Method and apparatus for measuring the reactivity of fuel assemblies

Purpose: To enable rapid measurement for the reactivity of fuel assemblies. Method: An irradiation case containing an external neutron source and a first neutron detector are provided on the opposing sides of a mounting box to which a standard fuel assemblies or measured fuel assemblies are mounted. A second neutron detector is disposed at such a position that the irradiation rate of neutrons emitted from the external neutron source is sufficiently small as compared with that of the neutrons emitted from the standard or measured fuel assemblies. Then, the difference value for the neutron fluxes between two conditions, that is, the external neutron source is located or not located to the side of the standard fuel assemblies, is divided by the difference value for the neutron fluxes similarly obtained with the measured fuel assemblies, to thereby measure the reactivity of the fuel assemblies. (Furukawa, Y.)