Nuclear reactor

Purpose: To eliminate the requirement of control-rod-pattern change during operation, as well as improve the fuel economy. Constitution: A plurality of fuel assemblies each having an equal outer size for constituting a reactor core comprise those of a plurality of fuel rods each with a smaller outer diameter and arranged by a more number in a sqaure lattice configuration, and those of fuel rods each with a larger outer diameter and arranged by a less number in a square lattice configuration. Control rods to be actuated during power operation are designated and fuel assemblies of fuel rods each with the smaller outer diameter and arranged by a more number in a square lattice configuration are charged around the designated control rods. Since nuclear fuels with the reduced linear power density are arranged around the designated control rods, requirement for the pattern exchange of the control rods during operation is eliminated the number of operation for control rods is decreased, the rod extraction can be facilitated and the operation is simplified. (Sekiya, K.)