Abstract: Improved shock absorber for slowing down the descent of a control rod in a liquid cooled reactor core at a controlled rate uses a thimble tube which has a corrugated section at its lower end to distribute the control rod deceleration forces over an extended distance in a "scram" situation. In a preferred embodiment, the corrugations have a lesser internal diameter at the lower end of the thimble tube than at the top to increase the rate of deceleration as the control rod aproaches the bottom of the core. In several modifications, the corrugation pitch and/or depth is varied over the length of the corrugated surface.