JP2018528421A - Assembly for an FNR-NA reactor having a housing with a spacer plate having improved stiffness - Google Patents

Abstract

  The present invention is an assembly for a nuclear reactor comprising a housing (10) having a longitudinal axis (X), in particular for a sodium-cooled fast reactor RNR-Na, each main surface of the housing being in the center. A spacer plate (2) with an adjacent assembly projecting outwards, the assembly being also added to the inside of the housing (10) and retained therein, next to the plate (2) Further comprising a reinforcing sleeve (3) arranged.

Description

  The present invention relates to fast neutron atoms that belong to a family of reactors that are cooled with liquid metal, in particular liquid sodium known as SFR ("sodium (cooled) fast reactor") and referred to as the fourth generation. The present invention relates to a fuel assembly for a furnace.

  The present invention firstly aims to provide a fuel assembly that can be used in a short time in a French technical fourth generation demonstration reactor project called ASTRID.

  The fuel assembly to which the present invention is directed is an integrated nuclear reactor, in other words, a nuclear reactor in which a primary sodium circuit having a pump means is completely accommodated inside a tank including a heat exchanger, and a loop type reactor. Both reactors, in other words intermediate heat exchangers and reactors where the main means for pumping sodium are located outside the tank can be used.

  “Fuel assembly” is understood to mean an assembly that contains fuel elements and is loaded into and / or removed from the reactor.

  "SFR type fuel assembly" is understood to mean a fuel assembly designed to be irradiated in a fast sodium neutron reactor cooled with liquid sodium known in SFR reactors.

  Despite being described with reference to the primary applications of interest, ie, fuel assemblies for nuclear reactors, the present invention provides reflectors, lateral neutron protection (LNP), control rods, experiments It can also be applied to any kind of reactor assembly, such as an assembly for assembly, complementary safety device.

  Fuel assemblies designed for use in liquid sodium cooled fast neutron reactors (SFRs) have special mechanical structures to allow, among other things, liquid sodium to pass therethrough .

  FIG. 1 shows a fuel assembly 1 already used in an SFR reactor known by the name “Pheonix”.

  Such an assembly 1 having an elongated shape along the longitudinal axis X first comprises a tube or housing 10 having a hexagonal cross-section, the upper part 11 of which forms the gripping head of the assembly and the upper neutron A shielding device (UNP) is housed, and its central portion 12 includes a fuel pin (not shown).

  In other words, the portions 11, 12 form the same tubular envelope 10 or housing having the same hexagonal cross section over its entire height. The aggregate head 11 includes a central opening 110 at the center thereof.

  The assembly 1 finally includes a lower portion 13 that forms the foot of the assembly in the extension of the housing 10. The assembly foot 13 has a distal end 155 that is conical or rounded so that it can be inserted vertically into the candle of the core floor (support). The legs 13 of the assembly have holes 16 that are open around it.

  In this way, in the configuration in which the fuel assembly is installed, in other words, at the position where the fuel assembly is loaded, the male feet 13 of the assembly 1 are inserted into the openings in the reactor floor. The assembly is held in the reactor with the longitudinal axis X thus vertically aligned.

  Primary sodium can circulate inside the housing 10 of the assembly 1 and thus can transport the heat released by the fuel pins by heat conduction. Therefore, the sodium is introduced through the opening 16 of the foot 13, passes through the bundle of fuel pins, and is discharged through the central opening 110 of the head 11.

  The central portion 12 of the assembly includes a plurality of nuclear fuel pins. Each pin takes the form of a leak-proof sheath tube, in which is stacked a column 14 of fissile fuel pellets that cause a nuclear reaction to generate heat. All columns 14 define a region usually referred to as a fission region that is approximately half the height of the assembly 1. FIG. 1 schematically shows the shape of a black rectangle.

  All assemblies of the same reactor are arranged vertically on the floor so as to form a dense array core with a hexagonal mesh.

  Aggregates in place on the floor are separated from each other at their bases (foot) and are typically separated by a few millimeters between opposing faces of two adjacent housings having a hexagonal cross section. Has been.

  This spacing needs to be kept substantially constant over the entire height of the assembly during reactor operation.

  In fact, bringing two adjacent fuel assemblies in close proximity results in a rapid increase in power that can have serious consequences such as reactivity insertion, i.e., overheating, clogging, etc., resulting in a core meltdown accident. There is a possibility of connection.

  To overcome this, a well-known solution for existing SFR reactors is to add a spacer device at the top of the assembly housing, just above the fissile pin area.

  Generally, these devices are placed at a height approximately equal to about 2/3 the height of the assembly protruding from the floor.

  These spacer devices, usually called “platelets”, consist essentially of bossing, in other words over-thickness, and protrude towards the outside of the assembly. Each surface of the hexagonal cross section of the housing is provided with a bossing (platelet).

Therefore, these platelets have the role of locally reducing play between adjacent assemblies,
-Ensure the tightness of the array of assemblies during normal operation of the reactor and during handling operations (at critical temperatures);
-During the earthquake, the core compresses, in other words the movement of the assembly together, or the core recompression following the elastic return of the assembly after stacking, in other words gas expansion, etc. Limit the segregation movement caused by the release of energy.

  The platelets known under the names "Pheonix", "Superpheonix" or "Rapsody" and already used in the SFR reactor assembly used in France, have the desired deformation towards the outside of the housing To obtain each of the six faces of the hexagonal housing by ram embossing.

  The functional parts of these platelets, in other words their flat contact surface 20 as illustrated in FIG. 3C, is generally at a maximum of a few millimeters across the platelet embossing depth, ie the surface. It is a 20 mm × 50 mm rectangle with the height of the bossing 20.

