Photo-induced changes in the Langmuir adsorption constants of metal oxide layers in self-cleaning cation sensors.

For the first time, we have used a metal oxide-coated quartz crystal microbalance (QCM) to measure Cs+ adsorption onto illuminated and un-illuminated mesoporous TiO2 (m-TiO2) films by microgravimetric means in-situ. In the simplest case, such experiments yield two parameters of interest: K, the Langmuir adsorption coefficient and mmax the maximum mass of adsorbate to form a complete monolayer at the m-TiO2-coated quartz crystal piezoelectric surface. Importantly, we have found that illumination of the m-TiO2 film with ultra bandgap light results in an increase in mmax i.e. illumination allows for greater adsorption of substrate to occur than in the dark. Our studies also show that under illumination, K also increases indicating a higher affinity for surface adsorption. The photoinduced change in mmax and K are thought to be due to an increase in surface bound titanol groups, thus increasing the number of available adsorption sites – and so providing evidence to support the notion of photoinduced adsorption processes in photocatalytic systems. These findings have implications for the development of a reversible adsorption based microgravimetric sensor for Cs+

The effect of acetohydroxamic acid on stainless steel corrosion in nitric acid

We present the first study of the effect of acetohydroxamic acid (AHA) on the corrosion behaviour of stainless steels. Particularly, studies have been performed using steels and physico-chemical conditions equivalent to those proposed for use in advanced nuclear reprocessing platforms. In these, AHA has been shown to have little effect on either steel passivation or reductive dissolution of both SS304L and SS316L. However, under transpassive dissolution conditions, AHA while in part electrochemically oxidised to acetic acid and nitroxyl/hydroxylamine, also complexes with Fe3 +, inhibiting secondary passivation and driving transpassive dissolution of both steels

The effect of SO3-Ph-BTBP on stainless steel corrosion in nitric acid

SO3-Ph-BTBP is a hydrophilic tetra-N-dentate ligand proposed for An(III)/Ln(III) separation by solvent extraction, and a candidate for use in future advanced reprocessing schemes such as GANEX and SANEX. We present the first study of the effect of SO3-Ph-BTBP on the corrosion behavior of stainless steels. Specifically, studies have been performed using steels and conditions equivalent to those found in relevant nuclear reprocessing flow sheets. SO3-Ph-BTBP has been shown to have little effect on either steel passivation or reductive dissolution. However, if driven cathodically into a region of hydrogen evolution at the electrode surface or conversely anodically into a region of transpassive dissolution, observed currents are reduced in the presence of SO3-Ph-BTBP, suggesting corrosion inhibition of the steel potentially through weak absorption of a SO3-Ph-BTBP layer at the metal-solution interface. The lack of any observed corrosion acceleration via complexation of Fe3+ is surprising and has been suggested to be due to the slow extraction kinetics of SO3-Ph-BTBP as a result of a requirement for a trans- to cis-conformational change before binding

A study of cerium extraction by TBP and TODGA using a rotating diffusion cell

For the study of the coupled interfacial-mass transfer kinetics of, inter alia, TBP, TODGA, CyMe4-BTBP and CyMe4-BTPhen based solvent extraction processes, a new rotating diffusion cell (RDC) apparatus has been established at Lancaster University. RDC studies of Ce(IV)/TBP and Ce(III)/TODGA extraction systems have been undertaken in order to improve the understanding of the chemical and kinetic processes involved. In each case, an interesting dependency on local hydrodynamics at the solution phase boundary with results suggesting that the organic extractant molecules migrate into the aqueous phase in order to capture Ce

Nitric acid reduction on 316L stainless steel under conditions representative of reprocessing

