Method of measuring the multiplication factors of spent fuels

Purpose: To measure the neutron multiplication factors of spent fuel assemblies, which undergo neutron irradiation in a reactor and whose composition becomes uncertain, with a good workability and a high accuracy. Constitution: Neutron fluxes of a spent fuel assembly are determined while putting it in water with a neutron source at one side and a neutron detector at the other side of the assembly and detecting to measure neutrons by the detector. The same measurement is carried out for a simulated fuel assembly prepared by disposing a not-spent fuel assembly of a known multiplication factor and a dummy rod together with water within a channel box to simulate the neutron transfer area. By utilizing the fact that the measured value thus obtained and the value prepared by subtracting the multiplication factor from 1 are in a hyperbola relation, a proportional coefficient is determined to thereby obtain the multiplication factor. (Kawakami, Y.)