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Short description of the work

Post-mortem examinations (XRD, Raman, XAS etc.) after laser melting experiments are needed to study the changes induced in the sample’s properties. We performed several melting experiments at ITU Karlsruhe on MOX samples with six compositions (Pu/U+Pu at% = 14, 24, 35, 46, 54 and 62). Two series of experiments were carried out – one in Ar and one in air (pressurized to 3.5 bars). The post-mortem examination at the ANKA synchrotron was divided in two parts. First, we collected data at the L edges, mainly to study local structure and electronic properties around each cations. Four pairs of samples were chosen (MOX14, MOX46, MOX54, MOX62), each with the same composition, with one sample melted in air and the other in Ar. We recorded spectra at room temperature at the UL3, UL2 and PuL3 edges.
In the case of the samples treated in Ar, slight changes were observed in the XANES spectra (at the PuL3 and UL3 edges) as compared to the stoichiometric references UO2 and PuO2. The results of linear combination fitting hint towards slight oxidation of U cations in MOX14 and slight reduction of Pu cations in MOX62. MOX46 and MOX54 contained both oxidized U and reduced Pu.
Uranium spectra collected on samples treated in air show strong deviations from UO2. It appears that in each case the white lines could be placed between those of U4O9 and U3O8 references. Data found in literature suggests that oxidation will readily occur in MOX in the considered temperature range (300-3000 K) under air. Thus, linear combination fitting using U3O8 and U4O9 was done to estimate the composition of each sample. With increasing Pu content, the level of oxidation decreased. Also, for samples rich in Pu (MOX54 and MOX62) the PuL3 spectra are shifted towards lower energies, indicating a slight reduction of Pu.

 

Short description of the work
There is a general consensus that high level radioactive waste (HLW) will be disposed in deep geological formations. In clay repositories, due to the low water content, the movement of the radionuclides is driven by diffusive processes, but sorption and precipitation of newly formed phases are the key limiting processes in case of cations. Boda Claystone Formation (BCF) was selected in Hungary as potential host rock of a HLW repository. Previous combined micro-XRF measurements using 5 µm spatial resolution, on microscopically heterogeneous clay-rich rock samples originating from BCF revealed that not only the argillaceous matrix is responsible for uranium uptake, but 5-10 µm thick U-rich fringes were observed around carbonate fillings. Experiments at SLS included micro-XRF/XRD mapping at 1-2 µm resolution and U-L3 micro-XANES measurements at selected positions of thin sections treated with U(VI) solution. High-resolution micro-XRF/XRD results clearly showed that the fracture infilling region of the BCF samples contained zoned dolomite rombohedra with outer Mn-rich areas. These rhombohedra were covered with ankerite rims that showed coincidence with U enrichment. U uptake induced by ankerite would require formation of FeOOH via Fe oxidation and U reduction, however significant reduction of U could not be proved from the micro-XANES results. The investigations provided essential information to clarify the uranium uptake processes at different mineral phases in argillaceous rocks of BCF on the microscale.

 

Short description of the work

The experiments carried out in HZDR-IRC were focused on determination of the mechanisms of interaction of neptunium(IV, V and VI) ions with ionic species containing technetium with various oxidation states in aqueous acidic media.
The chemical species containing Tc and Np with required oxidation states were generated electrochemically in an electrolyte containing potassium pertechnetate or neptunyl(V) sulfate. The optimal conditions required for generation of Tc and Np with a chosen oxidation state were determined on the basis of chronoamperometric and cyclic voltammetry experiments.
Both technetium and neptunium species were electrogenerated separately in electrochemical cells equipped with gold or platinum electrodes. Such generated Tc and Np species were used in preparation of the mixed solution for Tc and Np interaction studies. UV-Vis-NIR was employed to study of the interactions between both elements. The obtained results show that the interaction of technetium(III), technetium(IV) and technetium(III,IV) ionic species with neptunyl(VI) ions leads to formation of intermediate species characterized spectroscopically by a band with the maximum at 460 nm. Neptunium(IV) acts as a technetium reducing agent and is able to reduce pertechnetates ions in acidic media to the forms characterized spectroscopically by the band at 500 nm.

 

Short description of the work
The combination of the synchrotron-based microscopic methods μ-XRF, μ-XAFS and µ-XRD, which are available at the microXAS Beamline at the Swiss Light Source, made it possible to determine the Np diffusion pathways in natural clay (Opalinus Clay (OPA) from Mont Terri, Switzerland) simultaneously with the local Np speciation. The aim of this project was to bring our initial study (JPR TALI-C01-15) to the next level by measuring a time series of Np diffusion profiles. Microscopic chemical images of the evolving reactive transport pattern at multiple times are mandatory to develop a robust, dynamic reactive transport model of the migration of Np in host rocks such as OPA. Additionally, two consolidating diffusion samples based on an improved experimental technique were prepared. This allowed us, first, to improve the data quality and second, to confirm and validate the previously obtained results. Furthermore, the new diffusion cell approach was used for a pilot study with a diffusion cell related to higher salinities (1 M NaCl). This is of relevance regarding argillaceous rock formations in Northern Germany. Overall, the complementary information obtained will be important for modelling the long-term behaviour of Np(V) in possible nuclear waste repositories with an argillaceous host rock.

 
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