Patent Number: 056065825
Section: description

DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION FIG. 1 shows in front view the vessel 1 of a pressurized water nuclear reactor, sealed in its upper portion by a hemispherical cover 2 supporting tight enclosures 3. Within each of the tight enclosures 3 is placed a mechanism 4 permitting the displacement of a control rod 5 of an absorber fuel rod cluster 6. This mechanism is preferably of the screw-nut type with a separable screw. The absorber rod cluster 6 is placed in the lower portion of the control rod 5 in suspended manner. The fuel rods have a neutron absorbing power. The core of the reactor 7 is constituted by a juxtaposed fuel assembly system 8. The upper limit of the core 7, just above said fuel assemblies 8, is defined by a core plate 9. The regulation of the reactivity of the core 7 is consequently ensured by introducing to a more or less deep extent the absorber clusters 6, into certain of the fuel assemblies 8 of the core 7. Above the core 7, the absorber clusters 6 slide within cases referred to as "cluster guides" constituted by two superimposed elements 10a, 10b vertically suspended on an upper support plate 11 attached to the vessel and connected to the core plate 9 by fixing posts 12. These equipments located above the core form a rigid assembly called "upper internal equipments". Perforated transverse plates 13 are placed at different levels within the cluster guides 10b and ensure a discontinuous guidance of the different elements of the cluster. Another continuous guidance element 14 is installed in the lower portion of the lower cluster guide element 10b, in an area where the water passes radially out of the cluster guide in the direction of the vessel outflow tubes. In the right-hand part of FIG. 2 and in particular FIG. 4, showing the lower portion of the device 16 according to the invention, it is possible to see on a larger scale the continuous guidance element 14 constituted by slotted sleeves 15 positioned in a vertical manner. The device 16 connects the control rod 5 to a fixing part 17 for the absorber clusters 6. The fixture is constituted by a pommel 18 for assembling with the control rod 5 and equipped with fuel rod-supporting, radial arms 19. Core reloading or similar operations are performed in water, following the dismantling of the vessel sealing cover. During these operations, the temperature within the vessel must be kept below a certain level, e.g. 60.degree. C., and the absorber clusters 6 must remain completely inserted within the fuel assemblies 8, not shown in FIG. 2, their pommel 18 resting on the upper end piece 20 of the fuel assembly 8. A special tool for manipulating the device according to the invention can be stowed at the upper end of the control rod 5, once the vessel cover has been removed. The control rods 5 are removed by raising upper internal equipments, once the control rods 5 are disconnected and disengaged from the pommel 18 of the associated absorber cluster. During this removal operation, the control rods 5 are raised by upper internal equipments, by means of a shoulder 21 which they have in the lower portion and which is driven by attachment teeth 22 provided on the continuous guidance sleeves 15. The pommel 18 of the absorber cluster 6 has an opening 23 and a cylindrical recess 24 for receiving the lower end of the control rod 5. A cylindrical locking chamber 6 is machined within the recess 24. The upper portion of the locking chamber 26 is connected to the recess 24 by a conical contraction 27. The control rod 5 is constituted by a cylindrical sheath 28, contracted in its lower portion over a certain height and then terminated by a solid shaft 29 penetrating the opening 23. The sheath 28 of the control rod 5 is provided in the lower part of this contraction on the same axial side with a series of truncated cone-shaped radial notches 30 and whose base issues into the interior of the sheath 28. In the interior of each of these radial notches 30 is placed a ball 31, whose diameter exceeds the thickness of the sheath 28. Each ball 31 is mobile within its radial notch 30. The locking chamber 36 permits a disengagement of these balls 31 towards the outside of the control rod 5. The contracted portion of each of the radial notches 30 prevents a complete disengagement of each ball 31 in this sense. A locking member 32 is installed in the lower part of the control rod 5. This locking member 32 is central and coaxial with respect to the control rod 5 and displaceable in accordance with its longitudinal and vertical axis. An annular locking groove 33 is machined in the bottom of the locking member 32, which extends beyond the contraction of the sheath 28 of the control rod 5 and has, above said contraction, a widening 34 tightly connected to the lower end of a metal bellows 35, deformable in