Patent Number: 
Section: claims

1. A reactor core of a fast neutron reactor, said reactor core comprising:a plurality of fuel rods; anda reactivity control rod disposed at a central portion of the reactor core for controlling a reactivity therein, said reactivity control rod comprising a wrapper tube surrounded by said plurality of fuel rods, and a plurality of neutron absorber rods arranged in parallel to one another in the wrapper tube, wherein at least one of the plurality of neutron absorber rods includes a cladding tube and a mixture filled in the cladding tube, wherein said mixture is composed of a first percentage of a neutron absorber that absorbs a neutron and a second percentage of a neutron moderator that moderates the neutron, wherein only one reactivity control rod is provided within the reactor core, wherein the neutron absorber is gadolinium and the neutron moderator is zirconium hydride, and wherein the second percentage is larger than the first percentage in an initial structure of the reactor core. 2. A reactor core according to claim 1, wherein each of the plurality of neutron absorber rods includes a cladding tube and the mixture filled in the cladding tube. 3. A reactor core according to claim 1, wherein no fuel rod is provided within said wrapper tube. 4. A reactor core according to claim 1, wherein the fuel rods and the reactivity control rod are arranged so as to provide a hexagonal shape. 5. A reactor core according to claim 1, wherein the wrapper tube has a hexagonal shape, and seven neutron absorber rods are provided withinthe wrapper tube. 6. A reactor core according to claim 2, wherein the wrapper tube has a hexagonal shape, and seven neutron absorber rods are provided within the wrapper tube.