| 摘要 |
PURPOSE:To improve the reliability of the position information of a detector with a simple structure. CONSTITUTION:Marker materials 21-1 through 21-n locally changing the neutron flux level are arranged at the prescribed interval on or near a guide pipe 20, the output Of a neutron flux detector 8 is inputted to a position detecting means 23 to detect the portion where the output level is locally changed, and the position in the reactor core axial direction of the neutron flux detector 8 corresponding to the output level is judged from the local change.
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