JP6181067B2 - Reactor assembly comprising nuclear fuel and a system for activating and inserting at least one neutron absorption and / or mitigation element - Google Patents

Description

(Technical field and conventional technology)
The present invention relates to a nuclear mixing assembly comprising a nuclear fuel and at least one element to be inserted that can only be neutron absorbing and / or mitigating material in the case of overall core melting. The relaxation material is a material that can form a eutectic with a material forming the cladding of the nuclear fuel rod in the assembly at a low melting point and prevent formation of a plug that prevents discharge of the molten core or corium.

  This assembly is particularly intended for sodium-cooled fast reactors, hereinafter referred to as SFRs.

  It is planned that elements composed of neutron absorbers should be inserted into the reactor in order to coordinate the activity of the reactor core or limit the effects of reactor malfunction. These elements can be in the form of assembly bars suspended above the core during normal operation. If a need to reduce the reactor reactivity is detected, the absorbing element is inserted into the fission region.

  For example, in the case of a sodium cooled furnace, a plug that prevents the circulation of liquid sodium is formed, so a malfunction of the furnace can be a problem in a furnace cooling system such as a primary coolant system. The problem is the lack of a heat sink, i.e. the heat extracted through the furnace coolant system is no longer correctly discharged.

  In the absence of an inserted negative reactivity, these malfunctions can result in an increase in core temperature, which not only causes melting of one or more assemblies, but also causes overall melting of the core. It can lead to loss of health.

  The purpose of inserting an absorbing element in the core is to suppress the neutron reaction and stabilize the core at a temperature selected to meet the accepted criteria for considered malfunctions.

  In addition, several overlapping, diverse and independent shutdown systems are arranged to maintain maximum safety in furnace control and compensate for common mode defects.

  Shutdown systems (classified as conventional) used in the past for SFR reactors are based on active devices in the sense that the insertion of the absorption element is triggered by external power control or by loss of electrical signals. The shutdown systems used so far have a container head plug and a mechanical interface.

  For the next generation SFR reactor, it is planned to add a new shutdown system in case the conventional shutdown system fails. Therefore, this “emergency” shutdown system device must not be activated prior to the conventional shutdown system. Based on device diversification logic and to eliminate the risk of failure of electrical appliances, controls and logic systems, the insertion of absorbing elements starts directly following physical phenomena rather than electrical control It is envisaged to use passive devices in the sense that For example, it would be possible to envisage activation means that are sensitive to flow fluctuations or temperature increases. These passive devices have been studied in detail but have not yet been used in nuclear reactors.

  Patent document 1 is disclosing the fuel assembly provided with the box which stored the nuclear fuel rod, and the emergency operation stop apparatus. This shutdown device otherwise occupies the space occupied by the fuel rods. The device comprises a sealed casing that is secured to the assembly box. The shutdown device includes argon and boron carbide in the form of elongated elements suspended along the wire. The wire is suspended from the sealed casing by a temperature sensitive fuse, and when the temperature exceeds a threshold, the fuse melts and releases boron carbide, which falls to the bottom of the capsule at the core level. Since the casing is fixed to the assembly, the casing is not mechanically isolated from the assembly, so it can be deformed by such mechanical loads at the lattice pitch, and the arching and / or assembly plate It leads to crushing. This prevents the insertion of boron carbide into the core of the assembly.

  Thus, the reliability of the disclosed aggregate insertion is improved.

  Furthermore, it is impossible to check whether the emergency shutdown device is operating correctly for two reasons. First, the fuse system cannot be reequipped because this type of system can only operate once. Second, since the emergency shutdown device is fixed to the assembly, it cannot be assumed that a temperature start test is imposed on the carrier fuel assembly.

  Furthermore, since the casing is fixed to the carrier fuel assembly, it is impossible to separate the life of the absorber from the life of the carrier fuel assembly. Furthermore, it is difficult to handle the absorbent material in the fuel assembly production line and during the dismantling operation.

  Patent document 2 discloses a safety device for a reactor having an absorption element in the form of a cylindrical link. However, the cylindrical shape of the link is not optimal in terms of insertion reliability, temperature and neutron efficiency.

  Patent Document 3 discloses a nuclear fuel device having an integrated passive safety device. The passive safety device comprises an absorbent element in the form of a ball cast into a meltable matrix. First, stacked balls can cause mechanical blockage due to arching effects, which can reduce tripping and insertion reliability. Secondly, the distribution of the balls in the fission bundle is not controlled, especially at the end of the irradiation, especially when the bundle is significantly deformed by irradiation creep and is expanding under irradiation. There is also the risk of partial or complete blockage of the fuel rod bundle by the balls, and the risk of blockage of the grids located upstream and downstream from the bundle, which is very unlikely to occur. This would violate the unambiguous design rules for additional safety devices designed to cover. That is, the prevention level provided by the standard design should not be degraded. However, instantaneous full blockage (BTI) of the fuel assembly is one of the scenarios considered by the SFR as a fuel melt initiator, but it is delegated to the residual risk domain in the standard core design .

  Patent Document 4 discloses a temperature sensitive starter that allows a neutron absorber to be inserted into a nuclear reactor. This device is placed in the fuel assembly. The sensitivity and accuracy of this system with respect to temperature deviations is very limited. In addition, this device severely affects the neutron performance of the core during normal operation. If the shutdown system has minimal impact on the neutron performance of the core during normal operation, the neutron functional element (absorber or absorber) for each assembly is used to minimize the loss of fuel volume ratio in the core. The volume ratio of the fuel must be maximized. For assemblies dedicated to the control system, this typically means maximizing the capacity of the absorber per assembly in order to minimize the number of positions in the core. For mixed assembly designs where attempts are made to optimize the start-up reactivity, this will result in minimizing the reduction in fuel volume ratio compared to the power assembly. Patent Document 4 does not allow any of the above, which in Patent Document 4 is a maximum fuel volume ratio (combination of five fuel capsules and one absorber capsule) that is much lower than is possible with a standard fuel assembly. As well as providing a maximum absorbent volume ratio (a combination of five absorbent capsules and one fuel capsule) that is much lower than is possible with control rods.

French Patent Invention No. 2230984 Specification French patent invention 2251079 specification French Patent Invention No. 2683667 US Pat. No. 5,051,229

(Presentation of invention)
Accordingly, it is an object of the present invention to provide a nuclear assembly including a passively activated emergency shutdown system, i.e., without a container head plug and a mechanical link, with high accuracy and good activation reliability. It has good insertion reliability, and can test normal operation as many times as necessary.

  The above objective is accomplished by a nuclear assembly comprising a box having nuclear fuel rods disposed therein and an emergency shutdown system disposed within the box and occupying some space of the nuclear fuel rods. The emergency shutdown system comprises a capsule extending along the centerline of the box, said capsule being removably inserted into a sheath that delimits the housing within the fuel rod and into which the neutron absorption The assembly to be inserted, which can be a material and / or a relaxation material, is suspended.

  The assembly has an emergency shutdown system that is mechanically decoupled from the assembly, and when decoupled, the system can be relocated and removed from the assembly box and can be operating normally, repaired, or re-equipped It is checked whether or not the assembly to be inserted is replaced when the negative reactivity is extremely reduced.

