Patent Number: 052020851
Section: abstract

A fuel assembly has a plurality of first fuel rods and a plurality of second fuel rods having a shorter length in an axial direction than the first fuel rods. The second fuel rod is loaded with natural uranium in full length of its effective fuel length portion. The fuel assembly has a water rod having a larger horizontal cross sectional area at the upper region than the area at the lower region. The second fuel rods are arranged downward of the upper region of the water rod and adjacent to the lower region of the water rod. The width of the horizontal cross sectional area of the lower region of the water rod is set so as to locate the minimum values of both thermal neutron flux and resonance neutron flux in the horizontal direction of the fuel assembly at an outer side with respect the location of the second fuel rod in the horizontal direction. In accordance with the present invention, the H/U ratio in the axial direction becomes close to the optimum value, and increment of the resonance neutron absorption and flattening of the thermal neutron flux in the horizontal direction are achieved.