Development of Waste Monitor of Clearance Level to Ensure Social Reliance on Recycled Metal From Nuclear Facilities

Abstract

Metal and concrete wastes in the decommissioning of nuclear facilities are classified according to their radioactivity level after decontamination. Radioactive waste below the clearance level (e.g., 0.4Bq.g−1 for Co-60 in Japan) can be disposed of as general industrial waste or recycled. Metal wastes mainly originate from equipment in buildings, except for the metal bars in reinforced concrete. Since contaminated equipment must be decontaminated after dismantling, the main target of measurement would be fragments of equipment, of various shapes, numbers and sizes. In order to transport such metallic fragments out of controlled areas, a surface contamination survey must be performed to confirm that the contamination level is below the legal standard level (e.g., 4Bq.cm−2 for beta or gamma emitters in Japan) in addition to satisfying the clearance level. Taking account of social reliance on recycled metal after inspection of the clearance level and the surface contamination level, it is important to remove the possibility of overlooking contamination above these levels in the recycled metal. The measurement of beta rays is suitable for determining surface contamination on metal because almost none of the beta particles from inside the metal can be detected and the detected radiation can be mostly limited to that from the surface. This is the reason why a survey meter for measuring surface contamination has a detector with a higher sensitivity for beta particles than for gamma rays. Considering the characteristics of the survey meter, it may be difficult to measure the contamination level of the surface of a metal fragment, particularly when the surface is not flat. Moreover, in the case of internal contamination of a small metal pipe, measurement is impossible. The permeability of gamma rays is much greater than that of beta particles. Therefore, gamma rays can be detected even from internal contamination in metal. For gamma ray measurement, accurate and easy calibration of the actual radioactivity level and count rate obtained using a measurement instrument is important. If gamma ray measurement can confirm that the radioactivity level is less than about 400Bq, both the clearance level and the surface contamination level could be inspected simultaneously. In addition, the great amount of labor needed for manual inspection using a survey meter could be saved, and there will be no possibility of missing hot spots of radioactivity due to human error. In this study, a new technique for precise and automatic measurement of gamma emitters in metal waste has been developed using 3D noncontact shape measurement and Monte-Carlo calculation techniques to objectively confirm that the specific radioactivity level of metal waste satisfies the clearance level and furthermore, that the surface contamination level of the metal waste is below the legal standard level. The technique can yield a calibration factor for every measurement target automatically and realizes automatic correction for the reduction of the background count rate in gamma measurements due to the self-shielding effect of the measurement target. A practical monitor (Clearance Automatic Laser Inspection System, CLALIS) has been developed. The accuracy of the automatic calibration and correction of background reduction of the practical monitor has been clarified using mock metal wastes of various shapes, numbers and sizes. It was found that the values measured using the present monitor and the actual radioactivity level agreed within +/−20%, and the corrected and actual background reductions agreed within +/−2%. The detection limit of the present monitor was estimated as being 100Bq for Co-60, taking into consideration the calibration error and correction error of the reduction of the background count rate. The monitor accomplished precise measurements with a 100sec (30sec for gamma ray measurement, 30sec for background measurement) process time per inspection. This indicates that approximately 5 tons of metal waste can be measured per day (1,000 tons per year) in 20kg batches at that process speed.