  Since nearly all of the coolant flow circulating in the box passes through the device and is practically equal to the flow in a standard fuel assembly, insertion accuracy and start-up reliability are optimized. Since the power and coolant supply flow rate for a fuel assembly-only assembly or mixed assembly is significantly higher than the power and coolant supply flow rate for an absorbent-only assembly, insertion is an assembly dedicated to the absorber. Start up faster and more rigorously than you would.

  The release of the insertion target assembly can be triggered by any physical feature that represents the accident state of the assembly.

  Depending on the type of accident condition encountered, i.e. in the case of loss of reactor coolant flow and reactivity transitions, flow or neutron flux can be used as a physical phenomenon that triggers activation.

  Preferably, temperature is used as a physical phenomenon that triggers activation. The start-up device of the shutdown system can be magnetic. The insertion target assembly is released when the Curie temperature is reached. Preferably, the release can be triggered by a differential expansion phenomenon. The release device is directly in the coolant flow, and an increase in coolant temperature causes the assembly to be inserted to be released.

  Also advantageously, the emergency shutdown device comprises means for preventing undesired dropping of the insertion target assembly by preventing the release of the insertion target assembly when the coolant temperature does not exceed a given threshold. ing.

  Very advantageously, the assembly to be inserted is formed by a number of substantially spherical elements that are attached to the cable to form a series, reducing the risk that the elements will not be inserted.

  Next, the subject of the present invention is a box having a major axis oriented substantially along the vertical axis, a fission region located at the bottom of the box, a free volume located at the top of the box, and a major axis. A free space in the fission region that extends from the end of the fission region along the top side along at least part of the fission region along the long axis, a sheath that borders the free space, and activation and insertion A nuclear reactor carrier assembly, wherein the activation and insertion system includes a capsule having a long axis, an assembly to be inserted suspended in the capsule, and the assembly when the assembly is in an accident state. An activation and insertion device capable of releasing the assembly to be inserted, wherein the capsule is partially inserted into the sheath, and the activation and insertion system is removably installed in the carrier assembly. Is, the capsule is equipped with a gripping head, start and insertion system from the gripping head is suspended in the sheath above.

  The activation and insertion system is advantageously located at the top of the top area of the box.

  Preferably, the free space is arranged in the middle of the fission region so that the long axis of the activation and insertion system is coaxial with the axis of the assembly.

  The assembly to be inserted may be a type of neutron absorber and / or relaxation material.

  The longitudinal dimension of the assembly to be inserted can be selected, for example, to be equal to half the total longitudinal dimension of the capsule.

  Preferably, the capsule includes means for attenuating the fall of the assembly to be inserted at the end of the fall movement.

  According to another feature, the capsule comprises a coolant supply orifice at the end of the part located in the sheath.

  The carrier assembly may be provided with guide means for positioning the activation and insertion system in the fission region of the assembly, arranged at the end of the sheath located on the same side as the free volume of the carrier assembly.

  The box preferably has a hexagonal cross section, the sheath preferably has a hexagonal outer cross section and a hexagonal or circular inner cross section, and the capsule has a circular outer cross section.

  Advantageously, the assembly to be inserted comprises a plurality of elements mounted pivotally on one another, one end element forming an attachment head which cooperates with the means for holding the activation and insertion device. Advantageously, the element is fitted on the cable. For example, the cable can be a braided metal fiber or a braided ceramic fiber.

  Particularly advantageously, each element has a spherical shape.

  The carrier assembly may include damping means between at least a pair of elements.

  For example, the element is formed of several absorbent materials. The absorbent element may comprise at least some elements in the first absorbent element and another element in the second absorbent element.

  According to one advantageous feature, the element is hollow or comprises a central core and a peripheral casing composed of two different materials.

  Advantageously, the activation and insertion system is sensitive to temperature fluctuations. It is further advantageous if the activation and insertion system is of a different expansion type.

  For example, the start-up and insertion system includes a locking means that prevents insertion of the assembly to be inserted at a temperature lower than the furnace operating temperature.

  The carrier assembly preferably includes means for detecting insertion of the insertion target assembly by ultrasonic telemetry. For example, the activation and insertion device comprises a longitudinally fixed part formed of a capsule and a longitudinally movable part, the capsule comprising means for holding the assembly to be inserted in a suspended position above the fission region, said insertion The target assembly can be released by the movement of the movable part. The movable part includes locking means, means for holding the insertion target assembly in a suspended position, and means for releasing the insertion target assembly from the holding means, and the locking means is formed from at least one surface called a stop surface. The means for releasing the assembly to be inserted is formed of at least one second surface called a release surface, and means for displacing the stop surface and the release surface along the long axis. The displacement means is formed by a shell that can be expanded with a difference in the longitudinal direction with respect to the capsule under the influence of the temperature rise of the coolant. The arrangement of the stop surface and the release surface moves in the axial direction so that the stop surface moves away from the holding means when the temperature of the coolant rises, and the release surface moves in the axial direction so as to approach the holding means If the coolant is at normal furnace operating temperature, the stop surface is moved away from the holding means so that the holding means is unlocked, and the release surface is when the coolant temperature exceeds the threshold temperature. In addition, the holding means is applied with a thrust so that the insertion target assembly is released.

  The detection means may comprise at least one ultrasonic transducer disposed above the capsule head and a reflector placed on the capsule head facing the transducer. The longitudinal position of the reflector is controlled by whether or not the assembly to be inserted is held at a fixed position by the holding means, and the reflector slides freely within the longitudinal reaming that penetrates the capsule head. And is connected to the insertion target assembly by a long element that holds the reflector in its non-inserted state by being placed on the insertion target assembly.

  The carrier assembly may comprise elastic means that compresses in the presence of the insertion target assembly and expands in the absence of the insertion target element to apply a tensile force to the elongated element to move the reflector.

  In order to set the coolant circulation channel between the shell and the capsule, a radial clearance is preferably provided between the shell and the capsule, the shell being provided with an orifice for circulation of the coolant in the channel. .

  For example, the holding means comprises at least two, preferably three pins. Since the pin is distributed around the long axis and pivotally attached to the capsule, the pin is moved to a position near the long axis to hold the assembly to be inserted between the pins. It moves to a distant position, where the set to be inserted is released.

  The stop surface may be, for example, a surface that is disposed radially outside the pin and prevents the pin from moving in a direction away from the long axis. The release surface can be, for example, a surface perpendicular to the long axis. The pin has a cam surface, and the cam surface and the release surface cooperate to pivot the pin away from the long axis.

  For example, the shell is made of austenitic steel and the capsule is made of a tungsten base alloy. Alternatively, the shell is made of work hardened Z10CNDT15.15B steel and the fixed part is made of W-5Re.

The carrier assembly is advantageously used in a liquid metal cooled fast reactor, preferably a sodium cooled reactor, in which case the neutron absorber is, for example, boron carbide (B ₄ C) variably enriched in 10B, hafnium metal, eg It is selected from refractory boride type materials such as HfB ₂ and TiB ₂ and hexaboride. For water-cooled thermal neutron reactors, the material (s) used for the neutron absorber is selected from hafnium, Dy11B6, Gd11B6, Sm11B6 and Er11B4, natural HfB2 and natural TiB2.

  Another subject of the invention is a nuclear reactor comprising a nuclear fuel assembly and a carrier assembly according to the invention.

  The invention will be better understood after reading the following description and the accompanying drawings.