  The inventors have selected a platelet design for a previous nuclear fuel assembly as illustrated in FIGS. 2 to 3C, the existing platelet is required and shown below. Cannot be used in fuel assemblies for ASTRID because they are not sufficiently stiff for the safety goals to be achieved, or in other words their rigidity is not high enough. The rigidity of the platelet, indicated by K, characterizes the resistance of the platelet that is crushed by an external force. In the elastic region, it is defined as being equal to the ratio between the force applied to the platelets relative to the hexagonal housing axis and the displacement of the facets of the platelets.

  In fact, the known platelet stiffness K is insufficient in the ASTRID reactor framework described below.

  First of all, it is clear that the core compression limit in accident situations (earthquakes, energy release inside the core, etc.) is a safety goal that ASTRID reactors must follow the 4th generation reactor design. .

  In contrast to the ASTRID core, referred to as the “low emission factor core” (or CFV), which has the peculiarity of indicating a negative emission factor for sodium, research has shown that the core of the core is fixed at a maximum of $ + 1 ($) The criteria for increasing reactivity after compression is increased by a factor of 5 over known embossed platelets, while at the same time separate fuel assemblies designed to be used in ASTRID reactors. It is shown to be adhered to by implementing a platelet that keeps conforming to the specifications.

  These specifications are numerous and only have an impact on the platelet design process detailed below.

  The negative emission factor of the CFV core is the keystone for the safety demonstration of the ASTRID reactor. Basically, in the case of sodium “dumping”, the negative emission factor, which is characterized by a natural drop in the reactivity of the core, is notable for steel in an area called the “plenum” located directly above the fuel pin. This is achieved by minimizing the amount. This is because steel is a material that reflects neutrons.

  However, in an accident situation resulting in sodium boiling, for example, in a configuration where the plenum's sodium is discarded, a large amount of steel in the plenum results in reflections of leaking neutrons towards the fuel, resulting in increased reactivity, which is a CFV core. This is exactly the opposite of the desired effect on.

For reference, the fuel assembly of the CFV core is from the top to the bottom of the core
An upper absorption region made of neutron absorbing material,
-Liquid metal plenum region,
-The upper region of fissile material,
-The middle region of the parent substance,
-The lower region of fissile material.

  Furthermore, the lateral center plane of the intermediate region of the parent substance is located above the lateral center plane of the assembly formed by the upper region of the fissile material, the intermediate region of the parent material and the lower region of the fissile material, The ratio of the height of the intermediate region of the parent material to the height of the aggregate formed by the upper region of the material, the intermediate region of the parent material, and the lower region of the fissile material is a distance from 0.25 to 0.40 It is in.

  In addition, the platelets are located on a hexagonal housing just above the upper end of the fuel pin to ensure optimal spacing between the fission regions of two adjacent assemblies and thus limit compression. It is recalled that In other words, the platelet is located in the lower part of the plenum.

  Thus, the promise of maintaining a negative core discharge factor results in minimizing the amount of steel used in the platelets. In practice, only neutron calculations can estimate the effect of platelet shape on the emission factor.

  Furthermore, in any nuclear reactor, the force during handling of the assembly in the ASTRID reactor needs to be minimized.

  When they are placed on the floor, the aggregates contact or substantially contact via the platelets. The tightness of the core results in negative or zero play in the plane of the platelet. This tightness is desirable in normal operation corresponding to an average temperature of sodium of about 550 ° C. for platelets and about 400 ° C. for floors to ensure a static mechanical balance of the core.

  However, if the core is tight in normal operation, this is not necessarily the cold case during the handling operation of the assembly in the reactor vessel, and all of the sodium in the vessel is cooled to 200 ° C. Is done. Indeed, the level of core tightness at cold temperatures depends on the differential thermal expansion between the steel forming the floor and the steel forming the platelet.

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

  Platelet, on the other hand, is an AISI 316 austenitic steel whose expansion coefficient is the same as that of the floor, or EM10 type martensitic steel (9% Cr and 1% Mo) or ferritic steel whose expansion coefficient is lower than that of 316 steel. Can be made with either

Two situations are distinguished:
-When the platelet and floor are both made of austenitic steel: The tightness of the platelet in normal operation, i.e. at high temperature, suggests positive play to the platelet in the handling configuration, i.e. at low temperature. This positive play during handling is advantageous for array assembly insertion and extraction by minimizing frictional effects between the assemblies and hence handling forces. Any risk of blocking the aggregates in the array is avoided.
If the platelets are made of ferritic or martensitic steel while the floors are made of austenitic steel: the tightness of the platelets in normal operation is slightly lower or zero during handling Suggests the tightness of the platelets with equal play. Because this negative play involves the risk of exceeding the traction force of the handling machine or damaging the external surface of the platelet (scratches, scratches, etc.), it requires a large force required for extraction, It is disadvantageous. The forces for aggregate extraction depend on the forces applied to the platelets, and hence their stiffness against the imposed displacement, the surface area of contact between the platelets, and the coefficient of friction. In other words, having platelets that are very rigid and / or have a large contact surface area is disadvantageous for the purpose of minimizing forces during handling of the assembly.

  In fact, the installation method maintained for the assembly intended for ASTRID reactors is to insert a bundle of fuel pins through the top of a hexagonal housing with welded feet and then collect Providing insertion at the top of the assembly consisting of UNP and head to close the body.

  The bundle of fuel pins occupies the entire internal space of the hexagonal housing. Since the platelet is located on the pin bundle, the feasibility of inserting the fuel pin into the housing is therefore dependent on the shape of the platelet.

  Thus, if the platelets have a shape that does not reduce the inner diameter of the housing, in other words, a distance separating two opposing surfaces, they do not interfere with the insertion of the bundle. These platelets are said to be non-invasive. The platelets can then be installed or manufactured directly on the original hexagonal housing. This type of platelet is the simplest and is actually compatible with assembly installation.

