Patent GB-984656-A (GB)
Title: Improvements in or relating to nuclear reactors
IPC: G21C7/04
Filed: 1964-02-17
Abstract: 984,656. Reactors. GENERAL DYNAMICS CORPORATION. Feb. 17, 1964 [Feb. 18, 1963], No. 6542/64. Heading G6C. A nuclear reactor having a high prompt negative temperature coefficient of reactivity comprises a pressure vessel containing a liquid moderator coolant, and a core within the liquid, the core including fuel elements containing lutecium, erbium, or a mixture of both, and a mixture of zirconium hydride and fully enriched uranium. The reactor (Fig.3) is submerged in water in a concrete container (Fig. 2, not shown), this water being circulated and acting as a coolant and shield. The reactor is cooled internally by water, diphenyl, terpenyl, or a mixture of the last two. The stainless steel pressure vessel 26 is closed by a bolted cap 32, which includes lead shielding 33. The core, reading inwards, comprises the following bodies, all of annular plan view:- multisection stainless steel thermal shield 42, segmental radially movable beryllium reflector shield 15, a shroud 18, fuel assembly sheathing 91 containing a ring 12 of fuel assemblies, annular coolant channel 17, and central fuel assembly. Coolant enters at 20 and passes downwards through three annular channels outside the shroud 18; some coolant passes through pipes 111 to the inner annular channel 17. The coolant then passes up through the fuel assemblies and out of the pressure vessel through a pipe 173. The pressure vessel 10 may be surrounded by a second casing, the space between it and the pressure vessel being filled with a thermal insulator, such as glass wool. The reactor is controlled by moving the reflector shield 15 towards and away from the core. Reflector shield movement (Fig. 5). The shield comprises eight segments, each movable in a radial direction, each segment having an independent actuating system. Each segment 15 is guided by a pair of plain shafts, and moved by a driven nut 127 in conjunction with a leadscrew 122. For emergency shutdown, provision is made for releasing the driving nut, thereby enabling tension spring 133 to return each shield segment to its outermost position. A long shaft 134 connects each nut to its associated motor and drive gear unit 153 situated above the water level in the reactor containing vessel. A potentiometer 168 provides position indication signals which are fed to a reactor control console. Fuel assembly (Fig. 7). Top and bottom stainless steel grid plates having coolant circulation cutouts 100, are connected together by three of the fuel elements, by means of nuts 76. The outermost fuel elements are provided at both ends with plane spigots which locate in holes in the top and bottom grid plates. The majority of the fuel elements 13a are clamped to the bottom grid plate by nuts, their upper tapered ends being free. Various other shapes of fuel assembly are used in the core, all having the constructional basis of the one illustrated. The fuel elements are all of the same construction, other than the three types of end fitting described above, and comprise a stainless steel can having a single helical rib to improve heat transfer and to space the elements apart when assembled. A space is left in the can adjacent to each end for fission products. The fuel is a solid homogeneous mixture of zirconium hydride and fully enriched uranium, the ratio of zirconium to hydrogen atoms being preferably greater than 1. 5:1. Erbium is included in the fuel either homogeneously mixed or as lumps; in a preferred form, the erbium is present as a central axially extending wire. The erbium may be partially or entirely replaced by lutecium. Specification 852,877 is referred to.