Crystal and electronic structures of A2NaIO6 periodate double perovskites (A = Sr, Ca, Ba): candidate wasteforms for I-129 immobilization

The synthesis, structure, and thermal stability of the periodate double perovskites A2NaIO6 (A= Ba, Sr, Ca) were investigated in the context of potential application for the immobilization of radioiodine. A combination of X-ray diffraction and neutron diffraction, Raman spectroscopy, and DFT simulations were applied to determine accurate crystal structures of these compounds and understand their relative stability. The compounds were found to exhibit rock-salt ordering of Na and I on the perovskite B-site; Ba2NaIO6 was found to adopt the Fm-3m aristotype structure, whereas Sr2NaIO6 and Ca2NaIO6 adopt the P21/n hettotype structure, characterized by cooperative octahedral tilting. DFT simulations determined the Fm-3m and P21/n structures of Ba2NaIO6 to be energetically degenerate at room temperature, whereas diffraction and spectroscopy data evidence only the presence of the Fm-3m phase at room temperature, which may imply an incipient phase transition for this compound. The periodate double perovskites were found to exhibit remarkable thermal stability, with Ba2NaIO6 only decomposing above 1050 °C in air, which is apparently the highest recorded decomposition temperature so far recorded for any iodine bearing compound. As such, these compounds offer some potential for application in the immobilization of iodine-129, from nuclear fuel reprocessing, with an iodine incorporation rate of 25–40 wt%. The synthesis of these compounds, elaborated here, is also compatible with both current conventional and future advanced processes for iodine recovery from the dissolver off-gas

Design of a 4-DOF hybrid PKM module for large structural component assembly

This paper presents a novel 4-DOF hybrid parallel kinematic machine (PKM), named Bicept, comprising a 2-DOF parallel mechanism plus a 2-DOF rotating head. The PKM is designed as a rigid yet compact module that can act as a robot cell moving along a long track for aircraft structural component assembly, a wing box for example. Dimensional synthesis of the 2-DOF parallel mechanism is carried out to achieve a relatively good kinematic performance within a prescribed task workspace that has a large width/height ratio. Then, using commercial CAE software, rigid body dynamics and stiffness analyses are carried out for motor sizing and performance evaluation of a full-size virtual prototype

Proton-induced ionization of isolated uracil molecules: A theory/experiment confrontation

Proton-induced ionization of RNA-uracil target is here theoretically described by two quantum-mechanical models, namely, a first perturbative one developed within the 1st Born approximation and a second one based on the continuum distorted wave approximation. Comparisons between theory and experiments are reported in terms of differential as well as total cross sections exhibiting a very good agreement for the kinematics here investigated

Reactivity of HTcO4 with methanol in sulfuric acid: Tc-sulfate complexes revealed by XAFS spectroscopy and first principles calculations

The reaction between HTcO4 and MeOH in 13 M H2SO4 was investigated by 99Tc NMR, UV-visible and X-ray absorption fine structure (XAFS) spectroscopy. Experimental results and first principles calculations show the formation of Tc(+5) sulfate complexes. The results expand the fundamental understanding of Tc in high acid solutions

Damage evolution in a tensile specimen of a ductile stainless steel

Abstract This paper presents a study about the fracture mechanism of a ferritic stainless steel (UNS S44400 type) during a tensile test. The applied materials for the experimental procedures were 25 specimens of the steel, machined in the rolling direction. Each specimen was submitted to standard polishing procedures. One of the samples, in the original state, was structurally characterized by reflected light optical microscopy. The other samples were submitted to tensile tests with a constant displacement rate. Three samples were tested until failure (complete tests) and the others just until specific strain values, when the tests were interrupted and the samples were characterized by using optical and scanning electron microscopy. The main objective of these characterizations was to evaluate the structural damage evolution and to identify the fracture mechanism for the tested conditions. A methodology to quantify the damage evolution by surface roughness, identified by optical microscopy, was proposed. A new index - Damage by Diffuse Reflection Index (DRI) - was proposed to quantify the damage evolution in function of the specimen deformation. It was possible to confirm the ductile behavior of the studied steel and that the main fracture mechanism was the traditional dimpled rupture