In Japan, the clearance system has been in effect since December 2005, and at present, the cleared material is recycled at a site in affiliation with the nuclear installation. In the near future, it is assumed that a considerable amount of cleared material will be transported from nuclear facilities that are being decommissioned, so that the material can be recycled and used by general industry. In order to operate the clearance system with certainty, it is essential to devise a method for promptly measuring the radionuclide concentration of cleared materials that are suspected of exceeding the clearance level. That is, preparation for the contingency that an uncertain radiation source mixes with a clearance item by chance. In general, in-situ measurement of low-level radionuclide concentration is difficult since accurate information on the shape of the target, as well as on the distribution of radionuclide concentration in the target, is required. Therefore, we investigated a method for simple and rapid determination of radionuclide concentration, which does not require accurate information on the volume and density of the target. To avoid the influence of volume or density, we examined the adoption of a conversion factor with units of “(Bq/g)/cps”. ISOCS (In-situ Object Counting System) Calibration Software (Model S573 Version 4.0, CANBERRA Inc.) was used to determine the conversion factor. Metal or concrete was assumed to be stored and piled in a stockyard of a recycling facility. The piled objects were arranged as a cone with a gradient of 50%, in accordance with the limitations specified by the “Waste Management and Public Cleansing Law”. The volume and bulk density were assumed to be 0.1–10 m3 and 0.5–10 g/cm3, respectively. The conversion factor for a cone with volume of 1 m3 and density of 1 g/cm3 could be applied to the piled objects having a volume greater than or equal to 1 m3 and a density greater than or equal to 0.5 g/cm3, resulting in an increase in tolerance by a factor of two. These results show that in-situ measurements of radionuclide concentration can be simplified by using a conversion factor with units of “(Bq/g)/cps”. This approach can be used to carry out prompt measurements on suspected materials in the field.

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