Subcriticality evaluation formula with the use of detector importance function for the CF-driven neutron noise measurement

A compact formula is developed which converts a measured noise quantity, namely the spectral ratio of three-detector noise outputs, to subcriticality. It is a generalization of the one-point kinetics conversion formula which Mihalczo originally used in his Cf-driven neutron noise method. The present formulation includes modal effect of measurements, and reduces the spurious detector-position dependence of the resulting reactivity. The frequency-dependent detector importance function, which H.V. Dam used in BWR noise analysis, is employed. The function gives the field of view of a detector, taking account of time-delay and spatial attenuation that takes place between the neutron noise source and its detection. The present method is more compact and adequate for systems with deep subcriticality than the modal expansion method. The measured ratios are analyzed and compared with the Monte Carlo results. (Author)