17 simultaneously a down drive passage 36 is opened to a low pressure thus as the pressurized water drives piston 17 upward water in the annulus 33 is driven through the cross hole 32 and bore 31 down piston tube 26 and out of the passage 36 for downward motion of the drive the up drive passage 34 is opened to low pressure and pressurized water is supplied to the down drive passage 36 the pressurized water passes through piston tube 26 bore 31 and cross hole 32 to annulus 33 by which the water in annulus 33 between stop piston 23 and the top of drive piston 17 is pressurized to drive the piston 17 downward typically downward motion of the drive withdrawal of control rod is incremental notch by notch and at relatively low speed on the other hand upward drive control rod insertion is sometimes such as under scram conditions a continuous motion at high speed upon such rapid insertion as the drive piston 17 approaches the stop piston 23 near the end of the drive piston stroke it is necessary to arrest the drive piston motion and dissipate at least a substantial portion of the kinetic energy of the moving parts to avoid excessive mechanical shock on the drive system the.