A motor-driven centrifugal pump for circulating a coolant in a primary circuit of a nuclear reactor comprises a sealed motor unit, a hydraulic part and a shaft which is immersed in the coolant, turned by the sealed motor unit and pumping the coolant by means of an impeller of the hydraulic part secured to the shaft; the motor unit comprising a dry stator and an immersed rotor mounted securely on the shaft. The motor-driven pump also comprises an immersed flywheel mounted securely on the shaft between the rotor of the motor unit and the impeller of the hydraulic part, allowing a minimum slowing-down time after the electrical power supply is cut; the flywheel comprising a vaned wheel generating a circulation of the coolant to cool the motor unit.