POSITION DETECTING METHOD FOR IN-PILE NEUTRON DETECTOR

PURPOSE:To improve the reliability of the detection accuracy by generating a difference in a neutron absorption effect, generating a disturbance in an output of a detector at the time when an in-pile neutron detector passes through a prescribed position, and specifying a position of the in-pile neutron detector from this disturbance. CONSTITUTION:A fuel assembly 10 is provided with the upper nozzle 11, the lower nozzle 12, a fuel rod bundle 13 placed between these nozzles, and plural control rod guide tubes 14 attached to the nozzles 11, 12, and an instrumentation device 15 is inserted through the center of the fuel rod bundle 13. In the periphery of the instrumentation device 15, plural ring-like neutron absorbing members 16 consisting of a substance having a neutron absorbing effect, for instance, such as hafnium, etc., are attached separately in the axial direction in the upper end, the middle part and the lower end of a reaction area of the assembly 10 whose position is known. When a neutron detector 4 passes through a position of this member 16, a disturbance is generated in an output of the detector by the neutron absorbing effect by the member 16, and from this disturbance, a position in the axial direction of the neutron detector 4 in a detector guide tube can be specified.