Patent GB-1101600-A (GB)
Title: Improvements in or relating to fuel elements
IPC: G21C3/04
Filed: 1966-08-24
Abstract: 1,101,600. Fuel elements. GENERAL DYNAMICS CORPORATION. 24 Aug., 1966 [8 Sept., 1965], No. 37984/66. Heading G6C. A fuel element comprises a block of refractory material of good thermal conductivity and neutron moderating characteristics, e.g. dense graphite, having a cross-section taken parallel to its end faces of such a predetermined shape that a plurality of these blocks can be interfitted side-by-side to provide a continuous core array, the block containing a plurality of coolant holes which extend axially from end face to end face and also a plurality of integrally formed and axially extending fuel chambers for holding nuclear fuel material, each block having at one end face a plurality of hollow pins protruding from the end face, each pin being axially aligned with one of the coolant holes, and at the other end face a plurality of mating cavities for receiving the plurality of pins from an axially adjacent fuel element, each cavity being axially aligned with one of the coolant holes. When two of these fuel elements are engaged, one axially adjacent the other, the pins of one fuel element are received in the mating cavities of the other fuel element and precisely axially align the two fuel elements while the hollow centres of the pins connect coolant holes in the two fuel elements in fluid communication. A fuel element of the above type for use in a helium cooled nuclear reactor comprises a graphite block of hexagonal cross-section. The assembled columns of blocks provide good lateral support for one another, the space between the side faces of adjacent columns being no more than is necessary to accommodate thermal expansion. The fuel chambers may be filled with pyrolytic carbon coated nuclear fuel particles, e.g. fuel particles as disclosed in Specification 1,037,470. The fuel chambers 21, as shown in Fig. 4, do not extend the entire depth of the block 13 and are closed at their upper ends by plugs 27. The space between each plug 27 and the fuel material 25 is preferably filled with a plug 29 of heat-decomposable material, e.g. foamed polystyrene, which is carbonized and vaporized as the fuel element is brought up to reactor temperature. The fuel chambers 21 are located in a triangular array and the coolant holes 23 in a triangular array of greater pitch (Fig. 3, not shown) with a coolant hole 23 located generally in the centre of each six fuel chambers 21. To facilitate alignment of the individual fuel elements in a stacked column, the blocks 13 are provided with hollow pins 31 and cavities 33 at their upper and lower end faces, respectively, each aligned with a coolant hole 23. A space 37 between the horizontal upper end wall 39 of each cavity 33 and the upper end of each pin 31 of an assembled stack of fuel elements allows for ready handling of the fuel elements by a device having three depending arms. These arms may be lowered into the three coolant holes 23 with which the pins 31 are aligned. When fingers at the ends of the arms are in the spaces 37, the arms are extended radially so as to engage the end walls 39 of the cavities 33 and then raised. Each central block of a group of fuel elements is provided with two control rod holes slightly oversize with respect to the control rods, the resulting annular gaps providing vertical coolant passageways to cool the control rods. A reactor core made up of such an arrangement of fuel elements can be provided with differential coolant flow by means of orifices placed at the entry ends of the coolant passageways, a greater amount of coolant being allowed to flow through the central fuel elements so that the temperature of the coolant from these elements approximates the average coolant outlet temperature for the entire reactor.