An apparatus comprising: a control rod guide frame configured to support at least one control rod of a nuclear reactor when the control rod is partially or fully withdrawn from the reactor core, wherein the control rod guide frame comprises a stack of two or more columnar elements defining a central passage having a constant cross-section along the central passage, the apparatus further comprising: a control rod assembly comprising at least one control rod parallel aligned with the central passage of the control rod guide frame; wherein the at least one control rod is movable into and out of the central passage of the control rod guide frame; and wherein any portion of the at least one control rod disposed in the central passage is guided by the central passage over the entire length of the portion of the at least one control rod that is disposed in the central passage, wherein the control rod assembly comprises a plurality of control rods connected with a spider or other coupling element, and the spider or other coupling element is disposed in the central passage of the control rod guide frame and moves along the central passage as the plurality of control rods move into or out of the central passage, characterized in that the spider or other coupling element comprises: a frame comprising a first material and defining cavities; and a filler disposed in the cavities, the filler comprising a second material that is more dense than the first material. a control rod guide frame configured to support at least one control rod of a nuclear reactor when the control rod is partially or fully withdrawn from the reactor core, wherein the control rod guide frame comprises a stack of two or more columnar elements defining a central passage having a constant cross-section along the central passage, the apparatus further comprising: a control rod assembly comprising at least one control rod parallel aligned with the central passage of the control rod guide frame; wherein the at least one control rod is movable into and out of the central passage of the control rod guide frame; and wherein any portion of the at least one control rod disposed in the central passage is guided by the central passage over the entire length of the portion of the at least one control rod that is disposed in the central passage, wherein the control rod assembly comprises a plurality of control rods connected with a spider or other coupling element, and the spider or other coupling element is disposed in the central passage of the control rod guide frame and moves along the central passage as the plurality of control rods move into or out of the central passage, characterized in that the spider or other coupling element comprises: a frame comprising a first material and defining cavities; and a filler disposed in the cavities, the filler comprising a second material that is more dense than the first material. a control rod assembly comprising at least one control rod parallel aligned with the central passage of the control rod guide frame; wherein the at least one control rod is movable into and out of the central passage of the control rod guide frame; and wherein any portion of the at least one control rod disposed in the central passage is guided by the central passage over the entire length of the portion of the at least one control rod that is disposed in the central passage, wherein the control rod assembly comprises a plurality of control rods connected with a spider or other coupling element, and the spider or other coupling element is disposed in the central passage of the control rod guide frame and moves along the central passage as the plurality of control rods move into or out of the central passage, characterized in that the spider or other coupling element comprises: a frame comprising a first material and defining cavities; and a filler disposed in the cavities, the filler comprising a second material that is more dense than the first material. a frame comprising a first material and defining cavities; and a filler disposed in the cavities, the filler comprising a second material that is more dense than the first material.