JRP CALL information
Supported By

European Commission

Short description of the work

Under HLW repository relevant conditions (i.e., high temperature, anoxic), the corrosion of steel canister will lead to the formation of iron (hydr)oxides such as green rust and magnetite. Such neoformed phases may represent a sink for RN released upon waste matrix corrosion, delaying their migration out of the repository. RN retention may occur either by surface adsorption and/or by structural incorporation. Unfortunately, only limited information is available on the retention of long-lived and radiotoxic actinides by such phases. This project focuses on the trivalent actinides retention by green rust and magnetite.

 

 

Short description of the work

Four Pu-bearing borosilicate glasses have been synthesized in the laboratories of the JRC-ITU. Inactive glass frit, representing the VEK glass composition and already prepared in INE laboratories, was mixed in ITU with different amounts of PuO2, obtaining glass samples with PuO2 concentrations from 0.2 wt% to 1.5 wt%. Melting and annealing in a high-temperature furnace at 1200 °C (600 °C) for 3 h (3 h) was performed, as well as all the other manipulations, in a N2-inertgas glovebox. Removal of the cold glass material from the Pt/Rh took place without applying any mechanical force. The samples have been partly crushed for further investigations. Powder will be analysed by X-ray powder diffraction (XRD) to verify amorphous nature of the samples. From each of the four samples, two fragments have been embedded in resin for cutting and polishing, respectively. One set of fragments will be investigated at INE-BL by high energy resolution X-ray absorption near edge spectroscopy (HR-XANES), the other set will be removed from the resin and prepared for leaching experiments in reducing conditions. As-prepared glass fragments will be used for Raman, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analysis.

 

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 is a poly–hydroxocarboxylic acid, HGLU 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 Ca2+has been shown to facilitate complexation through the formation of ternary species, i.e., that comprise of An(III), Ca2+ and GLU−. Despite the relevance of Mg2+ in several repository concepts for radioactive waste disposal, no studies assessing the role of Mg2+ 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, Mg2+–An(III)–gluconate 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
The aim of this study is to investigate UxZr(1-x)SiO4 and ThxZr(1-x)SiO4 solid solutions regarding their mixing behavior. Complete solid solutions between USiO4 and ThSiO4 have been shown to be possible [1-3] and lattice parameters in the UxTh(1-x)SiO4 system exhibit behaviour according to Vegard's Law. ZrxHf(1-x)SiO4 solid solutions have also been described in detail recently [4] and exhibit a negative excess volume of mixing, indicator for non-ideal mixing behaviour. To the best of our knowledge neither has the successful synthesis of UxZr(1-x)SiO4 solid solutions nor that of ThxZr(1-x)SiO4 been reported so far. First principle calculations by Ferriss et al. [5] suggest that there is a miscibility gap between USiO4 and ZrSiO4 and ThSiO4 and ZrSiO4 (calculated for 500 K and 1000 K). For the HfSiO4 and ZrSiO4 system their paper proposes near ideal mixing with a weak tendency for phase separation. The latter was later confirmed by Cota et al [4].

 

Short description of the work
In the context of developing U-Pu-O type nuclear fuels with high contents of plutonium for the IVth generation nuclear reactors, much work is needed to provide the necessary accurate experimental data on the aforementioned ternary. The described project is devoted to developing a better understanding of the phases present in the UO2-PuO2-U3O8 region
We studied samples with three Pu contents (Pu/U+Pu): 14, 24 and 35 wt%. Prior to XAS experiments performed at the ESRF ROBL line in-situ high temperature X-ray diffraction has been performed at the LEFCA facility in CEA Cadarache, to achieve a certain oxygen to metal ratio (O/M). This was done by heating a stoichiometric powdered samples in air, and then reducing it at an elevated temperature using a solid state electrolyte oxygen pump, which allows for precise control of the oxygen partial pressure within the sample chamber. This way we were able to prepare samples with three different O/M ratios – O/M=2.00, O/M~2.25 and O/M corresponding to the maximum oxidation in air at 1300°C (max air). The O/M of ~2.25 was achieved by reducing the samples from the highest O/M, to a point at which all the high-oxygen hexagonal phase disappeared, leaving only a cubic M4O9 type phase, which does not exhibit strong departures from stoichiometry.

 

 

Short description of the work
The determination of the response of the redox sensitive radionuclides to the evolution of the system containing large amounts of iron and concrete is fundamental to the performance assessment of nuclear waste management systems. Redox transitions in the hyperalkaline pH range of for the radionuclides chosen in this study (Pu(III) ® Pu(IV) at pH+pe ~ –1, U(IV) ® U(VI) at pH+pe ~ +2, Tc(IV) ® Tc(VII) at pH+pe ~ +6) strategically cover the field of stability of the system Fe(0)/Fe(II)/Fe3O4, and thus are expected to provide a key understanding on the interaction of the waste with Fe corrosion products under repository conditions.

 

Short description of the work
The project undertaken was a continuation of a previous grant to visit JRC-ITU in Actinet 6. The ultimate goal is the better understanding of the interactions between actinyl cations and with other metal ions in nuclear waste by synthesising model systems held together by cation-cation interactions (CCI). In the short term, studying the formation and stability of actinide metal-oxo bonds in discrete molecules also offers the fascinating potential to make new single molecule magnets with enhanced properties. We have previously developed a readily-made Schiff-base polypyrrolic ligand (H4L; L= Pacman) which allows the stabilisation of the uranyl(V) monocation and its subsequent derivatisation by oxo-metalation.

The work carried out at JRC-ITU was the synthesis of low-valent uranium and neptunium starting materials for reductive metalation of the uranyl dication in the Pacman ligand [(UO2)(H2L)]. Prior to the visit, the mono-oxo functionalised potassium uranyl(V) Pacman [(K)(UO2)(H2L)] had been synthesised at the University of Edinburgh to be used as a starting material for subsequent salt metathesis reactions.

 
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