Understanding the Chemistry of Acetohydroxamic Acid (AHA) in the Presence of Fe(III) in the Context of an Advanced PUREX Process
Creators: name: Jones, Suzanne
Publisher: name: Lancaster University
Year: 2024

description: 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.