形状計測技術を用いたクリアランスレベル検認手法と装置の開発

抄録

We developed a clearance level inspection method using laser-based shape measurement and Monte Carlo calculations for quantifying very low radioactivity in metal wastes. The practical apparatus, the Clearance Automatic Laser Inspection System (CLALIS), is comprised of a shape measurement part and a radiation measurement part. The shape measurement part consists of four laser scanners and four controllers connected to a personal computer. The radiation measurement part consists of eight large plastic scintillation detectors at the upper and lower sides of the radiation measurement area, surrounded by a 5-cm-thick lead shield. Using the digital configuration data of wastes, the calibration factor is calculated by the Monte Carlo simulation code, assuming that the source type is either secondary contamination or activation. The correction factor for background (BG) reduction, due to the self-shielding effect of the waste during radiation measurement, is also calculated by the code. The uncertainties of calibration and BG count rate were experimentally evaluated using mock-metal wastes of various shapes, numbers and sizes with radioactive sources of ⁶⁰Co and ¹³⁷Cs. As a result, it was clarified that the detection limit of CLALIS, which is three times the measurement uncertainty, is approximately 100 Bq for ⁶⁰Co by considering the uncertainties of calibration and BG count rate, which makes it possible to achieve simultaneous inspections to confirm whether the radioactivity level is less than both the clearance level and the surface contamination level. In addition, the possibilities of overestimating and underestimating radioactivity due to the measurement method were evaluated. CLALIS can give objectivity to measured results through completely automatic operation without a manual survey, which would be helpful in the decommissioning of nuclear power plants.