Active neutron measuring method and device

The present invention provides a method and a device capable of correcting an influence of neutron absorbing materials in wastes and accurately measuring the mass of fission products upon nondestructive determination of fission materials in wastes generated from fuel manufacturing plants and reprocessing plants. Namely, the active neutron measuring device of the present invention comprises a collimator for selecting thermal neutrons transmitted from objective materials of the test and a neutron detector in the collimator for detecting thermal neutrons selected by the collimator in an irradiation chamber. The amount of the mass of neutron absorbing materials in the objective materials for the test to be corrected is determined based on the measured values of a thermal neutron detector in the irradiation chamber and the measured values of the neutron detector in the collimator. The mass of fission products in the material to be detected can be corrected based on the correction amount. As a result, the accuracy for the analysis of fission products can be improved. (I.S.)