PURPOSE: To simply and rapidly measure the degree of subcriticality of a system containing nuclear fuel. CONSTITUTION: A neutron source 2 (having predetermined relation of neutron and gamma ray generating ratio) is placed near the center in a fuel tank 1 containing solution-like nuclear fuel, and a gamma ray detector 3 is installed at a suitable distance from the tank 1. The output of the detector 3 is processed by a measurement signal processor 4 including a pulse height analyzer. The gamma ray intensity is measured in terms of the case where the fuel concentration is zero and the case where the fuel exists. In order to avoid the large dependence of the attenuation characteristics of the gamma ray upon the fuel conditions in the system, the measured value of the high energy of several MeV or more is employed as the measured value of the intensity. The subcriticality is obtained from S/(P+S) of the ratio of the measured value S of the former = (proportional to the number of neutrons supplied from the source to the system) to the measured value (P+S) of the latter = (proportional to the sum of the number of the neutrons generated by the nuclear fission in the system and the number of the neutrons supplied from the source to the system).
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