Patent Number: 048715107
Section: claims

1. A fuel assembly to be loaded in a reactor core with a pitch P from the other ones, said fuel assembly comprising: a plurality of fuel rods;  a water rod disposed within said fuel rods; and  a channel box surrounding the bundle of said fuel rods, said channel box having a width L between outer walls thereof and a width D between inner walls thereof, both of which satisfy the following equation: EQU 0.12.ltoreq.(P-L)/D.  a plurality of fuel rods;  a water rod disposed within said fuel rods; and  a channel box surrounding the bundle of said fuel rods, said channel box having a width L between outer walls thereof and a width D between inner walls thereof, both of which satisfy the following equation: EQU 0.12.ltoreq.(P-L)/D.  disloading a first fuel assembly from a reactor core of said reactor, which first fuel assembly is loaded in said reactor core with a pitch P from the other ones, and includes a plurality of first fuel rods, a first water rod disposed within said first fuel rods, and a first channel box surrounding the bundle of said first fuel rods, said first channel box having a width L1 between outer walls thereof and a width D1 between inner walls thereof, both of which satisfy the following equation EQU 0.12&gt;(P-L.sub.1)/D.sub.1 ; and  loading a second fuel assembly at the position at which said first fuel assembly disloaded has been positioned, said second fuel assembly includes a plurality of second fuel rods, a second water rod disposed within said second fuel rods, and a second channel box surrounding the bundle of said second fuel rods, said second channel box having a width L between outer walls thereof and a width D between inner walls thereof, both of which satisfy the following equation EQU 0.12.ltoreq.(P-L)/D. 2. A fuel assembly according to claim 1, wherein an average enrichment of said fuel assembly is 4 wt % or more. 3. A fuel assembly according to claim 1, wherein said water rod has a cross-sectional shape occupying a region in which five said fuel rods can be arranged. 4. A reactor core comprising a plurality of fuel assemblies which are loaded in said reactor core with a pitch P from the other ones, each of said fuel assemblies including: 5. A fuel assembly according to claim 4, wherein an average enrichment of said fuel assembly is 4 wt % or more. 6. A fuel assembly according to claim 4, wherein said water rod has a cross-sectional shape occupying a region in which five said fuel rods can be arranged. 7. A method for loading fuel to a reactor comprising the following steps of: 8. A method for loading fuel to a reactor according to claim 7, wherein said second fuel assembly having an average enrichment of 4 wt % or more is loaded in the position at which said first fuel assembly disclosed has been positioned in said reactor core. 9. A method for loading fuel to a reactor according to claim 8, wherein said second fuel assembly including said second water rod having a cross-sectional shape occupying a region in which five said second fuel rods can be arranged is loaded in the position a which said first fuel assembly disloaded has been positioned in said reactor core.