A neutron-absorber control rod tube includes end plugs welded at opposite ends to the tube with each end plug having a bore opening into the interior of the tube. A hafnium rod is disposed inside the absorber tube and has reduced diameter end portions providing an interference fit with the bores of the end plugs. The end faces of the hafnium rod terminate short of the inner end faces of the bores of the end plugs defining gas pockets therebetween. The interference fits between the bores of the end plugs and hafnium rod, respectively, afford substantial frictional resistance to relative movement of the end plugs and absorber rod, while the gas pockets provide a viscous dampening effect upon relative movement of the end plugs and absorber rod, in response to acceleration or deceleration loads on the structure in an axial direction of the absorber tube and the absorber rod.