A. M., Maksymenko1, M. D. Bondarkov1, B. Ya. Oskolkov1, V. A. Seida2,
V. N. Dubas2
1SSRI “Chornobyl Center for Nuclear Safety, Radioactive Waste and Radioecology”, Slavutych, 07101, Ukraine
2SSE “Chornobyl NPP”, Slavutych, 07101, Ukraine
The results of studies are presented for hard-to-measure radionuclides 55Fe, 59Ni, 63Ni, 90Sr, 239,240Pu, 238Pu, 241Pu, 241Am, 234U, 235,236U, 238U in the surface contamination of the metal of ChNPP equipment, being dismantled. The estimation of scaling factors for hard-to-measure radionuclides is given.
In the process of the Chornobyl NPP decommissioning, a large amount of equipment is dismantled, the metal of which has only the surface radioactive contamination. Under condition of successful decontamination of this metal, it is possible to release it from the regulatory control by clearance. The study of complete volume of the metal being dismantled for
hard-to-measure radionuclides (HMR) content is a complex and expensive task. Reducing the cost for HMR specific activity measurement is possible by measuring key radionuclides followed by calculating HMR using the empirical scaling factors (SF).
The main objectives of this work were to determine the activity of key radionuclides (KR) and HMR in the surface contamination layer of the decontaminated metal and to calculate the corresponding SF. To achieve the set objectives, it was necessary to solve the problem of measuring the specific activity of KR and HMR at the levels below the established exemption criteria.
These studies were carried out to the order of the SSE “Chornobyl NPP”, the main results are presented in the research report.
The measurements were carried out in four stages. The first stage was the preparation of the counting sample and measurement of the gamma-emitting radionuclides activity, including key radionuclides 60Co and 137Cs. The second stage was the dissolution of the counting sample after gamma spectrometry in the mixture of nitric and hydrochloric acids and the selection of an aliquot for the radiochemical analysis of 55Fe, 59Ni, 63Ni. The third stage was the selection of an aliquot for the radiochemical analysis of 239,240Pu, 238Pu, 241Pu, 241Am, 234U, 235,236U, 238U. The fourth stage is the selection of a sample weight of the analytical material and determination of 10Be activity.
Based on the criteria of referring radionuclides as key, of those listed in the Tables 2, 3,60Co and 137Cs can be considered as such radionuclides. The calculated correlation coefficients between these radionuclides and some of HMR are presented as the following values: 60Co – 63Ni – 0.60; 137Cs – 90Sr – 0.44; 137Cs – 239,240Pu – 0.61; 137Cs – 241Pu – 0.64; 137Cs – 241Am – 0.66.
The approaches to sampling used in this work and measurement methods made it possible to fairly reliably determine the activity of HMR in the metal samples of the ChNPP equipment being dismantled and to calculate SF for them and their total uncertainties that can be used in practice.
The large values of the total uncertainty of the scaling factors are due to the differences in the radionuclide ratios of radioactive contamination of the studied equipment fragments, the cause of which may be the differential use of decontamination (decontamination only of those fragments, which contamination levels exceeded the established levels, was carried out).
The obtained results allow us to conclude that, in order to determine SF for the purpose of exemption, it is necessary to apply the careful approach to the formation of the RM flux, for which the same method of decontamination is used.
Keywords: Chornobyl nuclear power plant, scaling factor, hard-to-measure radionuclide, specific activity, radioactive waste, release.
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