14 are apparently self excited and predominantly at the natural frequency and the first and second overtones of the control rod all frequencies less than 20 hz for a typical control rod these vibrations are believed to be the result of guide tube flow effects caused by driving forces related to the periodic interaction near the guide post exit 36 of the axial vortices 38 with the diffuser vortex 40 as described above a variety of proposed improvements were tested in a flow loop wherein the dimensions and flow rates were similar to typical reactor operating conditions most of the tested guide posts had very little effect in reducing the vibration of the control rod in the guide tube 14 but the present invention was effective in reducing vibration fig3 shows the preferred embodiment of the invention and fig2 shows the conventional structure which it replaces the control rod 28 being shown in the inserted position as in fig1 the cross section shown in fig2 and 3 is taken along the shoulder portion 46 of the guide post 20 shown in fig1 the prior art guide tube 14 and post 20 shown in fig1 and centered control rod 28 shown in fig2 a typically have a tube inner diameter of about 0 90 inches and a control rod outer diameter of 0 86 inches during full power flow conditions typically about 9 gallons per minute gpm the control rod was found to vibrate with an acceleration of about 0 20 g 39 s the.