PURPOSE: To reduce floating-up force by providing a pressure ring at an outer circumstance of lower shielding body.

CONSTITUTION: A high pressure and low temperature sodium discharged from a primary pump 8, flows into a high pressure prenum 19, flows out of an entrance nozzle 16, climbs up through an inside of a lower shielding body 15 and reaches a lower end part of fuel assembly element 12. Since there is no wrapper tube at upper flow paths than this region, free flow appears in a reactor core and the sodium tends to flow into gaps between adjoining fuel assemblies. However, the flowing-in flow is blocked by existence of a pressure ring 22. With this reason, all the flowing body climbs up through a region of the fuel assembly element. Since there is no flow into the gap between the adjoining fuel assemblies, the pressure P1 of an empty space part does not rise up. Even if there appears a small leakage, low pressure condition can be well kept because the pressure is released from a connection pipe of the space part to a low pressure prenum side 20. Accordingly, a condition that the pressure to a projection area of the lower shielding body is rather low, can be kept well, and therewith floating-up force of the fuel assembly does not appear.