Patent GB-1218764-A (GB)
Title: A core for a nuclear reactor
Inventor: TAS ALDERT
IPC: G21C3/328
Filed: 1968-06-12
Abstract: 1,218,764. Reactors. REACTOR CENTRUM NEDERLAND. 12 June, 1968 [12 June, 1967], No. 27943/68. Heading G6C. A reactor core contains fissile rods spaced apart and all extending in the same direction, the rods being divided into groups each en closed by a prismatic box and the rods within each group being arranged in a lattice formation at the intersections of sets of straight lattice lines parallel to the walls of the box but with vacant locations not occupied by fissile rods occurring at intervals in some of the lattice lines, the lattice lines adjacent the walls of each box being fully or almost fully occupied. This construction makes it possible to achieve uniform distribution of the neutron flux. As shown in Fig. 2, the fissile rods 1 are positioned at the points at which pairs of parallel lines intersect, one line of each pair being thinned-out and the majority of the vacant locations being occupied by hollow moderator pipes 30, the remainder of the vacant locations being occupied by guide tubes 31 housing slidable neutron-absorbing control rods 57. Various ratios of the number of moderator pipes 30 to the number of guide tubes 31 are possible and in special cases all the vacant locations may be occupied by moderator pipes 30 or guide tubes 31. The reactor core may be built up from fuel boxes provided with control rods as shown in Fig. 2 alternating with fuel boxes in which all the vacant locations are provided with moderator pipes. If the box walls are to be situated between two fully occupied lattice lines of adjacent boxes by enlarging the distance between the boxes, burnable poison may be introduced into the edge rods to reduce power peaks occurring due to the excess of water near the box walls. Two moderator pipes 30, 63 are shown in Fig. 5, enclosing between them a guide tube 31 and supported between lattice plates 64, 65 each having a large number of apertures 66, 67 for the passage of coolant. Some of the coolant penetrates small passages 69 into the interior of the pipe 30, leaving the pipe through similar passages 72 at the top. An alternative form of moderator pipe 63 has a moderator filling 73 consisting of rods or tablets including burnable poison stacked so as to obtain the maximum possible flux flattening in a vertical direction. In the case of the moderator pipe 30, the same result is obtained by making two groups of perforations in the side wall in such a way that the material composition of the moderator pipe wall reaches a minimum at two positions along the length of the pipe. Similar perforations may be made in the guide tubes 31. The control rod 57 has guide members 93, 94 at opposite ends which run with a smooth fit in the guide tube 31. They may be provided with small slits or apertures 95, 96 through which the coolant can pass. In the case of pressurized water reactors, these slits or apertures prevent the accumulation of vapour bubbles under the guide members, which might give rise to local overheating. An alternative form of moderator pipe (Fig. 6, not shown) has a thickness varying lengthwise, maximum thickness occurring in the middle and at the ends of the pipe and minimum thickness nearer to the ends than to the middle of the pipe. A pressurized-water reactor in which control rods according to Fig. 5 are used is shown in Fig. 8, a pressure vessel 101 housing the core 102 being incorporated in a water circuit consisting of duets 104, 105, a heat exchanger 106 and a pump 107. The pump 107 propels the water upwards through the core 102. Also included in the circuit is a set of jet tubes 113 whose suction orifices 114 open into the top part of a chamber 110 situated around the core. The jet tubes 113 cause the water to follow a second circuit 111, remaining inside the vessel. The amount of water flowing through the core per unit of time is therefore greater than that flowing through the external circuit 104, 105, 106 and 107. Situated in the top part of the pressure vessel 101 are a number of packing glands 155 through which pass operating rods 156 which are used for raising and lowering the control rods 157.