Possibility of applying the energy modal method to analysis of fast critical experiments

The possibility of analyzing the integral data obtained by fast critical experiments in the two-mode framework was investigated by numerical experiments. Nuclear model is 26-group zero-dimensional one, maintaining criticality by energy-independent geometrical buckling. Two energy modes used in this paper were chosen by observing the distribution of fundamental spectra of various assemblies on the reaction-rate space. By synthesizing these two modes, we can evaluate the eigenvalues of various dilute fast assemblies with the maximum error 0.7%. By the investigations, following conclusions were obtained. The modal combining coefficients can be estimated by measuring the spectral index, and the suitable combination of reactions for this purpose is the set va f of 238U/vσf; of 235U. The coefficients of importance spectra can be evaluated by measuring the material worth, on the basis of the first-order perturbation theory. Unknown mode-constants and adjoint-weighted mode-constants can be determined by measuring spectral indicies and material worths, respectively, in various assemblies where the modal combining coefficients were determined in advance. The errors in the spectral indicies and the material worths which are calculated by using these estimated mode-constants are less than a few percent and some 10%, respectively.