Patent Number: 048329061
Section: claims

1. A fuel assembly comprising: a channel box axially elongated;  upper and lower tie plates fixed to upper and lower end portions of said channel box, respectively;  first fuel rods each inserted in said channel box and retained by said upper and lower tie plates, each of said first fuel rods including a closed cladding and fuel pellets, enclosed in said closed cladding, containing fissile material and being free of burnable poison; and  second fuel rods each inserted in said channel box and retained by said upper and lower tie plates, each of said second fuel rods including a closed cladding and fuel pellets, enclosed in said closed cladding and containing fissile material and burnable poison, wherein the outer diameters of said fuel pellets of said second fuel rods are larger than that of said fuel pellets of said first fuel rods.  a channel box axially elongated;  upper and lower tie plates fixed to upper and lower end portions of said channel box, respectively;  first fuel rods each inserted in said channel box and retained by said upper and lower tie plates, each of said first fuel rods including a closed cladding and fuel pellets, enclosed in said closed cladding, containing fissile material and being free of burnable poison; and  second fuel rods each inserted in said channel box and retained by said upper and lower tie plates, each of said second fuel rods including a closed cladding and fuel pellets, enclosed in said closed cladding and containing fissile material and burnable poison, wherein the outer diameters of said fuel pellets of said second fuel rods are larger than that of said fuel pellets of said first fuel rods, wherein the outer diameter of each of said second fuel rods is larger than that of each of said first fuel rods. 2. The fuel assembly as defined in claim 1, wherein each of said fuel pellets of said second fuel rods containing gadolinia as said burnable poison, and has a hole at the central portion thereof. 3. A fuel assembly comprising a channel box axially elongated; upper and lower tie plates fixed to upper and lower end portions of said channel box, respectively; first fuel rods each inserted in said channel box, retained by said upper and lower tie plates and comprising a closed cladding and first fuel pellets disposed in said closed cladding, each of said first fuel pellets containing fissile material but not containing gadolinia and second gadolinia-containing fuel rods each inserted in said channel box, retained by said upper and lower tie plates and comprising a closed cladding and second gadolinia-containing fuel pellets in said closed cladding, each of said second gadolinia-containing fuel pellets containing fissile material and gadolinia, wherein the outer diameter of said each second gadolinia-containing fuel rod and the outer diameter of each second gadolinia-containing fuel pellet are larger than that of said first fuel rod and that of said each first fuel pellet, respectively. 4. The fuel assembly as defined in claim 1, wherein said gadolinia-containing fuel pellets each are hollow and have a central hole. 5. The fuel assembly as defined in claim 1, wherein said gadolinia-containing fuel pellets each contained in said cladding and having a density of fissile material which is greater in the outer preipheral portion than its central portion. 6. The fuel assembly as defined in claim 3, wherein said rod gadolinia-containing fuel is a solid moderating rod containing solid material of slowing-down for neutron and gadolinia in said cladding. 7. A fuel assembly comprising: 8. The fuel assembly as defined in claim 7, wherein the inner surface of each of said second fuel rods is lined with one of Zr and Cu.