Patent Number: 056065825
Section: summary

FIELD OF THE INVENTION The present invention relates to a disconnection device between a means for absorbing neutrons in the core of a pressurized nuclear reactor and a control rod for the same, disconnection taking place automatically during the phase leading the reactor to its cold shutdown state. The invention has a particularly important application in pressurized water reactors, by making it possible to reduce the duration of the core reloading operations and the like, as well as the exposure doses of personnel to radiation in this connection. PRIOR ART AND SET PROBLEMS For the control of pressurized water reactors, use is presently made of fuel rod clusters having an absorbing power with respect to neutrons and placed vertically in the reactor core, between the fuel elements, which, when grouped, is called a "fuel assembly", so as to regulate the reactivity of said core and therefore the power supplied by the reactor. These means are referred to as "absorber clusters" within the present application. The displacement of each of the absorber clusters respectively attached to a control rod is obtained by displacement mechanisms, e.g. of the screw-nut type, cooperating with the control rod and placed within tight tubular enclosures linked with the reactor vessel and arranged vertically above the reactor vessel sealing cover. The lower portion of the control rod is terminated by linking means, which engage in a gripping pommel of the absorber cluster and which can be remotely manually unlocked with the aid of a special tool, once the vessel sealing cover has been removed. The operation of a nuclear reactor requires the periodic replacement of the fuel assemblies of the core. Such an operation requires the removal of the sealing cover from the vessel, together with the control rod drive mechanisms carried by the same. Following the removal of the vessel cover, the control rods are manually disconnected individually from their respective absorber clusters with the aid of the special tool. The internal equipments of the reactor placed above the core are then removed at the same time as the group of control rods, the rods being raised by internal equipments. The fuel assemblies are then uncovered and can be removed from the reactor core. Obviously, the reactor is shutdown during the core reloading operations and the maintenance in said state and which is referred to as the "cold shutdown state", so that the absorber clusters must remain fully within the fuel assemblies in order to satisfy safety standards. The manual manipulation of each of the links, in order to disconnect the control rod from its associated absorber cluster, takes a relatively long time. Thus, the total duration of the manual disconnection operations is relatively long and during said time personnel is exposed to radiation. U.S. Pat. No. 2,261,595 discloses a linking device between a control rod or bar and its associated absorber cluster. This device permits disconnection prior to the removal of the sealing cover from the vessel making use of the displacement mechanism of the control rod. Disconnection and reconnection are carried out by lowering and then raising the control rod, whilst the absorber cluster is completely inserted in the fuel assembly. Such a device has the advantage of avoiding manual disconnection of the links following the opening of the vessel and therefore reduces the time taken for the core reloading operations and reduces the radiation exposure dose for personnel. However, the means for coupling the control rod base remain engaged in the absorber cluster gripping member, when they are in the inoperative position. Thus, it is necessary to fear the accidental raising of one or more absorber clusters during the removal of internal equipments covering the core if, in conventional manner, said equipments are above the control rods. This may e.g. be due to an alignment deficiency between the base of the control rods and the gripping member of their associated absorber cluster, producing an attachment or fastening between said elements. Such an event can produce a rise in the core reactivity, which can be completely incompatible with the safety criteria which provide for the maintenance of the absorber clusters in the fuel assemblies when the reactor vessel is open. Moreover, there is no system for disconnecting the link between the control rod and the absorber cluster in the case of the jamming of the device when the coupling means are in the operative position. German patent 42 12284 discloses a thermal control device automatically ensuring the uncoupling of a control rod from its associated absorber cluster and the propulsion in the core of said cluster in the case of an abnormal temperature rise within the reactor. This device has coupling means of the ball type maintained in the operative position by a bolt connected to a thermal expansion module constituted by one or more metal bellows and ancillary tanks communicating with one another and filled with primary fluid. The increase in the length of the bellows under the effect of the expansion of the fluid trapped in the expansion module displaces the bolt in such a way as to firstly authorize the disconnection and then pushes back the absorber cluster. However, such a device which has the advantage of requiring no external manipulation, only ensures the disconnection and propulsion into the core of the absorber cluster. The position of the bolt which conditions the putting into the inoperative or operative state of the coupling means is directly dependent on the temperature of the reactor and in fact the temperature of the expansion module. AIMS OF THE INVENTION The aim of the invention is to equip the link between the control rod and its associated absorber cluster with a device making it possible to automatically separate the rod from the cluster during the bringing of the reactor into the cold shutdown state prior to the removal of the vessel sealing cover in order to eliminate, without any external intervention and without encountering the difficulties which arise with the invention of U.S. Pat. No. 2,261,595, the manual manipulation in conventional form of the joints and consequently the reduction of the duration of the operations performed