Analysis Of Fluorides In The Aqueous Reprocessing Of The Tailings Of Pyroprocessing

As a part of the fuel processing of TMSR, hydrofluoric acid dissolution and pyrohydrolysis are put forward to recover the carrier salt and transform the insoluble fluorides to oxides. It is of great importance to make full use of resources and dispose the radioactive waste reasonably. The thesis focuses on the analysis of fluorides in the aqueous reprocessing of the tailings of pyroprocessing based on TMSR. The potential problems such as dissolution method of the insoluble fluorides, spectral interferences on trace elements caused by Th and U in ICP-AES, the matrix effect arose from high concentration matrix Li and Al in ICP-MS and the determination of fluorine in complex solution, all of them have been researched in the thesis. The following investigations with the main achievements have been completed.Firstly, we have studied the solubility of insoluble fluorides including rare earth flourides, LiF-AlF₃ and mixed fluorides molten salt. The results show that the solubility of rare earth fluorides in 16 mol/L nitric acid are among 2.5 g/L and 3.9 g/L while the solubility of LiF-AlF₃ in 8 mol/L nitric acid has been reach up to 2.0 g/L and the solubility of mixed fluorides molten salt in 16 mol/L nitric acid is about 1.0 g/L.The solubility difference of the fluorides and the oxides shows the pyrohydrolysis made the samples more soluble in nitric acid.Then the matrix effect arose from high concentration matrix Li and Al in ICP-MS has been studied when we determined some trace elements（U, Sr, Ce, La, Nd, Pr, Sm）. We found the elements including U, Pr, Nd, La, Ce can be determined at 0.0001% while Sr and Sm at 0.001% and 0.01% respectively（concentration of matrix are around 100 mg/L）. The simulative soluble liquid of the hydrofluoric acid dissolution process has been tested. Determination of trace elementsin LiF-AlF₃ also has researched, the recovery rate is between 94.4% and 101.9% while the relative standard deviation is below 6.7% with the detection limit is range from 0.039 μg/L to 0.126 μg/L.The spectral interferences on trace elements caused by Th and U in ICP-AES also has been studied when we determined some trace elements（Li, Al, Sr, Ce, La, Sm） in Th and U matrix. The prominent lines of each element free from the interference of Th and U has been chosen. The elements such as Li, Al, Sr, Ce, La, Sm can be detected up to 0.1 mg/L in 200 mg/L Th and U matrix under these lines. In addition, by combination of ICP-AES and ICP-MS method three simulated samples of the process has been directly tested; Finally, determination method of trace Li, Al, Ce, Sr in rich Th and U related to the raw material of pyrohydrolysishas been achieved. The recovery rate is between 94.2% and 108.9%, and the relative standard deviation is below 5.9% with the detection limit range from 0.1 μg/L to 2.9 μg/L.Lastly, the potential effect caused by the complex metal ions coordinated with Fhas been studied by fluorine ion selective electrode. The results shows that when the ratio of（Li, Al, Ce, Sr）/F, U/F, Th/F are no more than 1, 10 and 25 respectively, the measurement of F- are accurate. The recovery is between 91.5% and 104.0% with the relative standard deviation below 7.5%. The concentration of F- can be effectively stabilized by adding TISAB II both in samples and standard solutions with the recovery rate higher than 91.5% and the detection limit up to 0.012 mg/L.