System and methods are disclosed for controlling a nuclear reactor that uses fuel having plutonium-239. The nuclear reactor includes a neutron moderator, such as ZrH1.6, which behaves as an Einstein oscillator and increases the energy of thermal neutrons into the Pu-239 neutron absorption resonance as the temperature of the nuclear reactor increases. A neutron absorbing element with neutron absorption around 0.3 eV is added to the nuclear reactor to suppress any reactivity gain that can occur due to the increase in temperature. The amount of the neutron absorbing element to be added to the nuclear reactor is calculated such that the reactivity gain that may occur at any time during the life of the fuel is suppressed.