accordance with the axis of the control rod 5. According to a main feature of the invention, the top of the locking member 32 is engaged within an inner tube 36 terminated in its upper part by an attachment head 37 surmounted by a fastening 38. The tube 36 has a widening 39 tightly connected to the upper end of the bellows 35. A first spring 40 is placed within the inner tube 36 and tends to disengage the locking member 32 from the inner tube 36. The inner tube 36 has a vertical recess 41 receiving a guide and stop pin 42 radially fixed to the upper end of the locking member 32. The recess 41 links the internal volume of the inner tube 36 with that of the bellows 35. These two volumes contain an incompressible or almost incompressible substance, whose volume varies with the temperature to a significant extent. With these conditions satisfied, this substance must be compatible with the neutron flux radiation and also, in the case of a sealing loss of the bellows 35, with the water of the reactor and the chemical elements therein and with advantage use can be made of demineralized water. The attachment head 37, bellows 35 and locking member 32 constitute in this first embodiment a rigid, coaxial assembly able to slide within the control rod and whose length is consequently dependent on the temperature. An opening 43 within the locking member 32 and linked with the interior of the bellows 35 can also be filled with water and constitutes an additional volume making it possible to achieve the desired deformation characteristics of the thermal module. A second spring 44, interposed between a detachment 45 of the inner space of the control rod 5 and a projecting portion 46 of the attachment head 37, pushes towards the bottom said head. The stiffness of the second spring 44 is greater than that of the first spring 40. The attachment head 37 carries a finger 47 pushed back by a small spring 48. This attachment finger 47 is transversely mobile in order to project into the inner space of the control rod 5 or is partly or totally retracted into the inner space of a cylinder 49. The latter bears on a contraction 50 of the internal space of the control rod 5 and can turn by a fraction of a turn with respect thereto. The extent of this rotation is limited by the displacement of a radial guide and stop pin 51, integral with the cylinder 49 and in part engaged in a horizontal slot 52 made in the thickness of the sheath 28 of the control rod 5 over a circumferential sector of said sheath 28. In its upper portion, the internal space of the cylinder 49 has a widening 53 issuing onto the internal space of the control rod 5. This internal space of the cylinder 49 also has an axial, vertical notch 54, whose cross-section is that of a capital U, as illustrated in FIG. 3B. The lower end of the notch 54 is sealed by a shoulder 55. Its upper end issues into the thickness of the cylinder 49 level with the widening 53 and its edges 56 are rounded. An extractor tube 47 is mounted in sliding manner on the lower end of the control rod 5 and has a third internal spring 58 mounted on the contracted portion of the control rod 5 and interposed between the base of the contraction 59 and an internal surface 60 made within the extractor tube 57. This third spring 58 is prestressed in such a way as to produce a pressure on the control rod 5 which is at least adequate to raise it, the lower end of the extractor tube 57 being placed on the cluster pommel 18. Another guide and stop pin 61 is radially fixed in the contracted portion of the sheath 28 of the control rod 5 and projects inwards and outwards of said sheath 28. The portion of said guide pin 61 passing outside the sheath 28 is engaged in a vertical slot 62 made in the thickness of the extractor tube 57 parallel to its axis. The other end of the guide and stop pin 61 is engaged in a vertical recess 63 formed in the locking member 32 along the axis of the latter. With reference to FIG. 3, the extractor tube 57 has a slot 64 constituted by a vertical branch 64a, oriented parallel to the tube axis and issuing onto a second branch, oriented parallel to the tube axis and issuing onto a second, inclined branch 64b in the form of a helix portion. The slot 64 is engaged on the radial pin 51, which is integral with the cylinder 49 and collaborates with the horizontal slot 52 of the control rod 5. The horizontal slot 52 in the sheath 28 of the control rod 5 can be better seen in FIG. 4. It can be seen that it permits a rotation, whose extent is determined by its width for the cylinder 49 and the retraction of the abutment constituted by the shoulder 55 sealing the lower end of the notch 54. The device is completed by a long dismantling rod 66, positioned axially and centrally in the control rod 5 and terminated in its lower portion by a boss 67, which penetrates a fastening 38 of the attachment