  On the other hand, if the platelets have a shape that reduces the inner diameter, for example in the case of particularly rigid platelets that increase in thickness towards the inside of the housing, they are invasive and can be routed through the top of the housing. Blocks the insertion of the bundle. These platelets should be mounted on the housing once the bundle is installed.

  However, this operation is very critical. On the other hand, access to the approximately 8 to 10 cm platelet at the top of the fuel pin leads to very high temperatures in this region that could damage the pin during welding or even during thermal stabilization annealing. This prevents local welds from being welded between the platelet and the housing. On the other hand, this welding is an operation that is difficult to perform and confirm. Incomplete welding performed on the installed fuel bundle results in the entire loss of the assembly.

  Finally, the spacer platelets must be compatible with the reactor assembly and the core hydrothermal power.

  As already mentioned, the platelets are formed excessively thick on the outer surface of the housing, locally reducing or eliminating play between adjacent assemblies. Thus, the contact surface area, or more precisely the platelet width, should be too large until the risk of blocking the space between adjacent assemblies, thereby preventing sodium circulation between the assembly housings. is not.

  The sodium flow between the assemblies is very low and is not involved in cooling the assemblies during normal operation. On the other hand, the circulation of sodium between the housings of the assembly established by natural convection in certain accident situations, such as loss of primary flow, is necessary for the discharge of residual power from the assembly.

  Furthermore, the thermohydraulics inside the assembly are important at all stages of operation.

  In the case of non-invasive platelets, in other words platelets that do not reduce the inner diameter of the hexagonal housing, the flow of sodium through the interior of the assembly is not affected.

  In the opposite case of an invasive platelet, the decrease in inner diameter locally indicates an obstacle that increases the load loss in the assembly, which is the assembly in the monitoring device (sodium temperature and flow rate) located above the core. Can affect the flow of sodium at the outlet. In order to obtain a satisfactory flow rate of sodium, it is essential to adapt the shape of the invasive platelet.

  Accordingly, the inventors have identified among the known solutions for spacer devices between adjacent fuel assemblies in a reactor vessel, the exact between fuel assemblies for an ASTRID type 4th generation SFR reactor. An attempt was made to clarify what is suitable for guaranteeing a proper interval.

  U.S. Pat. No. 4,142,934 discloses a fuel assembly for a nuclear reactor, in particular an SFR nuclear reactor, including a platelet with its hexagonal housing attached to each side, each platelet being It consists of two half-platelets arranged adjacent to each other. The two half-platelets have the same dimensions but are made of different materials, one of which is selected to have a low coefficient of friction, for example stellite, the other is made of steel . The arrangement of half-platelets is such that when two half-platelets of an assembly are in contact with two half-platelets of an adjacent assembly, each half-platelet is in contact with a half-platelet of another material. It is structured in such a way. In the assembly according to US Pat. No. 4,142,934, the platelets divided into two half platelets cannot obtain an increase in rigidity, so that the platelets for the fourth generation SFR reactor of the ASTRID type Incompatible with the functional specifications of

  French Patent No. 2509896 also discloses a fuel assembly for a nuclear reactor, in particular an SFR reactor, in which platelets take the form of bosses at each corner of the hexagonal tube of the housing. ing. Although the corner platelets according to this French Patent No. 2509896 theoretically allow the platelet stiffness to be increased at a desired rate, the inventors have We believe that due to their lack of tolerances regarding the angular orientation errors of the assemblies, they cannot be retained as a spatial solution for the 4th generation SFR reactors of the ASTRID type. This is because a slight angular error in the orientation of the assembly according to this French patent invention 2509896 results in a greater displacement at the corners, which results in a contact pressure at the platelets between the assemblies, and It will therefore have a greater friction effect. This does not go in the direction of the function required to minimize the operating force on the aggregate.

  On the other hand, French Patent No. 2403626 discloses a fuel assembly for a nuclear reactor, in particular an SFR nuclear reactor, where the assembly is replaced by a regular tube of hexagonal cross section for the housing, A dodecagonal cross-section housing tube having flat or convex sides located at each corner of the tube is provided. The shape of this tube with a dodecagonal cross section makes it possible to limit the expansion of the assembly housing under irradiation. This assures core shape and neutron performance characteristics, as vibration is reduced due to maintenance of play between the fuel pin and the housing, thus limiting reactivity instability. Assuming that a dodecagonal cross-section is employed in the assembly housing as disclosed in this French Patent No. 2403626 having a platelet arranged individually in the center of each major surface, The increase in stiffness is actually effective due to the bend limitation of the main surface, however, since the steel thickness in the platelets does not increase, it is defined in the framework of the 4th generation SFR reactor of the ASTRID type It is probably insufficient for the purpose it is.

  U.S. Pat. No. 4,543,233 also discloses a fuel assembly for an SFR reactor having a circular cross-section platelet housed on the outer surface of a hexagonal housing and secured by a spring washer.

  JP 2006-145506 discloses a fuel assembly for an SFR reactor having a platelet attached to the outside of the housing of the assembly, similar to that of US Pat. No. 4,543,233. Platelets that are also circular in cross section are each housed in holes formed in each face of the housing tube and attached to the housing by screwing or welding. In any case that is not on the scale required for an ASTRID type 4th generation SFR reactor, the inventors attached to the outside of the housing surface according to US Pat. No. 4,543,233 and JP 2006-615506. We do not believe the resulting platelets provide a significant increase in stiffness.

