permanent magnet 104 mounted on shaft 68 tracks the safety rod by actuating reed switches 106 mounted on a tube 108 located outside the shaft this position detection is possible only during scrams when the entire safety rod translates as will be understood from the previous description when drive shaft 38 and control rod 32 of the first described embodiment of the invention are positioned as illustrated in fig1 the neutron absorber 58 on the end of shaft 54 is held above the nuclear reactor core 64 if a condition developes that requires shutdown of the reactor pressuring fluid in housing 28 below piston 30 is released through conduit 36 spring 34 then forces piston 30 and control rod 32 connected thereto downwardly which moves first cam elements 50 on the fingers 46 of latch 42 into engagement with the second cam element 56 on the end of drive shaft 38 since the oblique camming surface on cam element 56 is spaced farther from the longitudinal axis of drive shaft 38 than the oblique camming surface on each of the cam elements 50 the lower ends of fingers 46 are flexed away from shaft 54 and the detent elements 48 on the fingers are moved out of the recess 52 on the last mentioned shaft shaft 54 is thus allowed to move downwardly to a level wherein absorber.