Burnup degree measuring device

The device of the present invention can measure a burnup degree of spent fuel rods in water by a simple structure without decreasing the effect of shielding radiation ray's which releases to the outside. It comprises an insertion tube made of a flexible material and having a closed top end for inserting a radiation detector, a radiation detector inserted to the insertion tube at the counter-top end and movable therein and a moving means for moving the radiation detector in the insertion tube. With such a constitution, since the radiation detector is inserted in the insertion tube, the detector is free from direct immersion in water, giving no worry of radioactive contamination. If the insertion tube has such a structure that a portion is collapsible by hydraulic pressure, radiation is prevented from leaking to the outside. Further, since the radiation detector can freely move in the insertion tube, it can be taken out and replaced easily. (I.S.)