Uptake of actinides by calcium silicate hydrate (C-S-H) phases

The sorption of actinides (Th, U – Am) was studied in dependence of the solid-to-liquid (S/L) ratio (0.5–20.0 g/L) and the calcium-to-silicon (C:S) ratio. The C:S ratio was varied between 1.80 and 0.70 to simulate the changing composition of the C-S-H phases during cement degradation from high to low C:S ratios. The decrease of the calcium content in the C-S-H phases by time is accompanied by a decrease in pH in the corresponding suspensions from 12.6 to 10.2. X-ray photoelectron spectroscopy (XPS) of the C-S-H phases showed an increasing depletion of Ca on the surface with increasing C:S ratio in comparison to the composition of the solid phase as a whole. The sorption experiments were performed with the redox stable species Am(III), Th(IV) and U(VI), as well as the redox sensitive Np(V) and Pu(III). The average distribution coefficients Rd for all investigated actinides are around 105 L/kg. The oxidation state of Pu retained by the C-S-H phases was investigated with high-energy resolution X-ray absorption near-edge structure (HR-XANES) spectroscopy. Samples with C:S ratios of 0.75 and 1.65 showed that the initially added Pu(III) was oxidized to Pu(IV) in the course of the experiment

Comparative U, Np and Pu M edge high energy resolution X-ray absorption spectroscopy (HR-XANES) investigations of model and genuine active waste glass

Genuine radioactive glass sampled from the vitrification plant Karlsruhe and actinide doped model 2 glasses  are  investigated  by  U/Pu/Np  M4/M5  high  energy  resolution  X‐ray  absorption near edge structure (HR‐XANES), U L3 EXAFS and XPS spectroscopy techniques to characterize and compare the U, Pu and Np oxidation states and their local atomic environments. The importance of the results will be discussed in terms of the strategy of using simplified simulated waste glasses to understand more complex industrial glass samples. The final goal of these studies is to predict the long term behavior of vitrified nuclear waste stored in a nuclear waste repository. Highly active waste concentrate (HAWC) from nuclear fuel reprocessing is immobilized in borosilicate glass matrices to generate a disposable waste form [1]. Between 2009 and 2010, the vitrification plant Karlsruhe (VEK) was operated for vitrification of liquid process residues left over from operation of the former reprocessing plant Karlsruhe (WAK). About 56 m3 HAWC were processed, resulting in 50 t of waste  glass  [2].  The  long  term  radiotoxicity  of  U,  Np,  Pu  and  other  actinide  elements  (An),  minor constitute of the reprocessed waste, is of great concern in safety assessment studies of nuclear waste repositories. For example, in case of water intrusion and interaction with the glass matrix, corrosion processes will take place which might facilitate the release of radionuclides into the geosphere. The An redox state and bonding characteristics in the glass matrix determine their release mechanisms and retention processes taking place in near and far field of the repository [3]. Understanding the long term behavior of vitrified nuclear waste requires full and detailed characterization of the materials including their characteristics as synthesized and after exposure to  groundwater. Genuine radioactive waste glass has a complex chemical composition. Therefore we take a simplified approach by investigating and comparing the oxidation states of U, Pu and Np in high level waste (HLW) glass sampled from the VEK vitrification process (VEK glass) and in model glasses. The model glasses doped with U and Pu have the same borosilicate glass frit composition as the VEK glass, whereas  the  model  glass  doped  with  Np  has  a  base  glass  composition  (R7T7)  typically  used  for  vitrification of HLW in France. U/Pu/Np  M4/M5  edge  high  energy  resolution  X‐ray  absorption  near  edge  structure  (HR‐XANES)  spectroscopy technique [4] is applied to characterize the An oxidation states

Comparative U, Np and Pu M edge high energy resolution X-ray absorption spectroscopy (HR-XANES) investigations of model and genuine active waste glass

Understanding the long term behavior of vitrified nuclear waste requires a full and detailed characterization of the materials including their characteristics as synthesized and after exposure to groundwater. Genuine radioactive waste glass has a complex chemical composition. Therefore we take a simplified approach by investigating and comparing the oxidation states of U, Pu and Np in high level waste (HLW) glass sampled from the VEK vitrification process (VEK glass) and in model glasses. The model glasses doped with U and Pu have the same borosilicate glass frit composition as the VEK glass, whereas the model glass doped with Np has a base glass composition (R7T7) typically used for vitrification of HLW in France. U/Pu/Np M4/M5 edge high energy resolution X-ray absorption near edge structure (HR-XANES) spectroscopy technique [1] is applied to characterize the An oxidation states in model and genuine VEK HLW glass. The HR-XANES analyses suggest predominant existence of U(VI) and Pu(IV) in the HLW and the model glasses as expected from the oxidative vitrification conditions. Weak changes in U oxidation state as a function of the U loading (1.2 – 5 wt% UO2) are discussed on the basis of U M4 edge HR-XANES and X-ray photoelectron spectroscopy (XPS) results. One significant difference found between the model and the genuine HLW glasses is the strong radiation damage induced in the HLW glass by the soft X-ray beam (position of the U M4 edge: 3.73 keV) which was not observed for the U doped model glasses and the previous L3 edge investigations of the HLW glass sample. The dominant U(VI) oxidation state is reduced almost by 50% to U(IV) within 5 h of measurement. The complex chemical composition of the HLW glass leads to different local U atomic environments compared to the model glass as found by EXAFS investigations. EXAFS results confirm that U in the HLW glass is coordinated by Al/Si neighbors in the second coordination sphere, whereas no neighboring atoms are observed at this distance for the model glass. Differences in results obtained for the Np oxidation state for Np doped asprepared and leached R7T7 borosilicate model glasses and the HLW glass will be presented and discussed