Patent Number: 047864605
Section: summary

The invention relates to an installation for handling assemblies forming the core of a fast neutron nuclear reactor between the main reactor vessel in which the core is situated and an adjoining vessel situated alongside the main vessel and possibly used for the intermediate storage of the assemblies. BACKGROUND OF THE INVENTION An installation of the kind specified performs the double function of evacuating used assemblies from the main reactor vessel and introducing new assemblies into such vessel. The assemblies involved in such handling are all the assemblies forming the reactor core, such as the fuel assemblies, the lateral neutron screening assemblies and the absorbing assemblies. In the French Super Phenix reactor the installation for handling the assemblies comprises a transfer hood disposed above the slab surmounting both the main and adjoining vessels, so as to straddle the adjacent edges thereof. Two inclined ramps extend through the slab so as to produce a communication between the transfer hood and a primary station situated inside the main vessel and a secondary station situated inside the adjoining vessel. The transfer hood and ramps have guide rails along which two pots move each of which transports an assembly. The pots are attached to lifting means comprising chains each entrained by a winch disposed in the upper part of the hood. The installation for handling the assemblies of the Super Phenix reactor operates symmetrically, one of the pots being employed to evacuate a used assembly, while the other pot is employed to introduce a new assembly into the main reactor vessel. When the two points have been remounted inside the transfer hood, the latter performs a rotation of 180.degree. around its vertical axis, so that each of the pots can descend again via the ramp opposite that via which it was introduced into the hood. In the Super Phenix reactor assembly handling installation, the transfer hood is generally called the "transfer lock". This name is explained by the fact that slide valves are disposed between the hood and each of the ramps, thus enabling the hood to operate like a lock or sluice during the handling of the assemblies. When the reactor is operating, the slide valves are closed so as to ensure the containment of the reactor vessel. In practice the sluice-like operation of the transfer hood is not used, since the containment of the interior of the main reactor vessel with respect to the adjoining vessel during the handling of the assemblies is performed by a syphon of liquid metal disposed in the lower part of the ramp descending into the adjoining vessel. The only function of the slide valves, therefore, is to ensure the containment of the main reactor vessel during reactor operation. The presence of the slide valves between the ramps and the transfer hood means that the pot-guiding rails must be interrupted over a length which is considerable in relation to the diameter of the wheels via which the pots move over the rails. A first consequence of this break caused by the slide valves is that the pots must be given extra wheels to ensure that they continue to be guided when they pass over the break. In addition to the consequent increase in cost, the extra wheels make the guiding of the pot hyperstatic, so that there is the risk that the pot may be jammed in case of the deformation or angular deviation of the ramps. Moreover, the breaks introduced by the sidevalves must be crossed at low speed, so that the rate of handling is seriously impeded by the presence of these breaks. SUMMARY OF THE INVENTION The invention relates precisely to an installation for handling the assemblies of a fast neutron nuclear reactor which is free from the aforementioned disadvantages and more particularly enables any risk of the pots becoming jammed to be obviated by the use of an isostatic guiding system, while at the same time appreciably enhancing the handling rate, even though the installation is simplified so as to appreciably reduce its cost. To this end the invention provides an installation for handling the assemblies forming the core of a fast neutron nuclear reactor between a first station situated in a main vessel containing the core and a secondary station situated in an adjoining vessel, such installation comprising at least one pivotable transfer hood, means for pivoting the hood around a vertical axis, two inclined ramps adapted to connect the hood to the primary station and the secondary station respectively, at least one pot for transporting an assembly, means for lifting the pot along guide means inside each of the ramps and the pivotable hood between the primary station and the secondary station, and means for closing the ramps at their top ends when the reactor is operating, wherein the means for closing the ramps comprise two flaps borne by a rotary platform also supporting the transfer hood, such flaps being disposed in locations such that they can be simultaneously placed above the top ends of the ramps via the agency of the means for pivoting the hood when the reactor is operating, the pot cooperating with the guiding means isostatically. As a result of these features, when the reactor is operating, the top ends of the ramps are closed by flaps, so that containment is maintained. During handling the flaps are retracted into the rotary platform and the hood supported by the platform is successively moved opposite each of the ramps as a result of the platform being rotated. There are therefore practically no breaks between the guide rails formed in the ramps and inside the hood. The extra wheels indispensable in the prior art are therefore eliminated. As a result, the pot is guided along the rails isostatically and there is no risk of jamming, even if there should be a slight angular offsetting between the hood guide rails and the guide rails of a ramp. Moreover, the assembly-transporting pot can be moved at high speed over the majority of its travel. The rate of handling is therefore considerably enhanced, even if the installation comprises only one pot. By way of illustration, the handling installation according to the invention reduces a handling cycle to about 11 mn, in comparison with the figure of about 45 mn for the Super Phenix reactor. Preferably, to reduce the amount of material and therefore the cost of the installation the transfer hood comprises a thick tube inclined at an angle identical to that of the ramps, so that it can be placed in the prolongation of each of such ramps when the means for pivoting the hood are operated. The thick tube both ensures the hermeticity of the containment and the function of biological screening. Its positioning as close as possible to the radioactive source formed by the transporting pot when it is in the hood enables the volume and therefore the weight of the biological screening to be substantially reduced. The thick tube forming the biological screening is not heat-insulated internally, so that it forms a heat capacity which also performs thermal regulation, heat transfer taking place by conduction in the thickness of the tube. The thick tube therefore heats the pot when it contains a low-power assembly, while it ensures the cooling of a pot jammed in the hood and giving off a high power. The temperature in the hood can therefore be limited without using active means such as forced ventilation. To enable a flap to be interchanged quickly if the joints which it supports become worn, the rotary platform can also bear a spare flap disposed beneath a demountable plug extending through the platform, a flap-receiving station formed by a recess formed on a fixed base plate disposed below the platform enabling one of the ramp-closing flaps to be interchanged with the spare flap. In that case the spare flap and the hood are preferably located in two positions such that the spare flap faces one of the ramps, the hood facing the other ramp. In a preferred embodiment of the invention a closure member adapted to be attached to an open bottom end of the hood is located in a recess formed in the base plate (the recess preferably corresponding to the flap-gripping station), the hood being demountably mounted on the rotary platform. Due to this feature use can be made of a single hood to handle the fuels on a site comprising several fast neutron reactors. The feature also facilitates hood and pot maintenance operations, which can be carried out in a workshop separated from the actual reactor. To prevent any differential expansions which may take place in the ramps from causing breaks in the guiding of the pot, preferaby the ramps rest on the base plate via swivel links and can expand freely downwards. In the handling installation of the Super Phenix reactor the means for lifting the pots comprise a single chain for each of the pots. The pots have parachute systems to allow for the risk that a link of the chains may be broken. In an advantageous feature of the invention, which further simplifies the installation and therefore reduces its cost, the single chain is replaced by two cables simultaneously actuated by a motor assembly and means are provided for detecting any imbalance between the forces exerted via each of the cables. In this way any failure of one of the cables is detected immediately and the reliability of the system is strongly enhanced. The parachute system can therefore be eliminated, with the double advantage of reducing cost and eliminating another possible cause of the pot becoming jammed. The resulting kinematic chain also offers increased safety, since its design enables the load to be moved even after one of the two cables has broken.