Patent GB-1364770-A (GB)
Title: Nuclear reactor internals construction and failed fuel rod detect ion system
IPC: G21C7/117
Filed: 1972-12-11
Abstract: 1364770 Failed fuel rod detection system WESTINGHOUSE ELECTRIC CORP 11 Dec 1972 [21 Jan 1972] 57001/72 Heading G6C An arrangement for detecting a failed fuel rod in a nuclear reactor having axially movable control rods disposed in preselected fuel assemblies with drive shafts connected to the control rods extending through adapter tubes passing through the reactor vessel closure head, the reactor including a pressurized coolant, e.g. water, and mechanisms operable by the pressurized coolant for actuating the drive shafts, is characterized by means for conducting only coolant from selected fuel assemblies to the mechanisms and a radiation monitor adapted to receive coolant from preselected mechanisms to determine if a selected fuel assembly has a failed fuel rod. As shown in Fig. 1, the reactor core comprises fuel assemblies 14 mounted between lower and upper core plates 15, 16 within a core barrel 17, the core periphery being bounded by a form fitting baffle structure 21 which limits the core by-pass flow of the coolant. The coolant flows between the reactor vessel 10 and the barrel 17 before entering the core through apertures in the lower core plate 15. Support tubes 23 between the upper core plate 16 and a deepbeamed support plate 22 also house guide tubes 41 for the drive shafts 37 and control rods 38. As shown in Fig. 4, each drive shaft 37 is attached to a pair of control rods 38 by a spider 39 and the guide tubes 41 are alternatively triangular and oblong in cross-section. Coolant water exits from the support tubes 23 through a number of large windows 62 cut in the tube walls. The pressure drop required for operation of the control rod mechanisms is produced by a pump 86, the suction side of which is connected to a header 87 to which the individual mechanisms are connected through feeder lines 88. A radiation monitor 91 is mounted on the header 87. In order to check fuel assembly "A", for example, for a fuel rod failure, one of the valve operating coils "B" of the control rod mechanism located directly above the fuel assembly "A" is energized. The control rods are maintained in their raised position during normal operation of the reactor by electromagnets 34, thereby permitting the valve operating coils to be de-energized during normal operation of the reactor. Opening of the valve causes coolant to flow through guide tubes 41, seal assembly 73, adapter tubes 36 to reach the radiation monitor 91 via header 87. Since only one valve is permitted to open during the test period, the coolant reaching the monitor 91 is a true sample of that passing through the fuel assembly being tested. The monitor reading is compared to a simultaneous reading of the general radiation level of the reactor coolant system. Defects in other fuel assemblies can be located by indirect methods. By suppressing the output of a tested, non- defective fuel assembly by temporarily inserting all its control rods, coolant from the adjoining fuel assemblies is caused to mix with coolant from the tested assembly in sufficient quantities to determine if any of these has developed a defect. Pinpointing of a defective fuel assembly is then accomplished by testing in the immediate neighbourhood.