Application of clearance automatic laser inspection system to clearance measurement of concrete waste

Recently, the Clearance Automatic Laser Inspection System (CLALIS) has been developed for the clearance measurement of metal scraps. It utilizes three-dimensional (3D) laser scanning, y-ray measurement and Monte Carlo calculation, and has outstanding detection ability. For the clearance measurement of concrete segments, the effect of background (BG) gamma rays from natural radionuclides in the measurement target, such as K-40 and the radioactive decay products of Th-232 and U-238, should be compensated for to ensure adequate waste management. Since NE102A plastic scintillation detectors are used for y-ray measurement in CLALIS, it is impossible to distinguish between count rates of natural radionuclides and contaminants on the basis of gamma-ray energy information. To apply CLALIS to the clearance measurement of concrete segments, the original activity evaluation method was improved by adding a new compensation procedure. In this procedure, BG count rate due to natural radionuclides is estimated by a Monte Carlo calculation with pre-analyzed data of a representative sample of the measurement target. The activity concentration of natural radionuclides in concrete differs markedly depending on the production location of its components, such as cement and aggregates. In this study, using six mock concrete waste samples, which were composed of cement and fine aggregate from various production locations, the accuracy of BG compensation was experimentally estimated. In addition, the accuracy of calibration for concrete waste was also estimated using a number of mock concrete segments of small and large triangular prisms. By considering the uncertainties of BG compensation and calibration, the detection limit of CLALIS for concrete waste was estimated. As a result, it was revealed that CLALIS could be applied to the clearance measurement of concrete segments when the mass of the measurement target is greater than approximately 1.1 kg and the key radionuclide is Co-60. (authors)