Patent GB-869314-A (GB)
Title: Improvements in boiling reactor
IPC: G21D5/06
Filed: 1959-08-19
Abstract: 869,314. Nuclear reactors. NUCLEAR DEVELOPMENT CORPORATION OF AMERICA. Aug. 19, 1959 [Aug 19, 1958], No. 28364/59. Class 39(4). In a water boiling reactor the light water coolant flows through channels in heat transfer relationship with the fissionable material, the coolant existing both in the liquid and vapour phases; and a moderating substance surrounding the channels has a moderating ratio substantially greater than that of the coolant; further, the relative proportions of the moderating substance and the light water are such that the excess reactivity of the reactor, is substantially controlled by the absorption of neutrons by the coolant which acts as a poison and also are such. that a decrease in the average density of the coolant, due to variations in the proportions of the light water in the liquid and vapour phases, results in an increase in the excess reactivity of the reactor. Thus when the steam generated in the coolant is utilized for driving a turbine an increase in steam demand causes a reduction in pressure in the reactor coolant channels and the proportion of vapour to liquid increases and the reactivity of the reactor is increased since the capacity of vapour to absorb neutrons is less than that of liquid. Hence the reactor is self-regulating in relation to the load demand. To prevent the reactor from destroying itself the various factors which affect the reactivity are so arranged that the maximum excess reactivity is limited to less than one dollar. Fig. 1. The reactor 1 has a lattice of fuel and coolant tubes, those in the centre comprising a boiling region 2 and the outer tubes comprising an annular superheat region 3. The tubes of region 2, which are connected in parallel, are supplied with light water in liquid form through a pipe 4, the heated liquid and steam passing to a separator 6 through a pipe 5, the liquid being returned through pipe 7 and pump 8. The steam passes through a pipe 10 to the tubes, which are connected in parallel, of the superheat region 3, the superheated steam passing through a pipe 11 to a multistage turbine 12 whence it is exhausted to a condenser 15 and is delivered by a pump 16 to the coil 17 of a heat exchanger 18 having a heater coil 20. The moderator, which in this instance is heavy water, is circulated through the coil 20 by a pump 33 and pipes 31, 32 and thus gives up its heat to the coolant. Pumps 21, 26 deliver the feed water to low pressure and high pressure feed heaters 23, 28 heated by steam drawn off extraction lines 13, the heated water being returned by a pipe 30 to the separator 6 and combined with the liquid separated from the steam. Reactor construction. The reactor comprises a tank 35 closed by end plates 36, 37, Fig. 2, and traversed by coolant tubes, one such tube in the boiling region being denoted by 38b and one such tube in the superheat region being donated by 38s. The tank 35 is surrounded by a thermal shield consisting of a steel cylinder 40 and cylindrical tanks 41, 42 filled with water and steel ringlets 43. Each tube 38 receives a pressure tube 49, the spaces 55 between tubes 38, 49 and between the tank and thermal shield being filled with helium to provide heat insulation, the tank 35 being filled with heavy water. The upper ends 50 of the tubes 49 are connected to pipes 68b, 68s having removable caps 76. The pipes 68b are connected to leader pipes 70 connected to pipe 5 of Fig. 1, and the pipes 68s are connected to leader pipes 73 connected to pipe 10 of Fig. 1. The lower ends 53 of the tubes 49 of the superheat region are connected through pipes 81 to leader pipes 82 in turn connected to pipe 11 of Fig. 1. Pipes 80 connected to tubes 49 of the boiling region are connected to leader pipes 78 in turn connected to pipe 4 of Fig. 1. A basic fuel element comprises a rod 56, Fig. 6, having a metal jacket 58 with short and long axial extensions 59, 60, the jacket containing pellets 57 of UO 2 . Each tube 49 contains 190 rods 56 arranged in five end-to-end clusters, each cluster consisting of two end-to-end bundles of 19 rods each. In Fig. 4 only one rod of each bundle is shown. Each cluster comprises end support plates 61, 62 and a centre support plate 63, the plates 61-63 each having a large number of perforations (Fig. 5, not shown). Certain of the perforations receive the rod extensions 59, 60 and other perforations serve for the passage of the coolant. The short extensions 59 are brazed to the centre plate 63 whereas the extensions 60 freely engage the holes in the plates 61, 62. All five clusters are supported in the tube 49 by the lowermost plate 61 which abuts a shoulder 65. The reactor is refuelled by removing the caps 76 and concrete or steel plugs 77. In modifications (not shown) the moderator comprises beryllium or graphite. Further, the reactor may have no superheating region and the superheating may be performed by a separate reactor or by a superheater using chemical fuel.