Disposal of low- and intermediate-level activated waste generated at nuclear power plants is being planned or carried out in many countries. The radioactivity concentrations and/or total quantities of long-lived, difficult-to-measure nuclides (DTM nuclides), such as C-14, Ni-63, Nb-94, α emitting nuclides etc., are often restricted by the safety case for a final repository as determined by each country’s safety regulations, and these concentrations or amounts are required to be known and declared. With respect to waste contaminated by contact with process water, the Scaling Factor method (SF method), which is empirically based on sampling and analysis data, has been applied as an important method for determining concentrations of DTM nuclides. This method was standardized by the International Organization for Standardization (ISO) and published in 2007 as ISO21238 “Scaling factor method to determine the radioactivity of low- and intermediate-level radioactive waste packages generated at nuclear power plants”.
However, for activated metal waste with comparatively high concentrations of radioactivity, such as may be found in reactor control rods and internal structures, direct sampling and radiochemical analysis methods to evaluate the DTM nuclides are limited by access to the material and potentially high personnel radiation exposure.
In this case, theoretical calculation methods in combination with empirical methods based on remote radiation surveys need to be used to best advantage for determining the disposal inventory of DTM nuclides while minimizing exposure to radiation workers.
Pursuant to this objective a standard for the theoretical evaluation of the radioactivity concentration of DTM nuclides in activated waste, is in process through ISO TC85/SC5 (ISO Technical Committee 85: Nuclear energy, nuclear technologies, and radiological protection; Subcommittee 5: Nuclear fuel cycle). The project team for this ISO standard was formed in 2011 and is composed of experts from 11 countries. The project team has been conducting technical discussions on theoretical methods for determining concentrations of radioactivity, and has developed the draft International Standard of ISO16966 “Theoretical activation calculation method to evaluate the radioactivity of activated waste generated at nuclear reactors”.
This paper describes the international standardization process developed by the ISO project team, and outlines the following two theoretical activity evaluation methods:
— Point method
— Range 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.
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