Significant spatial dependence observed in inverse kinetics analysis for a loosely coupled-core system of the Kyoto University Critical Assembly

A loosely coupled-core system was constructed in the Kyoto University Critical Assembly to study the spatial effect observed in inverse kinetics analysis of control rod reactivity worth. In a rod drop experiment, the conventional inverse kinetic method resulted in a space- and time-dependent rod worth, which depended significantly on detector position and varied remarkably with the elapse of time. In another rod worth measurement, where a control rod was continuously inserted, the similar spatial dependence could be also observed. In this study, a modal expansion approach was proposed to reduce the above spatial dependence of the measured rod worth. Applying the present approach to inverse kinetics analysis, the troublesome dependence could be solved to obtain space-independent rod worth. This approach requires only the eigenfunctions of fundamental and higher modes for an unperturbed system but makes both static and transient calculations for various perturbed systems unnecessary.