The most obvious way to perturb a reactor for a dynamics test is to change the reactivity by moving a control rod. In this chapter, the most common control-rod drive mechanisms (CRDMs) will be considered to assess their suitability for dynamics testing. The key characteristics are the speed of motion, the accuracy of the position-measuring system, and the worth of the control rod. There are five different main CRDM types: (1) magnetic jack, (2) roller nut, (3) rack and pinion, (4) locking piston, and (5) winch.

Other input signals also may be used in power reactors. Steam-flow perturbations introduced with the steam valve cause significant responses in a number of plant process variables in all types of reactors. Feedwater flow to the steam generator (or to the reactor vessel for a boiling water reactor) may also be modulated. Modern boiling-water reactors use forced recirculation, and the recirculation flow may be perturbed to excite the system. Also, special tests involving minor loops may be useful. An example is a test involving modulation of the pressurizer pressure set point in a pressurized-water reactor.