Inputs other than control-rod perturbations are considered briefly in this section. The brevity is necessary because there is little experience with tests of this type. Nevertheless, in the long run, results from these tests may be much more useful than reactivity perturbation tests for many systems.

For reactors with separate steam generators (this includes all current power reactors except boiling-water reactors), the overall plant performance is strongly affected by steam generator characteristics. The dynamic perform­ance of the steam generator is influenced by its design (recirculation type or once-through), its size, and its controllers. Also, the heat-transfer and flow characteristics of steam generators can change significantly between cleanings because of crud deposits. Dynamics tests may be useful in assessing the consequences of these changes on system performance. Steam generators can be perturbed by changes in primary fluid temperature or flow, changes in feedwater flow, or by changes in steam flow. Changes in primary fluid temperature can be induced by reactivity perturbations, but these tempera­ture variations may be too small to give suitably large changes in steam generator conditions. In this case, feedwater flow or steam flow perturbations will be necessary to obtain steam generator test data. A possible exception is a once-through steam generator with superheat. These have rather large responses to primary fluid temperature.

Boiling-water reactors are even more strongly coupled to steam conditions than other reactor types. Steam flow and feedwater flow perturbations appear well suited as excitations for these systems. Also, the recirculation flow is readily modulated because it is varied for control purposes during normal operation.

Many of the early tests on reactor systems have focused on a single input-output pair, usually reactivity-neutron flux. However, if a number of important process variables are stimulated strongly enough by the input in a test, they can be analyzed to give frequency response results for each output relative to the selected input. These results may greatly increase the information available from the test.

A key factor in determining whether a particular process variable can be used is the sensitivity and response time of the plant sensor. For many plant sensors, the main duty is monitoring steady-state or slowly varying parameters. The tester must determine whether the sensor sensitivity is large enough to give suitably large signals relative to background noise for expected variations in the measured parameter. The response time is often longer than one might expect because the hostile environment in power reactors requires sturdy equipment, and this often leads to some isolation of the sensor from the process. Good examples are thermocouples and resistance thermometers used for fluid-temperature measurement. They are usually imbedded in a ceramic such as magnesium oxide and cased in a stainless steel sheath. A typical time constant is 3 sec.