Steels comprise the largest class of metal-based materials encountered on nuclear sites. An understanding of how process steels interact with HNO3 in spent fuel treatment plant environments is required to enable informed decisions to be made about the design and effective application of different steel types within nuclear environments. Stainless steels readily passivate in nitric acid. However, increasing the oxidising power of the media can lead to passive film dissolution, resulting in rapid transpassive corrosion. The corrosion of steels in nitric acid is further complicated by the autocatalytic reduction of HNO3 to aqueous HNO2 which attacks the steel surface. This paper describes the effect of this behaviour on process steels in stagnant and/or flowing conditions using electrochemical and microgravimetric based methods. We describe linear sweep voltammetry studies performed on a 316L stainless steel rotating disk electrodes in varying concentrations of nitric acid and rotation speeds and provide a qualitative interpretation of the results and what these imply about the mechanism of HNO3 reduction. These findings will be used in follow on studies to determine the kinetic parameters of the nitric acid reduction reaction at the surface of 316L stainless steel

Ruthenium volatilisation from reprocessed spent nuclear fuel - Studying the baseline thermodynamics of Ru(III)

Ruthenium is a fission product possessed of two relatively long lived isotopes, Ru and Ru, both of which form part of the Highly Active (HA) waste raffinate during spent nuclear fuel reprocessing. During reprocessing ruthenium, which may be in the form of the RuNO+ complex, encounters temperatures conducive to volatilization. Due rutheium's high specific radioactivity it is important to understand the mechanism by which volatilisation occurs. Here we use combined CV, RDE and electrochemical microgravimetry experiments in a study of the the RuCl3 system for the first time. We do this in the interest of establishing NO-free Ru(III) baseline behaviour so as to support future studies on NO complexed ruthenium. Using wide aqueous solvent window carbon electrodes we have observed discrete oxidations to a solution phase Ru(III)-Ru(IV)-Ru(III) trimer, to solid RuO2 and volatile RuO4. We have also observed and assigned discrete reductions of solid RuO2 back to Ru(III) and Ru(III) reduction to ruthenium metal

The Decontamination of 137Cs Contaminated Concrete Using Electrokinetic Phenomena and Ionic Salt Washes in Nuclear Energy Contexts

his work describes the first known use of electrokinetic treatments and ionic salt washes to remediate concrete contaminated with 137Cs. A series of experiments were performed on concrete samples, contaminated with K+ and 137Cs, using a bespoke migration cell and an applied electric field (60 V potential gradient and current limit of 35 mA). Additionally, two samples were treated with an ionic salt wash (≤ 400 mol m-3 of KCl) alongside the electrokinetic treatment. The results show that the combined treatment produces removal efficiencies three times higher (>60%) than the electrokinetic treatment alone and that the decontamination efficiency appears to be proportional to the initial degree of contamination. Furthermore, the decontamination efficiencies are equivalent to previous electrokinetic studies that utilised hazardous chemical enhancement agents demonstrating the potential of the technique for use on nuclear licensed site. The results highlight the relationship between the initial contamination concentration within the concrete and achievable removal efficiency of electrokinetic treatment and other treatments. This information would be useful when selecting the most appropriate decontamination techniques for particular contamination scenarios

Radiometric detection of non-radioactive caesium flux using displaced naturally abundant potassium

We report on a method that allows for the radiometric detection of non-radioactive caesium by the measurement of potassium ions displaced from an ion exchange barrier. Electrokinetic transport of K+ and Cs+ through concrete samples was measured using a bespoke scintillation detector to monitor electrolyte concentrations. Results show experimental ionic flux and diffusion parameters of non-active caesium (~1 × 10−5 mol m−3) were consistent with those recorded for potassium and also with values reported in relevant literature. This work demonstrates a novel concept that can be applied to proof-of-concept studies that help develop the next generation of nuclear decommissioning technologies

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

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

The Effects of Nitric Acid on Extraction Properties of TODGA During Fission Product Management

In the context of management of the wastes arising from the reprocessing of spent nuclear fuel, studies have been conducted into the solvent extraction kinetics of lanthanide fission products by TODGA and the effects of solvent phase acidity on the effectiveness of this organic extractant molecule. In this study we observe an increase in organic phase viscosity increasing acidity and, potentially by consequence, that the rate of cerium extraction is summarily decreased