超临界CO2中锕系镧系元素的络合行为及其分离

Reprocessing of spent nuclear fuel using supercritical fluid extraction technology has the advantages of minimizing secondary highly radioactive liquid waste and simplifying the process. In recent years, great progress has been made in this area, and it provides a promising option for future reprocessing technologies. However, lanthanides would also be extracted together with actinides during supercritical fluid extraction due to the low water feature of the system, this is different from traditional solvent extraction, in which, lanthanides in solution are preferentially distributed in the aqueous phase rather than complexed with TBP. Thus, it is a key scientific issue to separate actinide and lanthanide in supercritical fluid. It is a good start for us to study the complexation behavior of uranyl nitrate, neodymium nitrate, praseodymium nitrate and ammonium ceric nitrate mixture with tri-n-butyl phosphate (TBP) and the corresponding separation in supercritical fluid CO2, which is still not reported yet. In this project, the molecular interaction of nitrates with TBP and the competitive complexation reaction will be studied by using infrared spectroscopy in ambient temperature. In particular, the ultraviolet-visible spectra of the metal-TBP complex in supercritical phase will be collected by high-pressure optical cell, under variable of temperature, pressure, TBP dosage and chemical reagent added in the system. The UV-vis spectra will be analyzed by two dimensional correlation spectroscopy to investigate the influence of various factors on complexation behavior. In addition, linear least squares method will be used to quantify the content of various components in supercritical phase. This work will reveal the complexation reaction mechanism in actinides and lanthanides coexistence system, to find out a suitable method to fulfill the separation of actinide and lanthanide in supercritical fluid, thus provide a research foundation for application of supercritical fluid extraction in reprocessing of spent nuclear fuel.

超临界流体萃取用于乏燃料后处理具有减少放射性废液、简化流程的优点，发展前景良好。但与传统水法后处理不同，超临界流体萃取在实现对铀、钚萃取的同时，因体系的低水特性镧系元素也会一起被萃取，无法满足所需的净化要求。因此锕、镧系元素的分离是推进该技术工业化应用亟待研究的关键问题。研究锕、镧系元素络合行为的差异将有助于找到合适的分离方法，尚未见报道。为此本项目研究超临界CO2中铀、钕、镨和铈的固体硝酸盐与磷酸三丁酯(TBP)的络合行为及分离。采用常压红外光谱法研究硝酸盐与TBP的竞争络合作用。采用高压光学池采集不同温度、压力、TBP用量以及外加试剂下金属络合物的紫外光谱数据，并用二维相关光谱定性分析各因素对络合行为的影响规律，最小二乘法定量分析超临界相中各元素的含量。本研究将揭示锕、镧系元素共存时络合的差异性及影响规律，以找到有效的分离方法，进而为推动超临界流体萃取在后处理中的应用奠定理论基础。

超临界CO2萃取用于核燃料和核废物处理具有减少二次放射性废液的优点，主要有核国家都在持续开展相关研究。目前从废液、固体基质、固体氧化物中提取镧系锕系研究已经取得满意的结果。然而与传统的液液萃取体系不同，超临界CO2中镧系和锕系与络合剂的络合行为不尽相同，要实现其分离难度很大。目前国际上对多组分金属离子混合体系在超临界CO2中的分离研究不足，为此本课题开展了硝酸铀酰、硝酸镧系与络合剂TBP和TiAP的竞争络合及其分离研究。通过紫外可见、荧光、EXAFS光谱等方法研究了常压正己烷中络合物的结构信息，以及络合剂用量和外加试剂对镧系竞争络合的影响，结果表明重镧系元素与络合剂的络合能力大于轻镧系元素。建立了含在线光谱测量的超临界CO2络合反应系统，测定了络合物在超临界CO2中的吸光系数等数据。系统研究了超临界CO2中络合剂用量、温度、压力、外加试剂对竞争络合的影响。结果表明络合剂用量和外加试剂都可以显著影响竞争络合，并且竞争络合能力顺序为U(VI) > Ho(III) > Nd(III) ≈ Pr(III)。发现与单一硝酸盐体系相比，混合体系中超临界CO2对络合物的负载量更大，这是因为自由络合剂在体系中起双面作用，一方面促进络合反应的进行，提高络合物的浓度，另一方面又因其自身在超临界CO2中的溶解，会限制络合物的溶解度。另一重要结果是温度和压力变化会引起不同络合物间的配体交换反应，使一种络合物浓度升高，另一种络物浓度降低，从而可以通过调节温度和压力提高两种物质的分离度。本项目的开展在研究方法上丰富了超临界CO2中镧系锕系萃取化学的研究手段，实验发现也拓展和加深了对组分体系络合竞争行为的认识，为未来实现超临界CO2应用于核废物处理提供了理论基础。