To investigate the species and the distribution of the captured nuclides in a HEPA filter, it was dismantled into a metal part and a filter medium part to obtain a filter medium. From the nuclide analysis results for a filter medium part through pre-treatment of it, it was possible to obtain three kinds of typical distributions in view of the distribution of the captured nuclides in the filter medium. When considering these distribution characteristics of the captured radionuclide, the region showing the high concentration of the captured nuclides was the intake or the outlet part of the HEPA filter. On the other side, the middle part generally represented a uniform distribution below the average concentration of the captured nuclides. Therefore, in the event of taking a representative sample of a HEPA filter at the intake and the outlet part, the results of a nuclide analysis for that sample could be estimated as existing in the range from 1.0 to 1.5 times the real concentration of the nuclides captured by a filter medium. As a result, to conduct a radionuclide assessment of a spent HEPA filter without a dismantlement of it, the analysis results for a representative sample taken from both the intake and the outlet part of a spent HEPA filter could be regarded as an average value for the corresponding HEPA filter.
Spent filter wastes of about 2,160 units have been stored in the waste storage facility of the Korea Atomic Energy Research Institute since its operation. These spent filters have generally consisted of a HEPA filter after its filtering of all the contaminants in the gas stream generated during the operation of the HANARO research reactor and the nuclear fuel cycle facilities. At the moment, to secure enough storage space, it is necessary to make a volume reduction of the stored radioactive wastes through a compression treatment or a regulatory clearance. There have been many studies on a treatment and a clearance of the low level radioactive wastes generated from nuclear facilities. These methods are used in view of a reduction of a management cost and disposal cost and the security of free space for a waste storage facility approaching saturation. In order to dispose of the spent filters, it is first necessary to conduct a radionuclide assessment of them. To do that, a sampling procedure should be prepared to obtain a representative sample from a spent filter. As for conducting a nuclide analysis for this representative sample, a corresponding spent filter can be determined as either a regulatory clearance waste or a radioactive waste.
The dose-to-curie (DTC) conversion method is a very simple and easy way to estimate the radioisotope inventory of a drum by a non-destructive assay method. So, in order to make the dose-to-curie conversion method practical, the parameters affecting the measured dose rate were evaluated by using the MCNP code. The major uncertainties associated with measuring the dose rate from a drum could be divided into the impact of the elemental composition of the contained material, and of the spatial distribution of the radioactive sources in a drum. Also, the relative abundance of gamma emitters in a drum, another source of uncertainty at the DTC conversion, could be directly obtained from the measured pulse height spectrum. The dose rate of that drum could then be calculated by using a dose conversion factor which is widely used in the field of environmental radiation measurement. As a result, the impact of the elemental composition and the spatial distribution of radioisotopes within a 200-L drum with a low density could be reduced if the detection points are selected at three times the radius of a drum.
At KAERI (Korea Atomic Energy Institute), radioactive soil and concrete wastes with extremely low level of activity were regulatory cleared in 2008 and large amount of spent drums remained. After generation, drums having good physical integrity reused for packaging radioactive wastes and about 50 tons of drums unsuitable for reuse were stored as radioactive wastes. Having once been used for packaging regulatory cleared radioactive wastes, these spent drums were determined to be regulatory cleared. Before regulatory clearance, steel drums were radiation monitored, washed with pressurized water two times, compacted and stored at a designated area. Based on radiological dose assessment results using a recycling scenario derived from actual situation, the regulatory clearance of steel drums was permitted by the regulatory body. Treatment of the regulatory cleared drums was then committed to a scrap-metal dealer for recycling. In this study, a process of regulatory clearance for spent steel drums and a modified radiological dose assessment model for staff members of a scrap-metal dealer will be discussed.
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