Abstract: A fuel assembly comprising a plurality of fuel rods, where fuel rods containing a burnable poison element having a smaller neutron absorption cross-section such as boron are provided in a region of soft neutron energy spectrum and a large thermal neutron flux and fuel rods containing a burnable poison element having a larger neutron absorption cross-section such as gadolinium and fuel rods containing no such burnable poison element are provided in other region of average neutron energy spectrum, has a flat fuel rod power distribution throughout the fuel lifetime even against substantially uniform uranium enrichment distribution with reduced parasitic neutron absorption by gadolia, that is, with higher burnup and reduced fuel cycle cost.