when the vessel is opened, whilst reducing the radiation exposure doses for personnel. The aim of the invention is also to give said same link between the control rod and the absorber cluster an automatic separating device, which can be neutralized, unlike that of German patent 42 12 284. The neutralization of the automatic separation makes it possible to avoid on the one hand the needless uncoupling if the reactor is brought into the cold shutdown state for a reason other than an opening of the vessel and on the other the capacity to recouple, e.g. as in the prior art, during the cold shutdown prior to the sealing of the vessel and the reactor power rise. SUMMARY OF THE INVENTION To this end, the main object of the invention is an automatic uncoupling device between the pommel of an absorber cluster intended to be introduced between the fuel elements of the core of a pressurized water nuclear reactor and a control rod transmitting to the absorber cluster movements induced by a control mechanism located in a tight enclosure placed above the reactor vessel sealing cover, said uncoupling device incorporating an attachment head and a mobile locking member, able to occupy a locked state and an unlocked state, whereof an axial displacement leads to a change of state, characterized in that it also comprises a thermal module axially deforming under the effect of the temperature and which is positioned between the attachment head and the locking member to form an axially mobile assembly. In a preferred embodiment of the invention, the thermal module comprises a water-filled, tight bellows. The attachment head comprises at least one radial attachment finger projecting as a result of a small spring and sliding within a cylinder fitted into the cladding or shell of the control rod, said cylinder having in an inner portion and from top to bottom, in order to receive the attachment finger a chamfered widening and an axial notch, having a U-shaped cross-section, machined over a portion of its height. Above the attachment head is located a second spring, which pushes it permanently downwards. The locking member is a sliding cylindrical member coaxially mounted in the control rod cladding, said locking member having an annular locking groove which can be positioned in front of the balls, located in the control rod cladding, when the locking member is in a high position. The balls are received in radial, truncated cone-shaped cavities, located at the bottom of the cladding and whose thickness is less than the diameter of the balls, said cavities being shaped so as to prevent a radial, outwardly directed escape of the balls. An extractor tube is mounted in sliding manner on the lower end of the cladding and is pushed downwards by a third spring sufficiently stiff to raise the control rod. This extractor tube has in its upper portion a slot having an axial portion followed by a helical portion and which, by sliding on a guide pin integral with the cylinder, guides the displacement of the latter. Level with the guide pin, the control rod cladding has a horizontal slot allowing the rotation by a fraction of a turn of the cylinder with respect to the control rod, thus permitting the freeing of the attachment finger from the U-shaped, inner notch and, consequently, the free displacement of the attachment head in the downwards direction. The automatic uncoupling of the absorber cluster with respect to the control rod, controlled by the device according to the invention, occurs in the case of a drop in the temperature of the reactor vessel water below a certain threshold (e.g. approximately 80.degree. C.). The attachment finger then abuts in the U-shaped axial notch and the locking member rises under the effect of the shortening of the bellows, due to the contraction of the water, up to its upper position, which permits the radial engagement of the balls in the annular groove of the locking member. In this uncoupled position, when a pressure is exerted from the top on the control rod, the latter is firstly displaced downwards by a height equal to that of the axial portion of the cylinder slot and then, under the action of the helical portion of the slot on the guide pin, rotates the cylinder by a fraction of a turn disengagement the attachment finger from the U-shaped notch, thus permitting a downward displacement of the attachment head-thermal module-locking member assembly and consequently the recoupling of the control rod with the pommel of the absorber cluster. In a constructional variant, it is possible to complete the device with a barometric module positioned below the lower end of the locking member, which is then subdivided into two sections, which are interconnected by a fourth spring, whose displacement is limited by a guide and stop pin-axial slot system, said barometric module, kept under a vacuum, axially deforming as a function of the pressure variations prevailing in the reactor core giving rise to a change of state of the locking member. The barometric module is preferably constituted by a lower base surmounted by an axial bush, an upper cap covering the axial bush, a metal bellows tightly connecting the bush and the cap and a fifth spring placed within the bush. In this constructional variant, the automatic uncoupling is obtained on the one hand when the pressure within the reactor vessel is sufficiently reduced and on the other when the temperature within the vessel is sufficiently lowered. LIST OF DRAWINGS The invention and its various technical features will be better understood from a study of the following drawings, wherein show: FIG. 1 A sectional view of the upper portion of a reactor vessel where the invention is installed. FIG. 2 A sectional view of the device according to the invention in two parts. FIG. 3 A part front view of the notch permitting the rotation of the cylinder in order to retract the abutment in the device according to the invention. FIG. 3A A horizontal section along line A--A of FIG. 3. FIG. 3B A horizontal section along B--B of FIG. 3. FIG. 4 In section, the lower portion of the device according to the invention. FIGS. 5A to 5D Four simplified sectional views of the device according to the invention during four operating phases. FIG. 6 A frontal section of a variant of the device according to the invention.