<p>A fuel assembly (A1; A2; A3) is composed of first fuel rods (15B) containing nuclear fuel material and burnable poison material and second fuel rods (15A) containing nuclear fuel material but no burnable poison material. The fuel assembly (A1; A2; A3) is charged in a high conversion area (59) of a boiling water type nuclear reactor (50) having a reactor core (58) having the high conversion area (59) and a burner area (60) surrounding the high conversion area (59). A ratio VC/VF is not greater than 1.5 in the fuel assembly (A1; A2; A3) where VC is the volume occupied by coolant flow paths and VF is the volume occupied by the nuclear fuel material per unit length in an axial direction of the fuel assembly (A1; A2; A3). In addition, the burnable poison material is composed of neutron absorption nuclides having at least one resonant energy in a neutron energy region of one electron volt or less.</p>
