DEVELOPMENT OF THE ANALYSIS METHOD FOR THE RADIOACTIVITY CONCENTRATION ESTIMATION OF SOILS FORA REGULATORY CLEARANCE

摘要

In this study, for the analysis of a radioactivity concentration of soil, a sampling and an analyzing method were developed. To create homogeneity of the contents for each drum, big particles such as pebbles, rocks and scraps of concrete were removed after pouring the soil into a tray. And then the soil in the tray was mixed thoroughly. A 10 x 10 grid was used to partition the soil into 100 sections. 2 liter of soil was sampled out of 30 randomly pre-selected sections. Only 1 liter of the soil out of 2 liters was used for the γ-spectrometry analysis. The remaining 1 liter of soil was stored for a validation purpose. For a verification of the sampling process, 3 samples were taken from each drum and analyzed. 5 drums were used for the verification. And the results show that this sampling method has about a 9% sampling error. Also, the analysis results of the 865 drums of soil showed that the major nuclides in the soils were Co-60 and Cs-137 while a small amount of Mn-54, Fe-59, I-131, Cs-134 and Eu-152 were detected as γ-emitters. About 73% of the soils had a total radioactivity concentration below 0.1 Bq/g, while the soils with more than 0.4 Bq/g of a radioactivity concentration were only 3%. Based on the guide for the regulatory clearance criteria recommended by the IAEA (International Atomic Energy Agency), about 73% of the soils can be regulatory cleared without any treatment. Also, the remaining soils can be regulatory cleared after a further storage. Only 3% of the soils are considered to be necessary for a decontamination treatment. The results of this study can be applied to the treatment of radioactive soils generated in a large amount during the decommissioning of a nuclear facility.