Control rods and its driving method

Purpose: To prevent fuel failures due to fuel-cladding interaction and improve working efficiency of a reactor by the use of control rods containing those portions of greater neutron absorption performance and smaller neutron absorption performance alternately at an equal pitch in a fuel cladding tube in FBR type reactor. Constitution: Boron carbide powder layers and other absorption material layers with smaller neutron absorption performance are axially disposed alternately along the entire length of a cladding tube to form a control rod. Upon starting the reactor, reactor operation is conducted using such control rods and other control rods of greater neutron absorption performance. When the rated power is attained, only the foregoing control rods are inserted and extracted. Since this enables to vary the position of the smaller and greater neutron absorption performance portions, the power distribution is unified both in the axial and the radial directions in the fuel assembly to significantly eliminate the ununiformity in the fuel. Further, since the not uniform burning is also prevented for the insertion of control rods even for a long period, there is no need for changing the control rod pattern to thereby improve the working efficiency of the reactor. (Seki, T.)