JRP CALL information
Supported By

European Commission

Short description of the work

This research project focuses on the sorption of the redox sensitive element neptunium by montmorillonite under geochemical conditions relevant for radioactive waste repositories. In the near-field, corrosion of steel containers produces large amounts of Fe(II), which may influence Np sorption by the engineered barrier system. The sorption of the reduced Np(IV) in the absence Fe(II) will be studied by batch sorption experiments to verify the chemical analogy with Th(IV) and to provide Kd values for the performance assessment. For the oxidized Np(V), we will measure sorption edges and isotherms in the presence and absence of dissolved Fe(II). These experiments aim at elucidating the sorption mechanism in the presence of dissolved Fe(II), i.e. competitive sorption as well as surface mediated redox reactions between sorbed Fe(II) and Np(V). The 2SPNE SC/CE sorption model is complemented with the derived surface complexation reactions and associated thermodynamic constants for Np(IV/V) and with redox reactions. X-ray absorption spectroscopy (XAS) is carried out on the Np(V)-Fe(II)-montmorillonite system to derive structural information on the surface complexes. The results provide hitherto neglected data for the performance system of anoxic radioactive waste repositories.

 

Short description of the work
Heavy ion irradiation of U-Mo/Al fuel simulates in-pile irradiation; the interdiffusion layer (IDL) found after both kind of irradiations is similar in composition and microstructure. This similarity implies that the major contribution to the diffusion reaction comes from collisions of fission fragments. Based on this finding, studying the heavy ion induced diffusion behaviour will lead us to the diffusion mechanism of U-Mo/Al fuel. Atomic mixing at interfaces after irradiation is observed. This atomic mixing implies either the formation of solid solutions or of irradiation-induced new phases at the interfaces. Several parameters have been used in this study to provide overall understandings: U-Mo thickness, different protection layers and irradiation temperature. Different thickness of U-Mo layer induces different ion beam energy at interfaces. Application of different irradiation temperatures gives different amount of thermal energy contribution to the diffusion reaction. Up to now we have investigated the ion beam mixing effect (IBM) induced by heavy ion irradiation by means of SEM, EDX and RBS. Further understanding of the diffusion mechanism of U-Mo/Al fuels is currently limited by the resolution of these methods. First TEM measurements have indicated that “better” information can be gained by TEM. TEM measurements are able to clarify the compositions of intermixing. Selected area diffraction pattern (SADP) provide precise phase identifications and crystal structures. For instance it is important to know whether the IDL is amorphous or crystalline. Nano-EDX line-scan will be able to provide the elemental distributions across the interfaces. This line-scan can clarify the possible ion beam mixing model which cannot be achieved with limited resolution. The information at interfaces is important to evaluate the stability of the diffusion barrier and the adequacy.

 

Short description of the work
Storage of Spent Nuclear Fuel (SNF) in an underground repository is the favoured method for long term stewardship in the EU and US. Oxidation of UO2 in the moist oxidising environment has been shown to form uranyl phase transitions that have been identified on the surface of UO2, SNF and in the oxidative weathering of pitchblende in nature. Thus an understanding of how these phases can retard or enhance the migration of the highly radioactive trans-uranium elements is important for modelling of the environmental impact of repositories. Some work has shown that the structure of the mineral is important in neptunium migration, but the oxidation state of Np is not known with certainty. Np(V) and Np(VI) are soluble in water so of the most concern. In this project we have prepared a series of uranyl minerals in the presence of neptunyl(V) and used EXAFS and HR-XANES (High Energy Resolution X-ray Absorption Near Edge Spectroscopy) to confirm the oxidation state and give information on both the structure and electronic structure of the complexes formed. Thus, a co-precipitation reaction of Np(V), uranyl nitrate potassium carbonate and sodium carbonate affords Np incorporated grimselite {grimselite = K3Na[UO2(CO3)3]} that is rich in Np (by ICP-OES). Carbonate in the form of rock structures and the ions in solution are an important environmental ly. Whilst not fully analysed at this time, the EXAFS data at U and Np L3-edge suggests Np is in its +5 or +6 oxidation state (i.e. the presence of a Np=O bond) and the local environment around uranium is similar to the parent grimselite. Confirmation of the +5 oxidation state of Np was given by examination of the HR-XANES spectrum at the M edge, with comparison of known reference standards. Further analysis of this data will yield detailed information on the electronic structure of both the Np and U in the Np incorporated grimselite and U in grimselite. M-edge spectra directly probe the f-orbitals, compared to L-edge that give information on d-orbital populations. Further to the Np incorporation experiments a series of U minerals were also examined using M-edge HR-XANES. We have recently used L-edge HR-XANES to explore the differences in studtite [UO2(O2)(H2O)2].2H2O and meta-studtite [UO2(O2)(H2O)] and we have extended this to M-edge; in combination with XPS measurements also conducted at INE this will give a good description of the differences in these minerals. My lab has also synthesised bassetite, Fe[(UO2)2(PO4)2].7H2O and found that under the conditions of our experiment U(VI) is partially reduced to U(IV) – this has important implications as this can be used as a molecular model for the interactions of uranyl with Fe minerals such as goethite and haematite.

