Control rods

Purpose: To obtain a control rod using neutron absorption rods comprising solid metal having neutron absorbing performance such as hafnium, which is decreased with the vertical rigidity to the axis of the rod and reduced with the weight. Constitution: Neutron absorption rods prepared by winding hafnium solid metal wires spirally are contained in an elongate sheath of a V-shaped cross section and each sheath is connected to a cross-like tie rod to form a blade. Since the spiral neutron absorption rod has a decreased rigidity in the direction perpendicular to the axis, the rigidity of the control rod comprising a plurality of such absorption rods is also decreased in the direction perpendicular to the axis. Accordingly, if the control rod is in contact with a fuel assembly upon insertion of the rod, the control rod can be deformed to escape, whereby the rod does not uprise the fuel assembly to eliminate the need of reducing the emergency insertion speed of the control rod upon reactor shutdown. (Horiuchi, T.)