PROBLEM TO BE SOLVED: To accurately measure an effective delayed neutron fraction without need of special equipment.SOLUTION: An effective delayed neutron fraction measurement method includes: a test step S100 of acquiring, by an input device, measuring results of multiplication time at a time of withdrawing entire control rods from a critical state and an axial fission fraction distribution of fuel rods; a reactivity, etc., calculation step S200 of calculating a reactivity and an axial extrapolation distance ; a correction step S300 of calculating an infinite multiplication factor k, and correcting the infinite multiplication factor kfrom a measured value of the axial fission fraction distribution and a measured value of a critical water level zc; a critical-water-level calculation step S400 of calculating the critical water level zc at a time of withdrawal of the entire control rods; and a final-value calculation step S500 of calculating a final value of an effective delayed neutron fraction . In the reactivity, etc., calculation step S200, a provisional value of the effective delayed neutron fraction necessary for calculation is used.
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