普雷克斯流程中Np的行为和控制

Behavior and Routing of Neptunium in the Reprocessing of Irradiated Nuclear Fuel

在后处理流程的众多化学分离中 ,Np的走向和控制是国际后处理界关注的重点研究课题。根据我国和其他国家的研究成果 ,综合分析了后处理中Np的走向和控制。Np在辐照燃料溶解液中的价态分布主要取决于溶解液中HNO3与HNO2 之比 ,通常情况下 ,溶解液中Np(Ⅴ )占主要份额 ;Np在共去污阶段的走向有两种可能 ,一是将Np控制为Np(Ⅴ ) ,使其进入高放废液 (1AW ) ,二是将Np控制为Np(Ⅵ ) ,则Np将与U ,Pu一起进入有机相 ,但两者至今为止都难以实现定量分离。Np在U/Pu分离阶段部分随U ,部分随Pu。在U纯化循环中 ,理想的方法是采用低酸加热氧化Np(Ⅳ )至Np(Ⅴ ) ,以实现与铀的有效分离。

The process chemistry and routing of Np in current reprocessing are described. Based on the present chemical flowsheet, neptunium splits over the whole process, part in 1 AW, part in uranium stream and small part in plutonium stream. The reason leading to such unpresentable situation is the complex chemistry of neptunium in HNO 3 solution, which makes it very difficult to stabilize valence of neptunium required in different steps of reprocessing. In the co decontamination step, it is hard to overcome catalytic oxidation of Np(Ⅴ)to Np(Ⅵ) by HNO 3. So, neptunium will split partially into 1 AW and partially into uranium and plutonium stream. Best way to solve such problem is to add scavenger of HNO 2 before starting extraction. In the U/Pu separation step, it is better to drive neptunium into plutonium stream in order to minimize contamination of uranium by neptunium. A new technology must be found to selectively reduce Np(Ⅵ) only to unextractable Np(Ⅴ), but not to Np(Ⅳ). The research program in this direction is going now, nevertheless, the possible reductin of Np(Ⅴ) to Np(Ⅳ) by Pu(Ⅲ) must be taken into consideration if Pu(Ⅳ)/Pu(Ⅲ) ratio is less than 10 -3 . The separation of neptunium from uranium product is main task in second uranium purification cycle. A new technology, that is the oxidation of Np(Ⅳ) to Np(Ⅴ) by salt free process, has been developed and applied in reprocessing process having high DF(Np) in uranium.