The invention relates to a motor-driven centrifugal pump for circulation of heat transfer fluid in the primary circuit of a nuclear reactor. The motor-driven pump comprises a hermetic motor unit (12), a hydraulic section (13) and a shaft (17) immersed in the heat transfer fluid, rotated by the hermetic motor unit (12) and pumping the heat transfer fluid by means of an impeller (22) of the hydraulic section (13) rigidly connected to the shaft (17). The motor unit (12) comprises a dry stator (18) and an immersed rotor (19), rigidly connected to the shaft (17). The motor-driven pump also comprises an immersed flywheel (23) rigidly connected to the shaft (17) between the rotor (19) of the motor unit (12) and the impeller (22) of the hydraulic section (13), enabling a minimum slowing down time after cutting of the electrical power supply; and an immersed device for preventing rotation in the wrong direction, configured so as to mechanically block rotation of the shaft (17) in a predetermined direction.