1 is an overall view of an exemplary embodiment of a carrier assembly of the present invention with an activation and insertion system with a set of absorbent elements suspended. FIG. FIG. 2 is a view of the assembly of FIG. 1 with a set of absorbent elements inserted. FIG. 3 is a cross-sectional view through an absorption element in the fission region of the assembly of FIG. 2. FIG. 2 is a front view of a particularly advantageous embodiment of an activation and insertion system that can be used with the carrier assembly of the present invention, eg, at handling temperatures. FIG. 5 is a longitudinal cross-sectional view of FIG. 4 with the activation and insertion device at a handling temperature. FIG. 5 is a longitudinal cross-sectional view of FIG. 4 with the activation and insertion device at operating temperature. FIG. 5 is a longitudinal cross-sectional view of FIG. 4, showing a state in which the start-up and insertion device is at a start-up temperature just before the absorbent material is inserted into the core. FIG. 5 is a longitudinal cross-sectional view of FIG. 4 with the start-up and insertion device at a start-up temperature during insertion of the absorbent material into the core. FIG. 5 is a top view of the system of FIG. FIG. 5C is a cross-sectional view of the system of FIG. 4 along plane AA shown in FIG. 5C.

  In the following description, the terms “upper” and “lower” refer to the portion of the element located at the top and bottom of the drawing and correspond to the arrangement of the elements in the reactor. The terms “upstream” and “downstream” refer to the direction of coolant circulation within the assembly, ie, from the lower portion toward the upper portion.

(Detailed description of specific embodiments)
Throughout this description, “carrier assembly” refers to an assembly according to the present invention with both nuclear fuel and absorbing elements, and “standard assembly” refers to an assembly with only nuclear fuel.

  Furthermore, “normal operation” refers to the operation of the reactor under normal temperature conditions, and “accident state” refers to the state of the reactor that requires the insertion of an absorbent to delay the reaction or bring the reaction to a halt. Indicates. For example, this condition leads to a furnace temperature increase that causes an increase in coolant temperature above a given temperature threshold.

  Further, in the following description, it is described that the assembly to be inserted is a set of elements made of a neutron absorber, but the present invention can also be applied to insertion of a set of absorption elements and / or relaxation elements.

  Generally, a nuclear reactor includes a vessel, and a plurality of nuclear fuel assemblies are arranged adjacent to each other in the vessel. This assembly forms the reactor core. The coolant circulates within and between assemblies to extract the heat generated by the nuclear fuel and form the primary system. The assembly contains nuclear fuel distributed, for example, to fuel rods. The part of the assembly that contains nuclear fuel is called the fission zone.

  The carrier assembly A of the present invention shown in FIGS. 1 and 2 includes a box 40 having a long axis X1, which is a cylindrical shape having a hexagonal cross section. In general, in SFR, the assembly has a hexagonal external cross section. In other types of reactors, the assembly may have other types of external cross sections, for example circular or rectangular cross sections.

  The carrier assembly of the present invention replaces the standard nuclear fuel assembly. A nuclear reactor may comprise several carrier assemblies of the present invention.

The box 40 includes a central portion 42 called a fission region in which a nuclear fuel rod 41 is fitted.
The box 40 includes a lower portion called an assembly stand 44 that holds the assembly in the nuclear reactor, and the assembly stand 44 is designed to be installed on a support called an end track. Box 40 also includes an open upper portion 48.

  The assembly stand also includes a coolant supply orifice 46 from which coolant is supplied so that it can pass through the assembly.

  The carrier assembly A carries the coolant from the bottom to the top as indicated by the arrow F by a pump, and the coolant extracts the heat generated by the fuel rods. The coolant circulates outside the carrier assembly and between the standard assembly and the carrier assembly, in a so-called inter-assembly region.

  The carrier assembly also includes a housing 52 having a long axis that extends over the entire height of the fuel rod 41. The housing 52 is partitioned by a sheath 54, and the outer cross section of the sheath 54 is the same as the cross section of the box. The sheath 54 keeps the activation and insertion system SI described below within the fuel rod bundle, making the fuel rod bundle architecture consistent. Therefore, in the case of SFR, the sheath 54 has a hexagonal outer cross section similar to the box. In the example shown in FIG. 4, the inner cross section of the sheath 54 is circular. Alternatively, the inner cross section of the sheath 54 can be hexagonal.

  In the example shown, and as will be seen later, the axis of the housing 52 is aligned with the axis of the assembly. For example, the sheath 54 replaces the two fuel rod rings.

  The lower end of the sheath 54 is provided with one or several coolant supply orifices.

  The carrier assembly of the present invention also includes a neutron absorber activation and insertion system SI. This system SI forms an emergency operation stop system in case of an accident operation. The activation and insertion system SI comprises an activation and insertion device DI and an absorbent material assembly 2, said absorbent material group being suspended by the activation and insertion device DI during normal operation and released during operation in the event of an accident. .

  The activation and insertion system SI is removably mounted within the sheath 54. No securing means are provided between the activation and insertion system SI and the assembly.

  As can be seen from FIG. 4, the activation and insertion system SI comprises a capsule 10 formed from a cylindrical body having a long axis X and a circular cross section in the illustrated example. As described above in the example given, the inner cross section of the sheath 54 is circular, and the inner cross section of the capsule 10 is also circular.

  The capsule 10 has an upper zone ZI. As can be seen from FIG. 1, the absorbent assembly 2 is suspended in the upper zone ZI, suspended from the activation and insertion device DI arranged around the fuel rod when the system is installed in the carrier assembly. Be placed. The capsule 10 also comprises a lower zone ZII placed in the sheath 54, ie located in the fission zone in the fuel rod. The lower zone ZII accommodates the aggregate 2 when the aggregate 2 is released (FIG. 2). The diameter of the lower region ZII of the capsule 10 is slightly smaller than the inner diameter of the sheath 54 so that it can be inserted into the sheath 54.

  Apart from the fact that the sheath 54 delimits the housing of the capsule 10, the sheath 54 improves the mechanical isolation between the activation and insertion system SI and the assembly. That is because the rigid structure protects the activation and insertion system SI from expansion of the rod under irradiation. Thus, the sheath 54 generally contributes to mechanical isolation at the lattice pitch.

  The capsule 10 also includes a gripping head 13 that is used for handling the capsule 10 and more generally for handling the activation and insertion system SI. In FIG. 4, the gripping head 13 includes means for gripping the system by an external handling device (not shown). The head of the capsule 10 is held in the carrier assembly.

  A coolant, such as liquid sodium, flows along the long axis X from the bottom to the top in the assembly.

  The lower part of the capsule 10 is provided with a supply orifice used for filling the capsule 10 with coolant, and the lower supply orifice is provided with a porous vent with a very high pressure loss. Therefore, regardless of the size of the upper output orifice, filling can be performed without causing a large flow. Preferably, the assembly 2 has a low mass and sodium has a considerable viscosity so that the coolant flow in the capsule is as low as possible, so that it does not slow down the fall of the neutron absorber, Therefore, the fall time is increased.

  In the illustrated example, and advantageously, an annular component 61 is provided at the top of the sheath 54 to center the activation and insertion system during installation of the activation and insertion system. This part performs a secondary thermohydraulic function to mix the effluent from the sheath with the effluent from the fuel rod bundle. That is, this component provides important mixing to maintain a uniform temperature of the coolant surrounding the expansion shell. The head of the capsule 10 is suspended and the capsule is not supported at its lower end or at the shell. FIG. 3 shows a cross-sectional view of the assembly of FIG. 1 in the fission region and through the absorption assembly 2. The relative layout of the fuel rod 41, the sheath 54, the capsule 10 and the absorbent element 4 of the absorbent assembly 2 can be seen.

