Fuel assembly

A water rod comprises a coolant uprising channel, a coolant downcoming channel and a coolant flow channel introducing coolants to a discharge port of the water rod and discharging them so as to join with the flow channel from the coolant downcoming channel. With such a constitution, the difference pressure P between the exit and the entrance of the water rod is determined by the flow rate Q of the flow channel in which the coolants flow in a single liquid phase and the shape of the flow channel. The flow rate Q is determined by the pressure loss in the fuel assembly. Accordingly, the water level in the water rod uprising portion is made uniform in the reactor core. If the coolants flow rate passing through the reactor core is increased, the pressure loss in the fuel assembly is increased and Q is also increased and, accordingly, the water level of the water rod uprising portion is elevated, to reduce the volume of the steams. In other words, void amount in the water rod can be controlled by adjusting the flow rate of coolants passing through the reactor core. (I.S.)