Using N,N-dialkylamides for neptunium purification from other actinides for space applications

Abstract

ABSTRACT Tributylphosphate (TBP) has been reliably used in the solvent extraction of actinides for decades. One current application of TBP is the purification of plutonium-238 (238Pu), a heat source for radioisotope thermoelectric generators. Part of the 238Pu production process includes irradiating neptunium-237 (237Np) previously purified using di-(2-ethylhexyl)phosphoric acid (HDEHP). However, radiolytic decomposition of TBP and HDEHP forms several problematic phosphate-based degradation products. These non-incinerable byproducts contaminate the final 238Pu product and complicate the recycle of 237Np. In contrast, alternative extractants, such as the N,N-diakylamides, have incinerable byproducts. Nonbranched N,N-dialkylamides (N,N-dihexylhexanamide (DHHA) and N,N-dihexyloctanamide (DHOA)) and branched N,N-dialkylamides (N,N-di(2-ethylhexyl)butanamide (DEHBA) and N,N-di(2-ethylhexyl)isobutanamide (DEHiBA)) were considered for Np/Pu and Np/Th separations to replace TBP or HDEHP in 238Pu production. To understand the extraction potential of N,N-dialkylamides, all four N,N-dialkylamides were used to extract actinides (Th, U, Np, Pu, Am) across a range of solution-accessible oxidation states. Those results were then compared based on the actinides’ oxidation state (III - VI) to illustrate some of the similarities and differences in extraction behavior.