PURPOSE: To prevent the invasion of a clad in a nuclear reactor to an internal pump by providing a projection at the top of a diffuser surrounding the outer periphery of an impeller, to be inserted to an inverse U-shape groove provided at the boss of the impeller.

CONSTITUTION: In a water boiler type reactor, an impeller 3 is installed to the upper end of a pump shaft 1 which penetrates a pressure container 11 through a connecting rod 2. And a diffuser 7 provided at the lower stream side of the impeller 3 is fixed to a pump nozzle through a stretch tube 8. In this case, at the upper end of a boss 4 in the diffuser 7, a conical surface 4a is formed, while at the lower side of a boss 10 in the impeller 3, an inverse U-shape groove 10b having a conical surface 10a is formed. At the top of the boss 4, a projection 6 to be inserted to the groove 10b is formed. As a result, at the entrance of the pressure container 11, a flow passage facing upward is formed, and the area of the flow passage is restricted.