Estimation method for reactor core performance

A nuclear reactor core is divided into a plurality of small regions. Neutron fluxes at the inside of each of the small regions are determined by solving an equations expressing the transporting phenomenon of neutrons at the inside of each of the small regions under given boundary conditions at the outer boundary for each of the small regions. Neutron fluxes in each of the small regions in a case of actually present in the reactor core are approximated by using the neutron fluxes and a space coordinate function containing a plurality of parameters. The parameters are determined by using a variational method so as to reduce the error in the approximation. Thus, since the variational method is used so as to reduce the approximation error, a theoretical ground can be made distinct to greatly economize computer resources. Further, the accuracy for the resolution can be improved significantly. (N.H.)