| 摘要 |
PURPOSE:To shorten the cooling period of a used fuel element greatly by arranging the radiation passage of a bundle unit for radiation to the used fuel element within the effective visual field of its radiation converging unit except at a plenum spring. CONSTITUTION:A nondestructive measuring instrument for the discharge amount of FP gas is so constituted that the radiation passage 10 of the radiation converging unit 8 for the used fuel element 1 is arranged in the effective visual field of the radiation converging unit except at the plenum spring 5. It is confirmed that the signal-to-noise ratio obtained by using this device is improved by about 30 times as high as that of measurement using a conventional (lateral) type destructive measuring instrument. Consequently, when the gamma-ray spectrum of the plenum part of the used fuel element is measured by using this invented device, the measurement precision is improved greatly as compared with the conventional type, and consequently the cooling period of the used fuel element is made much shorter than before.
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