Browsing by Subject "decommissioning"

As nuclear reactors come to the end of their lifetime, they undergo decommissioning. All the construction materials and spend nuclear fuel must be characterized according to their activities before final disposal. For the analysis of difficult to measure radionuclides, such as beta emitters, sample decomposition methods are used. Acid digestion, alkaline fusion and thermal combustion are examples of sample decomposition methods. In this work, alkaline fusion methods were tested to gain knowledge on the suitability of the method for volatile and non-volatile radionuclides in decommissioning waste. The sample materials tested were concrete and two difficult to dissolve reference materials: soil and coal fly ash. Results obtained with alkaline fusion were compared with that of routinely used acid digestion method. In addition, activated graphite was studied to familiarize with the analysis of difficult to measure radionuclides. Non-volatile 63Ni and 55Fe and volatile 3H and 14C were studied. The results were compared with modelled activities. Alkaline fusions were performed with a borate flux in an automated fusion oven as well as with a carbonate flux in a muffle furnace. Acid digestion was performed in an open vessel with aqua regia. The dissolved samples were analysed with ICP-OES and ICP-MS for determination of elemental concentrations. Borate fusion seemed to be the most effective method, as it was suitable for all the sample materials and released even refractory elements. Some of the volatile elements were lost during the fusion methods (Cd and Zn), while some were not affected by the high temperatures used (Pb). Alkaline fusion methods appeared to be suitable for decommissioning studies, however further research is needed. In analysis of graphite, non-volatile elements were analysed by acid digestion. 55Fe and 63Ni were separated from the matrix by hydroxide precipitation and from each other by ion exchange in an AG-resin. Ni fraction was further purified by extraction chromatography with a Ni-resin. Volatile elements were analysed by thermal combustion in a pyrolyser. The activities of 55Fe, 63Ni, 3H and 14C were measured with liquid scintillation counting. The measured activity concentrations of 63Ni were partially in agreement with the modelled values, but some samples gave higher activities. No 55Fe activities were found in any samples, as was expected based on the modelled values and the short half-life of the radionuclide. Activities of the volatile 3H and 14C were accurate and close to the modelled values. In the future, the determined activity results will be used for improving scaling factors, for the assessment of radionuclide concentrations in graphite.