 

Short description of the work
In order to predict the helium behaviour under disposal condition, the mechanisms of helium diffusion, solubility and bubble nucleation in the glass have to be investigated. In this Talisman project, helium bubble nucleation and growth mechanisms were studied in 244Cm doped glass samples. The glass doping technique induces a homogeneous irradiation of all the glass volume, summing the two components of the alpha disintegration: recoil nuclei and alpha particles.
A SON68 glass doped with 1.2 wt.% 244Cm was prepared in 2001 in the Atalante facility of CEA Marcoule. After 14 years of storage corresponding to a dose of about 9.7×1018 α.g-1, isothermal annealing were performed at 693, 823, 923 K. SEM observations of glass surface (original and fractured) were performed after each thermal treatment. Moreover, after crushing, glass powders were also studied by TEM with a special modified FEI Tecnai G2 F20 XT. We analyzed the glass before and after annealing.
After storage at room temperature during 14 years, the glass microstructure is homogeneous, without any He bubbles. He bubbles could only be observed by SEM and TEM after annealing of the glass at temperature higher than the glass vitreous temperature. These helium bubble formation conditions are then far from the conditions of glass storage.

 

Short description of the work
The structure and bonding in hydrated and anhydrous actinyl (AnO22+; An = U, Np, U) nitrate complexes was studied by a combination of experimental methods (XRD, IR, Raman) on crystal compounds and theoretical methods (DFT) on small and large clusters and periodic crystals. Complete assignment of the experimental vibrational frequencies for [AnO2(NO3)2(H2O)2]×x(H2O) (x = 0, 1, 4) and M[UO2(NO3)3] (M = NH4, K, Rb, Cs) was achieved and force field studies of these compounds was undertaken. In all isotypic series the An-O2NO bond distance increases but bond strength down the actinide series. The most important contribution to this is the decrease of the actinide ionic radii. It was also found that the local structure plays an important role for the actinyl An=O bond strength. In the hydrated compounds the An=O bond strength was found to be weakened by hydrogen bonds to water in the second shell, while in the anhydrous compounds it was rather the interactions with the monovalent cations of higher polarization ability that weakened the An=O bond.

 

Short description of the work
An ongoing program of research performed at ITU includes the structural, thermomechanical and thermodynamic investigations of the reaction products between (U, Pu, Np)O2 fuels and sodium. Numerous sodium-uranates, -plutonates and -neptunates have been identified in the Na-An-O systems (An= U, Np, Pu) but none of the compounds, synthesized by solid-state synthesis, have ever been obtained as a single phase which is necessary for the measurements of their thermodynamic properties. In the present studies various routes of synthesis have been explored in order to get pure compounds. This work was part of a six months traineeship offered to a student of Chimie-Paristech. The first step of this research program has been to develop new methods of synthesis with identified surrogate elements to obtain pure compounds. This was performed by the student in the laboratories of Chimie-Paristech, Marie-Claire Illy, under the supervision of Pr. G. Wallez. The second step, which was the object of the TALISMAN proposal C0-05, was to apply those methods of synthesis to plutonium-bearing compounds. This has been done in the laboratories of ITU.

 

Short description of the work
Six MOX samples were studied with different Pu content: 14, 24, 35, 46, 54 and 62 mol%. The starting materials were sintered stoichiometric disks cut from pellets. For each composition we have performed up to 4 laser melting experiments – two in pressurized Ar and two in Air, each consisting of four shots of different duration and intensity. During the experiments we recorded the temperature at the surface of the sample with a pyrometer. The normal spectral emissivity of the sample was also studied with the help of a multi-channel spectro-pyrometer. Phase transitions were also qualitatively identified with the help of a reflected blue laser.
We noticed interesting differences between the melting behaviour observed in different atmospheres.
In Argon, the observed melting/freezing temperatures for the investigated compositions seem to be consistent with the stable near-stoichiometric fluorite phase across the thermal cycles. In this case the experimental points match the Calphad optimized phase boundaries well.
In Air, the observed melting/freezing transitions occur at a systematically lower temperature, the lower is the Pu content. This observation is consistent with the fact that Uranium dioxide is easily oxidized on the surface forming new chemical species (like MO2+x, M3O8, M4O9 etc.), which melt at a lower temperature and are more volatile.
Preliminary Raman spectroscopy investigation confirms these trends.
Further characterisation of the treated samples will be done using HR-XAS, XRD and SEM in the upcoming months.
With these results we hope to enhance the understanding of the melting behaviour of near-stoichiometric MOX materials and also gain insight into the behaviour under an oxidizing atmosphere.

 
More Articles...