Reduction of spatial dependence observed in least-squares inverse kinetics analysis

The least-squares inverse kinetics method (LSIKM) has been frequently applied to constant-reactivity insertion experiments, such as a control-rod drop experiment conducted to determine not only the reactivity but also the effective source strength of a subcritical system driven by a neutron source. When a large negative reactivity is inserted into such a loosely coupled core system consisting of two-core and one-large-core reactors, spatial higher-harmonic modes are probably significantly excited. Consequently, the excitation leads to the failure of the one-point kinetics model, and the LSIKM based on the model infers spatially dependent reactivity and source strength. In this paper, we present a reduction technique for the apparent spatial dependence and a demonstration of the applicability of such a technique to rod drop experiments carried out in both the subcritical and critical states of the two-core reactor. © Atomic Energy Society of Japan.