head 37. The dismantling rod 66 rises to the top of the control rod 5, where are located not shown, control means permitting the vertical actuation thereof and the fixed maintenance thereof in an axial position, where its terminal boss does not impede the displacements of the attachment head 37. The control rod 5 comprises not shown means making it possible to stop the rotation of said rod, e.g. with respect to the continuous guide 14 installed in the lower cluster guide 10b. The control rod 5 also comprises not shown means making it possible to link in rotation said rod and the cluster pommel 18 once these elements have been engaged. OPERATION OF THE DEVICE With reference to FIGS. 5A to 5D, the operation of the aforementioned device will now be explained. Consideration will be given to the case of a reactor during different phases between two core fuel resupplies. When the reactor is operating, i.e. at 350.degree., e.g. for producing electricity, the components of the device assume the position indicated in FIG. 2. FIG. 5A shows the main components of the device in this position. The control rod 5 is coupled in translation to the absorber cluster 6. The locking member 32 is in the lower position and has in front of the balls 31 a solid part keeping them in the "out" position, i.e. partly engaged in the thickness of the sheath 28 of the control rod 5 and in the locking chamber 26 of the pommel 18 of the absorber cluster 6. The locking member 32 is maintained in the lower position under the effect of the pressure of the second spring 44. Its lower end abuts against the bottom of the contraction of the sheath 28 and the control rod 5. Thus, the balls 31 transmit to the pommel 18 of the absorber cluster 6 the axial forces exerted by mechanism 4 in FIG. 1 on the control rod 5. The third spring 58 tends to permanently move apart the control rod 5 and the pommel of the absorber cluster 6 by pushing back the extractor tube 57. It therefore tends to maintain a permanent contact between the balls 31 and their recess 30 and the conical contraction 37 of the pommel 18. This avoids or renders acceptable any impacts between the balls 31 and the parts in question. Such impacts can e.g. result from vertical accelerations and decelerations transmitted to the control rod 5 by the mechanism during its operation and which are liable to produce a deterioration of the contacting surfaces. The attachment finger 47, which is pushed back by the small spring 48, is in a position projecting from the attachment head 37 and its end abuts against the interior of the sheath 28 of the control rod 5. In this position, a temperature rise or fall close to the device gives rise to a lengthening or shortening of the thermal module constituted by the water-filled bellows 35. These length changes are compensated by a crushing or an expansion of the second spring 44, the locking member 32 remaining in the lower position. They produce a rise or fall of the attachment head 37, the attachment finger 47 carried by the latter moving axially within the internal space of the sheath 28 of the control rod 5. If the temperature is sufficiently lowered, the attachment finger 47 descends within the cylinder 49. At the end of penetration, the entrance chamfer 58 pushes back the attachment finger 47 within the attachment head 37, until it comes into an intermediate radial position authorizing its displacement within the notch 54 of the cylinder 49. When a certain temperature is reached, e.g. 150.degree. C., the attachment finger 47 can abut, as shown in FIG. 5B, against the shoulder 55, which seals the notch 54 of the cylinder 49. If the temperature continues to drop, the attachment head 37 is stopped in its descent and the shortening of the thermal module, i.e. the bellows 35, is accompanied by a raising of the locking member 32. When a given temperature is reached, the groove 33 of the locking member 32 comes level with the balls 31 and allows the disconnection enabling said balls 31 to pass out of the locking chamber 26 of the pommel 18, in order to be placed in the "return" position within said groove 33. This disconnection temperature can e.g. be 80.degree. C. for a vessel cover raising temperature below 60.degree. C. During uncoupling, the clusters 6 can e.g. be suspended on the control rods 5, the base of the pommels 18 being a few centimeters above the upper end piece 20 of the fuel assembly. In this uncoupling position, the control rod 5 is disengaged from the pommel 18, which drops onto the upper end piece 20 of the fuel assembly under the combined effect of the weight of said cluster and the pressure of the third spring 38, to which it is exposed by means of the extractor tube 57. During the descent of the pommel 18, the extractor tube 57 is pushed downwards by the third spring 58 and slides on the control rod 5 so as to take the position