Fuel Assembly For An SFR Nuclear Reactor, Comprising A Housing Containing A Removably Fastened Upper Neutron Shielding Device

Abstract

Fuel assembly for a nuclear reactor comprising a housing of longitudinal axis (X) having a central section containing nuclear-fuel pins and an upper section, forming a portion of the head of the assembly, containing a upper neutron shielding device (NSD) including neutron absorbers and means for reversibly interlocking with the housing and a moveable weight forming the head of the NSD, which is mounted so as to be able to move translationally relative to the rest of the NSD over a given path, said interlocking means being configured so that the NSD and the housing can be interlocked and uninterlocked by moving the moveable weight along the longitudinal axis by means of a grapple for extraction of the NSD, the claws of this grapple engaging with the moveable weight and the rest of the NSD being in downward longitudinal abutment in the interior of the housing.

Claims

1. A fuel assembly for nuclear reactor, in particular for sodium-cooled SFR reactor, including: a shroud of longitudinal axis intended to be inserted vertically into the diagrid of the core of the reactor, the shroud comprising a central section housing nuclear fuel rods and an upper section forming the head of the assembly housing an upper neutron shield (UNS) device including neutron absorbers, means for reversibly locking with the shroud and; a weight forming a section of the head of the UNS, wherein said section are translationally movable with respect to the rest of the UNS over a given course, wherein said locking means are configured so that the UNS and the shroud can be locked and unlocked by moving the weight along the longitudinal axis by means of a UNS-extracting gripper with the fingers of the gripper hooked into the weight and the rest of the UNS being in downward longitudinal abutment in the interior of the shroud.

2. The fuel assembly as claimed in claim 1, the head of the assembly furthermore including holes or a groove that is or are suitable for interacting with the fingers of a handling gripper in order to allow the assembly to be handled whether it is or is not equipped with its UNS, the gripper for handling the assembly having the same operating movement as that of the UNS-extracting gripper.

3. The fuel assembly as claimed in claim 1, wherein the UNS head includes a part forming a plug of the neutron absorbers of the UNS and supporting the locking means.

4. The fuel assembly as claimed in claim 1, the locking means consisting of fingers that are mounted so as to be able to pivot in a vertical plane.

5. The fuel assembly as claimed in claim 4, each of the fingers being mounted so as to be able to pivot about a pivot pin fastened to the plug.

6. The fuel assembly as claimed in claim 4, the weight including fixed pins that are each suitable for sliding in the interior of a slot in a pivoting finger, a vertical translational movement of the weight causing the pins to slide in the slots and thus the fingers to pivot.

7. The fuel assembly as claimed in claim 1, the weight including an interior groove into which the fingers of the UNS-extracting gripper may be hooked.

8. The fuel assembly as claimed in claim 1, the shroud including an interior groove into which the fingers of the locking means may insert to form an upper stop for the UNS.

9. (canceled)

10. The fuel assembly as claimed in claim 1, wherein the UNS includes one or more hollow columns that is or are fastened to the plug and that pass through the weight, the one or more columns being suitable for being brought to bear against a translationally movable part of the UNS-extracting gripper, in order to create an ascendant relative movement between the weight and the rest of the UNS during the unlocking operation.

11. The fuel assembly as claimed in claim 1, wherein the UNS includes a ferrule that is exterior to the plug, the ferrule being suitable for being brought to bear against a translationally movable part of the gripper in order to create an ascendant relative movement between the weight and the rest of the UNS during the unlocking operation.

12. (canceled)

13. The fuel assembly as claimed in claim 1, wherein the UNS includes a sleeve housing and supporting blocks of neutron absorber, and a plug fastened to the top of the sleeve.

14. The fuel assembly as claimed in claim 1, wherein the UNS includes a wrapper rods of neutron absorber, and a plug fastened to the top of the wrapper and supporting the rods.

15. The fuel assembly as claimed in claim 1, including a part fastened to the interior of the shroud, forming the lower axial longitudinal downward stop of the UNS.

16. The fuel assembly as claimed in claim 1, wherein the neutron absorbers placed in the UNS are chosen from boron carbide (B.sub.4C), hafnium (Hf), hafnium diboride (HfB.sub.2), titanium diboride (TiB.sub.2), ferroboride (FeB), uranium dioxide (UO.sub.2), the rare earths.

17. A method for handling a fuel assembly whether it is or is not equipped with its UNS as claimed in claim 1, wherein a handling gripper that is of the same type as, and preferably identical to, that used for the extraction of the UNS is used.

18. A method for equipping a new fuel assembly not equipped with a UNS, with an irradiated UNS extracted from an irradiated fuel assembly as claimed in claim 1.

