A control rod drive unit for use with nuclear reactors having a motor for effecting movement of a drive rod coupled to the control rod between positions corresponding to full insertion and full withdrawal of the control rod with respect to the reactor. There is also provided a seal for providing a gas tight sealing between the drive rod and the cover of the nuclear reactor with the motor connected to the drive rod, and a further seal for sealing the drive rod to the reactor operative upon the means for disconnecting the motor from the drive rod, and a further means for partially disengaging the first seal from the reactor cover.