A nuclear reactor coolant pump (RCP) comprises a stator and a rotating assembly including a rotor, an impeller, and a flywheel configured to rotate about an axis of rotation in response to the stator being electrically energized. The flywheel comprises a first material (such as stainless steel) and has a plurality of mutually parallel tubular openings filled with a second material that is denser than the first material (such as tungsten or tungsten alloy). In some embodiments the mutually parallel tubular openings are cylindrical openings, and are filled with the second material comprising cylindrical rods. In a nuclear reactor including a reactor pressure vessel and a nuclear reactor core comprising fissile .sup.235U disposed in the reactor pressure vessel, the RCP is suitably disposed on or in the reactor pressure vessel with the impeller of the RCP arranged to engage coolant water disposed in the reactor pressure vessel.
 This application was conceived in the course of work supported by the Department of Energy Cooperative Agreement No. DE-NE0000583. The Department of Energy may have certain rights in this application.