Neutron source multiplication method (NSMM) is one of the well-known methods for subcriticality measurement, and the advantage of this method is the simplicity of measurement using an external neutron source and neutron detectors. To take into account of spatial dependence of neutron flux distribution in subcritical state, new NSMM with considering higher mode fluxes for subcriticality measurement is developed, where the neutron flux distribution in subcritical state is expanded in a series of higher mode fluxes of critical state based on assumption of uniform or local perturbation. To confirm the applicability of this method, subcriticality measurement experiments were carried out at Kyoto University Critical Assembly (KUCA). From the results, it was found that the results of the present method including higher mode fluxes showed that the discrepancy between different detectors became much smaller even in large subcritical state compared with ordinary experimental methods based on the one-point reactor theory.
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