PURPOSE: To obtain a boiling water type nuclear rotor which utilizes an internal space of a nuclear reactor pressure vessel very effectively as well as increases extent of inertia and pump efficiency of an internal pump, and enables keeping the pump performance at fully available level.

CONSTITUTION: A boiling water type nuclear rotor is equipped with a nuclear rotor pressure vessel 32 in which a reactor core 40 is provided, and internal pumps 75 provided with pump impellers 76 which are arranged in the nuclear reactor pressure vessel 32. A reactor core shroud 66 which extends downwardly penetrating through a center part of the rotor core 40, is installed, and the pump impellers 76 of the internal pumps 75 are provided at a lower part of the reactor core shroud 66. On the other hand, an opening part 67 reaching to the reactor core shroud 66 is opened at a center part of a shroud head 55, covering an upper part of the reactor core 40, and therewith reactor core outlet flow from the rotor core and downward flow in the rotor core shroud are well separated.