To facilitate radiation control, to eliminate contamination of an object to be measured, to facilitate measurement, and to increase the accuracy.

A neutron emitter 1 is arranged in a hydrogen containing material 3 of a thickness enough to absorb emitted neutrons 2, the absorbing reaction of neutrons 2 is produced in the hydrogen containing material 3, characteristic γ-rays 4 of 2.2 MeV produced by reaction with hydrogen, of generated γ-rays, are counted by a γ-ray detector 5 arranged at a position that does not affect the emitted neutrons 2, and the neutron emission rate of the neutron emitter 1 is determined based on the counting rate and an absolute value of the detection efficiency at which the characteristic γ-rays 4 of 2.2 MeV produced by the emitted neutrons 2 are counted by the γ-ray detector 5. The neutron emission rate of nuclear fuel is calculated based on the burnup of the nuclear fuel predicted in management and the composition based on the burnup, and actual burnup and composition of the nuclear fuel is validated by comparing the calculated value with the measured value of the neutron emission rate.

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