The power spectral density of the neutron density of a reactor is frequency dependent and related to the reactor transfer function and the power spectral density of the input disturbance. For a critical reactor, a power-spectral-density measurement can be used to evaluate the ratio (β/) where β is the effective delayed-neutron fraction and the effective neutron lifetime. For subcritical operation, an evaluation of the reactor shutdown margin can be obtained by determining the quantity where k, the effective reproduction constant, can be determined if the effective neutron lifetime and effective delayed neutron fraction are known. The output power spectral density of the University of Florida Training Reactor, operating in the subcritical region, has been measured using a plutonium/beryllium source to provide the input disturbance. The data are then fitted by a least-squares method to a theoretical model to obtain the quantity