Applicability of Clearance Automatic Laser Inspection System to Clearance Measurement of Concrete Segments -9129

In the decommissioning of a nuclear power plant, large amounts of metal scrap and concrete segments require disposal when dismantling the nuclear reactor and surrounding facilities. When their activity level is negligible or sufficiently small, they can be regarded as general industrial waste. To distinguish between radioactive and nonradioactive materials, the clearance level for each radionuclide has been determined in units of activity concentration. These clearance levels are indicated in the International Atomic Energy Agency (IAEA) Safety Standards Series No. RS-G-1.7. The Japanese regulatory authority decided in 2005 to adopt the values given in RS-G-1.7 as clearance levels in legislation. Recently, a Clearance Automatic Laser Inspection System (CLALIS) has been developed utilizing gamma-ray measurement, automatic laser shape measurement and Monte Carlo calculation. CLALIS comprises four laser scanners and eight large plastic scintillation detectors surrounded by a 5-cm-thick lead shield. Using three-dimensional (3D) laser scanning, a measurement target, which is placed on the measuring tray, is represented as a dot image. The dot image is converted into voxels after noise removal, and is written in MCNP (A General Monte Carlo N-Particle Transport Code System) input files. When the gamma-ray measurement is carried out, the MCNP calculations are also carried out to obtain the calibration factor and background (BG) correction factors. For the clearance measurement of concrete segments, the effect of 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 taken into account to ensure adequate waste management. Since NE102A plastic scintillation detectors are used for gamma-ray measurement in CLALIS, it is impossible to distinguish between the count rates of natural radionuclides and contaminants on the basis of gamma-ray energy information. To overcome this problem, the activity evaluation flow of CLALIS was improved to allow for the BG count rate due to natural radionuclides. The accuracies of calibration and BG correction for concrete segments were estimated using a number of mock -concrete waste samples and varying their position, number and size. As a result, it was found that CLALIS can estimate the calibration factor and BG correction factor within good accuracies of +/- 20% and +/- 2%, respectively. The main components of concrete are cement, fine aggregate and coarse aggregate, and it is well known that the natural activity concentration of these components depends on where they are produced. In CLALIS, the BG count rate due to natural radionuclides is estimated using preanalyzed data obtained from the representative sample, the mass of the measurement target and its efficiency calculated by the MCNP code. Here, the effect of an uneven distribution of natural radionuclides between the measurement targets should be considered because it may contribute to the uncertainty of the BG count rate. In this study, to discuss in detail the applicability of CLALIS to the clearance measurement of concrete segments, the standard equation for detection limit estimation was extended to separately treat the ambient BG count rate, the BG count rate due to natural radionuclides and their uncertainties. Consequently, the precise detection limit of CLALIS was evaluated and its range of application was clarified with respect to the natural activity concentration and its uncertainty.