Patent Number: 042736145
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1, reference numeral 1 diagrammatically designates the part of a fast neutron reactor with cooling by liquid sodium of a type well known in the art. The reactor comprises a core 2, formed by the juxtapositioning of a series of assemblies 3 maintained in the vertical position by the bottom or lower end thereof engaging in a supporting member 4. Core 2 is submerged beneath a suitable volume of liquid sodium 5 contained in an open container 6, whose upper end 7 is suspended beneath a thick horizontal slab 8 constituting the upper part of a protective caisson 9 surrounding the container 6. The sodium 5 in container 6 has a horizontal level 10 defining with the lower face 11 of slab 8 a space 12, which is normally filled with an inert gas, generally argon. Also in known manner, the reactor has within the container 6 a device 13 for transferring assemblies 3 and which is in particular able to take up each of the assemblies within the core 2 and bring them to a loading and unloading station, whilst maintaining them submerged beneath the level 10 of sodium 5. The loading and unloading station is illustrated in the drawing where it is possible to see assembly 3a removed from the core and arranged perpendicularly with respect to a handling pot 14, whose upper end is open and whose lower base is closed. This pot rests on a carrying carriage 15. After introducing assembly 3a into pot 14, the carriage 15 is moved along an inclined ramp 16 extended at its upper end so as to pass out of the container 6, whereby it firstly passes through space 12 containing the inert gas and then slab 8. During this movement, carriage 15 is pulled on the inclined ramp 16 by means of a pull chain 18 connected to the carriage via two straps such as 19a connected to safety device 19 of the parachute brake type permitting in the case of an accidental breakage of the chain to prevent a sudden dropping of the carriage and handling pot containing the assembly, thereby greatly reducing the serious risks of damage to the internal structures of the reactor which would result from this. The carriage 15 containing pot 14 can thus be removed in a sealed manner from caisson 9 through slab 8 in order to be brought into a chamber 20 positioned externally of the caisson above slab 8. This chamber, whose constructional details are of little importance to the present invention, makes it possible to pivot carriage 15 so that it can be passed from inclined ramp 16 to a second inclined ramp 21 where, by a reverse movement, the carriage and the pot carried on it can be lowered into a storage container 22 and, after traversing a gaseous space 12a identical to space 12, can again be submerged beneath a sodium volume. Following tilting, the pot and its carriage are placed in a position vertical to the not shown loading and unloading station. The pot containing the assembly is then unloaded from the carriage and rests in the storage container until the residual activity given off by the assembly has decreased sufficiently. As has been stated hereinbefore, the handling pot 14 into which has been introduced a fuel assembly 3a must be sealed once the pot and its carrying carriage emerge above the level 10 of the sodium and enter the atmosphere of the gas cushion 12 in communication with the inside of chamber 20. FIGS. 2 and 3 illustrate a first embodiment of the device which, according to the invention, makes it possible to ensure this automatic sealing of the handling pot. As can be seen in the drawings, the parachute brake 19 is provided at its lower end with an enclosure 24 having a transverse base 25 or upper part extended by a cylindrical collar terminated at its end directed towards pot 14 by an inwardly directed contraction 27. Enclosure 24 is able to house a hollow sphere 28, particularly made from stainless steel, filled with an inert gas, such as argon, whilst when carriage 15 does not carry pot 14 contraction 27 prevents the sphere 28 from escaping from the enclosure. According to a constructional variant, the inert gas atmosphere can be replaced by producing a vacuum within the hollow sphere 28. When the carriage 15 carrying the handling pot 14 is submerged beneath level 10 (FIG. 2) sphere 28 as a result of the hydrostatic buoyancy due to the sodium surrounding it is applied against a bearing surface 29 located in the bottom of enclosure 24 which has orifices 28 to permit the passage of sodium. However, and as illustrated in FIG. 3, when the carriage 15 and handling pot 14 emerge above level 10 into the atmosphere of the inert gas cushion 12, the hollow sphere 28, under the action of its own weight, bears against the upper end 30 of pot 14 having a conical bearing surface 31, thereby permitting a substantially tight closure of pot 14. According to another constructional variant, if sphere 28 is given the minimum appropriate thickness, it is possible to operate with pot 14 constantly closed by sphere 28, even in the submerged position. This result is achieved if the weight of the sphere exceeds the hydrostatic buoyancy due to the sodium. However, this variant has the disadvantage of leading to friction between sphere 28 and bearing surface 31 of the pot during the righting of the latter in order to bring it into a position which is vertical to one or other of the two loading and unloading stations. However, it is considered that the degree of friction occurring in sodium is relatively low. According to another variant illustrated in FIG. 4, parachute 19 is again associated with an enclosure 34 having a base 35 or upper part and a lateral collar 36, whereby the latter has a plurality of elongated longitudinal openings 37, which are advantageously diametrically opposed in pairs, there being four in the present embodiment. A hollow steel member 28 filled with argon is arranged within enclosure 34. This member substantially corresponds to a hemisphere and has a planar upper face 28a. Four transverse pins are respectively engaged in each of the openings 37 of enclosure 34. The upper end 30 of handling pot 14 again has a conical bearing surface 31. The device functions in the same way as the device described relative to FIGS. 2 and 3. The hollow member 28 is raised towards the upper part of enclosure 34 under the effect of hydrostatic buoyancy of the sodium when carriage 15 and pot 14 are submerged beneath sodium level 10, but when the carriage - pot assembly emerges above the sodium level and is located in the gaseous atmosphere, bearing takes place against the bearing surface 31 of pot 14 and the hollow member 28 then drops towards the bottom under the effect of its own weight. However, operation can also be obtained which is such that the pot is constantly sealed by the hollow member. In this embodiment, the lower the hydrostatic buoyancy the more easily this result is achieved (hemisphere). It should be noted that the overall dimensions in the vertical direction are reduced in this variant, making it possible to reduce the height of the handling chamber 20. In this way, a device for the substantially tight and automatic closure of a handling pot is obtained, in which the hollow member used acts in the manner of a float, the bearing force of said member against the upper end of the pot being a function of the weight of the member when the pot is in the gaseous atmosphere. For information purposes, it is pointed out that for a stainless steel sphere with a thickness of 4 mm and an external diameter of 300 mm, the bearing force is approximately 8.5 kg. One of the advantages of the present device is that it is completely independent of the fixed loading and unloading structures of the handling pot and in particular the construction of the ramps, the chamber and the storage container. Moreover, this device is not dependent on the orientation of the pot on its carriage. The invention is not limited to the embodiments described and represented hereinbefore and various modifications can be made thereto without passing beyond the scope of the invention.