hand prevents the insertion of the foot 62 of a fuel assembly 6.1 in this opening 400 (left part of Figures 3 and 4). It is specified here that one designated by a single reference 400, all the openings made in the upper plate 40A of the bed base and dedicated only to the insertion of a foot 181 of an insertion element 18 according to the invention.

As best illustrated in FIG. 3, the length L of the base 19 is dimensioned for both:

preventing the fissile zone 6 M from a fuel assembly 6 introduced by mistake into the base 19 to come to the level of the fissile zone 6 'of the other fuel assemblies properly inserted into the reactor core; that is to say to prevent the fissile zone 6'.1 from facing a fissile zone 6 'of an adjacent fuel assembly 6 already inserted in an opening of the bed base;

prevent any point of the base 19 to be facing a fissile zone 6 'of an adjacent fuel assembly 6 already inserted into an opening of the bed base;

blocking the fuel assembly 6.1 inadvertently inserted in a position such that it can be detected by a control system for handling fuel assemblies in the reactor vessel; typically the assembly 6.1 projects from a height of at least 1m above the assemblies 6 properly inserted into the core.

As best illustrated in Figure 4, there is provided a shoulder 183 shaped truncated cone or spherical between the extension 182 and the foot 181 of the insertion element. This shoulder allows a sphere / cone bearing against the inner wall of the opening 400 of the bed 40. FIG. 4A shows in detail a fuel assembly 6 between two adjacent pedestals 19 with the support of the right pedestal in the figure made by pressing the assembly 6 on a pin 190.

An advantageous example of a base 19 according to the invention is shown in FIG. 5. The base 19, in one piece, is hexagonal in section identical to that of a fuel assembly 6, and it comprises three locking lugs 190 to 120 ° one of the other transversely to its axis X2. Each of these three lugs 190 is jammed by an adjacent fuel assembly 6. Thus, such locking position of the base 19 ensures that the latter can not be extracted by the handling device at the same time as the insertion element 18. In case of reconfiguration of the heart is that is to say a relative change of position of the fuel assemblies 6 and nuclear control devices according to the invention 17, a base 19 can be released by extraction up the reactor of the three contiguous assemblies and resting on the locking lugs 190. Guidance during extraction or during placement and positioning is provided by the other three adjacent fuel assemblies held in place.

The base 19 is advantageously made of the same material constituting the foot 181, which makes it possible to avoid any seizure problem by relative thermal expansion between these two elements.

The base 19 advantageously has, in its upper end, a portion 191 which allows its gripping, preferably, by the same handling device used for the handling of the fuel assemblies 6 or insertion elements 18 in the tank 10 of the reactor 1.

The invention is not limited to the examples which have just been described; it is possible in particular to combine with one another characteristics of the illustrated examples within non-illustrated variants. Reference cited

[1]: Manual "Sodium-cooled fast neutron reactors" - The techniques of the Engineer B 3 171

Claims

A nuclear control device (17), intended to be inserted into a dedicated location within a plurality of fuel assemblies themselves within a reactor core (4) cooled to the liquid metal, said assemblies ( 6) of elongated shape along a longitudinal axis X each comprising an envelope (6A) with a portion (60) enveloping the fissile zone (6 ') and, a foot portion (62) in the extension of the envelope and whose cross-section at its axis X is less than that of the envelope (6A), the foot (62) being inserted into an opening of the bed base (40) of the heart while maintaining the assembly (6) in the latter with its axis X vertically,

the nuclear control device (17), comprising:

at least one nuclear control bar (16) containing neutron absorbers defining an absorbing zone;

an element (18), said insertion element, of elongate shape along a longitudinal axis XI comprising:

A casing (180) forming a guiding sheath of the nuclear control bar between an extreme position of withdrawal in which the absorbent zone is not housed in the sheath and an end position in which the absorbing zone is completely facing; a fissile zone of an adjacent fuel assembly when the nuclear control device is inserted into the plurality of fuel assemblies,

A foot portion (181) intended to be inserted into an opening of the bed base of the heart while maintaining the insertion element therein with its axis XI parallel to the axis X of the fuel assemblies (6),

• a foot extension portion (182) between the guide sleeve and the foot; the extension being of cross section to the axis XI greater than that of the foot; the extension of the foot and the foot being of male type,

a socket-like female element (19), of elongate shape along a longitudinal axis X2, dimensioned to allow interlocking of the extension and the foot (181) of the insertion element with projection of the latter beyond of the base and, where appropriate, the interlocking of a foot (62) of a fuel assembly but without projection of the latter beyond the base, the base being intended to be at least in abutment against the opening (400) of the bed base in the dedicated location so on the one hand to allow the insertion of the foot of the insertion element in this opening and on the other hand to prevent the insertion of the foot of a fuel assembly in this opening.

2. Nuclear control device according to claim 1, characterized in that the length of the base is further dimensioned to block, in case of inadvertent insertion of a fuel assembly into the opening at the dedicated location, said assembly in a position such that its fissile area is not opposite that of another adjacent fuel assembly already inserted into an opening of the bed base.

3. Nuclear control device according to claim 1 or 2, characterized in that the length of the base is further dimensioned so as not to be opposite a fissile zone (6 ') of an adjacent fuel assembly already inserted into a opening of the bed base.

4. Nuclear control device according to one of the preceding claims, characterized in that the length of the base is further dimensioned to block, in the event of inadvertent insertion of a fuel assembly into the opening at the dedicated location, said assembly in a position such that it can be detected by a fuel assembly handling control system in the reactor vessel.

5. nuclear control device according to one of the preceding claims, characterized in that the length of an insertion element considered between the end of the guide sleeve and the foot is equal to the length of a fuel assembly.

6. nuclear control device according to one of the preceding claims, the insertion member having a shoulder (183) in the form of a truncated cone or spherical between its foot extension (182) and its foot (181), shoulder allowing a sphere / cone bearing against the inner wall of the opening of the bed base.

7. nuclear control device according to one of the preceding claims, the base having at least one locking lug (190), preferably three pins angularly distributed at 120 ° from one another, which extends (ent) ) transversely to its axis X2 to lock the positioning of the base against the bed base (40) by wedging (s) the pin (s) by an (s) fuel (s) adjacent (s).

8. nuclear control device according to one of the preceding claims, the base having a hexagonal cross section transverse to its axis X2, identical to the hexagonal section of the casing (6A) of a fuel assembly (6).

9. nuclear control device according to one of the preceding claims, the guide sleeve having a hexagonal cross section transverse to its axis X2, identical to the hexagonal section of the casing (6A) of a fuel assembly (6).

10. Nuclear control device according to one of the preceding claims, the material constituting the base having the same coefficient of thermal expansion as that of the extension and the foot of the insertion element.

11. A method of implementing the nuclear control device according to one of the preceding claims, the vessel being the reactor vessel (2) of a fast neutron nuclear reactor cooled with liquid metal, such as liquid sodium ( SFR), a method according to which the foot (181) of the insertion element (18) is pressed against the bed base (40) around an opening (400) of the reactor at a dedicated location. insertion member in the opening (400) through the base (19) and then moves the nuclear control rod (16) to accommodate at least partially in the guide sleeve (180).

12. Liquid Sodium-cooled Fast Neutron Nuclear Reactor (1), comprising at least one nuclear control device (17) according to one of the claims

1 to 10, inserted in the bed base (40) of the heart being contiguously surrounded by six assemblies (6) SFR fuels.