  French Patent No. 2921509 discloses a fast reactor, in particular an SFR, comprising a star-mounted structure with six branches arranged on a bundle of fuel pins inside a hexagonal housing. A type of fuel assembly is disclosed. The structure may have branches of any given length, and additional lateral bars may be incorporated between the branches to reinforce the structure. The free end of each branch is extended by a spacer platelet that passes through a central opening on each side of the housing. The assembly according to FR 2921509 is not compatible with at least some functional features of the fuel assembly for an ASTRID type 4th generation SFR reactor. In particular, the star structure mounted inside the housing forms a significant amount of steel that is added to the plenum area, which necessarily results in an emission factor until the core CFV effect is no longer guaranteed. Bring about a decline. Furthermore, the star structure inevitably becomes an obstacle to sodium flow within the assembly, resulting in increased load loss and deleterious effects on flow at the exit of the assembly.

  U.S. Pat. No. 4,306,938 discloses a fuel assembly for an SFR reactor that includes platelets embossed in the form of contoured ridges or continuous strips all around a hexagonal housing. The embossed platelet is reinforced by the installation of a collar arranged inside the housing, simultaneously ram embossed as a platelet, and once deformed, it is received in the deformation recess of the platelet. Thus, the inner diameter behind the platelet is the same as the inner diameter of the hexagonal housing. The assembly according to U.S. Pat. No. 4,306,938 cannot meet the significant number of functional features of a fuel assembly for an ASTRID type 4th generation SFR reactor. First of all, the collar is precisely housed in the recess formed by the embossing of the platelet, so that its thickness is equal to half the distance between two adjacent assemblies to an embossing depth. This is typically on the order of 1.5-3 mm. This thickness is small and insufficient to allow the inherent rigidity of the platelet to be increased by a factor of 5 relative to the equivalent rigidity of the platelet and collar assembly. Subsequently, the platelets are continuously embossed over the entire circumference of the housing so that they completely block the space between the assemblies. This hinders obtaining sodium circulation or prevents the introduction of natural convection between aggregates. Finally, because the platelets are embossed across the entire width of the housing surface, the contact surface area on the platelets of two adjacent assemblies is large. Furthermore, the platelets are embossed in a housing made of ferritic steel and the play of the platelets at the handling temperature is close to zero. These two aspects are directed towards increasing handling forces during collection extraction / insertion into the array.

  U.S. Patent Application Publication No. 2014/185734 discloses a fuel assembly for an SFR nuclear reactor that includes a housing having a double-walled structure consisting of an inner tube housed in an outer tube of hexagonal cross section. . The inner tube deforms under the influence of internal loads until the inner tube contacts the outer tube. The outer tube absorbs the force transmitted by the inner tube. The correct distribution of deformation / constraint between the two tubes makes it possible to limit the deformation of the outer tube. Additional reinforcement may be placed between the two tubes to limit the deformation of the outer tube. In this patent application, the problem description relates to the transmission of force under the influence of coolant pressure from the inside to the outside of the inner assembly structure. The problem is for ASTRID type 4th generation SFR reactors, whose purpose is to limit the forces coming from the outside to the inside of the structure under the effect of propulsion from adjacent assemblies. It is the opposite of what was taken out for the fuel assembly.

U.S. Pat. No. 4,142,934 French patent invention No. 2509896 French Patent Application Publication No. 2403626 U.S. Pat. No. 4,543,233 JP 2006-145506 A JP 2006-615506 A French Patent Invention No. 2921509 Specification U.S. Pat. No. 4,306,938 US Patent Application Publication No. 2014/185734

Manual << Reacteurs a neutrons rapides refroidis au sodium >>-Les techniques de l’ Ingenieur B 3 171

  Therefore, the spacer device between adjacent fuel assemblies in a dense array in the reactor core is improved, in particular to meet the specifications of a fuel assembly spacer device for an ASTRID type 4th generation SFR reactor. There is a need.

  The object of the present invention is to at least partially meet this need.

  For this purpose, the subject of the present invention is an assembly for a reactor, in particular a sodium-cooled fast neutron reactor (SFR), comprising a housing having a longitudinal axis (X), each main surface of the housing Includes a platelet for defining an interval with an adjacent assembly including a portion projecting outward at the center thereof and internally delimiting a recess without material on the inside of the housing. The body is further placed and held inside the housing and arranged to face the platelets so as to allow reactor coolant to pass therethrough and form a cavity with each of them. Includes a reinforced collar consisting of a designed hollow tube.

  According to one embodiment, the housing has a hexagonal cross section.

  According to one advantageous embodiment, each platelet is an embossed platelet and the collar faces an embossed recess in the platelet.

  Preferably, each platelet has an external contact surface with an adjacent assembly having a rectangular shape.

  According to one advantageous embodiment, the reinforcing collar further comprises an inclined straight edge forming a converging surface connecting the inner circumference of the bottom of the collar and the outer circumference of the bottom of the collar, the inner circumference of the collar and the collar. And an inclined straight edge that forms a diverging surface connecting the outer periphery of the top of the substrate. The platelet reinforced by the collar according to the invention forms an intrusive platelet in the sense that the inserted collar constitutes an obstacle to the sodium flow in the assembly. Thus, by placing the converging surface upstream and the diverging surface downstream of the thick part of the collar, on the one hand, the load loss is minimized, which contributes to the realization of the heat flow inside the assembly, So, thanks to the reduced amount of steel, this contributes to maintaining the negative emission factor (CFV) required by the ASTRID reactor framework.

  Advantageously, the reinforcing collar has a height which is lower than the height of the platelet, the height being measured along the longitudinal axis (X).

  In the case of an embossed platelet, the reinforcing collar may conversely have a height that is higher than the height of the embossed platelet, the height being along the longitudinal axis (X). Measured. In this case, the collar includes means for allowing filling and draining of liquid and non-accumulation of gas within each cavity formed between the collar and the platelet. These means advantageously consist of at least two holes through the collar, each exiting into a respective cavity formed between the collar and the platelet, the holes being in each cavity. Located at the bottom and top respectively.

  According to a first different embodiment, the reinforcing collar comprises a hollow cylinder having a height equal to or greater than the height of the platelet and having an outer diameter substantially equal to the largest dimension of the inner cross section of the housing.