  The activation and insertion device DI is used to insert neutron absorbers in case of accident operation. Depending on the type of situation, this accidental motion can be detected, for example, by coolant flow fluctuations or neutron flux fluctuations. Advantageously, an accidental operation is detected by a rise in the coolant temperature in the assembly above a given threshold, resulting in a loss of reactor coolant flow, a heat sink formed by the secondary system. Major accident situations are detected: loss, transient change in reactivity. These three accident situations can cause an increase in coolant temperature. On the other hand, for example, even using flow fluctuations, only one accident situation can be detected.

  FIG. 2, and in particular FIG. 4, FIGS. 5A to 5D and FIG. 6, show a particularly advantageous exemplary embodiment of a differential expansion and insertion device for the carrier assembly of the present invention.

  The activation and insertion device DI is designed to keep the absorbent assembly 2 above the fission zone during normal operation and releases the absorbent assembly 2 in an accident situation.

  In the illustrated example, and very advantageously, the absorber assembly 2 is made up of a plurality of neutron absorbers fitted to the cable 6 (shown in broken lines) so as to form a series. A spherical or substantially spherical element 4 is provided. This set of absorbing elements will be described in detail in the following description.

  The upper end element 2.1 is distinguished from the other elements in that it is designed to cooperate with the activation and insertion device. The upper end element 2.1 forming the attachment head has a tapered shape and is formed from a large area base and side faces towards the spherical element.

  The shape of element 4 is in no way limiting and the use of elongate elements such as rotating columns may be appropriate. However, this elongated shape is less optimal than the spherical shape with respect to element insertion reliability.

  The series of pivoted structures is in no way limiting. For example, a structure formed from one or several rods composed of an absorbent material such as a control rod is suitable. However, the insertion reliability of the absorbent assembly in this structure is not as good as the series of pivoted elements.

  A coolant, such as liquid sodium, flows along the long axis X from the bottom to the top in the assembly.

  The activation and insertion device DI is arranged around the top region ZI of the capsule. The device DI comprises means 11 for holding the assembly 2, means for locking the holding means 11, and passive activation means for releasing the assembly 2 in an abnormal situation.

  The activation and insertion device DI is in the form of a rotating body with a long axis X.

  The activation and insertion device DI includes a shell 19 fixed to the capsule 10 via its upstream end in consideration of the coolant circulation direction from the bottom to the top, and extends the shell 19. A control head 18 that is fixed to the shell shell in the axial direction is provided at an upper portion thereof.

  The control head 18 is slidably installed around the capsule 10. A radial clearance is provided between the outer diameter of the capsule and the inner diameter of the control head 18.

  The holding means 11 also includes a pin 20 that is pivotally attached to an upper portion of the body of the capsule 10.

  Advantageously, the control head 18 and the pin 20 are arranged in the upper region of the capsule 10 in a region away from the fission core where the neutron flux is minimal.

  Preferably, three pins 20 are placed approximately 120 ° from each other to provide uniform support to the assembly. However, it is possible to provide two pins or more than three pins. In the holding position, the pin 20 is inclined in the major axis X direction.

  Each pin 20 is in contact with the attachment head 2.1 and a first longitudinal end 20.1 pivotally mounted on the body of the capsule 10 about a Y axis perpendicular to the long axis X. It has a second longitudinal end 20.2 that forms a support surface. Capsule 10 has a longitudinal slot in which pin 20 is mounted such that second end 20.2 of pin 20 is disposed within capsule 10.

  Advantageously, the second end 20.2 of each pin comprises a notch 22 delimited by two faces 22.1, 22.2, which can be seen particularly clearly in FIG. 5C. As can be seen particularly clearly in FIGS. 5A-5C, one surface 22.1 contacts and carries the large area base of the attachment head 2.1, while the other surface 22.2 contacts the side surface. And carry it.

  The control head 18 supports the means for locking the pins 20 at the holding position of the assembly 2, that is, the position inclined in the major axis X direction.

  The locking means includes a stop 24 disposed radially outward of the pin 20 to prevent the pin 20 from moving away from its holding position. In the example shown, each pin 20 comprises a protrusion at its edge 20.3 facing the stop surface 24. Advantageously, a radial clearance is provided between the projection and the stop surface 24 to prevent the risk of friction and biting.

  In the illustrated example, the stop surface 24 is supported by a single annular surface having an X axis formed inside the control head 18. In the example shown, this face is downstream from the pin rotation axis of the capsule 10.

  Furthermore, the control head 18 supports a passive means that triggers the release of the assembly 2 in an abnormal situation. The passive activation means is formed, for example, by a thrust surface 26 oriented along a transverse plane perpendicular to the major axis, the thrust surface 26 contacting and carrying the pin 20 to exert a thrust on the pin 20, 20 is pivoted about the axis of rotation of the pin 20.

  The thrust surface 26 abuts against the cam surface 28 of the pin 20 disposed radially inward from the rotation axis of the pin 20.

  In the illustrated example, the thrust surface 26 is disposed on the capsule 10 upstream of the rotational axis of the pin 20.

  In the illustrated example, the control head 18 includes a cavity 30 around its inner periphery, and the pin 20 fits inside the cavity 30.

  The outer surface of the cylindrical body of the capsule 10 includes three radially projecting tabs 32 that support the rotating hinge of the pin 20.

  Passive activation means are formed by the shell 19 and the control head 18. The shell 19 and the control head 18 are manufactured from a material with a high expansion coefficient, and are manufactured from a material that is higher than the material from which the capsule 10 is manufactured, preferably much higher.

  As can be seen in FIGS. 4-5D, the inner diameter of the shell 19 is selected to form a channel between the shell and the outer surface of the capsule 10 so that coolant can flow. Openings 36 are provided in the upstream and downstream portions of the shell 19 for supplying and discharging the coolant.

  For example, the radial distance between the shell and the capsule is on the order of 1 centimeter to several centimeters. Thus, it is ensured that most of the coolant flow circulates between the outer surface of the shell 19 and the inner surface of the capsule 10. The system temperature will then be close to the coolant temperature so that the system will start up with high accuracy.

  It is very advantageous if the axial dimension of the shell 19 is chosen to be very large and therefore has a very large heat exchange area with the coolant. In this way, possible local temperature non-uniformities can be managed, so that start-up reliability can be improved.

  Means for attenuating the fall of the neutron absorber to the end of its fall movement are provided in the lower zone ZII of the capsule. For example, this attenuation can be obtained by reducing the diametric gap between the absorbent assembly and the capsule at the bottom of the capsule.

  The operation of the activation and insertion device DI according to a preferred but non-limiting exemplary embodiment of the invention will now be described.

In the operation of the activation and insertion device according to the invention, four main states are distinguished depending on the applied temperature.
-Installation status (of system SI on the carrier assembly): Ambient temperature, called "installation temperature", eg 20 ° C;
-Handling state (in the core of the carrier aggregate in which the system SI is fitted): a temperature of about 180 ° C to 250 ° C, referred to as "handling temperature";
-Operating state: operating temperature around 550 ° C when the assembly is operating in the core;
-Start-up state: A threshold temperature of, for example, about 660 ° C. is required in the present invention, which requires insertion of an absorbent material into the fission core.

