BWR type reactor

Purpose: To increase the operation period and improve the operation efficiency of BWR type reactors. Constitution: Control rods are divided into two types of control rod groups having neutron absorption performances different from each other, and at least the axial distribution of neutron absorption of reactor shutdown control rods is adjusted previously corresponding to the neutron irradiation dose applied to the control rods. This enables to sufficiently maintain the shutdown margin upon reactor shutdown and it is also possible to make the axial distribution uniform, by which the reactor shutdown performance and the operation safety can be improved. In addition, since neutron absorbers of excellent neutron absorbing characteristics and long life are charged to control rods of certain ratio, the period of using the control rods is extended and the frequency of exchanging the control rods can be reduced, to enable continuous operation for a long period of time thereby improving the reactor operation efficiency. Further, it is possible to decrease the operators' radiation exposure dose upon fuel exchanging work, as well as reduce the amount of radio active waste formed. (Kamimura, M.)