Patent Number: 042736145
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to a handling device for an assembly, particularly for a fertile or fissile assembly of a fast neutron nuclear reactor with cooling by a liquid metal. It is known that in such a reactor the assemblies constituting the reactor core are formed by open enclosures containing a bundle of sheathed needles of fertile or fissile material. These assemblies are juxtaposed and rest in a vertical position on a supporting member. The complete core assembly is submerged in a suitable volume of liquid metal, generally sodium, contained in a container which is open at its upper end and is suspended beneath a thick slab, particularly made from concrete which extends horizontally and encloses a protective caisson surrounding the container. The liquid sodium circulates from bottom to top in the enclosures of the assemblies in contact with the needles in such a way as to extract the calories given off by the latter, the heated sodium being taken up by heat exchangers and then on leaving the latter by pumps which return the cooled sodium beneath the supporting member for a further passage through the core. In operation, the mass of liquid sodium in the container generally reaches a given level, separated from the lower face of the slab by an inert gas, generally argon cushion. At given intervals, variable according to the reactor operating conditions, it is necessary to replace the assemblies in the core. For this purpose, the installation has transfer means controlled from the outside of the protective caisson and which after traversing the slab are able to take up and extract one by one the assemblies in the core, whereby said assemblies are laterally displaced whilst maintaining them below the sodium level and finally they are deposited in a handling pot located at a loading and unloading station located on the side of the core and which is also beneath the sodium level. The handling pot is generally formed by an elongated receptacle open at its upper end via which the assembly is introduced and closed at its inner base, said pot generally being provided with means which, during its movement to an unloading station, permit the distribution and evacuation in acceptable proportions of the residual calories given off by the assembly. In a generally adopted construction, the handling pot is placed on a carriage against which it bears whereby after tilting said carriage is able to move in an inclined ramp by means of a cable or pull chain, thereby making it possible to remove the carriage, handling pot and the assembly contained therein from the container and caisson in order to bring them in tightly sealed manner in a chamber provided externally of said caisson to a position above the slab where the carriage is tilted and then returned over another inclined ramp. It then once again traverses the slab, externally of the caisson, and issues into an appropriate storage container where the pot is again submerged beneath an appropriate volume of liquid sodium and finally arrives at a loading and unloading station. Advantageously, the attachment straps for the carriage are provided with a parachute brake system, which prevents the sudden dropping of the pot containing the assembly in the case of an accidental breakage of the pull cable. Such a device is in particular described and claimed in French patent application No. 2,192,057. When the carriage and the pot are in a position vertical to the loading and unloading station of the reactor or storage container the attachment straps, whose lower end is connected to the carriage and whose upper end is connected to the parachute brake which always remains on the inclined ramp, are disposed obliquely and provide vertical access to the pot in order to permit the introduction or removal of an assembly. During an unloading operation the handling pot is initially completely submerged below the sodium level. When the carriage arrives in the vicinity of the slab, the upper end of the pot emerges above this level into the inert gas cushion and then the whole pot progressively emerges so that it is totally penetrating the gaseous atmosphere in the container above the sodium level and communicates with the inside of the chamber. Under these conditions the pot containing the assembly must be closed so as to prevent the evaporation of sodium, which may occur in the case of an accidental immobilisation of the pot in a partly or completely emerged position as a result of the rise in the sodium temperature. The drop in the sodium level may cause the circulation by convection of the sodium in contact with the assembly to stop, resulting in a local temperature rise which may even bring about a fusion of the fuel, which constitutes an inadmissible accident at the safety level. BRIEF SUMMARY OF THE INVENTION The present invention relates to a device for handling an assembly comprising a handling pot and a closing device ensuring the sealing of said pot in an automatic manner and without any mechanical action as soon as the upper end of the pot emerges from the sodium. To this end, the present device intended for a fast neutron reactor cooled by a liquid metal such as sodium and of the type comprising a container with a vertical axis containing the core and a given volume of sodium contained in said container, defining above the core a free level surmounted by an inert gas cushion, a thick horizontal slab placed above the gas cushion and constituting the upper closure of a protective caisson surrounding the container, an inclined ramp which passes into the container up to a loading and unloading station located in the vicinity of the core and extending at its opposite end into a handling chamber supported by the slab, a second inclined ramp connecting the inside of the handling chamber to a second loading and unloading station positioned externally of the caisson in a storage container, a carriage carrying a handling pot able to move along the said ramps and moving the pot from a position where it is submerged below the sodium level to a position where it is introduced into the inert gas atmosphere above the sodium level, said pot being open at its upper end so as to permit the introduction or removal of the assembly to be handled when it is positioned vertically on its carriage with respect to said loading and unloading stations provided for this purpose in the reactor vessel and in the storage container, wherein during its displacement between said stations the carriage is associated, to the right of the upper open end of the pot, with a generally cylindrical enclosure which is open towards the bottom and in which is mounted a hollow member, whereof at least the lower part has a spherical shape, whereby the hollow member bears via the spherical lower part against the upper open end of the pot when the latter emerges above said level. According to a first embodiment, the said member is constituted by a sphere. According to a second embodiment, said member is constituted by a hemisphere having a planar upper face. In a constructional variant of the device, the hollow member bears against the upper part of the enclosure when the pot and its carriage are submerged on the inclined ramps. According to another variant, the hollow member also bears against the upper end of the pot when the latter and its carriage are submerged on the inclined ramps. According to a special feature of the device, the upper open end of the handling pot has a conical bearing surface against which is applied the spherical part of the hollow member when said end emerges above the sodium level. Advantageously, the hollow member is filled with an inert gas. The interior of the hollow member may also be maintained under vacuum. According to a special embodiment of the invention, facing the open end of the pot the cylindrical enclosure has a contraction towards the inside of the enclosure able to retain the hollow member when there is no pot on the carriage. According to a special embodiment of the invention, the upper part of the enclosure is provided with orifices. According to another variant, the cylindrical enclosure laterally has longitudinal elongated openings diametrically opposed in pairs and in which are engaged transverse pins provided in the lateral surface of the spherical member.