Abstract: In a gas cooled pebble bed reactor the poison part of a control rod which s upper and lower portions and an interior cooling fluid channel is movable into and away from a bed of spherical fuel elements; the improvement includes a control rod security device which facilitates removal of the fractured control rod when the control rod breaks between its end portions. The security device includes an oblong member which is located and extends within the cooling channel and is slack relative to the control rod, a device attached to one end of the oblong member for preventing a fall of the oblong member into the lower portion of the control rod, and a device attached to the other end of the oblong member for preventing portions of the control rod from falling into the bed of fuel elements upon a fracture of the control rod between its end portions.