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

Relaxation Parameter Mapping Adapted for 7T and Validation against Optimized Single Voxel MRS

Previously published T1 relaxation times for brain tissue at 7T vary greatly in results and in methods. We present T1 and T2 mapping sequences applicable at 7T. For T1 mapping, a Look-Locker sequence with an adiabatic inversion prepulse and a modified fitting routine was implemented. For T2 mapping the vendor pre-implemented mixed imaging sequence [1] was validated. For cross validation purpose T1 and T2 relaxation times were measured at selected anatomical locations with SV-MRS parameter series. All measurements were acquired on a 7T MR system. Nine healthy volunteers were measured and gave informed consent in line with local ethics regulations. Unsuppressed single voxel water spectra using very small voxel sizes with flip angle-optimized outer volume suppression has been used. For T1 measurement an inversion recovery series and for T2 measurements an echo time series were recorded. A three parameter model was fitted to the areas of the water peak to determine T1, and a two parameter model to determine T2. For the Look-Locker sequence a highly adiabatic inversion pulse was used and the residual inversion imperfections due to B1 inhomogeneity were corrected by a modified fitting model. From the mixed sequence only the T2 maps were validated with MRS. Relaxation values from the spectroscopy experiments are summarized in Table 1. T1 values for white matter (wm) lie approximately in the mean of the literature data [2-8]. T1 values for nucleus caudate and gray matter (gm) are in a good agreement with published data. T2 values for wm, nucleus caudate and gm are in general lower than the few published data. Except T1 values in wm, which were higher (p < 0.05) in the Look-Locker scan, T1 and T2 values in the caudate nucleus and gm as well as T2 values in wm from the imaging sequences (Table 2) were in an excellent agreement with the MRS data. optimized adiabatic inversion and consideration of inversion imperfection during fitting and validated this with single voxel MRS. We compared the T2 map from the implemented mixed sequence with MRS echo time series and found an excellent agreement between the methods. This enables relaxation parameter mapping at 7T for T1 and T2 without the use of technically challenging B1 correction methods

Spatially Selective SRF Optimization for Improved Reduction of Residual Lipid Aliasing in SENSE-accelerated 1H MRSI at 7T

Purpose/Introduction: In SENSE-accelerated [1] Magnetic Resonance Spectroscopic Imaging (MRSI), lipid signal originating in the skull region is often found in the center of the brain due to voxel bleeding and/or residual spatial aliasing. By direct optimization of the Spatial Response Function (SRF) on an overdiscrete spatial grid [2,3] a better spatial specificity and higher SENSEacceleration can be achieved. In this work, we introduce anatomical spatial prior knowledge to further improve the suppression of residual lipid artifacts. Subjects and Methods: Using Cartesian k-space sampling with fourfold SENSE acceleration (R=2x2 in APxRL), a FIDLOVS [4] MRSI data set of a transversal brain slice of a healthy volunteer was acquired at 7T with a 32 channel head coil. A 200x160mm2 FOV with voxel size 10x10x10mm3 was chosen, and eddy current correction was perfomed using a non water-suppressed reference. The MRSI reconstruction operator F is calculated by minimizing where SRF optimization is regularized by the noise level for every pixel π [7]. E is the encoding matrix and T the Gaussian SRF target functions, both with a ζ2=9-fold overdiscretization in real space [2]. The new diagonal matrix A contains spatial weights and introduces a prioritization of SRF optimality across the FOV depending on the coarse tissue types f (fat), b (brain) and o (outside the object) (Fig.1). Results: With higher priority of SRF optimality on fat than on brain, a drop of lipid signal intensity in representative spectra is observed (Fig.2B). The effective spatial resolution decreases only slightly for moderate weighting factors f/b<10 (Table 1) and stays well below the value of 1.85 achieved with conventional Hamming filtering. In the brain region, a decrease of lipid signal can be achieved with spatial prior knowledge (f/b =10, Fig.3). Discussion/Conclusion: Adapted target-driven, overdiscretized reconstruction largely reduces lipid artifacts in accelerated FIDLOVS MRSI through direct optimization of the SRF [2]. We demonstrate that a further suppression of any residual lipid artifacts in 1H brain spectra can be achieved by assigning a moderately elevated priority to SRF optimality in the region of subcranial lipids of 1H MRSI

