Patent GB-974726-A (GB)
Title: Nuclear reactor
IPC: G21C3/36
Filed: 1963-10-28
Abstract: 974, 726. Nuclear reactors. UNITED STATES ATOMIC ENERGY COMMISSION. Oct. 28, 1963 [Dec. 26, 1962], No. 42351/63. Heading G6C. A nuclear reactor has a hexagonal cross section core with a central hexagonal hole in its centre, said core comprising a plurality of fuel elements of rhomboidal cross section arranged in a number of hexagonal rings around the central hole. The reactor is heterogeneous, water cooled and moderated, and it is intended for use as a research reactor; its principle object is to provide a very high thermal neutron flux in the central cavity. The reactor, Fig. 1, is submerged in water 26 within a biological shield 28. The pressure vessel comprises a central spherical region 21 surrounded by a heat shield 31, and upper and lower cylindrical portions 22, 23. The core 41 is surrounded by a shroud 50, and is supported by top and bottom plates 48, 49, welded to an outer hexagonal shroud 43 supported by the pressure vessel. Light water coolant enters the reactor through a single port 36 and leaves via a pair of radial ports 37. A portion of the coolant is allowed to leak at the seal 53 between the pressure vessel and the outer shroud 43, and also through holes in the outer shroud just below the seal. This flow leaves the spherical portion of the pressure vessel through small holes in a supporting skirt 45. Thus the core is surrounded by a water reflecting/ shielding region. Emergency shutdown is achieved by injecting boron into the core through nozzles 80. The research facilities comprise a horizontal beam tube 38, vertical sample thimbles 39, and rabbit facilities 40. The complete core is removable from the reactor as a single unit. The reactor core (Fig. 2) comprises three hexagonal rings of fuel elements 58 of rhomboidal cross section, surrounding the flux trap 57. The fuel elements are assemblies of flat plates most of which lie parallel to the core boundaries, and they are arranged so that the fisionable content of the plates increases in graduated steps from the boundaries. Six safety rods 59 and six control rods 60 are included in the core. The safety rods and control rods comprise an upper poison section of hafnium and a lower section of zircalloy 2. The rods are mounted in shrouds to prevent them being buffeted by coolant flow. All structural parts of the reactor are of stainless steel. The reflector (Fig. 2) is formed of a series of rings of beryllium blocks supported between plates 48, 49 (Fig. 1). The innermost reflector ring 90 comprises hollow blocks 91 with beryllium cylinders 93 inserted therein; this arrangement provides for easy removal of the reflector elements most liable to radiation damage, and, for special requirements, the beryllium rods in the holes may be replaced by further fuel elements. Small spaces are left between the reflector rings to allow for coolant to be circulated within the reflector assembly. The core includes a number of stainless steel flux suppressors to reduce flux peaking. The fuel elements, (Fig. 11), comprise assemblies of parallel plates 81 of highly enriched UO 2  particles in a matrix of stainless steel, and include 0.08 atomic per cent samarium as a burnable poison. Stainless steel cladding plates are attached to both sides of the fuel. The fuel plates are held together by dovetailed locking keys 82, and located in end pads 88. The upper end fitting includes an integral threaded stud 84 to facilitate bolting the fuel element to the top core plate 62, (Fig. 1).