Systems and methods for controlling reactivity in a nuclear fission reactor

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;
• and
• based 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.