Control rod worth measuring method for nuclear reactor in subcritical state and reactor reactivity measuring device for measuring control rod worth

The present invention concerns measurement for control rod worth of a nuclear reactor in a facility using the reactor. Namely, reactivity ρ is determined by the following steps: (1) After A/D conversion of a neutron flux signal n' of the reactor, it is converted to n by n = log n'/log 10. Reactivity ρ0 at neutron source intensity S = 0 is determined by using a relation: reactivity ρ = ρ0 - 1/n·S based on n and S = 0. (2) A regression analysis is performed on time sequential data of the n and ρ0 to approximate a true value, and when the value S is settled to a constant value, the value is determined as S. Next, control rods are successively inserted by every several steps, and the reactivity ρ in this case is determined from a relation ρ = ρ0 - 1/n·S by using the S determined by the steps (1) and (2), then a differentiation worth and an integration worth of the control rod worth are determined based on the change. As a result, the control rod worth can be measured simply in a state where only the reactor is in a critical state. (I.S.)