NEPTUNIUM SEPARATION THROUGH PHOTOCHEMICALLY INDUCED PROCESS

激光光化学过程在核燃料后处理中作为价态调整的替代手段提供了很大的希望。光化学诱导的Np氧化还原反应发生在硝酸溶液中，例如用于Purex过程。由于Np在有机相和水相之间的分布取决于Np的价态，因此可以利用激光光解来控制核燃料后处理工厂中的Np浓度分布。为了测定极低浓度下不能用吸收分光光度法测定的价态分数，研究了萃取色谱法的适用性。不同氧化态的Np离子可以在tbp色谱柱上用稀硝酸洗脱得到定性分离，价态的分数可以用α剂量法测定。该方法被应用于光解氧化还原比的测定。当氨基磺酸亚铁完全调整为Np（IV）的Np硝酸溶液暴露于KrF准分子激光辐射（249 nm）时，即使在起始溶液中存在还原剂，也会发生光化学氧化为Np(V)。实验结果表明，在适当的辐照时间后，平衡氧化比中存在光移。磺胺酸亚铁的初始量对光移影响不大，但会延迟光氧化的发生。反应温度对光移也没有影响。激光辐照后，残余的Np（VI）通过暗反应进一步氧化用KMnO4制备的Np（VI）在硝酸溶液中通过激光光解还原为Np(V)，即使初始溶液中存在强氧化剂。这个光化学反应最值得注意的方面是；1)还原比逐渐增大到受解条件支配的稳态值。2)较高的反应温度和较低的酸度可获得较高的还原比。3)当激光辐射关闭时，还原比突然变化为了获得光化学分离Np更实用的数据，进行了萃取实验。通过化学试剂预先将Np离子完全调整为NpnIV或Np（VI）。以30%TBP（70%正十二烷）为有机相，在激光照射前后进行溶剂萃取。实验结果表明，激光辐照使纳米粒子的分布系数显著降低。因此，激光光化学方法的Np价态调整可以与传统的Purex工艺相结合，而无需进行重大修改这些Np rebox反应的实验结果与Purex过程模拟代码“ADVANCED MIXSET”相结合。利用这一改进的计算机代码，激光照射在混合器-沉降器中对控制Np浓度分布和Purex过程中从U中去除Np具有重要的价值。少

1 Laser photochemical processes offer much promise as alternative means of valence state adjustment in nuclear fuel reprocessing The photochemically induced redox reactions of Np were shown to occur in nitric acid solution, such as used in the Purex process. Since Np distribution between organic and aqueous phases depends upon Np valence state, it is possible to control Np concentration profile in the nuclear fuel reprocessing plant using laser photolysis. Most practically important situations involve the adjustment of trace quantities of Np.2 In order to determine the valence state fraction under very low concentration where absorption spectrophotometry cannnot be used, the applicability of extraction chromatography has been studied. Np ions of different oxidation states can be qualitatively separated on chromatographic TBP-column by elution with dilute nitric acid and the fraction of valence state can be determined by alpha dosimetry. This method was used in the following experiments … More to determine the redox ratio by photolysis.3 when Np nitric acid solution adjusted completely to Np(IV) by ferrous sulfamate was exposed to KrF excimer laser radiation (249 nm), oxidation to Np(V) occurred photochemically even in the presence of a reducing reagent in the starting solutions. The experimental results show that the photo-shift exists in equilibrium oxidation ratio after adequate irradiation time. The initial amount of ferrous sulfamate had lettle effect upon the photo-shift but delayed initiation of photo-oxidation. Reaction temperature didn't affect the photo-hift either. The residual Np(VI) was oxidised further by dark reaction after the laser radiation bing off.4 Np(VI), which was prepared by KMnO4 was photochemically reduced to Np(V) in nitric acid solution by laser photolysis, even in the presence of a strong oxidising reagent in the initial solution. The most noticeable aspects of this photochemical reaction are; 1) Reduction ratio is increasing to a steady-state value governed by the solution condition. 2) Higher reaction temperature and lower acidity are preferred to obtain higher redution ratio. 3) Reduction ratio changes abruptly when laser radiation being off.5 Extraction experiments were accomplised, in order to obtain more practical data for photochemical separation of Np. Np ions were adjusted to NpnIV) or Np(VI) completely by means of chemical reagents in advance. Solvent extraction operations using 30%TBP(70%n-dodecane) as organic phase were conducted both before and after laser irradiation. The experimental results show that laser irradiation results in significant decrease of the distribution coefficients of Np. Therefore Np valence state adjustment by the laser photochemical method can be incorporated with the conventional Purex process without majour modification.6 These experimental results of Np rebox reactions were combined with the Purex process simulation code 'ADVANCED MIXSET'. Using this reformed computer code, laser irradiation in a mixser-settler has been shown of great value to control Np concentration profiles and to decontaminate Np from U in the Purex process. Less