  The installed state is not shown, but is very similar to the state shown in FIG. 5A. In the installed state, the various elements of the activation and insertion system are not deformed by thermal expansion. The pin 20 supports the assembly 2. The stop surface 24 faces the protrusion 20.3 of the pin 20 and the thrust surface 26 is spaced from the cam surface 28. Thus, the pin 20 is locked and the assembly 2 cannot be released. The system can be handled completely safely without any risk of unintentional insertion into the fuel rod bundle.

  In the handling state, the activation and insertion system is installed in a carrier assembly located in the reactor. The difference between the capsule 10 and the assembly composed of the shell 19 and the control head 18 due to the temperature in the furnace and the difference in expansion coefficient between the material of the capsule 10 and the material of the shell 19 and the control head 18. Expansion occurs. Therefore, there is a difference between the capsule 10 and the assembly composed of the shell 19 and the control head 18, and the pins of the stop surface 24 and the thrust surface 26 supported by the control head 18 are different. There is a relative displacement with respect to 20.

  Thus, in the handling state shown in FIG. 5A, the elements of the activation and insertion system are beginning to expand slightly. This deformation mainly occurs along the long axis.

  However, the difference in expansion between the installed state and the handling state is that even if the stop surface 24 moves relative to the pin 20, the stop surface 24 is still partially facing the protruding portion of the pin 20, It is as if it is still locked to the holding position of the assembly 2. Therefore, the pin 20 supports the assembly 2. Aggregates are not released. Thus, the system can be handled in a completely safe state without any risk of unintentional insertion into the fuel rod bundle.

  The operating state is shown in FIG. 5B. Various elements of the activation and insertion system are immersed in the coolant at the operating temperature. Since the shell 19 is surrounded by the coolant through a channel formed between the shell 19 and the capsule 10, it is sensitive to the operating state of the assembly.

  The rising temperature of the coolant between the handling state and the operating state means that the deformation of the activation and insertion system elements continues to increase due to thermal expansion. At the operating temperature, the degree of expansion difference between the shell 19 and the capsule 10 has reached the point where the stop surface 24 no longer faces the protrusion of the pin 20 and thus the pin 20 is released. The thrust surface 26 just barely contacts the cam surface 28, so that the pin 20 is inclined in the long axis direction at the holding position of the assembly 2.

  The expansion of the element continues as the coolant temperature rises between the operating temperature and the starting temperature. The thrust surface 26 applies a longitudinal upward thrust to the cam surface 28 of the pin 20, which tilts the pin 20 outward. The rotation of the pin 20 around the Y axis causes an upward axial displacement of the assembly 2. Due to this operational movement of the activation and insertion system, for example movement between the attachment part of the assembly 2 and the body of the capsule 10, the connection formed between the movable part and the fixed part by oxidation and aggregation of impurities. If there are, they are lost at the same time.

  The activation state when the temperature threshold is reached, ie when the assembly 2 is released, is shown in FIG. 5C (immediately before release) and FIG. 5D (during insertion). In FIG. 5C, the pin 20 is in the final rotational phase and the assembly 2 is effectively released. In FIG. 5D, the pins have been tilted and the assembly 2 has been released and is falling towards the fission core.

  The insertion of the absorption element suppresses the neutron chain reaction in order to prevent core melting in a short period of time.

  A suppression temperature that is compatible to maintain the integrity of the core support structure is ensured for a sufficiently long period of time to take corrective action.

  As mentioned above, the shell 19 and the control head 18 of the activation and insertion device are made of a material having a high expansion coefficient, such as steel, more particularly work hardened Z10 CNDT 15.15B (15/15 Ti) steel, Manufactured from austenitic steel such as used in fuel rod cladding. For capsules 10 made from a material having a coefficient of expansion sufficiently lower than that of the materials used for shell 19 and control head 18, a tungsten-based alloy, such as a W-5Re alloy, i.e. 5% rhenium, is used. The containing tungsten alloy can be selected. Alloys such as W-ODS (tungsten oxide dispersion strengthened alloy) can also be envisaged. In addition to a low expansion rate, tungsten has the advantage that, due to its refractory nature, there is very little expansion under irradiation at the expected temperature. Advantageously, the W-5Re alloy also has acceptable flexibility under the design rules considered. Alternatively, of course, a Z10 CNDT 15.15B alloy may be selected for the capsule and a W-5Re alloy may be selected for the shell, provided that the activation and insertion device is so adapted.

  In the illustrated example, the stop surface 24 forms a radial surface and the thrust surface 26 forms a surface perpendicular to the major axis. However, this configuration is by no means limiting.

  In order to test whether an unintended insertion of the assembly 2 has occurred, it is advantageous if means are provided for detecting the status of the activation and insertion device. The first technique is an indirect “core temperature treatment (TRTC) consisting of using a thermocouple placed directly on the top of the assembly to measure the coolant outlet temperature, either directly by a neutron detector. ”To detect the insertion of negative reactivity into the core. When the absorbent material falls, the power of the carrier assembly decreases, and the outlet temperature of the coolant from the support assembly decreases. As a result, when a coolant temperature drop is detected, a negative reactivity insertion is detected.

  Another technique consists of detecting the state of the absorbent element set (whether it is suspended or not).

  A detection device DT applying this technique is shown in FIGS. 5A to 5D. It is an ultrasonic telemetry device designed to measure the distance between one or more transducers 67 located above the assembly head and the reflector, the position of the reflector relative to the transducer being Depends on whether two sets have been inserted.

  The device DT comprises a gauge 64 installed so as to slide freely within a longitudinal reaming 65 formed in the gripping head 13 of the capsule 10. The length of the gauge 64 is such that its lower end rests on the attachment head of the absorbent element set 2 and its upper end protrudes from the upper end of the gripping head.

  The upper end of the gauge 64 is provided with a reflector 66. The lower end of the gauge 64 is simply in contact with the attachment head of the absorbent assembly, so that even if the gauge is blocked, it does not prevent continuous insertion. This is because the gauge and the ream are not fixed together. Since the small cross section of the gauge is not sufficient to form a reflector, the shape of the upper end of the gauge is consequently larger than that of the rod in the reaming. For example, it is conical and tapers upward, with the bottom surface of the cone forming the reflector 66. When the gauge falls, the cone contacts the upper part of the reaming and stops. However, it is also possible to drop several centimeters, which is sufficient for ultrasonic detection. For example, a distance of 13 mm is selected.

  A reflector 66 supported by a gauge penetrating the gripping head 13 is placed as close to the assembly head as possible (schematically shown in FIGS. 5A to 5D), which increases the ultrasonic stereoreflection angle. Limiting the echo to the structure surrounding the gripping head.

  A transducer 67 (shown schematically) is located above the assembly head. The axial displacement of the reflector during insertion of the assembly 2 enables the detection and positioning of the assembly. For example, the transducer is fixed on a core cover plug grid.

  It is advantageous if a spring 68, which is installed in compression, is provided between the bottom end of the gauge and the bottom end of the reaming. This spring is compressed in normal operation, ie when the assembly 2 is in the non-insertion position and the attachment head is held in place by the pin 20. When the assembly 2 falls while carrying the attachment head, the spring 68 expands and the gauge 64 is displaced downward. Advantageously, this spring 66 does not prevent the gauge 64 from falling. Since the mass of the gauge 64 is low, corrosion phenomenon, or biting due to the presence of impurities, prevents the gauge 64 from falling. Such occlusion is overcome by the force exerted by the spring 68 when expanding, the gauge 64 falls and the device DT detects that the ream 2 has fallen. The force exerted by the spring cannot be relaxed by irradiation creep due to the fact that the spring is away from the fission core. Thus, spring 68 improves the detection reliability of the telemetry device.

