Patent Number: 052992446
Section: claims

1. A fuel assembly comprising a plurality of fuel rods containing fissile material being arranged at predetermined pitches in first and second directions and being arranged in triangle lattices,  a plurality of water rods group arranged among said fuel rods,  an upper tie plate and a lower tie plate which respectively supports each of an upper end and a lower end of the fuel rod or a water rod in the water rods group, and  fuel spacers for maintaining intervals between the fuel rods,  each of the water rods group including no fuel rod and with a plurality of water rods which are arranged adjacent to each other in triangle lattices having substantially the predetermined pitches of the fuel rods,  the outer diameter of the water rod being smaller than the predetermined pitches, and  the water rods groups being arranged not adjacent to each other and being surrounded with the fuel rods.  a fraction of the number of said water rods to a sum of the number of said fuel rods and the number of said water rods included in the fuel assembly is at least 10%.  said fuel rods comprise first fuel rods and second fuel rods, the second fuel rods having a shorter length in an axial direction than the first fuel rods.  said fuel rods comprise first fuel rods and second fuel rods, the second fuel rods having a shorter length in an axial direction than the first fuel rods.  said second fuel rods are located not adjacently to said water rods group, and are surrounded with said first fuel rods. 2. A fuel assembly as claimed in claim 1, wherein 3. A fuel assembly as claimed in claim 1, wherein 4. A fuel assembly as claimed in claim 2, wherein 5. A fuel assembly as claimed in claim 3, wherein