Abstract: Certain of the nuclear fuel assemblies contained in a fast neutron nuclear reactor core have an integrated passive safety device.More specifically, said device is constituted by an annular block (48) forming at least the lower part of the upper neutron protection (42) of the assembly. The block (48) is formed by elements (50) of a neutron-absorbing material, embedded in a matrix (52) meltable at a temperature above the temperature of the sodium during the normal operation of the reactor. A perforated, inner envelope (54) mechanically secures the block (48), whilst enabling the elements (50) to drop in the assembly following the melting of the matrix (52).

Abstract: The clogging indicator makes it possible to continuously measure the purity of sodium circulating in the circuits of a fast neutron reactor without requiring the dismantling or completely changing the indicator should the measuring section becomes clogged. The clogging indicator essentially includes a bypass pipe comprising one coaxial external section (8) and one coaxial internal section (9). A measuring head (10) is mounted and able to move between these two sections and is mainly composed of at least one thermoelement (24) and one mobile grid (22) causing the measuring section to vary and thus the measured flow so as to favor uncloggings and the dissolving of any impurities crystallized during the measurements. This unit is completed by the other main elements of the clogging indicator in an elongated longitudinal body and surrounded by a thermic exchanger (11) for cooling the sodium.

Abstract: In order to ensure the cooling of the support ferrule of a vessel, such as the cylindrical internal vessel (14) of a fast neutron nuclear reactor, between said ferrule and an internal wall (20) containing a liquid metal at high temperature is placed a thermal protection apparatus (29). The latter comprises sealed collecting tanks (30) supplied with relatively cold, pressurized liquid metal by means of pipes (32), connected e.g. to the reactor bearing structure (16). This liquid metal is sprayed onto the ferrule by sprayers formed in the collecting tanks. On the side turned towards the wall (20), said tanks support elements (38) of a passive thermal protection structure.

Abstract: In a liquid metal-cooled fast neutron nuclear reactor which has a vessel sealed by a horizontal slab on which is suspended a core cover plug, the lower part of said plug comprises a deflecting grid, optionally duplicated by a thermal protection grid. Each of the grids is formed from a peripheral plate and modular plates defining between them passages for the liquid metal. The peripheral plates can be fixed to a perforated part of an external ferrule or can be suspended on a core cover plate by tie bolts and/or by certain of the control rod guidance sleeves. The modular plates are fixed to the other sleeves and traversed by sodium sampling tubes and tubes for housing thermocouples.

Abstract: A fast neutron nuclear reactor, in which the main vessel and the slab for sealing said vessel are separately suspended on the upper part of the vessel shaft. An outer ferrule of the slab is directly anchored in the vessel shaft by tie rods and is supported by an annular ring embedded in the concrete. The upper edge of the main vessel is welded beneath the ring, the vessel being supported by a second annular ring embedded in the concrete. The upper faces of the slab and the vessel shaft are consequently at the same level, thereby reducing the heights of the vessel shaft and the safety vessel.

Abstract: The invention relates to a nuclear reactor cooled by a liquid metal contained in a vessel obturated by upper closing means, said latter essentially being constituted by a slab and rotating plugs. The upper closing means comprise at least one circuit for circulation of a cooling gas or liquid, this circuit being provided with means for maintaining the closing means at a homogeneous temperature at least equal to 100.degree. C. and preferably close to 120.degree. C. The invention is applicable to the production of less cumbersome and less expensive fast neutron nuclear reactors.

Abstract: Thermal insulation device for insulating the upper area of the annular space separating the main vessel and safety vessel of a fast neutron nuclear reactor, said two vessels having a common vertical axis and are sealed in their upper part by a horizontal slab which is also responsible for the suspension of the two vessels, wherein the said device comprises a lower thermal insulation ring having a generally annular shape defining an inner peripheral edge spaced from the main vessel and fixed to the lower end of an inner thermal insulation baffle whose upper end is fixed to the main vessel, and an outer peripheral edge spaced from the safety vessel and fixed to the lower end of an outer thermal insulation baffle whose upper end is fixed to the safety vessel, each of the thermal insulation baffles being deformable so as to compensate any differential expansion or deformation of the vessels.

Abstract: Apparatus for homogenizing the circumferential temperatures of the vertically axes ferrule of a component passing through the upper slab of a nuclear reactor, wherein it comprises at least one assembly forming a heat pipe describing the entire circumference of said ferrule in order to ensure the homogenization of the temperatures of said ferrule level with the assembly, means for fixing the assembly or assemblies forming the heat pipe on the inner face of the ferrule and means for ensuring a thermal contact between the assembly or assemblies forming the heat pipe and the said ferrule.