19. The use of a fuel assembly as claimed in claim 1, in a fast neutron nuclear reactor.

20. The use as claimed in claim 19, the reactor being liquid-metal or gas-cooled, the liquid metal being chosen from sodium, lead or lead-bismuth.

Description

DETAILED DESCRIPTION

[0093] Other advantages and features of the invention will become more clearly apparent on reading the detailed description of the invention given by way of nonlimiting illustration with reference to the following figures, in which:

[0094] FIG. 1 is an external perspective view of a fuel assembly according to the prior art, already used in a sodium-cooled SFR nuclear reactor;

[0095] FIG. 2 is a longitudinal semi-cross-sectional view of the head of a fuel assembly according to the prior art showing the upper neutron shield (UNS) device, which has already been used in the “Phénix” nuclear reactor;

[0096] FIG. 2A is a transverse cross-sectional view of the UNS of the assembly in FIG. 2;

[0097] FIG. 3 is a longitudinal cross-sectional view of the head of a fuel assembly according to the prior art showing the upper neutron shield (UNS) device, which has already been used in the “Superphénix” nuclear reactor;

[0098] FIG. 3A is a transverse semi-cross-sectional view of the UNS of the assembly in FIG. 3;

[0099] FIG. 4 is a longitudinal cross-sectional view of the head of a fuel assembly according to the prior art showing the upper neutron shield (UNS) device, which was the preliminary version envisioned for the “ASTRID” nuclear reactor;

[0100] FIG. 4A is a transverse cross-sectional view of the UNS of the assembly in FIG. 4;

[0101] FIG. 5 is a partial longitudinal cross-sectional view of an exemplary fuel assembly according to the invention showing the upper neutron shield (UNS) device, which is intended to be used in the “ASTRID” nuclear reactor;

[0102] FIG. 5A is a partial longitudinal cross-sectional view of the assembly head in FIG. 5;

[0103] FIG. 5B is a top view of the assembly head in FIG. 5;

[0104] FIGS. 6A to 6E are partial longitudinal cross-sectional views illustrating various steps of handling, of inserting and of locking a UNS in an exemplary fuel assembly according to the invention such as shown in FIGS. 5 to 5B;

[0105] FIG. 7 is a longitudinal cross-sectional view level with the head of a variant of the shroud of a fuel assembly according to the invention;

[0106] FIG. 8 is a partial longitudinal cross-sectional view of another exemplary fuel assembly head according to the invention showing the upper neutron shield (UNS) device, which is intended to be used in the “ASTRID” nuclear reactor;

[0107] FIG. 8A is a top view of the assembly head in FIG. 8 showing the arrangement of the gripping head of the UNS;

[0108] FIG. 8B is a transverse cross-sectional view of the UNS in FIG. 8 showing the arrangement of the absorbing elements of the UNS;

[0109] FIGS. 9A and 9B are partial longitudinal cross-sectional views illustrating two steps of locking and unlocking a UNS in another exemplary fuel assembly according to the invention such as shown in FIGS. 8 to 8B.

[0110] For the sake of clarity, in FIGS. 1 to 9B the same references have been used to refer to the same elements of the fuel assembly and elements of the upper neutron shield (UNS) devices independently of whether they are according to the prior art or according to the invention.

[0111] Throughout the present application, the terms “vertical”, “lower”, “upper”, “bottom”, “top”, “below” and “above” are to be understood to be with reference to a fuel assembly such as it is in its vertical configuration in a nuclear reactor.

[0112] FIGS. 1 to 4A, which relate to the prior art, have already been described in detail in the preamble and are therefore not commented on below.

[0113] Now, with reference to FIG. 5, a fuel assembly 1 according to the invention, such as it is intended to be used in an ASTRID SFR nuclear reactor, will now be described. Just as for the fuel assemblies according to the prior art that were intended for fast neutron reactors, the assembly 1 according to the invention is elongate along a longitudinal axis X and comprises a shroud 10 of hexagonal cross-section, the upper section 11 of which forms the head of the assembly, and which contains a neutron shield device 2 called the UNS. The central section 12 of the assembly 1 contains fuel rods (not shown).

[0114] Lastly, the assembly 1 comprises a lower section 13 forming the nose of the assembly, in the continuation of the shroud 10. The nose 13 of the assembly has a cone-shaped or rounded distal end in order to be able to be inserted vertically into the diagrid of a reactor core. The nose 13 of the assembly also includes on its periphery orifices leading therein for the circulation of sodium in the interior of the assembly.

