GB897421A - Low power nuclear reactors - Google Patents

Abstract

897,421. Nuclear reactors. AEROJETGENERAL NUCLEONICS. Dec. 8, 1958, No. 39495/58. Class 39(4). The fuel of a nuclear reactor is uniformly embedded in finely divided form in a moderating organic material which is a solid hydrogenous binder material having a hydrogen density above 5 X 10<SP>22</SP> hydrogen atoms per cubic centimetre and being thermally stable at 100‹C. The fuel may be enriched uranium dioxide of a particle size from 10 to 30 microns containing about 20% by weight of U235 and dispersed in polyethylene which is preferably irradiated, for example by electron bombardment, before use in a reactor to raise its melting point; there may be about 3.16 grams of polyethylene for each gram of uranium dioxide. The fuel/moderator elements may be in the form of cylindrical discs such as 52, Fig. 5, and the upper elements are supported by a plate 48 which extends across the centre of the reactor. A graphite reflector block 64 is disposed above and around the upper elements and is surrounded by an annular graphite reflector 62; spaces are left between the block 64 and the top of the elements and between the block 66 and the top of the containing vessel. The lower fuel/moderator elements are supported on a lower graphite reflector block 80 supported by a compression spring 102 mounted on a nut screwed to a tube 108; at its upper end tube 108 is held in position over an inner tube 120 by a fuse member 116 so that if the fuse 116 melts the lower reflector member and its associated fuel/moderator elements fall to the bottom of the container. The member 80 and its surrounding reflector 76 are tapered to facilitate this movement, and the upper reflector 64 is similarly tapered so that if the reactor should be inverted the upper reflector member and its associated fuel/moderator elements would fall away from the centre. The fuse member 116 may comprise fuel dispersed in a polymer having a relatively low melting point, for example polystyrene, the loading density of the fuel being preferably twice as great as in the reactor core. Four tubes 129 (of which two are shown) pass through the lower half of the core and contain movable control rods such as that shown in Fig. 3. Each control rod comprises a number of fuel/moderator discs 130 and a graphite reflector member 132 strung on a rod 134 and supported by a compression spring 136 held by a nut screwed to the rod. At its upper end the rod 134 is held in place by a fuse disc 146, similar to fuse disc 116, so that if disc 146 melts due to excessive reactivity of the core the fuel and reflector elements within the control rods drop downwards. The reactor is provided with a tube 32 passing right through the centre of the core and may be surrounded by a graphite reflector (not shown in Fig. 5) through which access ports extend, and by a thick lead shield, the whole being immersed in a tank filled with water containing boric acid to act as a neutron shield. In alternative arrangements for preventing excessive heating of a reactor core, a uranium loaded fuse melts and allows a gaseous or liquid poison to pass into the core, Figs. 6 and 7 (not shown); or the fuse releases a spring which drives a core plug out of the core, Fig. 8 (not shown); or the fuse ignites an explosive which drives out a core plug, Fig. 10 (not shown); or the core is made in two halves secured together by uranium loaded strips which melt and allow the lower half to drop away from the upper half, Fig. 11 (not shown).