Patent GB-1025859-A (GB)
Title: Base support grid for nuclear reactors
IPC: G21C5/10
Filed: 1965-03-11
Abstract: 1,025,859. Reactors. UNITED KINGDOM ATOMIC ENERGY AUTHORITY. March 11, 1965 [March 11, 1964], No. 10385/65. Heading G6C. A base support grid for a gas-cooled nuclear reactor comprises a plate made up of at least one layer of connected blocks of a refractory material, e.g. graphite, supporting the reactor core and pierced by channels for the passage of the cooling gas, together with means for imposing sufficient prestressed radial pressure on the plate to prevent the development of tensile stress within the plate. As shown in Fig. 1, an integral, high temperature, heliumcooled nuclear reactor incorporating the above grid comprises an envelope A serving both as pressure vessel and as biological shield and consisting of a casing of prestressed concrete lined on the inside with a metallic shield 10. The reactor core B is supported by a base support grid C above heat exchangers D comprising six separate heat exchange loops 16. The circulation of the helium coolant is indicated by the arrows f, the helium at its lowest temperature starting from the top of the compartment 14 and passing down through the core B in which it becomes heated, then through the base support grid C and the heat exchangers D, returning through an annular compartment 24 between partitions 20 and the inner side wall of the envelope A. Blowers 22 connected one to each heat exchange loop 16 (only one of which is shown) force the helium round the circuit. The core B comprises an active central region 26 made up of one or several layers of fuel assemblies of hexagonal cross-section, each composed of fissile particles coated in pyrolytic graphite and dispersed in a graphite matrix pierced with a longitudinal channel for the helium to pass through. Surrounding the active region 26 of the core is a reflector 27 composed of moderator rods of similar shape to the fuel assemblies. The helium enters the core B at a temperature of about 300‹C. and leaves at a temperature of about 750‹ C. The central part of the plate 34, which has to be capable of withstanding this high outlet temperature, is composed of one or several layers of graphite rods of the same cross-section as the fuel assemblies and each pierced by an axial channel to allow passage of the helium, cotter pins being arranged between the rods in order to supplement the frictional forces between the rods and to avoid any displacement of the rods, in particular during rises in temperature. The graphite may include absorptive material such as boron in order to act as a biological shield for the heat exchangers D. Alternatively, alternate layers of graphite and of a graphite-boron mixture, the latter being of a very low order of thickness, may be used. The temperature is uniform throughout the central part of the grid with the result that there is a temperature drop of about 450‹ C. across the outer part 38 of the plate 34 which supports the reflector 27. This outer part 38 is composed of graphite bars of trapezoidal cross-section, the parallel sides of which are orientated in the direction of circles centred on the axis of the grid, the other two sides being orientated in radial directions. These bars do not require such intense cooling as do the rods supporting the active region 26 of the core and longitudinal channels of smaller diameter than those through the active region 26 pierce both the rods of the reflector 27 and the corresponding bars of the grid. These latter have transverse ribs which interleave in such a way as to supplement the frictional forces between the bars. The plate 34 is encompassed by a series of brackets 58 which are linked to each other by means of flexible metallic membranes so as to maintain at least an approximation of gas tightness between the brackets 58. As shown in Fig. 4, flanges 62 at the bottom of the brackets 58 support the outermost bars of the plate 34. Each bracket 58 is linked by a traction strip 64 to a circular beam 66 for stress distribution which is suspended from the upper part of the envelope A by a series of tie-rods 68. The means for prestressing the plate 34 comprises a rigid girdle 70 and sets of elastic pressure members 72, one set associated with each bracket 58 and compressed between the bracket 58 and a bearing plate 74, the distance of the latter from the girdle 70 being determined by an adjustable wedge device. This device comprises two wedges 76 integral with the bearing plate 74 and two wedges 78 which can be driven in to a greater or lesser extent between the wedges 76 and the girdle 70 (only one each of the wedges 76, 78 being seen in Fig. 4). Each of the members 72 contains a stack of Belleville washers or other type of spring. The application of the prestressing to the plate 34 is effected by first supporting the plate 34 and the girdle 70 by a temporary scaffolding and then placing two batteries of jacks, each battery applied to one bracket 58, adjacent radially opposite areas of the base support grid and putting the jacks under pressure in order to compress the members 72. Once the prestressing force is regulated, the wedges 78 are driven in and the jacks dismantled. The jacks are then placed in other positions around the grid and the same operation is repeated from place to place. Instead of being carried out in one single operation, the prestressing is achieved in successive tages as a result of a number of repeated adjustments to the same brackets 58.