PURPOSE: To flatten the output distribution in the longitudinal direction of a fuel rod and the output distribution in the horizontal direction at the terminal stage of an operating cycle.
CONSTITUTION: In the fuel assembly provided with a water rod wherein the water level is variable, a plurality of burnable poisonous elements of different neutron absorption cross sections are used as a burnable poison of a fuel rod 2. Burnable poisonous elements of a relatively small neutron absorption cross section, e.g. boron are arranged more at an upper half region of the fuel rod than at a lower half region. Meanwhile, burnable poisonous elements of a relatively large neutron absorption cross section are set more at the lower half region than the upper half region.
AOYAMA TADAO
MASUMI RIYOUJI
ISHIBASHI YOKO
