Patent Number: 
Section: claims

1. A fuel assembly comprising:a fuel rod array of a plurality of fuel rods including a plurality of first fuel rods including enriched uranium and not including a burnable poison, and a plurality of second fuel rods including said enriched uranium and said burnable poison; andat least one water rod;wherein said fuel rod array has a plurality of corner sections, each corner section being a region having three fuel rods in an outermost layer of said fuel rod array including one fuel rod at a corner of said outermost layer of said corner section and two fuel rods in said outermost layer directly adjacent said one fuel rod at said corner;wherein one of said second fuel rods is said one fuel rod at said corner of a first corner section of said plurality of corner sections;wherein another of said second fuel rods is at least one of said two fuel rods directly adjacent to said one of said second fuel rods at said corner of said first corner section;wherein others of said second fuel rods are placed adjacent to said at least one water rod;wherein a second corner section of said plurality of corner sections includes a fuel rod other than one of said second fuel rods at a corner in said outermost layer that faces a control rod when said fuel assembly is loaded in a core; andwherein the number of said second fuel rods placed in said second corner section is less than the number of said second fuel rods placed in said first corner section. 2. The fuel assembly according to claim 1, wherein said at least one water rod is a large-diameter water rod having a lateral cross section that occupies an area where at least two fuel rods are placeable. 3. The fuel assembly according to claim 1, wherein, in said second corner section, said second fuel rods are placed in a layer adjacent to said outermost layer of said fuel rod array. 4. The fuel assembly according to claim 1, wherein said plurality of corner sections includes two first corner sections. 5. The fuel assembly according to claim 1, wherein said fuel rod array has a substantially rectangular configuration in cross section and having four corner sections. 6. The fuel assembly according to claim 1, wherein when an active fuel length of said fuel assembly is denoted by L and a distance from a lower end of said active fuel length L in an axial direction of said fuel assembly is denoted by h, burnable poison regions including said burnable poison in said second fuel rods are located in a region defined by L/8≦h≦L/2. 7. The fuel assembly according to claim 1, wherein when an active fuel length of said fuel assembly is denoted by L and a distance from a lower end of said active fuel length L in an axial direction of said fuel assembly is denoted by h, and further when said active fuel length is divided into an upper region and a lower region at a position defined by h=L/2, said adjacent second fuel rods in said outermost layer include said burnable poison in most of said lower region and do not include said burnable poison in most of said upper region. 8. The fuel assembly according to claim 1, wherein an average enrichment in a lateral cross section of said fuel assembly is within a range from 3.7 wt % to 10.0 wt %.