  According to a second different embodiment, the reinforcing collar includes a component that includes a hexagonal portion whose outer periphery matches a hexagonal cross section inside the housing. According to this modification, the inner periphery may have a circular cross section.

  According to a third different embodiment, the reinforcing collar includes components whose outer periphery is a hexagonal cross section that fits into the hexagonal internal cross section of the housing. According to this modification, the inner peripheral surface may have a hexagonal cross section, and the height of the inner periphery of the component is lower than the height of the outer periphery.

  According to one advantageous embodiment, the reinforcing collar is made of a material whose coefficient of thermal expansion and radiation expansion is greater than that of the housing material comprising the aggregate platelets.

  According to this embodiment, the reinforcing collar is preferably made of austenitic steel and the housing is preferably made of ferritic steel or martensitic steel.

  According to one advantageous embodiment, in order to best secure the collar inside the housing, the reinforcing collar is attached to one or more structures of the assembly by attachment means arranged around the inner cross section of the housing. It has been.

  The assembly described above advantageously forms a fuel assembly, and the housing is designed to be inserted vertically into the support structure of the reactor core and is an upper neutron shield consisting of a neutron absorber. The spacer platelets are arranged in a plane located above the fuel pins, including an upper part forming the head of the assembly containing the device (UNP) and a central part containing the nuclear fuel pins.

  In this embodiment, the collar attachment means is a rod connected to the UNP substructure.

  The fuel assembly so defined allows the specification of the fuel assembly spacer device to be met for an ASTRID type 4th generation SFR reactor.

  The assemblies described above are also sodium, such as reflector assemblies, lateral neutron protection (LNP) assemblies, control rods, experimental assemblies, complementary safety devices, fast breeding assemblies, or transmutation assemblies. Any other type of non-fuel assembly that can be inserted into the cooled SFR core can be constructed.

The present invention also relates to a method for mounting the above-described aggregate including the following steps.
-Forming a reinforcing collar having external dimensions designed against the actual dimensions of the internal section of the assembly housing to ensure play for reduced installation-to increase play for installation Steps for preheating the housing-Inserting the collar with mounting means inside the preheated housing until the collar is positioned facing the platelet-Step for attaching the collar to the structure in the assembly-Cooling the housing Step to do.

  The present invention further provides the use of the fuel assembly described above in a fast neutron (FNR) reactor, such as a gas cooled or liquid metal cooled reactor, wherein the liquid metal is selected from sodium, lead or lead-bismuth. Is done.

  Other advantages and features of the present invention will become more clearly apparent when reading the detailed description of the invention given by way of non-limiting illustration with reference to the following drawings.

1 is an external perspective view of a conventional fuel assembly that is already used in a sodium-cooled fast reactor SFR. FIG. 1 is a perspective view of a prior art fuel assembly already used in a nuclear reactor “Pheonix”, showing the device in the form of spacer platelets, with adjacent fuel assemblies in the reactor core; FIG. ing. FIG. 4 is a longitudinal section of a fuel assembly assumed for ASTRID at the start of the project, showing a more accurate positioning of the platelets relative to other elements of the assembly. 2B is a perspective view of a portion of a housing having a hexagonal cross-section of a fuel assembly according to FIGS. 2 and 2A illustrating a platelet embossing formation. FIG. 2B is a cross-sectional view of a portion of a housing having a hexagonal cross-section of a fuel assembly according to FIGS. 2 and 2A illustrating a platelet embossing formation. FIG. 3B is a detailed view from the front of the platelet according to FIGS. 3A and 3B. FIG. FIG. 3B is the same as FIG. 3B, showing the deformation of the housing surface when the housing is subjected to a force applied to the platelet, whereas the three opposing surfaces of the housing are retained. A cross-section of a portion of a fuel assembly having a spacer device with platelets according to FIG. 2A and a hexagonal cross-section housing showing an attached collar according to the present invention, as envisioned for an ASTRID reactor FIG. FIG. 6 is a perspective view of a portion of a housing having a hexagonal cross section with embossed platelets, illustrating a first variation of the collar according to the present invention. FIG. 6 is a perspective view of a portion of a housing having a hexagonal cross section with embossed platelets showing a second variation of the collar according to the present invention. FIG. 6 is a perspective view of a portion of a housing having a hexagonal cross section with embossed platelets, illustrating a third variation of the collar according to the present invention. FIG. 6 is a perspective view of a part of a housing having a hexagonal cross section with an embossed platelet, showing a third variant of the collar according to the invention, with its attachment means inside the housing of the assembly. FIG. 3 is a detailed cross-sectional view of one advantageous variant of a reinforcing collar according to the invention, which can improve the thermal hydraulic power in a fuel assembly with a reinforcing collar. FIG. 4 is a schematic cross-sectional view of another advantageous variant of a reinforcing collar according to the invention, which can improve the thermal hydraulic power in a fuel assembly with a reinforcing collar. FIG. 4 is a schematic cross-sectional view of another advantageous variant of a reinforcing collar according to the invention, which can improve the thermal hydraulic power in a fuel assembly with a reinforcing collar. FIG. 3 is a schematic cross-sectional view of a reinforcing collar according to the present invention having a dodecagonal cross section secured within a housing of a hexagonal cross section assembly.

  For purposes of clarity, the same reference numerals indicating the same elements of the fuel assembly and of the spacer device with platelets according to the prior art and according to the present invention will be referred to for all of FIGS. Have been used.

  In the assembly of the present application, the terms “vertical”, “bottom”, “top”, “bottom”, “top”, “bottom” and “top” are in a vertical configuration within the reactor. As such, it is understood to refer to a fuel assembly.

  Figures 1 to 3C relating to the prior art have already been explained in detail in the introduction and will not be discussed below.