[0115] The head 11 of the assembly includes within it an interior passage 100 left free by the UNS 2 and that leads to a central orifice 110 that itself leads toward the exterior (FIGS. 5 and 5A). The head 11 of the assembly also includes a continuous interior groove 110 produced in the shroud 10 and a lower supporting part 102 fastened to the interior of the shroud 10.

[0116] Such as illustrated in FIGS. 5 to 6E, the head 11 of the assembly according to the invention includes holes 18 that are regularly distributed angularly and that are each suitable for interacting with a finger of a gripper for handling the assembly as explained below.

[0117] Such as illustrated in FIGS. 5 to 6E, the UNS 2 according to the invention includes a sleeve 20 housing blocks 21 of boron carbide B.sub.4C, by way of neutron absorbing materials.

[0118] The UNS 2 also includes a plug 23 fastened to the top of the sleeve 20 and that maintains the blocks 21 in the latter.

[0119] The UNS 2 also includes above the plug 23, a weight 24 forming the head of the UNS. The weight 24 is mounted so as to be free to move translationally with respect to the plug 23 but only over a given course, stops internal to the plug 23 and to the weight 24, formed by shoulders 231, 242, mutually interacting to maintain them together once the course has been travelled. The weight 24 has a continuous interior groove 240 that is suitable for interacting with the fingers of a UNS-extracting gripper 3 as explained below. The weight 24 lastly incorporates three fixed pins 241.

[0120] The head of the UNS 2 also includes locking fingers 25 that are mounted so as to be able to pivot about a fixed pivot pin 230 of the plug 23 in such a way that the fingers 25 pivot in vertical planes. As illustrated in FIG. 5B, the locking fingers 25 are three in number and distributed at 120° from one another. It goes without saying that the number of fingers 25 may be different, though they will preferably still be regularly distributed angularly around the periphery of the crown 24. Each of the fingers 25 includes a locking end 250 that is suitable for interacting with a continuous interior groove 110 produced in the shroud 10, and a through-slot 251 that is of oblong shape in the illustrated example.

[0121] According to the invention, the fact that the weight is mounted so as to be free to move translationally allows, when the weight 24 is moved toward the plug 23, each fixed pin 241 to slide in the interior of a slot 251 thereby causing a finger 25 to pivot in a vertical plane and toward the exterior of the UNS 2 and thus the finger 25 to be inserted into the interior groove 101 of the shroud 10, as detailed below. The weight 24, which then rests on the fingers 25 by way of the pins 241, prevents them from pivoting toward the interior of the UNS and locks them in position in the groove 101.

[0122] Thus, when the UNS 2 is in its locked position in a fuel assembly 1, i.e. when it is such as illustrated in FIGS. 5, 5A, 5B, 6C, 6D, 6E, 8, 8A, 8B and 9B, the lower portion of the UNS 2, i.e. the bottom of the sleeve 20, is supported by the supporting part 102 that is fastened to the interior of the shroud 10, thereby making it possible to ensure the UNS 2 is held laterally and that any downward translational movement is blocked, and the upper portion of the UNS 2 is locked in place, i.e. by its weight 24, via the insertion of the fingers 25 into the groove 101 of the assembly head 11, this making it possible to ensure any upward translational movement is blocked.

[0123] Advantageously, one or more hollow columns 26 is or are arranged and fastened to the plug 23 and also pass through the weight 24 (FIGS. 6A to 6E). Preferably, these columns 26 are three in number and distributed at 120° from one another. It goes without saying that the number of columns 26 may be different, though they are preferably regularly distributed angularly around the periphery of the plug 23. In the extreme position of separation of the plug 23 and the weight 24, as illustrated in FIG. 6A, the one or more columns protrude from the latter.

[0124] Each of these hollow columns 26 has the following functions: [0125] it forms a slide-connection between the plug 23 and the weight 24, in order to give a maximum robustness to the relative translational movement between these two components; [0126] it forms a vent allowing the sleeve 20 to be filled with sodium and helium to exit from the sleeve; [0127] it allows the head 30 of an extracting gripper 3 described below to mechanically force the fingers 25 to pivot during the operation of unlocking the UNS.

[0128] It will be noted here that in the context of the invention the expression “extracting gripper” is used to designate the gripper 3 used to grip the UNS 2 by way of the weight 24, because this gripper is not intended to be used to insert the UNS 2 into the rest of the assembly in the reactor vessel. In other words, the gripper 3 is not intended to be used in a reactor vessel for this inserting operation.

