In the presented work a comparison of conventional acid digestion with mixtures of acids HNO3, HClO4, and HF, thermal fusion using Li-borate as fusion agent and leaching with 8 M HNO3 was performed for solid samples. These techniques were used to analyse the decomposition of various samples and to determine the activity concentrations of uranium, thorium, americium, and plutonium, and to study their contents in possible residues obtained after decomposition. The results obtained showed that acid decomposition with an acid mixture is not complete and visible residues were observed after acid decomposition. A possible presence of uranium and thorium as natural radioisotopes was detected in the residues. They bind to the structure of minerals and are therefore more difficult to decompose. The activity concentrations of radionuclides were determined by preliminary separation by extraction chromatography using radioactive indicators and further detection by alpha and gamma spectrometry. The results obtained showed that acid digestion residues accounted from 1 to 50 % by weight of the total sample analysed, which represented 1 to 50 % of the total activity concentrations of uranium and thorium in the sample. The residual content and activity concentration in the residue depended strongly on the type and mineral composition of the samples.
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