Abstract: A nuclear internal pump is disclosed in which a flywheel and a thrust disc acting as an auxiliary impeller are fixed on a motor shaft. Fixed orifices X and Y are formed respectively between outer circumference section at larger and smaller diameter sides of the thrust disc and larger and smaller diameter rings disposed to face the outer circumference section respectively, thereby to form a pressurized chamber between the two fixed orifices X and Y. The thrust disc is equipped with an axial thrust reducing device which has two sets of channels for circulating motor cooling water and for boosting the pressure in the pressurized chamber.