[0129] Thus, when the UNS 2 is to be unlocked from the assembly head 11, the head 30 of the gripper 3 is brought to bear against each column 26 in order to create a relative ascending movement between the weight 24 and the rest of the UNS 2, and therefore mechanical seizure effects that are liable to be seen after a stay in sodium are mitigated. In other words, by virtue of these columns 26, it is possible to ensure the UNS may be reliably unlocked even in case of mechanical seizure.

[0130] All of the locking/unlocking means described are designed to minimize the risk of mechanical seizure. All the movements of the various means require no precise fits and there may be large amounts of play between all the parts. The function allowing an eventual seizure to be forced via the columns 26 allows the robustness of the unlocking assembly to be improved, and therefore on-line extraction of the UNS from its assembly to be guaranteed and hence the level of availability of the nuclear reactor containing the assemblies according to the invention to be guaranteed.

[0131] With reference to FIGS. 6A to 6E, the steps of lowering, of inserting and of locking the UNS 2 into the fuel assembly 1 will now be described in chronological order, these steps being carried out by means of the extracting gripper 3.

[0132] It will be noted that, as already mentioned, the gripper is not intended to be used to insert the UNS 2 into the fuel assembly 1 while it is in the ASTRID reactor, but rather to be used during mounting operations carried out outside the vessel. Nevertheless, the insertion of the UNS into the assembly with the extracting gripper 3 is described in order to describe the operation of the locking/unlocking means. Furthermore, this inserting operation may take place outside the reactor vessel, in particular in the external storage barrel, and it is the inverse of the extracting operation.

[0133] The extracting gripper 3 grips the UNS 2 by the weight 24 of the UNS. The extracting gripper 3 includes a head 30 in which gripping fingers 31 are mounted so as to be able to pivot in a vertical plane, and the head 30 of the gripper is mounted so as to be free to move translationally with respect to the fingers 31. Insertion of the fingers 31 in the interior groove 240 of the weight 24 allows it to be gripped and the fact that the head 3 is mounted so as to be free to move translationally with respect to the rest of the gripper 3 allows, when the UNS 2 is held by the fingers 31, a relative axial movement to be created between the weight 24 and the plug 23.

[0134] A phase of approach and insertion is first carried out in which the gripper 3 inserts the UNS 2 into the assembly 1 along its longitudinal axis X (FIG. 6A) until the bottom of the sleeve 20 makes contact with the supporting part 102 that is fastened to the shroud (FIGS. 5A and 6B).

[0135] The vertically downward translational movement of the head 30, which is free to move translationally, of the gripper 3 is continued, thus creating a relative axial movement between the weight 24 and the plug 23. The stops, formed by the lower shoulder of the weight 24 and the upper shoulder of the plug 23, respectively, then move further apart. Moreover, the downward vertical translational movement of the weight 24 causes the fingers 25 to pivot toward the exterior because this downward movement also causes each of the pins 241 fixed to the crown 24 to slide in a corresponding slot 251 of a finger 25. By pivoting toward the exterior, the fingers 25 insert into the interior groove 101 of the shroud 10, this preventing any relative upward translational movement of the UNS 2 in the fuel assembly 1, and thus locking the UNS 2 in place.

[0136] The lowering of the head 30 of the gripper 3 continues until the weight 24 abuts against the plug 23 (FIG. 6C).

[0137] The grip of the gripper 3 is then deactivated by pivoting the fingers 31 toward the interior (FIG. 6D). The gripper 3 may then be removed from the fuel assembly 1.

[0138] Lastly, the gripper 3 is raised, the UNS 2 being inserted and locked into the fuel assembly 1 by means of the fingers 25 inserted and held in the groove 101 of the shroud 10 (FIG. 6E). The weight of the weight 24 guarantees the UNS 2 is maintained and locked in the head 11 of the fuel assembly despite the ascending hydraulic thrust applied by the coolant in operation.

[0139] The steps used to unlock and extract the UNS 2 from the fuel assembly 1 will now be described in chronological order.

[0140] In the locked position, such as illustrated in FIG. 6E, the weight 24 and the plug 23 are in abutment and the columns 26 protrude from the weight 24. Provision is made for the height of the protrusion to be slightly smaller than the maximum relative axial movement between the weight 24 and the plug 23.

[0141] The handling gripper 3 is lowered until the translationally movable head 30 abuts against the columns 26.

[0142] After the weight 24 has been gripped by the pivoting fingers 31 of the gripper 3, i.e. the fingers inserted into the groove 240, it is possible to move the weight 24 translationally relative to the plug 23 and therefore to cause the locking fingers 25 to pivot toward the interior. The fingers 25 are made to pivot by the pins 241 sliding in the slots 251.

