Method of monitoring subcriticality

Purpose: To ensure critical safety by monitoring the subcriticality of a neutron multiplication system for spent nuclear fuel materials, etc. in a simple device. Method: Neutron detectors and γ-ray detectors are disposed surrounding a neutron multiplication system in which fissionable nuclides and spontaneous neutron-releasing nuclides are present together. Subcriticality is monitored by calculating the neutron multiplication state of the neutron multiplication system from the ratio between the neutron counting ratio and the γ-ray counting ratio. For instance, each of a plurality pair of neutron detectors and γ-ray detectors are disposed at the identical height position of the dissolving vessel of a reprocessing facility. The ratio between the neutron counting rate and the γ-ray counting rate counted by paired detectors disposed at an identical height is determined. The neutron multiplication constant is calculated by substituting the ratio into a predetermined equation, to recognize the neutron multiplication state, thereby monitoring the subcriticality. (Kawakami, Y.)