  Starting from the observation that the platelets for spacing between adjacent fuel assemblies do not exhibit sufficient stiffness K to conform to the specific characteristics of the fuel assemblies of the fourth generation SFR reactor, The inventors analyzed the morphology of the platelet deformation.

  Thus, the low stiffness of the platelet 2 formed by embossing is characterized by a significant displacement of the platelet, with a given crushing force on this platelet.

  The inventors have been able to demonstrate that this significant displacement or flattening of the platelets under force is mainly caused by bending deformation of the face of the hexagonal housing that incorporates the platelets in question. This phenomenon is reproduced by finite element calculation and is shown in FIG.

  The inventors find that the height of the embossing recess 21 of the platelet 2 is more precisely arranged and held inside the housing 10 having a hexagonal cross section and located facing the platelet 20. It is considered that the inner side of the embossing 2 is reinforced by the reinforcing collar 3 located at the top.

  A fuel assembly 1 incorporating such a collar 3 according to the present invention for use in an ASTRID type SFR reactor is shown in FIG. Just like a prior art fuel assembly designed for a fast neutron reactor, the assembly 1 according to the invention has an elongated shape along the longitudinal axis X and the end of its upper part 11 is an assembly. It comprises a housing 10 having a hexagonal cross section that forms a body head and contains a neutron protection device called UNP containing a neutron absorber 18. The central portion 12 of the assembly 1 includes fuel pins 14 that form the fission region of the assembly.

  The assembly 1 is finally provided with a lower part (not shown) forming the legs of the assembly in the extension of the housing 10, as in the prior art assembly. The feet of the assembly have a conical or round shaped distal end so that it can be inserted vertically into the core floor. The foot of the assembly also has an opening around it for sodium entry into the assembly.

  As shown in FIG. 5, the reinforcing collar 3 according to the present invention is housed and held inside the housing 10 on the fuel pin 14 and is located facing the embossed recess of the platelet 2.

  Each embossed platelet 2 always has an outer surface 20 in contact with an adjacent assembly, and this outer surface 20 has a rectangular shape and is not affected by the collar 3. The let 2 always has an embossed recess 21 without material inside the housing.

  Platelets can be envisioned in other shapes, such as circular, or shapes that are feasible and can meet the required space and stiffness constraints.

  The reinforcing collar 3 according to the invention limits the deformation due to bending of each of the six faces of the housing 10 and makes it possible to increase the rigidity of the platelet 2 embossed in this way. The total equivalent stiffness is the sum of the stiffness of the embossed platelet 2 and the stiffness of the collar 3.

  Since the reinforcing collar 3 according to the present invention is an element placed inside the housing 3, it has a number of shapes (cross-sectional shape, thickness, height) depending on the desired rigidity and manufacturing / installation constraints specific to the assembly. ) May be given.

  Collars with a circular, hexagonal or dodecagonal cross-section annular shape and collars with radial cross-sections such as square, trapezoidal, “T” shaped (rib), “U” shaped like this Can be envisaged. Generally speaking, the reinforcing collar 3 according to the invention can be easily adapted in terms of shape, dimensions and / or material.

Three different variations of the color shape are shown in FIGS.
A hollow cylinder 3 having an outer diameter substantially equal to the inner diameter of the housing 10 (FIG. 6)
A component 3 ′ whose outer periphery 30 has a hexagonal cross section adapted to the inner cross section of the housing 10 and whose inner periphery 31 has a circular cross section (FIG. 7);
The outer periphery 30 has a hexagonal cross section adapted to the internal cross section of the housing 10, and the inner periphery 31 is also a component 3 ″ having a hexagonal cross section, which is the inner periphery of the component; Component 3 ″ whose height is smaller than the height of its outer circumference. The outer periphery 30 can be connected to the inner periphery 31 by a central rib 32 (FIG. 8). The radial cross section of the collar has a “T” shape.

  The choice of color 3 material is free. Advantageously, the collar 3 is made of a material whose coefficient of thermal expansion and its expansion under irradiation is greater than that of the housing material containing the aggregate platelets. Preferably, the collar 3 is made of AISI 316 type austenitic steel because it has a housing made of EM10 type martensitic steel and is a fuel assembly for an ASTRID type SFR reactor This is because the material has the best compromise on the functions to be fulfilled in the body 1 framework.

  FIG. 9 shows one advantageous variant of the mounting of the collar 3 ″ in FIG. 8. According to this variant shown, the collars 3 ″ are each of a cross-section inside the hexagonal intersection of the housing 10. It is mounted (suspended) on the lower fixing structure of UNP (not shown) by mounting rods 4 arranged at the corners. The mounting rod 4 shown has a circular cross section, but may have any other cross section.

  By using such a mounting rod 4, the installation method for the fuel assembly is compatible with the standard installation procedure already provided for ASTRID reactors, and as a first step the mounting of the collar 3 together with UNP Only added.

  An advantage of the method according to the invention for attaching the collar 3 to the housing 10 is that no additional mechanical attachment such as welding, screwing, crimping etc. is required between the collar 3 and the housing 10.

  The collar 3 is inserted inside the housing 10 with play. An exact calibration of the play of the collar 3 with respect to the inner wall of the housing 10 needs to be performed.

Therefore, it is necessary to confirm the following conditions.
-The play must be strictly positive and must be large enough in the working environment of the assembly with the collar 3 being attached to the housing 10, in other words with an ambient temperature of about 20 ° C.
-The play must be zero or slightly negative (tight fit) at normal operation of the reactor, in other words, at an average temperature of 550 ° C in the platelet 2. This condition makes it possible to ensure that there is good contact between the collar 3 and the housing 10 in order to effectively obtain the increase in platelet rigidity required in the framework of the present invention. .

