PURPOSE: To improve cooling efficiency of a fuel rod and heighten the boiling transition output of nuclear fuel by providing a liquid membrane separating plate in the upstream side of a spacer at which boiling transition is produced and pressing it in the inside of a channel box.

CONSTITUTION: A spacer 1 maintains a fuel rod position of a fuel assembly and mutual intervals of fuel rods for a boiling water reactor in which the fuel rods 2 are arranged in approximately square lattice shape. Liquid membrane separating plates 27 are provided in the upstream side of the spacer 1 and pressed and brought into contact on the inner circumferential face 23 of a channel box arranged in the outside of the spacer 1. Since the liquid membrane flowing in the inner circumferential face 23 of the channel box is taken off and dispersed by the liquid membrane separating plate 27, the fuel rods 6 can effectively be cooled. Especially, since the position of the boiling transition produced is that of a few millimeters in the upstream side of the spacer 1 and the liquid droplets obtained by the liquid membrane separating plates 27 reach the fuel rods, the cooling effect of the fuel rods is increased and the boiling transition output of the fuel is enlarged.