超临界流体萃取是一种新型的“绿色”分离技术，应用于乏核燃料后处理可以简化处理流程，减少二次废物的产生，因此，有良好的潜在应用前景。本论文开展了超临界流体萃取技术应用于乏核燃料后处理的基础研究。 采用15.5 mol/L的HNO3与TBP或TRPO，按初始HNO3/TBP（或TRPO）体积比为1/7-5/1制备了TBP-HNO3络合剂和TRPO-HNO3络合剂，研究了这两种络合剂的硝酸浓度、水含量、组成、密度、表面张力和粘度，并对两种络合剂进行了NMR分析。两种络合剂都存在反胶团，在CDCl3中有抗溶剂效应。 TBP-HNO3络合剂常压下易溶解萃取Nd2O3粉末，在溶解萃取过程中有水析出，导致萃取率不高，加入纯TBP可络合析出来的水，提高萃取率。TRPO-HNO3络合剂由于粘度大，常压下很难完全溶解萃取Nd2O3粉末。TBP-HNO3络合剂和TRPO-HNO3络合剂常压下都很难溶解萃取CeO2粉末。 TBP-HNO3络合剂在超临界CO2（SC-CO2）中能有效地溶解萃取Nd2O3粉末，在一定的实验条件下，静态溶解萃取的萃取率可达95%，动态溶解萃取的萃取率可达98%以上，溶解萃取时间、络合剂用量、络合剂的硝酸浓度、温度和压力对萃取率都有影响。TRPO-HNO3络合剂在SC-CO2中静态溶解Nd2O3粉末的萃取率低于20%。两种络合剂在SC-CO2中均不能溶解萃取CeO2粉末。建立了TBP-HNO3络合剂在SC-CO2中动态溶解萃取Nd2O3粉末过程的动力学模型，计算出表观反应速率常数和表观活化能。 在一定的实验条件下，TBP-HNO3络合剂在SC-CO2中静态溶解萃取高温气冷堆燃料UO2微球的萃取率小于7%；当UO2微球研磨成粉末后，静态溶解萃取的萃取率可达到92%；而当UO2微球在通氧气和600 ℃的条件下变成U3O8粉末后，静态溶解萃取的萃取率达98%以上。 通过电解还原-共沉淀法制备了(U, Ce）O2固溶体来模拟(U, Pu）O2。在60 ℃和20 MPa下，TBP-HNO3络合剂在SC-CO2中静态溶解萃取(U,Ce）O2固溶体时，U的萃取率为98.61%，Ce的萃取率为98.1%。
Supercritical fluid extraction is a kind of new “green” separation technology. Application of supercritical fluid extraction in the reprocessing of spent nuclear fuel can simplify the process and minimize the generation of secondary waste，and therefore has good potential prospect. In this thesis, the fundamental studies on application of supercritical fluid extraction in the reprocessing of spent nuclear fuel were carried out. TBP-HNO3 complex and TRPO-HNO3 complex were prepared with 15.5 mol/L HNO3 and pure TBP or TRPO with initial HNO3/TBP or TRPO volume ratio from 1/7 to 5/1. The properties of both complexes were studied, which included acid content, water content, chemical composition, density, viscosity, surface tension and NMR spectra. Reverse micelles and antisolvent effect of CDCl3 exist in both complexes Nd2O3 powder can be easily dissolved and extracted by TBP-HNO3 complex at atmospheric pressure. In the process of dissolution extraction, there is some water to separate out, resulting in low extraction efficiency. The extraction efficiency can be improved by adding pure TBP to complex the separated water. It is difficult to dissolve Nd2O3 powder with TRPO-HNO3 complex at atmospheric pressure because of its high viscosity. CeO2 can not be dissolved by both TBP-HNO3 complex and TRPO-HNO3 complex at atmospheric pressure. Nd2O3 powder can be effectively dissolved and extracted by TBP-HNO3 complex in supercritical CO2 (SC-CO2). Under some experimental conditions, the static extraction efficiency of Nd can reach 95%, and the dynamic extraction efficiency of Nd can reach more than 98%. Some parameters including time of dissolution extraction, volume of the complexant, acidity of the complexant, temperature and pressure have effects on the extraction efficiency. The static extraction efficiency of Nd from Nd2O3 with TRPO-HNO3 complex in SC-CO2 is less than 20%. CeO2 can be not dissolved and extracted by both TBP-HNO3 complex and TRPO-HNO3 complex in SC-CO2. A kinetic model is proposed to describe the dynamic dissolution extraction process of Nd2O3 powder with TBP-HNO3 complex in SC-CO2. The apparent reaction rate constants and the apparent activation energy were calculated. The static extraction efficiency for direct dissolution extraction of the UO2 kernel microsphere of high temperature gas-cooled reactor (HTGR) with TBP-HNO3 complex in SC-CO2 is less than 7% under some experimental conditions. After the UO2 kernel is ground into UO2 powder, the static extraction efficiency can reach 92%. After the UO2 kernel is treated under O2 flow and 600 ℃, the UO2 kernel turns into U3O8 powder, and the static extraction efficiency can reach more than 98%. The (U, Ce)O2 solid solution was prepared to simulate the (U, Pu)O2 solid solution by the electroreduction – coprecipitation method. The static extraction efficiency of (U, Ce)O2 solid solution with TBP-HNO3 complex in SC-CO2 under 60 ℃ and 20 MPa is 98.61% and 98.1% for U and Ce respectively.
