A nuclear reactor control assembly includes a reactivity control device comprising a control rod cluster and a drive rod. The control rod cluster comprises an attaching head. The drive rod comprises an attaching device for connecting the drive rod to the attaching head. The attaching device is movable between a connection position and a disconnection position. The drive rod and the attaching device define an axial trough recess forming a sleeve. A checking device is used for checking separation between the control rod cluster and the drive rod. The checking device comprises a probe rod which is free to move translationally in the sleeve. A lower end of the probe rod can abut the attaching head. Relative displacement of the probe rod with respect to the drive rod can be used to determine whether the drive rod is separated from the control rod cluster.