Fuel assembly

The present invention provides a structure capable of reducing a pressure loss caused in a two phase flow region of water and steam while increasing the charging amount of fuels in a fuel assembly for a BWR type reactor. That is, fuel rods are constituted with four small assemblies arranged each in a triangle lattice. These small assemblies are disposed while rotated each by 90degC in a square channel box. A plurality of partial length fuel rods having such a structure that the length thereof is shorter than an ordinary fuel rod and the upper end portion thereof is cut out are disposed in adjacent with each other in the fuel assembly. Further, structural materials for promoting whirling flows are disposed in a space above a plurality of partial length fuel rods disposed in adjacent with each other. In the fuel assembly having the foregoing structure, frictional pressure loss of gas/liquid two phase flow of coolants flowing in the assembly is greatly decreased. As a result, flow stability of the reactor core is improved, pump power can be saved and the degree of freedom of operation is extended by the increase of rated flow rate. (I.S.)