Reactors

Purpose: To completely burn up burnable poisons and make the power distribution uniform in the reactor core by making a difference in the fuel charge patterns between the peripheral region and the central region in the core of BWR type reactors. Constitution: The reactor core is divided into a peripheral region comprising fuel assemblies situated from the outermost layer to the fourth layer for the reactor core and a central region comprising fuel assemblies situated from the fifth layer to the center of the reactor core, in which the charge pattern for the fuel assemblies in the peripheral region is made different from that in the central region, and a burnt fuel assemblies are charged in the third layer of the peripheral region. In the peripheral region, two unburnt fuel assemblies are charged in a unit lattice, whereby the excess increase in the excess reactivity and the neutron flux distribution in the third layer are offset to each other and the burnable poisons in the unburnt fuel assemblies charged in the peripheral region can completely be burnt up. (Moriyama, K.)