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The management of the storage of radioactive waste produced in the operation of nuclear power plants (NPPs) requires knowledge of the radionuclides present in the waste and their specific activities. The large majority of the radionuclides which are important for the long-term storage of the waste are difficult-to-measure nuclides (DTM nuclides) which activity cannot be measured from the outside of the waste package by non-destructive measurement techniques. On the other hand the waste packages contain also easy-to-measure nuclides (ETM nuclides) which activity can be measured directly from the outside of the waste containers. The scaling factor (SF) method is a widely accepted technique for assessment of the concentrations of DTM nuclides from the concentrations of the ETM nuclides, based on the presumption for a correlation between the ETM and DTM nuclide concentrations. The objective of this work is to give an overview and to present the results from a three year work program for determination of SFs for drummed dry active waste from the Kozloduy NPP in Bulgaria. The SF development includes: development of a sampling plan, sampling from the waste, radiochemical analysis of the samples and determination of the ETM and DTM concentrations, statistical analysis of the experimental data, choice of an appropriate key ETM nuclide, choice of an appropriate mathematical relationship to account for the correlation between the DTM and key nuclides and evaluation of the SFs. A description of the development of the sampling plan is presented. Next, we describe the sampling and the radiochemical analysis of the data. Results from the statistical analysis of the radiochemical data and the selection of key nuclides are given. Finally we give the values of some scaling factors determined in this study. The developed SFs will be used for evaluation of the radionuclide inventory of the waste prior to its final disposal at a dedicated disposal site.