Criticality monitor for neutron multiplying system

For example, a nuclear fuel dissolving step of a nuclear fuel reprocessing facility comprises a neutron multiplying system containing fissionable materials. The monitor of the present invention monitors for preventing the neutron multiplying system reaching criticality. That is, two or more kinds of gamma ray detectors having different gamma energy detection performances are disposed on the side of the neutron multiplying system containing fissionable nuclides and neutron releasing nuclides therein. The ratio of signals sent from each of the detectors is compared in a calculation circuit. Then, when the ratio exceeds a predetermined level, an alarm or an alarm signal is generated for preventing the neutron multiplying system from reacting criticality. Gamma rays generated by nuclear fission or capturing reaction of fissionable materials has a higher energy compared with gamma rays due to decay of radioactive isotopes (RI). The monitor of the present invention utilizes the difference to monitor criticality by using two or more kinds of detectors having different performances. (I.S.)