The locking piston CRDM is a hydraulically driven CRDM used by the General Electric Company in its boiling-water reactors. These CRDMs are mounted beneath the reactor core and use primary system water as the operating fluid. These CRDMs move in stepwise fashion between fixed stops. The distance between stops is 6 in. The nominal rod speed is 3 in./sec or less. This means that it takes at least 2 sec to accomplish the unlatching, latching, and rod moving associated with a step. Also, the CRDM is designed so that the rod must be raised slightly to disengage a latch before it can be lowered. The reactivity per step is quite adequate for dynamic testing, but the maximum frequency is less than 0.25 Hz. Furthermore, the small insert before a withdraw step gives a reactivity change that may be difficult to account for in the input-signal spectrum, since there is normally no continuous indication of rod position. The position indication consists of

magnetically operated switches that are actuated as the rod passes certain points (usually at latching points and halfway between latching points). These observations indicate that this CRDM is not very well suited for frequency response testing.