乏燃料后处理液-液萃取中第三相微观结构研究进展

乏燃料后处理液-液萃取中有机相容易在一定条件下发生相分离而形成上层轻相和下层重相,其中下层重相通常称之为"第三相"。第三相的形成不利于萃取工艺的连续稳定运行,而且Pu(Ⅳ)溶剂化物在第三相的聚集还有可能带来核临界的重大风险。自核工业建立以来,人们就针对第三相开展了大量的研究,发现操作温度、酸种类/浓度、金属离子种类/浓度、萃取剂和稀释剂种类/浓度等因素均能影响第三相的形成。通常认为,金属配合物在有机稀释剂中溶解度不足是造成第三相的主要原因,但形成第三相的微观分子机制仍然不清楚。20世纪70年代以来,胶体化学的概念和方法被引入到第三相的研究中,并逐渐证实萃取有机相中形成反胶束或微乳液可能是第三相生成的主要原因,这种以两亲分子自组装为基础的理论较好地解释了第三相形成的相分离过程和分子机制,并成为萃取领域的研究热点之一。本文主要从胶体化学的观点出发,综述了近期后处理萃取体系中第三相形成微观机制的研究进展。

Research Progress on the Microstructure of the Third Phase During the Liquid-Liquid Extraction of Spent Nuclear Fuel Reprocessing

In liquid-liquid extraction of the spent nuclear fuel reprocessing,the organic phase will usually split into a heavy phase and a light phase,and the heavy phase is well known as the third phase.Formation of the third phase restricts the stable and continuous operation in the reprocessing of spent fuel,and can potentially cause safety and even criticality concerns due to the high Pu(Ⅳ)concentrations.Great efforts have been devoted to investigate the process of the third phase formation since the foundation of nuclear industry,and it is now known that a series of parameters such as the temperature,acid,extractant,diluent,etc.affect the third phase formation.The formation of the third phase is often believed to be the result of limited solubility of metal-ligand complexes in the organic diluent,however,the exact molecular interpretation remains largely unknown.Since 1970s,the concept of colloidal chemistry has been introduced to further illustrate the third phase,growing experimental evidences indicate that the formation of the reversed micelles or micro-emulsions are responsible for the phase transition process.And this model based on the amphiphilic molecule self-assembly can satisfactorily explain the third phase formation.The recent progress of the mechanistic studies of the third phase formation concerning the spent fuel reprocessing on the basis of colloid chemistry is reviewed.