GB1253569A - Nuclear reactors - Google Patents

Abstract

1,253,569. Reactors. SIEMENS A.G. 2 May, 1969 [3 May, 1968], No. 22627/69. Heading G6C. A reactor core includes an array of rod-like fuel elements suspended from their upper ends to hang downwardly alongside one another from a support grid, the upper ends being arranged within a single cell of the grid in a manner enabling the elements to be removed upwardly for replacement through the grid, and the elements being bounded by flat lateral surfaces by virtue of which they fit closely together in the array. A gas-cooled, fast breeder reactor is described in which the core is situated in the upper part of a prestressed concrete pressure vessel closed at its upper end by a cover plate 102, a steam generator being located in the lower part of the pressure vessel. The core includes fuel elements suspended from a support grid 10, which in turn is secured by means of tie-rods to the cover plate 102. The fuel elements, which each contain a plurality of fuel or breeder material rods, are of generally square cross-section and arranged in sets of nine, with a central element 9 laterally enclosed by elements 1 to 8. The enclosing fuel elements 1 to 8 are narrowed along upper portions, the length of which approximately equals the height of the grid 10, on two adjacent lateral sides 12, 13. At the upper ends of these sides, the elements are provided with projecting support hooks 14, 15 and 16 for engagement with the support grid 10. In contrast, the enclosed fuel element 9 has a uniform square cross-section along its entire length. Provided at the upper end of this fuel element 9 are projecting support fillets 21 which bear against bevelled portions of the fuel elements 1 to 8. The fuel element 9 is formed with a longitudinal bore inside which is located a regulating rod 20. In order to avoid unwanted stressing of the tubular casings of the fuel rods inside the elements, gas is supplied to the interior of the fuel rods through pipes 17. Regulating valves 22 are provided at the upper ends of the fuel elements for regulation of the throughput of coolant through the fuel elements. A plug (not shown) in an aperture in the cover plate 102 and thermal shield 107 has a hollow cylindrical extension by means of which the fuel elements 1 to 8 are held down in position. Drive means for the regulating rod 20 are provided in the plug. A loading machine (not shown) travels over the cover plate on a co-ordinate chassis, so that each point can be reached. The plug is released and drawn into the loading machine and a holding-down tube 29 introduced into the aperture 23. The tube 29 has at its lower end a lateral aperture 30 extending around a quarter of its periphery, and the tube 29 and a manipulating rod 31 are rotatable by means of a drive in steps of 45 degrees so that a gripping head 32, which is displaceable through the aperture 30, can reach all the elements of the array which are suspended around the fuel element 9. The tube 29 holds the fuel elements 1 to 8 down against the grid 10 while the fuel element 9 is withdrawn. The fuel element 9 is engaged by pawls of the gripping head 32 and is drawn upwards into the loading machine with the aid of the manipulating rod 31. The loading machine has its own gas cooling system for cooling the spent fuel elements. Once the fuel element 9 has been removed, the fuel elements 1 to 8 are gripped by the gripping head 32, lifted to a small extent so that they are unhooked from the support grid 10, and then guided inwardly under the aperture in the cover plate 102, so that they can then be withdrawn into the loading machine. A net-like steel structure may be so disposed below the core that, in the event of inadvertent incorrect loading of the core with fuel elements, individual fuel elements are prevented from falling down on to the distributer cone disposed beneath the core. However, in case the whole core should break up, and a large number of fuel elements fall from the support grid, or the core melt and many fragments fall, the steel structure should be of such weak construction as to yield and permit the fallen elements or fragments to fall on to the distributer cone and thereby prevent accumulation on the steel structure of a critical mass.