Fuel assembly and water rod

In a BWR type reactor, a water rod is disposed at the central portion of a fuel assembly in which a water-to-fuel ratio is reduced relatively for flattening the horizontal distribution of thermal neutron fluxes in the fuel assembly. In a general water rod, a small through hole is perforated each at the upper end and the lower end, so that a portion of coolants flowing into the fuel assembly flows in the water rod to be served for the removal of heat from the fuel rod. However, since the effect of the spacer on the cooling of the fuel rod, in particular, on the boiling transient is not considered with respect to the position for the small holes, there are disadvantageous effects such as effective cooling can not be attained and the pressure loss is increased. In view of the above, a small hole is disposed at a position of the water rod facing to the spacer, so that a portion of the coolants flowing in the pipe of the water rod is effectively scattered as liquid droplets to supply the coolant to the peripheral fuel rods. As a result, the boiling transient for the fuel assembly is suppressed to improve the limit power. (N.H.)