Understanding the Chemistry of Acetohydroxamic Acid (AHA) in the Presence of Fe(III) in the Context of an Advanced PUREX Process

Jones, Suzanne and Boxall, Colin (2024) Understanding the Chemistry of Acetohydroxamic Acid (AHA) in the Presence of Fe(III) in the Context of an Advanced PUREX Process. PhD thesis, Lancaster University.

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Abstract

Since the 1950s, the majority of operating commercial nuclear fuel reprocessing plants, including those in the UK, France, Russia and Japan, have used the well-proven hydrometallurgical PUREX (plutonium uranium extraction) process, or a variant PUREXbased process to chemically separate uranium (U) and plutonium (Pu) from used nuclear fuel. However, enhancements to PUREX are needed for future fuel cycles to improve its proliferation resistance, its capability to handle higher burnup fuels and to minimize its waste arisings. A key objective within the development of an Advanced PUREX process is the effective control of the actinides U, neptunium (Np) and Pu within a single cycle flowsheet. Simple hydroxamic acids such as acetohydroxamic acid (AHA) have the ability to strip Pu(IV) and Np(IV) from tri-butyl phosphate into nitric acid and have thus been identified as suitable reagents for this purpose. Utilising this in an Advanced PUREX process will ultimately allow for the generation of a co-processed Pu/Np product and a high purity U product, addressing some of the shortcomings of traditional PUREX. There are however a few key knowledge gaps that must be addressed before AHA can be implemented in such a process. Firstly, it is known that simple hydroxamic acids hydrolyse to hydroxylamine (NH2OH) and the parent carboxylic acid in acidic media, the former product being known to react autocatalytically / explosively with nitric acid which is ubiquitous in reprocessing flowsheets. Whether the reaction mechanism or product distribution changes when the AHA is complexed to a metal ion is unclear. Additionally, observations that Pu(IV) is reduced to Pu(III) during complex hydrolysis have opened up the possibility of their use as replacements for U(IV)/N2H4 or NH2OH in advanced PUREX processes, but whether the reducing agent is the hydroxamate itself, or NH2OH, is still in question. To answer these questions, Fe(III) has been used as a non-active analogue to Pu(IV) and Np(IV), as it exhibits analogous complexation with AHA and whilst thermodynamically possible, redox chemistry mechanistically analogous to that of Pu(IV) is thought to be kinetically hindered at high hydrogen ion concentrations to the point where it can be ignored on the timescales of AHA hydrolysis. However, initial studies by Raman spectroscopy showed identical AHA hydrolysis products in the absence and presence of initial Fe(III), but with differing final yields. Further quantification techniques were then explored including a titrimetric method for hydroxylamine, UV-Vis spectroscopy for nitrous acid and Fe(II), and ion chromatography (IC) for multiple species, all of which suggested redox chemistry akin to Pu(IV). A library of data to describe these systems has been gathered utilising a single column ion chromatography system to measure a number of key ions over time in nitric acid solutions of varying temperatures and initial Fe(III) and AHA concentrations. These key species include the acetate ion (CH3COO- ) and protonated hydroxylamine (NH3OH+) from the hydrolysis of AHA, and the reduced form of the metal ion, Fe(II), which has been not previously been seen during hydrolysis of the Fe(III)-AHA complex. Our analysis therefore shows that the current definition of Fe(III) as a non-oxidizing metal ion with regards to AHA needs revising. Using CH3COOingrowth as a direct measure of AHA loss and assuming redox chemistry of Fe(III) mechanistically analogous to Pu(IV), these studies have additionally been combined with kinetic modelling in the software platform gPROMS (General PROcess Modelling System), and have thus provided key insights into the nature of the reducing agent in these systems.