Patent GB-1347349-A (GB)
Title: Nuclear reactor
IPC: G21C1/08
Filed: 1971-07-19
Abstract: 1347349 Controlling reactors ASEA-ATOM AB 19 July 1971 [20 July 1971] 33700/71 Heading G6C A nuclear reactor has within the central region of its core a plurality of similar cells each including nuclear fuel and a longitudinally displaceable, elongated neutron-absorber unit which comprises at least one absorber rod, the absorber units being mechanically connected in groups in such a way that they form a plurality of control rods in which a quotient a/b is at least 1/#2, a being the distance between the longitudinal axis passing through the centre of gravity of a control rod and that of an absorber unit belonging to the same control rod, and b being the distance between the same longitudinal axis of the same control rod and that of an adjacent absorber unit belonging to an adjacent control rod. As shown in Figs. 2 and 8, each absorber unit comprises one absorber rod 19, (19<SP>1</SP>) of cruciform cross-section, each control rod 15 comprising five of these absorber rods. The absorber rods, which may contain B 4 C, are attached at their lower ends to a common, transverse cruciform carrier 20 which is joined to the control drive by means of a shaft 31. The arms of the absorber rods 19 are located in the gaps 18 between adjacent square-section fuel assemblies 3, each of which comprises a shroud tube 22 surrounding a bundle of elongate canned fuel rods 23 arranged in a regular square lattice. A number of fuel rods, uniformly distributed through the central region of the lattice, are replaced by tubes 24 through which cooling water flows without boiling, the cooling water flowing through the shroud tubes 22 but outside the tubes 24 being caused to boil. Approximately 10% of the circulating cooling water is led up through the core in the gaps 18 to cool the control rods but does not boil. The non- boiling water in the tubes 24 compensates the effect caused by the water gaps between the fuel assemblies 3. The tubes 24 may also be arranged to contain burnable prisms. Although described in connection with a boiling water reactor, the invention can also be used in other nuclear reactors, e.g. in a pressurized water reactor. In the Fig. 8 arrangement (also Figs. 3 and 4, not shown), the quotient a/b is #2 whereas in Fig. 5 (not shown) it has its minimum value of 1/#2. In Figs. 6 and 7 (not shown), the control rods have a cross-section in the form of a crossed cross in which each of the four main arms ends in a small cross. Alternatively, as in Figs. 9 to 13 (not shown), each absorber unit comprises a group of absorber rods in the form of control rod fingers of circular cross-section. The control rod fingers are located in guide tubes in the fuel assemblies, each tube occupying the space which would otherwise have been occupied by four fuel rods. The gaps between the fuel assemblies are of such a width that the water in them and in the guide tubes is substantially uniformly distributed throughout the reactor core. Where there are no gaps between the fuel assemblies, additional tubes are provided in the fuel assemblies and these do not house control rod fingers but serve only to introduce an extra quantity of non-boiling water into the fuel assemblies. These latter tubes may even with advantage contain burnable prisms.