Patent Number: 
Section: claims

1. A method comprising:determining a neutron production coefficient of a controllably movable rod in a nuclear fission reactor, the controllably movable rod including fertile nuclear fission fuel material and neutron absorbing material;determining a neutron absorption coefficient of the controllably movable rod in the nuclear fission reactor;comparing a first combination of the determined neutron production coefficient and the determined neutron absorption coefficient with a target;based on the comparing of the first combination, determining a first application of the controllably movable rod to be a nuclear fission fuel rod when the combination is at least the target;again determining the neutron production coefficient of the controllably movable rod in the nuclear fission reactor after determining the first application of the controllably movable rod;again determining the neutron absorption coefficient of the controllably movable rod in the nuclear fission reactor after determining the first application of the controllably movable rod;comparing a second combination of the again-determined neutron production coefficient and the again-determined neutron absorption coefficient with the target; andbased on the comparing of the second combination, determining a second application of the controllably movable rod to be a reactivity control rod when the combination is less than the target. 2. The method of claim 1, wherein determining the neutron production coefficient, determining the neutron absorption coefficient, again determining the neutron production coefficient, and again determining the neutron absorption coefficient of the controllably movable rod are based on neutron exposure history of the controllably movable rod. 3. The method of claim 1, before comparing the first combination and before comparing the second combination, the method further comprising:yet again determining the neutron production coefficient of the controllably movable rod in the nuclear fission reactor; andyet again determining the neutron absorption coefficient of the controllably movable rod in the nuclear fission reactor. 4. The method of claim 3, further comprising:comparing a third combination of the yet-again-determined neutron production coefficient and the yet-again-determined neutron absorption coefficient with the target; andbased on the comparing of the third combination, determining a third application of the controllably movable rod to be a nuclear fission fuel rod when the combination is at least the target, and determining the third application to be a reactivity control rod when the combination is less than the target. 5. The method of claim 1, wherein determining the neutron production coefficient, determining the neutron absorption coefficient, again determining the neutron production coefficient, and again determining the neutron absorption coefficient of the controllably movable rod are based on a property of fertile nuclear fission fuel material of the controllably movable rod. 6. The method of claim 1, wherein determining the neutron production coefficient, determining the neutron absorption coefficient, again determining the neutron production coefficient, and again determining the neutron absorption coefficient of the controllably movable rod are based on a property of fissile nuclear fission fuel material of the controllably movable rod. 7. The method of claim 1, wherein determining the neutron production coefficient, determining the neutron absorption coefficient, again determining the neutron production coefficient, and again determining the neutron absorption coefficient of the controllably movable rod are based on a property of neutron absorbing poison of the controllably movable rod. 8. The method of claim 1, wherein determining the neutron production coefficient, determining the neutron absorption coefficient, again determining the neutron production coefficient, and again determining the neutron absorption coefficient of the controllably movable rod are based on a property of fission products of the controllably movable rod. 9. The method of claim 1, wherein determining the neutron production coefficient, determining the neutron absorption coefficient, again determining the neutron production coefficient, and again determining the neutron absorption coefficient of the controllably movable rod include monitoring at least one reactivity parameter of the controllably movable rod in the nuclear fission reactor. 10. The method of claim 1, wherein determining the neutron production coefficient, determining the neutron absorption coefficient, again determining the neutron production coefficient, and again determining the neutron absorption coefficient of the controllably movable rod includes predicting at least one of the neutron production coefficient, the neutron absorption coefficient, the again-determined neutron production coefficient, and the again-determined neutron absorption coefficient of the controllably movable rod in the nuclear fission reactor. 11. The method of claim 10, wherein predicting at least one of the neutron production coefficient, the neutron absorption coefficient, the again-determined neutron production coefficient, and the again-determined neutron absorption coefficient includes calculating at least one of the neutron production coefficient, the neutron absorption coefficient, the again-determined neutron production coefficient, and the again-determined neutron absorption coefficient of the controllably movable rod in the nuclear fission reactor.