previously occupied by the pommel 18. This renders impossible, in redundant manner with the conical contraction of the recess 30 of the balls 31, the loss of said balls 31 when the latter have been disengaged from the pommel 18. This movement of the extractor tube 57 is stopped when a radial, inner surface 58 machined in the interior of said extractor tube 57, comes into contact with an external shoulder 69 of the control rod 5. When the control rod 5 has become disengaged from the pommel 18 of the absorber cluster 6, the control rod is placed on the pommel 18 with the aid of mechanism 4 in FIG. 1. The position then assumed by the main components of the device and the cluster pommel is diagrammatically shown in FIG. 5C. The extractor tube 57 abuts the pommel 18 of the absorber cluster 6, which rests on the upper end piece of the cluster. The third spring 58 then raises the control rod 5 and maintains the sheath 28 and solid shaft 29 of rod 5, which terminates it respectively out of the cylindrical recess 24 and out of the opening 23 of the pommel 18. The deformation characteristics of the thermal module constituted by the bellows 35 and the geometry of the locking groove 26 are such that a temperature drop below the uncoupling temperature has no effect on the position of the balls 31. Once the vessel cover has been raised, the control rods 5 are discharged at the same time as the upper internal equipments, the shoulder 21 bearing on the attachment teeth 22 of the continuous guide 14 in FIG. 1, when the upper internal equipments are raised. The arrangement of the control rod 5 with respect to the pommel 18 of the absorber cluster 6, obtained by the device according to the invention, avoids the untimely raising of a cluster, during the retraction of the upper internal equipments. Following core reloading, the upper internal equipments are put back into place at the same time as the control rods 5 and the latter are in a relative position, with respect to their respective pommel 18, identical to those which they had before cover removal and then that of the upper internal equipments. This position is indicated in FIG. 5C. The upper end of the control rods 5, in this position, extends beyond the cluster guide elements 10. When it is necessary to recouple the control rods 5 and the absorber clusters 6, use is made of the device manipulating tool, which is engaged in the upper portion of each rod 5. In order to obtain its recoupling, the control rod 5 is lowered to a conical surface 70 connecting the lower end of its sheath 28 and its terminal shaft 29 abuts against the contraction 71 at the entrance of the opening 23 of the pommel 18. The descent takes place under the effect of the weight of the tool, which ensures that the equipment is not damaged by the application of an overload. The descent of the control rod 5 passes the radial pin 51, connected to the inner cylinder 49, into the longitudinal portion 64a of the slot 64 of the extractor tube 56 and then, when the balls 31 pass the recess 24 of the pommel 18, into the helical portion 64b of said same slot. The displacement of the radial pin 51 in said helical portion 64b gives rise to the rotation by a fraction of a turn of the cylinder 49, which retracts the shoulder 55 of said cylinder and brings the attachment finger 47 into the complete retraction position within the attachment head 37 to which it is fixed. This attachment head 37 is then freed and the assembly which it forms with the thermal module, i.e. the bellows 35, and the locking member 32 descends until the latter arrives in the lower position, under the effect of gravitational forces exerted on the different elements and the pressure of the second spring 44 applied to the attachment head 37. The descent of the locking member 32 pushes the balls 31 towards the outside of their conical recess 30 in the sheath 28 and passes them out into the locking chamber 26 of the pommel 18. When the force on the control rod 5 is slackened, the latter rises under the effect of the pressure of the third spring 58 until the balls 31 come into contact with the conical contraction 27. During this rise, the radial pin 51 passes into the helical portion 64b of the slot 64 and rotates the cylinder 49 until it is brought, when the longitudinal, vertical, portion 64a of the slot 64 is joined into a starting position where the notch 54 coincides with the trajectory of the attachment finger 47. Coupling is then implemented and the position assumed by the components of the device according to the invention is shown in FIG. 5D. The vessel cover can then be put back into place. The special manipulating tool of the device can also be used for manually uncoupling the control rod 5 from the absorber cluster 6, in the case of a malfunctioning of the automatic disconnection system, e.g. in the case of a sealing loss of the bellows 