In some cases, the position-measuring device on a control rod will not be accurate enough for use in a frequency response test. If the mechanical characteristics of the rod and drive are essentially the same at zero power and at full power, then it may be possible to overcome the position-measure­ment problem.

The basis of the calibration technique is that the zero-power frequency response is known with negligible uncertainty. The measurement of the zero-power frequency response using a CRDM with unknown dynamic response may be interpreted as in Fig. 7.2. The system output 50 and the

input to the CRDM controller 5R are measured. The zero-power transfer function G is obtained from theory. Then the frequency response of the rod and drive system is obtained using

F = (50/5R)/G (7.4.1)

This dynamic control-rod calibration is subsequently used for the at-power

tests. In that case the desired at-power frequency response Gp is given by

Gp = (ёОт/F (7.4.2)

Of course, the experimenter must be careful to insure that the response characteristics of the CRDM do not change with power level. For example, if the temperatures of CRDM components change, then clearances between parts could change. This would cause changes in frictional effects that would alter the CRDM response characteristics.