The two conditions regarding play described above may be satisfied with a color having the following characteristics.
The collar 3 is made of austenitic steel with a higher coefficient of thermal expansion and expansion under neutron flux than the housing 10 made of ferritic or martensitic steel. In this way, it becomes easy to block the play between the collar 3 and the housing 10 during operation at 550 ° C.
-For installation in the housing 10 of the collar 3, the housing 10 is preheated to a temperature of about 100 ° C to 200 ° C and the collar 3 is kept at about 20 ° C. The difference resulting from the expansion between the two steels makes it possible to increase the play to install.
The outer dimensions of the collar 3 defining the contact surface with the housing 10 are machined just before installation and adapted according to the actual measurement of the inner diameter of the housing. This makes it possible to overcome the relatively large manufacturing tolerance problem of the housing 10.

  The fact that it is mounted separately is in contrast to the color solution according to US Pat. No. 4,306,938, which has virtually the same height as the platelet, so that the collar 3 according to the present invention This means that the height is selected independently of the height.

  As an example, we have a useful height H1 collar 3 equal to 50 mm for an embossed platelet 2 having an embossed height h of 80 mm, as shown in FIG. We think that we can be satisfied with. At this useful height, the thickness E of the collar 3 required to increase the rigidity by a factor of 5 is 8.6 mm.

  In order to minimize the load loss introduced by the collar according to the invention, one advantageous variant shown in FIG. 10 forms a converging surface that connects the inner circumference 31 of the bottom of the collar to the outer circumference 30 of the bottom of the collar. It comprises a collar 3 having an inclined linear edge 33 and an inclined linear edge 34 forming a diverging surface connecting the inner periphery 31 of the top of the collar to the outer periphery 30 of the top of the collar 3.

  This deformation is even more effective as the collar is thicker.

  In a variant in which the total height H3 of the collar 3 including the converging surface 33 and the diverging surface 34 exceeds the height of the embossed platelet, the volume between the embossed platelet 2 and the collar 3 is The machining recess 21 delimits and forms a closed cavity. It is necessary to provide at least one through hole for filling the liquid sodium into the cavity and emptying the liquid sodium from the cavity at the bottom of the convergence surface 33. To avoid trapping gas while filling the cavity with sodium, at least one other through hole may be required at the top of the diverging surface.

  Instead of the converging surface 33 and the diverging surface 34, other means of limiting interference in the sodium flow within the aggregate may be provided.

  To illustrate the latter point, FIGS. 11A and 11B show two differently sized through-openings 35, 36 within the thickness of the collar that allow sodium to flow. In this regard, the inventors have found that this solution is less optimized than the solution using the converging / diverging surface and requires a high flow rate aggregate or platelet high stiffness. I think it is not applicable.

  In the case of the collar 3 having a small height compared to the height of the platelets, it is possible to guarantee an overall negative (CFV core) emission factor, which is a key point of the ASTRID reactor. In fact, in studies conducted by the inventors, the parameter that minimizes the increase in reactivity in the case of evacuation is that even if the thickness has to be increased to maintain equivalent stiffness, This indicates that the height is limited with respect to the color 3.

  Furthermore, calculations show that the stiffness of the collar 3 under compression varies linearly with its height, but varies with the cube of its thickness.

  In order to maximize the rigidity of the collar 3 according to the invention and consequently the rigidity of the embossed platelet 2, it is preferable to envisage the collar 3 having a small height and a large thickness.

  Through various studies, the inventors have been able to meet the need for a fuel assembly for an ASTRID reactor, i.e., a 5 times increase in platelet stiffness over the prior art, while at the same time being negative. Finally, it shows that it is compatible with nuclear reactor handling, manufacturing and thermohydraulics.

  Therefore, the fuel assembly 1 according to the present invention described above enables all functional specifications for spacing the fuel assemblies of the fourth generation FNR reactor such as ASTRID to be satisfied.

  The present invention has been described in the specific context of a fuel assembly for a fourth generation FNR reactor, such as an ASTRID, in other words, having a CFV core, including verifying a number of constraints simultaneously.

In other fast neutron reactors or non-fuel assemblies, some of these constraints may be relaxed or ignored. For example,
-Non-CFV core architecture allows a substantially free choice of collar dimensions (unlimited amount of steel);
The absence or low flow rate of sodium makes it possible to avoid the converging surface 33 and the diverging surface 34 on the collar 3 as shown in FIG.

Furthermore, generally speaking, in other words, outside the specific content of an ASTRID reactor, the present invention can be applied to any arbitrary kind of assembly.
-Specifically envisage a collar having a dodecagonal cross section in a housing having a hexagonal cross section as shown in FIG. 12, regardless of its shape, its cross section, its radial cross section, its dimensions. Can do.
-Regardless of the type of platelets, these may be platelets embossed in the housing, but they are non-embossed and non-intrusive platelets, eg attached to the outside of the housing Any other type such as a platelet may be used.
-Regardless of the shape of the platelets on the housing, their shape may be rectangular, circular, square, or any other shape.

  However, other variations and improvements can be provided without departing from the framework of the present invention.

  Therefore, although the above-described reinforcing collar 3 is placed in the fuel assembly for an ASTRID type fourth generation fast neutron reactor (FNR), the space between the assemblies is secured by the housing of the assembly. It can also be envisaged to apply this to other types of FNRs which must be ensured and a certain rigidity must be ensured. This can be any type of FNR that is cooled using gas, sodium, lead, lead bismuth, and the like.

  If the above-described reinforcing collar 3 is installed separately in the fuel assembly, it can also be any other type of assembly present in the FNR core, such as a reflective assembly, Lateral Neutron Protection (LNP). ) It can also be installed separately to reinforce assemblies, control rods, experimental assemblies, complementary safety devices, fast breeding assemblies, transmutation assemblies, etc.

