Abstract: A system and method for measuring and monitoring axial flux to determine subcriticality in a spent fuel pool of a nuclear power plant. In certain embodiments of this invention, one or more neutron detectors are operable to generate signals resulting from neutron interactions in the spent fuel pool, a counting device counts the signals which are generated by the one or more neutron detectors, a connecting means electrically connects the one or more neutron detectors to the counting device, a signal analyzer is used to determine reactivity of the fuel assemblies in the spent fuel pool based on the counted signals, a power supply provides power for the neutron detectors, the counting device and the system analyzer, and a software code containing an axial flux curve index is used to correlate the counted signals to determine the subcriticality of the spent fuel pool.