  As a variant, the transducer is not vertically aligned with the reflector. In order to direct the ultrasonic beam towards the reflector 66, a fixed reflector is arranged on the inner surface of the assembly head. Alternatively, the reflector 66 supported by the gauge 64 can have a face with several facets to form a three-sided mirror, for example to improve bundle guidance. These variants advantageously allow one transducer to be used in several assemblies.

  Here, the operation of the detection device DT will be briefly described.

  5A to 5C, when the assembly 2 is suspended, the gauge 64 is placed in contact with the attachment head of the absorbent assembly 2, the spring 68 is compressed, and the reflector 66 is a transducer ( It is arranged away from the plurality, which corresponds to the non-inserted state of the absorbent assembly.

  If the assembly falls because the threshold temperature has been reached or during unintentional activation (FIG. 5D), the gauge 64 is no longer supported on the attachment head under the expansion action of the spring 68 and gravity, and the gauge 64 Slides downward in the reaming 65 with the reflector 66 and moves to a second position on the gripping head 13. The transducer 67 measures the increase in the distance between the transducer 67 and the reflector 66 and thereby detects the insertion of the assembly 2.

  This detector is particularly reliable. Since the gauge 64 has a small cross-section, the result is flexible with respect to bending and a large mechanical clearance is formed in the reaming. Even if the gripping head 13 is significantly deformed due to shaft distortion and / or collapse of reaming, all risks of mechanical blockage can be prevented.

  This detection device guarantees the detection (and positioning in the core) of the fallen absorbing element under all conditions without any loss of reliability regarding the start-up and insertion of this series.

  This detection device can be used in addition to or instead of TRTC and / or fission chamber to diversify the means of detecting the inserted negative reactivity.

  According to the invention, the activation and insertion system SI is added to the assembly, in which case the activation and insertion system SI is completely independent of the carrier assembly and is therefore managed independently of the fuel assembly. sell.

  Therefore, for example, the start-up and drop test of the assembly 2 outside the field, that is, the operation test outside the reactor can be performed only on the capsule 10. These operational tests can be systematically performed prior to initial incorporation into Aggregation A.

  If necessary, the activation and insertion device can be inspected or replaced or re-equipped independently of other fuel assembly elements if there is a system malfunction. This replacement or re-equipment can be accomplished without removing the entire assembly. When this is possible, it has the advantage of managing the life of the insertion system independently of the life of the fuel assembly, which is useful when it is necessary to reduce assembly costs or minimize the amount of active waste for the post cycle. It can be.

  The activation and insertion device DI according to the invention is particularly suitable for removable activation and insertion systems. With this activation and insertion device, and more particularly with the stop surface 24 which ensures a locked state up to the handling temperature, all risks of unlocking the pins during handling are avoided and the set of absorbent elements is Unless the pins, attachment heads, or cables are broken, they cannot fall during placement of the capsule on the carrier assembly, for example if there is an impact. This is also advantageous during integration of the assembly into the core (the handling state described previously).

  Due to the fuel assembly structure of the present invention and the integration of the start-up and insertion system of the present invention, the fuel ratio per volume is slightly reduced, resulting in a slight decrease in the core neutron performance. The volume of the central space reduces the fuel volume fraction by about 7% in the carrier fuel assembly and about 0.6% in the core.

  Furthermore, the carrier assembly design allows state-of-the-art assembly fuel cycles to be applied with a minimum number of modifications, thus optimizing costs.

  Furthermore, since the influence of the structure of the assembly of the present invention on the pressure loss of the fuel assembly is very small, the core thermal hydraulic power is optimized.

  The assembly of the present invention associated with the activation and insertion device of the present invention optimally utilizes the flow of the fuel assembly, which ensures maximum activation speed and accuracy. Due to the central position of the shell in the assembly, as well as its structure, the flow in the shell is very similar to the flow in a standard fuel assembly, so its expansion represents the coolant temperature and therefore the assembly. Represents the state of the body.

  The present invention optimizes the reliability of negative reactivity insertions. The capsule is mechanically isolated from the deformation of the fuel rod bundle because it is protected from deformation by a sheath having significant rigidity and because of the large radial clearance after insertion of the capsule. The presence of a bundle of fuel rods between the sheath and the hexagonal tube also means that the capsule can be mechanically isolated from deformations that affect the lattice pitch, which means that the bundle of fuel rods is a hexagonal tube. This is because it has some ability to cope with the deformation of (a presence of clearance between the fuel rod and the spacer wire).

  Here, details of the absorbent assembly 2 will be described. The use of the absorbent assembly 2 further optimizes the insertion reliability, and the set of absorbent elements described later can be easily inserted into the deformed capsule. The assembly 2 includes a spherical absorbent element 4 fitted on the cable 6. This assembly is very flexible, which facilitates its insertion into the capsule.

  Dispersion along the run prevents clogging due to arching effects and / or sinter-type phenomena that can occur in the case of solid stacks.

  The spherical shape of the absorbent element 2 provides good insertion reliability of the absorbent element into the capsule, since the spherical shape can be more easily inserted into deformed structures and / or smaller dimensions. is there. Furthermore, considering the thermal and thermomechanical aspects associated with the absorbent element itself, the spherical shape provides optimal cooling conditions to minimize core temperature. For example, the temperature gradient between the core and the outer surface is one third lower than in the state of the art cylindrical shape.

  The temperature stress is also low due to the low temperature gradient between the core and the outer surface of the absorbent element. Therefore, the risk of cracking is also reduced.

  The amount of absorber for the insertion of negative reactivity is optimally utilized and the spherical shape minimizes the effect of neutron self-shielding per unit volume.

The spherical element is solid and can be made from a single absorbent material. As a variant, it would also be possible to make the element from two different materials in order to optimize the characteristics of the element. For example, a metal core can be made with a lower neutron absorption capacity than a ceramic material (such as B ₄ C), but has a higher thermal conductivity, thereby lowering the core temperature and increasing the melting margin, Material can be reserved for the surrounding wall. The two materials are selected such that the differential expansion between the two materials ensures the mechanical integrity of the element. Such an element is made, for example, from a metal sphere surrounded by two hollow hemispheres made of ceramic material.

  In another variant, the spherical element is hollow. This structure is advantageous from a thermal point of view, since it can lower the maximum temperature experienced by the absorbent, especially in the case of undesired insertions. This structure reduces the magnitude of secondary stress on the element due to temperature because there is no longer any differential expansion phenomenon between the core and the surroundings. From the neutron point of view, the core material is significantly less effective than the surrounding material due to its shielding effect. As a result, the lack of core material in the hollow sphere is not particularly important.

  The hollow spherical element can be made by assembling two hollow hemispheres or by reaming a solid sphere. In the latter case, metal inserts can be provided on both sides of the reaming to reduce mechanical diametric clearance with the series of cables.

  The cable 6 is made of braided metal fiber or braided dry ceramic fiber.

  The assembly includes the above-described attachment head at one of the end portions that cooperate with the pin 20.

