Patent GB-923905-A (GB)
Title: Improvements in or relating to nuclear reactor graphite structures
Inventor: FIRTH ARTHUR
IPC: G21C1/12
Filed: 1959-11-26
Abstract: 923,905. Moderator structures. UNITED KINGDOM ATOMIC ENERGY AUTHORITY. Nov. 21, 1960 [Nov. 26, 1959], No. 40171/59. Class 39 (4). A graphite moderator structure comprises a mass of elongate graphite blocks of prismatic form arranged end-to-end in columns with the grain in the blocks running parallel to the axes of the columns, the blocks being spaced apart at their side faces by spacing members disposed along planes perpendicular to the columns, the spacing members being of material dimensionally stable under irradiation and of low neutron absorption. In one embodiment (Fig. 1), the spacing members comprise ceramic tiles 8 of beryllia keyed to adjacent end faces of elongate graphite blocks 5 stacked end-to-end. The tiles 8 are of somewhat greater dimensions than the blocks 5 in the horizontal direction and contact one another on their side faces, the side faces of the blocks 5 being spaced from one another. The moderator structure is held together by peripheral restraint bands 17 (of the type disclosed in Specification 782,922) located in slots 18 in the outer side faces of the graphite blocks 6 forming the peripheral reflector of the structure. The structure is supported inside a pressure vessel 4 by means of a grid 3 mounted on supports 22 and coaxial ducts 19, 20 conduct inflowing and outflowing coolant, respectively. Duct 20 is connected to a header 21 penetrated by standpipes 27 extending from the refuelling face 26 of the reactor to the upper ends of the coolant channels 9 in the blocks 5. Coolant enters the pressure vessel 4 by way of the duct 19, passes down the outside of the moderator structure and up through the coolant channels 9, enters the header 21 through ports 28 in the standpipes 27, and is finally discharged through the duct 20. In a modification, the beryllia tiles are of cruciform outline and are keyed into transverse recesses in the end faces of the graphite blocks. According to a second embodiment (Fig. 6), the spacing members comprise beryllia dowels 53 which penetrate the end-stacked graphite blocks 51 along co-ordinate axes perpendicular to the longitudinal axes of the blocks. The dowels 53, which are in end-contact with one another, are retained in the blocks 51 by means of circlips, and of such a length that the blocks 51 are spaced apart by gaps 57. The blocks in each column are keyed together at their end faces by graphite sleeves 59. In modifications, the dowels end-contact both inside and outside the blocks, the ends outside the blocks having enlarged heads, or they extend wholly through the blocks and have an enlarged head at one end. The dowels may, as shown in Fig. 10, each comprise a steel tube 71 filled with a moderating material 72, e.g. beryllia, in the form of a compacted powder and end sealed with steel plugs 73. In any of the foregoing examples, alumina or magnesia may be used instead of beryllia as the ceramic. Beryllium metal may be used, in place of a ceramic, in the form of rods 84, 85 keyed to the blocks 82 by means of flanges 90, 91 located in recesses 92, 93, as shown in Fig. 11, the rods being attached to the inner walls of the pressure vessel and maintained in tension.