Suggestion to Waste Classification for Scaling Factor Method

Scaling factors (SF) are widely used to determine the nuclide specific radioactivity concentration of a waste package. In this paper, an appropriate waste classification for applying the same SF value is considered through a study of differences in physicochemical behavior of nuclides and a comparison of nuclide data obtained by a radiochemical analysis of actual wastes from several waste streams. Corrosion product (CP) nuclides show only minor differences in production/transportation behavior through all waste streams because they are generated by the activation of reactor materials and have low solubility in common. Therefore a unified SF for all waste streams is considered applicable, though the SF can at best be determined for each individual waste stream. Fission product (FP) nuclides and alpha-emitters are generated by neutron capture and nuclear fission and their solubility varies. If Cs-137 is selected as the key nuclide, distinct differences in nuclides ratios are recognized between homogeneous waste (e.g. resins, concentrates) and heterogeneous waste (e.g. filter cartridges, dry active waste). This is mainly because the release behavior and the solubility of alpha emitters and FP nuclides differ from those of Cs-137. Our study suggests that all waste streams can be divided into those two categories. On the other hand, some countries selected Co-60 as key nuclide for alpha-emitters and some FP nuclides. If Co-60 is selected as the key nuclide, it may be helpful to categorize power plants according to their fuel failure history. This is because the generation mechanism of the key nuclide differs from that of the difficult to measure (DTM) nuclides. Within each categorization, insignificant differences are recognized in terms of ratios of DTM nuclides to Co-60, for both nuclides have rather low solubility. Therefore a unified SF can be applicable, though further categorization of SF is possible for more accurate estimate.