This chapter gives a short description of the frequency response functions that describe nuclear reactor dynamics. A power reactor has inherent feed­back due to temperature (and possibly pressure) changes that accompany power changes. This closed-loop system may be modeled using a forward loop to describe the power response to a reactivity change and a feedback loop to describe the power-to-reactivity feedback processes.

The forward-loop transfer function gives the response to a reactivity input that would occur in the absence of any feedback. This transfer function is called the zero-power transfer function.

The most general form of model that has been used to calculate the zero — power frequency response includes the energy and spatial dependence of the neutron flux as well as the frequency dependence. In this case, the multi­group, time-dependent neutron diffusion equations are used. The space dependence of the neutron flux can be important at higher frequencies, but this usually does not influence tests on power reactors because the main emphasis in these tests is to identify feedback effects, and these influence the response at lower frequencies. In special measurements, the higher-frequency response may be measured to provide reactor physics data (such as neutron generation time). Space dependence is also important at very low fre­quencies (less than 10“5 Hz) for xenon spatial transients.

There is also some interest in the dynamics of subcritical reactors. The degree of subcriticality can be determined in tests in which the reactivity or a neutron source is modulated.

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