Patent Number: 
Section: claims

1. A fuel assembly comprising a plurality of first fuel rods and a plurality of second fuel rods having a length shorter than a length of the plurality of first fuel rods, said first and said second fuel rods being arranged in a fuel rod array of 10 rows by 10 columns; and a plurality of water rods occupying regions allowing arrangement of 8 fuel rods, said second fuel rods being not arranged in an outermost tier of the fuel rod array, which satisfies the following conditions: B xe2x89xa760,  15 xe2x89xa6n xe2x89xa620( n : integer),  Awr/Ach xe2x89xa60.149,  Lp/Lf xe2x89xa711/24,  Awr/Ach xe2x89xa7(3.00xc3x9710 xe2x88x924 xc3x97n 2 +6.00xc3x9710 xe2x88x924 xc3x97n xe2x88x921.2xc3x9710 xe2x88x922 )xc3x97( Lp/Lf xe2x88x921)+1.75xc3x9710 xe2x88x921 , and  Awr/Ach xe2x89xa6(8.63xc3x9710 xe2x88x924 xc3x97n 2 xe2x88x926.09xc3x9710 xe2x88x922 xc3x97n +1.33xc3x9710 1 )xc3x97( Lp/Lf xe2x88x928.32xc3x9710 xe2x88x921 )  where Awr is a total sum of horizontal sectional areas of said water rods, Ach is a horizontal sectional area of a coolant flow passage in a bottom portion of said fuel assembly, Lf is an effective fuel length of said first fuel rods, n is the number of said second fuel rods, Lp is an effective fuel length of said second fuel rods, and B (GWd/t) is an average unloading burn-up. 2. A fuel assembly according to  claim 1 , wherein some of said second fuel rods are arranged adjacent to said water rods and some of said second fuel rods are arranged in a second tier from the outer side in the fuel rod array. claim 1 3. A fuel assembly according to  claim 1 , wherein each of said water rods has a horizontal sectional area arranged in a region allowing arrangement of four fuel rods, and the number of said water rods is two. claim 1