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The problem of measuring fundamental characteristics of an inherently stable power-producing reactor is described. It is noted that, in the power range of operation of such a plant, reactivity is a function not only of control rod position, but also of power itself. The mechanism of the latter effect involves the variÃ²us inherent coefficients of reactivity. The result is that conventional, low-power methods can be used neither to obtain rod calibrations, nor to determine temperature coefficient in the very range of operation where these characteristics assume importance. It is shown that a number of parameters including those mentioned above may be measured by effecting small sinusoidal perturbations in control rod position over a range of frequencies. The resultant frequency responses are analyzed by the familiar transfer function-block diagram method. Certain inherent advantages of the method are cited including: 1) Ability to separate effects which predominate in different frequency regions. 2) Improved reproducibility of power range measurements. Disturbances induced by steam plant changes, inherently nonreproducible, are avoided. 3) Good accuracy of data. The measurement replies primarily on good differential accuracy, easier to obtain from operational instrumentation than good absolute accuracy. 4) Ability to check data reproducibility by obtaining several cycles of data at each frequency. A brief description is included of the manner by which the technique is used to determine the following nuclear properties: 1) Perturbing rod calibration, 2) Temperature coefficient of reactivity, 3) Neutron generation lifetime, 4) Fractional production of delayed neutrons, 5) Mean delayed neutron decay constant.