PURPOSE: To prevent the fluctuation in the liquid level of a coolant and to maintain the defectless operation of a nuclear reactor by providing a guide plate to attenuate the perpendicular flow rate component of the coolant flowing toward the coolant liquid surface to the inside peripheral face of a hanging body.

CONSTITUTION: The guide plate 40 having an inside circumferential cylindrical shape is provided under the liquid surface of the primary sodium 6 on the inside peripheral face of the hanging body 4. The primary sodium 6 flowing out of a reactor core part 2 changes its direction radially in the lower part of an upper mechanism 16 of the core and flows toward the inside wall of the body 4. The greater part of the sodium 6 flows out of a hanging body flow hole 8 and part of the sodium 6 rises along the inside peripheral wall of the body 4. The ascending flow of the sodium 6 passes the cylindrical guide plate 40 provided on the inside peripheral face of the body 4 and the annulus part of the body 4 and the greater part thereof flows from the flow hole 8 to the outside of the hanging body. The flow backward toward the inside of the body 4 by passing an orifice 41 is also possible. The sodium is attenuated in the perpendicular flow rate component after the passage through the orifice 41 so that the sodium 6 flows by changing its direction toward the lateral side. The oscillation in the free liquid surface of the sodium 6 is, therefore, prevented.