Patent GB-1023687-A (GB)
Title: Heavy water moderated organic cooled nuclear fission reactor and method
IPC: G21C19/20
Filed: 1964-08-13
Abstract: 1,023,687. Reactors. UNITED STATES ATOMIC ENERGY COMMISSION. Aug. 13, 1964 [Sept. 26, 1963], No. 33011/64. Heading G6C. A reactor has a moderator tank through which pass coolant tubes, each containing fuel assemblies which are maintained in their required axial position by spacer rods; the fuel assemblies are axially arranged and are not positively connected to each other; the spacer rods in each tube are axially aligned with the fuel assemblies, and are pushed to remove a given fuel assembly from the coolant tube; heavy water is circulated through the moderator tank, and a liquid coolant is pumped through the coolant tubes to remove fission heat from the reactor. The invention aims at providing a heavy water moderated power reactor which shows a considerable saving in the bulk of D 2 O required, which has good neutron economy, and a freedom from flux peaking. The coolant is preferably terphenyl and the fuel is natural or slightly enriched uranium. There is no excess reactivity to be controlled during steady state operation. The reactor (Fig. 1), comprises the moderator tank 12 through which pass the coolant tubes 26. Above and below the core are tanks 28, 32 which, in addition to the coolant tubes 26, contain shielding material, e.g. iron shot 31, 33. The moderator tank is provided with thermal shielding 18, 22, 24 and is surrounded by a concrete wall 34 which carries brackets 38 etc. to support the moderator and shielding tanks. The wall 34 is surrounded by a number of annular and semi-annular coolant headers, and jumper tubes 54, 56 pass through apertures in the wall to interconnect the headers and coolant tubes 26. Pipes 14, 16 provide for moderator circulation. Reactor control is achieved either with control rods inserted between the coolant tubes in the moderator tank, or by columns of boron steel balls inserted in the tank. Coolant circulation is essentially aimed at achieving uniform fuel temperatures throughout the core. Thus coolant is passed first through the central coolant tubes, mixed with fresh coolant, and then passed through the outer tubes, and finally to the circulation pumps and a heat exchanger. Coolant flows in opposite directions in adjcacent coolant tubes, and on-load refuelling is carried out from the upstream end of each tube so that the assemblies generating most heat are always at the coldest end of a given tube. Each coolant tube is provided with removable end closures at both ends, and is surrounded by a second concentric tube closely spaced from it; helium gas is circulated through each of these annular section passages to thermally insulate the moderator from the coolant. Fuel assemblies.-Each fuel assembly comprises a bundle of canned UO 2  rods, held in cylindrical array by straps passed round them. The rod ends are staggered at both ends of the assembly, to prevent an accumulation of absorbing material at any cross-section; however, the outermost rods in each assembly terminate in planes perpendicular to the assembly axis at both ends, so that they form suitable surfaces for self-supporting axial stacking in a given coolant tube. The fuel assemblies are not positively attached to each other, but both ends of the column of assemblies in a given channel are abutted and held in axial position by the spacer rods. To unload a fuel assembly from the downstream end of a coolant tube the entire stack is pushed from the opposite end.