Abstract: A method of determining an amount of fissile material in a reactor includes sensing neutrons or photons emitted from the reactor before and after a change in a reactor operating parameter and determining a mass of fissile material responsive to a difference between the intensities of the radiation. A system for determining an amount of fissile material includes a radiation detector and a computing system. The radiation detector may be configured to detect neutrons or photons. The computing system may be configured for calculating mass of the fissile material based at least in part on a change in the power output from the reactor as a function of time. Some methods include collecting radiation emitted by a reactor, generating an electrical signal responsive to the collected radiation, and evaluating mass of the fissile material responsive to the magnitude and response of the electrical signal after a change in the reactor.