Fuel assembly

In a fuel assembly for a BWR type reactor, more burnable poison elements having relatively small neutron absorbing cross section such as boron are disposed in a region of relatively greater moderator-fuel volume ratio, soft neutron energy spectrum and great thermal neutron fluxes. Alternatively, more burnable poison elements having relatively large absorbing cross section such as gadolinium are disposed in other regions of an average neutron energy spectrum. A plurality of burnable absorbers are mixed in separate fuel rods. Control for the concentration is necessary for burning over the same period of time is necessary, and it is not preferred that the power at the initial stage of burning is excessively suppressed. Since a part of excess reactivity and local power peaking can be controlled by boron, the mixing amount of gadolinia can be reduced and the reactivity can be increased. Further, even when there is a limit for the uranium enrichment degree, higher burnup degree can be attained. (N.H.)