Patent Number: 
Section: claims

1. A core of a light water reactor having a plurality of fuel assemblies, which are loaded in said core, having nuclear fuel material containing a plurality of isotopes of transuranium nuclides, an upper blanket zone, and a fissile zone, in which said transuranium nuclides are contained, disposed below said upper blanket zone;wherein a ratio of Pu-239 in all said transuranium nuclides contained in said fuel assembly is in a range of 5% or more but less than 40% when burnup of said fuel assembly is 0;an axial length of said upper blanket zone is in a range of 20 to 100 mm; anda lower end of said fissile zone in an axial direction substantially delimits a lower end of said core in the axial direction;wherein the lower end of said fissile zone substantially delimits the lower end of said core without a lower blanket zone between the lower end of the fissile zone and the lower end of the core. 2. The core of the light water reactor according to claim 1,wherein said core has an inner blanket zone between said upper blanket zone and said lower end of said core;said fissile zone includes an upper fissile zone containing said transuranium nuclides and disposed between said upper blanket zone and said inner blanket zone, and a lower fissile zone containing said transuranium nuclides and disposed between said inner blanket zone and said lower end of said core; andan axial length of said upper fissile zone is larger than an axial length of said lower fissile zone within a range of 10 to 25 mm. 3. The core of the light water reactor according to claim 1,wherein an upper end of a neutron absorber zone of a safety rod provided in said light water reactor is positioned between the lower end of the core and a position which is 5 mm below the lower end of the core. 4. A fuel assembly having nuclear fuel material containing a plurality of isotopes of transuranium nuclides, an upper blanket zone, and a fissile zone containing said transuranium nuclides and disposed below said upper blanket zone;wherein a ratio of Pu-239 in all said transuranium nuclides contained in said nuclear fuel material is in a range of 5% or more but less than 40% when burnup of said fuel assembly is 0;an axial length of said upper blanket zone is in a range of 20 to 100 mm; anda lower end of said fissile zone in an axial direction substantially delimits a lower end of an active fuel length of said fuel assembly in the axial direction;wherein the lower end of said fissile zone substantially delimits the lower end of the active fuel length of said fuel assembly without a lower blanket zone between the lower end of the fissile zone and the lower end of the active fuel length of said fuel assembly. 5. The fuel assembly according to claim 4,wherein said fuel assembly has an inner blanket zone between said upper blanket zone and a lower end of said active fuel length of said fuel assembly;said fissile zone includes an upper fissile zone containing said transuranium nuclides and disposed between said upper blanket zone and said inner blanket zone, and a lower fissile zone containing said transuranium nuclides and disposed between said inner blanket zone and said lower end of said active fuel length; andan axial length of said upper fissile zone is larger than an axial length of said lower fissile zone within a range of 10 to 25 mm.