Method for monitoring sub-critical reactor reactivity

The invention relates to a method for monitoring sub-critical reactor reactivity. The method includes the steps: placing neutron detectors at two different positions of a reactor; effective multiplication constant gradually changing the reactor reactivity, and recording a specific value C and an effective multiplication constant keff of neutron counting rate at the same moment; fitting calibratingcurve graphs of the specific values C and the effective multiplication constants keff of the neutron counting rate of various moments; recording neutron counting rates of the two neutron detectors inreal time to obtain specific values of the two neutron counting rates, and substituting the specific values into the calibrating curve graphs to obtain effective multiplication constants of the effective multiplication constant when reactor reactivity is practically monitored. The neutron detectors are used for acquiring neutron counting rates, and the specific values C of the neutron counting rates recorded by two detect