In reactor dynamics experiments the variable directly excited affects reactivity either directly, as in the motion of a control rod, or indirectly, as in the alteration of some system parameter (flow, pressure, etc.) Some ways in which reactivity can be varied are

1. By a specially installed rotary or reciprocating control rod.

2. By moving the normal control rod of the reactor through special switching or signal injection into the automatic control system.

3. By changing valve position, pump output, etc, usually by signal injection into its control system.

It is not surprising that 2 and 3 are more commonly encountered than 1, except perhaps in special-purpose experimental reactors, since they require relatively little modification of the existing system.

Among the specially installed rod oscillators, there is some preference for the rotary type over the reciprocating type, usually because of the higher frequencies attainable. Rotary types use the following methods to vary reactivity (see Fig 6.11) eccentric neutron absorbers rotating in a flux gradient,19,26 eccentric fuel rotating m a flux gradient,1 1 or neutron absorbers rotating past similar stationary absorbers which act as time varying shields 1 Usually neutron absorbers are oscillated with typical reactivity amplitudes being m the 0 5ф to 5$ range In zero-power fast reactors, fuel can be oscillated, however, this is unusual.

Regardless of the device used to perturb the reactor, there are a number of aspects to be considered, especially when high precision is important

1. Backlash and related effects causing phase un­certainty.

2. Random (long-term or short-term) variations in excitation frequency

3. Transfer function of the control device if only its input (rather than its output) is measured.

4. Unwanted harmonics when striving for a pure sinusoidal input

5. Reactor conditions affecting the excitation device.

In the last of these considerations, it should be remarked that more than just the integrity of materials is desired For example, the reactivity worth of an oscillator rod can depend on reactor power and flux distribution, for this

ECCENTRIC SOLID

ECCENTRIC SHEET

VARIABLE SELF SHIELDING (THE INNER RING HAVING VARYING ABSORBER AREAS)

ECCENTRIC VARIABLE SELF-SHIELDED SHEET

Fig. 6.11—Top view of some typical cylindrical rotary oscillator rods.

reason it is often desirable to have a sufficiently high frequency available to calibrate the oscillator rod against the zero-power transfer function in a frequency range where feedback is expected to be negligible.