Advantageously, the assembly comprises at least one, preferably several (e.g. three) metal elements at the end opposite to that end where the attachment head fits, so that the element made of absorbent material replace. First, these elements form a stop member for the absorbent element. Second, since neutron flux can degrade the properties of the absorbing elements, these elements form a partial neutron shield from the fission core for the absorbing elements. For example, in the case of B ₄ C, its thermal conductivity is reduced by the effect of irradiation, which leads to an increase in the core temperature of the element. The B ₄ C element is partially protected by the insertion of a metal element that forms a neutron shield.

Finally, ballast can be formed when the concentration of the absorbent element material is low. The presence of ballast can reduce the fall time of the aggregate and reduce the risk of blockage. Furthermore, these metal elements can absorb impacts at the bottom of the capsule, which is particularly beneficial in the case of B ₄ C, which has low impact resistance.

  Advantageously, mechanical damping means can be inserted between the absorbent elements. For example, a Belleville washer can be used. These means need not be located between each pair of elements.

  The cable is longer than the height of the sphere stack, which determines the flexibility of the chain. This mechanical clearance is dimensioned according to the deformation of the component due to the effect of irradiation, such as expansion, expansion, creep.

  Further, a radial clearance is provided between the cable and the reaming that penetrates the sphere.

If there is a risk of crushing of absorbent material such as B ₄ C due to differential deformation (expansion and irradiation expansion) in the furnace, the absorbent element is provided with a sleeve that forms a metal cladding, inside the metal cladding An absorbent material is disposed on the surface.

The absorber element can be made from any neutron absorber. For example, the absorbent element can be boron carbide (B ₄ C) with some ¹⁰ B enrichment.

Hafnium-based materials can also be used. These materials have a high density that can reduce the drop time and do not release gas under irradiation, so they do not cause expansion and their negative reactivity is not significantly impaired under irradiation. Therefore, neutron efficiency and detectability level are stable. Hafnium metal can also be used. Hafnium metal has a remarkably low neutron efficiency per unit volume compared to B ₄ C, but has an advantage that it has a significantly higher thermal conductivity than B ₄ C and is stable under the influence of irradiation. Hafnium hydrides that have high thermal conductivity in a non-irradiated state and are stable under irradiation, such as hafnium metal, can also be used.

For example, it is possible to use a refractory boride type absorber having a melting temperature of about 3300 ° C., such as TiB ₂ or HfB ₂ . It is also possible to use europium hexaboride EuB6. It is also possible to use Eu ₂ 0 ₃ . It does not produce a gaseous product under irradiation and has a high absorption capacity.

As a variant, it is possible to envisage absorbent elements made from different absorbent materials depending on the position along the run. For example, a hafnium-based element can be placed at the bottom of the run and a B ₄ C-based element can be placed at the top of the run. This dispersion provides the essence of the necessary negative reactivity with the B ₄ C element, while the hafnium element at the bottom of the run, in the uninserted state, is a B ₄ C element located at the top of the run. Form a neutron shield for At the same time, the hafnium element provides significant complementarity to negative reactivity at the beginning of insertion and contributes to the total additional negative reactivity at the end of insertion. Furthermore, hafnium elements do not pose a risk of melting in the inserted state, as the thermal conductivity under irradiation in the suspended position is not reduced.

  Hafnium can also be used as a moderator in the case of overall core melting.

In the case of a pressurized water reactor, the materials used for the absorber are, for example, hafnium, Dy ¹¹ B ₆ , Gd ¹¹ B ₆ , Sm ¹¹ B ₆ Er ¹¹ B ₄ , natural HfB ₂ and natural TiB ₂ .

The coolant can be composed of any suitable liquid metal, such as sodium. Lead and lead bismuth are other liquid metals that can be used in fast reactors. Sodium is preferentially used because of its good thermal conductivity. Furthermore, in the case of boriding absorbers, the liquid metal medium avoids the potential problem of strong pressurization of the container (bar, capsule, etc.) due to helium derived from ¹⁰ B. Finally, the high viscosity of the metallic medium also allows a significant progressive deceleration at the end of the fall distance, which strongly limits the risk of breakage of the absorbent ceramic.

  As an illustrative example, an example of the aggregate sizing of the present invention will be given.

The set of spherical absorbent elements can be 800 mm high. The size and mass of the absorbent element depends on its raw material.
For -48% 10B concentrated B ₄ C, the diameter is 35 mm and the mass is 1.8 kg.
For -71% 10B concentrated HfB _2, the diameter is 35 mm, the mass is 10.8 kg,
-In the case of hafnium, the diameter is 67 mm and the mass is 46.9 kg.
For -eu ₂ O _3, the diameter is 52 mm, the mass is 17.6 kg.

  The integration of an activation and insertion system based on a 35 mm diameter spherical absorbent element into the assembly corresponds to the elimination of two fuel rods. The exclusion affects the fuel volume fraction by 7% in the carrier fuel assembly and 0.6% in the core.

For the operation of the system, assuming a shell formed from Z10CNDT15.15B and a capsule made from W-5Re, the startup temperature is assumed to be 660 ° C. and is approximately 800 mm high with selected component dimensions. In the case of the shell, the axial displacement with the shell difference relative to the capsule is calculated as follows:
Between ambient and operating temperature: 5.65 mm,
-Between operating temperature and starting temperature: 1.44 mm.

  The displacement of the attachment head due to the displacement of the pin 2 between the operating temperature and the starting temperature can be calculated as follows. The linear displacement of the pin is 5.4 mm and the angular displacement is 7.2 °.

  The axial displacement of the attachment head of the assembly 2 between the operating temperature and the starting temperature is then 3.5 mm.

  The set of absorbing elements in the form of a series of carrier assemblies and spherical elements of the present invention is particularly suitable for use in sodium-cooled fast neutron reactors. They are also applicable to other types of nuclear reactors, such as fast reactors cooled with other liquid metals such as lead or lead bismuth, gas cooled fast reactors, pressurized liquid or boiling water reactors .

2 Assembly 2.1 Attachment Head 4 Spherical Absorbing Element 6 Cable 10 Capsule 11 Holding Means 13 Grip Head 18 Control Head 19 Shell 20 Pin 20.1 First Longitudinal End 20.2 Second Longitudinal End 20.3 Edge 22 Notch 24 Stop surface 26 Thrust surface 28 Cam surface 30 Cavity 32 Tab 36 Opening 40 Box 41 Nuclear fuel rod 42 Central part 44 Assembly stand 46 Supply orifice 48 Open upper part 52 Housing 54 Sheath 64 Gauge 65 Reaming 66 Reflector 67 Transducer 68 Spring A Carrier assembly X1 Long axis DI Activation and insertion device SI Activation and insertion system ZI Upper area ZII Lower area

Claims (30)

