Cross-Spectral Density Measurements in a Coupled-Core Reactor

A method is developed to determine the ratio of the reactivity coupling coefficient to the mean generation time in a two-slab reactor based on experimental measurements of the inherent reactor-noise spectrum. A matrix formulation of the cross-spectral density function of the fluctuating neutron density at two experimental access locations adjacent to the cores is used in conjunction with a two-point reactor model to show that the real part of the cross-spectral density vanishes at a particular frequency, termed the sink frequency. The sink frequency is a function of the ratio of the reactivity coupling coefficient to generation time in the cores and the times required for neutron disturbances to travel between the cores and the detector locations. Experimental results from the UTR-10 reactor verify the predicted behavior of the cross-spectral density function in the neighborhood of the sink frequency and provide an at-critical measurement of the reactivity coupling coefficient.