35. Therefore the device according to the invention is designed in such a way that, if the seal of the bellows 35 is broken, the locking member 32 is brought and/or immobilized in a bottom position. Thus, coupling is maintained when the reactor is cooled. This is obtained by means of the first spring 40 exerting a permanent downward pressure on the locking member 32 which, in the case of a rigidity loss of the thermal module, brings and/or maintains said member 32 in the bottom position. Emergency manual uncoupling takes place by raising the dismantling rod 66 with the aid of the special tool, the control rod 55 being kept fixed by said same tool. The rising of the dismantling rod 66 displaces the locking member 32 when a certain distance has been covered. The boss 67 terminating the dismantling rod 66 firstly raises the attachment head 37 engaging with its fastening 38 and then, if the bellows has fractured, the pin 42 of the locking member 32 abuts against the bottom of the recess 41 of the tube 36 of the attachment head 37, which raises the member 32. The latter is brought into the upper position, the lower end of the recess 63 thereof abutting against the pin 61 traversing the sheath 28 of the control rod 5. The groove 33 in the locking member 32 is then positioned in front of the balls 31, which allows uncoupling to take place. Finally, with the aid of the tool, the control rod 5 is raised under the effect of the pressure of the third spring 58 until it is disengaged from the pommel 18 and the dismantling rod 66 is lowered into the inoperative position. The position assumed by the components of the device and the pommel 18 of the absorber cluster 6 is the same as after an automatic disconnection followed by a placing of the rod on the pommel (FIG. 5C). The neutralization of the automatic uncoupling requires that the temperature is lowered below the locking temperature of the attachment head 37 by an abutment of the attachment finger 47 against the shoulder 55 of the inner cylinder 49. It is also necessary for the absorber cluster 6 to be brought into a position inserted in the fuel assembly, its pommel 18 resting on the upper end piece of the assembly. With these conditions fulfilled, neutralization takes place by exerting a pressure on the control rod 5 with the aid of its displacement mechanism until its conical surface 70 is made to abut against the contraction 71 of the pommel 18. The lowering of the control rod 5 causes, as in the manual recoupling operation with the aid of the special manipulating tool described hereinbefore, a complete return of the attachment finger 47 into the attachment head 37, which authorizes a lowering of the head 37 within the cylinder 49 and is accompanied by a lowering of the locking member 32 into its bottom position. Under these conditions, a subsequent temperature drop causes a descent of the attachment head 37, the locking member 32 remaining in the lower position. Thus, the automatic uncoupling is neutralized and the internal components of the device according to the invention assume, for the same temperature, the position shown in FIG. 5D. Automatic uncoupling is again obtained after an adequate temperature rise to bring the attachment finger 47 above the shoulder 55 of the cylinder 49. Finally, if the temperature has been brought beyond the uncoupling temperature without neutralization of automatic uncoupling, it is possible to recouple without having to open the vessel cover. This recoupling takes place with the aid of the displacement mechanism, the procedure being the same as when this operation is performed with the aid of the special manipulating tool. PROPOSED VARIANT A second embodiment of the device according to the invention is shown in FIG. 6. It only differs from the first in that use is made of a locking member in two parts, together with a second, pressure-sensitive module. Thus, the locking member 32 is here constituted by two superimposed sections. The upper section 32a is connected to the thermal module and the locking groove 33 is machined in the bottom of the lower section 32b. The latter has in the upper portion a contraction engaged within an opening made within the upper section 32a. These two sections are interconnected with the possibility of a relative translational displacement equal to the unlocking travel of the ball link, by means of a stop pin 72 radially fixed to the lower section 32b and engaged in an axial opening 73 made in the upper section 32a. A fourth spring 74 is mounted on the contraction of the lower section 32b and tends to move apart the two sections. A barometric module 100 is placed within the control rod sheath 28a beneath the lower section 32b of the locking member. This barometric module 100 is constituted by a central axial bush 75 coaxial to the control rod 5 and sealed in the lower portion by a base 76 and