[0143] The fingers 25 are then extracted from the groove 101 of the shroud 10 and the UNS 2 is unlocked from the rest of the fuel assembly 1.

[0144] When the upper transverse plane of the columns 26 reaches level with the upper transverse plane of the weight 24, the translationally movable head 30 can no longer create a relative axial movement between the weight 24 and the plug 23.

[0145] Then only the upward translational movement of the gripper 3 then allows the extraction of the weight 24 to continue until the shoulder 242 of the bottom portion of the weight 24 abuts against the shoulder 231 of the top portion of the plug 23. The UNS 2 is then raised by the gripper 3 then extracted out of the fuel assembly 1.

[0146] A fuel assembly 1 according to the invention with its connection for locking/unlocking its UNS 2 as just described allows the functional specifications of a fourth-generation fast neutron nuclear reactor such as ASTRID to be met.

[0147] Other variants and improvements may be provided without however departing from the scope of the invention.

[0148] Thus, it is advantageously possible to measure the axial movement of the gripper 3 during the steps of inserting and of locking the UNS 2 in the fuel assembly 2 in order to guarantee that the targeted lock is operational.

[0149] Such as illustrated in FIGS. 5 to 6B, the various components of the UNS 2 and the locking components are designed to minimize pressure losses in the flow of sodium. This also easily allows the lock to be made safer, i.e. to guarantee the absence of any risk of ejection of the weight 24 during operation of the nuclear reactor.

[0150] It is possible for the extracting gripper 3 not to comprise a translationally movable part. Specifically, in the absence of seizure, raising the weight 24 with the gripper alone may allow the locking fingers 25 to be rotated and thus the UNS 2 to be unlocked.

[0151] As regards the actual handling of the fuel assembly 1 by its shroud 10, instead of holes 18 provision may be made for a continuous groove 19 in the interior wall of the shroud 10, as illustrated in FIG. 7. This continuous groove 19 is also suitable for interacting with the fingers 31 of a handling gripper having the same operational movement as the extracting gripper 3.

[0152] As regards the actual form of the UNS 2, instead of a sleeve 20 housing the blocks 21 of neutron absorbers, provision may be made for a cylindrical wrapper 27 housing a plurality of neutron absorber rods 28 that are arranged in the form of a bundle, as illustrated in FIGS. 8 to 8B. As may be best seen in FIG. 8, the lower end 270 of the support 27 again abuts against the supporting part 102, in the inserted and locked position of the UNS 2 in the fuel assembly 1.

[0153] In the illustrated examples, the wrapper 27 has a circular transverse cross-section, but it could have a different cross-section, for example a hexagonal cross-section inter alia.

[0154] One of the functions of the wrapper 27 is to protect the rods 28, in particular during the extraction of the UNS 2 from the rest of the fuel assembly 1. Instead of the wrapper 27, other wrapperless structures could however be envisioned, for example grids for holding the rods inter alia.

[0155] Instead of the columns 26, provision may be made for a ferrule 29 securely fastened to the plug 23 and arranged on the periphery of the weight 24. The upper end of the ferrule 29 is located level with the upper plane of the weight 24 in the unlocked position (FIG. 9A) and it protrudes in the unlocked position (FIG. 9B). The arrangement of the bearing ferrule 29 on the periphery of the weight 24 allows a maximum of space to be created at the center and on the periphery of the weight 24 and therefore pressure losses to be limited, thereby promoting the flow of sodium through the crown 24. Orifices 271, which are advantageously three in number and regularly distributed spaced apart by 120° from one another, are provided in the upper portion of the weight 24 (FIG. 8A). Furthermore, in order to allow the sodium to flow over the rods 28, vents (not shown) are integrated into the upper portion of the plug 23.

[0156] As regards the structures for holding the bundles of rods, various options that have been developed for the fuel assemblies and assemblies for controlling reactivity of SFRs may be envisioned. It is possible to envision a system of rails for supporting rods, in general in the bottom portion for the bundles of fissile rods of the fuel assemblies, and in general in the top portion for the assemblies for controlling the reactivity (suspended bundle). Next, in particular depending on the number of rings of rods and their diameter, provision may in particular be made for: [0157] a cylindrical wrapper of circular cross-section associated with spacer wires, as shown in FIGS. 8 to 9B; [0158] a central shaft connecting the supporting rails to one or more spacer grids located along the bundle of rods; [0159] a toroidal ring deployed level with the plugs located at the other end with respect to the supporting rails.

CITED REFERENCE

[0160] [1]: Text book “Réacteurs à neutrons rapides refroidis au sodium”—Les techniques de l'Ingénieur B 3 171