AbstractAnalytical expressions describing the measured auto- and cross-spectral densities of the zero-power noise at the Measuring and Research Reactor Braunschweig (FMRB) were derived from the two-point reactor kinetics equations. By means of this theory, the following properties of the two fission zones of the assembly were obtained from measurements: (a) the shutdown reactivities of the isolated cores, (b) the coupling reactivity, and (c) the power in the fission zones.The efficiencies of the detectors, needed to evaluate the properties mentioned, were obtained from these measurements also. Furthermore, the influence of the delayed neutrons-which were neglected when estimating the properties of the FMRB–on the coherence function is shown. This function was used to detect coupling effects in extended cores.