Synthesis and characterization of homogeneous (U,Am)O₂ and (U,Pu,Am)O₂ nanopowders

This paper details the first dedicated production of homogeneous nanocrystalline particles of mixed actinide oxide solid solutions containing americium. The target compositions were $U_{0.75}Pu_{0.20}Am_{0.05}O_{2}$, $U_{0.90}Am_{0.10}O_{2}$ and $U_{0.80}Am_{0.20}O_{2}$. After successful hydrothermal synthesis and chemical characterisation, the nanocrystals were sintered and their structure and behaviour under self-irradiation were studied by powder XRD. Cationic charge distribution of the as-prepared nanocrystalline and sintered $U_{0.80}Am_{0.20}O_{2}$ materials was investigated applying $U M_{4}$ and $Am M_{5}$ edge high energy resolution XANES (HR-XANES). Typical oxidation states detected for the cations are $U_{(IV)}/U_{(V)}$ and $Am_{(III)}/Am_{(IV)}$. The measured crystallographic swelling was systematically smaller for the as-synthesised nanoparticles than the sintered products. For sintered pellets, the maximal volumetric swelling was about 0.8% at saturation, in line with literature data for $PuO_{2}$, $AmO_{2}$, $(U,Pu)O_{2}$ or $(U,Am)O_{2}$

In-situ high-temperature EXAFS measurements on radioactive and air-sensitive molten salt materials

The development at the Delft University of Technology (TU Delft, The Netherlands) of an experimental set-up dedicated to high-temperature in situ EXAFS measurements of radioactive, air-sensitive and corrosive fluoride salts is reported. A detailed description of the sample containment cell, of the furnace design, and of the measurement geometry allowing simultaneous transmission and fluorescence measurements is given herein. The performance of the equipment is tested with the room-temperature measurement of thorium tetrafluoride, and the Th—F and Th—Th bond distances obtained by fitting of the EXAFS data are compared with the ones extracted from a refinement of neutron diffraction data collected at the PEARL beamline at TU Delft. The adequacy of the sample confinement is checked with a mapping of the thorium concentration profile of molten salt material. Finally, a few selected salt mixtures (LiF:ThF4) = (0.9:0.1), (0.75:0.25), (0.5:0.5) and (NaF:ThF4) = (0.67:0.33), (0.5:0.5) are measured in the molten state. Qualitative trends along the series are discussed, and the experimental data for the (LiF:ThF4) = (0.5:0.5) composition are compared with the EXAFS spectrum generated from molecular dynamics simulations

In situ high-temperature EXAFS measurements on radioactive and air-sensitive molten salt materials

The development at the Delft University of Technology (TU Delft, The Netherlands) of an experimental set-up dedicated to high-temperature in situ EXAFS measurements of radioactive, air-sensitive and corrosive fluoride salts is reported. A detailed description of the sample containment cell, of the furnace design, and of the measurement geometry allowing simultaneous transmission and fluorescence measurements is given herein. The performance of the equipment is tested with the room-temperature measurement of thorium tetrafluoride, and the Th-F and Th-Th bond distances obtained by fitting of the EXAFS data are compared with the ones extracted from a refinement of neutron diffraction data collected at the PEARL beamline at TU Delft. The adequacy of the sample confinement is checked with a mapping of the thorium concentration profile of molten salt material. Finally, a few selected salt mixtures (LiF:ThF4) = (0.9:0.1), (0.75:0.25), (0.5:0.5) and (NaF:ThF4) = (0.67:0.33), (0.5:0.5) are measured in the molten state. Qualitative trends along the series are discussed, and the experimental data for the (LiF:ThF4) = (0.5:0.5) composition are compared with the EXAFS spectrum generated from molecular dynamics simulations

Novel non-invasive algorithm to identify the origins of re-entry and ectopic foci in the atria from 64-lead ECGs: A computational study.

Atrial tachy-arrhytmias, such as atrial fibrillation (AF), are characterised by irregular electrical activity in the atria, generally associated with erratic excitation underlain by re-entrant scroll waves, fibrillatory conduction of multiple wavelets or rapid focal activity. Epidemiological studies have shown an increase in AF prevalence in the developed world associated with an ageing society, highlighting the need for effective treatment options. Catheter ablation therapy, commonly used in the treatment of AF, requires spatial information on atrial electrical excitation. The standard 12-lead electrocardiogram (ECG) provides a method for non-invasive identification of the presence of arrhythmia, due to irregularity in the ECG signal associated with atrial activation compared to sinus rhythm, but has limitations in providing specific spatial information. There is therefore a pressing need to develop novel methods to identify and locate the origin of arrhythmic excitation. Invasive methods provide direct information on atrial activity, but may induce clinical complications. Non-invasive methods avoid such complications, but their development presents a greater challenge due to the non-direct nature of monitoring. Algorithms based on the ECG signals in multiple leads (e.g. a 64-lead vest) may provide a viable approach. In this study, we used a biophysically detailed model of the human atria and torso to investigate the correlation between the morphology of the ECG signals from a 64-lead vest and the location of the origin of rapid atrial excitation arising from rapid focal activity and/or re-entrant scroll waves. A focus-location algorithm was then constructed from this correlation. The algorithm had success rates of 93% and 76% for correctly identifying the origin of focal and re-entrant excitation with a spatial resolution of 40 mm, respectively. The general approach allows its application to any multi-lead ECG system. This represents a significant extension to our previously developed algorithms to predict the AF origins in association with focal activities

Thermodynamic Assessment of the AF–CrF3 (A = Li, Na, K) and CrF2–CrF3 Systems

Understanding the corrosion mechanisms and the effect of corrosion products on the basic properties of the salt (e.g., melting point, heat capacity) is fundamental for the safety assessment and durability of molten salt reactor technology. This work focused on the thermodynamic assessment of the CrF2−CrF3 system and the binary systems of chromium trifluoride CrF3 with alkali fluorides (LiF, NaF, KF) using the CALPHAD (computer coupling of phase diagrams and thermochemistry) method. In this work, the modified quasi-chemical model in the quadruplet approximation was used to develop new thermodynamic modelling assessments of the binary solutions, which are highly relevant in assessing the corrosion process in molten salt reactors. The agreement between these assessments and the phase equilibrium data available in the literature is generally good. The excess properties (mixing enthalpies, entropies and Gibbs energies) calculated in this work are consistent with the expected behaviour of decreasing enthalpy and Gibbs energy of mixing with the increasing ionic radius of the alkali cations

Nuclear Reactor Fuels: Materials with Highly Complex Behaviour

Radiation Science and TechnologyElectrical Engineering, Mathematics and Computer Scienc