Backup shutdown device for high temperature gas cooled reactor

In a nuclear reactor, a reactor shutdown back-up device is disposed in an independent system considering a case if the main reactor power system operation should fail. However, when the reactor shutdown back-up device is operated, it is necessary to recover control elements dropped in the control element insertion holes upon re-start up of the nuclear reactor. For the recovery, gases in the control element insertion holes are sucked by a blower, etc. and the gas stream is utilized. However, control elements remain in the bottom of the holes to cause troubles in the re-start up of the reactor. In view of the above, the bottom of the control element insertion hole in the reactor core is tapered so that the hole diameter at the lower end agrees with the hole diameter of a recovery pipe of the control element recovery device. Since the control elements are gathered at the lower end along the inclination of the side walls and aligned with the hole diameter of the recovery pipe, the control elements can be recovered with no residue. Further, there is no requirement for attaching a suction nozzle to the recovery pipe and the blower load is not increased. (K.M.)