Fuel assembly having emergency cooling mechanism

In a fuel assembly, an emergency metal pipe water channel having an opening which reaches the upper surface of a lattice member is disposed in parallel with fuel rods. The water channel has also a function of a water rod which keeps water stream flowing along the fuel rod during usual operation. Further, a large-diameter water rod having the cross sectional area increased at a portion below the lattice member is disposed. Upon emergency, fuel rods are exposed to cause uncontrollable thermal running and if cooling water injected to a reactor core stagnates on an upper plate due to steam pressure, coolants are flown down from the opening of the emergency metal pipe water channel having less blowing up, to cool the inner surface of the metal pipe water channel. As a result, since a state is that a cold metal pipe is inserted between overheated fuel rods, steams are rapidly condensated at the surface of the metal pipe to decrease the steam pressure in the fuel assembly, and blowing up to the opening of the upper tie plate is also decreased to promote the dropping of stagnating cooling water. Re-flooding of the core is also accelerated to decrease the temperature elevation of the fuel rods. (N.H.)