JRP CALL information
Supported By

European Commission

Short description of the work
Title of the proposed JRP: “Thermodynamic study on the complexation of gluconate with An(III) in diluted to concentrated MgCl2 solutions”

Gluconic acid (HGLU) is a poly–hydroxocarboxylic acid expected in repositories for low and intermediate–level radioactive waste as component of cementitious materials. Radionuclide solubility and sorption in cementitious and saline systems can be affected by the presence of this organic ligand. Formation of stable An(III)-GLU complexes has been reported in the literature. The presence of Ca has been shown to facilitate complexation through the formation of ternary Ca-An(III)-GLU species. Despite the relevance of Mg in several repository concepts for radioactive waste disposal, no studies assessing the role of Mg in An(III)-GLU complexation have been conducted so far. The development of complete and correct chemical, thermodynamic and activity models for the ternary system, Mg–An(III)–GLU is thus required to properly assess the impact of this ligand on the mobilization of An(III) under repository-relevant conditions.

 

Short description of the work
Studies on metal-ligand interactions allow us to determine chemical affinity of actinides to the ligand studied as we move across the 5f-series. Fundamental information on the behaviour of actinides in solvent extraction processes can be gathered from the studies on actinide speciation in liquid media, on the thermodynamic and structural properties of their complexes with organic ligands, etc. New hydrophilic sulphonated BTP ligands were synthesized and proven to selectively complex trivalent actinides in HNO3 solutions The objective of this proposal was to characterize the complexes of U(IV,VI) and Pu(IV) via NMR and ESI‐MS and to determine their thermodynamic properties via microcalorimetry. Unfortunately, some difficulties with determination of 1:3 complexes caused that it was decided to perform some additional extraction-liquid-liquid experiments with Pu(IV). In the present work, we studied the distribution ratio of Pu(IV) in the two-phase systems: TODGA (in kerosene) / SO3-Ph-BTP (0–30 mM) in aqueous HNO3 (0.5-3.0 M). Based on the distribution ratios of the metal ions we determined the apparent stability constants of their SO3-Ph-BTP complexes in the aqueous phase.

 

Short description of the work
Ionic liquids have very interesting properties for a potential application in solvent extraction for the separation of actinides from used nuclear fuel, e.g. the low volatility and high flash point. However, the extraction mechanism in ionic liquids differs from the extraction mechanism in conventional diluents and furthermore is even depending on the initial nitric acid concentration.
The extractant CyMe4BTPhen is currently under investigation in European collaborative projects and it was shown to be highly selective for trivalent actinides over lanthanides in conventional diluents. Therefore, the extraction of Am(III) was also studied in ionic liquids. A "boomerang" shaped extraction behavior was found as a function of the nitric acid concentration, implying a competition of different extraction mechanisms at the different nitric acid concentrations.
Samples for EXAFS measurements were prepared by extracting Am(III) from nitric acid solutions of different acidity into an ionic liquid phase containing CyMe4BTPhen. The ionic liquid phase was then separated and measured.

 

Short description of the work
The radiolytic behaviour of two innovative hydrophilic complexing agents, PyTri-Diol and PyTri-Tetraol, to be used within an i-SANEX/GANEX process for the selective stripping of minor actinides, was studied after γ-irradiation up to 200kGy. The main objects of the research activity have been:

  • Evaluation of the stripping solvent efficiency.

Batch stripping tests were performed at 25°C on organic phases loaded with 152Eu(III), 239Pu(IV), 241Am(III), 244Cm(III) and 2,4g/L of Lanthanides; the distribution coefficients and the separation factors were determinated, [H+] was monitored at each step.

  • Evaluation of the stripping solvent degradation.

H,C-NMR and 2D-NMR spectroscopy and UV-VIS spectrophotometry analyses were performed on fresh and irradiated stripping solvents to quantify the ligand consumption. The main by-products were detected by ESI-MS and some identification hypotheses have been proposed.

  • Evaluation of the stability constants.

The stability constants of complexes between PyTri-Diol (fresh and irradiated at 200kGy) and trivalent cations (Am and Eu) were estimated by means of UV-VIS spectrophotometry. The protonation of the pyridine nitrogen of PyTri-Diol was confirmed by N-NMR and the protonation constant was estimated from the shifts observed in the H,C NMR spectra.

