Patent GB-913653-A (GB)
Title: Improvements in or relating to nuclear reactors
IPC: G21C11/08
Filed: 1959-01-27
Abstract: 913,653. Nuclear reactors. BABCOCK & WILCOX CO. Jan. 27, 1959 [Jan. 31, 1958], No. 2981/59. Class 39 (4). In a nuclear reactor of which the core is contained in a pressure vessel and includes a plurality of upright fuel element assemblies, the core is surrounded by a sleeve located within the pressure vessel, the sleeve is formed by an upper part and a lower part, the lower end of the lower part and the upper end of the upper part are maintained in position relative to the pressure vessel, the lower end of the upper part and the upper end of the lower part overlap, and means is provided for locating the overlapping ends of the two parts laterally relative to each other. The reactor shown in Fig. 2 comprises a cylindrical pressure vessel 33 surrounded by stainless steel wool thermal insulation contained in a can 44 which in turn is surrounded by a water tank 40 forming a neutron shield. Within the pressure vessel two heat snields 54, 56 and a frusto-conical support skirt 52 are permanently disposed, the other parts being removable through the top of the vessel. The fuel elements 94 extend between a lower grid assembly 47, which rests on the support 52, and an upper grid assembly 45. The active part of the core is surrounded by a heat shield 132 which, together with the cylinder 156 and the upper flow baffle 166, forms a longitudinally extensible sleeve that contains the core and is fixed at its upper and lower ends. The upper end of shield 132 fits outside cylinder 156 and the two are pressed apart by springs 184. In operation, water flows through the pressure vessel from an inlet 20 to an outlet 22, most of the water passing through the reactor core but part of it passing round the heat shields 132, 54 and 56. The lower grid assembly 47 and the support ring 60 for the heat shields 54 and 56 have orificed plugs in them to control the flow of coolant; the plugs are replaceable by plugs of different orifice size to provide for variation of coolant flow. Each fuel element assembly comprises a casing 94, Fig. 3, of generally square crosssection containing a number of fuel pins 98 extending between horizontal support plates such as 112A, 112B. Each fuel pin comprises a stainless steel tube containing a column of cylindrical fuel pellets stacked end to end. The fuel pellets comprise a mixture of fully enriched uranium dioxide (i.e. the uranium is at least 90% U235) and thoria which has been compressed and sintered to be of high density. An inert insulating pellet of magnesium oxide or alumina is placed at each end of the column of fuel pellets to reduce the temperature gradient between the tube and the end plugs, and a heat transfer medium such as helium, lead, or sodium is provided within the tube. The lower end of casing 94 is attached to an inlet nozzle extension adapter 116 which fits into the lower grid assembly 47. The upper end of casing 94 is removably attached to an extension 121 by a bayonet lock 122; an interior transition sleeve 119 is slidably engaged at one end within extension 121, and its other end bears on plate 112B under the pressure of a spring 120. As shown in Fig. 6, the fuel element assemblies are not quite square but have recessed corners so that each group of four assemblies provides a central cross-shaped control rod channel 126. The reactor core has two fuel concentration zones, the fuel assemblies in the centre zone (marked A) containing 5.3%, UO 2  and the remainder ThO 2 , and the assemblies in the outer zone (marked B) containing 6.6% UO 2 ; this provides a more uniform distribution of power in the core. Outside the outer zone are dummy assemblies 93 which have the same dimensions as active assemblies and provide uniformity of the flow of coolant in the core; the control rod channels in this area contain stainless steel dummy rods of the same dimension as the control rods to equalize coolant flow. The core is surrounded by a shroud 134 and coolant flow in the space between the shroud 134 and the shield 132 is controlled by a horizontal baffle plate 138 provided with openings for the passage of coolant. Specifications 775,156, 913,654 and 913,655 are referred to.