Patent Number: 050698613
Section: summary

This invention relates to an apparatus and a process for the remote unscrewing and extraction of an assembly screw, the head of which is damaged, particularly of a screw of which the head is separated from the threaded body by breakage. The fuel assemblies of nuclear reactors cooled with light water and, in particular, the fuel assemblies of pressurized-water nuclear reactors comprise a bundle of parallel fuel rods arranged in a framework having end connectors. The fuel assemblies, in the operating position and during their deactivation storage, are placed in a position in which the fuel rods are arranged vertically, the fuel assemblies resting on a support by means of their lower connector. The upper connector has leaf springs, on which the upper plate of the reactor core comes to rest when the assemblies are in operation in the vessel of the nuclear reactor. The leaf springs are fastened to the upper connector by means of flanges which are screwed to a frame forming the upper part of the connector. The flanges are brought to bear on the end part of the springs and have countersinks, into which it is possible to introduce the assembly screws passing through the end parts of the springs and possessing a threaded part screwed into an internally threaded hole passing through the frame of the connector. When the assembly screws have been tightened, the heads of these screws are seated completely in the countersinks above the springs, and their end part opposite the head projects slightly relative to the lower surface of the connector frame. The springs of the upper connector of the fuel assemblies ensure that these assemblies are retained in the operating reactor as a result of the bearing force of the upper core plate transmitted to their framework by means of the springs. In the operating reactor, the springs, called anti-flyoff springs, can experience some damage, with the result that they no longer satisfactorily perform their function of retaining the fuel assembly. In particular, the springs can suffer breaks or lose their elasticity characteristic under irradiation. It is then necessary to replace the damaged spring leaves and, for that purpose, remove the flange of the upper connector of the assembly. This operation is carried out remotely, the assembly being in the storage position in the bottom of a pool, by using a tool of conventional type having a hexagonal socket interacting with the head of the fastening screw of the spring. The hexagonal socket of the tool is fastened to the end of a pole actuated by operators working from the operating platform of the pool. After a certain operating time of the fuel assembly, the fastening screws of the springs are corroded and the threaded shank of the screw can be jammed in the thread of the socket. Furthermore, the screws have detent pins ensuring that they are blocked in terms of rotation. In order to extract a screw, it is necessary beforehand to machine the detent pin of the screw with a bell-type cutter and then unscrew it by means of a tool which ensures that the screw is gripped at the end of the removal operation. The screw is injected into a discharge container held underwater in the fuel-assembly storage pool. This procedure therefore makes it necessary to employ suitable cutting and unscrewing tools of complex structure and use. Likewise, to ensure the installation, tightening and locking of the new fastening screw of the flange and springs, suitable tools have to be used. Moreover, it may be necessary to exert a considerable tightening torque on the tool, with the result that the screw head is liable to break in a zone forming an incipient breaking point at the intersection of the head and the shank of the screw, part of which is threaded. Studies have been conducted to prevent, as far as possible, the screws from breaking and in particular it has been proposed to use nickel alloys resistant to corrosion and to the effects of irradiation in order to produce these screws. The replacement of the screws of older type by screws of the more recent nickel-alloy type can be carried out during the shutdowns of the reactor for the refueling of the core. If the head of a screw has undergone a break or damage making it impossible to exert the unscrewing torque on the screw head, the customary procedure for removing and extracting the screw is no longer practicable. On the other hand, there are known devices for the unscrewing and extraction of screws with a damaged head, which comprise a C-shaped frame and an extraction screw terminating in a punch and engaged in an internally threaded hole passing through one branch of the frame. A bearing piece is fastened to the frame branch opposite the branch in which the extraction screw is engaged. However, such apparatus as described in U.S. Pat. Nos. 1,380,071, 2,535,638 and 1,639,111 are not suitable to be put in place and actuated remotely, for example in order to carry out the unscrewing and extraction of a fastening screw of an element of a fuel assembly of a nuclear reactor from the platform of the fuel storage pool of the reactor. The object of the invention is, therefore, to provide an apparatus for the remote unscrewing and extraction of a fastening screw of an element of a fuel assembly of a nuclear reactor, the head of which is damaged, the screw being engaged by means of its head part in a bore of a first piece of the assembly and by means of its threaded body in an internally threaded bore passing through a second piece, this apparatus comprising a C-shaped frame having two branches substantially parallel to one another and adjoining part between the two branches, and an extraction screw engaged in an internally threaded hole passing through one branch of the frame and having one end forming a punch directed towards the inside of the frame, and a bearing and centring piece, the diameter of which is smaller than the diameter of the internally threaded bore of the second piece and which is fastened to the second branch of the frame and is directed towards the inside of the frame in the axial direction of the extraction screw, this apparatus being usable whatever the state of the screw head, and, in particular, when this screw head is separated from the body completely by breakage. To achieve this, the apparatus according to the invention also possesses: a pole of great length, to the end of which the C-shaped frame is fastened, a ring for centring the extraction screw relative to the screw to be extracted, arranged round the extraction screw over a smooth part of this screw located between its threaded part and its end part forming the punch, and a means for the remote actuation of the extraction screw by screwing or unscrewing. In a preferred embodiment, the apparatus furthermore possesses a bearing and centring piece, the diameter of which is smaller than the diameter of the threaded bore of the second piece and which is fastened to the second branch of the frame and is directed inwards in the axial direction of the extraction screw.