  Attachment means for attaching the reinforcing collar 3 other than the attachment rod 4 to the UNP can be envisaged in the framework of the present invention. In this way, the inventors have used ball joints in the mounting means to overcome the problem of statically uncertain positioning, which is a potential source of undesired forces, particularly during installation or during operation. Any linkage used is considered preferred.

DESCRIPTION OF SYMBOLS 1 Aggregate 2 Platelet 3 Collar 3 ', 3 "Component 4 Mounting rod 10 Housing 11 Head 12 Central part 13 Foot part 14 Fuel pin 16 Hole 18 Neutron absorber 20 Platelet 21 Indentation 30 Outer circumference 31 Inner circumference 32 Center Rib 33 Converging surface 34 Diverging surface 35, 36 Through opening

Claims (23)

  An assembly for a nuclear reactor, in particular a sodium-cooled fast reactor SFR, comprising a housing (10) having a longitudinal axis (X), each main surface of the housing being adjacent to the central part thereof A platelet (2) for spacing and including a portion (2) projecting outwardly and including an inner portion defining a recess free of material inside the housing; The assembly is a reinforcing collar (3) consisting of a hollow tube designed to allow the passage of coolant for the reactor and is located separately and held inside the housing (10). And an assembly further comprising a reinforcing collar (3) disposed facing the platelet (2) to form a cavity with each of them.   The assembly of claim 1, wherein the housing (10) has a hexagonal cross section.   Each platelet is an embossed platelet (20), and the collar is arranged facing the embossed recess (21) of the platelet (2). Or the aggregate | assembly of 2.   4. An assembly according to any one of the preceding claims, wherein each platelet has an external contact surface (20) with an adjacent assembly having a rectangular shape.   The reinforcing collar (3) includes an inclined linear edge forming a converging surface (33) connecting an inner periphery (31) of the bottom of the collar to an outer periphery (30) of the bottom of the collar, and a top of the collar 5. An inclined linear edge forming a diverging surface (34) connecting the inner circumference (31) to the outer circumference (30) of the top of the collar, according to any one of the preceding claims. Aggregation.   The reinforcing collar (3) has a height that is lower than the height of the embossed recess (21) of the embossed platelet (2), the height being the longitudinal axis (X). The assembly according to claim 3, which is measured along the line.   The reinforcing collar (3) has a height equal to or higher than the height of the embossed recess (21) of the platelet (2), the height being along the longitudinal axis (X). The collar includes means for allowing filling and emptying of liquid and non-accumulation of gas inside each cavity formed between the collar and a platelet. Item 6. The aggregate according to any one of Items 3 to 5.   The means comprises at least two holes extending through the collar, each opening into a respective cavity formed between the collar and a platelet, the holes being at the bottom and top of the cavity. The assembly according to claim 7, which is located in each of the above.   The reinforcement collar has a height equal to or greater than the height of the platelet (2), and has an outer diameter substantially equal to the largest dimension of the internal cross section of the housing. 9. An assembly according to any one of claims 1 to 8, comprising a cylinder (3).   10. An assembly according to any of claims 2 to 9, wherein the reinforcing collar comprises a component (3 ') whose outer periphery (30) comprises a hexagonal part that fits into the hexagonal cross section inside the housing.   11. Assembly according to claim 10, wherein the component (3 ') has an inner circumference (31) with a circular cross section.   10. The reinforcing collar according to any one of claims 2 to 9, wherein the reinforcing collar comprises a component (3 ") whose outer periphery (30) has a hexagonal cross section that matches the hexagonal cross section inside the housing. The aggregate described in the paragraph.   13. The component (3 '') has an inner circumference that also has a hexagonal cross section, and the height of the inner circumference of the component is lower than the height of its outer circumference. Aggregation of   14. The reinforcement collar is made of a material whose coefficient of thermal expansion and expansion under irradiation is greater than that of the housing material containing the platelets of the assembly. An assembly according to any one of the above.   15. An assembly according to claim 14, wherein the reinforcing collar (3) is made of austenitic steel.   16. Assembly according to claim 14 or 15, wherein the housing (10) is made of ferritic steel or martensitic steel.   17. The reinforcement collar (3) is attached to one or more structures of the assembly by attachment means (4) arranged around the inner cross section of the housing. The assembly described in.   The housing is designed to be inserted vertically into the reactor core support structure, the housing forming the head of the assembly containing the upper neutron shielding device (2) (UNP). An upper portion (11) and a central portion (12) containing a nuclear fuel pin (14), wherein the spacer platelet (2) forms a fuel assembly disposed above the fuel pin. An assembly (1) according to any one of the preceding claims, in particular an assembly (1) for a sodium-cooled fast reactor SFR.   19. Fuel assembly (1) according to claim 18, in combination with claim 17, wherein said collar attachment means (4) is a rod connected to the UNP substructure.   18. A reflector assembly, a lateral neutron shielding (LNP) assembly, a control rod, an experimental assembly, a complementary safety device, a fast breeding assembly or a transforming assembly. 4. The assembly (1) described in 1 above, especially the assembly (1) for the sodium-cooled fast reactor SFR. A method for producing an aggregate according to any one of claims 1 to 20,
-Forming a reinforcing collar (3) having an outer dimension adapted to the actual dimension of the inner section of the housing (10) of the assembly to ensure reduced play placement;
-Preheating said housing (10);
-Inserting a collar (3) with its attachment means (4) inside the preheated housing (10) until it is located facing the platelet;
-Attaching the collar to a structure inside the assembly;
-Cooling the housing (10);
Including methods.   Use of a fuel assembly (1) according to claim 18 or 19 or any other non-fuel assembly according to claim 20 in a fast neutron reactor.   23. Use according to claim 22, wherein the reactor is gas cooled or has a liquid metal, the liquid metal being selected from sodium, lead or lead bismuth.