Fuel assembly for BWR type reactor

In a fuel assembly for a BWR type reactor comprising long fuel rods and short fuel rods having a short fuel effective fuel portion, short fuel rods of fuel rod group situated at a second layer from the outermost circumference which contain burnable poisons are disposed at corner positions. In addition, the fuel rod is axially divided into upper and lower two regions by defining the region above the upper end of the effective fuel portion of the short fuel rod as an upper portion, and defining the region corresponding to the effective fuel portion as a lower portion to provide long fuel rods containing more burnable poisons in the upper portion than in the lower portion. Then, local power at the peripheries of the corners of the fuel assembly in adjacent with the short fuel rods can be reduced by the thermal neutron absorbing effect of the burnable poisons, and the enrichment degree of the fuel rods at the peripheries of the corners can be increased by so much. If the number of the burnable poisons are made different between the upper and the lower portions appropriately, the axial difference of the reactivity between upper and lower portions can be controlled at the initial stage of the burning thereby enabling to suppress power peak at the lower region in the former half of the operation cycle. (N.H.)