Essential Role of Heterocyclic Structure of N-Alkylated 2-Pyrrolidone Derivatives for Recycling Uranium from Spent Nuclear Fuels

• Published in: Bulletin of the Chemical Society of Japan 2020-07, Vol.93 (7), p.846-853
• Main Authors: Inoue, Tsubasa, Kazama, Hiroyuki, Tsushima, Satoru, Takao, Koichiro
• Format: Article
• Language: English
• Subjects: Alkylation; Amides; Carbonyls; Corrosion resistance; Derivatives; Hydrolysis; Nuclear fuel reprocessing; Nuclear fuels; Recycling; Spent nuclear fuels; Stability; Uranium; Uranium dioxide
• ISSN: 0009-2673; 1348-0634
• DOI: 10.1246/bcsj.20200061

In a simple and versatile reprocessing method for recycling U and Pu from spent nuclear fuels, cyclic amides like N-alkylated 2-pyrrolidone derivatives (NRPs) are exclusively employed. However, there has been no convincing rational to explain why such a heterocyclic structure is required. To answer this question, we employed N-cyclohexyl-2-pyrrolidone (NCP) and N-cyclohexylformamide (NCF) as cyclic and acyclic monodentate amides, and focused on the following 3 topics in this study; (1) structural chemistry of their uranyl dinitrato complexes, (2) precipitation behavior of UO22+ from HNO3(aq) by using these amides, and (3) their chemical stability in HNO3(aq) simulating the reprocessing conditions for spent nuclear fuels. Fundamental coordination chemistry of UO2(NO3)2(L)2 (L = NCP, NCF) was found to be common to both L, regardless of the presence or absence of the pyrrolidone ring. Furthermore, both L exhibit comparable capability in precipitation of UO22+ from HNO3(aq). The most critical difference between NCP and NCF was found in their chemical stability in HNO3(aq), where NCF was gradually decomposed through acid-catalyzed hydrolysis, while NCP remained intact for at least 4 h. In conclusion, the pyrrolidone ring of NRPs plays an important role to sterically protect the carbonyl C from nucleophilic hydrolysis which initiates the amide C(=O)–N bond cleavage.