engaged in the upper portion in a cap 77. A metal bellows 78 is mounted on the axial bush 75 and tightly connects the base 67 of the bush 75 to the edge of the cap 77. The vacuum is formed in the interior of the barometric module 100. A guide and stop pin 79 is radially fixed to the cap 77 and engaged in an axial slot 80 machined in the axial bush 75. A fifth spring 81 is placed within the axial bush 75 and tends to raise the cap 77. The stiffness of the fifth spring 81 exceeds that of the fourth spring 74 and is less than that of the first and second springs 40, 44 of FIG. 2. When the reactor is under full power, the temperature within the vessel reaches several hundred .degree.C. and the pressure exceeds 100 bars. The position assumed by the components of the device according to the second embodiment is shown in FIG. 5. The pressure exerted on the barometric module 100 compresses the fifth spring 81 and maintains the module in the maximum shortening position, the bottom of the cap 77 bearing on the upper end of the tube 75. The attachment finger 47 is in the projection position into the interior of the internal space of the control rod 5. The second spring (not shown in FIG. 6) exerts a downward pressure on the attachment head, which is transmitted by means of the thermal device, i.e. the bellows 35, to the upper section 32a of the locking member. This pressure compresses the fourth spring 74 until the upper end of the contraction of the lower section 32b is brought to the bottom of an opening of the upper section 32a. The lower section 32b has a solid portion facing the balls 31, which maintains the coupling. If the temperature is lowered beyond said temperature for abutting the attachment finger against the cylinder shoulder, the upper section 32a of the locking member is raised by the thermal module, whereas the lower section 32b remains in engagement on the cap 77 and is maintained in this position by the pressure of the fourth spring 74. Automatic uncoupling is obtained if the operating temperature is sufficiently low for the upper section 32a of the locking member to rise to a level equal to the unlocking travel and if the pressure is sufficiently decreased for the pressure permanently exerted by the fifth spring 81 on the cap 77 to become adequate to raise said cap 77 and the lower section 32b by a height equal to the unlocking travel. As a result the unlocking groove 33 faces the balls 31. For example, unlocking can be obtained in this way when the temperature is lowered to 80.degree. C. and the pressure to a few bars. When the unlocking travel has been covered, the cap 77 abuts at the top against a surface 82 in the control rod sheath 28a. The barometric module 100 is then in the maximum elongation position and a subsequent pressure drop has no effect on the locking member position. Finally, if the seal of the metal bellows 78 of the barometric module 100 is broken and if that of the thermal module bellows 35 is maintained, uncoupling is obtained when the uncoupling temperature is reached. If the seal of the thermal module is broken and that of the barometric module 100 maintained or if the seal of said two modules is lost, the lower section 32b of the locking member remains in the lower position when the uncoupling pressure and temperature are reached and coupling is maintained. The special manipulating tool then makes it possible to obtain the uncoupling, the process being similar to that described during the description of the operation of the first embodiment of the device according to the invention. It is possible to raise the lower section 32b of the locking member by means of the pin 72. In the same way, the recoupling and automatic uncoupling neutralization procedures remain unchanged compared with those described in connection with the first embodiment of the device according to the invention. The invention is not limited to the embodiments described hereinbefore and has several variants. Thus, it is possible to use other means for coupling the lower end of the control rod to the absorber cluster pommel. It is also possible to use other means for attaching the locking member to the control rod. The attachment means can also be carried by the control rod and not by the internal part thereof. Finally, the device according to the invention is applicable not only to pressurized water nuclear reactors, but to any reactor type where the control takes place by translations of absorber elements within the core and where said control is ensured by mechanisms located outside the vessel and transmitted to the absorber elements by a vessel-traversing transmission member. It is also possible to use the invention in nuclear installations, where use is made of an elongated vessel with an exceptionally long transmission member, e.g. to permit the installation within said vessel of a heat exchanger.