Essential role of heterocyclic structure of N-alkylated 2-pyrrolidone derivatives for recycling uranium from spent nuclear fuels

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. (author)