Relaxation Parameter Mapping Adapted for 7T and Validation against Optimized Single Voxel MRS

Previously published T1 relaxation times for brain tissue at 7T vary greatly in results and in methods. Only two publications have assessed in vivo T2 relaxation times in the human brain at 7T so far. We present the development and validation of T1 and T2 mapping sequences applicable at 7T in the presence of B1 inhomogeneity. For T1 mapping, a Look-Locker sequence with an adiabatic inversion prepulse and a modified fitting routine was implemented. For T2 mapping the vendor pre-implemented mixed imaging sequence was validated. For cross validation purpose T1 and T2 relaxation times were measured at selected anatomical locations by optimized SV-MRS parameter series

Whole-brain estimates of directed connectivity for human connectomics

Connectomics is essential for understanding large-scale brain networks but requires that individual connection estimates are neurobiologically interpretable. In particular, a principle of brain organization is that reciprocal connections between cortical areas are functionally asymmetric. This is a challenge for fMRI-based connectomics in humans where only undirected functional connectivity estimates are routinely available. By contrast, whole-brain estimates of effective (directed) connectivity are computationally challenging, and emerging methods require empirical validation. Here, using a motor task at 7T, we demonstrate that a novel generative model can infer known connectivity features in a whole-brain network (>200 regions, >40,000 connections) highly efficiently. Furthermore, graph-theoretical analyses of directed connectivity estimates identify functional roles of motor areas more accurately than undirected functional connectivity estimates. These results, which can be achieved in an entirely unsupervised manner, demonstrate the feasibility of inferring directed connections in whole-brain networks and open new avenues for human connectomics.ISSN:1053-8119ISSN:1095-957

XANES characterization of UO2UO_{2}/Mo(Pd) thin films as models for ϵ\epsilon -particles in spent nuclear fuel

International audienceX-ray absorption near edge structure (XANES) is recorded for nano-and micro-particles formed in Mo doped UO 2 thin films (TFs) prepared by in situ sputter co-deposition and 1000°C tempering. These UO 2 /Mo TFs are intended to serve as synthetic models for ε-particles in spent nuclear fuel. We find that when Si is used as substrate, nano-sized zero-valent Mo particles form as desired. However, these are embedded in USi 3 , which forms at the high temper temperatures. Micron-sized Mo-particles are formed when SiO 2 is used as substrate. Using focussed X-ray beams of varying size (500µm, 25µm and 5µm), these particles are characterized to be predominantly hexavalent Mo oxides, potentially with tetrahedral coordinated [Mo(VI)O 4 ] 2-at the surface and a MoO 3-like phase in the bulk. These TFs are poor synthetic models for spent fuel ε-particles but do offer the opportunity to study changes in surface structures in response to stress/charge as a function of particle size. 1. Introduction Spent nuclear fuel (SNF) consists of 95% UO 2 and 5% mixture of radionuclides (RNs) and fission products, heterogeneously distributed throughout the fuel matrix in different phases, e.g., gases (Xe, Kr, I), oxides (including transuranic elements Pu, Np, etc.) and metallic precipitates or so-called ε-particles (Pd, Mo, Rh, Ru, etc.)[1,2]. Systematic investigations of SNF matrix corrosion as an important source term for the mobilization of RNs are essential for evaluating SNF disposal safety [3,4]. We prepare UO 2 /Mo thin films (TFs) for potential use as models to study the influence of ε-particles on the surface corrosion of SNF. The TFs provide a less complex system and lower radiation fields than actual SNF samples, which allow systematic mechanistic investigations even in non-radioactive laboratories. We determine how well these TF models mimic the SNF surface by thoroughly characterizing their composition, structure and morphology using numerous methods. Studies of UO 2 /Mo TFs having varying Mo content prepared via in situ sputter co-deposition on different substrates (Si, SiO 2 or LaAlO 3) by means of 2-dimensional (2D) and 3-dimensional synchrotron-based imaging methods (scanning nano-/micro-XRF, scanning nano-XRD and XRF, holographic and ptychographic tomography) reveal that nano-and micro-sized Mo-particles form in TFs prepared on Si and SiO