Abstract: A core construction for boiling water nuclear reactor in which a multiplicity of core units each having four fuel assemblies disposed in a square pattern adjacent to a control rod are arranged to form a reactor core, wherein the reactor core comprises control cells comprising four fuel assemblies which have been in the core more than three cyclic periods. The fuel assemblies located in the outer peripheral portion of the core are fuel assemblies which have been in the core more than three cyclic periods. Most of the core units comprise four fuel assemblies of first-, second-, third- or fourth-burning cyclic periods. The core is rotationally symmetric by 90 degrees. The core can be divided into four identical 1/4 cores by two vertical planes passing axial center lines of the core. Each of the 1/4 cores is substantially mirror symmetric about a plane passing the axial center line of the core.

Abstract: A fuel assembly to be charged in a reactor core comprises a channel box extending vertically in the core and having a substantially square cross section, a number of fuel rods arranged in the channel box in the form of a lattice in cross sections thereof, the fuel rods extending vertically in the channel box with spaces between each other, a plurality of spacers for supporting the fuel rods in the channel box so as to maintain axial spaces between the fuel rods and upper and lower tie plates for supporting upper and lower end portions of the fuel rods. The fuel rods include first type fuel rods arranged at and near four corner portions of the channel box, each of the first type fuel rods being composed of upper and lower areas, except for extreme upper and lower ends, in which a fuel enrichment of the upper area is made lower than that of the lower area.