 

Short description of the work
This JRP's aim is a fundamental study of the interaction of Am with stainless steels encountered in repository and waste separations, treatment and storage. The specific objectives are:
To determine whether and in what form Am incorporates into passivating layers on Cr/Ni rich low carbon steels in acid and neutral media (relevant to waste separation processes and repository).
To determine whether and in what form Am incorporates into corrosion products formed in carbon steels under alkaline conditions (of relevance to waste storage) and on low carbon steels under acid conditions (of relevance to HAL evaporators)
To determine whether Am affects steel corrosion, either as a corrosion accelerator or by changing the corrosion product (as per U uptake in iron oxyhydroxides)
A large amount of cyclic voltammograms have been generated in the various nitric acid concentrations using the glassy carbon and various sized gold microelectrodes. Active and inactive experiments have been carried to isolate carbon/gold electrochemistry from Am electrochemistry. Background subtract experiments were carried out in order to deconvulate Am peaks. A 8 M perchloric acid solution has been made up to be used with the glassy carbon electrode to widen the potential window and separate out any oxygen evolution that has been seen in nitric acid media.

 

Short description of the work
This JRP's aim is a fundamental study of the interaction of Am with stainless steels encountered in repository and waste separations, treatment and storage. The specific objectives are:
To determine whether and in what form Am incorporates into passivating layers on Cr/Ni rich low carbon steels in acid and neutral media (relevant to waste separation processes and repository).
To determine whether and in what form Am incorporates into corrosion products formed in carbon steels under alkaline conditions (of relevance to waste storage) and on low carbon steels under acid conditions (of relevance to HAL evaporators)
To determine whether Am affects steel corrosion, either as a corrosion accelerator or by changing the corrosion product (as per U uptake in iron oxyhydroxides)
Various cyclic voltammetries and open circuit potential measurements were carried out using the glassy carbon and various sized gold microelectrdoes.
The following peak assignments in 1 M HNO3 with glassy carbon were made:
Oxidation wave observed during forward sweep assigned to conversion of Am(III) to Am(VI)
Reductive peak at 1.2 V has been assigned to carbon electrochemistry
Am reduction peak observed at ~ 0.7 V
Am reduction peak observed at ~ 0.7 V - corresponding to Am(V) to Am(IV) reduction. Randle-Sevcik analysis reveals this to be the reduction of a solution phase species. It also reveals that this peak corresponds to the reduction of Am(V) derived from the stoichiometric oxidation of Am(III).

 

Short description of the work
This JRP's aim is a fundamental study of the interaction of Am with stainless steels encountered in repository and waste separations, treatment and storage. The specific objectives are:
To determine whether and in what form Am incorporates into passivating layers on Cr/Ni rich low carbon steels in acid and neutral media (relevant to waste separation processes and repository).
To determine whether and in what form Am incorporates into corrosion products formed in carbon steels under alkaline conditions (of relevance to waste storage) and on low carbon steels under acid conditions (of relevance to HAL evaporators)
To determine whether Am affects steel corrosion, either as a corrosion accelerator or by changing the corrosion product (as per U uptake in iron oxyhydroxides)
A 150 ppm (concentration of PUREX raffinates) stock solution of Am was prepared to study Am(III) electrochemistry in acid, neutral and basic media on inert noble metal microelectrodes to establish baseline behaviour. The following electrolytes were made up containing 6.2 x 10-4 M Am:
1, 3 & 5 M HNO3
0.1 M KNO3
0.01 mM KCl
Modified simplified groundwater (0.01 M NaCl & 0.002 M NaHCO3)
'Pondwater' – representative of interim storage ponds, Na2SO4 electrolyte adjusted to pH 11 with NaOH
Electrodes used were: 10, 50 100, 250 & 500 um Gold microelectrodes, Graphene ring nanoelectrode, glassy carbon electrode and 100 um platinum microelectrode.
It was found that only the glassy carbon and various gold microelectrodes yielded useful results, therefore only these electrodes have been used in subsequent experiments. It was not possible to use the pondwater in these experiments as the americium precipitated out of the solution due to the basic nature of the electrolyte.

 
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