Investigation On Extraction Behavior Of Eu³⁺ And UO₂²⁺ Using CMPO/[C_nmim][NTF₂] Systems

With the massive increasing of nuclear power,a plenty of spent fuel will be inevitablely generated.How to deal with these spent fuel has become a central point concerned about by many countries.However,Minor Actinides（MAs）contained by High Level Liquid Waste（HLLW）have long half-life and strong radioactivity,which were produced in spent fuel reprocessing.The ideal treatment method is to separate and transform them into a low level radioactive or non radioactive element,so that the potential harm in the subsequent treatment will be reduced effectively.But before the treatment,MAs need to be separated from the Lanthanide（Ln）carried very high neutron absorption cross section and existed in HLLW with great amount.Yet,the chemical properties between Mas and Ln are very similar,which will bring great difficulties to the separation.For decades,n-Octyl（phenyl）-N,N-diisobutylcarbamoylmethylphosphine oxide（CMPO）has acquired recognized outstanding performance in the extraction of actinides.Nevertheless,the organic extractant and diluent used together with CMPO in the extraction process will occur hydrolyzation and radiation decomposition,leading to the decline of extraction performance.These problems lead to serious doubts about the application of the extractant in practical use.Thus,it is necessary to find a new extraction system that is good at extracting and resisting radiation.Ionic liquids（ILs）have been regarded as perfect substitutive solvents in liquid-liquid extraction because of their splendid character,for instance,inappreciable vapor pressure,fine thermal,chemical durability and fine dissolving ability,etc.Moreover,it has been noted that Ln cations can be extracted from aqueous phase into the ILs phase.Therefore,in this paper,we choose CMPO as extractant,ionic liquids as diluent,combineing them into a kind of new extraction system,in order to study the extraction behavior of Europium and Uranium.We hope our new system can open up a new way for the study of Lanthanide/actinide separation and be applied in actual waste disposal.Nevertheless,in-depth and systematic investigation on the extraction behavior of CMPO/IL is crucial.The extraction behavior of Eu³⁺and UO₂²⁺from nitric acid solution with CMPO dissolved in hydrophobic ionic liquid[C_nmim][NTf₂]（n=2,6,8,12）was studied.The extraction efficiency（E）of Eu³⁺and UO₂²⁺in CMPO/[C_nmim][NTf₂]was measured as a function of various parameters such as contact time,the concentrations of nitric acid,CMPO and temperature.The influence of ionic liquid structure change on the extraction behavior is also studied.CMPO/[C₂mim][NTf₂]system was selected for irradiation stability study and for lanthanide and actinide partitioning from simulated high level waste（SHLW）.The thesis concludes five chapters.The main contents of every chapter are listed as follows:In chapter 1,the background of this thesis is introduced firstly.Nuclear energy is the inevitable choice for the future energy development,and it will become an important part of the future energy structure.Nuclear electticity--the main way of nuclear energy utilizing,its development prospects are limitless.However,the development of nuclear power will lead to nuclear fuel reprocessing,which the prime task is to dispose HLLW.And aqueous reprocessing is the most popular method amoung nuclear fuel reprocessing.The application of extraction agent CMPO in lanthanum/actinide separation is introduced continuously,and finally the ionic liquids are elaborated,including the basic concepts,properties and recent progress in the study of spent fuel reprocessing based on them.of is introduced,and this kind of solvents in the spent fuel postprocessing research achievements in related fields.Based on the background and literature researches,the purposes and contents of the thesis are presented.In chapter 2,the extraction behavior of Eu³⁺from nitric acid solution with CMPO dissolved in hydrophobic ionic liquid[C_nmim][NTf₂]（n=2,6,8,12）was elaborated.The effect of acidity,contact time,on the extraction performance of CMPO/[C_nmim][NTf₂]system were investigated at first.Under the optimal experimental conditions,the extraction mechanism and thermodynamics of the extraction reaction were studied.In addition,we discussed the influence of the[C_nmim][NTf₂]structure changing on the extraction behavior and the cause of the third phase formation in the extracting process.The advantages and disadvantages between the new extraction system and the traditional organic solvent extraction system were evaluated under the same experimental conditions.In chapter 3,the extraction behavior of UO₂²⁺from nitric acid solution with CMPO dissolved in hydrophobic ionic liquid[C_nmim][NTf₂]（n=2,6,8,12）was elaborated.The effect of acidity,contact time,on the extraction performance of CMPO/[C_nmim][NTf₂]systems were investigated at first.Then,for UO₂²⁺,the extraction mechanism and thermodynamics of the extraction reaction were studied in each system.In the same experimental conditions,the experimental results using traditional organic solvent extraction system was compared with that of CMPO/[C_nmim][NTf₂]systems.In this process,we discussed the influence of the alkyl chain length of[C_nmim][NTf₂]on the extraction behavior of UO₂²⁺.In chapter 4,based on the previous experimental results,the relatively superior CMPO/[C₂mim][NTf₂]system in extraction performance was selected,and the radiation stability of this new extraction system was preliminarily studied.The results show that the new system has better radiation protection performance compared with the traditional organic system.In order to inspect the application effect of the new system,we used it to extract and separate Eu³⁺and UO₂²⁺in the simulated high level waste liquid,and found that the extraction performance of the new system was better than that of the traditional organic system whether at high acidity or low acidity.In chapter 5,the conclusions and innovations of the thesis are summarized.Finally,the prospects in future work are proposed.