Determination of Reactor Transfer Functions by Statistical Correlation Methods

An experimental investigation has been made of the self-fluctuations in neutron density in a nuclear reactor, and the response in neutron density for random reactivity inputs, using analog correlation techniques. The analysis of self-fluctuations was based on ion chamber measurements of the fluctuations of neutron intensity at various points. Autocorrelation analysis was then used to find the power spectrum of the fluctuations, which has the shape of square modulus of transfer function. A random reactivity input was realized by using an electromechanical system to convert the white noise of a radioactive source into linear motion of a small neutron absorber. Analysis of the response was made by autocorrelating the reactivity input and cross-correlating the reactivity input and the response in neutron density, and determining their spectra. The amplitude and phase of the reactor transfer function were determined from these spectra. Results are presented on some measurements made on a small reactor at Brookhaven National Laboratory. The measured transfer function agrees with the calculated transfer function.