Patent Number: 
Section: claims

1. A method of managing excess reactivity in a nuclear fission reactor, the method comprising:achieving criticality with a quantity of excess reactivity in a central core region of a reactor core of a nuclear fission reactor;increasing the quantity of excess reactivity until a predetermined burnup level is achieved in selected ones of fuel assemblies in the central core region of the reactor core; andcompensating for the increase in excess reactivity, wherein compensating includes inserting neutron absorbing material into a central core region,wherein inserting neutron absorbing material in a central core region includes inserting a plurality of breeder fuel assemblies in the reactor core and radially spaced peripherally from the central core region of the reactor core; the breeder fuel assemblies containing fertile fuel and at least one of the plurality of breeder fuel assemblies containing substantially no fissile fuel before inserting,wherein compensating for the increase in excess reactivity includes breeding up the plurality of breeder fuel assemblies to contain fertile fuel,wherein compensating for the increase in excess reactivity includes replacing the selected ones of fuel assemblies in the central region of the reactor core having achieved the predetermined burnup level with a selected plurality of fertile fuel assemblies of the bred up plurality of breeder fuel assemblies, the selected ones of fuel assemblies being radially spaced inwardly from the selected plurality of fertile fuel assemblies of the bred up plurality of breeder fuel assemblies. 2. The method of claim 1, wherein increasing the quantity of excess reactivity until a predetermined burnup level is achieved in selected ones of fuel assemblies in the reactor core includes monotonically increasing the quantity of excess reactivity until a predetermined burnup level is achieved in selected ones of fuel assemblies in the reactor core. 3. The method of claim 1, wherein increasing the quantity of excess reactivity until a predetermined burnup level is achieved in selected ones of fuel assemblies in the reactor core includes increasing amount of fissile material in ones of the fuel assemblies of the reactor core until a predetermined burnup level is achieved in selected ones of fuel assemblies in the reactor core. 4. The method of claim 3, wherein increasing amount of fissile material in ones of the fuel assemblies of the reactor core until a predetermined burnup level is achieved in selected ones of fuel assemblies in the reactor core includes breeding fissile fuel material from fertile fuel material. 5. The method of claim 1, wherein inserting neutron absorbing material into the central core region includes inserting control rods into the central core region. 6. The method of claim 1, wherein inserting neutron absorbing material into the central core region includes replacing selected fissile fuel assemblies in the central core region with fertile fuel assemblies from a peripheral region of the reactor core.