PURPOSE: To make improvement in S/N and to make nondestructive and accurate measurement of the released amt. of fission product gases by disposing a radiation shielding material into the radiation passage between a used fuel element and a radiation detecting unit.

CONSTITUTION: This measuring instrument is constituted of a radiation converging unit 1, the radiation detecting unit 2, the radiation shielding material 3, and a radiation passage 4. The shielding material 3 is required to have the property of a large difference between the coefft. of radiation absorption of the shielding material 3 to krypton-85 and the coefft. of radiation absorption of the shielding material 3 to the induced radioactivity of a plenum spring 9; for example, an uranium shielding material is used. The influence of the Compton background occurring in the induced radioactivity is lessened in this way and improved S/N is attained. The nondestructive and accurate measurement of the released amt. of the fission product gases existing in the free space of the used fuel element is executed.