Patent GB-1076549-A (GB)
Title: Improvements in or relating to fuel elements
IPC: G21C3/16
Filed: 1965-04-28
Abstract: 1,076,549. Fuel elements. GENERAL DYNAMICS CORPORATION. April 28, 1965 [May 5, 1964; July 22, 1964], No. 17849/65. Heading G6C. A fuel element for a high-temperature, gascooled nuclear reactor comprises an elongated body section 13 of graphite having a predetermined gas permeability in which are formed an annular fuel chamber or a plurality of longitudinally extending fuel chambers 17 arranged in the general shape of a ring and also a central purge gas passageway 29, a radially inward flow of purge gas from the outer surface of the body section 13 through the or each of the fuel chambers 17 to the passageway 29 being established when the fuel element is disposed in an operating nuclear reactor and the passageway is connected to a region of lower pressure than the pressure of the coolant gas, e.g. helium, at the outer wall of the body section 13, which region is isolated from the coolant gas stream. In the embodiment shown in Fig. 1, internal threads 25, 27 are provided at the top and bottom, respectively, of the body section 13 to join it to a top reflector 21 and a bottom connector 23. The top reflector 21 is made of graphite and has a spacer ring 31 provided intermediate its ends which serves to laterally abut spacer rings on adjacent fuel elements when the elements are mounted within the reactor core. Additional spacer rings (not shown) may be provided, if desired, intermediate the ends of the body section 13 to prevent bowing of the element. The fuel chambers 17 may be of any desired cross-section. The central bore 15 of the body section 13 houses a central moderator 28, preferably of beryllium oxide, spaced from the bore 15 to provide the purge gas passageway 29 which extends in annular form the full length of the body section 13. A coupling element 35 which is provided to allow the assembly of a fission-products trap 37 inside the bottom connector 23 contains a plurality of passageways 39 registering with the annular passageway 29. The lower end of the bottom connector 23 contains a central hole 49 which receives a hollow stand-off pin (not shown) that positions the element in the reactor core, and above the central hole 49 is a cylindrical cavity 51 which houses the fission-products trap 37, the latter consisting of a graphite block fitting within the cavity 51 and having an indentation in its lower surface in which a cup-shaped filter 53 is seated. The filter 53 prevents solid particles from being carried out of the fuel element with the purge gas. A plurality of radial slots 61 extend inwardly from the outer surface of the trap 37 and are filled with charcoal which removes fission products from the purge gas. The fuel material 19 may comprise packed beds of fuel particles, in which case the heat transfer is improved by filling in the void spaces with a material having good thermal conductivity. In the embodiment shown in Fig. 1, the fuel material 19 comprises a paste-like composition of fuel particles of a solid solution of ThC 2  and enriched UC 2  (in a 10 : 1 atom ratio, Th to U), either coated or uncoated, and a viscous liquid having good thermal conductivity, preferably a mixture of graphite and coal tar pitch. The paste is squeezed into the fuel chambers 17 and the body section 13 is then heated to carbonize the pitch and remove any volatile substances present. Alternatively, the fuel material comprises spherical packets of fuel granules, less than 10 microns in their largest dimension and having an average size of about 2 microns, and a compatible refractory substance such as graphite, disposed in a supporting matrix of a moderating material, preferably graphite. As a result of recoil, fission products are deposited in the graphite portion of the packets, outside the fuel granules, and readily migrate into the supporting matrix where they are picked up by the purge gas. The damage resulting from the fission product recoil is substantially confined to the graphite in the packets, leaving the matrix free from any significant recoil damage.