Fuel assembly for BWR type reactor and water tube for passing cooling water

The present invention concerns a fuel assembly for BWR type reactor and relates to an improvement of a water rod for passing coolants. That is, the entire axial length of a water rod in the fuel assembly is made substantially equal with the effective fuel length. That water rod is made shorter by a length corresponding to the plenum region of the fuel rod as compared with an existent fuel rod. Accordingly, the water rod is not present in portion of the axial direction region of the assembly corresponding to the plenum portion. Therefore, the flow channel of the inchannel region in the axial region of the assembly (plenum-inchannel flow channel area) is enlarged by so much as the transverse cross sectional area of the water rod. Two phase flow at a high void ratio is caused in the inchannel region corresponding to the plenum region. Reduction of the pressure loss can be attained by the enlargement of the flow channel area. As a result, the channel stability can be improved remarkably in a BWR type reactor loaded with the fuel assemblies according to the present invention. (I.S.)