1,150,730. Moderator structures. UNITED KINGDOM ATOMIC ENERGY AUTHORITY. 21 July, 1966 [28 July, 1965], No. 53908/68. Divided out of 1,150,729. Heading G6C. A nuclear reactor core structure comprises an assembly of prismatic blocks 4 of neutron moderating material, e.g. graphite, arranged end to end in columns, having fuel element channels 8 and keying members 14 fitted in keyways radial relative to the axes of the blocks to key the blocks together, the keying members 14 being less than half the length of the blocks 4 to which they key and being supported within the length of the blocks 4 by loosely fitting spacing members 16 which serve no keying function in relation to the blocks. The structure also comprises an outer peripheral reflector region 1 of solid reflector blocks 2 coupled to each other and to adjacent core blocks 4 by keying members 14. The outer reflector blocks 2 are, however, coupled by sealing keys (not shown) in the form of rectangular section graphite bars which extend the full length of each column of blocks 2. Each of the core blocks 4 has a spigot 10 at its lower end which engages with a recess 11 in the upper end of the lower adjacent core block 4. The spigot 10 at the lower end of the bottom core block in each column engages with a recess 12 around an aperture 13 in the bottom support plate 6. The reflector blocks 2 and the core blocks 4 are arranged on a square lattice and the interspace between each group of four adjacent blocks is filled by a column of interstitial graphite blocks 17, 18. The interstitial blocks 17 are keying members of the same length as the keying members 14 and have an integral keying projection formed on each face, these projections engaging with longitudinal keyways in the adjacent blocks 2, 4. The interstitial blocks 18 are positioned alternately with the blocks 17 and serve no keying functions in relation to the blocks 2, 4. The interstitial blocks 17, 18 are joined end to end by graphite dowels 21 which engage with corresponding sockets 22 in the end faces of the blocks.