A box (40) having a major axis (X1) oriented substantially along a vertical axis, a fission region located at the bottom of the box (40), and a free volume located at the top of the box (40) And within the fission region extending over at least a portion of the fission region along the long axis (X1) from the end of the fission region disposed on the apex side along the long axis (X1). A nuclear reactor carrier assembly comprising a free space (52), a sheath (54) bordering the free space (52), and an activation and insertion system (SI), the activation and insertion system comprising: Capsule (10) having a long axis (X), an insertion target assembly (2) suspended in the capsule, and the insertion target assembly can be released when the assembly is in an accident state Activation and insertion device (DI), the capsule (10) is partially inserted into the sheath (54), the activation and insertion system is removably installed in the carrier assembly, and the capsule comprises a gripping head And the activation and insertion system (SI) is suspended above the sheath (54) from the grasping head ,
The expansion coefficient of the material of the parts constituting the activation and insertion device (DI) is different from the expansion coefficient of the material of the capsule (10),
When the assembly is in an accident state, an expansion with a difference occurs between the capsule (10) and the activation and insertion device (DI), thereby releasing the insertion target assembly. Carrier aggregate.   The carrier assembly of claim 1, wherein the activation and insertion device is located at the top of the top region of the box.   The free space (52) is located at the center of the fission region such that the long axis (X) of the activation and insertion system is coaxial with the axis (X1) of the assembly. The carrier aggregate according to claim 1 or 2.   The carrier assembly according to any one of claims 1 to 3, wherein the insertion target assembly is a type of neutron absorbing material and / or relaxation material.   The carrier assembly according to any one of claims 1 to 4, wherein a longitudinal dimension of the insertion target assembly (2) is not more than half of a total longitudinal dimension of the capsule (10).   The carrier assembly according to any one of claims 1 to 5, wherein the capsule includes means for attenuating the fall of the assembly to be inserted at the end of the fall movement.   7. A carrier assembly according to any one of the preceding claims, wherein the capsule (10) comprises a coolant supply orifice at the end of its portion located in the sheath.   A guide means for positioning the activation and insertion system (SI) in the fission region of the aggregate is arranged at the end of the sheath located on the same side as the free volume of the carrier aggregate, The carrier aggregate according to any one of claims 1 to 7.   The box has a hexagonal cross section, the sheath (54) has a hexagonal outer cross section and a hexagonal or circular inner cross section, and the capsule (10) has a circular outer cross section. The carrier aggregate according to claim 8.   The assembly to be inserted (2) comprises a plurality of elements (4) installed pivotally on one another, one of the end elements being means (11) for holding the activation and insertion device (DI) 10. A carrier assembly according to any one of the preceding claims, forming an attachment head (2.1) that cooperates with the carrier.   11. A carrier assembly according to claim 10, wherein the element (4) is fitted on a cable (6).   The carrier assembly according to claim 11, wherein the cable is made of braided metal fiber or braided ceramic fiber.   13. A carrier assembly according to claim 10, 11 or 12, wherein each of the elements (4) has a spherical shape.   The carrier assembly according to any one of claims 10 to 13, further comprising a damping means between at least a pair of elements.   15. A carrier assembly according to any one of claims 10 to 14, wherein the element (4) is formed from several absorbent materials.   16. The element (4) according to any one of claims 10 to 15, wherein the element (4) comprises at least some elements in a first absorbent element and another element in a second absorbent element. Carrier aggregate.   17. A carrier assembly according to any one of claims 10 to 16, wherein the element is hollow or comprises a central core and a peripheral casing made of two different materials.   18. A carrier assembly according to any one of claims 1 to 17, wherein the activation and insertion system is sensitive to temperature variations. The carrier assembly according to claim 18, wherein the activation and insertion system comprises a locking means for preventing the insertion of the insertion target assembly at a temperature lower than the furnace operating temperature. The carrier assembly according to any one of claims 1 to 19 , further comprising means for detecting insertion of the assembly to be inserted by ultrasonic telemetry. The activation and insertion device comprises a longitudinally fixed part formed by a capsule (10) and a longitudinally movable part, the capsule (10) placing the assembly to be inserted (2) in the fission region Means (11) for holding in an upwardly suspended position, wherein the assembly to be inserted can be released by operation of the movable part, and the movable part comprises a locking means and the assembly to be inserted. Means for holding in a suspended position and means for releasing the assembly to be inserted from the holding means, the locking means being formed from at least one first surface (24) called a stop surface, The means for releasing the assembly to be inserted includes at least one second surface (26) called a release surface, and means for displacing the stop surface (24) and the release surface (26) along the major axis ( 9), wherein the displacement means is formed by a shell (19) that can be expanded with a difference in the longitudinal direction with respect to the capsule (10) under the influence of a coolant temperature rise, and the stop surface (24) And the release surface (26) is configured such that when the temperature of the coolant rises, the stop surface (24) moves axially away from the holding means (11), and the release surface (26) When moving axially in a direction approaching the holding means (11) and the coolant is at a normal furnace operating temperature, the stop surface (24) is moved away from the holding means (11) and The holding means is unlocked, and when the coolant temperature exceeds a threshold temperature, the release surface (26) applies a thrust to the holding means so that the insertion target assembly is released. It is a configuration to The carrier assembly according to claim 19. The detection means comprises at least one ultrasonic transducer disposed above the capsule head, and a reflector (66) placed on the capsule head facing the transducer, the longitudinal length of the reflector (66). The directional position is controlled by whether or not the assembly to be inserted is held at a fixed position by the holding means (11), and the reflector (66) is freely within the longitudinal reaming that penetrates the capsule head. Is connected to the insertion target assembly via a long element (64) that holds the reflector (66) in its non-inserted state by being placed on the insertion target assembly. The carrier assembly of claim 21 in combination with claim 21 . Elastic means that compresses when the assembly to be inserted is present, expands when the element to be inserted is absent, and applies a tensile force to the elongated element (64) to move the reflector (66) ( The carrier assembly according to claim 22, comprising 68). A radial clearance is provided between the shell (19) and the fixed part (10) to delimit a coolant circulation channel between the shell and the capsule (10), the shell (19) 24. A carrier assembly according to any one of claims 21 to 23 , comprising an orifice for circulation of coolant therein. Said holding means (11) comprises at least two pins (20), preferably three pins, said pins being distributed around a long axis (X) and pivotably pivoted on said capsule (10). Since it is worn, it moves to a position closer to the long axis (X), holds the insertion target assembly (2) between the pins (20), and is separated from the long axis (X). The carrier assembly according to any one of claims 21 to 24 , wherein the carrier assembly is moved to a position and the insertion target assembly (2) is released at that position. The stop surface (24) is a surface that is disposed radially outward of the pin (20) and prevents the pin (20) from moving in a direction away from the long axis (X), and the release surface (26) is a surface perpendicular to the long axis (X), and the pin (20) can be provided with the cam surface (28), the cam surface and the release surface (26) being linked. 26. The carrier assembly according to claim 25, wherein the pin is pivoted in a direction away from the major axis. The shell (19) is made of austenitic steel, and the capsule (10) is made of a tungsten base alloy, or the shell (19) is made of work-hardened Z10 CNDT15.15B steel, and the capsule (10 27) The carrier assembly according to any one of claims 21 to 26 , wherein the carrier assembly is made of W-5Re. The material of the neutron absorber is selected from boron carbide (B ₄ C) variably enriched with 10B, hafnium metal, refractory boride type materials such as HfB ₂ and TiB ₂ , europium hexaboride EuB6 or Eu2O3 28. A carrier assembly according to any one of claims 1 to 27 , for a liquid metal cooled fast reactor, preferably for a sodium cooled reactor. 29. Any one of claims 1 to 28 for a water-cooled thermal neutron reactor, wherein the material used for the neutron absorber is selected from hafnium, Dy11B6, Gd11B6, Sm11B6 and Er11B4, natural HfB2 and natural TiB2. The carrier assembly according to item. A nuclear reactor comprising the carrier assembly according to any one of claims 1 to 29 .

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