Verification Test of Clearance Automatic Laser Inspection System for Surface Contamination Measurement

Abstract

Recently, a clearance automatic laser inspection system (CLALIS) has been developed for clearance measurement of scrap metals and concrete debris. It utilizes three-dimensional laser scanning, gamma-ray measurement and Monte Carlo calculation, and its outstanding detection ability has been verified. In Japan, when an object is removed from a radiation-controlled area, the activity level must be lower than the surface contamination density standard of 4 Bq/cm2 for beta and gamma emitters, which is one-tenth of the surface contamination density limit. According to the clearance inspection report published by the Nuclear Safety Commission of Japan, the activity level of waste must be compared with not only the clearance level but also the surface contamination density standard for clearance inspection. To demonstrate that CLALIS can also be used for the measurement of surface contamination, a verification test was carried out using actual metal waste samples of various shapes, sizes and activity levels at Kashiwazaki-Kariwa Nuclear Power Station of Tokyo Electric Power Company. As a result, it was clarified that CLALIS gives a conservative value for surface contamination compared with the conventional GM survey meter measurement. This because the activities of metal waste samples were estimated using the total count rate, a fixed average surface area of 100 cm2 and the conservative source position assumed in the Monte Carlo calculation for the calibration factor. In a nuclear power plant, the actual judgment of whether an object can removed from a radiation-controlled area is based on whether the result of surface contamination measurement is lower than the detection limit, which is significantly lower than 4 Bq/cm2. According to this criterion, CLALIS provides an almost identical judgment to the GM survey meter, which means that CLALIS can be used as a rational clearance monitor to carry out clearance level and surface contamination inspections in a single radiation measurement. The prospective detection limit for CLALIS at nuclear power plants is also discussed and compared with that for the conventional surface contamination monitors.