Patent GB-951625-A (GB)
Title: Nuclear fuel element
IPC: G21C1/12
Filed: 1960-07-15
Abstract: 951,625. Nuclear fuel elements. UNITED STATES ATOMIC ENERGY COMMISSION. July 15, 1960 [March 7, 1960; April 19, 1960], No. 24756/60. Heading G6C. A fuel element for a nuclear reactor comprises a container of neutron moderating material having a permeation coefficient of not more than 1 x 10-<SP>5</SP> cm2/sec. as measured by a helium leakage method, a fuel body inside and in contact with the container, and continuous purge gas channels within the container extending between a gas inlet and a gas outlet. As shown in Fig. 1, fuel compacts 53 are enclosed in a can 55 and an outer container 58, both of low permeability graphite. The can 55 also contains a fission product trap 71 in which charcoal or alumina mixed with copper or silver is arranged in channels 111 drilled in a graphite block 109. Part of the coolant gas enters the element at 99 as a purge gas stream, passing up through grooves 93 between the container and the can and then down through grooves 77 between the fuel compacts and the can; fission products are removed by the trap 71, gas leaving the element through passageways 117, 119. An alternative form of fission product trap is described in Specification 883,678. The elements 11 are arranged in a high temperature reactor as shown in Fig. 2 in which helium coolant gas enters at 43 and leaves at 45. Purge gas from the elements leaves the reactor through pipes 51 and passes through external fission product traps, returning to a low pressure point in the primary cooling circuit. In a modified element (Fig. 4, not shown), purge gas enters through a porous plug at the top of the element and flows downwards in parallel streams between the container and the can and between the can and the fuel. In Fig. 5 (not shown) purge gas does not pass through the fuel can, but only between the can and the container. Fig. 6 (not shown) illustrates an element having a singlewalled graphite container enclosing fuel compacts and a fission product trap, purge gas flowing downwardly through the element. The element illustrated in Fig. 7 (not shown) has longitudinally apertured fuel compacts in a closed can and does not include a fission product trap; purge gas flows between an outer container and the fuel can and carries the fission products to external traps. Elements of this type are provided near the top with graphite positioning rings 345 having spokes 353, 357 which fit into the spokes of adjacent elements as shown in Fig. 8; the outer sleeve of each element also has graphite spacing pads. The fuel compacts may consist of a mixture of uranium 235 carbide, uranium 238 or thorium 232 carbide, and graphite, beryllium, or beryllium oxide as moderator. The mixture may be cold pressed with pitch or petroleum and then densified by hot pressing; the procedure described in Specification 933,552 may be used. The compact may consist of a central cure of moderator alone surrounded by a ring of the fuel mixture. The permeability of the graphite may be lowered by coating or impregnation, for example as described in Specification 933,552 or U.S.A. Specification 2,995,471; reference is made to coating with silicon carbide or zirconium carbide. Specifications 779,134, 789,022, U.S.A.  Specifications 2,714,577, 2,782,158, 2,799,642, 2,827,429 and 2,831,807 also are referred to.