Abstract: The fast reactor structure to be protected comprises a wall and a parallel cylindrical shell having a free top edge which forms an overflow sill. A diverted flow of liquid metal withdrawn from a principal volume is circulated upwards through a vertical space formed between the wall and the shell. The shell surface which is remote from the space is provided with a packing of heat-insulating elements so that the overflowing sheet of liquid metal which falls on the downstream side of the overflow sill passes through the packing before returning into the principal volume of liquid metal.

Abstract: Each panel of a heat-insulating structure contains a stack of wire fabric elements and is maintained applied against the surface to be protected by means of a double fixing system comprising a tubular member which carries a clamping nut and a bearing plate, said member being rigidly fixed to the surface, and a threaded stud mounted coaxially within the tubular member. The stud carries a nut and a washer for preventing the panel from falling in the event of failure of the tubular member.

Abstract: A protective device for a vessel suspended beneath the closing plate of a nuclear reactor in which the wall of the vessel is covered in the vicinity of its connection to the plate towards the inside of the vessel by a metal heat insulating structure constituted by at least one layer of juxtaposed panels. Each of the panels is formed by two parallel cover plates between which is located a layer of thin superimposed metal members. The connection between the cover plates is provided by pins which are perpendicular to the plane of the plates fixed at one end to one of the plates and screwed at the opposite end to a nut which bears against the other plate, thus compressing the layer. The edges of the cover plates of two adjacent panels in the structure are separated by a gap. The layer of thin metal members located between the plates of one panel projects laterally beyond the outline of the latter so as to engage between the plates of at least one adjacent panel while filling the gap.

Abstract: The heat-insulating panel designed to be applied against a wall to be protected consists of two parallel sheet-metal sealing elements of small thickness which are welded together at their periphery and form an enclosed space in which is mounted a stack of thin wire-fabric elements and perforated plates or grids in alternate relation. The free spaces between the wire-fabric elements and the perforated plates communicate with each other and are filled with an inert gas. A leak-tight welded joint is formed between the non-perforated peripheral portion of the plates and the sheet-metal sealing elements.

Abstract: A thermal insulation structure mounted within the annular space between the primary vessel and the inner vessel of a liquid metal cooled fast reactor is constituted by a plurality of heat-insulating elements extending circumferentially around the common axis of the two vessels and consisting of half-casings each formed by two interengaged L-section members packed with flexible and compressible heat-insulating material.

Abstract: Structural elements which are each formed by two sectional members of substantial length are mounted in juxtaposed relation in the line of extension of each other. The two sectional members of each structural element are engaged one inside the other with a small clearance space between their parallel faces. A flexible packing of metallic material is enclosed between the sectional members so as to ensure that they are capable of working in pairs both along the length of said members and in the direction of engagement of these latter.

Abstract: This invention relates to a device for heat-insulating the top part of the annular space separating the main vessel from the safety vessel from a fast-neutron nuclear reactor, the two vessels having a common vertical axis and being open at their top part and covered by a horizontal slab in which the ends are fitted so as to be suspended by the slab.According to the invention a substantially cylindrical heat-insulating ring is mounted under the slab and cooperates with the slab and with the facing walls of the vessel to bound the region to be insulated. The ring extends through the annular space in a horizontal plane and is carried by the slab by means of tie-rods, and a top insulating ring divided into two coaxial elements rests on the bottom ring and is secured to the walls of the two vessels respectively by an assembly of spacing springs disposed radially and bearing against the two elements.

Abstract: A thermal protection device which prevents the formation of neutral gas thermosiphons within the annular space formed between a reactor vessel wall and the shell of a top closure structure from which the vessel wall is suspended essentially consists of one or a number of packing rings fitted and compressed within the annular space, each ring being made up of a series of elements placed in adjacent relation at the periphery of the closure structure.

Abstract: Heat-insulating panels applied against a bearing wall and extending parallel to the wall to be protected are supported by means of a device comprising a yoke rigidly fixed to the wall to be protected and fitted with a cross-pin, the link of a panel-supporting element being mounted freely around the cross-pin. A second yoke secured to the bearing wall is supported by means of a second cross-pin on which the last link of the supporting element is freely mounted.

Abstract: A heat-insulating lining for a fast reactor shield slab comprises a set of panels each consisting of a mat of superposed wire mesh or fabric elements enclosed within at least one metallic casing which is secured against the shield slab. The end walls of the casing are formed by sealing plates applied against the panels and the side walls are formed by at least two interengaged L-section members so arranged that one flange of each member covers the edge of one sealing plate whilst the other two flanges leave a narrow lateral gap which forms a capillary seal.

Abstract: A series of heat-insulating panels each consisting of a mat of superposed wire-fabric or wire-mesh elements enclosed within a casing formed of two parallelepipedal metallic half-boxes engaged one inside the other is applied against the sole-plate of a fast reactor shield slab. The lower half-box is of slightly larger size than the upper half-box, a narrow gap being thus formed between the opposite lateral faces of the two half-boxes and a narrow gap being also formed between adjacent casings, the gaps being intended to form capillary sealing joints.