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A study on the nuclear characteristics of an axially decoupled core was performed through a series of experiments using the solid moderator core in the Kyoto University Critical Assembly (KUCA). In the axially decoupled core which has an internal blanket between the upper and lower cores, the measured differential reactivity worth of a control rod moving axially was apparently asymmetric between the two core regions. Since it was considered that this phenomenon was attributed to the neutron flux tilt caused by the axial movement of control rods during the measurement of differential worth, the axial reaction rate distributions were measured by gold wires with and without a cadmium tube. It was found that the reaction rate distribution was remarkably tilted by changing the pattern of control rods inserted in the core to maintain the critical state, and that the flux distribution was tilted without energy dependency. The value of first-order eigenvalue separation was obtained using the relation between the neutron flux tilt and the first-mode eigenvalue separation based on the Explicit Higher-order Perturbation (EHP) method. This value agreed with the calculated result by the SRAC system.

The general expression of noise coherence function including the energy dependence of neutron detection cross section and the higher harmonic contributions in multi-dimensional model is developed on the basis of the modal expansion technique. Applicability of the method is demonstrated by numerical calculations carried out for a two-dimensional model of large fast reactor assemblies ZPPR-9 and −13C. The agreement between the theory and the measurement is satisfactory, which indicates the validity of the theory and the calculational model employed. In the assemblies, the coherence for a detector pair in a specific location can be approximately described by including the fundamental and the first harmonic modes.