| The fuel salts of thorium based molten salt reactor?TMSR?are composed of fluoride raw material salts such as 7LiF,BeF2,ZrF4,UF4 and ThF4 in a certain ratio,which are mixed and further purified before use.To some extent,the quality of raw salts affects the preparation technology of the fuel salts and also the neutron economy of the reactors.Therefore,it is necessary to carry out a quality control process of the raw salts.To achieve this goal,the establishment of reliable and convenient chemical/spectroscopical analysis methods is a top priority.On the other hand,a small amount of actinides and fission products(10-5-10-8)are produced in the fuel salts after the molten salt reactors are running for some period.On-line pyroprocessing technologies,such as fluorization and decompress distillation,not only remove some amounts of the neutron poisons to keep the reactors running,but also extract the fissile materials or their precursors effectively.The mass flow of the fission products is the key to evaluate the pyroprocessing processes.Therefore,the establishment of reliable analysis methods of the fission products is also very important for the pyroprocessing research.The particularity and/or difficulty of the analysis of the raw salts for TMSR and/or of the outputs in the pyroprocessing process mainly lie in:?1?A large amount of impurities are concerned,including alkali metals,alkaline earth metals,rare earth elements and several oxygen-containing anions;?2?the solubilities of fluorides or molten fluorides are usually very poor,especially convenient dissolution methods for ThF4 and ReFx are lacking;?3?the concentration levels of the impurities are very wide(10-1?10-9);?4?as the matrix,the heavy metals of uranium and thorium are both multispectral line elements,which often cause interferences during the ICP-OES analysis of certain impurities.Therefore,special separation methods are often needed to remove most of the matrix elements before instrumental analysis.However,some impurities will inevitably be lost in such chemical separation processes,while some other foreign impurities may be introduced to the solutions.As a result,the number of impurities that could be effectively analyzed in the corresponding samples are limited.According to the requirements of the quality control of the raw fluoride materials and the needs of the pyroprocessing research,and from the perspective of simplifying the pretreatment process and of analyzing multi-elements simultaneously,the standard addition-inductively coupled plasma mass spectrometry?ICP-MS?method,without matrix separation,is established and systematically evaluated for the determination of trace impurities in the raw salts,while for the samples from the pyroprocessing studies,the spectral interference in the ICP-OES measurements and the mass spectrometry interference in the ICP-MS measurements are thoroughly studied,and different analytical procedures for the direct determination of the target elements of different levels are established.The main research contents and results of this thesis are as follows:1.Study on the simultaneous determination of 42 trace impurities in uranium tetrafluoride by a combined microwave digestion-standard addition-ICP/MS method.The results show that the microwave digestion of the uranium tetrafluoride is a simple,rapid and effective process,and that the standard addition method can effectively compensate for the matrix inhibition effects of ICP-MS.The accuracy of the method is verified by using certified reference materials?CRMs?of uranium dioxide due to the lack of uranium tetrafluoride CRMs.The recovery rates of the standard addition are in the range of 92%-111%,the relative standard deviations are less than 10%,and the detection limits are between 0.0004-0.072?g.g-1.2.Study on the direct and simultaneous determination of 40 trace impurities in thorium tetrafluoride by a procedure of repeated microwave digestion followed by standard addition-ICP/MS analysis.The results show that 0.1g of thorium tetrafluoride can be completely dissolved by repeated microwave digestion with 15 ml nitric acid and evaporation to remove acid using a heating plate for four times.The results indicate also that the proposed method had good accuracy and repeatability at the trace level.The recoveries of 0.1?g.g-11 individual impurities?except 10?g.g-11 for Bi?are in the range of 85%-111%,the corresponding relative standard deviations are less than 10%and the detection limits are in the range of 0.0001-0.08?g.g-1.3.Study on the direct and simultaneous determination of 15 trace rare earth impurities in lithium fluoride,beryllium fluoride and zirconium fluoride,respectively by ICP-MS.The detection limits of the method are in the range of 0.0003-0.009?g.g-1.The recoveries are from 85%to 111%at different levels of rare earth impurities(0.05,0.1 and 1.0?g.g-1),and the relative standard deviations are better than 10%.The measurement uncertainties of the proposed rare earth analysis methods are evaluated.The main contributions to the measurement uncertainty are sample preparation,reference materials,calibration and repeatability,among which the sample preparation process had the greatest impact on the results.4.Study on the dissolution and analysis procedures for the feeds and discharges of the typical pyroprocessing units,i.e.,fluorization and decompress distillation.The samples include simulated irradiated fuel salts,the outputs of the fluorization process?uranium hexafluoride and the remained fluoride salts?and the outputs of the decompress distillation?the recovered salts and the remained salts?.For the simulated irradiated fuel salts and the remained salts from the fluorization process,the spectral interference in ICP-OES analysis and mass spectrum interference in ICP-MS analysis are both studied systematically.As a result,the spectral lines and mass numbers are optimized.Different analytical procedures are studied for different levels(from 10-88 to 10-1)of various elements.The detection limits,analytical sensitivity,linear coefficients,recovery rates and working scope of the methods are all investigated,and the detection limits of typical fission products are between 10-7 and 10-8. |
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